Shotcrete is a process where concrete is projected or "shot" under pressure using a feeder or "gun" onto a surface to form structural shapes including walls, floors, and roofs. The surface can be wood, steel, polystyrene, or any other surface that concrete can be projected onto. The surface can be trowelled smooth while the concrete is still wet.
History
Shotcrete was invented in the early 1900s by American taxidermist Carl Akeley, used to fill plaster models of animals. He used the method of blowing dry material out of a hose with compressed air, wetting it as it was released. This was later used to patch weak parts in old buildings. In 1911, he was granted a patent for his inventions, the "cement gun", the equipment used, and "gunite", the material that was produced. Until the 1950s when the wet-mix process was devised, only the dry-mix process was used. In the 1960s, the alternative method for gunning by the dry method was devised with the development of the rotary gun, with an open hopper that could be fed continuously. Shotcrete is also a viable means and method for placing structural concrete.
The nozzleman is the person controlling the nozzle that delivers the concrete to the surface. The nozzle is controlled by hand on small jobs, for example the construction of small swimming pools. On larger work the nozzle is held by mechanical arms and the nozzleman controls the operation by a hand-held remote control.
Benefits
Shotcrete has high strength, durability, low permeability, excellent bond and limitless shape possibilities. These properties allow shotcrete to be used in most cases as a structural material. Although the hardened properties of shotcrete are similar to conventional cast-in-place concrete, the nature of the placement process provides additional benefits, such as excellent bond with most substrates and instant or rapid capabilities, particularly on complex forms or shapes. In addition to building homes, shotcrete can also be used to build pools.
Methods of Application
Wet Mix - All ingredients, including water, are thoroughly mixed and introduced into the delivery equipment. Wet material is pumped to the nozzle where compressed air is added to provide high velocity for placement and consolidation of the material onto the receiving surface.
Dry Mix - Pre-blended dry or damp materials are placed into the delivery equipment. Compressed air conveys material through a hose at high velocity to the nozzle, where water is added. Material is consolidated on the receiving surface by the high-impact velocity.
Features
The properties of both wet and dry process shotcrete can be further enhanced through the addition of many different additives or admixtures such as:
Silica Fume - Provides reduced permeability, increased compressive and flexural strength, increased resistance to alkali and chemical attack, improved resistance to water washout, reduced rebound levels and allows for thicker sngle pass applications.
Air-Entraining Admixtures - Improve pumpability and adhesion in wet-process shotcrete and freeze-thaw durability in both wet and dry processes.
Fibers - Control cracking, increase toughness values and improve impact resistance and energy absorption.
Accelerators - Improve placement characteristics in adverse conditions, allow for thicker single pass applications, increase production capabilities and reduce the occurrence of fallouts on structures subjected to vibration.
Shotcrete and gunite are two commonly used terms for substances applied via pressure hoses. Shotcrete is concrete (or sometimes mortar) conveyed through a hose and pneumatically projected at high velocity onto a surface. Shotcrete undergoes placement and compaction at the same time due to the force with which it is projected from the nozzle. It can be impacted onto any type or shape of surface, including vertical or overhead areas.
The nozzleman is the person controlling the nozzle that delivers the concrete to the surface. The nozzle is controlled by hand on small jobs, for example the construction of small swimming pools. On larger work the nozzle is held by mechanical arms and the nozzleman controls the operation by a hand-held remote control.
DRY MIX VS WET MIX
The dry mix method involves placing the dry ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle. The nozzleman controls the addition of water at the nozzle. The water and the dry mixture is not completely mixed, but is completed as the mixture hits the receiving surface. This requires a skilled nozzleman, especially in the case of thick or heavily reinforced sections. Advantages of the dry mix process are that the water content can be adjusted instantaneously by the nozzleman, allowing more effective placement in overhead and vertical applications without using accelerators. The dry mix process is useful in repair applications when it is necessary to stop frequently, as the dry material is easily discharged from the hose.
Wet-mix shotcrete involves pumping of a previously prepared concrete, typically ready-mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The wet-gun procedure generally produces less rebound, waste (when material falls to the floor), and dust compared to the dry-mix procedure. The greatest advantage of the wet-mix process is that larger volumes can be placed in less time.
SHOTCRETE VS. GUNITE
Shotcrete is today an all-inclusive term that describes spraying concrete or mortar with either a dry or wet mix process. However, it may also sometimes be used to distinguish from gunite as a wet-mix. The term shotcrete was first defined by the American Railway Engineers Association (AREA) in the early 1930s.[1] By 1951, shotcrete had become the official generic name of the sprayed concrete process.[1]
Gunite refers only to the dry-mix process, in which the dry cementitious mixture is blown through a hose to the nozzle, where water is injected immediately before application. Gunite was the original term coined by Akeley, trademarked in 1909 and patented in North Carolina. The concrete is blasted by pneumatic pressure from a gun, hence "gun"-ite.
The term "Gunite" became the registered trademark of Allentown, the oldest manufacturer of gunite equipment. Other manufacturers were thus compelled to use other terminology to describe the process such as shotcrete, pneumatic concrete, guncrete, etc. Shotcrete emerged as the most commonly used term other than gunite, and after the later development of the wet process came to be used for both methods.
REINFORCEMENT
A 76 mm borehole in fibre reinforced shotcrete on a tunnel wall
Sprayed concrete is reinforced by conventional steel rods, steel mesh, and/or fibers. Fiber reinforcement (steel or synthetic) is also used for stabilization in applications such as slopes or tunneling.
Rabu, 11 Agustus 2010
Senin, 09 Agustus 2010
Application Techniques of Structural Repair and Strengthening of Concrete
A significant number of facilities in the world were constructed during the first half of the 20th Century using reinforced or prestressed concrete materials. Now, at the beginning of the next century, many of these buildings have reached the end of their planned service life, and deterioration in the form of steel corrosion, concrete cracking and spalling is frequently observed. In addition, many of these structures were built to carry loads that are significantly smaller than the current needs.
The result of these factors leaves many owners with the challenge of evaluating and implementing effective and economical repair and strengthening programs. Such repair and retrofit though is complicated. Most of these structures are occupied and the mainstream engineering and construction community expertise is centered on new construction, not retrofit or rehabilitation.
However, success can be obtained if the repair and strengthening systems are tailored to serve the structure's intended use without interfering with its occupants or function. Key to success is the combination of the different design skills and application techniques - structural strengthening and structural repair - necessary for such projects.
Embracing the jargon
Concrete experts commonly use the terms structural concrete repair and strengthening to describe building renovation activities. Although the two terms sound similar, they refer to slightly different concepts. Structural repair describes the process of reconstruction and renewal of an existing building or its structural elements. This involves determining the origin of the distress, removing damaged materials and cause of distress, as well as selecting and applying appropriate repair materials that extend the structure's useful life.
Structural strengthening, on the other hand, describes the process of upgrading the structural system of an existing building to improve performance under existing loads or to increase the strength of the existing structural components to carry additional loads. For upgrade projects, design engineers must deal with structures in which every element carries a share of the existing load.
The effects of strengthening or removing part or all of a structural element - such as penetrations or deteriorated materials - must be carefully analyzed to determine its influence on the global behavior of the structure. Failure to do so may overstress the structural element surrounding the affected area, which can lead to a bigger problem and even localized failure. With upgrade projects, contractors also must deal with critical issues related to access to the work area, constructability of the repair, noise and dust control, and type of construction materials that may not be as critical for new construction projects.
Environmental factors
Although durable, buildings constructed using reinforced and prestressed concrete have a finite service life. When exposed to harsh environments, de-icing salts and chemicals, these structures may experience significant deterioration, which typically occurs in the form of steel corrosion, concrete spalls, delamination and cracks.
Interestingly, one of the most severe and widespread problems in concrete is the internal damage caused by the corrosive action of external chlorides on reinforcing or prestressing steel embedded in concrete. Corrosion problems are basically caused by corrosion-process by-product (rust) that expands up to eight times its original volume, thus creating internal pressure, which causes the concrete to crack and spall. If not addressed at early stages, corrosion will continue to grow rapidly, ultimately creating a safety issue due to falling concrete and loss of strength.
The assessment, design and implementation of a durable repair to an existing structure is indeed more complex than for new construction. In addition to the unknown state of existing structural materials, the degree to which repair materials and the existing material will act as a composite and share loads must be addressed.
Before establishing a repair strategy, the concrete-repair expert must diagnose the problem's root cause, which enables prescribing repairs that are long-lasting and durable. In addition, by better understanding the repair material properties, it may be possible to produce repairs that will constitute a composite behavior with the existing structure and protect it from environmental factors. Failing to follow this process may result in a frustrating but common cyclical outcome known as "repairing the repair."
Figure 1a shows a graph of the Cost of Concrete Repair Versus Time. This figure illustrates the three observed phases describing the natural evolution of the concrete deterioration process and the influence of maintenance on this process:
Fig. 1a: Typical repair cost history diagram
• Preventive Maintenance Phase: In this phase, the owner may spend a fixed annual maintenance cost to install systems such as protective coatings to slow down the deterioration process. Money spent in this phase will delay the ingress of aggressive materials, thus delaying the start of active deterioration (Repair Phase).
• Repair Phase: In this phase, the concrete deterioration has begun, and the repair cost curve increases exponentially over time. The reason for the rapid increase in cost is that once aggressive materials that cause deterioration have sufficiently permeated into the concrete (a process that may take 20 to 30 years), the deterioration rate is rapid and irreversible.
• Replacement Phase: In this phase, a "wholesale" deterioration occurs throughout the structure at such a rapid rate that repair costs may exceed the costs of replacing the entire structure. However, total replacement of the structure may not be an option because of interruption to the function of the structure.
Incurring additional costs at early years to ensure well-protected concrete and addressing deterioration problems as soon as they are observed would delay excessive deterioration and may increase significantly the service life of the structure, as shown in Figure 1b.
Fig. 1b: Alternate repair cost history diagram
Preparation essential
A crucial step in achieving a durable repair is surface preparation. Multiple close-up observations of spalled concrete that had been repaired previously indicate that in many cases repairs have failed not because of material or technology, but because of the poor quality of surface preparation. The care with which deteriorated concrete is removed and the concrete and steel reinforcement surfaces are prepared often will determine whether a repair project will turn out to be successful.
Equipment selection is important, because the concrete-removal method should not weaken or crack the surrounding sound concrete. Care also should be exercised to avoid further damage to the reinforcing and prestressing steel. This is achieved by using only industry-specified, lightweight demolition equipment. The effect of concrete removal on the structural integrity should be investigated carefully. A temporary shoring system may be required to relieve the loads on repaired elements in cases where removal of concrete or corroded reinforcing steel is significant enough to affect the structure's load-carrying capacity.
Once concrete has been removed, the reinforcing steel should be carefully inspected to determine whether the steel should be simply cleaned, repaired or replaced. The final step in concrete and steel surface preparation is cleaning using abrasive or water-blasting techniques to remove loose materials and achieve an open-pore structure for the exposed concrete substrate and to remove the rust from the steel. This results in an appropriate surface to bond the new repair material and stop the corrosion process.
Fig. 2a: Surface preparation
Fig. 2b: Failing of repair on a building
Figure 2b shows an unsuccessful concrete repair project in which the spalled concrete edge was patched with a repair material without determining the cause of spalling or adequately preparing the steel and concrete surfaces. The repair materials started to delaminate just a few short months after the first repair was completed. Rust products can be seen trickling through the interface between the existing concrete and the repair material. Adequate surface preparation could have prevented this repair failure. All deteriorated concrete should have been removed above and around the steel bars, and the corroded steel should have been removed prior to installing the repair material.
Structural strengthening
Many school buildings that were originally constructed for a specific use now are being renovated or upgraded for a different application that may require higher load-carrying capacity. Typical examples of changing uses include the upgrade of parking garages and access ramps to carry the heavier loads of fire trucks and emergency vehicles; the conversion of administrative buildings to storage areas or classes with heavier load demands; and the installation of high-density filing systems in schools and education administrative offices.
As a result of these higher load demands, existing structures need to be reassessed and may require strengthening to meet heavier load requirements.
In general, structural strengthening may become necessary because of code changes, seismic upgrade, deficiencies that develop because of environmental effects (i.e., corrosion), changes in use that increase service loads, or deficiencies within the structure caused by errors in design or construction. The structural upgrade of concrete structures can be achieved using one of many different upgrading methods such as span shortening, externally bonded steel, fiber-reinforced polymer (FRP) composites, external or internal post-tensioning systems, section enlargement, or a combination of these techniques.
Similar to concrete repair, strengthening systems must perform in a composite manner with the existing structure to be effective and share the applied loads. The following gives a brief description of these methods and case-study applications.
Fig. 3: Parking garage using span shortening
Span shortening
Span shortening is accomplished by installing additional supports underneath existing members to reduce the span length. Materials used for span-shortening applications include structural steel members and cast-in-place reinforced concrete members, which are quick to install.
Connections can be designed easily using bolts and adhesive anchors. Span shortening may result in loss of space and reduced headroom. An example of this upgrading method is shown in Figure 3. The structural steel system shown was installed on a parking deck to shorten the span and carry part of the load, transferring it to the existing supporting system.
Strengthening with FRP composites
Fiber-reinforced polymer (FRP) systems are paper-thin fabric sheets bonded to concrete members with epoxy adhesive to increase their load-carrying capacity significantly. Usually carbon-based, these systems have been used extensively in the aerospace, automotive and sport-equipment industries, and are now becoming a mainstream technology for the structural upgrade of concrete structures. Important characteristic of FRPs for structural repair and strengthening applications include their non-corrosive properties, speed and ease of installation, lower cost, and aesthetic appeal.
Fig. 4a: Tunnel slab FRP strengthening
Fig. 4b: Carbon FRP fabric on slab underside
Fig. 4c: Installation of FRP rod
As with any other externally bonded system, the bond between the FRP system and the existing concrete is critical, and surface preparation is very important. Typically, installation is achieved by applying an epoxy adhesive to the prepared surface, installing the FRP fabric into the epoxy and then applying a second layer of the epoxy adhesive. After curing, the FRP composite will add considerable capacity to the element despite the fact that it is a very thin laminate. This is because the carbon FRP has tensile strength approximately 10 times that of steel.
Figure 4a shows a schematic for the structural strengthening of a utility tunnel at a university in South Florida. The utility tunnel roof originally functioned as a pedestrian walkway.
A new dormitory structure required the walkway to be the primary access for emergency vehicles. Analysis of the tunnel's top slab revealed it did not have adequate strength to carry loads from fire trucks and other emergency vehicles. The school needed an innovative approach to strengthen the tunnel slab to bring it up to required strength. A structurally efficient, easy to install and cost-effective strengthening option was achieved by using externally bonded FRP sheets.
The strengthening solution consisted of carbon FRP sheets bonded to the bottom of the slab, serving as additional bottom tension reinforcement, as shown in Figure 4b.
In addition, the overhanging portions of the slab were strengthened using carbon FRP bars epoxy-bonded in grooves made on the slab's top side. The latter technique is more appropriate than FRP sheets, because the bars were bonded below the surface, thereby avoiding traffic damage to the externally bonded reinforcement (see Figure 4c).
Bonded steel elements
Strengthening concrete members by using bonded steel plates was developed in the 1960s in Switzerland and Germany. In this method, steel elements are glued to the concrete surface by a two-component epoxy adhesive to create a composite system. The steel elements can be steel plates, channels, angles or built-up members. Steel elements bonded to the sides or bottom of a structural member can improve its shear or flexural strength.
In addition to epoxy adhesive, mechanical anchors typically are used to ensure the steel element will share external loads in case of adhesive failure. The exposed steel elements must be protected with a suitable system immediately following installation. Regardless of the specified corrosion protection system, its long-term durability properties and maintenance requirements must be fully considered.
Fig. 6: Schematic of the hybrid strengthening system
Figure 6 illustrates a schematic for the strengthening of a roof system of an elementary school in New Jersey. The school administration wanted to install skylights on the existing roof. The roof consisted of prestressed concrete hollow planks. Installation of the skylights required cutting openings in the planks that would reduce their load-carrying capacity.
This issue was resolved by designing a hybrid strengthening system composed of FRP fabric and steel elements. The externally bonded FRP strengthened the planks adjacent to the one to be cut, while the steel elements tied the plank to the adjacent ones, thus creating a new unit consisting of three planks with adequate capacity. In addition to the fast application of this system, this was a less expensive solution that was also aesthetically pleasing.
External post-tensioning
Fig. 7: Schematic for external post-tensioning system
The external post-tensioning technique has been effectively used to increase the flexural and shear capacity of both reinforced and prestressed concrete members since the 1950s. With this type of upgrading, active external forces are applied to the structural member using post-tensioned (stressed) cables to resist new loads. Because of the minimal additional weight of the repair system, this technique is effective and economical, and has been employed with great success to correct excessive deflections and cracking in beams and slabs, parking structures and cantilevered members.
The post-tensioning forces are delivered by means of standard prestressing tendons or high-strength steel rods, usually located outside the original section. The tendons are connected to the structure at anchor points, typically located at the ends of the member. End-anchors can be made of steel fixtures bolted to the structural member, or reinforced concrete blocks that are cast in-situ. The desired uplift force is provided by deviation blocks, fastened at the high or low points of the structural element. Prior to external prestressing, all existing cracks are epoxy-injected and spalls are patched to ensure prestressing forces are distributed uniformly across the section of the member.
Figure 7 illustrates an external post-tensioning system used to strengthen prestressed double tees damaged by vehicular impact. Four double-tee stems on an overpass located on the premises of a university in Washington, D.C., were damaged when the driver of an over-height truck failed to observe the posted height restriction.
The four stems suffered excessive concrete cracking and spalling, and damage occurred to some of the internal prestressing steel.
Proposed solutions included replacing the damaged double tees with new ones and installing a steel frame underneath for support. Both options would render the overpass out of service for a longer-than-desired period. The option of an external post-tensioning system was more economical, required less time to complete, and allowed for a strengthening system that provided active forces and therefore was more compatible with the existing construction.
After all cracks were injected, the sides of the stems were formed and new concrete was cast to restore the integrity of the stems. The strengthening system was then installed, and - after the concrete cured - the external strands were stressed according to the engineer-specified forces. This structural-strengthening option was fast and effective, saving the owner a considerable amount in construction and operation costs.
Section enlargement
Fig. 8: Beam strengthening using section enlargement
This method of strengthening involves placing additional "bonded" reinforced concrete to an existing structural member in the form of an overlay or a jacket. With section enlargement, columns, beams, slabs and walls can be enlarged to increase their load-carrying capacity or stiffness. A typical enlargement is approximately 2 to 3 inches for slabs and 3 to 5 inches for beams and columns.
Figure 8 depicts details of a section enlargement used to increase the capacity of a main girder in a university parking garage. The girder was re-evaluated because of a change in the required loading and found to be deficient in flexure and shear. To correct the deficiency, additional flexural and shear steel were added. The entire beam was then formed and a 4-inch jacket of concrete was cast to enlarge the section.
Do it right the first time
Regardless of the experience and experimental knowledge gained in more than 100 years of reinforced concrete construction, educational structures require repair and/or strengthening because of natural causes, human error and change in loading conditions.
Further, it is important to recognize that concrete repair and strengthening is a "scientific art form" that involves the use of conventional cement-based materials, as well as new techniques and materials.
A variety of factors including technical (engineering), constructability (construction methods), aesthetics (architectural), and economics (ROI) each play a role.
Many opportunities exist for engineers, contractors and material suppliers who can work together to supply their perspectives to an upgrade project. This explains the trend of design/build-type teams for delivering cost-effective solutions to school districts.
Contrary to industry perception, strengthening assessment and design is far more complex than new construction, and thus should not be treated lightly. Challenges usually arise because of unknown actual structural states such as load path, material properties, as well as the size and location of existing reinforcement or prestressing. The degree to which the upgrade system and the existing structural elements share the loads must be evaluated and properly addressed in the upgrade design, detailing and implementation methods.
In addition, facility engineers should consider the procurement process for specialty repair and strengthening projects to be different from new construction services.
Engaging specialty engineering and contracting firms that are familiar on a day-to-day basis with all of the critical aspects highlighted here will ensure the most cost-effective and long-lasting results. Although it may appear there is an up-front financial benefit to obtaining these specialty services from firms with experience in new construction, the real risk is that the repairs will cause an endless "repair of repairs" cycle resulting in additional disruption and expenditure to owners. When it comes to structural repair and strengthening, the mantra "do it right the first time" pays dividends
The result of these factors leaves many owners with the challenge of evaluating and implementing effective and economical repair and strengthening programs. Such repair and retrofit though is complicated. Most of these structures are occupied and the mainstream engineering and construction community expertise is centered on new construction, not retrofit or rehabilitation.
However, success can be obtained if the repair and strengthening systems are tailored to serve the structure's intended use without interfering with its occupants or function. Key to success is the combination of the different design skills and application techniques - structural strengthening and structural repair - necessary for such projects.
Embracing the jargon
Concrete experts commonly use the terms structural concrete repair and strengthening to describe building renovation activities. Although the two terms sound similar, they refer to slightly different concepts. Structural repair describes the process of reconstruction and renewal of an existing building or its structural elements. This involves determining the origin of the distress, removing damaged materials and cause of distress, as well as selecting and applying appropriate repair materials that extend the structure's useful life.
Structural strengthening, on the other hand, describes the process of upgrading the structural system of an existing building to improve performance under existing loads or to increase the strength of the existing structural components to carry additional loads. For upgrade projects, design engineers must deal with structures in which every element carries a share of the existing load.
The effects of strengthening or removing part or all of a structural element - such as penetrations or deteriorated materials - must be carefully analyzed to determine its influence on the global behavior of the structure. Failure to do so may overstress the structural element surrounding the affected area, which can lead to a bigger problem and even localized failure. With upgrade projects, contractors also must deal with critical issues related to access to the work area, constructability of the repair, noise and dust control, and type of construction materials that may not be as critical for new construction projects.
Environmental factors
Although durable, buildings constructed using reinforced and prestressed concrete have a finite service life. When exposed to harsh environments, de-icing salts and chemicals, these structures may experience significant deterioration, which typically occurs in the form of steel corrosion, concrete spalls, delamination and cracks.
Interestingly, one of the most severe and widespread problems in concrete is the internal damage caused by the corrosive action of external chlorides on reinforcing or prestressing steel embedded in concrete. Corrosion problems are basically caused by corrosion-process by-product (rust) that expands up to eight times its original volume, thus creating internal pressure, which causes the concrete to crack and spall. If not addressed at early stages, corrosion will continue to grow rapidly, ultimately creating a safety issue due to falling concrete and loss of strength.
The assessment, design and implementation of a durable repair to an existing structure is indeed more complex than for new construction. In addition to the unknown state of existing structural materials, the degree to which repair materials and the existing material will act as a composite and share loads must be addressed.
Before establishing a repair strategy, the concrete-repair expert must diagnose the problem's root cause, which enables prescribing repairs that are long-lasting and durable. In addition, by better understanding the repair material properties, it may be possible to produce repairs that will constitute a composite behavior with the existing structure and protect it from environmental factors. Failing to follow this process may result in a frustrating but common cyclical outcome known as "repairing the repair."
Figure 1a shows a graph of the Cost of Concrete Repair Versus Time. This figure illustrates the three observed phases describing the natural evolution of the concrete deterioration process and the influence of maintenance on this process:
Fig. 1a: Typical repair cost history diagram
• Preventive Maintenance Phase: In this phase, the owner may spend a fixed annual maintenance cost to install systems such as protective coatings to slow down the deterioration process. Money spent in this phase will delay the ingress of aggressive materials, thus delaying the start of active deterioration (Repair Phase).
• Repair Phase: In this phase, the concrete deterioration has begun, and the repair cost curve increases exponentially over time. The reason for the rapid increase in cost is that once aggressive materials that cause deterioration have sufficiently permeated into the concrete (a process that may take 20 to 30 years), the deterioration rate is rapid and irreversible.
• Replacement Phase: In this phase, a "wholesale" deterioration occurs throughout the structure at such a rapid rate that repair costs may exceed the costs of replacing the entire structure. However, total replacement of the structure may not be an option because of interruption to the function of the structure.
Incurring additional costs at early years to ensure well-protected concrete and addressing deterioration problems as soon as they are observed would delay excessive deterioration and may increase significantly the service life of the structure, as shown in Figure 1b.
Fig. 1b: Alternate repair cost history diagram
Preparation essential
A crucial step in achieving a durable repair is surface preparation. Multiple close-up observations of spalled concrete that had been repaired previously indicate that in many cases repairs have failed not because of material or technology, but because of the poor quality of surface preparation. The care with which deteriorated concrete is removed and the concrete and steel reinforcement surfaces are prepared often will determine whether a repair project will turn out to be successful.
Equipment selection is important, because the concrete-removal method should not weaken or crack the surrounding sound concrete. Care also should be exercised to avoid further damage to the reinforcing and prestressing steel. This is achieved by using only industry-specified, lightweight demolition equipment. The effect of concrete removal on the structural integrity should be investigated carefully. A temporary shoring system may be required to relieve the loads on repaired elements in cases where removal of concrete or corroded reinforcing steel is significant enough to affect the structure's load-carrying capacity.
Once concrete has been removed, the reinforcing steel should be carefully inspected to determine whether the steel should be simply cleaned, repaired or replaced. The final step in concrete and steel surface preparation is cleaning using abrasive or water-blasting techniques to remove loose materials and achieve an open-pore structure for the exposed concrete substrate and to remove the rust from the steel. This results in an appropriate surface to bond the new repair material and stop the corrosion process.
Fig. 2a: Surface preparation
Fig. 2b: Failing of repair on a building
Figure 2b shows an unsuccessful concrete repair project in which the spalled concrete edge was patched with a repair material without determining the cause of spalling or adequately preparing the steel and concrete surfaces. The repair materials started to delaminate just a few short months after the first repair was completed. Rust products can be seen trickling through the interface between the existing concrete and the repair material. Adequate surface preparation could have prevented this repair failure. All deteriorated concrete should have been removed above and around the steel bars, and the corroded steel should have been removed prior to installing the repair material.
Structural strengthening
Many school buildings that were originally constructed for a specific use now are being renovated or upgraded for a different application that may require higher load-carrying capacity. Typical examples of changing uses include the upgrade of parking garages and access ramps to carry the heavier loads of fire trucks and emergency vehicles; the conversion of administrative buildings to storage areas or classes with heavier load demands; and the installation of high-density filing systems in schools and education administrative offices.
As a result of these higher load demands, existing structures need to be reassessed and may require strengthening to meet heavier load requirements.
In general, structural strengthening may become necessary because of code changes, seismic upgrade, deficiencies that develop because of environmental effects (i.e., corrosion), changes in use that increase service loads, or deficiencies within the structure caused by errors in design or construction. The structural upgrade of concrete structures can be achieved using one of many different upgrading methods such as span shortening, externally bonded steel, fiber-reinforced polymer (FRP) composites, external or internal post-tensioning systems, section enlargement, or a combination of these techniques.
Similar to concrete repair, strengthening systems must perform in a composite manner with the existing structure to be effective and share the applied loads. The following gives a brief description of these methods and case-study applications.
Fig. 3: Parking garage using span shortening
Span shortening
Span shortening is accomplished by installing additional supports underneath existing members to reduce the span length. Materials used for span-shortening applications include structural steel members and cast-in-place reinforced concrete members, which are quick to install.
Connections can be designed easily using bolts and adhesive anchors. Span shortening may result in loss of space and reduced headroom. An example of this upgrading method is shown in Figure 3. The structural steel system shown was installed on a parking deck to shorten the span and carry part of the load, transferring it to the existing supporting system.
Strengthening with FRP composites
Fiber-reinforced polymer (FRP) systems are paper-thin fabric sheets bonded to concrete members with epoxy adhesive to increase their load-carrying capacity significantly. Usually carbon-based, these systems have been used extensively in the aerospace, automotive and sport-equipment industries, and are now becoming a mainstream technology for the structural upgrade of concrete structures. Important characteristic of FRPs for structural repair and strengthening applications include their non-corrosive properties, speed and ease of installation, lower cost, and aesthetic appeal.
Fig. 4a: Tunnel slab FRP strengthening
Fig. 4b: Carbon FRP fabric on slab underside
Fig. 4c: Installation of FRP rod
As with any other externally bonded system, the bond between the FRP system and the existing concrete is critical, and surface preparation is very important. Typically, installation is achieved by applying an epoxy adhesive to the prepared surface, installing the FRP fabric into the epoxy and then applying a second layer of the epoxy adhesive. After curing, the FRP composite will add considerable capacity to the element despite the fact that it is a very thin laminate. This is because the carbon FRP has tensile strength approximately 10 times that of steel.
Figure 4a shows a schematic for the structural strengthening of a utility tunnel at a university in South Florida. The utility tunnel roof originally functioned as a pedestrian walkway.
A new dormitory structure required the walkway to be the primary access for emergency vehicles. Analysis of the tunnel's top slab revealed it did not have adequate strength to carry loads from fire trucks and other emergency vehicles. The school needed an innovative approach to strengthen the tunnel slab to bring it up to required strength. A structurally efficient, easy to install and cost-effective strengthening option was achieved by using externally bonded FRP sheets.
The strengthening solution consisted of carbon FRP sheets bonded to the bottom of the slab, serving as additional bottom tension reinforcement, as shown in Figure 4b.
In addition, the overhanging portions of the slab were strengthened using carbon FRP bars epoxy-bonded in grooves made on the slab's top side. The latter technique is more appropriate than FRP sheets, because the bars were bonded below the surface, thereby avoiding traffic damage to the externally bonded reinforcement (see Figure 4c).
Bonded steel elements
Strengthening concrete members by using bonded steel plates was developed in the 1960s in Switzerland and Germany. In this method, steel elements are glued to the concrete surface by a two-component epoxy adhesive to create a composite system. The steel elements can be steel plates, channels, angles or built-up members. Steel elements bonded to the sides or bottom of a structural member can improve its shear or flexural strength.
In addition to epoxy adhesive, mechanical anchors typically are used to ensure the steel element will share external loads in case of adhesive failure. The exposed steel elements must be protected with a suitable system immediately following installation. Regardless of the specified corrosion protection system, its long-term durability properties and maintenance requirements must be fully considered.
Fig. 6: Schematic of the hybrid strengthening system
Figure 6 illustrates a schematic for the strengthening of a roof system of an elementary school in New Jersey. The school administration wanted to install skylights on the existing roof. The roof consisted of prestressed concrete hollow planks. Installation of the skylights required cutting openings in the planks that would reduce their load-carrying capacity.
This issue was resolved by designing a hybrid strengthening system composed of FRP fabric and steel elements. The externally bonded FRP strengthened the planks adjacent to the one to be cut, while the steel elements tied the plank to the adjacent ones, thus creating a new unit consisting of three planks with adequate capacity. In addition to the fast application of this system, this was a less expensive solution that was also aesthetically pleasing.
External post-tensioning
Fig. 7: Schematic for external post-tensioning system
The external post-tensioning technique has been effectively used to increase the flexural and shear capacity of both reinforced and prestressed concrete members since the 1950s. With this type of upgrading, active external forces are applied to the structural member using post-tensioned (stressed) cables to resist new loads. Because of the minimal additional weight of the repair system, this technique is effective and economical, and has been employed with great success to correct excessive deflections and cracking in beams and slabs, parking structures and cantilevered members.
The post-tensioning forces are delivered by means of standard prestressing tendons or high-strength steel rods, usually located outside the original section. The tendons are connected to the structure at anchor points, typically located at the ends of the member. End-anchors can be made of steel fixtures bolted to the structural member, or reinforced concrete blocks that are cast in-situ. The desired uplift force is provided by deviation blocks, fastened at the high or low points of the structural element. Prior to external prestressing, all existing cracks are epoxy-injected and spalls are patched to ensure prestressing forces are distributed uniformly across the section of the member.
Figure 7 illustrates an external post-tensioning system used to strengthen prestressed double tees damaged by vehicular impact. Four double-tee stems on an overpass located on the premises of a university in Washington, D.C., were damaged when the driver of an over-height truck failed to observe the posted height restriction.
The four stems suffered excessive concrete cracking and spalling, and damage occurred to some of the internal prestressing steel.
Proposed solutions included replacing the damaged double tees with new ones and installing a steel frame underneath for support. Both options would render the overpass out of service for a longer-than-desired period. The option of an external post-tensioning system was more economical, required less time to complete, and allowed for a strengthening system that provided active forces and therefore was more compatible with the existing construction.
After all cracks were injected, the sides of the stems were formed and new concrete was cast to restore the integrity of the stems. The strengthening system was then installed, and - after the concrete cured - the external strands were stressed according to the engineer-specified forces. This structural-strengthening option was fast and effective, saving the owner a considerable amount in construction and operation costs.
Section enlargement
Fig. 8: Beam strengthening using section enlargement
This method of strengthening involves placing additional "bonded" reinforced concrete to an existing structural member in the form of an overlay or a jacket. With section enlargement, columns, beams, slabs and walls can be enlarged to increase their load-carrying capacity or stiffness. A typical enlargement is approximately 2 to 3 inches for slabs and 3 to 5 inches for beams and columns.
Figure 8 depicts details of a section enlargement used to increase the capacity of a main girder in a university parking garage. The girder was re-evaluated because of a change in the required loading and found to be deficient in flexure and shear. To correct the deficiency, additional flexural and shear steel were added. The entire beam was then formed and a 4-inch jacket of concrete was cast to enlarge the section.
Do it right the first time
Regardless of the experience and experimental knowledge gained in more than 100 years of reinforced concrete construction, educational structures require repair and/or strengthening because of natural causes, human error and change in loading conditions.
Further, it is important to recognize that concrete repair and strengthening is a "scientific art form" that involves the use of conventional cement-based materials, as well as new techniques and materials.
A variety of factors including technical (engineering), constructability (construction methods), aesthetics (architectural), and economics (ROI) each play a role.
Many opportunities exist for engineers, contractors and material suppliers who can work together to supply their perspectives to an upgrade project. This explains the trend of design/build-type teams for delivering cost-effective solutions to school districts.
Contrary to industry perception, strengthening assessment and design is far more complex than new construction, and thus should not be treated lightly. Challenges usually arise because of unknown actual structural states such as load path, material properties, as well as the size and location of existing reinforcement or prestressing. The degree to which the upgrade system and the existing structural elements share the loads must be evaluated and properly addressed in the upgrade design, detailing and implementation methods.
In addition, facility engineers should consider the procurement process for specialty repair and strengthening projects to be different from new construction services.
Engaging specialty engineering and contracting firms that are familiar on a day-to-day basis with all of the critical aspects highlighted here will ensure the most cost-effective and long-lasting results. Although it may appear there is an up-front financial benefit to obtaining these specialty services from firms with experience in new construction, the real risk is that the repairs will cause an endless "repair of repairs" cycle resulting in additional disruption and expenditure to owners. When it comes to structural repair and strengthening, the mantra "do it right the first time" pays dividends
Selasa, 02 Maret 2010
PERBAIKAN DAN PERKUATAN STRUKTUR BETON
PERBAIKAN DAN PERKUATAN STRUKTUR BETON
Ir. Hartono/Ir Jeffry F Tumatar
ABSTRAK
Dengan makin banyaknya struktur bangunan yang mengalami kerusakan pada saat masa layannya ataupun pada saat proses konstruksi, maka diperlukan pengetahuan mengenai teknologi perbaikan dan perkuatan yang tepat guna.
Pada paper ini akan disajikan beberapa metode serta material perbaikan dan perkuatan yang dapat digunakan, dan kontrol kualitas selama dan sesudah pelaksanaan proses perbaikan dan perkuatan. Selain itu juga dibahas mengenai penggunaan Self Compacting Concrete (SCC) pada metode perkuatan
1. PENDAHULUAN
Seperti kita ketahui semua, pada saat ini dengan makin banyaknya bangunan yang mengalami kerusakan struktur maupun non-struktur; pada saat masa layannya ataupun pada saat proses pembangunan; yang diakibatkan oleh faktor dari bangunan itu sendiri maupun faktor dari luar. Dimana bentuk dan tingkat kerusakan yang terjadi mulai dari yang ringan sampai berat.
Dengan adanya tuntutan bahwa bangunan yang mengalami kerusakan harus sudah dapat secepatnya difungsikan kembali, maka perlu adanya penanganan terhadap kerusakan-kerusakan yang terjadi, baik dengan melakukan perbaikan ataupun perkuatan. Seringkali dengan terbatasnya waktu, maka perbaikan atau perkuatan yang dilakukan tidak memperhatikan beberapa kaidah yang berkaitan dengan kapasitas struktur dan prosedur pelaksanan serta kontrol kualitas.
Oleh sebab itu untuk mendapatkan hasil perbaikan dan perkuatan yang tepat guna dan mencapai sasaran yang telah ditetapkan, maka perlu dilakukan investigasi untuk mendapatkan data-data kerusakan baik melalui pengamatan visual ataupun dengan bantuan pengujian non-destructive maupun semi destructive dan mereview dokumen dari struktur yang ada. Dari hasil investigasi tersebut, kemudian dilakukan analisa dan evaluasi pada struktur tersebut untuk menetapkan apakah kerusakan yang terjadi hanya perlu perbaikan atau perlu perkuatan atau dalam kondisi yang terjelek struktur yang mengalami kerusakan harus dilakukan pembongkaran dan dibangun struktur baru.
Dalam paper ini akan dibahas mengenai beberapa metode perbaikan dan perkuatan yang dapat digunakan dalam penanganan terhadap kerusakan yang terjadi.
2. METODE DAN MATERIAL PERBAIKAN
Penentuan metode dan material perbaikan umumnya tergantung pada jenis kerusakan yang ada, disamping besar dan luasnya kerusakan yang terjadi, lingkungan dimana struktur berada, peralatan yang tersedia, kemampuan tenaga pelaksana serta batasan-batasan dari pemilik seperti keterbatasan ruang kerja, kemudahan pelaksanaan, waktu pelaksanaan dan biaya perbaikan.
Jenis kerusakan yang sering terjadi adalah kerusakan berupa keretakan dan spalling (terlepasnya bagian beton).
A. Keretakan
Keretakan dibedakan retak struktur dan non-struktur. Retak struktur umumnya terjadi pada elemen struktur beton bertulang, sedang retak non-struktur terjadi dinding bata atau dinding non-beton lainnya.
Untuk retak non-struktur, dapat digunakan metode injeksi dengan material pasta semen yang dicampur dengan expanding agent serta latex atau hanya melakukan sealing saja dengan material polymer mortar atau polyurethane sealant.
Sedang pada retak struktur, digunakan metode injeksi dengan material epoxy yang mempunyai viskositas yang rendah, sehingga dapat mengisi dan sekaligus melekatkan kembali bagian beton yang terpisah.
Proses injeksi dapat dilakukan secara manual maupun dengan mesin yang bertekanan, tergantung pada lebar dan dalamnya keretakan.
B. Spalling
Metode perbaikan pada kerusakan spalling, tergantung pada besar dan dalamnya spalling yang terjadi.
i. Patching
Untuk spalling yang tidak terlalu dalam (kurang dari selimut beton) dan area yang tidak luas, dapat digunakan metode patching.
Metode perbaikan ini adalah metode perbaikan manual, dengan melakukan penempelan mortar secara manual. Pada saat pelaksanaan yang harus diperhatikan adalah penekanan pada saat mortar ditempelkan; sehingga benar-benar didapatkan hasil yang padat.
Material yang digunakan harus memiliki sifat mudah dikerjakan, tidak susut dan tidak jatuh setelah terpasang (lihat maksimum ketebalan yang dapat dipasang tiap lapis), terutama untuk pekerjaan perbaikan overhead. Umumnya yang dipakai adalah monomer mortar, polymer mortar dan epoxy mortar.
ii. Grouting
Sedang pada spalling yang melebihi selimut beton, dapat digunakan metode grouting, yaitu metode perbaikan dengan melakukan pengecoran memakai bahan non-shrink mortar.
Metode ini dapat dilakukan secara manual (gravitasi) atau menggunakan pompa.
Pada metode perbaikan ini yang perlu diperhatikan adalah bekisting yang terpasang harus benar-benar kedap, agar tidak ada kebocoran spesi yang mengakibatkan terjadinya keropos dan harus kuat agar mampu menahan tekanan dari bahan grouting.
Material yang digunakan harus memiliki sifat mengalir dan tidak susut. Umumnya digunakan bahan dasar semen atau epoxy.
iii. Shotcrete (Beton Tembak)
Apabila spalling yang terjadi pada area yang sangat luas, maka sebaiknya digunakan metode Shot-crete. Pada metode ini tidak diperlukan bekisting lagi seperti halnya pengecoran pada umumnya.
Metode shotcrete ada dua sistim yaitu dry-mix dan wet-mix.
Pada sistim dry-mix, campuran yang dimasukkan dalam mesin berupa campuran kering, dan akan tercampur dengan air di ujung selang. Sehingga mutu dari beton yang ditembakkan sangat tergantung pada keahlian tenaga yang memegang selang, yang mengatur jumlah air. Tapi sistim ini sangat mudah dalam perawatan mesin shotcretenya, karena tidak pernah terjadi ‘blocking’.
Pada sistim wet-mix, campuran yang dimasukkan dalam mesin berupa campuran basah, sehingga mutu beton yang ditembakkan lebih seragam. Tapi sistim ini memerlukan perawatan mesin yang tinggi, apalagi bila sampai terjadi ‘blocking’.
Pada metode shotcrete, umumnya digunakan additive untuk mempercepat pengeringan (accelerator), dengan tujuan mempercepat pengerasan dan mengurangi terjadinya banyaknya bahan yang terpantul dan jatuh (rebound).
iv. Grout Preplaced Aggregat (Beton Prepack)
Metode perbaikan lainnya untuk memperbaiki kerusakan berupa spalling yang cukup dalam adalah dengan metode Grout Preplaced Aggregat. Pada metode ini beton yang dihasilkan adalah dengan cara menempatkan sejumlah agregat (umumnya 40% dari volume kerusakan) kedalam bekisting, setelah itu dilakukan pemompaan bahan grout, kedalam bekisting.
Material grout yang umumnya digunakan adalah polymer grout, yang memiliki flow cukup tinggi dan tidak susut.
3. METODE DAN MATERIAL PERKUATAN
Dalam pemilihan metode perkuatan, harus diperhatikan beberapa hal yaitu kapasitas struktur, lingkungan dimana struktur berada, peralatan yang tersedia, kemampuan tenaga pelaksana serta batasan-batasan dari pemilik seperti keterbatasan ruang kerja, kemudahan pelaksanaan, waktu pelaksanaan dan biaya perkuatan.
Metode perkuatan yang umumnya dilakukan adalah :
- Memperpendek bentang dari struktur dengan konstruksi beton ataupun dengan konstruksi baja.
Tujuannya adalah memperkecil gaya-gaya dalam yang terjadi, tetapi harus dianalisa ulang akibat dari perpendekan bentang ini yang menyebabkan perubahan dari gaya-gaya dalam tersebut.
Umumnya dilakukan dengan menambah balok atau kolom baik dari beton maupun dari baja.
- Memperbesar dimensi daripada konstruksi beton.
Umumnya digunakan beton sebagai material untuk memperbesar dimensi struktur; dengan adanya admixture beton generasi baru, dimungkinkan untuk menghasilkan beton yang dapat memadat sendiri (self compacting concrete), dibahas di bagian 4 – Self Compacting Concrete.
Akibat dari penambahan dimensi tersebut, maka harus diperhatikan bahwa secara keseluruhan beban dari Bangunan tersebut bertambah, sehingga harus dilakukan analisa secara menyeluruh dari struktur atas sampai pondasi.
- Menambah plat baja.
Tujuan dari penambahan ini adalah untuk menambah kekuatan pada bagian tarik dari struktur Bangunan.
Didalam penambahan plat baja tersebut, harus dijamin bahwa plat baja menjadi satu kesatuan dengan struktur yang ada, umumnya untuk menjamin lekatan antara plat baja dengan struktur beton digunakan epoxy adhesive.
- Melakukan external prestressing.
Dengan metode ini, kapasitas struktur ditingkatkan dengan melakukan prestress di luar struktur, bukan didalam seperti pada struktur baru.
Yang perlu diperhatikan adalah penempatan anchor head, sehingga tidak menyebabkan perlemahan pada struktur yang ada.
Material yang umumnya digunakan adalah baja prestress, tetapi pada saat ini sudah mulai digunakan bahan dari FRP (Fibre Reinforced Polymer).
- Menggunakan FRP (Fibre Reinforced Polymer)
Prinsip daripada penambahan FRP sama seperti penambahan plat baja, yaitu menambah kekuatan di bagian tarik dari struktur.
Tipe FRP yang sering dipakai pada perkuatan struktur adalah dari bahan carbon, aramid dan glass. Bentuk FRP yang sering digunakan pada perkuatan struktur adalah Plate / Composite dan Fabric / Wrap
Bentuk plate lebih efektif dan efisien untuk perkuatan lentur baik pada balok maupun plat serta pada dinding; sedang bentuk wrap lebih efektif dan efisien untuk perkuatan geser pada balok serta untuk meningkatkan kapasitas beban axial dan geser pada kolom.
4. Self Compacting Concrete
Self Compacting Concrete atau yang umum disingkat dengan istilah SCC adalah beton segar yang sangat plastis dan mudah mengalir karena berat sendirinya mengisi keseluruh cetakan yang dikarenakan beton tersebut memiliki sifat-sifat untuk memadatkan sendiri, tanpa adanya bantuan alat penggetar. Beton SCC yang baik harus tetap homogen, kohesif, tidak segregasi, tidak terjadi blocking, dan tidak bleeding.
Pemakaian beton SCC sebagai material repair dapat meningkatkan kualitas beton repair oleh karena dapat menghindari sebagian dari potensi kesalahan manusia akibat manual compaction. Pemadatan yang kurang sempurna pada saat proses pengecoran dapat mengakibatkan berkurangnya durabilitas beton. Sebaliknya dengan beton SCC struktur beton repair menjadi lebih padat terutama pada daerah pembesian yang sangat rapat, dan waktu pelaksanaan pengecoran juga lebih cepat.
Workability
Berdasarkan spesifikasi SCC dari EFNARC, workabilitas atau kelecakan campuran beton segar dapat dikatakan sebagai beton SCC apabila memenuhi kriteria sebagai berikut yaitu:
§ Filling ability
§ Passing ability
§ Segregation resistance
Filling ability, adalah kemampuan beton SCC untuk mengalir dan mengisi keseluruh bagian cetakan melalui berat sendirinya.
Passing ability, adalah kemampuan beton SCC untuk mengalir melalui celah-celah antar besi tulangan atau bagian celah yang sempit dari cetakan tanpa terjadi adanya segregasi atau blocking.
Segregation resistance, adalah kemampuan beton SCC untuk menjaga tetap dalam keadaan komposisi yang homogen selama waktu transportasi sampai pada saat pengecoran.
Metoda Test
Metoda test pengukuran workability telah dikembangkan untuk menentukan karakteristik beton SCC dan sampai saat ini belum ada satu jenis metoda test yang bisa mewakili ketiga syarat karakteristik beton SCC seperti tersebut di atas. Dari beberapa metoda test yang telah dikembangkan akan dibahas hanya tiga macam metoda yang dianggap dapat mewakili ketiga kriteria workability tersebut di atas.
Slump-Flow
Slump-flow test dapat dipakai untuk menentukan ‘filling ability’ baik di laboratorium maupun di lapangan; dan dengan memakai alat ini dapat diperoleh kondisi workabilitas beton berdasarkan kemampuan penyebaran beton segar yang dinyatakan dengan besaran diameter yaitu antara 60 cm – 75 cm.
Kebutuhan nilai slump flow untuk pengecoran konstruksi bidang vertikal berbeda dengan bidang horisontal. Kriteria yang umum dipakai untuk penentuan awal workabilitas beton SCC berdasarkan tipe konstruksi adalah sebagai berikut :
Untuk konstruksi vertikal, disarankan menggunakan slump-flow antara 65 cm sampai 70 cm.
Untuk konstruksi horisontal disarankan menggunakan slump-flow antara 60 cm sampai 65 cm.
Slump-Flow test
L-Shape-Box
Dipakai untuk mengetahui kriteria ‘passing ability’ dari beton SCC. Dengan menggunakan L-Shape Box, dapat diketahui kemungkinan adanya blocking beton segar saat mengalir, dan juga dapat dilihat viskositas beton segar yang bersangkutan. Selanjutnya dengan L-Shape-Box test akan didapat nilai blocking ratio yaitu nilai yang didapat dari perbandingan antara H2 / H1. Semakin besar nilai blocking ratio, semakin baik beton segar mengalir dengan viskositas tertentu. Untuk test ini kriteria yang umum dipakai baik untuk tipe konstruksi vertikal maupun untuk konstruksi horisontal disarankan mencapai nilai blocking ratio antara 0.8 sampai 1.0
L-Shape-Box test
V - funnel
Dipakai untuk mengukur viskositas beton SCC dan sekaligus mengetahui ‘segregation resistance’ . Kemampuan beton segar untuk segera mengalir melalui mulut di ujung bawah alat ukur V-funnel diukur dengan besaran waktu antara 6 detik sampai maksimal 12 detik.
V-funnel test
Pouring dan Formwork
Beberapa hal yang perlu diperhatikan sebelum pengecoran dengan beton SCC adalah sebagai berikut:
Durasi waktu pengecoran disesuaikan dengan waktu ikat awal beton untuk menghindari terjadinya cold joint.
Cara terbaik untuk pengecoran beton SCC adalah dari bawah cetakan/formwork untuk menghindari udara terjebak (dengan eksternal hose adalah sangat efektif).
Beton SCC dapat mengalir sampai jarak 10 meter tanpa hambatan.
Elemen tipis 5 – 7 cm dapat diisi oleh beton SCC tanpa hambatan.
Tidak memerlukan keahlian yang spesifik saat pelaksanaan pengecoran.
5. PELAKSANAAN PERBAIKAN DAN PERKUATAN
Sebelum dilakukan pelaksanaan perbaikan atau perkuatan, perlu dilakukan pengecekan terakhir apakah metode dan material yang sudah ditentukan sesuai dengan kondisi lapangan dan dapat dilaksanakan.
Pada saat pelaksanaan yang perlu mendapat perhatian adalah :
- Persiapan permukaan.
Permukaan beton yang akan diperbaiki atau diperkuat perlu dipersiapkan, dengan tujuan agar terjadi ikatan yang baik; sehingga material perbaikan atau perkuatan dengan beton lama menjadi satu kesatuan.
Permukaan beton yang akan diperbaiki atau diperkuat, harus merupakan permukaan yang kuat dan padat, tidak ada keropos ataupun bagian lemah lainnya (kecuali bila menggunakan metode injeksi untuk mengisi celah keropos); serta harus bersih dari debu dan kotoran lainnya.
Apabila ada tulangan yang sudah berkarat, maka perlu dilakukan pemotongan beton hingga + 20 mm dibawah tulangan yang berkarat. Dan karat tersebut harus dibersihkan, serta diberi lapisan anti karat.
Permukaan yang sudah dipersiapkan, apakah harus dalam keadaan kering atau harus dijenuhkan terlebih dahulu sebelum dilakukan pelapisan berikutnya. Hal ini sangat tergantung pada material yang digunakan. Untuk material berbahan dasar semen atau polymer, permukaan beton harus dijenuhkan terlebih dahulu; tetapi bila material yang digunakan berbahan dasar epoxy, maka permukaan beton harus dalam keadaan kering.
- Perbandingan campuran.
Untuk menghasilkan mutu dari material perbaikan atau material bonding yang digunakan dalam perkuatan sesuai dengan yang direkomendasikan dari pabrik, maka perbandingan campuran dari material harus diikuti dengan tepat, apalagi bila menggunakan material berbahan dasar epoxy.
Bila menggunakan beton yang dapat memadat sendiri, perlu diperhatikan jumlah air, flow dari beton serta dipastikan tidak adanya bleeding dan segregasi.
- Pot life.
Adalah waktu yang dibutuhkan dari pengadukan hingga material tersebut terpasang. Apabila waktu telah melebihi pot life-nya, maka material yang sudah tercampur jangan digunakan.
- Kekuatan tekan.
Seperti pada pelaksanaan kontruksi baru, dimana dilakukan kontrol kualitas pada mutu beton yang ada; maka saat pelaksanaan dari perbaikan dan perkuatan, juga harus dilakukan hal yang sama, dengan melakukan pengambilan sample sesuai standard yang ada. (ASTM C39 – beton, ASTM C109 – mortar semen dan ASTM D495 – epoxy)
Setelah pelaksanaan juga perlu dilakukan kontrol kualitas, untuk melihat apakah pelaksanaan perbaikan dan perkuatan sudah sesuai dengan standard yang ada.
- Injeksi.
Tujuan dari kontrol kualitas setelah pekerjaan injeksi dilakukan adalah untuk melihat apakah bahan injeksi sudah mengisi celah keretakan yang ada, dan juga melihat kualitas lekatan dari bahan injeksi dalam mengikatkan celah keretakan.
Dilakukan dengan melakukan coring f 50 mm (ASTM C42) untuk melihat penetrasi bahan injeksi, kemudian hasil core tersebut ditest tekan (ASTM C39) atau splitting (ASTM C496) untuk mengetahui kualitas lekatan yang terjadi. Atau dapat juga dilakukan kontrol kualitas dengan non-destruktif test yaitu UPV (Ultra Pulse Velocity) – ASTM C597 atau Impact Echo.
- Patching, Grouting, Shot-crete, Beton Prepack dan Beton SCC.
Tujuan dari kontrol kualitas pada pekerjaan ini adalah untuk melihat lekatan yang terjadi antara beton lama dengan material perbaikan.
Dilakukan dengan Direct tensile bond test -ACI 503R Appendix A atau Pull-Off Test - ICRI Technical Guideline 03739.
- Perkuatan dengan FRP.
Tujuan dari kontrol kualitas pada pekerjaan ini adalah untuk melihat lekatan antara epoxy adhesive yang digunakan untuk melekatkan FRP.
Dilakukan dengan Direct tensile bond test -ACI 503R Appendix A atau Pull-Off Test - ICRI Technical Guideline 03739.
6. PENUTUP
Untuk mendapatkan hasil perbaikan dan perkuatan yang tepat guna dan mencegah hasil-hasil yang tidak diharapkan, maka diperlukan koordinasi antara pihak-pihak yang melakukan investigasi, pengujian, evaluasi dan pelaksanaan. Oleh sebab itu diperlukan keterlibatan semua pihak terkait mulai dari konsultan perencana, konsultan pengawas, kontraktor spesialis dan supplier dari bahan-bahan perbaikan atau perkuatan. Karena tanpa adanya koordinasi yang baik, maka tidak dapat diharapkan hasil yang maksimal.
Dan yang terpenting masing-masing tahapan harus dilakukan oleh pihak-pihak yang berkompeten dan berpengalaman di bidangnya.
7. DAFTAR PUSTAKA :
1. Peter H. Emmons, Concrete Repair and Maintenance Illustrated, USA, 1994.
2. FIP, Inspection and Maintenance of Reinforced and Pre-stressed Concrete Structures, London, 1986.
3. FIP, Repair and Strengthening of Concrete Structures, London, 1991.
4. ACI 201.1R, Guide for Making a Condition Survey of Concrete in Service, 1992.
5. ACI 364.1R, Guide for Evaluation of Concrete Structures Prior to Rehabilitation, 1993.
6. ACI 228.2R, Nondestructive Test Methods for Evaluation of Concrete in Structures
7. ICRI Technical Guidelines No. 03731, 1996
8. ICRI Technical Guidelines No. 03733, 1996
9. ICRI Technical Guidelines No. 03739, 2004
10. ICRI Technical Guidelines No. 03742, 2006
11. Repair Application Procedure – ACI Committee 706, 2003
12. Specification and Guidelines for Self-Compacting Concrete, February 2002 EFNARC, Association House, 99 West Street, Farnham, Surrey GU9 7EN, UK
Ir. Hartono/Ir Jeffry F Tumatar
ABSTRAK
Dengan makin banyaknya struktur bangunan yang mengalami kerusakan pada saat masa layannya ataupun pada saat proses konstruksi, maka diperlukan pengetahuan mengenai teknologi perbaikan dan perkuatan yang tepat guna.
Pada paper ini akan disajikan beberapa metode serta material perbaikan dan perkuatan yang dapat digunakan, dan kontrol kualitas selama dan sesudah pelaksanaan proses perbaikan dan perkuatan. Selain itu juga dibahas mengenai penggunaan Self Compacting Concrete (SCC) pada metode perkuatan
1. PENDAHULUAN
Seperti kita ketahui semua, pada saat ini dengan makin banyaknya bangunan yang mengalami kerusakan struktur maupun non-struktur; pada saat masa layannya ataupun pada saat proses pembangunan; yang diakibatkan oleh faktor dari bangunan itu sendiri maupun faktor dari luar. Dimana bentuk dan tingkat kerusakan yang terjadi mulai dari yang ringan sampai berat.
Dengan adanya tuntutan bahwa bangunan yang mengalami kerusakan harus sudah dapat secepatnya difungsikan kembali, maka perlu adanya penanganan terhadap kerusakan-kerusakan yang terjadi, baik dengan melakukan perbaikan ataupun perkuatan. Seringkali dengan terbatasnya waktu, maka perbaikan atau perkuatan yang dilakukan tidak memperhatikan beberapa kaidah yang berkaitan dengan kapasitas struktur dan prosedur pelaksanan serta kontrol kualitas.
Oleh sebab itu untuk mendapatkan hasil perbaikan dan perkuatan yang tepat guna dan mencapai sasaran yang telah ditetapkan, maka perlu dilakukan investigasi untuk mendapatkan data-data kerusakan baik melalui pengamatan visual ataupun dengan bantuan pengujian non-destructive maupun semi destructive dan mereview dokumen dari struktur yang ada. Dari hasil investigasi tersebut, kemudian dilakukan analisa dan evaluasi pada struktur tersebut untuk menetapkan apakah kerusakan yang terjadi hanya perlu perbaikan atau perlu perkuatan atau dalam kondisi yang terjelek struktur yang mengalami kerusakan harus dilakukan pembongkaran dan dibangun struktur baru.
Dalam paper ini akan dibahas mengenai beberapa metode perbaikan dan perkuatan yang dapat digunakan dalam penanganan terhadap kerusakan yang terjadi.
2. METODE DAN MATERIAL PERBAIKAN
Penentuan metode dan material perbaikan umumnya tergantung pada jenis kerusakan yang ada, disamping besar dan luasnya kerusakan yang terjadi, lingkungan dimana struktur berada, peralatan yang tersedia, kemampuan tenaga pelaksana serta batasan-batasan dari pemilik seperti keterbatasan ruang kerja, kemudahan pelaksanaan, waktu pelaksanaan dan biaya perbaikan.
Jenis kerusakan yang sering terjadi adalah kerusakan berupa keretakan dan spalling (terlepasnya bagian beton).
A. Keretakan
Keretakan dibedakan retak struktur dan non-struktur. Retak struktur umumnya terjadi pada elemen struktur beton bertulang, sedang retak non-struktur terjadi dinding bata atau dinding non-beton lainnya.
Untuk retak non-struktur, dapat digunakan metode injeksi dengan material pasta semen yang dicampur dengan expanding agent serta latex atau hanya melakukan sealing saja dengan material polymer mortar atau polyurethane sealant.
Sedang pada retak struktur, digunakan metode injeksi dengan material epoxy yang mempunyai viskositas yang rendah, sehingga dapat mengisi dan sekaligus melekatkan kembali bagian beton yang terpisah.
Proses injeksi dapat dilakukan secara manual maupun dengan mesin yang bertekanan, tergantung pada lebar dan dalamnya keretakan.
B. Spalling
Metode perbaikan pada kerusakan spalling, tergantung pada besar dan dalamnya spalling yang terjadi.
i. Patching
Untuk spalling yang tidak terlalu dalam (kurang dari selimut beton) dan area yang tidak luas, dapat digunakan metode patching.
Metode perbaikan ini adalah metode perbaikan manual, dengan melakukan penempelan mortar secara manual. Pada saat pelaksanaan yang harus diperhatikan adalah penekanan pada saat mortar ditempelkan; sehingga benar-benar didapatkan hasil yang padat.
Material yang digunakan harus memiliki sifat mudah dikerjakan, tidak susut dan tidak jatuh setelah terpasang (lihat maksimum ketebalan yang dapat dipasang tiap lapis), terutama untuk pekerjaan perbaikan overhead. Umumnya yang dipakai adalah monomer mortar, polymer mortar dan epoxy mortar.
ii. Grouting
Sedang pada spalling yang melebihi selimut beton, dapat digunakan metode grouting, yaitu metode perbaikan dengan melakukan pengecoran memakai bahan non-shrink mortar.
Metode ini dapat dilakukan secara manual (gravitasi) atau menggunakan pompa.
Pada metode perbaikan ini yang perlu diperhatikan adalah bekisting yang terpasang harus benar-benar kedap, agar tidak ada kebocoran spesi yang mengakibatkan terjadinya keropos dan harus kuat agar mampu menahan tekanan dari bahan grouting.
Material yang digunakan harus memiliki sifat mengalir dan tidak susut. Umumnya digunakan bahan dasar semen atau epoxy.
iii. Shotcrete (Beton Tembak)
Apabila spalling yang terjadi pada area yang sangat luas, maka sebaiknya digunakan metode Shot-crete. Pada metode ini tidak diperlukan bekisting lagi seperti halnya pengecoran pada umumnya.
Metode shotcrete ada dua sistim yaitu dry-mix dan wet-mix.
Pada sistim dry-mix, campuran yang dimasukkan dalam mesin berupa campuran kering, dan akan tercampur dengan air di ujung selang. Sehingga mutu dari beton yang ditembakkan sangat tergantung pada keahlian tenaga yang memegang selang, yang mengatur jumlah air. Tapi sistim ini sangat mudah dalam perawatan mesin shotcretenya, karena tidak pernah terjadi ‘blocking’.
Pada sistim wet-mix, campuran yang dimasukkan dalam mesin berupa campuran basah, sehingga mutu beton yang ditembakkan lebih seragam. Tapi sistim ini memerlukan perawatan mesin yang tinggi, apalagi bila sampai terjadi ‘blocking’.
Pada metode shotcrete, umumnya digunakan additive untuk mempercepat pengeringan (accelerator), dengan tujuan mempercepat pengerasan dan mengurangi terjadinya banyaknya bahan yang terpantul dan jatuh (rebound).
iv. Grout Preplaced Aggregat (Beton Prepack)
Metode perbaikan lainnya untuk memperbaiki kerusakan berupa spalling yang cukup dalam adalah dengan metode Grout Preplaced Aggregat. Pada metode ini beton yang dihasilkan adalah dengan cara menempatkan sejumlah agregat (umumnya 40% dari volume kerusakan) kedalam bekisting, setelah itu dilakukan pemompaan bahan grout, kedalam bekisting.
Material grout yang umumnya digunakan adalah polymer grout, yang memiliki flow cukup tinggi dan tidak susut.
3. METODE DAN MATERIAL PERKUATAN
Dalam pemilihan metode perkuatan, harus diperhatikan beberapa hal yaitu kapasitas struktur, lingkungan dimana struktur berada, peralatan yang tersedia, kemampuan tenaga pelaksana serta batasan-batasan dari pemilik seperti keterbatasan ruang kerja, kemudahan pelaksanaan, waktu pelaksanaan dan biaya perkuatan.
Metode perkuatan yang umumnya dilakukan adalah :
- Memperpendek bentang dari struktur dengan konstruksi beton ataupun dengan konstruksi baja.
Tujuannya adalah memperkecil gaya-gaya dalam yang terjadi, tetapi harus dianalisa ulang akibat dari perpendekan bentang ini yang menyebabkan perubahan dari gaya-gaya dalam tersebut.
Umumnya dilakukan dengan menambah balok atau kolom baik dari beton maupun dari baja.
- Memperbesar dimensi daripada konstruksi beton.
Umumnya digunakan beton sebagai material untuk memperbesar dimensi struktur; dengan adanya admixture beton generasi baru, dimungkinkan untuk menghasilkan beton yang dapat memadat sendiri (self compacting concrete), dibahas di bagian 4 – Self Compacting Concrete.
Akibat dari penambahan dimensi tersebut, maka harus diperhatikan bahwa secara keseluruhan beban dari Bangunan tersebut bertambah, sehingga harus dilakukan analisa secara menyeluruh dari struktur atas sampai pondasi.
- Menambah plat baja.
Tujuan dari penambahan ini adalah untuk menambah kekuatan pada bagian tarik dari struktur Bangunan.
Didalam penambahan plat baja tersebut, harus dijamin bahwa plat baja menjadi satu kesatuan dengan struktur yang ada, umumnya untuk menjamin lekatan antara plat baja dengan struktur beton digunakan epoxy adhesive.
- Melakukan external prestressing.
Dengan metode ini, kapasitas struktur ditingkatkan dengan melakukan prestress di luar struktur, bukan didalam seperti pada struktur baru.
Yang perlu diperhatikan adalah penempatan anchor head, sehingga tidak menyebabkan perlemahan pada struktur yang ada.
Material yang umumnya digunakan adalah baja prestress, tetapi pada saat ini sudah mulai digunakan bahan dari FRP (Fibre Reinforced Polymer).
- Menggunakan FRP (Fibre Reinforced Polymer)
Prinsip daripada penambahan FRP sama seperti penambahan plat baja, yaitu menambah kekuatan di bagian tarik dari struktur.
Tipe FRP yang sering dipakai pada perkuatan struktur adalah dari bahan carbon, aramid dan glass. Bentuk FRP yang sering digunakan pada perkuatan struktur adalah Plate / Composite dan Fabric / Wrap
Bentuk plate lebih efektif dan efisien untuk perkuatan lentur baik pada balok maupun plat serta pada dinding; sedang bentuk wrap lebih efektif dan efisien untuk perkuatan geser pada balok serta untuk meningkatkan kapasitas beban axial dan geser pada kolom.
4. Self Compacting Concrete
Self Compacting Concrete atau yang umum disingkat dengan istilah SCC adalah beton segar yang sangat plastis dan mudah mengalir karena berat sendirinya mengisi keseluruh cetakan yang dikarenakan beton tersebut memiliki sifat-sifat untuk memadatkan sendiri, tanpa adanya bantuan alat penggetar. Beton SCC yang baik harus tetap homogen, kohesif, tidak segregasi, tidak terjadi blocking, dan tidak bleeding.
Pemakaian beton SCC sebagai material repair dapat meningkatkan kualitas beton repair oleh karena dapat menghindari sebagian dari potensi kesalahan manusia akibat manual compaction. Pemadatan yang kurang sempurna pada saat proses pengecoran dapat mengakibatkan berkurangnya durabilitas beton. Sebaliknya dengan beton SCC struktur beton repair menjadi lebih padat terutama pada daerah pembesian yang sangat rapat, dan waktu pelaksanaan pengecoran juga lebih cepat.
Workability
Berdasarkan spesifikasi SCC dari EFNARC, workabilitas atau kelecakan campuran beton segar dapat dikatakan sebagai beton SCC apabila memenuhi kriteria sebagai berikut yaitu:
§ Filling ability
§ Passing ability
§ Segregation resistance
Filling ability, adalah kemampuan beton SCC untuk mengalir dan mengisi keseluruh bagian cetakan melalui berat sendirinya.
Passing ability, adalah kemampuan beton SCC untuk mengalir melalui celah-celah antar besi tulangan atau bagian celah yang sempit dari cetakan tanpa terjadi adanya segregasi atau blocking.
Segregation resistance, adalah kemampuan beton SCC untuk menjaga tetap dalam keadaan komposisi yang homogen selama waktu transportasi sampai pada saat pengecoran.
Metoda Test
Metoda test pengukuran workability telah dikembangkan untuk menentukan karakteristik beton SCC dan sampai saat ini belum ada satu jenis metoda test yang bisa mewakili ketiga syarat karakteristik beton SCC seperti tersebut di atas. Dari beberapa metoda test yang telah dikembangkan akan dibahas hanya tiga macam metoda yang dianggap dapat mewakili ketiga kriteria workability tersebut di atas.
Slump-Flow
Slump-flow test dapat dipakai untuk menentukan ‘filling ability’ baik di laboratorium maupun di lapangan; dan dengan memakai alat ini dapat diperoleh kondisi workabilitas beton berdasarkan kemampuan penyebaran beton segar yang dinyatakan dengan besaran diameter yaitu antara 60 cm – 75 cm.
Kebutuhan nilai slump flow untuk pengecoran konstruksi bidang vertikal berbeda dengan bidang horisontal. Kriteria yang umum dipakai untuk penentuan awal workabilitas beton SCC berdasarkan tipe konstruksi adalah sebagai berikut :
Untuk konstruksi vertikal, disarankan menggunakan slump-flow antara 65 cm sampai 70 cm.
Untuk konstruksi horisontal disarankan menggunakan slump-flow antara 60 cm sampai 65 cm.
Slump-Flow test
L-Shape-Box
Dipakai untuk mengetahui kriteria ‘passing ability’ dari beton SCC. Dengan menggunakan L-Shape Box, dapat diketahui kemungkinan adanya blocking beton segar saat mengalir, dan juga dapat dilihat viskositas beton segar yang bersangkutan. Selanjutnya dengan L-Shape-Box test akan didapat nilai blocking ratio yaitu nilai yang didapat dari perbandingan antara H2 / H1. Semakin besar nilai blocking ratio, semakin baik beton segar mengalir dengan viskositas tertentu. Untuk test ini kriteria yang umum dipakai baik untuk tipe konstruksi vertikal maupun untuk konstruksi horisontal disarankan mencapai nilai blocking ratio antara 0.8 sampai 1.0
L-Shape-Box test
V - funnel
Dipakai untuk mengukur viskositas beton SCC dan sekaligus mengetahui ‘segregation resistance’ . Kemampuan beton segar untuk segera mengalir melalui mulut di ujung bawah alat ukur V-funnel diukur dengan besaran waktu antara 6 detik sampai maksimal 12 detik.
V-funnel test
Pouring dan Formwork
Beberapa hal yang perlu diperhatikan sebelum pengecoran dengan beton SCC adalah sebagai berikut:
Durasi waktu pengecoran disesuaikan dengan waktu ikat awal beton untuk menghindari terjadinya cold joint.
Cara terbaik untuk pengecoran beton SCC adalah dari bawah cetakan/formwork untuk menghindari udara terjebak (dengan eksternal hose adalah sangat efektif).
Beton SCC dapat mengalir sampai jarak 10 meter tanpa hambatan.
Elemen tipis 5 – 7 cm dapat diisi oleh beton SCC tanpa hambatan.
Tidak memerlukan keahlian yang spesifik saat pelaksanaan pengecoran.
5. PELAKSANAAN PERBAIKAN DAN PERKUATAN
Sebelum dilakukan pelaksanaan perbaikan atau perkuatan, perlu dilakukan pengecekan terakhir apakah metode dan material yang sudah ditentukan sesuai dengan kondisi lapangan dan dapat dilaksanakan.
Pada saat pelaksanaan yang perlu mendapat perhatian adalah :
- Persiapan permukaan.
Permukaan beton yang akan diperbaiki atau diperkuat perlu dipersiapkan, dengan tujuan agar terjadi ikatan yang baik; sehingga material perbaikan atau perkuatan dengan beton lama menjadi satu kesatuan.
Permukaan beton yang akan diperbaiki atau diperkuat, harus merupakan permukaan yang kuat dan padat, tidak ada keropos ataupun bagian lemah lainnya (kecuali bila menggunakan metode injeksi untuk mengisi celah keropos); serta harus bersih dari debu dan kotoran lainnya.
Apabila ada tulangan yang sudah berkarat, maka perlu dilakukan pemotongan beton hingga + 20 mm dibawah tulangan yang berkarat. Dan karat tersebut harus dibersihkan, serta diberi lapisan anti karat.
Permukaan yang sudah dipersiapkan, apakah harus dalam keadaan kering atau harus dijenuhkan terlebih dahulu sebelum dilakukan pelapisan berikutnya. Hal ini sangat tergantung pada material yang digunakan. Untuk material berbahan dasar semen atau polymer, permukaan beton harus dijenuhkan terlebih dahulu; tetapi bila material yang digunakan berbahan dasar epoxy, maka permukaan beton harus dalam keadaan kering.
- Perbandingan campuran.
Untuk menghasilkan mutu dari material perbaikan atau material bonding yang digunakan dalam perkuatan sesuai dengan yang direkomendasikan dari pabrik, maka perbandingan campuran dari material harus diikuti dengan tepat, apalagi bila menggunakan material berbahan dasar epoxy.
Bila menggunakan beton yang dapat memadat sendiri, perlu diperhatikan jumlah air, flow dari beton serta dipastikan tidak adanya bleeding dan segregasi.
- Pot life.
Adalah waktu yang dibutuhkan dari pengadukan hingga material tersebut terpasang. Apabila waktu telah melebihi pot life-nya, maka material yang sudah tercampur jangan digunakan.
- Kekuatan tekan.
Seperti pada pelaksanaan kontruksi baru, dimana dilakukan kontrol kualitas pada mutu beton yang ada; maka saat pelaksanaan dari perbaikan dan perkuatan, juga harus dilakukan hal yang sama, dengan melakukan pengambilan sample sesuai standard yang ada. (ASTM C39 – beton, ASTM C109 – mortar semen dan ASTM D495 – epoxy)
Setelah pelaksanaan juga perlu dilakukan kontrol kualitas, untuk melihat apakah pelaksanaan perbaikan dan perkuatan sudah sesuai dengan standard yang ada.
- Injeksi.
Tujuan dari kontrol kualitas setelah pekerjaan injeksi dilakukan adalah untuk melihat apakah bahan injeksi sudah mengisi celah keretakan yang ada, dan juga melihat kualitas lekatan dari bahan injeksi dalam mengikatkan celah keretakan.
Dilakukan dengan melakukan coring f 50 mm (ASTM C42) untuk melihat penetrasi bahan injeksi, kemudian hasil core tersebut ditest tekan (ASTM C39) atau splitting (ASTM C496) untuk mengetahui kualitas lekatan yang terjadi. Atau dapat juga dilakukan kontrol kualitas dengan non-destruktif test yaitu UPV (Ultra Pulse Velocity) – ASTM C597 atau Impact Echo.
- Patching, Grouting, Shot-crete, Beton Prepack dan Beton SCC.
Tujuan dari kontrol kualitas pada pekerjaan ini adalah untuk melihat lekatan yang terjadi antara beton lama dengan material perbaikan.
Dilakukan dengan Direct tensile bond test -ACI 503R Appendix A atau Pull-Off Test - ICRI Technical Guideline 03739.
- Perkuatan dengan FRP.
Tujuan dari kontrol kualitas pada pekerjaan ini adalah untuk melihat lekatan antara epoxy adhesive yang digunakan untuk melekatkan FRP.
Dilakukan dengan Direct tensile bond test -ACI 503R Appendix A atau Pull-Off Test - ICRI Technical Guideline 03739.
6. PENUTUP
Untuk mendapatkan hasil perbaikan dan perkuatan yang tepat guna dan mencegah hasil-hasil yang tidak diharapkan, maka diperlukan koordinasi antara pihak-pihak yang melakukan investigasi, pengujian, evaluasi dan pelaksanaan. Oleh sebab itu diperlukan keterlibatan semua pihak terkait mulai dari konsultan perencana, konsultan pengawas, kontraktor spesialis dan supplier dari bahan-bahan perbaikan atau perkuatan. Karena tanpa adanya koordinasi yang baik, maka tidak dapat diharapkan hasil yang maksimal.
Dan yang terpenting masing-masing tahapan harus dilakukan oleh pihak-pihak yang berkompeten dan berpengalaman di bidangnya.
7. DAFTAR PUSTAKA :
1. Peter H. Emmons, Concrete Repair and Maintenance Illustrated, USA, 1994.
2. FIP, Inspection and Maintenance of Reinforced and Pre-stressed Concrete Structures, London, 1986.
3. FIP, Repair and Strengthening of Concrete Structures, London, 1991.
4. ACI 201.1R, Guide for Making a Condition Survey of Concrete in Service, 1992.
5. ACI 364.1R, Guide for Evaluation of Concrete Structures Prior to Rehabilitation, 1993.
6. ACI 228.2R, Nondestructive Test Methods for Evaluation of Concrete in Structures
7. ICRI Technical Guidelines No. 03731, 1996
8. ICRI Technical Guidelines No. 03733, 1996
9. ICRI Technical Guidelines No. 03739, 2004
10. ICRI Technical Guidelines No. 03742, 2006
11. Repair Application Procedure – ACI Committee 706, 2003
12. Specification and Guidelines for Self-Compacting Concrete, February 2002 EFNARC, Association House, 99 West Street, Farnham, Surrey GU9 7EN, UK
Kamis, 25 Februari 2010
Durability of Concrete
Durability is the ability to last a long time without significant deterioration. A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement. Construction and demolition waste contribute to solid waste going to landfills. The production of new building materials depletes natural resources and can produce air and water pollution.
The heavily traveled Wacker Drive replacement in downtown Chicago was designed for a 75 to 100-year life. (PCA No. 16263)
The design service life of most buildings is often 30 years, although buildings often last 50 to 100 years or longer. Most concrete and masonry buildings are demolished due to obsolescence rather than deterioration. A concrete shell can be left in place if a building use or function changes or when a building interior is renovated. Concrete, as a structural material and as the building exterior skin, has the ability to withstand nature’s normal deteriorating mechanisms as well as natural disasters.
Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of concrete.
These 3x5-ft concrete panels with decorative finishes were displayed outdoors in the relatively severe weather in the Skokie, Illinois, area (near Chicago). With only a few exceptions, their appearance changed very little after more than 40 years of exposure to bright sun-light, wind, snow, acid rain, freezing and thawing, hot summers, and cold winters (PCA No. 2101)
High Humidity and Wind-Driven Rain: Concrete is resistant to wind-driven rain and moist outdoor air in hot and humid climates because it is impermeable to air infiltration and wind-driven rain. Moisture that enters a building must come through joints between concrete elements. Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Good practice for all types of wall construction is to have permeable materials that breathe (are allowed to dry) on at least one surface and to not encapsulate concrete between two impermeable surfaces. Concrete will dry out if not covered by impermeable treatments.
Portland cement plaster (stucco) should not be confused with the exterior insulation finish systems (EIFS) or synthetic stucco systems that have become popular but may have performance problems, including moisture damage and low impact-resistance. Synthetic stucco is generally a fraction of the thickness of portland cement stucco, offering less impact resistance. Due to its composition, it does not allow the inside of a wall to dry when moisture gets trapped inside. Trapped moisture eventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metal attachments. There have been fewer problems with EIFS used over solid bases such as concrete or masonry because these substrates are very stable and are not subject to rot or corrosion.
Ultraviolet Resistance: The ultraviolet portion of solar radiation does not harm concrete. Using colored pigments in concrete retains the color in concrete long after paints have faded due to the sun’s effects.
Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some softer materials are inedible but still provide pathways for insects. Due to its hardness, vermin and insects will not bore through concrete. Gaps in exterior insulation to expose the concrete can provide access for termite inspectors.
Moderate to Severe Exposure Conditions for Concrete: The following are important exposure conditions and deterioration mechanisms in concrete. Concrete can withstand these effects when properly designed. The Specifier’s Guide for Durable Concrete is intended to provide sufficient information to allow the practitioner to select materials and mix design parameters to achieve durable concrete in a variety of environments.
Resistance to weathering, including freezing and thawing
Resistance to Freezing and Thawing: The most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of deicing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste, the aggregate particles, or both.
With the addition of an air entrainment admixture, concrete is highly resistant to freezing and thawing. During freezing, the water displaced by ice formation in the paste is accommodated so that it is not disruptive; the microscopic air bubbles in the paste provide chambers for the water to enter and thus relieve the hydraullic pressure generated. Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete with a low water-cementitious ratio and an air content of 5 to 8% will withstand a great number of cycles of freezing and thawing without distress.
Chemical resistance
Chemical Resistance: Concrete is resistant to most natural environments and many chemicals. Concrete is virtually the only material used for the construction of wastewater transportation and treatment facilities because of its ability to resist corrosion caused by the highly aggressive contaminants in the wastewater stream as well as the chemicals added to treat these waste products.
However concrete is sometimes exposed to substances that can attack and cause deterioration. Concrete in chemical manufacturing and storage facilities is specially prone to chemical attack. The effect of sulfates and chlorides is discussed below. Acids attack concrete by dissolving the cement paste and calcareous aggregates. In addition to using concrete with a low permeability, surface treatments can be used to keep aggressive substances from coming in contact with concrete. Effects of Substances on Concrete and Guide to Protective Treatments discusses the effects of hundreds of chemicals on concrete and provides a list of treatments to help control chemical attack.
Resistance to Sulfate Attack: Excessive amounts of sulfates in soil or water can attack and destroy a concrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste. These reactions can induce sufficient pressure to cause disintegration of the concrete.
Like natural rock such as limestone, porous concrete (generally with a high water-cementitious ratio) is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of concrete include sodium carbonate and sodium sulfate.
Confederation Bridge, spanning the Northumberland Strait between Prince Edward Island and New Brunswick, was specifically designed for high durability in a severe environment and a 100-year life. The bridge has to resist freezing and thawing, seawater exposure, and abrasion from floating ice. (PCA No. IMG15714)
Sulfate attack and salt crystallization are more severe at locations where the concrete is exposed to wetting and drying cycles, than continuously wet cycles. For the best defense against external sulfate attack, design concrete with a low water to cementitious material ratio (around 0.40) and use cements specially formulated for sulfate environments.
Seawater Exposure: Concrete has been used in seawater exposures for decades with excellent performance. However, special care in mix design and material selection is necessary for these severe environments. A structure exposed to seawater or seawater spray is most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying and/or freezing and thawing. Sulfates and chlorides in seawater require the use of low permeability concrete to minimize steel corrosion and sulfate attack. A cement resistant to sulfate exposure is helpful. Proper concrete cover over reinforcing steel must be provided, and the water-cementitious ratio should not exceed 0.40.
Corrosion resistance
Chloride Resistance and Steel Corrosion: Chloride present in plain concrete that does not contain steel is generally not a durability concern. Concrete protects embedded steel from corrosion through its highly alkaline nature. The high pH environment in concrete (usually greater than 12.5) causes a passive and noncorroding protective oxide film to form on steel. However, the presence of chloride ions from deicers or seawater can destroy or penetrate the film. Once the chloride corrosion threshold is reached, an electric cell is formed along the steel or between steel bars and the electrochemical process of carrions begins.
The resistance of concrete to chloride is good; however, for severe environments such as bridge decks, it can be increase by using a low water-cementitious ratio (about 0.40), at least seven days of moist curing, and supplementary cementitious materials such as silica fume, to reduce permeability. Increasing the concrete cover over the steel also helps slow down the migration of chlorides. Other methods of reducing steel corrosion include the use of corrosion inhibiting admixtures, epoxy-coated reinforcing steel, surface treatments, concrete overlays, and cathodic protection.
Resistance to alkali-silica reaction (ASR)
Resistance to Alkali-Silica Reaction (ASR): ASR is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures, and mixing water. The reactivity is potentially harmful only when it produces significant expansion. Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or movement of portions of a structure. ASR can be controlled through proper aggregate selection and/or the use of supplementary cementitious materials (such as fly ash or slag cement) or blended cements proven by testing to control the reaction.
Abrasion resistance
Abrasion Resistance: Concrete is resistant to the abrasive affects of ordinary weather. Examples of severe abrasion and erosion are particles in rapidly moving water, floating ice, or areas where steel studs are allowed on tires. Abrasion resistance is directly related to the strength of the concrete. For areas with severe abrasion, studies show that concrete with compressive strengths of 12,000 to 19,000 psi work well.
Why does concrete crack?
Concrete, like most materials, will shrink slightly when it dries out. Common shrinkage is about 1/16th of an inch in a 10-foot length of concrete. The reason contractors place joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint, where concrete cracks due to shrinkage are expected to occur. Control or construction joints are also placed in concrete walls and other structures.
Why do concrete surfaces spall?
Concrete spalling (or flaking) can be prevented. It occurs due to one or more of the following reasons.
1.) In cold climates subjected to freezing and thawing, concrete surfaces have the potential to spall if the concrete is not air-entrained.
2.)Too much water in the concrete mix will produce a weaker, more permeable and less durable concrete. The water-cementitious ratio should be as low as possible (0.45 or less).
3.) Concrete finishing operations should not begin until the water sheen on the surface is gone and the excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because finishing operations have begun too soon, the concrete on the surface will have too high of a water content and this surface will be weaker and less durable.
The heavily traveled Wacker Drive replacement in downtown Chicago was designed for a 75 to 100-year life. (PCA No. 16263)
The design service life of most buildings is often 30 years, although buildings often last 50 to 100 years or longer. Most concrete and masonry buildings are demolished due to obsolescence rather than deterioration. A concrete shell can be left in place if a building use or function changes or when a building interior is renovated. Concrete, as a structural material and as the building exterior skin, has the ability to withstand nature’s normal deteriorating mechanisms as well as natural disasters.
Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of concrete.
These 3x5-ft concrete panels with decorative finishes were displayed outdoors in the relatively severe weather in the Skokie, Illinois, area (near Chicago). With only a few exceptions, their appearance changed very little after more than 40 years of exposure to bright sun-light, wind, snow, acid rain, freezing and thawing, hot summers, and cold winters (PCA No. 2101)
High Humidity and Wind-Driven Rain: Concrete is resistant to wind-driven rain and moist outdoor air in hot and humid climates because it is impermeable to air infiltration and wind-driven rain. Moisture that enters a building must come through joints between concrete elements. Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Good practice for all types of wall construction is to have permeable materials that breathe (are allowed to dry) on at least one surface and to not encapsulate concrete between two impermeable surfaces. Concrete will dry out if not covered by impermeable treatments.
Portland cement plaster (stucco) should not be confused with the exterior insulation finish systems (EIFS) or synthetic stucco systems that have become popular but may have performance problems, including moisture damage and low impact-resistance. Synthetic stucco is generally a fraction of the thickness of portland cement stucco, offering less impact resistance. Due to its composition, it does not allow the inside of a wall to dry when moisture gets trapped inside. Trapped moisture eventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metal attachments. There have been fewer problems with EIFS used over solid bases such as concrete or masonry because these substrates are very stable and are not subject to rot or corrosion.
Ultraviolet Resistance: The ultraviolet portion of solar radiation does not harm concrete. Using colored pigments in concrete retains the color in concrete long after paints have faded due to the sun’s effects.
Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some softer materials are inedible but still provide pathways for insects. Due to its hardness, vermin and insects will not bore through concrete. Gaps in exterior insulation to expose the concrete can provide access for termite inspectors.
Moderate to Severe Exposure Conditions for Concrete: The following are important exposure conditions and deterioration mechanisms in concrete. Concrete can withstand these effects when properly designed. The Specifier’s Guide for Durable Concrete is intended to provide sufficient information to allow the practitioner to select materials and mix design parameters to achieve durable concrete in a variety of environments.
Resistance to weathering, including freezing and thawing
Resistance to Freezing and Thawing: The most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of deicing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste, the aggregate particles, or both.
With the addition of an air entrainment admixture, concrete is highly resistant to freezing and thawing. During freezing, the water displaced by ice formation in the paste is accommodated so that it is not disruptive; the microscopic air bubbles in the paste provide chambers for the water to enter and thus relieve the hydraullic pressure generated. Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete with a low water-cementitious ratio and an air content of 5 to 8% will withstand a great number of cycles of freezing and thawing without distress.
Chemical resistance
Chemical Resistance: Concrete is resistant to most natural environments and many chemicals. Concrete is virtually the only material used for the construction of wastewater transportation and treatment facilities because of its ability to resist corrosion caused by the highly aggressive contaminants in the wastewater stream as well as the chemicals added to treat these waste products.
However concrete is sometimes exposed to substances that can attack and cause deterioration. Concrete in chemical manufacturing and storage facilities is specially prone to chemical attack. The effect of sulfates and chlorides is discussed below. Acids attack concrete by dissolving the cement paste and calcareous aggregates. In addition to using concrete with a low permeability, surface treatments can be used to keep aggressive substances from coming in contact with concrete. Effects of Substances on Concrete and Guide to Protective Treatments discusses the effects of hundreds of chemicals on concrete and provides a list of treatments to help control chemical attack.
Resistance to Sulfate Attack: Excessive amounts of sulfates in soil or water can attack and destroy a concrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste. These reactions can induce sufficient pressure to cause disintegration of the concrete.
Like natural rock such as limestone, porous concrete (generally with a high water-cementitious ratio) is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of concrete include sodium carbonate and sodium sulfate.
Confederation Bridge, spanning the Northumberland Strait between Prince Edward Island and New Brunswick, was specifically designed for high durability in a severe environment and a 100-year life. The bridge has to resist freezing and thawing, seawater exposure, and abrasion from floating ice. (PCA No. IMG15714)
Sulfate attack and salt crystallization are more severe at locations where the concrete is exposed to wetting and drying cycles, than continuously wet cycles. For the best defense against external sulfate attack, design concrete with a low water to cementitious material ratio (around 0.40) and use cements specially formulated for sulfate environments.
Seawater Exposure: Concrete has been used in seawater exposures for decades with excellent performance. However, special care in mix design and material selection is necessary for these severe environments. A structure exposed to seawater or seawater spray is most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying and/or freezing and thawing. Sulfates and chlorides in seawater require the use of low permeability concrete to minimize steel corrosion and sulfate attack. A cement resistant to sulfate exposure is helpful. Proper concrete cover over reinforcing steel must be provided, and the water-cementitious ratio should not exceed 0.40.
Corrosion resistance
Chloride Resistance and Steel Corrosion: Chloride present in plain concrete that does not contain steel is generally not a durability concern. Concrete protects embedded steel from corrosion through its highly alkaline nature. The high pH environment in concrete (usually greater than 12.5) causes a passive and noncorroding protective oxide film to form on steel. However, the presence of chloride ions from deicers or seawater can destroy or penetrate the film. Once the chloride corrosion threshold is reached, an electric cell is formed along the steel or between steel bars and the electrochemical process of carrions begins.
The resistance of concrete to chloride is good; however, for severe environments such as bridge decks, it can be increase by using a low water-cementitious ratio (about 0.40), at least seven days of moist curing, and supplementary cementitious materials such as silica fume, to reduce permeability. Increasing the concrete cover over the steel also helps slow down the migration of chlorides. Other methods of reducing steel corrosion include the use of corrosion inhibiting admixtures, epoxy-coated reinforcing steel, surface treatments, concrete overlays, and cathodic protection.
Resistance to alkali-silica reaction (ASR)
Resistance to Alkali-Silica Reaction (ASR): ASR is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures, and mixing water. The reactivity is potentially harmful only when it produces significant expansion. Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or movement of portions of a structure. ASR can be controlled through proper aggregate selection and/or the use of supplementary cementitious materials (such as fly ash or slag cement) or blended cements proven by testing to control the reaction.
Abrasion resistance
Abrasion Resistance: Concrete is resistant to the abrasive affects of ordinary weather. Examples of severe abrasion and erosion are particles in rapidly moving water, floating ice, or areas where steel studs are allowed on tires. Abrasion resistance is directly related to the strength of the concrete. For areas with severe abrasion, studies show that concrete with compressive strengths of 12,000 to 19,000 psi work well.
Why does concrete crack?
Concrete, like most materials, will shrink slightly when it dries out. Common shrinkage is about 1/16th of an inch in a 10-foot length of concrete. The reason contractors place joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint, where concrete cracks due to shrinkage are expected to occur. Control or construction joints are also placed in concrete walls and other structures.
Why do concrete surfaces spall?
Concrete spalling (or flaking) can be prevented. It occurs due to one or more of the following reasons.
1.) In cold climates subjected to freezing and thawing, concrete surfaces have the potential to spall if the concrete is not air-entrained.
2.)Too much water in the concrete mix will produce a weaker, more permeable and less durable concrete. The water-cementitious ratio should be as low as possible (0.45 or less).
3.) Concrete finishing operations should not begin until the water sheen on the surface is gone and the excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because finishing operations have begun too soon, the concrete on the surface will have too high of a water content and this surface will be weaker and less durable.
High Performance Concrete
High-Performance Concrete
2.1 Introduction
At least one type of HPC, that is, High-Strength Concrete (HSC), has been used in many locations for well over a decade. Since the completion of the Strategic Highway Research Program, other types of HPC have moved more and more from the research or limited field trial stage into full scale commercial use. However, significant new developments in the materials or production of HPC have been few since the publication of the SHRP C-205 State-of-the-Art Report (SAR) [Zia et al. 1991].
Most of the research in HPC since the publication of the State-of-the-Art Report in 1990 has concentrated on increasing basic knowledge regarding HPC performance and properties rather than developing new types of HPC. HPC research and utilization continues to be concentrated in HSC or in improved durability, primarily using the materials and methods developed prior to the publication of the SAR. Changes have come primarily in modification of existing mix components and proportions.
While additional research on the mechanical properties of HSC has been conducted, much of the research of the last five years has concentrated on particular applications in buildings and bridges rather than the properties of the concrete itself. With time, data regarding the long term properties of HSC have been reported. Mechanical properties and durability of lightweight HSC have been reported, in many cases for use in extreme environments where concerns include abrasion resistance of the concrete to sea ice, as well as frost durability.
In the area of durability, substantial research has been conducted on the frost resistance of HPC. Much of the research has investigated the behavior of non-air entrained concretes with very low (<0.35) water to cementitious material ratio (W/CM), concentrating on HSC containing silica fume and high range water reducers. Research has also been conducted on the permeability, passage of chloride ions and carbonation of various concretes, many of which are HSC by virtue of their low W/CM ratios. The impact of high cement contents on heat of hydration, and the consequent effects on strength and durability in place have also been investigated. Some work on the abrasion resistance behavior of HPC concrete and its use to improve pavement durability have been reported, as has work on the fire endurance of HSC.
An important improvement in constructibility has been the further development of super-workable concrete. This has added another important, practical dimension to the concept of HPC. These concretes with improved constructibility have been developed using materials which, while not exotic, have not been routinely used, in some cases, and have not been used in the combinations and proportions reported. Other investigations concerned with constructibility issues have involved the relative sensitivity of HPC to curing conditions, including external temperature.
In this chapter, materials and methods used to produce HPC are reviewed which were not addressed previously or in which there has been a shift in focus in the last five years.
2.2 Raw Materials and Proportions
2.2.1 New Materials
Few new raw materials have been introduced into the technology in the last five years which promise to make a substantial difference in HPC production or use. However, some existing chemical admixtures have evolved. Modifications of high range water reducers to reduce slump loss have produced additional alternatives for the engineer, however, these HRWR's typically involve extended set times which may not be advantageous for early strength applications.
Another change in composition has been noted with air entraining admixtures. Air entraining agents are now frequently based on compounds other than "neutralized vinsol resin", in many cases producing finer and more uniform air void systems than were common earlier. Neeley et al. [1992] presented results of a new air entraining admixture intended to provide adequate frost durability at low air contents. Their research, not based on HPC, indicated that low air contents provided insufficient durability to rapid freezing and thawing.
Cements and combinations of cementitious materials have been the subject of continuing research but there have been few new materials used outside the laboratory. Some blended cements, specifically developed for niche markets, have been investigated, but their use in practice has been somewhat limited. Neeley [1995] reports promising results for one product based primarily on Class C fly ash. A key ingredient of the product was three admixtures used in various quantities, added at various times and in different sequences to regulate setting characteristics. Concrete made with this product had very low w/c ratios and setting times which varied from approximately a half hour to over three hous.
In preliminary testing at the U.S. Army Engineer Waterways Experiment Station, Ash Bonding Chemicals Corporation Cement was found to provide relatively high strengths at early ages with good frost resistance, low to very low permeability as measured by Chloride Ion Permeability tests and reduced shrinkage when compared to a conventional Type III portland cement based concrete containing water reducing and high range water reducing admixtures. However, the concrete was more variable and admixture effects are still neither completely understood nor completely predictable. In addition, costs and control issues due to the addition of the admixtures on the job site, possibly including air entraining agents, have apparently not been fully investigated.
2.2.2 Review of Other Selected Raw Materials
2.2.2.1 Cementitious MaterialsRenewed interest in shrinkage compensating cements was generated several years ago in jointless slabs, including bridge decks. While concretes made with these cements have good strength characteristics and exceptional sulfate durability, there have been serious concerns with the durability of the concrete exposed to deicer salts. Reports presented in sessions at the 1995 Transportation Research Board indicated very different experiences in Ohio and New York with shrinkage compensating cement. Bridge members produced with shrinkage compensating cement in Ohio were sealed and have apparently performed well to date. Structures in New York which were not sealed exhibited some premature scaling and deterioration. There was speculation that differences in performance may be due to sealing of the concrete.
The blended cement Pyrament was investigated as part of the SHRP C-205 research. This material could be used to produce concretes with exceptional early strength characteristics and very good later age properties. Further research by Husbands et al. [1994] found that performance and durability were generally good. Concerns with alkali-silica reactivity have not been completely resolved but appear to be manageable. Unfortunately, Pyrament never gained broad market acceptance due to availability and cost, and its production has been suspended. Super-fine cements continue to be unavailable on a commercial basis. Regulated set cements are acceptable for early strength applications except where sulfate exposure is likely.
2.2.2.2 Mineral AdmixturesWhile additional research continues with mineral admixtures, especially silica fume, most of the new research involves different proportions rather than new materials. However, a few other mineral admixtures have also received attention. The use of zeolitic admixtures, a natural pozzolan, were examined by Feng et al. [1990] as was metakaolin, a reactive alumino-silicate pozzolan by Walters and Jones [1991]. The use of 5% and 10% metakaolin was found to be very similar to the use of similar percentages of silica fume in terms of permeability, frost durability and mechanical properties. The major differences noted were in color (the metakaolin was much lighter in color) and in HRWR dosages (concrete containing silica fume had a much higher demand).
A number of researchers have confirmed earlier reports that mineral admixtures typically reduce the permeability of concrete. Detwiler et al. [1994] have reported on this phenomenon in steam cured concrete. Geiker et al. [1991] noted that both silica fume and fly ash reduced the permeability of concrete to the penetration of chloride ion without changing the total porosity greatly. Bijen and van Selst [1991] found higher rates of carbonation in concrete with typical commercial quantities of ground granulated blast furnace slag compared with concretes containing typical quantities of fly ash. However, this study was not conducted with HPC.
Silica fume (also called condensed silica fume or microsilica) continues to be a popular element of high performance concrete, and especially high strength concrete. Not only does it provide an extremely rapid pozzolanic reaction, but researchers including Detwiler and Mehta [1989], and Goldman and Bentur [1993] found that its very fine size also appears to provide a beneficial contribution to concrete. Detwiler and Mehta, and Goldman and Bentur examined the effects of silica fume on mechanical behavior. Luther's [1989] review examined durability effects, while Fidjestol [1993], and Khayat and Aitcin [1993], have provided general reviews of the effects. These reports confirm findings that silica fume tends to improve both mechanical properties and durability.
However, St. John et al. [1994] report that deleterious expansion due to alkali silica reactivity is possible under wetting and drying conditions when particles of the densified form of silica fume admixtures are not sufficiently dispersed during mixing. In addition, a number of research efforts have attempted to answer important and unresolved questions in long term strength gain and frost resistance of silica fume concrete.
Maage et al. [1990] report that silica fume concretes continue to gain strength under a variety of curing conditions, including unfavorable conditions. They further indicate that concretes with silica fume appear to be more robust to early drying than similar concretes which do not contain silica fume.
A number of issues with frost resistance of concrete containing silica fume have been investigated, including the need for any entrained air when working with very low W/CM ratio concretes. Due to the dramatic reduction in permeability which accompanies the use of silica fume, concerns with frost durability in general, and with the usefulness of rapid freezing and thawing tests, have complicated the interpretation of research results. These issues are discussed in more detail in section 2.2.4, however it is useful to note here that ACI 318-95 [1995] limits the quantity of silica fume in concrete exposed to deicing salts to no more than 10 percent.
Attempts to improve the performance of systems of cementitious material in HSC have led researchers to examine mixes with multiple cementitious components. The use of ternary cementitious systems has received attention in recent years. Kashima et al. [1993] report on HSC produced with a blend of cement with large amounts of fly ash and ground granulated blast furnace slag in order to reduce heat of hydration. Their report is important because it reviews both experimental work and construction results.
Sarkar et al. [1991], examining the microstructural development in HSC using both silica fume and fly ash, found that strength at twelve hours was improved over similar mixes with silica fume alone. They state that this phenomenon may be related to the liberation of soluble alkalies from the surface of the fly ash. Baalbaki et al. [1993], reported on the properties of HPC produced with an extremely finely ground Type V cement with various mineral admixtures. Mixes with prolonged working times and very high strengths at ages out to one year were produced.
The use of cementitious systems with very high quantities of fly ash have also been investigated. Carette et al. [1993], and Bilodeau and Malhotra [1994], report that mixes have been developed which provide acceptable plastic and hardened properties, although strengths were not high, especially at early ages. In other studies, Bilodeau et al. [1994], and Malhotra [1990] report that performance in rapid freezing and thawing of concrete with high volumes of class F fly ash was adequate but that the concrete with very high quantities of fly ash performed poorly in deicer scaling tests. However, Nasser and Lai [1993] found that high volume, class C fly ash concrete was not frost durable even with a 6% air and after prolonged moist curing. They found that 20% fly ash mixes showed no difficulties in this respect. Kukko and Matala [1991] noted that frost resistance of non-air entrained concrete was reduced for very low W/CM ratio concrete produced with slowly hardening portland cement or containing slag, compared to rapid hardening portland cement with or without silica fume.
Naik et al. [1994] found that although concrete made with high volumes of class C fly ash passed ASTM C-944 for abrasion resistance, better abrasion resistance was obtained for concrete without the high fly ash content. Gjorv et al. [1990] also found that the abrasion resistance of conventional HSC pavements is exceptionally good. It would appear that high volume fly ash mixes have limited applicability to highway structures, although additional research appears warranted.
2.2.2.3 AggregatesHigh Performance Lightweight Concrete (HPLC) has been extensively investigated for, among other applications, use in oil drilling platforms in severe environments. Hoff [1991], and Tachibana et al. [1990], have presented information on the behavior of HSLC in extreme conditions. Hoff has demonstrated that HSLC containing both lightweight aggregate and conventional weight stone is both frost resistant and resistant to abrasion by ice. Holm and Bremner [1991], have provided additional information on the long term durability of lightweight concrete, indicating that in general when well-known prophylactic measures are taken to insure durability, long term durability is good.
Zhang and Gjorv [1991a, 1991b, 1991c, 1991d] have investigated both the mechanical properties and the permeability of lightweight concretes with strengths ranging from 50 to 100 MPa (about 7,000 to about 14,500 psi). Elastic modulus and the tensile-compressive strength ratio were lower than would be expected with conventional stone concrete at the same strength levels. While permeability of the HSLC's was very low, it was noted that permeability might be higher with lightweight aggregate than with conventional aggregate at the same strength. This, of course, would depend on the porosity and permeability of the aggregate itself.
2.2.3 Proportioning Methods
Modifications to conventional proportioning methods have been proposed by several researchers. Mehta and Aitcin [1990], and ACI Committee 363 report [1990] provide an excellent review of proportioning considerations for HSC. Selection of the proper raw materials and adjustment of proportions based on experience, using mixes conducted both in the laboratory and in the field, have typically proven adequate to achieve the desired concrete characteristics, at least within the limits allowed by the available raw materials. With adequate control of production and placement, routine use of concrete with compressive strengths in excess of 70 MPa (10,000 psi) is practical in many areas.
Several articles have been published with suggestions on methods of optimizing the development of particular mixes by reducing the number of trial mixes necessary. Campbell and Detwiler [1993], for example, have provided guidance for proportioning and producing steam-cured concrete. While de Larrard [1990] has provided suggestions for HSC mixes based on rheological considerations, Domone and Soutsos [1994] have reexamined the maximum density theory for applicability to HSC.
Field trials of High Early Strength (HES), Very Early Strength (VES) and Very High Strength (VHS) concretes in SHRP C-205 and C-206 indicated that existing proportioning methods remain valid, with minor modifications, for these mixes. Routine precautions such as those regarding minimum water contents and appropriate quantities or combinations of chemical admixtures, contained in numerous publications and discussed in the previous State-of-the-Art Report, remain valid. Development or adaptation of new types of high performance concrete or combinations of raw materials are better served by engineering judgement than by more sophisticated proportioning techniques.
2.2.4 Air Entrainment
The need for any air entrainment at all in concrete with very low W/CM ratio has been questioned. This issue has been complicated by the problem of interpreting test results of one of the most commonly used test methods in practice. ASTM C 666 measures the resistance of concrete to rapid freezing and thawing. The rate of freezing is much higher in this test than is found in practice, and C 666 has been criticized in this respect even for conventional concrete.
Concretes with a low W/CM ratio, such as HPC and HSC, have a lower permeability than conventional concrete. A rapid freezing and thawing rate may induce additional damage to concretes with low W/CM ratio simply due to the lower permeability. On the other hand, the very low w/c ratio, for an adequately cured concrete, can reduce or even eliminate the amount of freezable water in the pores for practical temperature ranges. These mixes will also dramatically reduce the ingress of water during the test, therefore reducing the amount of damage due to physically freezing water in the concrete. The time required to achieve an internal moisture content sufficient to contribute to frost damage is less than the time required for the C 666 test for concretes with very low W/CM ratios. However, since hydraulic pressure due to freezing of water is only one of several mechanisms of frost damage, the use of very low W/CM may not be adequate in all cases. Experimental results have been mixed.
Damage to concrete specimens may be due to thermal shock or to disequilibrium between energy states during cooling rather than expansion associated with the presence of freezing water for the mixes of very low W/CM ratio. Hanson et al. [1993], found that the electrical impedance of air entrained concrete of very low W/CM ratio actually increased during ASTM C 666 testing while the dynamic elastic modulus decreased. The increase in impedance could only have come from internal drying associated with the loss of free moisture during curing. Therefore, the decrease in elastic modulus was not related to the formation of ice.
Many believe that C 666 testing is still a valid discriminant for frost durability even though it is extremely severe for concretes of low W/CM ratio. Others have chosen to rely on deicer scaling as a more useful and informative test, particularly for highway and pavement applications.
Hammer and Sellevold [1990] report that salt scaling resistance is acceptable for HPC with W/CM ratios below 0.37, even without entrained air but that rapid freezing and thawing is accompanied by deterioration for all non-entrained air concrete tested down to a W/CM of 0.25. However, they also note that calorimeter data indicates very little ice formation until -20 c. They state that this indicates that much of the deterioration may be due to thermal incompatibility of the components rather than the formation of ice.
In research conducted by Kashi and Weyers [1989], non-air entrained concrete with a W/CM ratio of less than 0.30 was found to be resistant to rapid freezing and thawing based on ASTM C 666, Method A. For concretes with a W/CM ratio of 0.32, the concrete was frost resistant only if silica fume was not used in the mix. Cohen et al. [1992] similarly found that non-air entrained concrete with a W/CM ratio of 0.35 and containing 10% silica fume were not resistant to rapid freezing and thawing when tested in accordance with ASTM C 666 (A), even when curing had been extended to 56 days. They also noted that although there was a dramatic drop in the elastic modulus, the reduction in compressive strength was much less severe. Li et al. [1994], on the other hand, found that a maximum W/CM ratio of 0.24 was necessary for adequate frost protection of non-air entrained concrete when based on ASTM C 666 (A) testing.
Tests conducted by Pigeon et al. [1991], and by Gagne et al. [1991], using both ASTM C 666 (A) and ASTM C 672 deicer scaling tests, indicated that non-air entrained concrete containing silica fume and good quality coarse aggregate, with a W/CM ratio of 0.30 generally, but not uniformly, had good resistance to deicer scaling. When tested using ASTM C 666 (A), the W/CM ratio required to provide acceptable performance ranged from less than 0.25 to over 0.30. The cement used was found to play a significant role in the performance of otherwise similar mixes. It was also noted that the air void system produced by the use of water reducing or high range water reducing admixtures, commonly used in all concretes with low W/CM ratio, may be contributing in a fashion not yet well documented.
Additional research in these areas, perhaps concentrating more on test methods such as resistance to deicer scaling and critical dilation test concepts rather than ASTM C 666 (A), would appear to be useful. At the present time, the use of at least minimal quantities of entrained air appear prudent for concrete exposed to severe freezing, especially when exposed to deicing salts, unless that concrete has a W/CM ratio less than 0.24.
2.2.5 Other Types of HPC
Another type of HPC has been developed for use in situations where vibration is difficult or impossible and where reinforcing steel is highly congested. "Super-workable" or "flowable" concrete has been developed in Japan and used in both bridge structures and buildings. Although not specifically developed for high strength, low water to "powder" ratios are common. Kuroiwa et al. [1993], report super-workable concrete with strengths in excess of 50 MPa, a marginally high strength concrete.
Low W/CM ratio, flowing concretes utilizing HRWR's are well known. Super-workable mixes are an extension of this concept which have been specifically formulated to resist segregation. Super-workable concretes also derive partly from concrete developed for underwater placement. Research has led in several different directions. Paste and aggregate volumes, and paste composition, admixture type and dosages, and testing methods have been investigated.
A number of different "powder" combinations are reported including portland cement, fly ash, GGBFS and silica fume. In addition, where a low heat of hydration was a concern, finely ground limestone powder was used as a partial replacement for cement. This material was reported [Tanaka et al. 1993] to have both a chemical and physical effect similar to that reported for silica fume. Low heat of hydration mixes will typically exhibit considerably extended set times, with final set at about twenty hours, and low early strengths. This is due both to large quantities of HRWR in conjunction with a blended cement composed of 30% low heat of hydration portland cement and 70% GGBFS. The aggregate paste ratio was somewhat lower compared to conventional concrete.
Cellulose based products have been known to improve the cohesion of concrete and to reduce segregation. Sogo et al. [1987] report that a polymer based on cellulose ether can be used to increase both water reduction and cohesiveness. Many long chain organic molecules have water reducing capability and, if air content and setting time can be controlled, can be successfully used in concrete [Mehta 1975] with various effects on cohesiveness. Kuroiwa et al. [1993] report on findings using a polysaccharide polymer. Ozawa et al. [1990] describe studies to optimize the combination of HRWR and other admixtures affecting the viscosity of this type of mix.
Self compacting capability and resistance to segregation were determined by so-called "slump-flow", by a modified grout cone flow test and by self-leveling flow through reinforcing bars, as well as by mock-ups of particular members. Self-leveling, non-segregating performance was reported with several different mixtures. Details of these tests are discussed in the next chapter.
The addition of materials to compensate for increased drying shrinkage due to the lower aggregate content has been suggested, although Kuroiwa et al. state that drying shrinkage is equal to or better than comparable, conventional concretes. Frost durability was also found to be adequate for super-workable concrete containing at least 4% entrained air. Testing was based on ASTM C 666, although it was not stated whether Procedure A or B was used.
It is significant to note that these types of mixes have been successfully used, without vibration, in field placements. Applications include the Akashi Kaikyo Bridge near Kobe, Japan, and in the heavily reinforced concrete core and shear wall of a 20-story building. Concrete delivery was by pump. The use of super-workable concretes in certain applications appears promising.
2.3 Production Considerations
For high strength concrete where early strength is not a critical consideration, the use of conventional production methods and facilities appears adequate, as long as well recognized practices for the production of good quality concrete are enforced. Howard and Leatham [1989] and Sanchez and Hester [1990] discuss the production and delivery of HSC noting the importance of a team approach. Kakizaki et al. [1993], note that the mixing sequence can affect the slump and compressive strengths of very high strength concretes. Leming et al. [1993], emphasize that pre-pour conferences and field trials are necessary with any HPC prior to actual use.
In situations where early strength is critical, particular care must be taken to insure that temperature is closely controlled and that high dosages of water reducing or high range water reducing admixtures are avoided, since these can extend the time of set. While strength after one day may not be significantly affected, strengths at less than twenty-four hours and particularly before twelve hours, can be significantly reduced. If mineral admixtures are included, caution must be exercised in dosing the entire quantity of cementitious material. When significant percentages of mineral admixtures are employed, the result may be to effectively overdose the portland cement, again resulting in extended set times.
Much of the VES and HES concrete used in field trials for SHRP C-205 and C-206 contained a corrosion inhibiting admixture as a non-chloride accelerator, added at the job site to mitigate rapid slump loss [Hanson et al. 1994]. The admixture was added either by hand or by pump typically from trailer-mounted tanks. In either case, provision for adding the admixture, adequate quality control and sufficient remixing time are necessary.
Due to the high water content of this admixture and the low W/CM ratio of the paving mixes investigated, it was necessary to employ a HRWR in the initial batching. Inadvertent use of large dosages of HRWR caused low strengths at ages up to and including one day. The low water content and rich paste of these mixtures required strict control of the batching sequence. Although only minor adjustments to conventional practice were required, some adjustment must be anticipated. Exact procedures will vary from one production facility to another, but there are several keys to a successful placement. The plant itself should be equipped with an automatic moisture indicator for the aggregate.
A pre-placement conference, including all parties who will be involved in the slab-on-grade placement, is required. A practice placement to adjust operations, if necessary, to develop estimates for slump and air loss in transit, and to acquaint the crews involved in the placement is highly recommended. Inspection of trucks, especially in a dry batch operation, is necessary to insure that only trucks with clean fins and adequate mixing speed are used. Trucks should carry no more than two-thirds of their rated mixing capacity to insure adequate mixing on the job site if there is to be any addition of admixtures on the job site. Trucks should discharge their entire load as soon as possible. A time limit of ten minutes after arrival on the job site should be used for planning purposes but may vary depending on the type and composition of the concrete being used. Special ready-mixed concrete trucks, intended for paving operations, should be used if discharge of very low slump is anticipated.
Since most HPC's are paste rich with low water content, bleeding is typically very low. This can potentially create difficulties with plastic shrinkage cracking. Therefore, it is necessary to apply curing compound or take other precautionary measures to reduce evaporation as soon as possible for slabs or members with large exposed surfaces. However, field trials for SHRP 205 [Leming et al. 1993] of HES and VES concretes in slabs found that cracks due to plastic shrinkage were rare. The concrete was apparently gaining strength faster than it shrank. Another consequence of the rapid setting and strength gain was that time prior to sawing the slabs was reduced. It was critical that joints be sawed as early as possible. Delays past eight hours were found to cause cracking of 20 cm depth (8 in.) pavements at approximately 7.5 m (approximately 25 ft) intervals.
2.4 Fiber Reinforced Concrete
Although there has been continued interest and research in the use of fiber-reinforced concrete (FRC), there have been few major innovations in proportioning or production of high performance fiber reinforced concrete (HPFRC) since the last State-of-the-Art Report. In addition, while research in FRC has examined the influence of modifications of existing fibers, fibers with larger aspect ratios, and higher fiber volumes, and there continues to be interest in non-metallic fibers or combinations of fibers, these researches were based on existing fiber materials.
One of the few new approaches in this area has been the development of SIMCON, or Slurry Infiltrated Mat Concrete, described by Hackman et al. [1992] and Krstulovic-Opara et al. [1994]. They noted that SIMCON is a different material from SIFCON, or Slurry Infiltrated Fiber-Reinforced Concrete, which is based on the use of prepacked discontinuous steel fibers. SIMCON, on the other hand, uses a manufactured continuous mat of interlocking discontinuous steel fibers, placed in a form, and then infiltrated with a flowable cement-based slurry. The use of continuous mats, typically made with stainless steel to control corrosion in very thin members, permits development of high flexural strengths and very high ductility with a reduced volume of fibers than SIFCON.
The use of SIMCON appears to be very promising for at least two reasons. First, the very high volume of fibers required to provide significant increases in mechanical properties such as SIFCON can create a problem with economic justification in a large number of practical applications. However, with SIMCON, direct tensile strengths of 15.9 MPa at 1.1% strain have been reported with only a 5% volume fraction of fibers. Secondly, in situations where normal FRC may be economically justified, such as in pavements, the addition of fibers to the mix and the placement of the fiber-reinforced mix required special care, and considerable extra time and expense. SIMCON overcomes many of these limitations since the fiber mat, normally delivered in large rolls, can be laid out by hand and the slurry simply pumped into place. The use of SIMCON permits fabrication of thin, complex shapes with very high ductility and flexural strength.
Another interesting and useful development in FRC construction has been to provide non-metallic fibers in small, cylindrical bundles, approximately 50 mm high (the length of the fiber) and 55 mm in diameter, wrapped in a water soluble compound. This permits the easy addition of the fibers, by hand, into the mixing drum of a truck mixer, either during charging or at the job site. The wrapper disintegrates, allowing the fibers to disperse into the concrete mixture with little balling or segregation. Quality control is improved by making the quantity of fibers added easy to determine and easy to check, and by minimizing problems in dispersion in the mixer. Further, production rates are maintained with little additional effort. Successful field applications in a full-depth pavement, a thin bridge-deck overlay, a Jersey barrier, and white-topping on scarified asphalt pavement have been reported from South Dakota
2.1 Introduction
At least one type of HPC, that is, High-Strength Concrete (HSC), has been used in many locations for well over a decade. Since the completion of the Strategic Highway Research Program, other types of HPC have moved more and more from the research or limited field trial stage into full scale commercial use. However, significant new developments in the materials or production of HPC have been few since the publication of the SHRP C-205 State-of-the-Art Report (SAR) [Zia et al. 1991].
Most of the research in HPC since the publication of the State-of-the-Art Report in 1990 has concentrated on increasing basic knowledge regarding HPC performance and properties rather than developing new types of HPC. HPC research and utilization continues to be concentrated in HSC or in improved durability, primarily using the materials and methods developed prior to the publication of the SAR. Changes have come primarily in modification of existing mix components and proportions.
While additional research on the mechanical properties of HSC has been conducted, much of the research of the last five years has concentrated on particular applications in buildings and bridges rather than the properties of the concrete itself. With time, data regarding the long term properties of HSC have been reported. Mechanical properties and durability of lightweight HSC have been reported, in many cases for use in extreme environments where concerns include abrasion resistance of the concrete to sea ice, as well as frost durability.
In the area of durability, substantial research has been conducted on the frost resistance of HPC. Much of the research has investigated the behavior of non-air entrained concretes with very low (<0.35) water to cementitious material ratio (W/CM), concentrating on HSC containing silica fume and high range water reducers. Research has also been conducted on the permeability, passage of chloride ions and carbonation of various concretes, many of which are HSC by virtue of their low W/CM ratios. The impact of high cement contents on heat of hydration, and the consequent effects on strength and durability in place have also been investigated. Some work on the abrasion resistance behavior of HPC concrete and its use to improve pavement durability have been reported, as has work on the fire endurance of HSC.
An important improvement in constructibility has been the further development of super-workable concrete. This has added another important, practical dimension to the concept of HPC. These concretes with improved constructibility have been developed using materials which, while not exotic, have not been routinely used, in some cases, and have not been used in the combinations and proportions reported. Other investigations concerned with constructibility issues have involved the relative sensitivity of HPC to curing conditions, including external temperature.
In this chapter, materials and methods used to produce HPC are reviewed which were not addressed previously or in which there has been a shift in focus in the last five years.
2.2 Raw Materials and Proportions
2.2.1 New Materials
Few new raw materials have been introduced into the technology in the last five years which promise to make a substantial difference in HPC production or use. However, some existing chemical admixtures have evolved. Modifications of high range water reducers to reduce slump loss have produced additional alternatives for the engineer, however, these HRWR's typically involve extended set times which may not be advantageous for early strength applications.
Another change in composition has been noted with air entraining admixtures. Air entraining agents are now frequently based on compounds other than "neutralized vinsol resin", in many cases producing finer and more uniform air void systems than were common earlier. Neeley et al. [1992] presented results of a new air entraining admixture intended to provide adequate frost durability at low air contents. Their research, not based on HPC, indicated that low air contents provided insufficient durability to rapid freezing and thawing.
Cements and combinations of cementitious materials have been the subject of continuing research but there have been few new materials used outside the laboratory. Some blended cements, specifically developed for niche markets, have been investigated, but their use in practice has been somewhat limited. Neeley [1995] reports promising results for one product based primarily on Class C fly ash. A key ingredient of the product was three admixtures used in various quantities, added at various times and in different sequences to regulate setting characteristics. Concrete made with this product had very low w/c ratios and setting times which varied from approximately a half hour to over three hous.
In preliminary testing at the U.S. Army Engineer Waterways Experiment Station, Ash Bonding Chemicals Corporation Cement was found to provide relatively high strengths at early ages with good frost resistance, low to very low permeability as measured by Chloride Ion Permeability tests and reduced shrinkage when compared to a conventional Type III portland cement based concrete containing water reducing and high range water reducing admixtures. However, the concrete was more variable and admixture effects are still neither completely understood nor completely predictable. In addition, costs and control issues due to the addition of the admixtures on the job site, possibly including air entraining agents, have apparently not been fully investigated.
2.2.2 Review of Other Selected Raw Materials
2.2.2.1 Cementitious MaterialsRenewed interest in shrinkage compensating cements was generated several years ago in jointless slabs, including bridge decks. While concretes made with these cements have good strength characteristics and exceptional sulfate durability, there have been serious concerns with the durability of the concrete exposed to deicer salts. Reports presented in sessions at the 1995 Transportation Research Board indicated very different experiences in Ohio and New York with shrinkage compensating cement. Bridge members produced with shrinkage compensating cement in Ohio were sealed and have apparently performed well to date. Structures in New York which were not sealed exhibited some premature scaling and deterioration. There was speculation that differences in performance may be due to sealing of the concrete.
The blended cement Pyrament was investigated as part of the SHRP C-205 research. This material could be used to produce concretes with exceptional early strength characteristics and very good later age properties. Further research by Husbands et al. [1994] found that performance and durability were generally good. Concerns with alkali-silica reactivity have not been completely resolved but appear to be manageable. Unfortunately, Pyrament never gained broad market acceptance due to availability and cost, and its production has been suspended. Super-fine cements continue to be unavailable on a commercial basis. Regulated set cements are acceptable for early strength applications except where sulfate exposure is likely.
2.2.2.2 Mineral AdmixturesWhile additional research continues with mineral admixtures, especially silica fume, most of the new research involves different proportions rather than new materials. However, a few other mineral admixtures have also received attention. The use of zeolitic admixtures, a natural pozzolan, were examined by Feng et al. [1990] as was metakaolin, a reactive alumino-silicate pozzolan by Walters and Jones [1991]. The use of 5% and 10% metakaolin was found to be very similar to the use of similar percentages of silica fume in terms of permeability, frost durability and mechanical properties. The major differences noted were in color (the metakaolin was much lighter in color) and in HRWR dosages (concrete containing silica fume had a much higher demand).
A number of researchers have confirmed earlier reports that mineral admixtures typically reduce the permeability of concrete. Detwiler et al. [1994] have reported on this phenomenon in steam cured concrete. Geiker et al. [1991] noted that both silica fume and fly ash reduced the permeability of concrete to the penetration of chloride ion without changing the total porosity greatly. Bijen and van Selst [1991] found higher rates of carbonation in concrete with typical commercial quantities of ground granulated blast furnace slag compared with concretes containing typical quantities of fly ash. However, this study was not conducted with HPC.
Silica fume (also called condensed silica fume or microsilica) continues to be a popular element of high performance concrete, and especially high strength concrete. Not only does it provide an extremely rapid pozzolanic reaction, but researchers including Detwiler and Mehta [1989], and Goldman and Bentur [1993] found that its very fine size also appears to provide a beneficial contribution to concrete. Detwiler and Mehta, and Goldman and Bentur examined the effects of silica fume on mechanical behavior. Luther's [1989] review examined durability effects, while Fidjestol [1993], and Khayat and Aitcin [1993], have provided general reviews of the effects. These reports confirm findings that silica fume tends to improve both mechanical properties and durability.
However, St. John et al. [1994] report that deleterious expansion due to alkali silica reactivity is possible under wetting and drying conditions when particles of the densified form of silica fume admixtures are not sufficiently dispersed during mixing. In addition, a number of research efforts have attempted to answer important and unresolved questions in long term strength gain and frost resistance of silica fume concrete.
Maage et al. [1990] report that silica fume concretes continue to gain strength under a variety of curing conditions, including unfavorable conditions. They further indicate that concretes with silica fume appear to be more robust to early drying than similar concretes which do not contain silica fume.
A number of issues with frost resistance of concrete containing silica fume have been investigated, including the need for any entrained air when working with very low W/CM ratio concretes. Due to the dramatic reduction in permeability which accompanies the use of silica fume, concerns with frost durability in general, and with the usefulness of rapid freezing and thawing tests, have complicated the interpretation of research results. These issues are discussed in more detail in section 2.2.4, however it is useful to note here that ACI 318-95 [1995] limits the quantity of silica fume in concrete exposed to deicing salts to no more than 10 percent.
Attempts to improve the performance of systems of cementitious material in HSC have led researchers to examine mixes with multiple cementitious components. The use of ternary cementitious systems has received attention in recent years. Kashima et al. [1993] report on HSC produced with a blend of cement with large amounts of fly ash and ground granulated blast furnace slag in order to reduce heat of hydration. Their report is important because it reviews both experimental work and construction results.
Sarkar et al. [1991], examining the microstructural development in HSC using both silica fume and fly ash, found that strength at twelve hours was improved over similar mixes with silica fume alone. They state that this phenomenon may be related to the liberation of soluble alkalies from the surface of the fly ash. Baalbaki et al. [1993], reported on the properties of HPC produced with an extremely finely ground Type V cement with various mineral admixtures. Mixes with prolonged working times and very high strengths at ages out to one year were produced.
The use of cementitious systems with very high quantities of fly ash have also been investigated. Carette et al. [1993], and Bilodeau and Malhotra [1994], report that mixes have been developed which provide acceptable plastic and hardened properties, although strengths were not high, especially at early ages. In other studies, Bilodeau et al. [1994], and Malhotra [1990] report that performance in rapid freezing and thawing of concrete with high volumes of class F fly ash was adequate but that the concrete with very high quantities of fly ash performed poorly in deicer scaling tests. However, Nasser and Lai [1993] found that high volume, class C fly ash concrete was not frost durable even with a 6% air and after prolonged moist curing. They found that 20% fly ash mixes showed no difficulties in this respect. Kukko and Matala [1991] noted that frost resistance of non-air entrained concrete was reduced for very low W/CM ratio concrete produced with slowly hardening portland cement or containing slag, compared to rapid hardening portland cement with or without silica fume.
Naik et al. [1994] found that although concrete made with high volumes of class C fly ash passed ASTM C-944 for abrasion resistance, better abrasion resistance was obtained for concrete without the high fly ash content. Gjorv et al. [1990] also found that the abrasion resistance of conventional HSC pavements is exceptionally good. It would appear that high volume fly ash mixes have limited applicability to highway structures, although additional research appears warranted.
2.2.2.3 AggregatesHigh Performance Lightweight Concrete (HPLC) has been extensively investigated for, among other applications, use in oil drilling platforms in severe environments. Hoff [1991], and Tachibana et al. [1990], have presented information on the behavior of HSLC in extreme conditions. Hoff has demonstrated that HSLC containing both lightweight aggregate and conventional weight stone is both frost resistant and resistant to abrasion by ice. Holm and Bremner [1991], have provided additional information on the long term durability of lightweight concrete, indicating that in general when well-known prophylactic measures are taken to insure durability, long term durability is good.
Zhang and Gjorv [1991a, 1991b, 1991c, 1991d] have investigated both the mechanical properties and the permeability of lightweight concretes with strengths ranging from 50 to 100 MPa (about 7,000 to about 14,500 psi). Elastic modulus and the tensile-compressive strength ratio were lower than would be expected with conventional stone concrete at the same strength levels. While permeability of the HSLC's was very low, it was noted that permeability might be higher with lightweight aggregate than with conventional aggregate at the same strength. This, of course, would depend on the porosity and permeability of the aggregate itself.
2.2.3 Proportioning Methods
Modifications to conventional proportioning methods have been proposed by several researchers. Mehta and Aitcin [1990], and ACI Committee 363 report [1990] provide an excellent review of proportioning considerations for HSC. Selection of the proper raw materials and adjustment of proportions based on experience, using mixes conducted both in the laboratory and in the field, have typically proven adequate to achieve the desired concrete characteristics, at least within the limits allowed by the available raw materials. With adequate control of production and placement, routine use of concrete with compressive strengths in excess of 70 MPa (10,000 psi) is practical in many areas.
Several articles have been published with suggestions on methods of optimizing the development of particular mixes by reducing the number of trial mixes necessary. Campbell and Detwiler [1993], for example, have provided guidance for proportioning and producing steam-cured concrete. While de Larrard [1990] has provided suggestions for HSC mixes based on rheological considerations, Domone and Soutsos [1994] have reexamined the maximum density theory for applicability to HSC.
Field trials of High Early Strength (HES), Very Early Strength (VES) and Very High Strength (VHS) concretes in SHRP C-205 and C-206 indicated that existing proportioning methods remain valid, with minor modifications, for these mixes. Routine precautions such as those regarding minimum water contents and appropriate quantities or combinations of chemical admixtures, contained in numerous publications and discussed in the previous State-of-the-Art Report, remain valid. Development or adaptation of new types of high performance concrete or combinations of raw materials are better served by engineering judgement than by more sophisticated proportioning techniques.
2.2.4 Air Entrainment
The need for any air entrainment at all in concrete with very low W/CM ratio has been questioned. This issue has been complicated by the problem of interpreting test results of one of the most commonly used test methods in practice. ASTM C 666 measures the resistance of concrete to rapid freezing and thawing. The rate of freezing is much higher in this test than is found in practice, and C 666 has been criticized in this respect even for conventional concrete.
Concretes with a low W/CM ratio, such as HPC and HSC, have a lower permeability than conventional concrete. A rapid freezing and thawing rate may induce additional damage to concretes with low W/CM ratio simply due to the lower permeability. On the other hand, the very low w/c ratio, for an adequately cured concrete, can reduce or even eliminate the amount of freezable water in the pores for practical temperature ranges. These mixes will also dramatically reduce the ingress of water during the test, therefore reducing the amount of damage due to physically freezing water in the concrete. The time required to achieve an internal moisture content sufficient to contribute to frost damage is less than the time required for the C 666 test for concretes with very low W/CM ratios. However, since hydraulic pressure due to freezing of water is only one of several mechanisms of frost damage, the use of very low W/CM may not be adequate in all cases. Experimental results have been mixed.
Damage to concrete specimens may be due to thermal shock or to disequilibrium between energy states during cooling rather than expansion associated with the presence of freezing water for the mixes of very low W/CM ratio. Hanson et al. [1993], found that the electrical impedance of air entrained concrete of very low W/CM ratio actually increased during ASTM C 666 testing while the dynamic elastic modulus decreased. The increase in impedance could only have come from internal drying associated with the loss of free moisture during curing. Therefore, the decrease in elastic modulus was not related to the formation of ice.
Many believe that C 666 testing is still a valid discriminant for frost durability even though it is extremely severe for concretes of low W/CM ratio. Others have chosen to rely on deicer scaling as a more useful and informative test, particularly for highway and pavement applications.
Hammer and Sellevold [1990] report that salt scaling resistance is acceptable for HPC with W/CM ratios below 0.37, even without entrained air but that rapid freezing and thawing is accompanied by deterioration for all non-entrained air concrete tested down to a W/CM of 0.25. However, they also note that calorimeter data indicates very little ice formation until -20 c. They state that this indicates that much of the deterioration may be due to thermal incompatibility of the components rather than the formation of ice.
In research conducted by Kashi and Weyers [1989], non-air entrained concrete with a W/CM ratio of less than 0.30 was found to be resistant to rapid freezing and thawing based on ASTM C 666, Method A. For concretes with a W/CM ratio of 0.32, the concrete was frost resistant only if silica fume was not used in the mix. Cohen et al. [1992] similarly found that non-air entrained concrete with a W/CM ratio of 0.35 and containing 10% silica fume were not resistant to rapid freezing and thawing when tested in accordance with ASTM C 666 (A), even when curing had been extended to 56 days. They also noted that although there was a dramatic drop in the elastic modulus, the reduction in compressive strength was much less severe. Li et al. [1994], on the other hand, found that a maximum W/CM ratio of 0.24 was necessary for adequate frost protection of non-air entrained concrete when based on ASTM C 666 (A) testing.
Tests conducted by Pigeon et al. [1991], and by Gagne et al. [1991], using both ASTM C 666 (A) and ASTM C 672 deicer scaling tests, indicated that non-air entrained concrete containing silica fume and good quality coarse aggregate, with a W/CM ratio of 0.30 generally, but not uniformly, had good resistance to deicer scaling. When tested using ASTM C 666 (A), the W/CM ratio required to provide acceptable performance ranged from less than 0.25 to over 0.30. The cement used was found to play a significant role in the performance of otherwise similar mixes. It was also noted that the air void system produced by the use of water reducing or high range water reducing admixtures, commonly used in all concretes with low W/CM ratio, may be contributing in a fashion not yet well documented.
Additional research in these areas, perhaps concentrating more on test methods such as resistance to deicer scaling and critical dilation test concepts rather than ASTM C 666 (A), would appear to be useful. At the present time, the use of at least minimal quantities of entrained air appear prudent for concrete exposed to severe freezing, especially when exposed to deicing salts, unless that concrete has a W/CM ratio less than 0.24.
2.2.5 Other Types of HPC
Another type of HPC has been developed for use in situations where vibration is difficult or impossible and where reinforcing steel is highly congested. "Super-workable" or "flowable" concrete has been developed in Japan and used in both bridge structures and buildings. Although not specifically developed for high strength, low water to "powder" ratios are common. Kuroiwa et al. [1993], report super-workable concrete with strengths in excess of 50 MPa, a marginally high strength concrete.
Low W/CM ratio, flowing concretes utilizing HRWR's are well known. Super-workable mixes are an extension of this concept which have been specifically formulated to resist segregation. Super-workable concretes also derive partly from concrete developed for underwater placement. Research has led in several different directions. Paste and aggregate volumes, and paste composition, admixture type and dosages, and testing methods have been investigated.
A number of different "powder" combinations are reported including portland cement, fly ash, GGBFS and silica fume. In addition, where a low heat of hydration was a concern, finely ground limestone powder was used as a partial replacement for cement. This material was reported [Tanaka et al. 1993] to have both a chemical and physical effect similar to that reported for silica fume. Low heat of hydration mixes will typically exhibit considerably extended set times, with final set at about twenty hours, and low early strengths. This is due both to large quantities of HRWR in conjunction with a blended cement composed of 30% low heat of hydration portland cement and 70% GGBFS. The aggregate paste ratio was somewhat lower compared to conventional concrete.
Cellulose based products have been known to improve the cohesion of concrete and to reduce segregation. Sogo et al. [1987] report that a polymer based on cellulose ether can be used to increase both water reduction and cohesiveness. Many long chain organic molecules have water reducing capability and, if air content and setting time can be controlled, can be successfully used in concrete [Mehta 1975] with various effects on cohesiveness. Kuroiwa et al. [1993] report on findings using a polysaccharide polymer. Ozawa et al. [1990] describe studies to optimize the combination of HRWR and other admixtures affecting the viscosity of this type of mix.
Self compacting capability and resistance to segregation were determined by so-called "slump-flow", by a modified grout cone flow test and by self-leveling flow through reinforcing bars, as well as by mock-ups of particular members. Self-leveling, non-segregating performance was reported with several different mixtures. Details of these tests are discussed in the next chapter.
The addition of materials to compensate for increased drying shrinkage due to the lower aggregate content has been suggested, although Kuroiwa et al. state that drying shrinkage is equal to or better than comparable, conventional concretes. Frost durability was also found to be adequate for super-workable concrete containing at least 4% entrained air. Testing was based on ASTM C 666, although it was not stated whether Procedure A or B was used.
It is significant to note that these types of mixes have been successfully used, without vibration, in field placements. Applications include the Akashi Kaikyo Bridge near Kobe, Japan, and in the heavily reinforced concrete core and shear wall of a 20-story building. Concrete delivery was by pump. The use of super-workable concretes in certain applications appears promising.
2.3 Production Considerations
For high strength concrete where early strength is not a critical consideration, the use of conventional production methods and facilities appears adequate, as long as well recognized practices for the production of good quality concrete are enforced. Howard and Leatham [1989] and Sanchez and Hester [1990] discuss the production and delivery of HSC noting the importance of a team approach. Kakizaki et al. [1993], note that the mixing sequence can affect the slump and compressive strengths of very high strength concretes. Leming et al. [1993], emphasize that pre-pour conferences and field trials are necessary with any HPC prior to actual use.
In situations where early strength is critical, particular care must be taken to insure that temperature is closely controlled and that high dosages of water reducing or high range water reducing admixtures are avoided, since these can extend the time of set. While strength after one day may not be significantly affected, strengths at less than twenty-four hours and particularly before twelve hours, can be significantly reduced. If mineral admixtures are included, caution must be exercised in dosing the entire quantity of cementitious material. When significant percentages of mineral admixtures are employed, the result may be to effectively overdose the portland cement, again resulting in extended set times.
Much of the VES and HES concrete used in field trials for SHRP C-205 and C-206 contained a corrosion inhibiting admixture as a non-chloride accelerator, added at the job site to mitigate rapid slump loss [Hanson et al. 1994]. The admixture was added either by hand or by pump typically from trailer-mounted tanks. In either case, provision for adding the admixture, adequate quality control and sufficient remixing time are necessary.
Due to the high water content of this admixture and the low W/CM ratio of the paving mixes investigated, it was necessary to employ a HRWR in the initial batching. Inadvertent use of large dosages of HRWR caused low strengths at ages up to and including one day. The low water content and rich paste of these mixtures required strict control of the batching sequence. Although only minor adjustments to conventional practice were required, some adjustment must be anticipated. Exact procedures will vary from one production facility to another, but there are several keys to a successful placement. The plant itself should be equipped with an automatic moisture indicator for the aggregate.
A pre-placement conference, including all parties who will be involved in the slab-on-grade placement, is required. A practice placement to adjust operations, if necessary, to develop estimates for slump and air loss in transit, and to acquaint the crews involved in the placement is highly recommended. Inspection of trucks, especially in a dry batch operation, is necessary to insure that only trucks with clean fins and adequate mixing speed are used. Trucks should carry no more than two-thirds of their rated mixing capacity to insure adequate mixing on the job site if there is to be any addition of admixtures on the job site. Trucks should discharge their entire load as soon as possible. A time limit of ten minutes after arrival on the job site should be used for planning purposes but may vary depending on the type and composition of the concrete being used. Special ready-mixed concrete trucks, intended for paving operations, should be used if discharge of very low slump is anticipated.
Since most HPC's are paste rich with low water content, bleeding is typically very low. This can potentially create difficulties with plastic shrinkage cracking. Therefore, it is necessary to apply curing compound or take other precautionary measures to reduce evaporation as soon as possible for slabs or members with large exposed surfaces. However, field trials for SHRP 205 [Leming et al. 1993] of HES and VES concretes in slabs found that cracks due to plastic shrinkage were rare. The concrete was apparently gaining strength faster than it shrank. Another consequence of the rapid setting and strength gain was that time prior to sawing the slabs was reduced. It was critical that joints be sawed as early as possible. Delays past eight hours were found to cause cracking of 20 cm depth (8 in.) pavements at approximately 7.5 m (approximately 25 ft) intervals.
2.4 Fiber Reinforced Concrete
Although there has been continued interest and research in the use of fiber-reinforced concrete (FRC), there have been few major innovations in proportioning or production of high performance fiber reinforced concrete (HPFRC) since the last State-of-the-Art Report. In addition, while research in FRC has examined the influence of modifications of existing fibers, fibers with larger aspect ratios, and higher fiber volumes, and there continues to be interest in non-metallic fibers or combinations of fibers, these researches were based on existing fiber materials.
One of the few new approaches in this area has been the development of SIMCON, or Slurry Infiltrated Mat Concrete, described by Hackman et al. [1992] and Krstulovic-Opara et al. [1994]. They noted that SIMCON is a different material from SIFCON, or Slurry Infiltrated Fiber-Reinforced Concrete, which is based on the use of prepacked discontinuous steel fibers. SIMCON, on the other hand, uses a manufactured continuous mat of interlocking discontinuous steel fibers, placed in a form, and then infiltrated with a flowable cement-based slurry. The use of continuous mats, typically made with stainless steel to control corrosion in very thin members, permits development of high flexural strengths and very high ductility with a reduced volume of fibers than SIFCON.
The use of SIMCON appears to be very promising for at least two reasons. First, the very high volume of fibers required to provide significant increases in mechanical properties such as SIFCON can create a problem with economic justification in a large number of practical applications. However, with SIMCON, direct tensile strengths of 15.9 MPa at 1.1% strain have been reported with only a 5% volume fraction of fibers. Secondly, in situations where normal FRC may be economically justified, such as in pavements, the addition of fibers to the mix and the placement of the fiber-reinforced mix required special care, and considerable extra time and expense. SIMCON overcomes many of these limitations since the fiber mat, normally delivered in large rolls, can be laid out by hand and the slurry simply pumped into place. The use of SIMCON permits fabrication of thin, complex shapes with very high ductility and flexural strength.
Another interesting and useful development in FRC construction has been to provide non-metallic fibers in small, cylindrical bundles, approximately 50 mm high (the length of the fiber) and 55 mm in diameter, wrapped in a water soluble compound. This permits the easy addition of the fibers, by hand, into the mixing drum of a truck mixer, either during charging or at the job site. The wrapper disintegrates, allowing the fibers to disperse into the concrete mixture with little balling or segregation. Quality control is improved by making the quantity of fibers added easy to determine and easy to check, and by minimizing problems in dispersion in the mixer. Further, production rates are maintained with little additional effort. Successful field applications in a full-depth pavement, a thin bridge-deck overlay, a Jersey barrier, and white-topping on scarified asphalt pavement have been reported from South Dakota
Sabtu, 09 Januari 2010
Selasa, 15 Desember 2009
Shotcrete atau Beton Semprot ( Spray Concrete )
Shotcrete atau Beton Semprot ( Spray Concrete )
Shotcrete atau gunite pertama kali ditemukan oleh Carl Ethan Akeley (1864-1926) pada 1910. Arsitek Amerika ini telah terinspirasi untuk mewujudkan reproduksi yang nyata dari dinosaurus untuk sebuah taman wisata. Mengingat ukuran struktur, ia mempunyai ide untuk mengembangkan "semen gun" mesin yang memungkinkan penyemprotan dari cementitious mortar, maka dinamakanlah Shotcrete.
Pada tahun yang sama, Kaspar Winkler mendirikan Sika. Sejak saat itu Sika telah memberikan kontribusi sangat besar bagi perkembangan teknologi Shotcrete , baik bahan kimia dari material shotcrete itu sendiri ( Sigunit ) maupun peralatannya yang dikenal dengan nama Alliva Machine.
Bidang Aplikasi Shotcrete
Shotcrete terutama digunakan dalam proyek konstruksi bawah tanah sebagai perkuatan struktural awal ataupun permanen untuk bangunan struktur seperti jalan bawah tanah, terowongan kereta api, pembangkit listrik tenaga air (PLTA), , tambang bawah tanah, kereta bawah tanah, dll tempat penyimpananNamun shotcrete juga dapat digunakan untuk stabilisasi lereng mencegah supaya tidak longsor , kolam renang, saluran air, perbaikan beton, inner lining arsitektur dan struktur. Kira-kira 90% dari shotcrete akan diterapkan ke dalam proyek-proyek konstruksi bawah tanah. Total volume shotcrete yang diaplikasikankan di seluruh dunia adalah lebih dari 12 juta meter kubik per tahun.
Definisi Shotcrete
Menurut American Concrete Institute (ACI), shotcrete dapat didefinisikan sebagai mortar atau beton yang diberikan tekanan dengan kecepatan tinggi .
Komponennya campurannya terdiri atas semen, pasir, agregat, air, dan tambahan admixtures.
Perbedaan shotcrete dengan beton normal dapat dilihat dari 3 hal :
1.ukuran agregat maksimum yang digunakan.
2.Prosesnya
3.Campuran dari shotcrete bisa kering atau basah.
Mengenai terminologi kita dapat menjelaskan Gunite sebagai mortar yang disemprotkan sementara Shotcrete sebagai beton yang disemprotkan.Gunite adalah campuran antara semen dengan partikel/aggregat ukuran diameter yang dibatasi sampai 8mm.Sedangkan untuk Shotcrete penggunaan maksimum diameter aggregat adalah 16 mm. Namun, dalam 10 tahun terakhir ada kecenderungan untuk membatasi maksimal ukuran agregat sampai 10 mm.
Ada 2 cara proses shotcrete :
1. Proses kering
2. Proses basah
Shotcrete atau gunite pertama kali ditemukan oleh Carl Ethan Akeley (1864-1926) pada 1910. Arsitek Amerika ini telah terinspirasi untuk mewujudkan reproduksi yang nyata dari dinosaurus untuk sebuah taman wisata. Mengingat ukuran struktur, ia mempunyai ide untuk mengembangkan "semen gun" mesin yang memungkinkan penyemprotan dari cementitious mortar, maka dinamakanlah Shotcrete.
Pada tahun yang sama, Kaspar Winkler mendirikan Sika. Sejak saat itu Sika telah memberikan kontribusi sangat besar bagi perkembangan teknologi Shotcrete , baik bahan kimia dari material shotcrete itu sendiri ( Sigunit ) maupun peralatannya yang dikenal dengan nama Alliva Machine.
Bidang Aplikasi Shotcrete
Shotcrete terutama digunakan dalam proyek konstruksi bawah tanah sebagai perkuatan struktural awal ataupun permanen untuk bangunan struktur seperti jalan bawah tanah, terowongan kereta api, pembangkit listrik tenaga air (PLTA), , tambang bawah tanah, kereta bawah tanah, dll tempat penyimpananNamun shotcrete juga dapat digunakan untuk stabilisasi lereng mencegah supaya tidak longsor , kolam renang, saluran air, perbaikan beton, inner lining arsitektur dan struktur. Kira-kira 90% dari shotcrete akan diterapkan ke dalam proyek-proyek konstruksi bawah tanah. Total volume shotcrete yang diaplikasikankan di seluruh dunia adalah lebih dari 12 juta meter kubik per tahun.
Definisi Shotcrete
Menurut American Concrete Institute (ACI), shotcrete dapat didefinisikan sebagai mortar atau beton yang diberikan tekanan dengan kecepatan tinggi .
Komponennya campurannya terdiri atas semen, pasir, agregat, air, dan tambahan admixtures.
Perbedaan shotcrete dengan beton normal dapat dilihat dari 3 hal :
1.ukuran agregat maksimum yang digunakan.
2.Prosesnya
3.Campuran dari shotcrete bisa kering atau basah.
Mengenai terminologi kita dapat menjelaskan Gunite sebagai mortar yang disemprotkan sementara Shotcrete sebagai beton yang disemprotkan.Gunite adalah campuran antara semen dengan partikel/aggregat ukuran diameter yang dibatasi sampai 8mm.Sedangkan untuk Shotcrete penggunaan maksimum diameter aggregat adalah 16 mm. Namun, dalam 10 tahun terakhir ada kecenderungan untuk membatasi maksimal ukuran agregat sampai 10 mm.
Ada 2 cara proses shotcrete :
1. Proses kering
2. Proses basah
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