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