The information provided below is intended for guidance in planning and executing shotcrete applications. This information is intended only for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations, and who will accept responsibility for the application of the material it contains. The American Shotcrete Association provides this information based on the best knowledge available to them and disclaims any and all responsibility for the information provided. The American Shotcrete Association will not be liable for any loss or damage arising therefrom.
1. What is the difference between shotcrete and Gunite?
2. How should I design joints for shotcrete?
3. I want to specify ACI Nozzleman Certification in my next project. A contractor has told me that there are no certified nozzlemen in the project area. How can I verify that information? What should I direct this contractor to do?
4. We have a project that calls for new 6 in. concrete shearwalls formed and placed against the existing structure from the basement up to the fourth floor to enable an additional seven floors to be added to the structure. Our engineer has suggested that the new shearwalls be constructed using shotcrete. We are not familiar with using this system for structural applications. Most of the information we have gotten relates to using shotcrete for swimming pools and cosmetic applications. What advice can you provide?
5. We are building a new home. Foundation contractors who place traditional basement walls tell us they would never go into a house built with walls constructed using shotcrete. When used for walls, can shotcrete be of equivalent strength as placed concrete?
6. We will be using shotcrete to repair a concrete box culvert that has some minor spalling. Do we need to apply a bonding agent before applying the shotcrete? How should we prepare the surface?
7. I am a structural engineer working on a project in Southern California. We are creating specifications for the use of shotcrete for basement walls. However, I cannot find any information on compressive strength requirements for shotcrete in the building code. We are basing our design on compressive strengths ranging from 3500 to 4500 psi. Are there minimum and maximum allowable compressive strengths for shotcrete?
8. Our firm has no experience designing for shotcrete applications. We have been investigating the process and would like to know what we should be looking for as the shotcrete is placed. Are there special features or problems in shotcreting?
9. I know air entrainment is required in concrete exposed to cycles of freezing and thawing while saturated. However, the shotcrete I am going to be applying on a project in Chicago is on a vertical surface where the water will essentially run off the surface. Do I still need to worry about air content?
10. We are going to be using shotcrete for repairs in a parking structure. We have no experience performing this work and will be subcontracting this portion of the job. What should we be watching for when the shotcrete is being applied?
11. I am currently involved in the design of a large retaining wall for a job in Boston. One option under consideration is the use of soil nails with shotcrete lagging. The design anticipates a 100-year service life. What can I tell my client to realistically expect from the shotcrete option? Is shotcrete durable in the freezing-and-thawing conditions in this area? What is the best way to improve the longevity of the product?
12. I am a civil engineer working on the rehabilitation of a low fixed crest concrete dam of 6 foot height. After stitching of cracks and patch repairs, we want to specify a 2.5" shotcrete facing on the down-stream side to protect from high velocity-induced erosion. The up-stream side will be sealed with a betonite-clay liner to save costs. To get a very dense concrete, we are thinking of 8000 psi airentrained, fiber-reinforced mixture. Should we use a WWF reinforcement? Should this be a wet or dry application?
13. I am an architecture student and would like any information you could provide in regard to the proper and typical mix ratios of cement to sand.
14. I have come across the term "spacing factor" and have been unable to find a definition. What is a spacing factor?
15. I am looking for any information regarding the use of construction joints for permanent shotcrete wall facing. I have found information on placing shotcrete over existing construction joints but none regarding the use of construction joints for the shotcrete wall facing itself.
16. I have a project wherein some 25,000 sq ft of existing shotcrete is to undergo varying degrees replacement, repair and restoration.
* It is on slopes varying from 1:1 to 1:10 or so.
* It is approximately 40 years old in most cases.
* It is in a fairly arid climate (Southern New Mexico) with little rainfall and typically low humidity.
* The subgrade is non-plastic gravelly sandy material.
* It was reinforced with wire mesh (looks like 6x6x10x10).
I'm interested in any techniques and/or materials that might be applicable.
17. Can shotcrete be painted like other concrete? Can an elastomeric paint, 100% acrylic latex house paint or solvent acrylic be used? I have a customer who wants to paint a tank which uses shotcrete. With normal concrete the surface must be 30 days or older, pH is approximately 7-8 and moisture content is low, remove efflorescence or laitance, etc., then it is ready to paint or coat. Do the same restrictions for shotcrete?
18. I am a general contractor who hired a company to shotcrete a new swimming pool. They began on Friday, a very hot day, and they were placing concrete very slowly (27 yards in 4 hours). Their pump broke down and they were unable to complete the job that day so they returned on Monday. My question is about the "cold joint" between the work on Friday and the work on Monday. What is your opinion of this situation?
19. My company manufactures a polyester geogrid that is coated with PVC. We sell these grids into underground mines, as well as many aboveground civil engineering products. We have a new grid that may work very well as an auxiliary reinforcement for shotcrete-type products. Can you tell me what the pH is for these products? The type that we would be exposed to is used in underground mines to reinforce the mine roofs.
20. Is there any reference that differentiates between temporary shotcrete work and permanent shotcrete work, as far as inspection/testing requirements?
21. I am looking for design information for shotcreting a steel sheet pile wall to create a composite structure for a lift station wet well. I can design the sheet piling, which would be driven into the ground in a plan circle of 12 feet diameter, followed by excavation. I need to know the practicality of then applying a layer of shotcrete, primarily as a means of sealing the joints of the sheet pile, protecting the sheet pile from the wastewater, and providing additional wall strength. The lift station will be above the water table during construction, but would be periodically below the water table under groundwater conditions.
22. What is the minimum thickness that shotcrete can be applied? We are currently using shotcrete on a restoration project and have a concern at the corner locations are returning to tight recessed steel framed windows. There is an exterior wood molding approximately 1 inch from the tight corner that needs to be preserved. Do you have any suggestions as to how we can address this? Do we need to provide caulking between the wood molding and the shotcrete?
23. When used on walls, can shotcrete be of equivalent strength as poured concrete?
24. Can you provide any information on insulating gunite in spa installations?
25. Is it possible to put a texture on the application side of a shotcrete wall? I understand that I can shoot against a form, but what about the side that gets screeded?
26. I would like to get expert opinions regarding a proposal. I am reviewing from a contractor to replace precast concrete wall panels with shotcrete wall. The wall acts as a retaining wall and the precast panels were specified to span between the soldier piles (with tiebacks), driven and anchored into the rock at a spacing of 10 feet. Shotcrete walls over 3-inch wood lagging have been proposed to replace the precast panels and they have been designed exactly the same way as reinforced concrete walls. Using ACI Code working strength design for 4000 psi concrete, and fs= 24000 psi steel, the reinforcing in the shotcrete walls have been determined using value of a = 1.76 . ( As= M / 1.76. d ) I do not feel comfortable accepting the same equations and numbers for a shotcrete wall as for a cast-in-place or precast concrete wall with all the quality controls and rigid specifications per ACI 318 Code concerning mixing, formwork, placement, vibration and curing. Could you please provide an expert opinion on the matter? What would be the reasonable values of coefficient to determine the reinforcing in shotcrete walls?
27. I am interested in constructing my home using shotcrete applied over polystyrene panels. There are several systems for this, but I'm most interested in avoiding "thermal bridging" that occurs when metal reinforcement passes from the inside of the home to the outside through the foam insulation. I am also interested in fabricating the panels myself, if possible. There was a system utilizing metal reinforcement grids on each side of the polystyrene panel connected by plastic components. Can you point me toward a company that offers this system in the US?
28. Do you have any publications on shotcrete curing, specifically in tunneling? How is shotcrete cured in tunnel constructions with the temperature and moisture problems?
29. I am trying to find an article on the bond strength between two layers of shotcrete. My company is placing a 22" thick shotcrete retaining wall and, at a later date, we are placing a small amount of shotcrete over the existing shotcrete wall. The Engineer thinks the shotcrete will just falls off over time. Is this true? Can you point me in a direction that might have information on the bond strength between two layers of shotcrete?
30. Can you provide input on the applicability of the shotcrete placement method for the structural repair of existing concrete walls? These walls (two) are conventionally reinforced, 31 feet in height and are parallel with a clear spacing of 5'-0". There length is 150 feet. Structural repair is required at many locations that have experienced spalled concrete with corroded reinforcing bars. Depth of repairs will range from 2" to approximately 6". Concrete substrate will have exposed aggregate with a significant amplitude. From a production and cost viewpoint, shotcrete appears to be more applicable than a form and pour or form and pump repair method.
31. We have a project that our subcontractor would like to change from concrete liner for a box culvert to a shotcrete liner it is a C.O.E. project. The C.O.E. has questions of durability. Could you help?
32. We are building a home where some of the outside walls are bricked. Is there a way to use shotcrete over strand board (chipboard)? If so, how and what cost would there be approximately a square foot for the actual shotcrete installed?
33. We are having a pool built with shotcrete. The pool company has asked us to change the contract to allow them to use the wet method instead of the dry method of shotcrete. I have read through your website and found it helpful in understanding the difference between the two, but I would like to know if one is better or more sound than the other.
34. We have a design/build drainage channel project that requires a concrete lining over secant piles in which the secant piles form the main structural walls of a box culvert. The box culvert discharges into the ocean. We proposed a shotcrete concrete liner but there are concerns about the life service durability of shotcrete in a saline environment. Do you have any reference information on this matter that we could use to support our position?
35. Our company is developing alkali-free accelerator, both powder and liquid types. Since our information and knowledge is limited, please answer the following questions:
1. What is the formal definition of alkali-free in DIN, ASTM, or other specifications?
2. What is the lowest pH value of alkali-free accelerator? In which pH value that the product won't harm to the human tissue or vascular system? Please also advise where we can find the related information.
3. Is it acceptable to use Aluminum Sulfate as the main component of alkali-free accelerator?
36. I wish to request expert advice from ASA in regard to the Gunite Contractor's Association method, that we are using to make test cylinders (i.e. 6" diameter and 12" high shot into a form of 3/4" square mesh hardware cloth). Since we are currently in the process of guniting a silo and have today received 3,250 psi rather than the mix designed 4,000 psi 7-day strengths, we would appreciate your prompt response.
37. I have a special request for a shotcrete mix design. My company has been using shotcrete for about three years, here in Alaska. I have recently had a request to shotcrete a 60'x50' duck pond to make it waterproof. The problems I am running into are that moose keep walking into the pond, and the pond is on the side of a hill with built up edges around the outside. The mix design I am looking for needs to have an epoxy or some kind of adhesive to help stop the water from running out the cracks. Last, are there any fabric or plastic materials that I could lay down and spray the wet shotcrete on to put on the sides of the pond?
38. We are shotcreting our first wall and the contractor tells us that in shotcrete, the lapping of the bars is not done by putting the bars alongside each other as in conventional pouring of concrete but rather a gap is left between the bars in order to avoid voids behind bars bundles. A two-inch gap is being used on our job. Is there a publication that deals with reinforcing steel placement in shotcrete in general and one that deals with bar laps in particular?
39. I'm looking for information as to the thickness design of shotcrete for ditch slope lining purposes. Can you direct me?
40. Our development has 8 recirculating water ponds of various sizes. All are vinyl liner under concrete construction. Some ponds have developed leaks due to cracking of the concrete. Will shotcrete provide an adequate seal to stop the leaks for an appreciable time? Can you recommend a contractor in the Denver, CO area for this type of work?
41. We are a construction company and are currently executing a cathodic protection work for the reinforced concrete pile caps of a jetty. After the application of concrete repairs and placement of CP system over the R/C surfaces, we are to cover the concrete surfaces with a waterproofing material. The engineer of the project recommends the shotcrete application with a thin layer in order to provide with the protection of the buried anode strips and as well as waterproofing of the surface. We use strip type CP anodes and we place them into the sound/repaired concrete by saw cutting the surface. Saw cuts are 1/4" width by 1" depth and located top, mid and bottom sections of the 40" depth vertical pile cap surface. Our Questions are:
1. Can we apply a thin layer of shotcrete over the repaired concrete surfaces without having any reinforcement and would it be a good solution as far as the stability of the shotcrete is concerned?
2. Would it be a safe solution to apply the shotcrete over the repaired surfaces just to provide with the protection of the CP anodes placed in saw cuts as described above?
3. Would it be enough to make a waterproof coating instead of applying shotcrete to the whole concrete surface so that the waterproofing of the surface shall be provided?
42. We are currently in the process of doing a seismic upgrade to one of our parking structures using shotcrete. During this process, the murals that are painted on the interior walls are being removed and will be repainted at a later date. How long do I wait before it is cured enough to begin painting?
43. I am trying to find out if there is any research or literature regarding the drying shrinkage of shotcrete. Can you help?
44. I've been a pool builder all my life and I use your magazine as a technical source and I really enjoy it. I found a conflict: In Shotcrete Summer 2004, page 30, the answer to the second question suggests the use of 8% as batched air content with max sized coarse aggregate of 3/8 inch. The conflict I have is that a) won't 8% as batched drop to 1-2% after wet gunning? and b) previous articles suggested the use of 15-22% air as batched to help get it through the hose and to achieve 8% in place. Can you clarify?
45. As a specifier, should I specify which process—dry or wet—should be used on my projects? What are the significant differences?
46. My firm is a general contracting entity that frequently uses shotcrete subcontractors. When project specifications are not clear on testing, I have been relying on the advice of my shotcrete subcontractors on the frequency of taking tests for compliance with strength requirements. We always shoot a test panel prior to starting construction. How much testing should we be doing during construction?
47. Is there a U.L. (Underwriters Laboratories) certification for shotcrete?
48. I have a client who may be interested in using shotcrete for walls in a radiosurgery unit requiring radiation shielding. Could you please tell me the typical density of shotcrete?
49. Are there specific benefits in using silica fume in shotcrete beyond reduced permeability in the hardened shotcrete?
50. The Park District Department of our city is in the process of designing a new swimming pool. One of the prospective bidders made a presentation in which they said they would use shotcrete instead of conventional cast in place concrete. Their design is to use 6 in.-thick walls instead of the 12 in.-thick walls as proposed for the cast in place design. They claim that 6 in. of shotcrete is as strong as 12 in. of formed concrete. Is this a true statement?
51. I am working on repairing some mildly deteriorated walls in a drinking-water treatment plant. There are no chlorides used in the treatment process. I would like to apply a 1 in.-thick shotcrete layer over the existing concrete utilizing a mix containing silica fume, which will achieve a compressive strength of 5000 psi at 28 days. I am having difficulty formulating a mix to meet those requirements that also has a water soluble chloride content of less than 0.10 % chloride ion concentration by mass of cement. I cannot get the chloride ion concentration below 0.15%. What adjustments can I make to get to my goal of 0.10% or less?
52. Our firm is working as a consultant for a project. We have very little experience with shotcrete. What is the life span for a shotcrete wall?
53. What is the best reference when specifying aggregate gradations for shotcrete projects?
54. I want to apply a 3.5 in. (89 mm) veneer of shotcrete over an existing cast-in-place wall. I am concerned about how well the shotcrete will bond to the existing wall. This wall is 50 ft (15.25 m) in height. What are the keys to doing this work successfully?
55. We are having a swimming pool built with shotcrete. Our question is, what is the required curing time for shotcrete prior to exposure to heavy rain? We are trying to plan the shotcrete installation when the weather looks most favorable.
56. I will be shotcreting an existing structure that has some diesel fuel and oil stains on the existing concrete. How should I treat them before shotcreting?
57. Our firm is preparing to use the shotcrete method on a project for the first time. What type of prequalification work should we be specifying?
58. We are hearing a lot of discussion about performance versus prescription specifications? What do we need to know about this discussion?
59. I am doing a wet-process shotcrete project. The shotcrete mixture is being delivered by a ready mixed concrete company. Recently we had some delays on the site. The inspector told us that any concrete not unloaded within
90 minutes of arrival on the site would be rejected. Where does that rule come from?
60. I am bidding a tunnel project and am uncertain about part of the specifications. Are specifications for shotcrete temperature different for the wet and dry processes? Are there separate requirements for the shotcrete, ambient, and surface temperatures? Can you refer me to industry standards?
61. What wire size and opening are recommended for repair of bridge substructures? We realize the mesh would not be for restoring or improving structural capacity, merely to help control cracking.
62. Our general contracting firm is working on a project with a very tight schedule and significant penalties for missing the completion date. It has been suggested that we consider using shotcrete for the below-grade foundation walls. We have been told that we can save significant time by using shotcrete instead of cast-in-place construction. These walls are heavily reinforced. Has this been done successfully elsewhere?
63. Is a bonding agent recommended when placing shotcrete on an existing substrate?
64. My firm just completed a 2 in. (51 mm) overlay of shotcrete in an existing storage tank. Almost immediately after the shotcrete was applied, we noticed spider web cracking on almost the entire surface. The weather was very hot during shotcreting, and we suspect this caused the cracking. The project engineer is concerned about permeability and is thinking of having the shotcrete removed. Is removal really required or can we live with this cracking?
65. What is the recommended core size for shotcrete? Are there unique characteristics of shotcrete cores?
66. Is the core grading scale in the ACI CP-60(02) manual used as an acceptance tool on projects?
67. Our construction management firm is relatively new in allowing shotcrete on our projects. In the most recent issue of Shotcrete magazine, there was a discussion of cores taken from shotcrete in the FAQ feature. Is there additional critical information we should be aware of when determining our coring plan?
68. We have a large pond (12,000 ft2 [1115 m2]) 12 ft (4 m) deep with 2-to-1 sloped sides. It currently has an old PVC liner that is ripped and cannot be repaired. We have no shotcrete experience and wonder if shotcrete would be a better option than installing a new PVC liner?
69. Is there any specified finish for shotcrete?
70. I am a project engineer. Recently I received a mixture design for a shotcrete project that included limestone coarse aggregate. This is a first for me. All other shotcrete mixtures I have seen have had pea gravel as a coarse aggregate or no coarse aggregate at all. Is limestone commonly used in shotcrete?
71. We are concerned about the compressive strengths of shotcrete recently placed on one of our projects. The specification calls for 8000 psi (55 MPa). Test results indicate we are only at 5200 psi (36 MPa) at 28 days. Ambient temperatures are constant at about 45 °F (7 °C) at the point of placement. Should we be considering removal of the shotcrete?
72. I am in the process of designing a 6" shotcrete overlay for an existing wall that is approximately 1,250 square feet. The shotcrete subcontractor has proposed to use a dry-mix shotcrete. What are the advantages and disadvantages to the dry-mix process? The design includes dowels on 24" centers and 4x4 W4xW4 wire mesh. Can the entire 6 inch thickness be placed at one or will it require a number of different lifts to build up to the 6 inch thickness?
73. We are currently designing a retaining wall, sloped at 1H:0.5V, 5.5 high. We want to use shotcrete for this 12 inch (300mm) thick structural wall. For strength requirements, we are able to use a 10mm mesh, however this does not satisfy for crack control requirements. For crack control, it is required that we us 1/2 inch (12mm) individual rebars. Obviously for cost and ease of construction, the mesh is a favorable choice for reinforcing. Is there a typical section for this type of application? Will shotcrete shrink less than poured concrete?
74. We are looking at lining an existing 20 ft (6.1 m) diameter brick sewer with shotcrete that is 15 in. (0.4 m) or more thick and fairly heavily reinforced. Can this be done? The existing sewer is about 3 mi (4.8 km) long and 100 years old. Would shotcrete be a suitable method of rehabilitation? The rehabilitation is not just a liner, but the owner wants the shotcrete designed as a replacement pipe inside the existing brick sewer, designed for all earth and other superimposed loads as though the brick sewer were not there.
75. We are currently designing a retaining wall, sloped at 1H:0.5V, 18 ft (5.5 m) high. We want to use shotcrete for this 12 in. (300 mm) thick structural wall. For strength requirements, we are able to use a 0.4 in. (10 mm) mesh; however, this does not satisfy for crack control requirements. For crack control, it is required that 1/2 in. (12 mm) individual reinforcing bars are used. Obviously, for cost and ease of construction, the mesh is the favorable choice of reinforcing. Is there a typical section for this type of application? Will shotcrete shrink less than placed concrete?
76. We have a 6 in. (152 mm) thick tilt-up concrete wall that needs to be upgraded to achieve a 4-hour fire rating. We would like to add shotcrete to achieve that rating. What is the hourly rating per inch of shotcrete? We were hoping that 2 in. (51 mm) of shotcrete would provide the desired rating.
77. I am interested in testing bond strength between shotcrete and the substrate being sprayed. This is a 12 in. (300 mm) thick section, however, and I believe it will be difficult to get a slender diameter core using the EFNARC test method without damaging the core. Do you have any suggestions?
78. Can brackish or salt water be used to make shotcrete for a pool and will it have any negative effect on the quality of a shotcrete pool?
79. We are constructing a canopy for a mine entrance. We need to attach some type of wire mesh to the wood fillers to give the shotcrete some surface to bond to. What type of wire would be the best for this application? The mine canopy is self-supporting and the shotcrete is strictly to be used as a sealant.
80. What can we add to dry-process shotcrete mixtures for cold weather operations?
81. How can I maintain a 2 in. (50 mm) thickness of shotcrete in a rock excavated tunnel?
82. Can shotcrete be applied to a slope to act as a retaining wall without a moisture barrier? If a moisture barrier is recommended, what type should we use?
83. We are having a swimming pool constructed. The pool consultant is concerned about cold joints during construction if walls and the floor are shotcreted on different days. The shotcrete subcontractor states that there is no problem as the next layer of shotcrete will knit itself to the previous placement and form a solid bond. Is the shotcrete subcontractor correct?
84. What is the maximum thickness for shotcrete used for shear walls? Can we use more than 12 ft (3.7 m) if we use a double layer of reinforcing?
85. We would like to get approval to use shotcrete on the perimeter walls of an existing laboratory building. We would be shooting against a waterproofing membrane and shoring lagging. The project engineer is concerned that the shotcrete will damage the membrane, resulting in leaking into the occupied space. Are there any examples where this type of shotcrete placement has been used?
86. I am repairing a concrete masonry unit (CMU) block wall that was partially damaged when a portion of the roof collapsed. The engineer on the project is proposing to apply shotcrete to one side of the wall to help structurally reinforce the wall. I would like to know if there is a way to finish the wall so it is cosmetically pleasing, especially since this is on the inside of an existing building with the other walls being a painted CMU. Also, were can I get some conceptual pricing for applying the shotcrete?
87. I have an unfinished (dirt) basement with a stacked stone and mortar foundation. Can I shotcrete the existing dirt walls and floor with shotcrete MS (micro silica enhanced) and have it adhere to the dirt portion of the basement? If so, what method would be best?
88. I am lining a below-ground conical shaped excavation with shotcrete. Dimensions are approximately 90 ft (27.4 m) diameter by 45 ft (13.7 m) depth. Sand will be moved in and out of the container daily. Temperature range is 590 to 740 °F (310 to 393 °C). Can you tell me if a mixture is available that can meet the following specific conditions:
• Withstand the temperature ranges noted above without spalling, cracking, etc.; and
• Resist abrasion assuming hot sand is flowing over the surface area daily?
89. I am reconfiguring the interior of a spa and am wondering if drains and jets can be relocated without compromising the overall structure and getting cold cracks. Can the entire interior be re-shot to maintain the monolithic form and guarantee against failure? Is there an independent professional who could conduct an on-site inspection and recommend a next step?
90. I am a civil engineer looking to use shotcrete in a culvert rehabilitation project. Due to flow constraints, we are forced to have a maximum wall thickness of 3 in. (76 mm). For the typical 96 in. (2438 mm) precast concrete culvert, the walls are approximately 9 in. (228 mm). What can I do to obtain a near similar product with only 3 in. (76 mm) of wall thickness? Can shotcrete be applied at higher compressive strengths, 10 psi (0.07 MPa), or is it better to use fiber-reinforced shotcrete? The intent of the retrofit is to at least obtain a 10-year service life to this temporary solution.
91. Is it feasible and economical to construct floodwalls approximately 5 ft (1.5 m) high with shotcrete?
92. I am a pool builder who favors dry-mix shotcrete. I have a project requiring: a) cast-in-place concrete retaining walls, where there will be exposed downhill faces (that are not necessarily meant to be seen). Should my shotcrete contractor be able to finish the exposed face in some sort of reasonable finished appearance? and b) placing a pool house foundation (about 4 ft [1.2 m] high). Would I be able to shoot these? I am thinking not because there is no place for the rebound to go.
93. Our client has a retaining wall that has experienced movement in the precast concrete panels and has asked us to research a product that could be applied to give a smooth look to the retaining wall. Is shotcrete a possible option? I would also like information on the recycled content of shotcrete.
94. We recently stained a shotcrete wall. After we placed the staining on the wall, the stain came out in different shades across the wall, in effect bring out the different curing of the concrete. What can be done to eliminate this inconsistency?
95. I have a seawall with a gunite (dry-mix shotcrete) outer layer. The gunite layer has cracked in multiple locations on the seawall resulting from years of exposure to the harsh environment. The original gunite was not part of a soil nail system. I am considering a re-coat of shotcrete probably 3 to
4 in. (76 to 100 mm) thick with wire mesh and L-anchors on a 2 to 3 ft (0.6 to 0.9 m) grid. I know the importance of surface treatment for bonding, etc., but I am not sure if I should remove the original gunite layer (which is still sound in some places) or apply the re-coat. The new overlay needs to be structurally effective. I know that a soil nail system is the most dependable solution, but cost is a major concern. Do you have
any suggestions?
96. We will be tiling a pool. The pool’s shotcrete walls and floor were placed approximately 10 days ago. What is the earliest we can begin gauging the pool walls and floors?
97. We are considering the use of bentonite in a blind-side waterproofing situation to waterproof a basement with shotcrete as the confinement material. The basement has a 8.2 ft (2.5 m) head of water permanently against it (approximately 6.5 ft [2 m] higher than the slab/shotcrete wall construction joint).
In brief, we intend to construct as follows:
1. Pump the area dry;
2. Place secant piles, and then apply shotcrete over the piles. The shotcrete will be troweled to accept the bentonite;
3. Apply the bentonite sheet membrane to the troweled shotcrete;
4. Tie two rows of reinforcing steel at 11.8 ft (300 mm) centers in each direction;
5. Shoot shotcrete through the steel onto the bentonite tanking; and
6. Turn the pumps off once the curing period is complete.
We have been advised this will be effective. Any advice on this system would be greatly appreciated, as we believe using shotcrete rather than cast-in-place concrete as the confinement material would result in significant cost savings. We know little, however, of the confinement properties of shotcrete.
98. I have reviewed ACI 506R, “Guide to Shotcrete,” and 506.5R, “Guide for Specifying Underground Shotcrete,” but was unable to find specific criteria pertaining to shotcrete protection for reinforcing steel. Would the clear cover then be based on ACI 318 Section 7.7.1 for cast-in-place concrete? For underground structures, would 3 in. (76.2 mm) of clear cover from ground be required?
99. I have a question on cold weather shotcreting. I have heard that for shotcrete operations, the ambient temperature has to be 40°F (4.4°C) and rising. I am on a job, and the inspector said it only needs to be 35°F (1.67°C) and rising. The high for the day is expected to be around 45°F (7.2°C), then fall back into the high 20s°F (–4 to –1.67°C). What would be your advice?
100. One of our clients has a 65.6 ft (20 m) tall mechanically supported earth (MSE) wall (to dump the ore from the mine into the crushers). The wall is about 984.25 ft (300 m) long and has approximately 30-degree slopes on both ends, like a pyramid. These slopes have eroded over the last 8 years of operation and some of the wall reinforcing is exposed. We want to stop the erosion and stabilize the slopes. The instructions issued to the contractor are: level the slopes; fill the voids; compact; apply shotcrete (maximum 1 in. [25 mm]). The area in question is 6.6 x 65.6 x 131.2 ft (2 x 20 x 40 m). Is shotcrete application in this case appropriate? Can you forward information on experts we could consult on?
101. Type CA and FA shotcrete are two classifications listed in ASTM C1480. What is the application of these two types of shotcrete?
102. How soon after shooting a pool shell can formwork be removed? How soon can tiling begin?
103. We have an approximately 9500 ft2 (882.6 m2) pool that was built and finished in midsummer. Four weeks later, the pool has developed “spider web” cracking in the bottom. We need to have a compressive strength test done. Our crew is on site now and is going to pull a 4 in. (101.6 mm) core sample for testing. I need to know what procedure to follow and where to send the sample for testing.
104. We have demolished two radioactively contaminated buildings down to their concrete slabs. One of the slabs has a concrete pit that is 26 ft (8 m) deep. The slabs have not been removed because the soil beneath the slabs is contaminated and we’re using the slabs as a cover to protect the spread of contamination in the soil until the soil remediation begins. We’d like to use shotcrete to temporarily (up to 5 years) fix the contamination on the slabs and the 5 ft (1.5 m) area surrounding them. The questions we have are: 1) Will shotcrete adhere to the concrete slabs and pit walls for up to 5 years without special preparations? (Portions of the radioactively contaminated concrete are painted and it is dirty from demolition activities); and 2) What is the minimum thickness of shotcrete needed to last for 5 years in this type of application? We do not want to use any wire or fabric mesh as it would require personnel to work in a radiologically controlled environment to install the material.
105. What is the recommended cure time for shotcrete pools and spas so that shrinkage cracks in finished tile work can be avoided?
106. We are designing underground support for a hydropower tunnel. I want to know whether wire mesh-reinforced shotcrete or steel fiber-reinforced shotcrete will be better and more economical. What are the advantages and disadvantages of both of these types of reinforcement if used for supporting a tunnel for hydropower? Also, for slope protection work, which type of shotcrete is better in terms of reliability, durability, and cost?
107. We are repairing a culvert in Dallas, TX. The concrete wall of the structure is pre¬maturely disintegrating. We are considering a process to temporarily support the ceiling, remove the wall, place a form on one side, and use shotcrete to replace the wall. Does this sound like a reasonable use for shotcrete? What kind of specifications should be used?
108. We’re looking at adding approximately 4 in. (100 mm) of shotcrete to an existing 8 in. (200 mm) wall to meet new load requirements. What’s the minimum cover between the rein¬forcement and existing wall for proper encapsulation of the reinforcement?
109. I am working on a water feature formed out of cast-in-place reinforced concrete with a hot-fluid-applied waterproofing system over the concrete. To protect the waterproofing, we plan to install shotcrete over it. What minimum thickness of shotcrete is required? Would welded wire fabric or fiber mesh be required as well?
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Question 1: What is the difference between shotcrete and Gunite?
Answer: Shotcrete is an all-inclusive term to describe the spraying of concrete or mortar that may be accomplished through either a dry- or wet-mix process. Gunite refers only to the dry-mix process in which the dry cementitious mixture is blown through a hose to the nozzle, where the water is injected immediately prior to application. Because complete mixing of the water and dry ingredients is not possible in the nozzle, mixing is completed as the material impinges on the receiving surface, through manipulation of the nozzle. This requires a very highly skilled nozzleman, especially in the case of thick or heavily reinforced sections. Large aggregate is seldom used with the dry-mix process. Wet-mix shotcrete involves pumping of a previously prepared mixture, typically ready mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The mixture usually contains minus 1/2 in. aggregate, although larger-size aggregate has also been used.
The use of the term “shotcrete” first occurred in Railroad Age magazine more than 50 years ago in place of the then proprietary word “Gunite,” and has been used by the American Concrete Institute since at least 1967 to describe all sprayed concrete or mortar. (back to top)
Question 2: How should I design joints for shotcrete?
Answer: Shotcrete is concrete forced or impelled through a hose using a pressurized air system. Therefore, the guidelines for jointing concrete are no different than for concrete placed by other methods. (back to top)
Question 3: I want to specify ACI Nozzleman Certification in my next project. A contractor has told me that there are no certified nozzlemen in the project area. How can I verify that information? What should I direct this contractor to do?
Answer: Go to the ACI website, www.concrete.org, and click on the Certification tab. A button will appear for the Certified Personnel Directory. Click this button. Using Search Option 2, customize the search by type of certification and location. Please note, ACI will identify the individual by name, city, and state only. The individual address, telephone, or employer is not available from ACI or the ASA. Education for ACI Certification is available through the ASA office. Contact ASA for the roster of ASA Educators. Certification exams are conducted by ACI-approved examiners in strict compliance with ACI certification policies. (back to top)
Question 4: We have a project that calls for new 6 in. concrete shearwalls formed and placed against the existing structure from the basement up to the fourth floor to enable an additional seven floors to be added to the structure. Our engineer has suggested that the new shearwalls be constructed using shotcrete. We are not familiar with using this system for structural applications. Most of the information we have gotten relates to using shotcrete for swimming pools and cosmetic applications. What advice can you provide?
Answer: The use of shotcrete for structural applications has been documented in numerous articles in Shotcrete, Concrete International, and other publications. The key is to find a shotcrete contractor experienced in structural applications. Investigate the contractor’s project history to determine his/her experience. A contractor experienced in this type of structural enhancement will be most helpful in achieving the desired result in an economical and timely manner. (back to top)
Question 5: We are building a new home. Foundation contractors who place traditional basement walls tell us they would never go into a house built with walls constructed using shotcrete. When used for walls, can shotcrete be of equivalent strength as placed concrete?
Answer: Shotcrete is a method of building a structure using a concrete mixture. A shotcrete mixture likely would exceed the compressive strength of most mixtures used for placed walls because the application of shotcrete requires a much lower water-cementitious material ratio than commonly found in residential wall mixtures. A shotcrete mixture will have a water-cementitious material ratio of approximately 0.50, yielding a compressive strength of about 4000 psi at 28 days. Poured wall mixtures have ratios of approximately 0.70 and compressive strengths of 2500 to 3000 psi. The lower water-cementitious material ratios of shotcrete mixtures produce other benefits such as reduced shrinkage and lower permeability. Additionally, the greater compaction of shotcrete achieved through the velocity of placement improves compressive strength and durability. (back to top)
Question 6: We will be using shotcrete to repair a concrete box culvert that has some minor spalling. Do we need to apply a bonding agent before applying the shotcrete? How should we prepare the surface?
Answer: No bonding agent is required. A key to a successful repair is proper surface preparation. The surface receiving the shotcrete must have the deteriorated material completely removed, be thoroughly cleaned, and in a saturated surface-dry condition (SSD) at the time of shotcrete appli¬cation. Another key item is proper curing and protection following shotcreting. Details can be found in the Task Force 37 Report “Guide Specification for Shotcrete Repair of Highway Bridges.” The document is available from the American Associ¬ation of State Highway and Transportation Officials (AASHTO), Washington, DC. (back to top)
Question 7: I am a structural engineer working on a project in Southern California. We are creating specifications for the use of shotcrete for basement walls. However, I cannot find any information on compressive strength requirements for shotcrete in the building code. We are basing our design on compressive strengths ranging from 3500 to 4500 psi. Are there minimum and maximum allowable compressive strengths for shotcrete?
Answer: To the best of our knowledge, there is no maximum compressive strength limitation. The minimum compressive strength would be dictated by your structural calculations as it would be with any structural concrete design. The most common compressive strength specifically encountered by ASA members in your area is a minimum of 4000 psi at 28 days. (back to top)
Question 8: Our firm has no experience designing for shotcrete applications. We have been investigating the process and would like to know what we should be looking for as the shotcrete is placed. Are there special features or problems in shotcreting?
Answer: Proper placement is the most important element in achieving good shotcrete results. Most defects that occur in shotcrete are due to poor placement. Shotcrete success depends largely on the skill and actions of the nozzleman. The nozzleman’s goal is to achieve adequate compaction and good encasement of the reinforcement (if present) with no entrapped rebound or hardened overspray. For this reason, it is important to require that the nozzleman be ACI certified for the application. There are specific certifications for both wet and dry processes as well as vertical and overhead applications. If the nozzleman is certified, the probability that you will get the desired results is significantly increased. For more information on certification, visit the ASA website, www.shotcrete.org, and click on Certification. (back to top)
Question 9: I know air entrainment is required in concrete exposed to cycles of freezing and thawing while saturated. However, the shotcrete I am going to be applying on a project in Chicago is on a vertical surface where the water will essentially run off the surface. Do I still need to worry about air content?
Answer: You are correct in stating that entrained air is necessary in concrete that is exposed to freezing and thawing while critically saturated. Even vertical walls can get critically saturated in places. Because you are working in a part of the country that experiences significant freezing and thawing, it is imperative that you maintain sufficient air content in the shotcrete. Remember, you are going to lose some air content in the placement process so the air content of the shotcrete mixture going into your pump must be higher than the desired in-place air content. It is a wise idea to do some testing in advance of the actual shotcreting to determine how much air content you will lose. (back to top)
Question 10: We are going to be using shotcrete for repairs in a parking structure. We have no experience performing this work and will be subcontracting this portion of the job. What should we be watching for when the shotcrete is being applied?
Answer: Surface preparation is a critical operation. The substrate must be prepared properly. All deteriorated concrete must be removed. This is generally accomplished with light-duty chipping hammers, scarifiers, or scabblers. The remaining concrete is then sandblasted or waterblasted to remove the concrete “bruised” by the initial removal operation. The objective is to create a clean, sound surface with the proper surface roughness to receive the shotcrete.
After the surface preparation, the substrate must be saturated with clean water and then allowed to dry to a saturated, surface-dry condition immediately prior to shotcreting. Shotcrete should not be applied to a bone-dry surface as the substrate will absorb water in the shotcrete mixture intended for hydration of the cement. Also, a bone-dry surface will tend to allow plastic and drying shrinkage cracks to form. Conversely, a surface that is wet at the time of shotcreting will result in a high water-cement ratio (w/c) at the interface between the substrate and the shotcrete. High w/c at the interface will result in significantly lower bond strengths.
As with all concrete, proper curing and protection is critical. Failure to cure properly will result in lower shotcrete strengths and may cause some delaminations if drying shrinkage causes stresses that exceed early bond strength. Plastic shrinkage cracking and “crazing” may also result from failure to cure and protect properly. Moist curing is the preferred method of curing. If moist curing is not feasible, membrane curing compounds may be used.
Finally, be sure the nozzleman who will be applying shotcrete on your project is certified by the American Concrete Institute (ACI). Certified nozzlemen have been trained and tested on the requirements for proper shotcrete application. Insisting on this certification dramatically increases the probability that you will get the desired results. (back to top)
Question 11: I am currently involved in the design of a large retaining wall for a job in Boston. One option under consid¬eration is the use of soil nails with shotcrete lagging. The design anticipates a 100-year service life. What can I tell my client to realistically expect from the shotcrete option? Is shotcrete durable in the freezing-and-thawing conditions in this area? What is the best way to improve the longevity of the product?
Answer: The simplest way to clarify things is to advise your client that shotcrete is not a product but a process. Shotcreting is a process of installing concrete at a high velocity. Because the concrete is installed at a high velocity, it will have a higher density than conventional concrete in most cases. The increased density will provide reduced permeability and higher durability.
A shotcrete mixture can be designed and proportioned to meet virtually any job requirement. In this case, air entrainment must be specified. Whenever any concrete mixture (shotcrete mixtures included) will be exposed to freezing and thawing while critically saturated, air entrainment must be part of the mixture. The amount of air entrainment required depends on the maximum size of the coarse aggregate used. In general, for a mixture with a maximum-sized coarse aggregate of 3/8 in. (10 mm), the air content should be about 8% as-batched for a severe exposure condition.
Another key to longevity is reduction of permeability. As a mixture becomes denser, the transmission of fluids through the mixture becomes more difficult. This is especially critical when trying to protect reinforcing steel. When chloride ions and oxygen reach reinforcing steel, corrosion is initiated. Increasing the density by using products like silica fume, slag cement, and fly ash dramatically decreases permeability.
Discuss the curing and protection plan with the contractor prior to the start of shotcreting. Failure to cure and protect properly is the most common reason for poor concrete or shotcrete performance.
Another often overlooked element in obtaining an extended type of service life is maintenance of the concrete structure. By periodically cleaning the concrete and applying an appropriate surface sealer, materials that may lead to deterioration are removed from the surface and not allowed to penetrate the pore structure of the concrete. (back to top)
Question 12: I am a civil engineer working on the rehabilitation of a low fixed crest concrete dam of 6 foot height. After stitching of cracks and patch repairs, we want to specify a 2.5" shotcrete facing on the down-stream side to protect from high velocity-induced erosion. The up-stream side will be sealed with a betonite-clay liner to save costs. To get a very dense concrete, we are thinking of 8000 psi airentrained, fiber-reinforced mixture. Should we use a WWF reinforcement? Should this be a wet or dry application?
Answer: Whether to use the wet or dry process depends primarily on your production schedule. With wet you will get much higher production; it will be easier to entrain air; and rebound and dust will be less. It is suggested that you use a wet-mix, steel fiber reinforced, air entrained, silica fume shotcrete, mechanically connected with L-bar anchors and small diameter bars (not mesh) spanning between the anchors. For precedence with this type of retrofit of the face of a dam, see the publication on "Seismic Retrofit of Littlerock Dam, by Forrest, Morgan in ACI, Concrete International, November, 1995, pp. 30-36, or an abbreviated version of the paper in the ASA Shotcrete Magazine, May,1999, pp. 46-55. If you must specify the shotcrete you can use ASTM C 1436, “Specification for Materials for Shotcrete”, which will cover all the materials mentioned, including fibers. For a general shotcrete specification you should review ACI 506.2. You should not use welded wire fabric and fibers together. Fibers will hang up on the mesh causing voids behind the mesh. I recommend a steel fiber meeting ASTM C 1436, Type I, Deformed at approximately 85 lbs/c.y. (50 kgs/c.m.). The steel fibers will tend to lie in the plain of the shotcrete surface; however, you should be aware that some fibers may protrude from the surface, and over time will corrode. Thirty years of experience shows corrosion is only to carbonation depth (2-3 mm), and corrosion of one fiber does not effect other fibers nor disrupt the shotcrete. Staining of the shotcrete surface is a possibility. Some spray a thin ( ½ in.) layer of non-fibrous shotcrete as a final finish to cover fibers. (back to top)
Question 13: I am an architecture student and would like any information you could provide in regard to the proper and typical mix ratios of cement to sand.
Answer: The best reference for shotcrete questions in general is ACI 506 - Specification for Shotcrete. It is available from the American Concrete Institute.
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Question 14: I have come across the term "spacing factor" and have been unable to find a definition. What is a spacing factor?
Answer: The term "spacing factor" refers to the distance between air bubbles in hardened concrete. All concrete has some air bubbles, usually in the range of 1 or 2%, referred to as "entrapped air". These bubbles provide no freeze/thaw protection. Where freeze/thaw protection is desired, air bubbles are intentionally introduced, or entrained, into the plastic concrete mixture. These microscopic bubbles protect the mortar portion of the concrete by providing space for water in the concrete to expand during the freezing process. If these bubbles were not available for this purpose, the expansion of the water would damage the mortar. An important characteristic of a good air-void system is the spacing factor. Bubbles need to be in close proximity so the water migrating through the concrete does not have to travel far to find a bubble in which the water can expand. Ideally the spacing factor will be less than 0.008 in. This analysis is performed on hardened concrete by a trained petrographer using test method ASTM C 457. There usually is some slight variance between petrographers evaluating the same concrete sample. (back to top)
Question 15: I am looking for any information regarding the use of construction joints for permanent shotcrete wall facing. I have found information on placing shotcrete over existing construction joints but none regarding the use of construction joints for the shotcrete wall facing itself.
Answer: In many experiences, the spacing and design of the joints are the same as you would expect for a cast in place wall. Walls have been constructed with no joint, with contraction and expansion joints, with a joint that is caulked, with joints containing waterstop, and just about anything else you might see in a cast in place wall. In short, it is suggested to look to the direction given for cast in place concrete. The construction joint should be designed similar to the needs of any cast in place wall. (back to top)
Question 16: I have a project wherein some 25,000 sq ft of existing shotcrete is to undergo varying degrees replacement, repair and restoration.
* It is on slopes varying from 1:1 to 1:10 or so.
* It is approximately 40 years old in most cases.
* It is in a fairly arid climate (Southern New Mexico) with little rainfall and typically low humidity.
* The subgrade is non-plastic gravelly sandy material.
* It was reinforced with wire mesh (looks like 6x6x10x10).
I'm interested in any techniques and/or materials that might be applicable.
Answer: I recommend reading the following publications in Shotcrete Magazine:
"Shotcrete for Ground Support: Current Practices in Western Canada", by C.Chan, R Heere, & D. R. Morgan, Part I printed in Winter 2002, and Part II printed in Spring 2002. "Soil and Rock Slope Stabilization Using Steel Fiber Reinforced Shotcrete in North America", by M.Ballou & M Niermann, Summer 2002. (back to top)
Question 17: Can shotcrete be painted like other concrete? Can an elastomeric paint, 100% acrylic latex house paint or solvent acrylic be used? I have a customer who wants to paint a tank which uses shotcrete. With normal concrete the surface must be 30 days or older, pH is approximately 7-8 and moisture content is low, remove efflorescence or laitance, etc., then it is ready to paint or coat. Do the same restrictions for shotcrete?
Answer: Shotcrete is pneumatically applied concrete. All surface prep work for concrete will be the same for shotcrete applications. Before a recommendation can be made, is this tank going to be painted on the outside or the inside? Second if this tank is to be painted on the inside, what will be put in it? The environment in which this tank is located also plays a key part in determining what type of paint or coating application. If this a tank that has been in operation, what was stored in it? Testing of the concrete in this case is important, in order to determine what method of surface prep would be needed to achieve a good coating bond. (back to top)
Question 18: I am a general contractor who hired a company to shotcrete a new swimming pool. They began on Friday, a very hot day, and they were placing concrete very slowly (27 yards in 4 hours). Their pump broke down and they were unable to complete the job that day so they returned on Monday. My question is about the "cold joint" between the work on Friday and the work on Monday. What is your opinion of this situation?
Answer: On large swimming pools, it is not unusual to have joints that are left over a weekend or longer. The key is the means by which the joint is dealt with. As with any concrete joint, the surface needs to be clean and free of laitance or other contamination. This can be accomplished by cleaning the joint while it is green on the first day or by cleaning with waterblasting, sandblasting, or wire brushing after the surface has gotten hard. As long as the joint is clean, all gloss has been removed, and the joint is dampened the structure should not be impacted by the joint. Also, 27 cubic yards in 4 hours is not necessarily slow production. Depending upon the circumstances, I would think that 27 cy in 4 hours was quite productive. (back to top)
Question 19: My company manufactures a polyester geogrid that is coated with PVC. We sell these grids into underground mines, as well as many aboveground civil engineering products. We have a new grid that may work very well as an auxiliary reinforcement for shotcrete-type products. Can you tell me what the pH is for these products? The type that we would be exposed to is used in underground mines to reinforce the mine roofs.
Answer: The most commonly used estimates for pH of concrete are 13 for plastic (fresh) concrete and about 10 for hardened concrete with a little age to it.
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Question 20: Is there any reference that differentiates between temporary shotcrete work and permanent shotcrete work, as far as inspection/testing requirements?
Answer: Temporary lagging of shotcrete must meet some standard as it is the shoring holding back the earth. If reinforcing is used in the design of the temporary shoring it must be fully encapsulated to provide the design strength of the lagging as specified in the design. A temporary structure may have a low safety factor but the strength of the rebar and shotcrete must meet the design specifications. Many times it is more important to do good shotcrete for the temporary shoring just because it has a lower factor of safety and therefore less allowance for poor construction practices.
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Question 21: I am looking for design information for shotcreting a steel sheet pile wall to create a composite structure for a lift station wet well. I can design the sheet piling, which would be driven into the ground in a plan circle of 12 feet diameter, followed by excavation. I need to know the practicality of then applying a layer of shotcrete, primarily as a means of sealing the joints of the sheet pile, protecting the sheet pile from the wastewater, and providing additional wall strength. The lift station will be above the water table during construction, but would be periodically below the water table under groundwater conditions.
Answer: There are four common types of sheet pile sealing: 1.) all seams were welded to keep the ground water from seeping in, 2.) the sheet pile surface was sandblasted for bonding, 3.) wire mesh was tack welded to the sheet pile and 4.) rebar was tack welded to the sheet piles. This was done prior to the shotcrete layer. In each case the shotcrete is used as a coating to keep the water from touching the piles and in the third and fourth examples, it is used as a structural coating as well. (back to top)
Question 22: What is the minimum thickness that shotcrete can be applied? We are currently using shotcrete on a restoration project and have a concern at the corner locations are returning to tight recessed steel framed windows. There is an exterior wood molding approximately 1 inch from the tight corner that needs to be preserved. Do you have any suggestions as to how we can address this? Do we need to provide caulking between the wood molding and the shotcrete?
Answer: Thicknesses depend on the structure and surface (surface prep is the key to proper bonding of shotcrete) the shotcrete is being applied to. Depending on the application 1/4 flash coat to 1 inch thickness can be the minimum. As far as shotcrete up to the steel windows, you have to consider that cracking may occur off of each corner. This can be minimized by adding additional reinforcement at those locations. It is common to tool in a joint around the windows so that we could apply a caulk later. The caulking will assure a waterproof seal between the window and the concrete during temperature changes that may create some expansion and contraction. You do not have to depend on the trim work to create the weather and water tight seal the architect requires. (back to top)
Question 23: When used on walls, can shotcrete be of equivalent strength as poured concrete?
Answer: Basically, shotcrete is a method of placing concrete that does not require forms. As a matter of fact, shotcrete requires the concrete mix to be proper every time. With formed concrete walls, the ready mixed concrete going in can be substandard and still appear to be okay. Shotcrete also provides a more dense concrete less susceptible to water penetration. The most glaring difference will be the quality of the materials used. Most poured walls are designed for a compressive strength of 2500 to 3000 psi. Typically they are placed with a water/cementitious material ratio of 0.60 and higher. Curing is almost unknown in the poured wall sector. Protection only occurs in the coldest weather. By the very nature of the process, shotcrete will have a much lower w/cm ratio. This will produce a wall with higher compressive strength and have the attributes of lower w/cm ratio concrete, i.e. reduced permeability, less shrinkage, increased durability. With proper curing and protection, the shotcrete mixture will produce significantly better long-term performance. The shotcrete process should allow for easier addition of insulation to the walls as well. This is especially important if the basement is to be used for more than just storage. (back to top)
Question 24: Can you provide any information on insulating gunite in spa installations?
Answer: There are two ways to insulate the outside of concrete spas. The first way is to shotcrete the spa and then glue Styrofoam to the outside of the concrete shell or to spray the insulated foam to the outside surface. The second way is to use the ICF (insulated concrete form). You would only have to use one side of this form system. This system would act as the outside form so that the shotcrete could bond to the foam. This type of system has foam insulation thicknesses from 1 to 4 inches thick. Yes, it can be fitted to form circles. Each ICF system is different, so some research would be needed to see which system would work the best. Since most spas are formed up before they are shot, the ICF system would serve two purposes: forming and insulation in one step. (back to top)
Question 25: Is it possible to put a texture on the application side of a shotcrete wall? I understand that I can shoot against a form, but what about the side that gets screeded?
Answer: There are many textures that can be applied to the finish surface of the shotcrete. The least expensive is the natural nozzle finish which is rough and tends to absorb light as opposed to reflecting light and standing out. On the other extreme is carved and stained simulated rock as found in zoos and amusement parks. Stamping or rolling also creates a great finish. The broom finish is also very common. Color and textures are options and the owner or designer needs to decide on the value and effect he/she is looking for. Whatever finish, texture, pattern, color, stain, lump, bump, or crease that can be applied to concrete also applies here. (back to top)
Question 26: I would like to get expert opinions regarding a proposal. I am reviewing from a contractor to replace precast concrete wall panels with shotcrete wall. The wall acts as a retaining wall and the precast panels were specified to span between the soldier piles (with tiebacks), driven and anchored into the rock at a spacing of 10 feet. Shotcrete walls over 3-inch wood lagging have been proposed to replace the precast panels and they have been designed exactly the same way as reinforced concrete walls. Using ACI Code working strength design for 4000 psi concrete, and fs= 24000 psi steel, the reinforcing in the shotcrete walls have been determined using value of a = 1.76 . ( As= M / 1.76. d ) I do not feel comfortable accepting the same equations and numbers for a shotcrete wall as for a cast-in-place or precast concrete wall with all the quality controls and rigid specifications per ACI 318 Code concerning mixing, formwork, placement, vibration and curing. Could you please provide an expert opinion on the matter? What would be the reasonable values of coefficient to determine the reinforcing in shotcrete walls?
Answer: We often use shotcrete in lieu of cast in place concrete without using different design factors. Shotcrete is simply a method of placing concrete. Properly designed and constructed, the same reinforcing steel used for cast-in-place concrete or precast concrete should be able to be used with shotcrete constructed retaining walls. The only differences would be in the reinforcing detailing, in that the rebars should be tied in a configuration that makes them suitable for proper encapsulation with shotcrete. Avoid bundled bars or other conditions not conducive to proper shotcrete encapsulation. See "ACI 506R-90 Guide to Shotcrete" for guidance, except that it is possible to use much larger diameter bars than indicated in that document, as has been described in several articles. (See for example the article by James Warner on "Dealing with Reinforcing" in the Winter 2001 of Shotcrete magazine.) (back to top)
Question 27: I am interested in constructing my home using shotcrete applied over polystyrene panels. There are several systems for this, but I'm most interested in avoiding "thermal bridging" that occurs when metal reinforcement passes from the inside of the home to the outside through the foam insulation. I am also interested in fabricating the panels myself, if possible. There was a system utilizing metal reinforcement grids on each side of the polystyrene panel connected by plastic components. Can you point me toward a company that offers this system in the US?
Answer: ICS, 3-D panels are structurally reinforced styrofoam panels that, in conjunction with properly applied shotcrete, become a superior building system. This is a proven panel with a global track record and much experience among ASA members. They are located in Brunswick, GA. (back to top)
Question 28: Do you have any publications on shotcrete curing, specifically in tunneling? How is shotcrete cured in tunnel constructions with the temperature and moisture problems?
Answer: All concrete must be cured to ensure full and proper hydration of cementitious components control of shrinkage. Shotcrete is concrete placed pneumatically, therefore must be cured, as all concrete must be. The tunnel environment presents positive and negative conditions. The humidity in an underground space is generally high in humidity and constant in a moderate to cool temperature. Both conducive to slow egress of moisture from the concrete and "natural" curing. The negative in tunnel construction is ventilation air which is generally of high volume and high speed, which tends to dry the surface and "pull" important moisture out of the sprayed concrete. Most tunnels can tolerate extra water in the work space, therefore misting or spraying water onto the concrete surfaces, especially overhead, is the most practical method of curing. Sprayed on liquid membranes are effective as long as their interference with bonding of additional layers of concrete, sprayed or cast, is not an issue. Recommended reading: "Understanding and Controlling Shrinkage and Cracking in Shotcrete" by D.R. Morgan and C.Chan, published in the ASA Shotcrete magazine. (back to top)
Question 29: I am trying to find an article on the bond strength between two layers of shotcrete. My company is placing a 22" thick shotcrete retaining wall and, at a later date, we are placing a small amount of shotcrete over the existing shotcrete wall. The Engineer thinks the shotcrete will just falls off over time. Is this true? Can you point me in a direction that might have information on the bond strength between two layers of shotcrete?
Answer: There is a paper by Denis Beaupre about this issue in the May 1999 issue of Shotcrete magazine. The simple answer to bonding layers of shotcrete is the same as bonding layers of concrete in typical repair applications. Bonding agents are not recommended. The bond strength between shotcrete layers is generally superior to cast interface because of the impact of velocity and the matrices that form at the bond plane and provide a denser, therefore stronger interface. The key in any bonding situation is primarily dependent on the surface preparation before application of the next layer. The surface must be clean and free of latence and any other unsound materials and should be roughened or textured (gun finish is sufficient) to provide sufficient keying or mechanical locking as required. The surface should be SSD and overspray from progressive application should be controlled. ACI International and the International Concrete Repair Institute can provide direction for surface prep. AASHTO/AGC/FHWA Task Group 37 Report, "Guide for Shotcrete Repair of Bridges and Structures" contains spec and procedure information that should be useful. (back to top)
Question 30: Can you provide input on the applicability of the shotcrete placement method for the structural repair of existing concrete walls? These walls (two) are conventionally reinforced, 31 feet in height and are parallel with a clear spacing of 5'-0". There length is 150 feet. Structural repair is required at many locations that have experienced spalled concrete with corroded reinforcing bars. Depth of repairs will range from 2" to approximately 6". Concrete substrate will have exposed aggregate with a significant amplitude. From a production and cost viewpoint, shotcrete appears to be more applicable than a form and pour or form and pump repair method.
Answer: From the limited info given, it sounds like an ideal shotcrete application. But, with many caveats, such as: TOTAL deteriorated substrate removal, thorough removal of all aggregate/substrate that may have been fractured during removal of deteriorated concrete (heavy sandblasting and/or high pressure washing), using a replacement concrete mix with similar properties as the original, thorough cleaning or removal and replacement of corroded rebar, etc. It is suggested to discuss this with a shotcreter in the area that has experience with a similar application. It is also recommended to review related ACI and ICRI publications. (back to top)
Question 31: We have a project that our subcontractor would like to change from concrete liner for a box culvert to a shotcrete liner it is a C.O.E. project. The C.O.E. has questions of durability. Could you help?
Answer: If the shotcrete is applied correctly, the durability factor is better than cast in place concrete. The 506 and the ASTM documents have references on this subject. There have been papers written on durability and permeability. Countless culverts have been very successfully relined with shotcrete, not only concrete culverts but also brick lined and galvanized metal culverts. If you broaden the definition of culvert to include tunnels you would most likely be identifying where the largest volume of shotcrete is used as a rehabilitation method. To answer questions of durability, shotcrete should be thought of a process or method of placing concrete. Shotcrete in place is concrete. The higher cement content of shotcrete and the impaction of its placement mix design for mix design of other placement methods create a higher strength and more dense, thus less permeable concrete. (back to top)
Question 32: We are building a home where some of the outside walls are bricked. Is there a way to use shotcrete over strand board (chipboard)? If so, how and what cost would there be approximately a square foot for the actual shotcrete installed?
Answer: More information is needed before answering this correctly. Shotcrete will stick to strand board, but you need some type of reinforcement (wire mesh) to hold it all together. A good cement plaster mix at a lower velocity would be more economical. Contractors who have shotcreted a house in the past will tell you that it is too time consuming for the money involved. (back to top)
Question 33: We are having a pool built with shotcrete. The pool company has asked us to change the contract to allow them to use the wet method instead of the dry method of shotcrete. I have read through your website and found it helpful in understanding the difference between the two, but I would like to know if one is better or more sound than the other.
Answer: Pools are built with both processes. Some find it easier to shoot pools with the wet method. But, when properly done, there should be no difference in performance between wet and dry process shotcrete. Depending on the complexity of the pool, the wet method placement can be faster than the dry method. It comes down to the experience of the contractor and their crews, for a good quality placed pool shell. The nozzleman plays a key role in the placement of well placed shotcrete in both methods. The geographical area may determine the economics of which method is used. Curing of the in-place concrete shell is the same for both processes (water curing for 7 days). Wet concrete has a 90 minute window from the time it is batched at the plant until it placed. Temperature of the material and the air temperature can increase or decrease the set times of the concrete. Typically Ready-Mix companies hold back 10-15 gallons of water in the mix so that the contractor can adjust the slump of the concrete on site. Adding 1 gallon of water over the design mix (amount of gallons of water per yard of concrete) can decrease the strength of the concrete by 200psi.
If you have additional concerns, the following questions should be asked:
*Does the contractor have a good track record of shooting pools with the wet method?
*How many pools have they completed with the wet method?
*Can you provide a list of past completed jobs?
*How do they plan to incorporate the trimmed concrete into the shell? (The rebound and the trimmed concrete play a key role in the final quality of the pool shell.)
*What concrete mix design do they plan to use? (back to top)
Question 34: We have a design/build drainage channel project that requires a concrete lining over secant piles in which the secant piles form the main structural walls of a box culvert. The box culvert discharges into the ocean. We proposed a shotcrete concrete liner but there are concerns about the life service durability of shotcrete in a saline environment. Do you have any reference information on this matter that we could use to support our position?
Answer: Please refer to the following articles:
Morgan, D.R. "Freeze-Thaw Durability of Shotcrete" Concrete International, Vol. 11, No.8, August 1989, pp 86-93.
Shotcrete magazine Vol. 4, No. 5, Fall 2002, pp. 32-38
Shotcrete magazine Vol. 5, No. 2, Spring 2003, pp. 30-37, “Freeze-Thaw Durability of Shotcrete,”
Gilbride,P., Morgan, D.R. and Bremner,T.W. "Deterioration and Rehabilitation of Berth Faces in Tidal Zones at the Port of Saint John", ACI, Concrete in Marine Environment, SP-109, 1988, pp.199-227.
Gilbride, P. Morgan, D.R. and Bremner T.W. "Performance of Shotcrete Repairs to the Berth Faces at the Port of Saint John", Third CANMET/ACI International Symposium on Performance of Concrete in Marine Environment,1996, pp 163-174.
Morgan,D.R., Rich L. and Lobo, A, "About Face-Repair at Port of Montreal", Concrete International, Vol. 20, No.9, September,1998, pp. 66-73.
The bottom line is that with a properly designed, air-entrained shotcrete, properly applied by qualified nozzlemen, you should be able to get a good quality product, with long-term freeze thaw durability every bit as good as a quality, air-entrained cast-in-place concrete. (back to top)
Question 35: Our company is developing alkali-free accelerator, both powder and liquid types. Since our information and knowledge is limited, please answer the following questions:
1. What is the formal definition of alkali-free in DIN, ASTM, or other specifications?
2. What is the lowest pH value of alkali-free accelerator? In which pH value that the product won't harm to the human tissue or vascular system? Please also advise where we can find the related information.
3. Is it acceptable to use Aluminum Sulfate as the main component of alkali-free accelerator?
Answer: Question 1. - Na2O (sodium oxide) equivalent, below 1.0%
Question 2. – 3 is the lowest; anywhere between 3 and 10, most European specifications state a range between 3 and 8 for better performance.
Question 3. - Yes
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Question 36: I wish to request expert advice from ASA in regard to the Gunite Contractor's Association method that we are using to make test cylinders (i.e. 6" diameter and 12" high shot into a form of 3/4" square mesh hardware cloth). Since we are currently in the process of guniting a silo and have today received 3,250 psi rather than the mix designed 4,000 psi 7-day strengths, we would appreciate your prompt response.
Answer: The method of using 6" diameter by 12" long wire mesh cylinders has not been used regularly in several years. The most accepted means of taking samples is as specified in ACI 506 documents which generally require a sample panel of approximately 18"X18" by 4" thick from which cores are taken. The cores should be taken at a minimum distance from the edge of the thickness of the panel to yield fair test results. ACI 506.4R-94 references under testing of shotcrete, ASTM C 1140-03 (Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels. Also ASTM C42/C 42M-03 (Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. Standard 18"X18"X4" panels are typically made. ASTM C 1140-03 states a 24"X24"X4", cores are to be taken 1 core diameter plus one inch from any side of the test panel. (back to top)
Question 37: I have a special request for a shotcrete mix design. My company has been using shotcrete for about three years, here in Alaska. I have recently had a request to shotcrete a 60'x50' duck pond to make it waterproof. The problems I am running into are that moose keep walking into the pond, and the pond is on the side of a hill with built up edges around the outside. The mix design I am looking for needs to have an epoxy or some kind of adhesive to help stop the water from running out the cracks. Last, are there any fabric or plastic materials that I could lay down and spray the wet shotcrete on to put on the sides of the pond?
Answer: This inquiry involves a lot more than just mix design. First, additives to the mix by themselves will not keep the shotcrete from cracking. To minimize leakage for the proposed application, he will have to use either a waterproofing membrane on top of the shotcrete, or plaster like would be used on a swimming pool. Putting a membrane behind the shotcrete would only serve to keep ground water from entering the pond through the back side. The other aspect to be addressed is the fact that all concrete shrinks, and that is what causes the cracks. So anything that can be done to minimize shrinkage should help. To name just a few items: avoid shooting on a windy and or low humidity day; use aggregates in the mix that have a good record regarding shrinkage; avoid excessive cement content in the mix; use reinforcing steel (mesh or rebar); synthetic fibers help reduce early plastic shrinkage; proper curing is absolutely essential! (back to top)
Question 38: We are shotcreting our first wall and the contractor tells us that in shotcrete, the lapping of the bars is not done by putting the bars alongside each other as in conventional pouring of concrete but rather a gap is left between the bars in order to avoid voids behind bars bundles. A two-inch gap is being used on our job. Is there a publication that deals with reinforcing steel placement in shotcrete in general and one that deals with bar laps in particular?
Answer: The ACI 506R-90 Guide to Shotcrete, Section 5.4.2 is the publication you are looking for. Amongst other things it states: "If the design allows, lapping of the reinforcing splices should be avoided. Lapped bars should be spaced apart at least three times the diameter of the largest bar at the splice". If laps are not permitted by the design, then it is best to lap the bars one on top of the other (relative to the shooting orientation), rather than side-by-side, to facilitates proper encapsulation with shotcrete. (back to top)
Question 39: I'm looking for information as to the thickness design of shotcrete for ditch slope lining purposes. Can you direct me?
Answer: Typically, the thickness is a minimum of 3 inches and slope lining in the 6 to 8 inch range is often installed. The reinforcing is also variable with the lightest sections with no reinforcing or a low dosage of polyfibers or light welded wire fabric and the heavier sections with rebar. Basically, a lot of different designs can be used. We are not aware of any widely used standards. (back to top)
Question 40: Our development has 8 recirculating water ponds of various sizes. All are vinyl liner under concrete construction. Some ponds have developed leaks due to cracking of the concrete. Will shotcrete provide an adequate seal to stop the leaks for an appreciable time?
Answer: When trying to find a contractor in your area, please visit the Corporate Member page of this website. When constructing water ponds, the liner is always under the concrete just in case the concrete cracks not on top. Master Builders makes a product called Master Seal 345 which is designed to waterproof the concrete before the shotcrete is placed. Using a macro synthetic fiber for strength, flexural and to control shrinkage cracking will help. It comes down to proper prep work prior to placement and curing of the concrete (7 days of water) to control cracks. Bentonite shotcrete could be a possibility or perhaps plastic shotcrete (cement and bentonite shotcrete). (back to top)
Question 41: We are a construction company and are currently executing a cathodic protection work for the reinforced concrete pile caps of a jetty. After the application of concrete repairs and placement of CP system over the R/C surfaces, we are to cover the concrete surfaces with a waterproofing material. The engineer of the project recommends the shotcrete application with a thin layer in order to provide with the protection of the buried anode strips and as well as waterproofing of the surface. We use strip type CP anodes and we place them into the sound/repaired concrete by saw cutting the surface. Saw cuts are 1/4" width by 1" depth and located top, mid and bottom sections of the 40" depth vertical pile cap surface. Our Questions are:
1. Can we apply a thin layer of shotcrete over the repaired concrete surfaces without having any reinforcement and would it be a good solution as far as the stability of the shotcrete is concerned?
2. Would it be a safe solution to apply the shotcrete over the repaired surfaces just to provide with the protection of the CP anodes placed in saw cuts as described above?
3. Would it be enough to make a waterproof coating instead of applying shotcrete to the whole concrete surface so that the waterproofing of the surface shall be provided?
Answer: When involved with The High Level Bridge in Fairmount, WV we had several aspects of shotcrete repair on this project. This also included the largest installation of cathodic mesh on top and bottom of each arch span which was then covered by shotcrete. Several cathodic design issues affected the shotcrete application. Surface profiling had to be conducted prior to mesh installation for bonding purposes. The anchor spacing had to be drastically reduced because of the small gauge wire and the vibration it caused during shooting. A thin layer of silica fume dry shotcrete (Gunite-MS) from the Quikrete Company was applied over the mesh. A natural gun finished was chosen over a trowel/broom finish because of the delaminations it created during the finishing. Curing of this thin layer was very important in preventing it from delaminating from the surface.
1. Yes, but the surface needs to be profiled for bonding of the shotcrete to the old surface.
2. Yes, but more information is needed. Repair all bad areas, profile the entire surface that will receive the cathodic system, shotcrete the entire surface, waterproof the entire surface.
3. With out seeing the job or design, it would be hard to make any suggestions on this subject. You may need to review what the manufacture for the cathodic system recommended? (back to top)
Question 42: We are currently in the process of doing a seismic upgrade to one of our parking structures using shotcrete. During this process, the murals that are painted on the interior walls are being removed and will be repainted at a later date. How long do I wait before it is cured enough to begin painting?
Answer: The easy answer is that shotcrete material is the same as concrete material and that the same rules or guidelines would apply to shotcrete as to concrete. We usually tell our customers to present this question to the painters. The curing process and chemical reactions are greatest in the first 28 days. Generally a paint or coating is not applied until after the curing of the shotcrete is complete, or mostly so, and the moisture content of the shotcrete is below a point specified by the coating manufacturer. (back to top)
Question 43: I am trying to find out if there is any research or literature regarding the drying shrinkage of shotcrete. Can you help?
Answer: See ACI 506R, Sec. 1.7 (ACI document). Typical shrinkage varies in the range of 0.06 to 0.10 percent after 28 days drying. It is typically slightly higher than similar strength concrete, mostly due to less and/or smaller coarse aggregate in the shotcrete mix. (back to top)
Question 44: I've been a pool builder all my life and I use your magazine as a technical source and I really enjoy it. I found a conflict: In Shotcrete Summer 2004, page 30, the answer to the second question suggests the use of 8% as batched air content with max sized coarse aggregate of 3/8 inch. The conflict I have is that a) won't 8% as batched drop to 1-2% after wet gunning? and b) previous articles suggested the use of 15-22% air as batched to help get it through the hose and to achieve 8% in place. Can you clarify?
Answer: For over 30 years in Canada we have been designing wet mix shotcrete for exterior exposure (rock-slope stabilization, tunnel portals, canals and beams, infrastructure rehabilitation, etc.) to have air content at the point of discharge into the pump to be in the 7 to 10% range. Pumping and the impact on shooting reduces the air content in the in-place shotcrete by about half. i.e. we find the in-place air content in the shotcrete to consistently be in about the 3.5 to 5.0% range. (Only about 1 to 2% air content is lost in pumping; the rest is lost in impacting on the receiving surface).
The air content is measured either by digging out the in-place shotcrete (or dig it out of a shot test panel) and reconsolidating it in the base of the air pressure meter in the ASTM C231 test and conducting the test. Alternatively the shotcrete can be shot directly into the air pressure meter base. It provides virtually the same value as obtained with dug-out shotcrete (as described above), provided the nozzle is held perpendicular to the air pressure meter base, and at the appropriate distance for proper consolidation of the shotcrete.
Testing on numerous projects has demonstrated that shotcrete with 3.5 to 5% in-place air content has a good air voids system ( air content, spacing factor and specific surface), when analyzed in the ASTM C457 test. Such shotcrete has been demonstrated to have good freeze/thaw durability in the ASTM C666 test and deicing salt scaling resistance in the ASTM C672 test. More importantly, feedback from the field demonstrates that such air entrained shotcrete with many thousands of cycles of freezing and thawing in the field over several decades display good durability. There are many research and case-history examples in the published shotcrete literature to support these observations. (See references 1 and 2 below)
With respect to the use of very high air contents at the pump (15-22%), this has been more of a research initiative, used on only a few projects in Quebec, and is not common practice, nor in this writer's opinion, necessary.
There is another benefit which accrues from the use of air entraining admixtures to get 7-10% air content in the shotcrete discharged at the pump. As any concrete user knows, as the air content increases, the slump goes up. For shotcrete mixes (which have high cementitious contents and low rock contents compared to concretes) this makes the mix easier to pump and shoot. Thus it is common to shoot air entrained wet mix shotcrete at 100 to125mm (4 to 5 inch) slump. On impacting on the receiving surface, as the air content is reduced by about half, the slump of the in-place shotcrete is also instantaneously reduced by about half. (This can be demonstrated by digging the shotcrete out of the in-place material, or a test panel and conducting a slump test on it). We refer to this phenomenon as the "slump killing "process and have used it to advantage on many shotcrete projects. With a good air entrained shotcrete mix design (particularly when silica fume is used) we commonly shoot vertical sections as much as 500mm (20in) thick at 100 to 125mm (4 to 5 inch) slump in a single pass with no problems of sagging or sloughing (fall-out), without having to resort to the use of accelerators.
Finally, there are a few situations where 7 to 10% air content in the shotcrete at discharge into the pump may not work. These are situations where excess air content reduction could occur during shotcrete conveyance, such as dropping shotcrete down a pipe from the surface in an underground mine and catching it in a kettle or remixer unit. In this case, air, if needed, is best added underground in the remixer. Also, pumping shotcrete long distances (particularly pumping shotcrete downhill) may result in excessive loss of air content in the line, which could cause a slump reduction in the line and possible pumping problems. Other than for situations such as these, we always use 7-10% air content in the shotcrete at the point of discharge into the pump (even if it is not needed for frost resistance reasons) because of its enhanced pumping and "slump killer effects".
Reference 1: Morgan, D.R., “Freeze-Thaw Durability of Shotcrete”, Concrete International, Vol. 11, No. 8, August, 1989, pp 86-93
Reference 2: Morgan, D.R., Kirkness, A.J., McAskill, N. and Duke, N., “Freeze-Thaw Durability of Wet-Mix and Dry-Mix Shotcretes with Silica Fume and Steel Fibers”, ASTM Cement, Concrete Aggregates, Vol. 10, No. 2, Winter 1988, pp 96-102.
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Question 45: As a specifier, should I specify which process—dry or wet—should be used on my projects? What are the significant differences?
Answer: The application of shotcrete can be done successfully with either method. The dry-mix shotcrete process tends to be more favorable for lower volume placements. It is also a more flexible method, allowing for more frequent relocations of equipment. Equipment is more easily cleaned at the end of the placement. The nozzleman must exercise great care in adding the necessary amount of water while shooting.
The wet-mix shotcrete method is more favorable for larger volume placements. Rebound is substantially less than in the dry-mix shotcrete process. The nozzleman does not have to be concerned with controlling the water addition. This method is less efficient when there is a requirement for frequently starting and stopping placements. The wet shotcrete mixture has a limited “pot-life.”
Remember, shotcrete is not a special product. It is a method of placing concrete. All the recommended practices for concrete placed by any other method, such as curing and protection, also apply to shotcrete. (back to top)
Question 46: My firm is a general contracting entity that frequently uses shotcrete subcontractors. When project specifications are not clear on testing, I have been relying on the advice of my shotcrete subcontractors on the frequency of taking tests for compliance with strength requirements. We always shoot a test panel prior to starting construction. How much testing should we be doing during construction?
Answer: ACI 506.2, “Specification for Shotcrete,” recommends that a test panel be produced for every 50 yd3 (38 m3) of shotcrete placed or one per day, whichever is less. A minimum of three cores are to be cut from the test panel for compressive strength testing in accordance with ASTM C 42, “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.” Testing must be performed in accordance with ASTM C 1140, “Standard Practice for Preparing and Testing Specimens from Shotcrete Panels.” The average of the strength results from the cores must be at least 85% of the specified strength with no individual core less that 75% of the specified strength. (back to top)
Question 47: Is there a U.L. (Underwriters Laboratories) certification for shotcrete?
Answer: No. Shotcrete is a method of placing concrete. Therefore, any applicable certifications would apply to concrete regardless of the method of placement.
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Question 48: I have a client who may be interested in using shotcrete for walls in a radiosurgery unit requiring radiation shielding. Could you please tell me the typical density of shotcrete?
Answer: Shotcrete made with normalweight aggregates will have a density of approximately 145 lb/ft3 (2323 kg/m3).
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Question 49: Are there specific benefits in using silica fume in shotcrete beyond reduced permeability in the hardened shotcrete?
Answer: Shotcrete containing silica fume will tend to be more adhesive (sticking to substrate surfaces) and cohesive (adhesion to itself). This will result in quicker build-up (greater thicknesses per pass) and possibly reduced need for acceler¬ators. Silica fume additions also result in dramatic reductions in rebound, particularly with the dry-mix process. (back to top)
Question 50: The Park District Department of our city is in the process of designing a new swimming pool. One of the prospective bidders made a presentation in which they said they would use shotcrete instead of conventional cast in place concrete. Their design is to use 6 in.-thick walls instead of the 12 in.-thick walls as proposed for the cast in place design. They claim that 6 in. of shotcrete is as strong as 12 in. of formed concrete. Is this a true statement?
Answer: If this statement was true, there would be a lot more shotcrete projects! The truth is that shotcrete is a method of concrete placement, not a special material. The materials, mix designs, and mix proportions may vary between the shotcrete method and the conventional concrete form and pour method, but the thickness and reinforcing of the structure will be very similar.
There is a subtle difference between the two methods that might affect thickness requirements. Shotcrete is generally placed directly onto the undisturbed soil, joining with the soil to provide the shell for the pool. To use the form and pour method, over-excavation would be required to accommodate two-sided forming. The walls would then have to withstand the forces of backfilling. This may result in a thicker wall requirement. The final decision regarding wall thickness, however, should be made by a structural engineer.
Shotcrete is widely used for swimming pool construction. In some areas it is virtually the only method used. Successful shotcrete swimming pool construction is a result of having an appropriate design, selecting a qualified contractor with certified nozzlemen, selecting appropriate materials and shotcrete mixture design, and following industry recommendations for placing, finishing, and curing. (back to top)
Question 51: I am working on repairing some mildly deteriorated walls in a drinking-water treatment plant. There are no chlorides used in the treatment process. I would like to apply a 1 in.-thick shotcrete layer over the existing concrete utilizing a mix containing silica fume, which will achieve a compressive strength of 5000 psi at 28 days. I am having difficulty formulating a mix to meet those requirements that also has a water soluble chloride content of less than 0.10 % chloride ion concentration by mass of cement. I cannot get the chloride ion concentration below 0.15%. What adjustments can I make to get to my goal of 0.10% or less?
Answer: There are areas that have no problem getting values lower than the most stringent ACI requirement of 0.06% for prestressed concrete with no special adjustments. It would be prudent to test each of the proposed shotcrete constituents to determine their soluble chloride ion content. The most likely suspects are the aggregate and water sources. Typically portland cement and silica fume would contribute little, if any, detectable chloride ions. Assuming this would be a dry-process application, the only admixture other than the silica fume might be an air entraining agent, which would not provide any chloride ions. This leaves only the aggregates and water as the sources. At a minimum, the aggregates and water should be tested by a qualified laboratory for soluble chloride ion content. Alternate sources of aggregates and water may be required based on the laboratory results. (back to top)
Question 52: Our firm is working as a consultant for a project. We have very little experience with shotcrete. What is the life span for a shotcrete wall?
Answer: Shotcrete is a method of concrete placement, not a product. Therefore, concrete placed by the shotcrete method will exhibit the same characteristics as concrete placed by other methods. Mixture designs and proportions for shotcrete are modified for high-velocity placement. The high velocity provides some performance improvements over conventional cast-in-place methods when properly placed.
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Question 53: What is the best reference when specifying aggregate gradations for shotcrete projects?
Answer: ASTM C 33 contains a variety of aggregate gradations. Gradations recommended for shotcrete applications can be found in ASTM C 1436, Standard Specification for Materials for Shotcrete, or ACI 506, Guide to Shotcrete. Note that ACI 506 includes the caveat that “aggregates failing to comply with gradations shown in Table 2.1 may be used if preconstruction testing proves that they give satisfactory results or if acceptable service records are available.” (back to top)
Question 54: I want to apply a 3.5 in. (89 mm) veneer of shotcrete over an existing cast-in-place wall. I am concerned about how well the shotcrete will bond to the existing wall. This wall is 50 ft (15.25 m) in height. What are the keys to doing this work successfully?
Answer: This is a common use for shotcrete. The key elements are as follows.
1. Proper surface preparation. To establish suitable surface roughness, use heavy-duty sandblasting, high-pressure water blasting, or mechanical methods such as scabblers or scarifiers, followed by sandblasting or high-pressure water blasting to remove the “bruised” surface material. Refer to ICRI Guideline No. 03732, concrete surface profile Chip 6 (CSP 6), or greater.
2. Provide mechanical connection between the shotcrete and concrete by installing L-bar anchors (epoxy or portland cement grouted) on a systematic pattern, with reinforcing bar (or heavy-duty mesh) spanning vertically and horizontally between the anchor bars. Size and spacing of the bars to be determined by the structural engineer. Position anchors and reinforcing bar to ensure adequate shotcrete cover to them. Nonmechanically connected veneers are not recommended.
3. Wash concrete surface with clean water to remove dust or any other contaminants to achieve a good bond and presaturate concrete. Allow concrete to dry back to a saturated surface dry (SSD) condition immediately prior to shotcrete application. If concrete dries excessively, bring back to SSD condition with fogging. (A 3000 psi [21 MPa] water pressure sprayer works well for this purpose).
4. Apply the shotcrete from the bottom up, taking care not to entrap rebound/hardened overspray. Use proper shotcreting techniques to encase reinforcing bar and anchors. Use 45-degree construction joints (do not construct long tapered joints).
5. Use shooting wires, guide forms, or other suitable methods (for example, rods with alignment bubbles) to establish proper line and grade. When the shotcrete has stiffened sufficiently, trim it to line and grade with cutting rods and then finish using fresnos or floats to provide the desired surface texture (wood floats for more textured finish, rubber/sponge floats or magnesium floats for intermediate texture finish, or steel floats with steel toweling for smooth finish). Note: very smooth finishes are not recommended as they tend to show imperfections from hand-finishing procedures. Avoid over-finishing of shotcrete or procedures/timing which could pull tears or sags/sloughs/delaminations in the fresh shotcrete.
6. Cure the freshly placed shotcrete using one of the methods prescribed in ACI 506R-90. Our preferred method is fogging/misting until the shotcrete has reached initial set, followed by wet curing for 7 days using presaturated plastic-coated geotextile fabric (for example, Transguard 4000), which is kept wet with soaker hoses. Curing compounds are a (second best) alternative, but should not be used if a paint or coating is to be applied, unless they are approved by the coating/paint supplier for such purposes. (back to top)
Question 55: We are having a swimming pool built with shotcrete. Our question is, what is the required curing time for shotcrete prior to exposure to heavy rain? We are trying to plan the shotcrete installation when the weather looks most favorable.
Answer: Shotcrete needs to be protected from rain until it obtains its final set, usually 4 or 5 hours. Following final set, it should be wet cured for at least 4 days, preferably 7 days if possible. The exposure to rain would prove beneficial as the rain would assure the presence of moisture for continued curing. (back to top)
Question 56: I will be shotcreting an existing structure that has some diesel fuel and oil stains on the existing concrete. How should I treat them before shotcreting?
Answer: There are a number of ways to treat these stains. Successful treatment will depend on the specific material in the stain and the depth of the stain. The first step would be to try to draw out the material from the surface by applying a poultice of finely ground kitty litter, cement powder, or talc and allow the surface to dry. Repeat this application if necessary.
Next, try a scrubbing a nominally dry detergent powder into the surface. Allow the powder to dry and rinse off the surface. Follow this treatment with a liquid detergent scrubbed with a bristle brush into the surface. Allow the liquid to remain in the surface for 1 to 2 days, then rinse thoroughly. Should the staining persist, you may want to try a proprietary stain remover specifically intended for use on concrete.
Muratic acid is also an option. However, muratic acid can have deleterious affects on the concrete if not thoroughly removed. Because of its potential to attack concrete aggregates and mortar, along with the hazards inherent with applying and removing acid, muratic acid should only be used with the guidance of an experienced consultant.
Following a thorough power washing, the surface should be mechanically roughened to ensure proper bond with the shotcrete. (back to top)
Question 57: Our firm is preparing to use the shotcrete method on a project for the first time. What type of prequalification work should we be specifying?
Answer: There are four basic reasons to require preconstruction qualification testing:
1. To prove the suitability of the fresh shotcrete mixture design for the intended use;
2. To verify the proposed mixture will produce the required strength and any other specified hardened shotcrete properties;
3. To prove the ability of the nozzleman (and blowpipe oper¬ator, if required) to place dense, homogeneous shotcrete completely encasing the reinforcing steel under field conditions; and
4. To prove the desired surface finish can be achieved.
This testing must be discussed in detail with the shotcrete contractor in advance with a clear understanding of the expected outcomes and the process for any required adjustments. Requiring ACI Nozzleman certification is an important requirement in screening for qualified nozzle operators. However, it is not a guarantee that the nozzleman has applied shotcrete under the same conditions to be encountered on your project. Therefore, a preconstruction plan is an important part of critical projects.
Other prequalification testing may be necessary depending on the nature of the work.
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Question 58: We are hearing a lot of discussion about performance versus prescription specifications? What do we need to know about this discussion?
Answer: The short version of this discussion is that performance specifications provide a list of desired results. The contractor takes this list and selects materials and methods to produce the desired results. The contractor assumes responsibility for results. Prescriptive specifications are very specific as to what materials, proportions, and methods of installation are to be used. The specifier assumes responsibility for the results. The contractor must be able to demonstrate compliance with the specification.
Which method is better? The answer to this question is highly dependent on the nature of the project. However, in general, performance specifications produce a higher probability of achieving the desired results as the contractor is better able to use his expertise as it applies to project conditions. (back to top)
Question 59: I am doing a wet-process shotcrete project. The shotcrete mixture is being delivered by a ready mixed concrete company. Recently we had some delays on the site. The inspector told us that any concrete not unloaded within 90 minutes of arrival on the site would be rejected. Where does that rule come from?
Answer: ASTM C 94, “Standard Specification for Ready Mixed Concrete” states that concrete must be unloaded within 90 minutes of contact between water, cement, and aggregates, or before the mixer drum has revolved 300 revolutions—whichever comes first. This limit, however, may be waived by the purchaser if the concrete has sufficient workability that it can be placed without the addition of water. In hot weather, the 90-minute limit may be reduced by the purchaser. (back to top)
Question 60: I am bidding a tunnel project and am uncertain about part of the specifications. Are specifications for shotcrete temperature different for the wet and dry processes? Are there separate requirements for the shotcrete, ambient, and surface temperatures? Can you refer me to industry standards?
Answer: The requirements for material temperatures are the same for both wet and dry shotcreting. Refer to Sections 8.7 and 8.8 of ACI 506R-90, “Guide to Shotcrete,” for recommended shotcrete temperatures during placement. Additional information is available in ACI 506.2-95, “Specification for Shotcrete,” in the sections on hot and cold weather shotcreting. Generally, concrete mixtures should be maintained at temperatures above 50 °F (10 °C) and below 100 °F (38 °C). Ambient temperatures should be maintained in a similar range.
Regarding surface temperatures, concrete should never be placed on a frozen substrate. Practical experience in Canadian mines has lead to a suggested minimum temperature of 40 °F (4 °C) for the rock receiving the shotcrete. Without special measures, cold temperatures will cause the shotcrete to set more slowly and result in slower strength development. Remember that in thin sections, the shotcrete will lose its heat more quickly in cold conditions. (back to top)
Question 61: What wire size and opening are recommended for repair of bridge substructures? We realize the mesh would not be for restoring or improving structural capacity, merely to help control cracking.
Answer: The inclusion of wire mesh must be considered on a case-by-case basis, depending on the thickness and orientation of the shotcrete. Thin sections may well not have any wire mesh. In aggressive environments, at least 2 in. (50 mm) of shotcrete must cover the mesh. The mesh size should be at least 2 x 2 in. (50 x 50 mm) and preferably 4 x 4 in. (100 x 100 mm) to allow for proper encapsulation. Overhead shotcrete usually includes wire mesh for thicknesses greater than 2 in. (50 mm) in case the shotcrete debonds from the substrate. The mesh must be mechanically anchored.
Some designers are eliminating wire mesh and relying on synthetic fiber reinforcement for shrinkage crack control. The use of synthetic fiber eliminates the concern over cover and corrosion in aggressive environments. Specific recommendations on the amount and type of fiber should come from the manufacturer. (back to top)
Question 62: Our general contracting firm is working on a project with a very tight schedule and significant penalties for missing the completion date. It has been suggested that we consider using shotcrete for the below-grade foundation walls. We have been told that we can save significant time by using shotcrete instead of cast-in-place construction. These walls are heavily reinforced. Has this been done successfully elsewhere?
Answer: Yes. Heavily-reinforced shotcrete has been used in California for over 50 years in response to the need to retrofit structures to resist earthquake damage. The shotcrete contractor must demonstrate his ability to shoot test panels with the same reinforcement as designed into the project. By using an experienced and qualified shotcrete contractor, it is possible to achieve cost savings of almost 30% and time savings approaching 50%. (back to top)
Question 63: Is a bonding agent recommended when placing shotcrete on an existing substrate?
Answer: A bonding agent is not required or recommended. A properly prepared substrate in a saturated surface-dry condition (SSD) is the optimum condition for application of shotcrete. Bonding agents may act as a bond breaker in some circumstances. (back to top)
Question 64: My firm just completed a 2 in. (51 mm) overlay of shotcrete in an existing storage tank. Almost immediately after the shotcrete was applied, we noticed spider web cracking on almost the entire surface. The weather was very hot during shotcreting, and we suspect this caused the cracking. The project engineer is concerned about permeability and is thinking of having the shotcrete removed. Is removal really required or can we live with this cracking?
Answer: Removal is probably not called for in this situation. Spider web cracking usually is an indication of crazing, a form of plastic shrinkage cracking. Crazing generally occurs when the combination of temperature and humidity creates a rate of evaporation at the surface of the concrete that is higher than the rate of bleed water exiting the concrete. Because the surface has very little, if any, tensile strength at this time, crazing cracks start to form. The good news is that crazing is an aesthetic problem. It affects only the very top surface and does not extend deeply into the concrete. Crazing cracks are more apparent when the surface is damp.
To avoid or limit crazing, be conscious of the weather conditions during placement. If there will be high temperature, low humidity, and moderate to high winds, measures such as fogging and/or erection of windbreaks may be required during placement. Synthetic fibers will help inhibit the formation of crazing cracks. Curing must begin as soon as possible, especially in these conditions. (back to top)
Question 65: What is the recommended core size for shotcrete? Are there unique characteristics of shotcrete cores?
Answer: Regarding sample size for compressive strength, the core length-to-diameter ratio should be in the range of 1:1 to 2:1, with length-to-diameter core strength correction factors applied as per the requirements in ASTM C 42, Clause 7.9.1. Shotcrete test panels are typically between 3.5 to 5 in. (89 to 127 mm) deep. Thus, either 3 or 4 in. (76 to 102 mm) diameter cores should be drilled for compressive strength testing, depending on test panel thickness. We would also suggest referring to ASTM C 1604/C 1604M for securing and testing cores of shotcrete. This new test method allows smaller core diameters for shotcrete in an effort to provide for increased length-to-diameter ratios. Care should be taken when interpreting the compressive strengths using smaller-diameter cores because of the possible presence of voids, which may result in compressive strengths that are not representative of the actual in-place shotcrete. (back to top)
Question 66: Is the core grading scale in the ACI CP-60(02) manual used as an acceptance tool on projects?
Answer: According to ACI 506R-05, the core grading method in ACI CP-60(02) is only to be used for nozzleman evaluation. (This is typically done in ACI Shotcrete Nozzleman Certification sessions and/or in preconstruction testing.) The core grading method should not be used to evaluate structures. (back to top)
Question 67: Our construction management firm is relatively new in allowing shotcrete on our projects. In the most recent issue of Shotcrete magazine, there was a discussion of cores taken from shotcrete in the FAQ feature. Is there additional critical information we should be aware of when determining our coring plan?
Answer: ASTM C 1604, Standard Test Method for Obtaining and Testing Drilled Cores, covers cores that are obtained for determination of length, compressive strength, or split tensile strength. In addition to discovering the thickness of the applied shotcrete and its strength, a visual assessment can be made to evaluate the shotcrete quality, workmanship, shotcrete-to-substrate bond, and condition of the reinforcement. Shotcrete core strength is affected by core orientation relative to the direction of the shotcrete application. Therefore vertical, sub-horizontal, and overhead application of the same shotcrete may show variability. If obtaining cores for determination of compressive strength, cores containing wire mesh or reinforcing bars may not be used. Also, if a sample has been damaged in the process of removal, it cannot be used for strength determination. Cores must have a diameter of at least 3.0 in. unless otherwise permitted by the specifier. Cores with diameters less than 3.0 may demonstrate somewhat lower strengths and have greater variability. They may also be more sensitive to length-diameter ratio. Cores with length-diameter (L/D) ratios greater than 2.1 must be sawed to produce a capped or ground specimen with a L/D ratio between 1.9 and 2.1. Strength results from cores with L/D ratios less than 1.75 must be corrected as detailed in ASTM C42. A core having a length of less than 95% of its diameter before capping or a length less than its diameter after capping or grinding shall not be tested unless otherwise directed by the specifier. To avoid introducing the effects of moisture gradients of wetting and drying, extracted cores are to be stored in a sealed plastic bag at all times except during end preparation and a maximum of 2 hours prior to capping. Prior to capping, it is a good idea to determine the density of each core. Reported results should include the following: length of the core as drilled reported to the nearest ¼” (5 mm); length of the test specimen before capping or grinding reported to the nearest 0.1 in. (2 mm) and average diameter to the nearest 0.01 in. (0.2 mm); compressive strength reported to the nearest 10 psi (0.1 mpa) if the diameter is reported to the nearest 0.01 in. (0.2 mm) or nearest 50 psi (0.5 mpa) if the diameter is reported to the nearest 0.1 in. (2 mm); direction of the application of the load with respect to the horizontal plane of the shotcrete as placed; moisture conditioning history; date and time of test; nominal maximum size of the shotcrete aggregate; if determined, the estimated density; and any deviation from the stated test method and the reason for the deviation. (back to top)
Question 68: We have a large pond (12,000 ft2 [1115 m2]) 12 ft (4 m) deep with 2-to-1 sloped sides. It currently has an old PVC liner that is ripped and cannot be repaired. We have no shotcrete experience and wonder if shotcrete would be a better option than installing a new PVC liner?
Answer: Shotcrete is used extensively for zooscapes, water parks, museum exhibits, swimming pools, and spas. A shotcrete water feature, although more expensive than PVC liner, would provide a long-term, more aesthetically pleasing alternative to a new PVC pond liner. Shotcrete is very versatile and can be shaped to replicate natural rock ledges or boulders. A properly designed and built water feature would provide a low-maintenance, durable solution. (back to top)
Question 69: Is there any specified finish for shotcrete?
Answer: There are several different finishes that are specified for shotcrete. One is a natural gun finish, which is the natural finish as sprayed (often used in slope protection). Another is a cut-down finish, which is cut-to-grade with the edge of a trowel or cutting rod (this finish is often flashed and sealed with a light gun finish to seal and texture the surface). Often in concrete repair, a trowel finish is specified where the shotcrete is cut down with the edge of a trowel or cutting rod to grade after the initial set of the material, and the surface is lightly flashed and toweled. Several different finishes can be achieved with shotcrete, but it should not be pushed or floated with the flat part of the trowel, as is done with poured concrete. It is important to wait for the initial set of the material and to use the edge of the trowel to cut the high points or shave the surface to achieve the grade or effect desired.
Several excellent articles describing shotcrete finishes and finishing techniques are available as free downloads from the ASA website: www.shotcrete.org. One article, Technical Tip: Technical Tips on Shotcrete Finishes, written by Denis Beaupre, describes the different finishes that can be applied to shotcrete. Another article of interest would be Finishes for Retaining Walls by Marcus H. von der Hofen. Go the Publications section of the ASA website, click on “Click here to search the archive of Shotcrete Publications” and type “Shotcrete Finish” in the search window.
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Question 70: I am a project engineer. Recently I received a mixture design for a shotcrete project that included limestone coarse aggregate. This is a first for me. All other shotcrete mixtures I have seen have had pea gravel as a coarse aggregate or no coarse aggregate at all. Is limestone commonly used in shotcrete?
Answer: A limestone coarse aggregate will generally be harder and more angular than what you are used to seeing in shotcrete mixtures. It really shouldn’t be a problem to use. In dry-process gunning, it is considerably more abrasive so there is more wear and tear on equipment, such as hoses, bowls, and wear plates, but it generally guns fine. In wet-process gunning, a sharper aggregate may not flow as easily through the hoses as smoother sand and pea gravel aggregates would. These are issues that the shotcrete contractor will have to address. They should have no effect on the quality of the in-place shotcrete. (back to top)
Question 71: We are concerned about the compressive strengths of shotcrete recently placed on one of our projects. The specification calls for 8000 psi (55 MPa). Test results indicate we are only at 5200 psi (36 MPa) at 28 days. Ambient temperatures are constant at about 45 °F (7 °C) at the point of placement. Should we be considering removal of the shotcrete?
Answer: Shotcrete, like any other concrete mixture, will continue to gain strength as long as there are unhydrated cement particles present along with sufficient temperature and moisture. Strength development will generally be quite slow at the ambient temperature reported. The inclusion of supplementary cementitious materials in this mixture is a benefit in this instance as strength will increase as long as calcium hydroxide is available from the hydration of the cement. The specified strength should eventually be attained as long as the ambient temperature does decrease further and some form of moisture is available to the shotcrete. (back to top)
Question 72: I am in the process of designing a 6" shotcrete overlay for an existing wall that is approximately 1,250 square feet. The shotcrete subcontractor has proposed to use a dry-mix shotcrete. What are the advantages and disadvantages to the dry-mix process? The design includes dowels on 24" centers and 4x4 W4xW4 wire mesh. Can the entire 6 inch thickness be placed at one or will it require a number of different lifts to build up to the 6 inch thickness?
Answer: The overlay can be placed successfully with either a dry-mix or wet-mix shotcrete process. The preference of the shotcrete subcontractor is likely related to his/her past experience and what they are best suited doing. The advantages of dry-mix process are beyond the scope of a simple answer. The process is well described in ACI 506R Guide to Shotcrete. The entire 6 inch thickness can be placed in one layer using the bench gunning technique. The number of vertical lifts would depend upon the height of the wall and the nature of the surface that the shotcrete is being placed against. (back to top)
Question 73: We are currently designing a retaining wall, sloped at 1H:0.5V, 5.5 high. We want to use shotcrete for this 12 inch (300mm) thick structural wall. For strength requirements, we are able to use a 10mm mesh, however this does not satisfy for crack control requirements. For crack control, it is required that we us 1/2 inch (12mm) individual rebars. Obviously for cost and ease of construction, the mesh is a favorable choice for reinforcing. Is there a typical section for this type of application? Will shotcrete shrink less than poured concrete?
Answer: Each retaining wall needs to be engineered for the specific job conditions. However it is fairly common to see two layers of reinforcing bars in a wall of this thickness. In addition to reinforcing the wall, the steel would help support the shotcrete during placement. If drying shrinkage crack control is an issue, synthetic fibers may be added. Shrinkage in shotcrete mixes may be higher than a poured concrete with a 1" (25mm) maximum sized coarse aggregate content, and higher cement/cementitious material content. This may be partially offset by a slightly lower water/cementitious material ratio in a shotcrete mixture. (back to top)
Question 74: We are looking at lining an existing 20 ft (6.1 m) diameter brick sewer with shotcrete that is 15 in. (0.4 m) or more thick and fairly heavily reinforced. Can this be done? The existing sewer is about 3 mi (4.8 km) long and 100 years old. Would shotcrete be a suitable method of rehabilitation? The rehabilitation is not just a liner, but the owner wants the shotcrete designed as a replacement pipe inside the existing brick sewer, designed for all earth and other superimposed loads as though the brick sewer were not there.
Answer: Yes, this can and should be done in shotcrete. Shotcrete has been used to successfully line brick sewers for 75 years. Shotcrete has been used to line over $40 million worth of brick sewers in Atlanta alone. Large brick sewers have been lined with shotcrete in most of the major midwestern cities. All of them were designed using the existing sewer as a one-sided form. Properly designed and constructed, shotcrete will provide the owner with a new concrete pipe or permanent tunnel lining and the associated expected longevity. (back to top)
Question 75: We are currently designing a retaining wall, sloped at 1H:0.5V, 18 ft (5.5 m) high. We want to use shotcrete for this 12 in. (300 mm) thick structural wall. For strength requirements, we are able to use a 0.4 in. (10 mm) mesh; however, this does not satisfy for crack control requirements. For crack control, it is required that 1/2 in. (12 mm) individual reinforcing bars are used. Obviously, for cost and ease of construction, the mesh is the favorable choice of reinforcing. Is there a typical section for this type of application? Will shotcrete shrink less than placed concrete?
Answer: Each retaining wall needs to be engineered for the specific job conditions. It is fairly common, however, to see two layers of reinforcing bars in a wall of this thickness. In addition to reinforcing the wall, the steel would help support the shotcrete during placement. If drying shrinkage crack control is an issue, synthetic fibers may be added. Shrinkage in shotcrete mixtures may be higher than placed concrete with a 1 in. (25 mm) maximum-sized coarse aggregate due to smaller coarse aggregate size in shotcrete mixtures, higher fine aggregate content, and higher cement/cementitious material content. This may be partially offset by a slightly lower water-cementitious material ratio in a shotcrete mixture. (back to top)
Question 76: We have a 6 in. (152 mm) thick tilt-up concrete wall that needs to be upgraded to achieve a 4-hour fire rating. We would like to add shotcrete to achieve that rating. What is the hourly rating per inch of shotcrete? We were hoping that 2 in. (51 mm) of shotcrete would provide the desired rating.
Answer: Shotcrete is a method of concrete placement, not a special type of concrete. The fire-rating of a concrete wall constructed by shotcreting or pouring will be the same. The important consideration is the requirements of the Underwriters Laboratory (UL) Fire Resistance Directory. The directory will provide guidance. UL ratings provide the most widely accepted criteria. (back to top)
Question 77: We would like to apply a 2 in. (50 mm) layer of shotcrete on 10 ft (3 m) diameter steel pipes including wire mesh. Is this practical? If so, how do we do this successfully?
Answer: This type of application is very common. Either wet- or dry-process shotcrete can be used successfully. The mixture should contain a minimal amount of coarse aggregate and be rich in cementitious material to minimize rebound. Generally either 2 x 2 in. (50 x 50 mm) 14 gauge or 2 x 4 in. (50 x 100 mm) 12 or 14 gauge welded wire fabric is used. The wire fabric needs to be spaced off the surface of the steel pipe to allow the shotcrete to encase the wire properly. This can be accomplished by welding studs or nuts on the pipe surface and securing the wire to them. (back to top)
Question 78: Can brackish or salt water be used to make shotcrete for a pool and will it have any negative effect on the quality of a shotcrete pool?
Answer: As a general rule of thumb, brackish or salt water should not be used as shotcrete mixing water. High chloride ion contents can cause rapid setting of the shotcrete (which can make finishing difficult) and longer-term reinforcing steel corrosion-induced cracking, delamination, and spalling. Other components of brackish water can also be damaging to the fresh and hardened shotcrete. For a detailed statement on what constitutes acceptable contents of various dissolved chemicals for concrete/shotcrete mixing water, refer to the Portland Cement Association publication Design and Control of Concrete Mixtures, Chapter 4, “Mixing Water for Concrete.”
One could also consult ASTM C1602/C1602M for limits on the composition of nonpotable water for use in the production of shotcrete. (back to top)
Question 79: We are constructing a canopy for a mine entrance. We need to attach some type of wire mesh to the wood fillers to give the shotcrete some surface to bond to. What type of wire would be the best for this application? The mine canopy is self-supporting and the shotcrete is strictly to be used as a sealant.
Answer: AA typical wire mesh for such applications is 2 x 2 in. (51 x 51 mm) by 12 or 14 gauge; 3 x 3 in. (76 x 76 mm) by 11 gauge; or 4 x 4 in. (102 x 102 mm) by 10 gauge. It is important that the mesh be secured such that it does not move during the shotcrete placement. The mesh will tend to be pushed away from the back surface by the pressure of the shotcrete application. (back to top)
Question 80: What can we add to dry-process shotcrete mixtures for cold weather operations?
Answer: Successful cold weather placements require more than just modifying a mixture. The mixture temperature, condition of the substrate, and the placing and curing environment are also important considerations. Generally, one is discouraged from trying to apply shotcrete if substrate temperatures are too cold and the ambient temperature is at 40 °F (5 °C) and falling. There are, however, exceptions for extreme situations such as shotcreting in permafrost ground conditions, where it is not possible (or advisable) to heat up the substrate. In such conditions, special accelerated dry-mix shotcretes (in conjunction with the use of heated materials) have been successfully used. This type of work is highly specialized and not recommended for the novice.
Accelerators can be added to shotcrete mixtures to help overcome cold weather conditions. The accelerator can be either a liquid accelerator added with the mixing water at the nozzle or a dry-powdered accelerator in prebagged dry-mix shotcrete. Caution is advised when using accelerators containing calcium chloride, as the use of these materials may accelerate corrosion of reinforcing steel.
More information can be found in ACI 306R, “Cold Weather Concreting,” available from the American Concrete Institute, www.concrete.org. (back to top)
Question 81: How can I maintain a 2 in. (50 mm) thickness of shotcrete in a rock excavated tunnel?
Answer: There are many ways of maintaining the thickness of shotcrete. When placing shotcrete over a rough rock excavation, the thickness will vary with more material filling in the voids than covering the high points. Some methods of checking or maintaining the thickness are as follows: stabbing the plastic shotcrete with a depth gauge; preinstalling pins to the desired thickness; and using groundwires or shooting wires that would create an even plane over the length of the wires. (back to top)
Question 82: Can shotcrete be applied to a slope to act as a retaining wall without a moisture barrier? If a moisture barrier is recommended, what type should we use?
Answer: Most shotcrete slopes are placed without moisture barriers and are constructed to ensure that water pressure does not build up behind the slope and create hydrostatic pressure on the backside of the shotcrete. This is generally done with drainage material and weep holes or vents near the base of the shotcrete slope. Please bear in mind that shotcrete slope paving alone is not generally considered as a retaining wall. If shotcrete slope paving is to be used as a retaining structure, it is generally done in conjunction with soil nailing, tie backs, or some type of structural footing. If the shotcrete is intended to be used as a structural wall, a structural engineer must be consulted to be sure all structural issues are addressed. (back to top)
Question 83: We are having a swimming pool constructed. The pool consultant is concerned about cold joints during construction if walls and the floor are shotcreted on different days. The shotcrete subcontractor states that there is no problem as the next layer of shotcrete will knit itself to the previous placement and form a solid bond. Is the shotcrete subcontractor correct?
Answer: Yes, if care is taken to prepare the receiving surface properly. The receiving shotcrete edge must be sound (no loose or unconsolidated material), clean (no traces of laitance or gloss), rough, and dampened to a saturated surface-dry condition. If these steps are followed, there should be no concern about the soundness of the joint. (back to top)
Question 84: What is the maximum thickness for shotcrete used for shear walls? Can we use more than 12 ft (3.7 m) if we use a double layer of reinforcing?
Answer: There is no stated maximum thickness for shotcrete used in shear walls or any other type of wall. Walls have been successfully placed to a thickness of 36 in. (914 mm) for some time. The two main concerns are the heat of hydration and proper encapsulation of the reinforcing steel. Because shotcrete mixtures typically contain more cement per cubic yard or cubic meter than formed and poured placements, there will be more heat generated by the shotcrete mixture. The ability of the nozzleman to encapsulate the reinforcing will be a function of proper mixture design, proper selection of shotcrete equipment, and the skill level of the nozzleman and the crew. (back to top)
Question 85: We would like to get approval to use shotcrete on the perimeter walls of an existing laboratory building. We would be shooting against a waterproofing membrane and shoring lagging. The project engineer is concerned that the shotcrete will damage the membrane, resulting in leaking into the occupied space. Are there any examples where this type of shotcrete placement has been used?
Answer: This is a commonly used technique in the Western U.S. and Canada, and has been used successfully from Stanley Hall at the University of California at Berkeley, Berkeley, CA, to the Baltimore Hilton Convention Center near Camden Yards, Baltimore, MD. There are a number of suppliers of waterproofing materials to choose from for this application. In selecting a supplier, be sure there is field service available to inspect the project before placement of the shotcrete. (back to top)
Question 86: I am repairing a concrete masonry unit (CMU) block wall that was partially damaged when a portion of the roof collapsed. The engineer on the project is proposing to apply shotcrete to one side of the wall to help structurally reinforce the wall. I would like to know if there is a way to finish the wall so it is cosmetically pleasing, especially since this is on the inside of an existing building with the other walls being a painted CMU. Also, were can I get some conceptual pricing for applying the shotcrete?
Answer: Shotcrete can, and often is, finished to provide nice printable wall surfaces. To be the same general texture of the concrete block wall, you should specify a wood or rubber float finish. You can access the ASA Buyers Guide at www.Shotcrete.org/BuyersGuide to locate organizations regarding budget or conceptual pricing. (back to top)
Question 87: I have an unfinished (dirt) basement with a stacked stone and mortar foundation. Can I shotcrete the existing dirt walls and floor with shotcrete MS (micro silica enhanced) and have it adhere to the dirt portion of the basement? If so, what method would be best?
Answer: Shotcrete would work well for the overlay of the walls. In most cases, floors are placed by a conventional cast-in-place method. Either the wet- or dry-mix procedure would work well for the walls. To ensure good bond of the shotcrete to the walls, the walls should be cleaned and prepared to assure that the shotcrete is bonding to sound material rather than contaminates such as dirt or weathered material. You might also want to consider reinforcing the walls, but you should consult with an engineer on how to do this and with what material. If you were to use the wet-mix process, you could use the same equipment to place the floor as you are using for the walls. (back to top)
Question 88: I am lining a below-ground conical shaped excavation with shotcrete. Dimensions are approximately 90 ft (27.4 m) diameter by 45 ft (13.7 m) depth. Sand will be moved in and out of the container daily. Temperature range is 590 to 740 °F (310 to 393 °C). Can you tell me if a mixture is available that can meet the following specific conditions:
• Withstand the temperature ranges noted above without spalling, cracking, etc.; and
• Resist abrasion assuming hot sand is flowing over the surface area daily?
Answer: You certainly have adverse conditions to work with! There are products on the market based on calcium aluminate cements that will tolerate the temperatures you mention and are durable. These products can be placed using the shotcrete process. A list of companies who supply this product can be found at www.Shotcrete.org/BuyersGuide. (back to top)
Question 89: I am reconfiguring the interior of a spa and am wondering if drains and jets can be relocated without compromising the overall structure and getting cold cracks. Can the entire interior be re-shot to maintain the monolithic form and guarantee against failure? Is there an independent professional who could conduct an on-site inspection and recommend a next step?
Answer: We are not able to advise you on the structural integrity of a remodel of a spa or any other structure and would suggest you consult with a local engineer who is familiar with pools and spas. Shotcrete is often used to overlay or patch structures and the success of such overlays and patches is highly dependent upon the quality of the surface preparation prior to the application of the shotcrete. With respect to referrals of independent professionals, we would suggest that you use the directory of Corporate Members in the ASA Buyers Guide. (back to top)
Question 90: I am a civil engineer looking to use shotcrete in a culvert rehabilitation project. Due to flow constraints, we are forced to have a maximum wall thickness of 3 in. (76 mm). For the typical 96 in. (2438 mm) precast concrete culvert, the walls are approximately 9 in. (228 mm). What can I do to obtain a near similar product with only 3 in. (76 mm) of wall thickness? Can shotcrete be applied at higher compressive strengths, 10 psi (0.07 MPa), or is it better to use fiber-reinforced shotcrete? The intent of the retrofit is to at least obtain a 10-year service life to this temporary solution.
Answer: This is an engineering question, not an application question. Precast pipe is sized for multiple uses and services. Depending on this service (depth of cover or loads), creative reinforcing bar placement and higher compressive strengths can reduce the wall thickness significantly. For example, success has been realized using elliptical steel to reduce concrete section thickness. Fiber reinforcement is secondary reinforcing and is not a suitable replacement for reinforcing steel. Given the short life required of the culvert, and assuming fairly equal loading on the circumference, a 3 to 4 in. (76 to 101 mm) section with judiciously placed reinforcing bar, and silica fume (8 to 10% of cement for higher strength up to 10,000 psi (69 MPa)]) would be sufficient. The resulting culvert’s life would probably be much longer than 10 years. In the end, an engineering call should be made, but the material will perform. (back to top)
Question 91: Is it feasible and economical to construct floodwalls approximately 5 ft (1.5 m) high with shotcrete?
Answer: Yes, it is feasible and economical to construct structural walls such as a 5 ft (1.5 m) high floodwall. Shotcrete is a method of placing concrete and has similar, if not identical, properties after placement. As you can imagine, shotcrete needs to be shot against something such as a one-sided form, gabion baskets, earth, expanded metal lath, or just about any structurally sound thing you can think of. The economy of the system is dependent upon the site conditions and the ingenuity of the contractor. An example of a similar structure is on the east side of I-880, south of Dixon Landing Road in Milpitas, CA. This project, a flood control channel, involved trapezoidal channel sections, vertical wall sections, and a combination of sloped walls with a vertical extension. If you have further interest, you should contact an organization with experience in this area. An excellent source is the directory of Corporate Members in the ASA Buyers Guide. (back to top)
Question 92: I am a pool builder who favors dry-mix shotcrete. I have a project requiring: a) cast-in-place concrete retaining walls, where there will be exposed downhill faces (that are not necessarily meant to be seen). Should my shotcrete contractor be able to finish the exposed face in some sort of reasonable finished appearance? and b) placing a pool house foundation (about 4 ft [1.2 m] high). Would I be able to shoot these? I am thinking not because there is no place for the rebound to go.
Answer: a) Shotcrete can be finished in a wide variety of ways. It can be left with anywhere from a very rough to a very smooth finish and a huge variety of other finishes. We suggest you visit ASA’s Web site, click on the tab for Shotcrete magazine, and search the previous articles for finishes and swimming pools. You will find a lot of photos of great-looking walls. Not all shotcrete contractors are proficient in providing these attractive finishes. You need to discuss this with your current shotcrete contractor and/or interview other shotcrete contractors to make sure the chosen contractor can provide what you are looking for. We also suggest you look at work these contractors have previously completed. You can also locate contractors online at ASA’s Buyers Guide, www.Shotcrete.org\BuyersGuide. b) If the pool house foundation is a footing trenched into the ground 4 ft (1.2 m) deep, dry-mix shotcrete would not be a good solution. If the foundation is 4 ft (1.2 m) above grade, then it could be done with shotcrete against a one-sided form. This would be considered structural shotcrete and not all shotcrete contractors are qualified to place shotcrete for structural walls. Again, we suggest you ensure the chosen contractor is qualified to do the work. (back to top)
Question 93: Our client has a retaining wall that has experienced movement in the precast concrete panels and has asked us to research a product that could be applied to give a smooth look to the retaining wall. Is shotcrete a possible option? I would also like information on the recycled content of shotcrete.
Answer: Shotcrete is basically concrete that is pneumatically applied. Shotcrete can be used as an overlay for an existing wall to provide structural strengthening and a smooth look. Again, we suggest that you visit ASA’s Web site and search previous Shotcrete magazine articles for finishes. Before the shotcrete is applied, the wall must be stabilized from any anticipated future movement. Relatively thin layers of shotcrete or concrete will not withstand future wall movements without distress and cracking. The recycled content of most shotcrete mixtures is limited to the substitution of fly ash or other pozzolans for a percentage of the cement in the mixture. To properly place shotcrete, this substitution is generally limited to approximately 25% of the cement content. (back to top)
Question 94: We recently stained a shotcrete wall. After we placed the staining on the wall, the stain came out in different shades across the wall, in effect bring out the different curing of the concrete. What can be done to eliminate this inconsistency?
Answer: It is not unusual to have variations in the tone of color for shotcrete or concrete walls that have been stained due to variation of the texture or density of the surface being stained. An acid-based stain typically results in more consistent shading. When anticipating that a wall will be stained, extra care needs to be taken in the curing process. It is generally recommended that walls to be stained should be water-cured to avoid any interaction between a curing compound and the stain material. If a curing compound is used, it must be completely removed prior to applying the stain material. Consult the stain supplier for more information. (back to top)
Question 95: I have a seawall with a gunite (dry-mix shotcrete) outer layer. The gunite layer has cracked in multiple locations on the seawall resulting from years of exposure to the harsh environment. The original gunite was not part of a soil nail system. I am considering a re-coat of shotcrete probably 3 to 4 in. (76 to 100 mm) thick with wire mesh and L-anchors on a 2 to 3 ft (0.6 to 0.9 m) grid. I know the importance of surface treatment for bonding, etc., but I am not sure if I should remove the original gunite layer (which is still sound in some places) or apply the re-coat. The new overlay needs to be structurally effective. I know that a soil nail system is the most dependable solution, but cost is a major concern. Do you have
any suggestions?
Answer: The new shotcrete layer can be added to the existing shotcrete or installed after the existing shotcrete is removed. The decision to remove or not remove the existing shotcrete is beyond the scope of what we can comment on. If the existing shotcrete is left in place and overlayed, it should be thoroughly cleaned and roughened to create a good bonding surface. Because this is in a marine environment and you are considering the use of wire mesh, you need to make the new layer thick enough to have sufficient cover on the reinforcing steel. Alternately you could consider the use of fiber-reinforced shotcrete and silica-fume-enhanced fibrous shotcrete. Please note that there are many types of fibers on the market. We recommend that you review some of the Shotcrete magazine articles on fibrous shotcrete and on shotcrete in a marine environment on the ASA Web site. We suggested two papers for reference. The first is by Gilbride, Bremner, and Morgan on the Port of Saint John, and the other is by Morgan on the use of fibers that cover marine repairs. You mentioned using “L-anchors” at 2 to 3 ft (0.6 to 0.9 m) spacing. The use of grouted anchors with a reasonable embedment is quite common, but the design of such anchors is again beyond the scope of what we can advise. (back to top)
Question 96: We will be tiling a pool. The pool’s shotcrete walls and floor were placed approximately 10 days ago. What is the earliest we can begin gauging the pool walls and floors?
Answer: It is generally good practice to let the shotcrete cure for the full 28 days before attempting to apply coatings or overlays. We would recommend you get a recommendation on the cure time from the manufacturer of the gauging product before doing the work. (back to top)
Question 97: We are considering the use of bentonite in a blind-side waterproofing situation to waterproof a basement with shotcrete as the confinement material. The basement has a 8.2 ft (2.5 m) head of water permanently against it (approximately 6.5 ft [2 m] higher than the slab/shotcrete wall construction joint).
In brief, we intend to construct as follows:
1. Pump the area dry;
2. Place secant piles, and then apply shotcrete over the piles. The shotcrete will be troweled to accept the bentonite;
3. Apply the bentonite sheet membrane to the troweled shotcrete;
4. Tie two rows of reinforcing steel at 11.8 ft (300 mm) centers in each direction;
5. Shoot shotcrete through the steel onto the bentonite tanking; and
6. Turn the pumps off once the curing period is complete.
We have been advised this will be effective. Any advice on this system would be greatly appreciated, as we believe using shotcrete rather than cast-in-place concrete as the confinement material would result in significant cost savings. We know little, however, of the confinement properties of shotcrete.
Answer: The use of shotcrete over waterproofing in blind-side applications is not uncommon; and, as you note, it is generally very efficient from a cost and schedule standpoint. It should be noted that the shotcrete applicator (shotcrete contractor) should be very experienced in high-quality structural shotcrete work. The application of shotcrete in tunnels, canals, channels, or swimming pools is very different from the application of shotcrete for structural walls. The experienced structural shotcrete contractor will use experienced and knowledgeable tradesmen including a certified ACI nozzleman.
There are many types of waterproofing material including sodium bentonite, as you mentioned. Other membrane material and additives can be added to the shotcrete mixture as delivered. It is not within our scope to comment on the choice of these materials. You can contact the various manufactures or engage a waterproofing professional to give you the proper advice. The ASA online Buyers Guide is a great starting point in locating qualified professionals. (back to top)
Question 98: I have reviewed ACI 506R, “Guide to Shotcrete,” and 506.5R, “Guide for Specifying Underground Shotcrete,” but was unable to find specific criteria pertaining to shotcrete protection for reinforcing steel. Would the clear cover then be based on ACI 318 Section 7.7.1 for cast-in-place concrete? For underground structures, would 3 in. (76.2 mm) of clear cover from ground be required?
Answer: Shotcrete is concrete, and therefore if designing structures based on the ACI 318 Code, cover for conventional reinforcing steel should be those suggested in ACI 318 for concrete against ground. If the shotcrete is a “temporary” support, with further placement of “final” support, then these requirements do not apply. (back to top)
Question 99: I have a question on cold weather shotcreting. I have heard that for shotcrete operations, the ambient temperature has to be 40°F (4.4°C) and rising. I am on a job, and the inspector said it only needs to be 35°F (1.67°C) and rising. The high for the day is expected to be around 45°F (7.2°C), then fall back into the high 20s F (–4 to –1.67°C). What would be your advice?
Answer: Shotcrete is concrete and the same rules apply with respect to cold weather applications. Cold weather is defined in ACI 306R, “Cold Weather Concreting” as “a period when, for more than 3 consecutive days, the following conditions exist: 1) the average daily air temperature is less than 40°F (4.4°C) and 2) the air temperature is not greater than 50°F (10°C) for more than one-half of any 24-hour period.” ACI 306R is an excellent reference that provides recommendations for cold weather concrete placement and protection. A copy of ACI 306R can be purchased online at ACI’s Web site, www.concrete.org, from the Bookstore and Publications tab. You can also download articles regarding cold weather placement from ASA’s Web site, www.shotcrete.org—click on “Shotcrete magazine,” go to the article search page, and type in “cold weather.” (back to top)
Question 100: One of our clients has a 65.6 ft (20 m) tall mechanically supported earth (MSE) wall (to dump the ore from the mine into the crushers). The wall is about 984.25 ft (300 m) long and has approximately 30-degree slopes on both ends, like a pyramid. These slopes have eroded over the last 8 years of operation and some of the wall reinforcing is exposed. We want to stop the erosion and stabilize the slopes. The instructions issued to the contractor are: level the slopes; fill the voids; compact; apply shotcrete (maximum 1 in. [25 mm]). The area in question is 6.6 x 65.6 x 131.2 ft (2 x 20 x 40 m). Is shotcrete application in this case appropriate? Can you forward information on experts we could consult on?
Answer: Shotcrete is well suited to the application you have described. You need to determine the characteristics that you want from the shotcrete (strength, toughness, freeze-thaw durability) and include these in the specification.
The 1 in. (25 mm) seems very thin for a long-term installation. Please be aware that the material costs (in most cases) will be a small part of the total cost of the installation. You should also make sure that you have a good specification for surface preparation. If the existing surface is not properly prepared, the added shotcrete will not bond well and the installation will not last very long.
The ASA Online Buyers Guide (www.Shotcrete/BuyersGuide) is an excellent source to locate members within the field of shotcrete whom are listed as shotcrete consultants. (back to top)
Question 101: Type CA and FA shotcrete are two classifications listed in ASTM C1480. What is the application of these two types of shotcrete?
Answer: Type FA shotcrete uses a fine aggregate meeting the requirements of ASTM C1436 Gradation #1. Type CA shotcrete uses a combined aggregate gradation meeting the requirements of ASTM C1436 Gradation #2. The decision on which type to use depends on the application, shotcrete thickness, specification requirements, and perhaps the shotcrete equipment to be used, that is, wet- or dry-process. For example, one may want to use Type FA if using dry-process equipment and placing thin sections, or when a smooth finished surface texture is required. For thicker sections, Type CA shotcrete may provide the best properties for the application. The choice of which to use depends on the application, equipment, and experience of the contractor. (back to top)
Question 102: How soon after shooting a pool shell can formwork be removed? How soon can tiling begin?
Answer: Vertical formwork can generally be removed the day following the shotcrete installation. If the formwork is supporting a load like a soffit form, the form should not be removed until the shotcrete has attained full strength such that it can support the weight of the member.
Your question regarding the installation of the tile should be directed to a professional who installs tile. (back to top)
Question 103: We have an approximately 9500 ft2 (882.6 m2) pool that was built and finished in midsummer. Four weeks later, the pool has developed “spider web” cracking in the bottom. We need to have a compressive strength test done. Our crew is on site now and is going to pull a 4 in. (101.6 mm) core sample for testing. I need to know what procedure to follow and where to send the sample for testing.
Answer: Consult with a local engineering firm that is qualified to develop a coring plan, obtain cores, and perform testing in accordance with ASTM C42/C42M or ASTM C1604/C1604M. Please refer to ASTM C42/C42M for further guidance. (back to top)
Question 104: We have demolished two radioactively contaminated buildings down to their concrete slabs. One of the slabs has a concrete pit that is 26 ft (8 m) deep. The slabs have not been removed because the soil beneath the slabs is contaminated and we’re using the slabs as a cover to protect the spread of contamination in the soil until the soil remediation begins. We’d like to use shotcrete to temporarily (up to 5 years) fix the contamination on the slabs and the 5 ft (1.5 m) area surrounding them. The questions we have are: 1) Will shotcrete adhere to the concrete slabs and pit walls for up to 5 years without special preparations? (Portions of the radioactively contaminated concrete are painted and it is dirty from demolition activities); and 2) What is the minimum thickness of shotcrete needed to last for 5 years in this type of application? We do not want to use any wire or fabric mesh as it would require personnel to work in a radiologically controlled environment to install the material.
Answer: Shotcrete, like concrete, likely will not adhere to surfaces that are painted and dirty from the demolition activities. There should be no issue to the time durability. Shotcrete is pneumatically placed concrete and has great long-term durability characteristics if placed properly.Shotcrete has been installed in many adverse environments at a thickness of 2 in. (50 mm) with fibrous reinforcement and provided a long service life. Many irrigation districts line their canals with shotcrete and it has provided decades of great service in freeze-thaw exposures. (back to top)
Question 105: What is the recommended cure time for shotcrete pools and spas so that shrinkage cracks in finished tile work can be avoided?
Answer: Concrete, when applied using the shotcrete process, or cast-in-place, needs to cure for 7 days. Water is the best curing method (7 continuous days). Curing compound can be applied, but the membrane film that is formed will have to be removed by sand or water blasting (5000 psi [34.5 MPa] is recommended) before the plaster or tile can be set (it will create a bond breaker if not removed). There are curing compounds with a dissipating resin, which means after about 30 days in the sun, the material will break down. In either case, it is a good practice to pressure wash the concrete surface to remove the grit and dust out of the pours so that the plaster and tile will have a good bond. This is normal, everyday concrete curing practice that helps to prevent shrinkage cracks. The concrete being applied should have a water-cement ratio (w/c) of 0.35 to 0.45. Having the w/c at 0.40 at a 2 to 3 in. (50 to 75 mm) slump will keep the water demand low to help minimize the shrinkage. Wet-fogging freshly placed concrete before the curing process begins will also help prevent shrinkage cracks. (back to top)
Question 106: We are designing underground support for a hydropower tunnel. I want to know whether wire mesh-reinforced shotcrete or steel fiber-reinforced shotcrete will be better and more economical. What are the advantages and disadvantages of both of these types of reinforcement if used for supporting a tunnel for hydropower? Also, for slope protection work, which type of shotcrete is better in terms of reliability, durability, and cost?
Answer: There are really two questions here: 1) Underground fiber-reinforced versus mesh reinforced; and 2) slope protection fiber reinforced versus mesh reinforced.
1. Underground fiber reinforced versus mesh reinforced: it is not clear what the alternatives are that you are considering, but sprayed concrete has a good, solid track record for ground support. If it is a simple comparison of steel mesh versus steel fiber reinforcement, then the issue is one of a design approach.
Wire mesh and bolts have a longer history and are simple to design as a rigid structure. To install mesh and bolts, however, requires working under unsupported ground. Mechanized spraying of concrete is done with the operator under supported ground and therefore is intrinsically safer.The design of fiber-reinforced sprayed concrete as ground support is approached differently. The sprayed concrete is allowed to deform to a certain extent before coming to rest with the ground forces finding a new equilibrium. The extent of this deformation depends on the energy absorption of the sprayed concrete structure, which is provided for by the fibers.Steel fiber-reinforced sprayed concrete is by far faster to place and therefore has economic benefits. As the fibers are discontinuous, there is merit in considering this structure less susceptible to corrosion and consequential durability issues. We recommend consulting ACI 506.1R and ACI 506.5R.
2. Slope protection fiber reinforced versus mesh reinforced: for slope protection, both fiber-reinforced and wire-mesh-reinforced shotcrete work well and are durable, reliable, and cost effective if done properly. Care must be taken with wire mesh reinforcing to ensure that it is maintained in the middle of the section and not on the ground where it is not effective. Wire mesh can also be difficult to install on an irregular surface and require more shotcrete material to cover the area and the mesh. The wire mesh can be an asset to the installer in providing a grid to support a scaffold system. In many applications, the choice of wire mesh or fibers should be left to the installer with the engineer specifying the minimum requirement for each. (back to top)
Question 107: We are repairing a culvert in Dallas, TX. The concrete wall of the structure is pre¬maturely disintegrating. We are considering a process to temporarily support the ceiling, remove the wall, place a form on one side, and use shotcrete to replace the wall. Does this sound like a reasonable use for shotcrete? What kind of specifications should be used?
Answer: Yes, this sounds like a good use of the shotcrete process. Your sequence sounds like a good plan. A sample Structural Shotcrete Specification is available from the Shotcrete magazine archive on the ASA Web site (www.shotcrete.org). (back to top)
Question 108: We’re looking at adding approximately 4 in. (100 mm) of shotcrete to an existing 8 in. (200 mm) wall to meet new load requirements. What’s the minimum cover between the rein¬forcement and existing wall for proper encapsulation of the reinforcement?
Answer: A minimum clearance for the reinforcment off the existing surface should be 0.75 in. (19 mm) or one bar diameter, whichever is greater, to allow a good flow of material around the reinforcing steel. (back to top)
Question 109: I am working on a water feature formed out of cast-in-place reinforced concrete with a hot-fluid-applied waterproofing system over the concrete. To protect the waterproofing, we plan to install shotcrete over it. What minimum thickness of shotcrete is required? Would welded wire fabric or fiber mesh be required as well?
Answer: In general, we would recom¬mend a minimum of 2 in. (50 mm) of shotcrete.Either fibers or wire mesh or both should be used in this application. Please note that there are different types of synthetic fibers (microsynthetic and macrosynthetic). Refer to ACI 506.1R for information on fiber-reinforced shotcrete.
If the surfaces are steep or vertical, wire mesh should be used, but provisions need to be included to stabilize the wire mesh. This would likely require attachment points through the waterproofing syste .
Sent by American Concrete Association.
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