The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.
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UndergroundQ I am an engineer working on a project involving shotcrete and earthwork. The shotcrete that was placed has some expansion cracks, which we expected. I would like to know the best way to repair them. Is there some type of waterproof coating/grout that can be applied between the cracks? Part of the cracks will be continuously under water. The shotcrete is the surfacing material for a diversion ditch at a mine, and we need to recommend some remediation solutions to our client.
There are many products in the marketplace for repairing cracks. Because shotcrete is simply a method for placing concrete, any method for concrete crack repair would be applicable. It would be wise to use a product that filled the cracks and is able to tolerate thermal movement in the future (not a brittle product). Many injectable polyurethane grouts can accomplish this. Surface-applied coatings would need an adequate thickness and elasticity to tolerate moving cracks. We suggest that you contact one of our corporate members who is familiar with your area and get their specific advice. Please refer to “ASA’s Buyers Guide”.
UndergroundQ I am an engineering technologist working on a landslide project where shotcrete had been applied to stabilize the sandstone head scarp at the crest of the slope. The shotcrete was applied in 1998. After a recent inspection, it was noted that the surface of the shotcrete had some cracking in some sections. How can this be repaired? Can the cracks simply be filled with a grout/mortar mixture of some sort or do the cracked sections have to be removed entirely and shotcrete be reapplied?
Shotcrete can and has been used to overlay previously installed shotcrete or concrete that has cracked over time. It would be advisable that you engage an engineer knowledgeable in geotechnical engineering and concrete properties to formalize a solution. It is important that the cause of the cracks be determined and adequate reinforcing be designed to ensure that the cracks do not propagate through the overlaid shotcrete.
UndergroundQ 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?
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.
UndergroundQ I am considering the use of shotcrete as an alternative to grouted riprap for slope stabilization. The project involves a basin with varying slopes and easily erodible soils. Water will cascade down the side slopes. I was going to specify shotcrete with welded-wire reinforcement but am now considering fiber-reinforced shotcrete. Is fiber-reinforced shotcrete the better choice and, if so, is 3 in. (76 mm) thickness sufficient?
Structurally, using proper quantities of either welded wire or fibers should work well. If fibers are used, they should be specified by an engineer who has the experience to specify the type of fiber and either performance requirements or dosage levels. The advantage of fibers is that they are uniformly distributed through the section, whereas the welded-wire reinforcement can be difficult to maintain in the proper location within the pavement section. The proper thickness should also be determined by a qualified engineer, as soil and groundwater pressures can impact the required thickness.
UndergroundQ 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?
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.
UndergroundQ I am evaluating a community in central Colorado that contains shotcrete slope reinforcement ranging from 14 to 44 ft (4 to 13 m) in height. Assuming the installation met all required guidelines, what should I anticipate as a useful life for this product?
Shotcrete is a method of placing concrete and properly placed shotcrete should have a service life similar to cast concrete. Generally, concrete structures in normal environmental exposures are expected to have service lives from 50 to 100 years. With particular attention to materials and construction methods, some concrete structures, such as the new San Francisco Bay Bridge, have been designed for a service life up to 150 years. The first step in achieving a long-lasting, high-quality installation is to engage a highly qualified and experienced shotcrete contractor. There are many other factors that influence service life, including using the right mixture design for the anticipated exposure conditions.
UndergroundQ I am lowering the roof of the draft tube on a hydropower dam. The work is all overhead and has a slope to it. The new roof profile will be lowered from 2 in. (51 mm) (initial edge) to 6.25 ft (approximately 2 m) thick. The plan is to use rock anchors to transfer the load to the existing concrete, and tie in a reinforcing bar grid at the lower section of the new roof profile (4 in. [102 mm] cover). Shotcrete is being planned for the infill material. The traditional ACI 318 design method was used for sizing reinforcing bar. The concern I have in the design is the application of the shotcrete. The plan is to allow the contractor to install a wire mat (or reinforcing bar mat) approximately 2 in. (51 mm) from the existing concrete roof, then apply shotcrete until reaching the reinforcing bar location, up to 6 ft (1.8 m) thick, then apply the finish layers. I have concerns about how thick shotcrete can be applied overhead. I have received feedback from some shotcrete companies that one can apply up to 4 ft (1.2 m) thick overhead layers, yet others say to never apply more than 4 in. (100 mm) layers. I also have concerns of delamination between the shotcrete and the existing concrete during the installation process, and potential shotcrete falling under its own weight in the thicker locations. Do you know of any situation where shotcrete was installed overhead to thicknesses of 6 ft (1.8 m)? How thick can the shotcrete be before additional reinforcement is required to hold it for overhead applications? Is there a recommended maximum thickness for overhead application of shotcrete being placed before additional reinforcement is required? For the area that is 6.25 ft (2 m) thick, should I be using multiple layers of reinforcement (or fiber reinforcement) to prevent fallout?
This is a very challenging potential installation and there could be several potential approaches. For the shotcrete to bond to the existing concrete, the existing surface should be properly prepared, removing any unsound concrete, then roughened and cleaned to allow for a good bonding interface. You mentioned using rock anchors or bolts. These should be installed before any shotcrete.
For each 6 in. (152 mm) layer, a layer of welded wire reinforcement or structural fibers should be used. These, in conjunction with the rock bolts, should ensure the stability of each layer of the shotcrete from falling.
The surface reinforcement should not be installed before most of the area is within 6 to 8 in. (152 to 203 mm) of the final surface.
There is currently similar thicknesses being placed on the East Side Access in New York City to build back the overbreak for the initial tunneling to the “A-line” or profile that was intended for the mining.UndergroundGeneralQ I am working on a project that has existing tunnels made with shotcrete. I am needing to hang 12 in. (300 mm) duct and trying to figure out the best type or suggested anchors to use.
Shotcrete is just a placement method for concrete. Shotcrete placement with proper materials and application techniques should provide monolithic concrete with a 28-day compressive strength of at least 4000 lb/in2 (28 MPa). Thus, any systems that work in concrete should be fine. Either mechanically-fixed or epoxy-set anchors are commonly used in concrete. You should consult with the anchor suppliers for the size and type of anchor appropriate for your specific application.
UndergroundQ I will be placing a large amount of concrete via the shotcreting process onto a river bed. There are minimal forces and the only reason I need reinforcing is for temperature and shrinkage. If I add fibers to the mixture design, what percent of steel will I still need (if any) or, in other words, how much fiber do I need to include so that any other form of mesh or reinforcing bar is not required to meet temperature and shrinkage requirements? In addition, will too much fiber have any unwanted effects?
Fibers are typically added to shotcrete linings for canals, channels, and other water structures in lieu of conventional reinforcing, such as welded wire reinforcement (WWR). For your “large amount of concrete via the shotcreting process,” we assume that you are using the wet-mix shotcrete process.
Temperature/shrinkage reinforcement is typically placed in thin sections governed by the structural concrete provisions of ACI 318 at a rate of 0.15 to 0.18%. Please be aware that if the lining is intended to be liquid-tight and has movement joints spaced at greater than 40 ft (12 m) apart, a reinforcement ratio of at least 0.50% is recommended by ACI 350 for concrete liquid-containing structures. Assuming that the section does not need to be liquid-tight and using the ACI 318 requirements, let’s consider the tensile capacity of a conventionally reinforced section and provide an equal or greater tensile capacity with fibers. Assuming a 3 in. (75 mm) thick lining with an assumed 28-day compressive strength of 4000 psi (28 MPa), using a WWR of 6 x 6 x W2.9/W2.9 in this section provides a percentage of steel of 0.161% and a tensile capacity of 3770 lb/ft (5610 kg/m). (Asfy = 0.058 in.2/ft [0.12 mm] x 65,000 psi [448 MPa] = 3770 lb/ft [5610 kg/m].)
Then, we assume that the direct tensile strength is 75% of the flexural strength (modulus of rupture [MOR]). For 3770 lb/ft (5610 kg/m) in a section 3 x 12 in. (75 x 300 mm), we have 3770/(12 × 3) = 105 psi (0.72 MPa). Then, we need an average residual strength (ARS) (ASTM C1399) of 105/0.75 = 139.6 psi (0.963 MPa) = 140 psi (0.965 MPa).
Using a macrosynthetic fiber, one can achieve these results using 4 to 5 lb/yd3 (2.4 to 3.0 kg/m3) in wet-process shotcrete. Fiber manufacturers will provide exact dosages to meet the ARS requirements.
Using steel fibers, approximately 43 lb/yd3 (25.5 kg/m3) will provide an equivalent area of steel to the WWR of 6 x 6 x W2.9/W2.9 in a 3 in. (75 mm) thick concrete section. Using steel fibers, however, may require a flash coat to cover the fibers that will protrude and rust over time. The corrosion of the fibers will only reach a carbonation depth of 0.05 to 0.10 in. (1 to 2 mm) but may result in staining the lining.
These calculations assume a thickness and strength. You must adjust for your conditions.
UndergroundQ 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?
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.)