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 We are, and have been, designing and constructing permanent soil nail and shotcrete retaining walls. Typically, our designs consist of a primary nozzle-finished shotcrete facing to shore during our top-down construction, followed by a secondary shotcrete facing that is shot and sculpted once the full height of the wall has been excavated, drilled, and shot with the primary facing. We had a comment recently that only the secondary facing thickness can be used in our design for the wall’s flexural capacity because the shotcrete layers may delaminate. Our general practice is to pressure-wash the primary nozzle-finished shotcrete facing before our approved and experienced nozzlemen place the secondary layer. From our experience, this procedure has been very effective and we have not experienced any delamination between shotcrete layers on any of the millions of square feet of shotcrete we have placed this way. If installed correctly with our general practice, is there any reason the shotcrete layers would delaminate? If not, have any studies been done to prove this to our reviewer?
All of your points are valid, but the Engineer of Record or the owner makes the final decision on recognizing a composite system or ignoring the value of the initial layer. As your experience shows, shotcrete provides an excellent bond between freshly placed layers and properly prepared concrete or shotcrete substrates. There are many articles available in the Shotcrete magazine archives—found on our website, —that may provide the designer or owner more information to allow them to make their design decision.
UndergroundQ We have a 17 mile (28 km) long TBM tunnel for water that will drive our underground powerhouse. Is there a recommended shotcrete surface texture we could use? Our contractor is using 0.31 in. (8 mm) aggregate, but they are getting an undulating surface. Can you provide some clarity as to what we should ask our contractor to try and achieve?
Shotcrete can be applied with many different textures. The nozzle finish shown is very rough, even for a natural gun finish. Nozzle finishes can be done smoother than this. Another technique would be to use a broom to make it smoother after it is shot. Other finishes include wood float, rubber or sponge float, broom, and smooth trowel finishes. There are many examples of finishes shown in articles in Shotcrete magazine.
UndergroundQ We have a client who is looking to make a relatively deep cut in a mixture of soils and rock (approximately. 50 ft [15.2 m] high, maximum; the structures will be placed on a pad at the bottom of the cut). We are looking to provide a shotcrete facing for the entire cut area. The upper portion of the cut will be in soil; therefore, the design of a soil nail wall with temporary and permanent facing in the soil region seems to be relatively straightforward using design guidance in FHWA publications, Geotechnical Engineering Circular #7, and some software programs. Significant portions of the exposed cut face, however, consist of nondurable bedrock (claystone). We want to stabilize this area with shotcrete to prevent weathering and the generation of overhang conditions where the claystone is overlain by a more durable sandstone. I have been unable to find design procedures or guidance on specifying shotcrete (thickness, reinforcement type, etc.) and whether or not rock bolts should be used. If so, how do you select the size, spacing, resin type, etc.?
Soil and rock stabilization is an excellent application for shotcrete. However, ASA as an association does not provide engineering design. We recommend consulting with a geotechnical engineer familiar with the local geology and soil conditions to evaluate potential lateral earth forces from the claystone. Once potential loads are established, a consulting engineer experienced with shotcrete in soil nailing applications will be able to design the soil nail facing. You can check our online Buyers Guide to find a consulting engineer experienced with shotcrete.
UndergroundQ We have a concrete tunnel repair project, where expansion joints in a 10 x 10 ft (3 x 3 m) tunnel need to be repaired. The joints are on the top, bottom, and both sides of the tunnel; therefore, we have vertical and overhead applications. The detail calls for deteriorated concrete to be removed to a depth of about 6 in. (152 mm), existing reinforcing steel to be cleaned and preserved, and old waterstop removed and new PVC waterstop installed. How far can shotcrete be pumped for an application in a tunnel? We are looking at about 200 ft (61 m) for the shotcrete to be pumped from the supply to the repairs. Is this constructible?
Yes, this is definitely a great application for shotcrete. Either wet- or dry-mix shotcrete can easily be used in tunnels with hose lengths of 200 ft (61 m). An experienced shotcrete contractor will be able to select the appropriate process based on the site, availability of material, and their particular equipment and trained shotcrete crews.
UndergroundQ We have a question about the shotcrete setup strength for the exclusion zone in underground shotcrete work. New York and others are requiring a shotcrete exclusion zone (an area excluded from personnel) based upon either time or strength. The UK asks for an engineered approach to this minimum strength. Do you have any information on this?
The minimum strength for safety must be established by the designer based on the specific structural and geotechnical aspects of the project. The minimum strength value may also be influenced on whether using fiber-reinforced shotcrete or plain shotcrete. The early-age strength was tested with a Meyco Needle Penetrometer after creating a time (early) strength curve with minimum tests at 10 minutes and 30 minutes with Needle Penetrometer; then at 3 hours and 6 hours with Hilti Studs; then at 1, 7, and 28 days with cores. Thus, the curve was calibrated for the specific mixture and environmental conditions. It was also useful to identify when early strength (and potentially long-term strength) was lacking.
RepairUndergroundStructuralQ We have a wet-mix shotcrete steel fiber overhead application progressing in our state. The question is about the use of a steel trowel finish, as opposed to say a magnesium or wood float finish. In the ASA Shotcrete Inspector seminar, it was stated that a steel trowel is less durable, reduces freeze-thaw resistance and shows cracking more proximately. As this particular application is overhead and, in a tunnel, there is not as much of a concern with water infiltration and the associated freeze-thaw exposure. We usually don’t allow steel trowels for flat work, due to deicing salts, but that concern wouldn’t apply here. My superintendent has asked me to reach out to you to see if you might have any further detailed advice on this type of application. Construction is wanting a smooth finish and looks do matter here as it is a high-profile project. If the DOT were to allow the steel trowel for finishing, what would be your concerns or suggestions to this approach?
Freeze-thaw deterioration is dependent on the concrete being saturated in multiple freezing/thawing cycles. In an overhead application, where water can’t stand on the surface, the concrete can’t be saturated unless water permeates through from the upper surface. And with good quality concrete in the tunnel, water shouldn’t permeate through, so it should be functionally watertight. As a result, freeze-thaw likely isn’t a critical durability issue.
A steel trowel finish does require extra working of the surface and would require the contractor to be very attentive to the proper time to obtain the finish yet not overly disturb the fresh concrete. Gravity is working against the overhead concrete staying in place.
Having a smooth steel trowel finish would make minor shrinkage cracks more noticeable. However, in the tunnel without exposure to sunlight or much wind exposure, and with proper attention to curing, perhaps surface cracking will be minimal.
UndergroundQ We will be shotcreting the inside of a tunnel entrance. The plan is to apply 3 ft (0.9 m) of shotcrete on the walls and ceiling to support a large section of limestone rock 22 ft (6.7 m) high, 30 ft (9.1 m) wide, and 20 ft (6.1 m) deep above the tunnel at the entrance that has moved and is wedged and supported with an existing steel structure. We would like to test the shotcrete and are wondering what type and quantity of tests you recommend and which testing labs are in our area that would be able to conduct the testing?
The article “Shotcrete Testing—Who, Why, When, and How” in ASA’s Summer 2011 issue of Shotcrete magazine should help answer most of your questions on testing of shotcrete. ACI 506R, “Guide to Shotcrete,” and ACI 506.2, “Specification for Shotcrete,” also have helpful information on shotcrete testing. Most competent testing labs should be able to test the compressive strength of cores extracted from shotcrete panels or sections, as they are very similar to concrete cylinder tests. If conducting more advanced testing, you may want to consider selecting a lab experienced with shotcrete.
UndergroundQ We would like to place 4 in. (100 mm) thick shotcrete reinforced with welded wire reinforcement and anchoring bolts in a water pressure tunnel. The water velocity would be between 10 and 16.4 ft/s (3 and 5 m/s). We would like to know if there is a possibility of erosion or cavitation of the shotcrete at this range of velocity. It is mentioned in our concrete manual that cavitation and destructive erosion begin when water velocities reach about 40 ft/s (12 m/s). Because the roughness of the shotcrete surface is higher than the concrete surface, is erosion more likely to occur? Do you know what may be the maximum water velocity acceptable for reinforced shotcrete?
Shotcrete is a method of placing concrete and the surface finish can be as smooth as that of cast concrete. Even with a nozzle finish, shotcrete erosion or cavitation should not be an issue at the stated velocities. Examples of smooth shotcrete surfaces can be found in many Shotcrete magazine articles and in particular (“Restoring the Century-Old Wachusett Aqueduct”).