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.
User agreement: The answers provided to submitted questions are 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.
If you are unable to find what you are looking for in the archive, then submit a new technical question.
UndergroundInternationalGeneralQ I am a TBM Tunnel Engineer from India, and I was looking for information on the applicable compressed air pressure range required for a wet-mix shotcrete application (small shotcrete pump-capacity 7 CU.M/Hr) hand spraying with a 30m hose for a better-compacted mix. I would kindly request you to please send me information on the pressure range to be expected for good quality shotcrete placement of the concrete mix on the rock substrate in NATM Tunneling.
Wet-mix shotcrete depends on air flow at the nozzle to accelerate the concrete to 60 to 80 mph (100 kph to 130 kph). Most air compressors produce their air flow capacity at 100 to 120 psi (7 to 8.4 kg/cm2) at the compressor. However, depending on the size length and couplings in the air hose, there may significant pressure drops when the air reaches the nozzle. Here’s what ACI 506R-16 Guide to Shotcrete Section 4.4.2 states for wet-mix: “The recommended ft3/min (m3/min) needed for the wet-mix process is between 200 to 400 ft3/min (5.7 to 11.3 m3/min) air volume at 100 psi (7 bar). Higher air volume capacities are needed for higher volume and higher-velocity shotcrete applications. If a blowpipe is to be used during the shooting process, more air will be required to run both operations simultaneously. Conducting a test during the preconstruction testing phase using a blowpipe while gunning the wet-mix material will indicate if the air compressor has enough air volume capacity to perform both tasks at the same time. Long, small-diameter lines may not provide sufficient air volume capacity, even with a large air compressor. Test and consider increasing the size of the air line.” Though there is no direct guidance for air pressure at the wet-mix nozzle you may consider the guidance for dry-mix air pressure in ACI 506R Section 4.4.1: “The operating air volume (ft3/min [m3/min]) drives the material from the gun into the hose, and the air pressure is measured at the material outlet or air inlet on the gun. The operating pressure varies directly with the hose length, the density of the material mixture, the height of the nozzle above the gun, and the number of hose bends. Experience has shown that operating pressures should not be less than 60 psi (4 bar) when 100 ft (30 m) or less of material hose is used, and the pressure should be increased 5 psi (0.34 bar) for each additional 50 ft (15 m) of hose and 5 psi (0.34 bar) for each additional 25 ft (7.5 m) the nozzle is above the gun.” The minimum 60 psi (4 bar) necessary for dry-mix could be applied to the wet-mix air supply as the velocity created by the air flow is similar.InternationalQ Shotcrete is very rarely used in Belize. I am building a new home and purchased a shotcrete machine to apply exterior finishes to houses, and most importantly to my new pool. A very good contractor here (who has built many hand-plastered pools) has told me that we cannot use shotcrete in Belize because of the type of sand that we have. Is this true? Can the consistency of the sand make it impossible to use shotcrete?
Shotcrete is a placement method for concrete. If the sand can be used for concrete, it should be acceptable for shotcrete placement. To some extent, the type of shotcrete equipment may make a difference. If shooting wet-mix shotcrete, the concrete mixture must be able to be pumped. For pumpability, we do recommend a smooth gradation of the fine aggregate. If shooting dry-mix shotcrete, the sand gradation has much less effect, and you should be able to shoot most any sand in your concrete mixture.
InternationalQ We are a shotcrete contractor in Gold Coast, Australia. We have noticed that in the United States, you use different types of tools for cutting the shotcrete. How do you maintain a plum wall with the shotcrete rods? When a project requires a smooth finish or steel trowel finish, what are the tools and processes that are typically used? Finally, for a structural wall, what is the typical psi (MPa) and size of aggregate used?
The face or surface of shotcrete walls as described are typically established with ground wires or screeds, which assist the person using the shotcrete rod in cutting the wall to the proper plane. The tools typically used to achieve a troweled surface are the shotcrete rod, wood floats, and steel trowels. Typically, shotcrete walls are a minimum of 4000 psi (27.6 MPa) 28-day compressive strength and the aggregate varies from sand only to a blend of sand and 0.375 to 0.5 in. (9.6 to 13 mm) aggregate. Consult our Shotcrete magazine archives for examples at www.shotcreteweb.wpengine.com.
InternationalQ We are constructing a new custom roundabout with water running through the bridges on the Coast of Zintan, Tripoli, Libya, using a three-dimensional (3-D) panel system. It’s basically a system with an expanded polystyrene (EPS) panel with a wire mesh and shotcrete on both sides. Because of the heat, sea salt, and high humidity of the region, we are looking for a mixture formula for a waterproof shotcrete for the exterior coating with the right aggregate size to help prevent moisture migration to the interior and prevent cracks. Do you have any recommendations for the shotcrete?
If looking for a low-permeability concrete mixture for shotcrete placement, you should consider using supplementary cementitious materials (SCMs) such as silica fume, fly ash, or slag to reduce the permeability. You should also be sure to require a minimum 4000 psi 28-day compressive strength to assure good paste content and the ability to fully encase your reinforcement. The addition of microfibers and early wetting of the finished shotcrete surface will help to reduce the potential for early-age plastic shrinkage cracking. Proper curing for at least 7 days is also important to help increase strength gain and reduce the potential for drying shrinkage cracking. You can consult our Buyer’s Guide to locate our corporate members, who may consult with you on the mixture design. However, please be aware that many of the panel systems with an EPS core don’t use high-velocity (60 to 80 mph) shotcrete for consolidation and compaction of the sprayed concrete, but use a low-velocity sprayed mortar (LVSM). Because LVSM doesn’t have the compaction of shotcrete impact, it depends on a more sophisticated and expensive cementitious mixture, often with a latex or other admixture to improve adhesion and reduce permeability. You’ll need to consult with the material supplier of the LVSM product to verify the permeability of their in-place product.