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?

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.

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?

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.

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.

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.

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?

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.

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?

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%.

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?

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.

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?

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.

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?

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.

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.

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.