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 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.

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?

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.

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?

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.

Do you have any publications on shotcrete curing, specifically in tunneling? How is shotcrete cured in tunnel constructions with the temperature and moisture problems?

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.

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?

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.

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.)

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?

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.