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%.
I have a client who may be interested in using shotcrete for walls in a radiosurgery unit requiring radiation shielding. Could you please tell me the typical density of shotcrete?
Shotcrete made with normalweight aggregates will have a density of approximately 145 lb/ft3 (2323 kg/m3).
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.
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.
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 am looking for design information for shotcreting a steel sheet pile wall to create a composite structure for a lift station wet well. I can design the sheet piling, which would be driven into the ground in a plan circle of 12 feet diameter, followed by excavation. I need to know the practicality of then applying a layer of shotcrete, primarily as a means of sealing the joints of the sheet pile, protecting the sheet pile from the wastewater, and providing additional wall strength. The lift station will be above the water table during construction, but would be periodically below the water table under groundwater conditions.
There are four common types of sheet pile sealing: 1.) all seams were welded to keep the ground water from seeping in, 2.) the sheet pile surface was sandblasted for bonding, 3.) wire mesh was tack welded to the sheet pile and 4.) rebar was tack welded to the sheet piles. This was done prior to the shotcrete layer. In each case the shotcrete is used as a coating to keep the water from touching the piles and in the third and fourth examples, it is used as a structural coating as well.