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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StructuralQ 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.
StructuralQ We have a project in the Los Angeles, CA, area that we have designed as poured-in-place concrete. The contractor would like to shotcrete all basement walls. However, we have columns that are integrated with the wall. A City of Los Angeles technical bulletin does not address shotcrete columns in a wall where the bar spacing varies from the wall. Do you have any experience with this issue and is there a way to shotcrete both the wall and the column?
There have been many prior projects in the city of Los Angeles where columns and pilasters are incorporated into perimeter basement walls. This would likely require a preconstruction mockup panel and might require a waiver from the City Engineering Department. It is extremely important to ensure that the work is done by a qualified and experienced shotcrete specialty contractor who has documented experience doing similar work.
StructuralQ We have a project that calls for new 6 in. concrete shearwalls formed and placed against the existing structure from the basement up to the fourth floor to enable an additional seven floors to be added to the structure. Our engineer has suggested that the new shearwalls be constructed using shotcrete. We are not familiar with using this system for structural applications. Most of the information we have gotten relates to using shotcrete for swimming pools and cosmetic applications. What advice can you provide?
The use of shotcrete for structural applications has been documented in numerous articles in Shotcrete magazine, Concrete International, and other publications. The key is to find a shotcrete contractor experienced in structural applications. Investigate the contractor’s project history to determine his/her experience. A contractor experienced in this type of structural enhancement will be most helpful in achieving the desired result in an economical and timely manner.
StructuralQ 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.
PoolStructuralQ We have a two-story shotcrete wall enclosing an indoor community pool. We are specifying a board-form finish for the interior and the exterior will have a parge finish coat. Are there any issues with the consistent moisture from the pool that should be addressed in the concrete mixture or topical sealant? How should we deal with the exterior versus interior finishes in regards to water intrusion protection and allowing the green concrete to “dry out” over time?
Shotcrete is a method of placing concrete and the characteristics of shotcrete are those of cast concrete. Although the enclosed swimming pool will increase the interior humidity, the high humidity should have no detrimental effects on the exposed shotcrete, and may even be beneficial in reducing long-term drying shrinkage of the wall.
Both cast-in-place and shotcreted concrete are commonly used for construction of water tanks with constant exposure to water under significant hydraulic pressure. Using good construction techniques with good-quality concrete to build the tank’s walls produces walls with no moisture evident on the exterior face of the tanks. Simply having a high-humidity atmosphere is a much less severe exposure and should not result in interior air moisture being transmitted into and through the shotcrete wall. Any coatings considered for aesthetics should follow the manufacturer’s recommendations for drying time of concrete before application. If there is a concern about the permeability of the shotcrete wall, a premium shotcrete mixture including silica fume might prevent some issues on this application.StructuralQ We have a unique situation where we need to apply shotcrete around a steel plate that is surrounding a beam supporting a floor. Can you provide any UL listings for applying shotcrete to a steel beam, column, or plate?
UL designs are typically for the hourly fire proofing ratings on structural steel members such as I-beams, wide flange beams, and vessel skirts. The beams and columns are tested for specific fireproofing products, beam sizes, and configurations. The thickness of the steel and other considerations factor in the evaluation; therefore, there is no blanket UL design number that you can use for steel plate. You can get guidance on the cover needed for different fire ratings in ACI 216.1-97/TMS 0216.1-97, “Standard Method for Determining Fire Resistance of Concrete and Masonry, Construction Assemblies.”
Remember that shotcrete is a process for applying concrete. You may also consider looking for a similar concrete UL design and submit it for consideration. Applying the shotcrete at a greater thickness to compensate for any variances should be proposed and presented to engineer or the owner for consideration.StructuralQ We have a vertical shaft that is (right now) 70 ft (21.3 m) deep and we do blasting every 5 ft (1.5 m) after applying shotcrete to the vertical surface for protection. My concern is that if we have less than 48 hours between successive blasting, is it allowable? How does one measure if the shotcrete reaches the required percentage of strength?
The best guidance on this subject can be found in ACI 506.5, “Guide for Specifying Underground Shotcrete” (available through the ASA Bookstore), and some articles from past issues of Shotcrete magazine might be of interest:
- “Shotcrete Spraying Machines for Immediate Support in Tunnels”
- “Slope Stabilization in an Open Pit Mine”
- “Where Are We Now with Sprayed Concrete Lining in Tunnels?”
- “The Danger of Fallouts in Overhead Shooting”
- “Incline Tunnel—S&S Quarries, Inc.”
- “Reaching 20 MPa (2900 psi) in 2 Hours is Possible”
- “Rapid-Setting Cement in Shotcrete”
With properly qualified nozzlemen, a good shotcrete mixture, and high-quality accelerator added at the nozzle, the re-entry time can be minimal—normally 24 hours.
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.
StructuralQ We have an existing cut slope approximately 328 ft (100 m) high (3:1 vertical:horizontal) with cut benches and need to apply shotcrete onto the slope surface at a height of approximately 230 ft (70 m) from the road level. Is it possible to reasonably transfer and apply shotcrete mixture at such a height from the road level? What type of transfer hoses and equipment is preferable? Is dry-mix or wet-mix shotcrete preferable?
Yes, either dry-mix or wet-mix process can and have been used at this height. Because either process can be used in these conditions, you should use an experienced shotcrete contractor who will pick the best method based on their firm’s shotcrete capabilities. Factors such as the shotcrete contractor’s specific shotcrete equipment, material availability, site constraints, remoteness of the location, and crew experience will influence their choice.
StructuralQ We have demolished two radioactively contaminated buildings down to their concrete slabs. One of the slabs has a concrete pit that is 26 ft (8 m) deep. The slabs have not been removed because the soil beneath the slabs is contaminated and we’re using the slabs as a cover to protect the spread of contamination in the soil until the soil remediation begins. We’d like to use shotcrete to temporarily (up to 5 years) fix the contamination on the slabs and the 5 ft (1.5 m) area surrounding them. The questions we have are: 1) Will shotcrete adhere to the concrete slabs and pit walls for up to 5 years without special preparations? (Portions of the radioactively contaminated concrete are painted and it is dirty from demolition activities); and 2) What is the minimum thickness of shotcrete needed to last for 5 years in this type of application? We do not want to use any wire or fabric mesh as it would require personnel to work in a radiologically controlled environment to install the material.
Shotcrete, like concrete, likely will not adhere to surfaces that are painted and dirty from the demolition activities. There should be no issue to the time durability. Shotcrete is pneumatically placed concrete and has great long-term durability characteristics if placed properly.Shotcrete has been installed in many adverse environments at a thickness of 2 in. (50 mm) with fibrous reinforcement and provided a long service life. Many irrigation districts line their canals with shotcrete and it has provided decades of great service in freeze-thaw exposures.