We are currently working on a Request for Deviation to use shotcrete in lieu of cast-in-place concrete. The engineer is requesting additional information/confirmation. The application locations are structural, using No. 6 and No. 8 reinforcing bars on 1.5 ft (0.5 m) thick walls approximately 40 ft (12.2 m) high. The engineer’s comments refer to detailing construction joints, curing, and plastic shrinkage gaps (work done in July). We have also requested a slump to be reduced to 2 ± 1 in. (51 ± 25 mm) and the use of 3/8 in. (9.5 mm) aggregate. What methods would you suggest to address each issue?

The project as described sounds very feasible for a structural shotcrete application. As we understand, the concerns are:

  1. Detailing construction joints—Please refer to ACI 506R, “Guide to Shotcrete,” Paragraph 5.7, Joints. Typically, shotcrete joints are beveled to increase the surface area of the bonding surface and reduce the likelihood of trapping rebound. Other considerations for construction joints should follow the principles of cast-in-place concrete. Shotcrete is a method of placing concrete.
  2. Curing—Shotcrete is concrete consisting of smaller aggregates and generally higher cement content. Good curing practices should be followed as they should be with cast-in-place concrete. Considerations should include the temperature and humidity when evaluating a curing program. High temperatures with low humidity will require significantly more effort than high temperature with high humidity. The key is to ensure that sufficient moisture is available to hydrate the cement during the curing period.
  3. Plastic shrinkage gaps or cracking—The shrinkage characteristics of shotcrete are similar to cast-in-place concrete. Shotcrete is composed of smaller particles and higher cement but generally places at a low water-cement ratio (w/c) or less than 0.45. Shotcrete is somewhat more prone to plastic shrinkage cracking due to the surface not being protected by a form in its early stages. If the finished surface is subjected to high ambient temperatures, low humidity, or high winds, it will tend to dry quickly on the surface and exhibit more plastic shrinkage cracking. In these environmental conditions, fogging of the exposed shotcrete surfaces soon after shotcreting may help to reduce or eliminate the plastic shrinkage cracks. Plastic shrinkage cracks are generally superficial in nature and can be repaired if necessary.
  4. Slump to be reduced to 2 ± 1 in. (51 ± 25 mm)—This is a good range if measured and treated properly. It is important to ensure that the shotcrete material has enough slump at the nozzle to properly encapsulate the reinforcing steel and is stiff enough to stay in place without sloughing or sagging. The slump at the nozzle is far more relevant than the slump at the pump.

The important factors influenced by slump are maintaining the proper water-cementitious material ratio (w/cm) and consistency at the nozzle to ensure good placement. The most important consideration is to ensure that you have an experienced shotcrete contractor who has a history of success with similar projects with respect to the size and complexity of the installation. You can locate shotcrete contractors on the ASA online Buyers Guide at shotcrete.org.

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.

I have reviewed ACI 506R, “Guide to Shotcrete,” and 506.5R, “Guide for Specifying Underground Shotcrete,” but was unable to find specific criteria pertaining to shotcrete protection for reinforcing steel. Would the clear cover then be based on ACI 318 Section 7.7.1 for cast-in-place concrete? For underground structures, would 3 in. (76.2 mm) of clear cover from ground be required?

Shotcrete is concrete, and therefore if designing structures based on the ACI 318 Code, cover for conventional reinforcing steel should be those suggested in ACI 318 for concrete against ground. If the shotcrete is a “temporary” support, with further placement of “final” support, then these requirements do not apply.

Is it feasible and economical to construct floodwalls approximately 5 ft (1.5 m) high with shotcrete?

Yes, it is feasible and economical to construct structural walls such as a 5 ft (1.5 m) high floodwall. Shotcrete is a method of placing concrete and has similar, if not identical, properties after placement. As you can imagine, shotcrete needs to be shot against something such as a one-sided form, gabion baskets, earth, expanded metal lath, or just about any structurally sound thing you can think of. The economy of the system is dependent upon the site conditions and the ingenuity of the contractor. An example of a similar structure is on the east side of I-880, south of Dixon Landing Road in Milpitas, CA. This project, a flood control channel, involved trapezoidal channel sections, vertical wall sections, and a combination of sloped walls with a vertical extension. If you have further interest, you should contact an organization with experience in this area. An excellent source is the directory of Corporate Members in the ASA Buyers Guide.

I am lining a below-ground conical shaped excavation with shotcrete. Dimensions are approximately 90 ft (27.4 m) diameter by 45 ft (13.7 m) depth. Sand will be moved in and out of the container daily. Temperature range is 590 to 740 °F (310 to 393 °C). Can you tell me if a mixture is available that can meet the following specific conditions: Withstand the temperature ranges noted above without spalling, cracking, etc.; and Resist abrasion assuming hot sand is flowing over the surface area daily?

You certainly have adverse conditions to work with! There are products on the market based on calcium aluminate cements that will tolerate the temperatures you mention and are durable. These products can be placed using the shotcrete process. A list of companies who supply this product can be found at www.shotcreteweb.wpengine.com/pages/products-services-information/buyers-guide/.

I have an unfinished (dirt) basement with a stacked stone and mortar foundation. Can I shotcrete the existing dirt walls and floor with shotcrete MS (micro silica enhanced) and have it adhere to the dirt portion of the basement? If so, what method would be best?

Shotcrete would work well for the overlay of the walls. In most cases, floors are placed by a conventional cast-in-place method. Either the wet- or dry-mix procedure would work well for the walls. To ensure good bond of the shotcrete to the walls, the walls should be cleaned and prepared to assure that the shotcrete is bonding to sound material rather than contaminates such as dirt or weathered material. You might also want to consider reinforcing the walls, but you should consult with an engineer on how to do this and with what material. If you were to use the wet-mix process, you could use the same equipment to place the floor as you are using for the walls.

We would like to get approval to use shotcrete on the perimeter walls of an existing laboratory building. We would be shooting against a waterproofing membrane and shoring lagging. The project engineer is concerned that the shotcrete will damage the membrane, resulting in leaking into the occupied space. Are there any examples where this type of shotcrete placement has been used?

This is a commonly used technique in the Western U.S. and Canada, and has been used successfully from Stanley Hall at the University of California at Berkeley, Berkeley, CA, to the Baltimore Hilton Convention Center near Camden Yards, Baltimore, MD. There are a number of suppliers of waterproofing materials to choose from for this application. In selecting a supplier, be sure there is field service available to inspect the project before placement of the shotcrete.

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.