Our firm is preparing to use the shotcrete method on a project for the first time. What type of prequalification work should we be specifying?

There are four basic reasons to require preconstruction qualification testing:

  1. To prove the suitability of the fresh shotcrete mixture design for the intended use;
  2. To verify the proposed mixture will produce the required strength and any other specified hardened shotcrete properties;
  3. To prove the ability of the nozzleman (and blowpipe oper­ator, if required) to place dense, homogeneous shotcrete completely encasing the reinforcing steel under field conditions; and
  4. To prove the desired surface finish can be achieved.

This testing must be discussed in detail with the shotcrete contractor in advance with a clear understanding of the expected outcomes and the process for any required adjustments. Requiring ACI Nozzleman certification is an important requirement in screening for qualified nozzle operators. However, it is not a guarantee that the nozzleman has applied shotcrete under the same conditions to be encountered on your project. Therefore, a preconstruction plan is an important part of critical projects. Other prequalification testing may be necessary depending on the nature of the work.

I will be shotcreting an existing structure that has some diesel fuel and oil stains on the existing concrete. How should I treat them before shotcreting?

There are a number of ways to treat these stains. Successful treatment will depend on the specific material in the stain and the depth of the stain. The first step would be to try to draw out the material from the surface by applying a poultice of finely ground kitty litter, cement powder, or talc and allow the surface to dry. Repeat this application if necessary.

Next, try a scrubbing a nominally dry detergent powder into the surface. Allow the powder to dry and rinse off the surface. Follow this treatment with a liquid detergent scrubbed with a bristle brush into the surface. Allow the liquid to remain in the surface for 1 to 2 days, then rinse thoroughly. Should the staining persist, you may want to try a proprietary stain remover specifically intended for use on concrete.

Muratic acid is also an option. However, muratic acid can have deleterious affects on the concrete if not thoroughly removed. Because of its potential to attack concrete aggregates and mortar, along with the hazards inherent with applying and removing acid, muratic acid should only be used with the guidance of an experienced consultant. Following a thorough power washing, the surface should be mechanically roughened to ensure proper bond with the shotcrete.

We are having a swimming pool built with shotcrete. Our question is, what is the required curing time for shotcrete prior to exposure to heavy rain? We are trying to plan the shotcrete installation when the weather looks most favorable.

Shotcrete needs to be protected from rain until it obtains its final set, usually 4 or 5 hours. Following final set, it should be wet cured for at least 4 days, preferably 7 days if possible. The exposure to rain would prove beneficial as the rain would assure the presence of moisture for continued curing.

What is the best reference when specifying aggregate gradations for shotcrete projects?

ASTM C 33 contains a variety of aggregate gradations. Gradations recommended for shotcrete applications can be found in ASTM C 1436, Standard Specification for Materials for Shotcrete, or ACI 506, Guide to Shotcrete. Note that ACI 506 includes the caveat that “aggregates failing to comply with gradations shown in Table 2.1 may be used if preconstruction testing proves that they give satisfactory results or if acceptable service records are available.”

Our firm is working as a consultant for a project. We have very little experience with shotcrete. What is the life span for a shotcrete wall?

Shotcrete is a method of concrete placement, not a product. Therefore, concrete placed by the shotcrete method will exhibit the same characteristics as concrete placed by other methods. Mixture designs and proportions for shotcrete are modified for high-velocity placement. The high velocity provides some performance improvements over conventional cast-in-place methods when properly placed.

I am working on repairing some mildly deteriorated walls in a drinking-water treatment plant. There are no chlorides used in the treatment process. I would like to apply a 1 in.-thick shotcrete layer over the existing concrete utilizing a mix containing silica fume, which will achieve a compressive strength of 5000 psi at 28 days. I am having difficulty formulating a mix to meet those requirements that also has a water soluble chloride content of less than 0.10 % chloride ion concentration by mass of cement. I cannot get the chloride ion concentration below 0.15%. What adjustments can I make to get to my goal of 0.10% or less?

There are areas that have no problem getting values lower than the most stringent ACI requirement of 0.06% for prestressed concrete with no special adjustments. It would be prudent to test each of the proposed shotcrete constituents to determine their soluble chloride ion content. The most likely suspects are the aggregate and water sources. Typically portland cement and silica fume would contribute little, if any, detectable chloride ions. Assuming this would be a dry-process application, the only admixture other than the silica fume might be an air entraining agent, which would not provide any chloride ions. This leaves only the aggregates and water as the sources. At a minimum, the aggregates and water should be tested by a qualified laboratory for soluble chloride ion content. Alternate sources of aggregates and water may be required based on the laboratory results.

The Park District Department of our city is in the process of designing a new swimming pool. One of the prospective bidders made a presentation in which they said they would use shotcrete instead of conventional cast in place concrete. Their design is to use 6 in.-thick walls instead of the 12 in.-thick walls as proposed for the cast in place design. They claim that 6 in. of shotcrete is as strong as 12 in. of formed concrete. Is this a true statement?

If this statement was true, there would be a lot more shotcrete projects! The truth is that shotcrete is a method of concrete placement, not a special material. The materials, mix designs, and mix proportions may vary between the shotcrete method and the conventional concrete form and pour method, but the thickness and reinforcing of the structure will be very similar.

There is a subtle difference between the two methods that might affect thickness requirements. Shotcrete is generally placed directly onto the undisturbed soil, joining with the soil to provide the shell for the pool. To use the form and pour method, over-excavation would be required to accommodate two-sided forming. The walls would then have to withstand the forces of backfilling. This may result in a thicker wall requirement. The final decision regarding wall thickness, however, should be made by a structural engineer.

Shotcrete is widely used for swimming pool construction. In some areas it is virtually the only method used. Successful shotcrete swimming pool construction is a result of having an appropriate design, selecting a qualified contractor with certified nozzlemen, selecting appropriate materials and shotcrete mixture design, and following industry recommendations for placing, finishing, and curing.