I have been asked to come up with a 5000 psi (35 MPa) in 24 hours shotcrete mixture, using cement, fly ash, silica fume, and fine aggregate. I need some advice on a mixture.

Design of a concrete mixture to be placed by the wet-mix shotcrete method is essentially the same as normal cast-in-place concrete mix design. The major differences with shotcrete mixtures are:

  • The maximum coarse aggregate size is generally limited to about 3/8 in. (9.5 mm);
  • They use a fairly low water-cementitious material ratio (w/cm) and slump to allow shooting on vertical surfaces without sloughing;
  • The potential to use an accelerator that can be added at the nozzle; and
  • The pumpability is an important workability characteristic.

Since you desire a high-early-strength mixture, using fly ash as a supplemental cementitious material (SCM) wouldn’t be recommended because it slows set and strength gain at early ages. Microsilica may be beneficial for early strength gain. Consideration should be given to using accelerator added at the nozzle. There is some guidance on concrete mixture design in ACI 506R-05, “Guide to Shotcrete”; however, because local materials (aggregates, cements, SCMs) can vary significantly, you should consult with an engineer or concrete testing laboratory familiar with shotcrete to produce and test a mixture design to meet your requirements.

I have a customer who would like to place 2 in. (51 mm) of shotcrete onto our geotextile canal liner, which has been used for many years with 2 to 4 in. (51 to 102 mm) of shotcrete. In all of these previous projects, contraction joints were installed. For this project, the customer is asking whether this is an absolute requirement, as the geocomposite canal liner beneath is the water containment component. Does it make a difference in terms of cracking and joints whether the shotcrete is 2 or 4 in. (51 or 102 mm) thick? What is the typical finishing that is done on canal projects?

Long expanses of concrete canal lining exposed to the sun and weather would experience significant internal tensile drying shrinkage stresses. Regular contraction joints help to relieve the internal tension created by concrete shrinkage. If no contraction joints are provided, shrinkage will still occur and the concrete lining will produce its own contraction joints, better known as “cracks.” Unfortunately, the resulting cracking will be random and can vary significantly in size and length. Thus, contraction joints are a good approach to help induce cracking at regular, controlled locations. If the client doesn’t want contraction joints, they need to understand that cracking will be much more extensive and likely more noticeable.
Theoretically, with the same percentage of embedded reinforcement, cracking between a 2 or 4 in. (51 or 102 mm) should not be substantially different. Of course, the 4 in. (102 mm) thick shotcrete section would require twice the concrete material and twice the embedded reinforcement to maintain the same percentage of reinforcement. A 2 in. (51 mm) thick section could have some difficulty in maintaining adequate cover over embedded reinforcing bars. The designers could also consider using fiber-reinforced shotcrete to help control shrinkage and temperature stresses, although fairly high dosages are needed for effective elimination of reinforcing bars. More guidance on fiber-reinforced shotcrete is available in ACI 506.1R-08, “Guide to Fiber-Reinforced Shotcrete”. A 2 in. (51 mm) overlay is absolutely the least possible and 3 or 4 in. (76 or 102 mm) is far more normal in practice.
Canals are generally specified to have a natural gun finish, a rough broom finish, or a light broom finish.

I am a structural engineer and I am supposed to design structures for shotcrete applications. Should I calculate and check its stability by the “working stress method?” Or, could I use the “ultimate limit design?” Are there regulations or specifications about the application of method on ACI? Finally, is elastic coefficient different between normal concrete and shotcrete?

Shotcrete is a method for placing concrete. Thus, the concrete placed by the shotcrete method has the same physical properties as cast concrete with the same mixture proportions. Either working stress or ultimate strength methods used for concrete design are applicable. Local building codes may require a particular design approach.

I am an engineering technologist working on a landslide project where shotcrete had been applied to stabilize the sandstone head scarp at the crest of the slope. The shotcrete was applied in 1998. After a recent inspection, it was noted that the surface of the shotcrete had some cracking in some sections. How can this be repaired? Can the cracks simply be filled with a grout/mortar mixture of some sort or do the cracked sections have to be removed entirely and shotcrete be reapplied?

Shotcrete can and has been used to overlay previously installed shotcrete or concrete that has cracked over time. It would be advisable that you engage an engineer knowledgeable in geotechnical engineering and concrete properties to formalize a solution. It is important that the cause of the cracks be determined and adequate reinforcing be designed to ensure that the cracks do not propagate through the overlaid shotcrete.

We have a 17 mile (28 km) long TBM tunnel for water that will drive our underground powerhouse. Is there a recommended shotcrete surface texture we could use? Our contractor is using 0.31 in. (8 mm) aggregate, but they are getting an undulating surface. Can you provide some clarity as to what we should ask our contractor to try and achieve?

Shotcrete can be applied with many different textures. The nozzle finish shown is very rough, even for a natural gun finish. Nozzle finishes can be done smoother than this. Another technique would be to use a broom to make it smoother after it is shot. Other finishes include wood float, rubber or sponge float, broom, and smooth trowel finishes. There are many examples of finishes shown in articles in Shotcrete magazine.

How might one add fibers to a gunite (dry-mix) application? I have heard of some companies adding them by hand at the base of the auger and others who poured them over their sand and mixed them in with a loader before loading it into the truck. Is there a more efficient way to add them to a dry mixture so that they are distributed evenly throughout?

Many of our members add them by hand at the mixer and have had good success when using an adequate mix time. Another method is to have the mix blended at a bag mix plant with the fibers.

I am an engineer working on a project involving shotcrete and earthwork. The shotcrete that was placed has some expansion cracks, which we expected. I would like to know the best way to repair them. Is there some type of waterproof coating/grout that can be applied between the cracks? Part of the cracks will be continuously under water. The shotcrete is the surfacing material for a diversion ditch at a mine, and we need to recommend some remediation solutions to our client.

There are many products in the marketplace for repairing cracks. Because shotcrete is simply a method for placing concrete, any method for concrete crack repair would be applicable. It would be wise to use a product that filled the cracks and is able to tolerate thermal movement in the future (not a brittle product). Many injectable polyurethane grouts can accomplish this. Surface-applied coatings would need an adequate thickness and elasticity to tolerate moving cracks. We suggest that you contact one of our corporate members who is familiar with your area and get their specific advice. Please refer to “ASA’s Buyers Guide”.

I am interested to know if any shotcrete contractors have shot a magnesium phosphate material (dry-process) before and, if so, could you detail the special requirements necessary in placing such a unique product?

Phosphate-bonded refractory materials were routinely shot in cyclone boilers in the 1970s. These phosphate-bonded materials don’t have a cement bond, but achieve a chemical bond when heat is applied. Without knowing the precise formulation of the mixture and grain sizes involved, we cannot tell you definitively that your specific material can be shotcreted. However, there is a long history of successful past experience with phosphate-bonded refractory materials being shot with the dry-mix process. You may want to consider a field trial before construction to verify your specific mixture works with your dry-mix shotcrete equipment.

We just shot a wet-mix swimming pool for a customer. The shallow end depth starts at 39 in. (991 mm) to the top of the beam and over 10 ft (3 m) linear slopes down to 54 in. (1372 mm). From there we maintain our 1 to 3 ft (0.3 to 0.9 m) slope down to 8 ft (2 m) for the diving end of the pool. The customer would like to raise the entire shallow end pool floor up to the 39 in. (991 mm) depth. We prefer to use wet-mix shotcrete. The overlay would be tapered from the 39 in. (991 mm) start to 15 in. (381 mm) thick at the 54 in. (1372 mm) depth. What would you recommend for this overlay to bond and not “pop loose” or cause crack transfer to pool plaster?

The proposed overlay will be similar to any repair where shotcrete is placed over existing concrete. Proper surface preparation is essential for allowing good bond. Guidance on surface preparation can be found in ACI 506R-05, “Guide to Shotcrete”. It also appears you are suggesting tapering the thickness from 15 to 0 in. (381 to 0 mm). Feathering thickness down to 0 in. (0 mm) is not encouraged, and a minimum thickness should be established. Because the overlay section will be quite thick and experience differential shrinkage from the previously shot material, the overlay will require additional reinforcement to accommodate temperature and shrinkage stresses. You should consult with an engineer experienced in shotcrete design to establish the proper amount of reinforcement. The required reinforcement and cover over the reinforcement will control your minimum overlay thickness.

I have a new construction project where I want to apply shotcrete to cast-in-place concrete columns and an elevated, post-tensioned concrete slab as a finish material. The finished application is intended to be in varying depths from 3 to 12 in. (76 to 305 mm) or more. The desired end result is a smooth, curvilinear, sculptural form. Is this type of application achievable?

Shotcrete can and has been used to increase the size of columns and thicken overhead slabs while providing great-looking linear or curvilinear finishes. Examples of curvilinear finishes can be found in past Shotcrete magazine articles. You can search the Shotcrete magazine archives.