Surface preparation is a critical operation. The substrate must be prepared properly. All deteriorated concrete must be removed. This is generally accomplished with light-duty chipping hammers, scarifiers, or scabblers. The remaining concrete is then sandblasted or waterblasted to remove the concrete “bruised” by the initial removal operation. The objective is to create a clean, sound surface with the proper surface roughness to receive the shotcrete.
After the surface preparation, the substrate must be saturated with clean water and then allowed to dry to a saturated, surface-dry condition immediately prior to shotcreting. Shotcrete should not be applied to a bone-dry surface as the substrate will absorb water in the shotcrete mixture intended for hydration of the cement. Also, a bone-dry surface will tend to allow plastic and drying shrinkage cracks to form. Conversely, a surface that is wet at the time of shotcreting will result in a high water-cement ratio (w/c) at the interface between the substrate and the shotcrete. High w/c at the interface will result in significantly lower bond strengths.
As with all concrete, proper curing and protection is critical. Failure to cure properly will result in lower shotcrete strengths and may cause some delaminations if drying shrinkage causes stresses that exceed early bond strength. Plastic shrinkage cracking and “crazing” may also result from failure to cure and protect properly. Moist curing is the preferred method of curing. If moist curing is not feasible, membrane curing compounds may be used.
Finally, be sure the nozzleman who will be applying shotcrete on your project is certified by the American Concrete Institute (ACI). Certified nozzlemen have been trained and tested on the requirements for proper shotcrete application. Insisting on this certification dramatically increases the probability that you will get the desired results.