Question: Is shotcrete as strong as cast-in-place concrete?

Answer: Well, first off, shotcrete IS concrete; it's just the method of application that’s different. A shotcrete mixture likely would exceed the compressive strength of most mixtures used for placed walls because the application of shotcrete requires a much lower water-cementitious material ratio than commonly found in wall mixtures. A shotcrete mixture will have a water-cementitious material ratio of approximately 0.50, yielding a compressive strength of about 4000 psi at 28 days. Poured wall mixtures have ratios of approximately 0.70 and compressive strengths of 2500 to 3000 psi. The lower water-cementitious material ratios of shotcrete mixtures produce other benefits such as reduced shrinkage and lower permeability. Additionally, the greater compaction of shotcrete achieved through the velocity of placement improves compressive strength and durability.

Question: What is the most important factor for achieving excellent shotcrete results?

Answer: Proper placement is the most important element in achieving good shotcrete results. Most defects that occur in shotcrete are due to poor placement. Shotcrete success depends largely on the skill and actions of the nozzleman. The nozzleman’s goal is to achieve adequate compaction and good encasement of the reinforcement (if present) with no entrapped rebound or hardened overspray. For this reason, it is important to require that the nozzleman is ACI certified for the application. There are specific certifications for both wet and dry processes as well as vertical and overhead applications. If the nozzleman is certified, the probability that you will get the desired results is significantly increased. All Torrent nozzlemen are ACI certified.

Question: Are there special preparation considerations when utilizing shotcrete?

Answer: 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 water-blasted 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 application. 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 application 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 are critical. Failure to cure properly will result in lower shotcrete strengths and may cause some delamination 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). All Torrent nozzlemen are ACI certified and 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.

Question: Is shotcrete durable to the long-term?

Answer: Shotcreting is a process of installing concrete at high velocity utilizing compressed air. Because the concrete is installed at a high velocity, it will have a higher density than conventional concrete in most cases. The increased density will provide reduced permeability and higher durability. As a mixture becomes denser, the transmission of fluids through the mixture becomes more difficult. Increasing the density by using products like silica fume, slag cement, and fly ash that is often included in the shotcrete concrete mixture dramatically decrease permeability.

Question: What is the difference between wet and dry shotcrete?

Answer: In a wet application, all ingredients, including water, are thoroughly mixed and introduced into the delivery equipment. Wet material is pumped to the nozzle where compressed air is added to provide high velocity for placement and consolidation of the material onto the receiving surface.

In a dry application, pre-blended dry or damp materials are placed into the delivery equipment. Compressed air conveys material through a hose at high velocity to the nozzle, where water is added. The material is consolidated on the receiving surface by the high-impact velocity.

Although both wet and dry shotcrete have specific benefits. Wet shotcrete tends to produce significantly less waste byproduct and is the favoured application method of Torrent.

Question: Isn’t there a lot of excess waste in shotcrete applications?

Answer: All concrete application technologies produce a certain amount of waste. As mentioned previously, wet shotcrete applications tend to produce significantly less waste than dry shotcrete. Properly applied wet shotcrete can reduce waste levels to amounts approaching that of CIP. An important factor to consider in waste management that is often overlooked is what actually happens to the waste byproduct after application. Unfortunately, many contractors treat the waste as just that – waste that has no subsequent use and must be disposed of. Torrent, however, recycles a very high percentage of waste into reusable custom blocks.  The blocks we make are high quality due to the premium mix used in shotcrete applications and can be used for a wide variety of purposes.