To prepare shotcrete, various materials must be mixed in specific proportions and then sprayed onto the intended surface.
There are many possible mix ratios, but the most common is a 1:3 ratio of cement to sand; other materials may be added depending on the specific situation.
The mix ratio depends on the type of shotcrete being used. For example, if using rapid-setting shotcrete, you would typically need approximately two parts sand and one part cement for every 10 liters of water.
Other ingredients may include:
An accelerator to speed up the curing process.
A bonding agent to enhance adhesion.
A water-reducing agent to facilitate rapid moisture evaporation at the job site.
High-efficiency water reducers (superplasticizers) to reduce the amount of water required in the mix.
A stabilizer to increase the strength of the shotcrete mixture.
When applying shotcrete, the first layer should be thick enough to cover the entire surface, and it must cure for at least 24 hours before the second layer is applied. The final layer should be thinner to allow for faster curing, thereby preventing cracking and warping upon complete drying.
The shotcrete curing process
The shotcrete curing process can take up to 72 hours, depending on the thickness of the concrete and the temperature.
Curing time depends on the curing agent used. Water is the most common curing agent; it requires at least 4 hours to achieve full strength. Calcium chloride or magnesium chloride solutions require 12 hours to reach a strength sufficient to support foot traffic.
The shotcrete curing process consists of three stages: wet curing, air curing, and hardening. Wet curing takes place after the concrete has been sprayed onto the surface; depending on the curing agent used, this stage can last up to 24 hours.
Air curing is the process where the wet concrete dries and hardens in the air. This stage lasts anywhere from two days to three weeks, depending on temperature and humidity.
Once air drying is complete, the hardening process continues over time. It can take months or even years for the concrete to reach its full strength and hardness.
Essential Tools for Shotcrete
Many of the tools and materials used for shotcrete are also employed in conventional concrete construction. However, the shotcrete process itself requires specialized spraying equipment.
The machinery used by shotcrete crews includes combination mixer-pumps, shotcrete machines, metering pumps, and mixers. They also require tools such as trowels, scrapers, and moisture barriers.