While shotcrete and gunite may not mean much to most people, those in the construction industry will certainly have heard of them before. If you do a Google search, there will be a lot of conflicting information about these terms and what they really are. While they are similar in that they are both forms of concrete, the processes used in to produce each is often misquoted. To help shed some light on the topic, we’ve decided to give you an overview of these terms along with the materials and equipment used in both processes.
Shotcrete and gunite are both essentially concrete that is pneumatically sprayed through the nozzle of a hose at high velocity onto a reinforced surface. Used mainly as a construction technique, shotcrete and gunite are both revolutionary processes in the application of concrete.
Gunite originated before shotcrete in the early 1900’s by an American taxidermist, Carl Akely. Mr Akely devised a way of shooting the dry materials of concrete through a hose using compressed air, while water was injected at the nozzle as the material was released. While he invented the idea for his own work, namely creating model animal replications, the process was first used commercially to repair the Field Columbian Museum in Chicago. In 1911, Mr Akely patented his idea as the ‘cement gun’, being the equipment used in the process, and shortly afterwards patented the materials used as ‘gunite’.
Gunite (dry mix process)
The use of gunite is now known as the ‘dry mix’ process, where the dry components of concrete are pushed through a hose at high velocity and mixed with water at the nozzle to complete the application process. This required a skilled nozzleman to control the injection of water so that when the materials hit the receiving surface, they are completely mixed. By the 1950’s however, the ‘wet mix’ process was introduced by using the same technique with ready mix concrete.
Shotcrete (wet mix process)
Shotcrete is essentially gunite, however the way in which the concrete is mixed and applied are quite different. Shotcrete uses the wet mix process, where the pumping equipment hydraulically shoots the wet (ready mix) concrete through the hose, but instead of injecting water at the nozzle, compressed air is used to accelerate the velocity of the materials so it would hit the receiving surface at a greater force. Since the introduction of shotcrete through the wet mix process, the term ‘gunite’ has been slowly faded out and currently ‘shotcrete’ has become the accepted term to describe both the dry and wet mix processes.
Are both processes still used today?
Yes, both the dry mix and wet mix processes are still used in the construction industry today. The dry mix process has evolved significantly and now uses a pre-dampening system so that the crystallisation process begins before the materials are sprayed through the nozzle. This ensures the materials are more evenly mixed which significantly reduces any errors by the nozzleman. Each process has advantages and disadvantages which are outlined in the following:
Dry mix advantages
- Quick dismantling process and cleaning of hoses and equipment
- Materials can be transported to areas that are difficult to access
- Hoses are easier to manoeuvre
- Better at patching and repairing concrete
Dry mix disadvantages
- Large area required to store the concrete materials and CFM compressor
- Controlling the injection of water at the nozzle can be difficult
- More waste
- In some cases, additional labour is required to operate the equipment
Wet mix advantages
- Greater control of the mixing process so the materials are more consistent
- Allows a higher volume of concrete to be poured in each session
- Additional materials can be easily obtained
- Better documentation process
Wet mix disadvantages
- Setting time of the concrete is increased
- Materials can separate within the hose
- Left-over concrete can harden within the hose
- Difficult to get mixed concrete to areas that are hard to access
It’s clear to see now that the difference between shotcrete and gunite is simply within the mixing and application processes, and the term ‘shotcrete’ encompasses both these terms today. Common applications of shotcrete are for swimming pool construction, tunnels, water tanks, slope stabilisation, and drainage. Any structure that requires customisation in its design can greatly benefit from using shotcrete, either through the dry or wet mix process.
If you’ve decided that shotcrete would be the best approach to apply concrete for your next project, reach out to the professionals at Evolution Shotcrete by phoning 07 5561 8885. Alternatively, visit their website for further information: https://www.evolutionshotcrete.com.au