Top Mistakes When Spraying Shotcrete

Not surprisingly, the use of shotcrete in the construction industry has become increasingly common due to a rising number of applications where shotcrete has unique benefits over conventional cast-in-place concrete. When compared to traditional concrete, shotcrete is stronger, less permeable, cheaper, and uses less resources and materials. As a result, shotcrete is ideal for hard to reach areas where traditional concrete has been deemed impractical.

Shotcrete, originally known as gunite, was invented over a century ago by an American taxidermist. Using a pneumatic hose to spray concrete materials onto a receiving surface, both the wet mix and dry mix methods are used in construction projects all over the world. The primary distinction between the two methods is that the wet mix approach uses ready mix concrete while the dry mix approach introduces water at the nozzle to complete the mixing process. In both scenarios, an experienced nozzleman is needed to direct the shotcrete onto the receiving surface with the correct amount of additives (for wet mix) and water (for dry mix).

While shotcrete enjoys many advantages over conventional concrete, the process of applying shotcrete requires a large degree of skill and experience. It’s not uncommon for mistakes to be made during the shotcrete process as there are many unique factors and variables that must be considered. The most important component of applying shotcrete lies with spraying the concrete onto the receiving surface, so to give you some insight, we've compiled a list of the top errors contractors make when spraying shotcrete.

Air compressors

One of the most common mistakes contractors will make is using an undersized air compressor. To ensure there is enough velocity for the materials to mix properly, it’s recommended that contractors use a compressor that can produce at least 250 CFM for wet mix and 700 CFM for dry mix. Anything less than this may result in poorly mixed shotcrete and substandard finishes. It’s important to keep in mind that if your air compressor is too powerful, simply take a step back or turn down the output.

Air temperature

Shotcrete can be applied using a normal ratio of materials when the air temperature is above 5⁰C and less than 37⁰C. If the air temperature is outside this range, it’s imperative that contractors use either retarding agents or accelerants to ensure the shotcrete can be applied and mixed correctly. A common mistake that arises is shotcrete being sprayed when the air is too hot or cold, or the new ratio of materials confuses the crew and other mistakes are made.

The concrete loses strength

Maintaining the proper moisture content is imperative for both the wet mix and dry mix approach. When there’s too much or too little water, the concrete loses strength and this typically happens when the concrete mix truck has been waiting too long (in wet mix) or injecting too much water at the nozzle (in dry mix). Contractors must always start spraying the materials within 90 minutes from when the truck was loaded, or 45 minutes from when the materials were placed in the dry mix gun. The proper ratio of cement, water, and aggregate is paramount to how well the materials mix and bond with the receiving surface.

Incorrect equipment setup

Having the shotcrete pump and nozzle too far from each other is another common mistake that is made. When the pump is working too hard, heat builds up and causes the composition of the concrete to change where the liquid, fines, cream, and aggregate get separated and the shotcrete is improperly mixed when it is applied. While this only happens during the wet mix approach, it’s crucial that the pump is positioned as close to the nozzle as appropriate to ensure the entire process runs smoothly.

Nozzleman using a flawed technique

The nozzleman should always spray the shotcrete at a 90-degree angle to the receiving surface, and the distance between the nozzle and the surface must not exceed 6 feet. The nozzleman should also start spraying in the corners first to decrease trapped rebound. If using a layered approach to shooting, it's vital that each previous layer has been appropriately prepared with substrate before any new materials are applied.

Although spraying concrete through a hose sounds easy, a great deal of skill and expertise is required to ensure the shotcrete is applied correctly. Contractors will commonly make some of the mistakes outlined above, specifically using an undersized air compressor, which result in costly errors. If you're interested in using shotcrete for your next project, it's always advisable to use qualified shotcrete specialists for the best results. If you have any enquiries relating to the application of shotcrete, or you just need some friendly advice, talk with one of the staff at Evolution Shotcrete by phoning 07 5561 8885.