Every concrete application methodology results in a certain amount of waste, but the inflated numbers sometimes associated with shotcrete are most often the result of inaccurate calculations based upon false assumptions.
There is waste in the shotcrete process – but the actual amount is much less than the percentages attributed to it. Some inexperienced observers believe shotcrete involves up to 25% waste. This just isn’t true.
The actual waste generated by Torrent’s wet mix shotcrete process is in the 6% to 10% by weight range. The original shotcrete or gunnite methodology utilized a dry mix process and had waste in the 20% range by weight. Torrent rarely uses the dry mix process – only in very specific applications. The lower waste, wet mix process is utilized for the vast majority of our work including all large structures.
In many cases, people don’t fully understand how site conditions can affect the amount of shotcrete actually used. For instance, if a wall is spec’d for 12 inches, but the site conditions don’t match what was assumed in the specification and the wall ends up having to be 14 inches, this doesn’t mean the extra concrete required is waste. But that’s exactly what the inexperienced person thinks; that the additional concrete used was “waste”. In any case, if extra concrete is required to do the job, it doesn’t matter if the application method was shotcrete or cast-in-place.
Another waste calculation mistake occurs when unused concrete is designated as waste. For instance, if the last truck was ordered with 9 cubic feet of concrete, but only 4 was needed to complete the job, it’s not correct to include the remaining 5 cubic meters in shotcrete waste calculations. This is, in reality, an ordering issue which the shotcrete contractor doesn’t control. It’s not shotcrete waste, it’s just a bad order. Inexperienced people would include this in the shotcrete waste calculations and this could drive the waste number to 25%. The actual waste generated by Torrent’s wet mix shotcrete process is actually very similar to the waste due to “wall creep” in cast-in-place applications (assuming a typical ½” creep of a 10’ high wall when forming and placing).
Incorrect waste percentage calculations are then pushed up the chain to the project owners and shotcrete is seen as the culprit. Project owners are getting bad data and their perceptions of shotcrete are negatively impacted as a result.
It’s therefore very important to discuss and review the ordering process with the cement supplier when negotiating terms to ensure the highest levels of accuracy. Better oversight with regards to the ordering process will significantly reduce overall waste volume.
The shotcrete contractor can also help with waste management by adding accurate waste calculations to their quality reports and pump logs. This is again something to be discussed during the specification stage to ensure compliance and the very best results.
The reality is that shotcrete waste isn’t a more of an issue than it is with the form and place method. Knowing the facts allows you to make the best choice – and that’s shotcrete every time!