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Mixing the Strongest Shotcrete

Research

When I started building Monolithic Domes, I wanted to know how to make the strongest possible concrete. How much and what kind of cement, water, rock, sand, admixtures, etc. should we use? I went to the Portland Cement Association for advice. I asked other shotcreters.

Conclusions

Over the years, I have tried every mix we could think of. The following are my conclusions:

Aggregate: Concrete is only as strong as its aggregate. If you use granite aggregate, you will surely get stronger concrete than if you use limestone. Obviously, you can get special aggregates – such as slag – to get even stronger mixes. The angular shapes of the aggregate may also add to the strength of the final mix (caveat: crushed stone has latent fractures). We prefer to use natural river rock and sand for shotcrete.

The best concrete has the most even gradation of its aggregate. It has small rocks of 3/8" minus size to very, very fine 1/100" in size.

Cement: In general, the more cement in the mix, the stronger the concrete will be. This is only true up to an 8-bag mix. Above an 8-bag mix, nothing is gained and more shrinkage will result. There is one exception: when using ready-mix and the plant is far from the job site, we use a 9-bag mix to allow more time for transit and placement.

Obviously the cost of cement is much more than the cost of sand in a yard of concrete. You may think, “If I leave out a few bags of cement I can save money.” But consider this: because of their shape, Monolithic Domes only need a relatively small amount of concrete. So always use the best materials and the optimum amount of cement to achieve the strengths we all want.

Admixtures: We use an 8-bag mix. We replace one bag of cement with admixtures that give us the same resultant strength, and at the same time, have other added benefits. These benefits include “air entrainment,” easier pumping, water reduction, etc. These admixtures include Kelcrete, or Easy-Spred. Always use the admixture per directions. We will post any additional admixtures on our Marketplace as we find them and can verify their abilities.

Fly Ash: If fly ash is available and it usually is at most ready-mix plants, we replace 100 pounds of cement with 120 pounds of fly ash per cubic yard. Fly ash makes the concrete pump easier and will eventually make the set concrete stronger. It is also more chemical resistant, and that’s important for salt and fertilizer storages as well as against some corrosive soils. In general, we do not use the fly ash for on-site mixing.

Water: Only very small amounts are needed to properly set concrete. In fact, you only need to show cement the water bucket and setting seems to happen! The more water in the mix, the weaker the concrete.

In order to effectively spray shotcrete, however, you need to do more than merely show the mix the bucket. But as an industry, we have learned that if all the other elements in the mix are correct, the additional water is compensated for.

Our goal is to apply the very best shotcrete with efficiency. We advise you to NOT cut back on the cement to save money. Your aim should be to use mixes which will yield maximum strength concrete. Should an airplane land on your house, it will make a difference!