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DIY Shotcrete

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Shotcrete is a wet mix concrete used in pump applications for vertical and other nonflat structures. Shotcrete is mixed before it is pumped through the wet-gun. The process produces less waste, dust and clumping than other concrete pumping methods and allows you to more closely control the moisture content of the final concrete. If you are doing a large area, the concrete supply company will premix it for you in the delivery truck. For a smaller batch, you can mix it yourself with materials available from a concrete supplier. Here's the formula for a 9-cubic-foot mixer.

The Formula

Pour cement and half the water into the mixer and start it turning. Most lumber yards carry type I or II Portland Cement in large bags.

Add 2.5 ounces of Kel-Crete. You can add up to 4 ounces. You'll have to adjust from load to load for best results . Kel-Crete improves dispersion of the mixture and adds air entrainment qualities to the final product. This is particularly crucial in areas where the concrete freezes and thaws frequently.

Alternate slowly adding sand and water. Leave about 2 gallons of water remaining to make sure the mixture is not too soupy. The concrete must be wet enough to pass through the concrete pump and gun without blocking up the pipe, but not so thin that it slides out of the form or produces a weak final product.

Add the two buckets of pea gravel slowly. You shouldn't need to add water. The mixture should be pretty wet. You may leave out the pea gravel in certain applications. This will help the concrete flow more smoothly through the gun. For large surfaces, however, the aggregate strengthens the final product.

Add nylon fibers to the mix and enough water to produce the desired thickness of the wet concrete. The kind of fibers used in shotcrete are monofilament fibers, single strand. Tests show that the fibers improve shrinkage tolerance and crack control in the final product, but some shotcrete users don't use them at all. They are an optional ingredient.

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