As a structural material, most people are familiar with concrete. However, not everyone is familiar with the form of concrete known as shotcrete. Here is some background on shotcrete, including how it is used in building projects today.
Essentially, shotcrete is projected concrete. Originally devised in the early 20th century, shotcrete was created as a means of using concrete to fill out molds. The concrete itself was a dry mix that was blown directly into the mold using compressed air.
As the concrete was released, the dry mix was moistened, allowing it to settle and set in the mold. The inventor of this method, Carl Akeley, received a patent in 1911 for both the concrete gun he developed, as well as the material that was produced, which he dubbed gunite.
Other applications for shotcrete were immediately obvious. Because the shotcrete could be applied to a horizontal or a vertical facing, the shot concrete could easily be applied to the sides of buildings, the shotcrete could be used as a patch and as a filler where cracks may have appeared in walls or in foundations. The method also hastened the construction of walkways in many front yards and backyard gardens as well, since shotcrete could be laid out in a fraction of the time it took to mix and apply concrete by hand.
The dry method of creating shotcrete remained in place until the middle of the 20th century, and continued to be refined. Still in use today, the dry method involves placing the dry mix into a hopper, where it is ran through a hose with a water attachment at the end of the hose. As the concrete is shot out of the hose and into the gun mechanism, the operator adjusts the amount of water that is added to the dry mix. The result is a concrete mixture that is easy to direct and will dry and cure in the same amount of time as any method using concrete.
By the middle of the 20th century, an alternate method to creating shotcrete was developed. Referred to as the wet method, this process involves using ready-mixed concrete. As with the dry method, compressed air is used to force the concrete mixture through a hose and out of a nozzle. With wet mixing, the operator does not have the ability to adjust the mix of water and dry concrete, since that process has already taken place. Fans of this method point to the fact there is no chance of introducing too much water into the mix, creating a poor concrete texture. Supporters of the original dry method state that the properly mixed concrete and water produce a finished product that is superior to the ready-mix used with the wet method.
With both applications, steel rods or webs of steel mesh are often used to help retain and reinforce the surface that is receiving the shotcrete treatment. By providing something extra for the shotcrete to cling to while it dries into place, the shotcrete patches or facades tend to last much longer.