Since its introduction in 1971, steel fibrous shotcrete has found many interesting and practical uses. New applications and improvements in materials and equipment made since 1975 are described in this article. One reason for the increasing popularity of this material is its potential to lower the inplace cost of shotcrete by eliminating wire mesh. This is possible because the improvements in properties over those of plain shotcrete are often great enough for fibrous concrete to do the job without mesh. These improved properties higher flexural strength, greater ductility, more toughness and higher post-cracking strength work to especially good advantage in thin-shell applications, mine and tunnel linings and general ground support.
An example of thin-shell use is a series of dome-shaped shelters built by the U.S. Army Construction Engineering Research Laboratory. Using a technique called an inflation/foam/shotcrete system, five shelters were built by applying urethane foam 3 to 6 inches thick to the outside of an inflated membrane and then immediately shotcreting with steel fibrous shotcrete. Ballistic testing of these hardened shelters showed that a 2-inch-thick steel fibrous shotcrete shell provided complete protection against hand grenades. The domes were portable, the foam could be fireproofed, and an 18-foot-diameter, 3-inch-thick dome would sustain a 70-ton load, simulating burial.
In a similar system, the insulating foam is applied to the inside of the inflated membrane. Then the shotcrete is applied to the inside of the polyurethane foam. After the inflatable membrane has been removed the outside of the insulation is coated with a weather-resistant coating. An important advantage of this system is that the building is extremely energy efficient, having a very small winter heat loss and summer heat gain. Also, once the inflatable membrane is in place, nearly all the construction work is done inside, out of the weather.