Spray it again, Uncle Sam

The environmental impact of logging in the US is perceived to be high, wood prices are inconsistent and there is a shortage of high quality timber: these factors are providing opportunities for suppliers of concrete building systems to establish their products and capture significant market share very rapidly. The 120 000 concrete homes built last year represent a 10% share of the US new build market and a 350% increase in around five years.

Concrete block construction has been available in the US for many years, but has been discounted for homebuilding on cost grounds. The innovations more recently incorporated into concrete homebuilding systems are allowing them to compete better with timber on price as well as on performance. Concrete homes still cost more than timber homes but American homebuyers are clearly prepared to pay the 2-5% premium.

Many of the concrete systems now used in US homebuilding originate from Europe, with suppliers from the Netherlands, Germany, Austria and France all active in the market. Compared to US timber ‘stick build’, concrete has considerable appeal for homebuyers. As well as sound and thermal insulating properties and resistance to fire, the durability of concrete homes in extreme weather and immunity to insect attack are perceived advantages. After Hurricane Andrew hit Florida in 1992, the area’s concrete homes gained national attention because they were the only ones left standing.

Several concrete technologies are used in US homebuilding: block, permanent and removable formwork, panel and Shotcrete. Shotcrete is a process where concrete is sprayed from a hose and nozzle onto a surface to form structural shapes including walls, floors, and roofs. The surface can be trowelled smooth while the concrete is still wet. This technology has more commonly been used to coat steel bridges or form swimming pools, but it has recently been applied to homebuilding.

To build traditional-looking homes, Shotcrete is used to form wall panels. The Shotcrete is sprayed onto both sides of polystyrene insulation, which is sandwiched between two sheets of wire mesh. The 4 ft by 8 ft panels weigh only 38 pounds, so can be assembled without heavy erection equipment, and by relatively unskilled workmen in minutes. Another plus of the system is that the wall can be textured or painted to resemble brick or stone.

Shotcrete can also be used to build far less conventional-looking houses like the dome home of Bill Click in Bandera, Texas. These too are quick to construct. A fabric balloon is placed on a concrete ring foundation and inflated with the help of blower fans to create an airform in the future shape of the home. Foam insulation is sprayed onto the inside of the fabric before steel reinforcing bars are tied to the foam. Special mix concrete is then sprayed onto the interior surface. Dome homes might seem like oddities, but there are already hundreds of them in the US.