High-temperature fire resistance for concrete

Concrete Construction, August, 2004

In the late 1990s the rapid rise in temperature from fires in concrete-lined tunnels was causing explosive spalling of large areas of concrete. The large, falling concrete chunks were nearly as dangerous to trapped motorists as the smoke and fumes. The problem had to be addressed.

The process of heat-induced spalling is fairly simple. When concrete is exposed to temperatures above 212[degrees] F, the boiling point of water, the moisture in the concrete turns to steam. If the temperature rises more rapidly than the steam can escape, the rising pressures cause the concrete to spall. In the extreme, this spalling can be explosive.

UGC International, a Zurich, Switzerland-based division of MBT International, a Degussa company, developed a cementitious-based passive fire protection barrier that shield underground concrete structures from heat up to 1350[degrees] C (2462[degrees] F). The mortar product, known as Fireshield 1350, is based on standard concrete technology and replaces the normal aggregate with another natural resource. The mixture consists of a mineral/organic main component, portland cement, water, and admixtures. It has relatively high compressive strength (up to 4350 psi) and bonds well to most substrates.

Fireshield is typically spray-applied in a layer up to 2 inches thick, using either a wet mix or dry mix method. Its bond strength allows application without anchor systems or steel mesh, but either can be used if desired. When properly applied, the Fireshield barrier prevents both mechanical deterioration of the underlying concrete and spalling due to high heating rates.

Typically fire resistance ratings in the United States are based a model fire as described in ASTM E119, "Standard Test Methods for Fire Tests of Building and Construction Materials." The E119 fire climbs to 1000[degrees] F in the first five minutes, then rises to 2000[degrees] F at the four-hour mark. However, because Fireshield was developed specifically to protect tunnel linings, a more demanding European test was used.

The material was tested using the RWS time-temperature curve developed by the Rijkswaterstaat Ministry of Transport in the Netherlands to simulate a gasoline tank truck burning out over a two-hour period. Such a fire in a typical 40-foot diameter tunnel would seriously damage 1500 square feet of tunnel lining. But more significant is the initial thermal shock applied to the barrier, consisting of a temperature rise of 1200[degrees] C (2192[degrees] F) in the first 10 minutes.

Research in the mid-1990s showed that heating rate plays a large role in determining how severely concrete will spall when exposed to high temperatures. NIST Special Publication 919, published in 1997, notes that heat-induced spalling is also affected by original compressive strength, concrete moisture content, concrete density, specimen dimensions and shapes, and loading conditions. But all other things being equal, the Dutch time-temperature curve provides a significantly more stringent test of fire resistance than E119.

The Fireshield testing showed no cracking, spalling, or debonding at the end of the two-hour test. Additionally, the interface temperature ranged from 400[degrees] C to 180[degrees] C (752[degrees] F-356[degrees F] for test specimens from 1.6 to 2 inches, respectively. Those temperatures are well below the point of inducing spalling for most concrete types.

The product has been successfully used in Europe for several years and will soon be available for use in the United States. Beside using it for concrete and masonry tunnel linings, the company expects to see it in a variety of fire protection applications both underground and above grade. For more information, visit www.degussa-ugc.com, e-mail askmbt@masterbuilders.com or Circle 15.