Q. How thick should concrete be to withstand four hours of temperatures of 2000 degree F without degrading or failing? The concrete barrier likely will be exposed to more than 2000 degrees F. The barrier would be more than 30 feet tall and more than 30 feet long. It will be outdoors and subject to rain and other weather.

A. The thickness of the fire barrier may not be as important as the material you put on the side facing the fire. Standard reinforced concrete loses much of its strength when exposed to high temperatures over a long period; it basically dehydrates. More importantly, the faster the temperature rises, the more likely the concrete will spall, sometimes explosively.

The mechanism behind 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 through the concrete matrix, the rising pressures exceed the strength of the concrete and it begins to spall. This spalling can be explosive in extreme cases.

UGC International, a Zurich, Switzerland-based division of MBT International, a Degussa company, developed a cementitious-based passive fire protection barrier that shields underground concrete structures from temperatures up to 2462 degrees F (1350 degrees C). The mortar product, known as Fireshield 1350, is based on standard concrete technology with another natural resource replacing the normal aggregate. 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.

2-inch-thick layer
This material is typically applied by spraying on a layer up to 2 inches thick. Fireshield’s bond strength permits application without anchor systems or steel mesh, but they can be used if desired. The Fireshield barrier prevents mechanical deterioration of the underlying concrete and spalling due to high heating rates.

Fire resistance ratings in the United States typically are based on 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 F in the first five minutes, then rises to 2000 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, which was developed by the Rijkswaterstaat Ministry of Transport in the Netherlands to simulate a gasoline tank truck burning more than two hours. 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 2192 degrees F (1200 C) in the first 10 minutes.

Research in the mid-1990s showed heating rate plays a large role in how severely concrete will spall when exposed to high temperatures. NIST Special Publication 919 notes that heat-induced spalling is also affected by original compressive strength, concrete moisture content, concrete density, specimen dimensions and shapes, and loading conditions. (The investigation found the problem was worse with the higher strength concrete because its increased density made it harder for the moisture to escape.)

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

The product has been successful in Europe for several years and will soon be available in the United States. In addition to concrete and masonry tunnel linings, the company expects it to be used in a variety of underground and above grade fire protection applications.

For more information, visit www.degussa-ugc.com and look under "fire protection." You can also email to askmbt@masterbuilders.com.

Keeping Up With the Specs
CRSI endorses shop-based fusion welding

With the increased use of rebar in high seismic zones such as California, more weld shops are introducing fusion-welded assemblies for beam and column cages. The Concrete Reinforcing Steel Institute (CRSI) has published "Assembling Reinforcing Bars by Fusion Welding in the Fabricating Shop," Engineering Data Report Number 53.

In the report, CSRI clarifies how to assemble reinforcing bar. CRSI traditionally has strong recommend that rebar be tied, as tack welding can seriously weaken a bar at the point of the weld.

But in this new report, CRSI took a new look at assemblages crafted with fusion welding. Laboratory testing showed computer-operated fusion welding machines produced assemblages without affecting the mechanical or tensile strength of the reinforcing bars.

It also indicates shop-fabricated cages can help the concrete construction process by reducing field labor costs, increasing the accuracy of stirrup positions in the cage, and producing assemblies with tighter dimensions.

The report also explains how its position on shop-produced, fusion-welded elements is consistent with the current ACI 318 Building Code and the Uniform Building Code.

To learn more, visit the free report section at www.crsi.org, or telephone CRSI at 847-517-1200.