HEAT-RESISTANT CONCRETE

The behavior of concrete exposed to high temperatures is complex. Concrete pavement exposed to high temperatures from aircraft jet blast or from auxiliary power units can suffer damage. Typical concrete pavement damage resulting from high temperatures of jet blast includes spalling, aggregate popouts, scaling, cracking, and loss of joint sealant. The time that the concrete is exposed to the jet engine or auxiliary power unit exhaust is critical, since there is considerable thermal lag in concrete.

Behavior of concrete

If the concrete is wet when the heat is suddenly applied, the production of steam within the concrete can cause spalling. If the concrete is dry or the heat is applied slowly, relatively little permanent damage is done with concrete temperatures up to 400 to 500 degrees Fahrenheit (204 to 260 degrees Celsius). At concrete temperatures above this, water of hydration is lost, and the concrete strength decreases. At about 1,000 degrees Fahrenheit (538 degrees Celsius), compressive strength loss can be 55 to 80 percent of the original strength. At the time of heating, the degree of saturation of the concrete influences the severity of strength loss, and repetitions of heating and cooling cycles further degrade the concrete. At a temperature of around 1,060 degrees Fahrenheit (571 degrees Celsius), silica in the concrete aggregates undergoes a crystal change and expands, and in the range of 1,300 to 1,800 degrees Fahrenheit (704 to 982 degrees Celsius), carbonate aggregates undergo a chemical change. As the concrete surface is heated, a large temperature gradient develops between the surface concrete and the cooler slab depths that can lead to separation and spalling.

Properly designed pavements generally have not suffered heat damage from aircraft. Power check pads where extensive engine operations occur for maintenance are specially detailed to minimize the exhaust plume’s contact with the pavement surface. Where existing pavements, particularly if at a shallow slope, are converted to use as power check pads, extensive thermal damage can occur. Particular problems are posed by aircraft with vectored thrust such as the Navy’s Harrier or aircraft such as the B-1 or FA-18 with auxiliary power units that exhaust downward on the pavement for extended periods of time. AFCESA and TSMCX should be consulted for the most up-to-date guidance on how to deal with these problems.

Exposure Time and Temperature.

Concrete slabs exposed to an ASTM E119 standard fire for 2 hours indicated that after the temperature of the concrete at about 3/4 inch (19 millimeters) below the surface was 1,200 degrees Fahrenheit (649 degrees Celsius), at 1 1/2 to 2 inches (38 to 51 millimeters) it was 800 degrees Fahrenheit (427 degrees Celsius), and at about 3 1/2 inches (89 millimeters) it was 400 degrees Fahrenheit (204 degrees Celsius). (The atmosphere temperature for a standard ASTM E119 fire rises to 1,000 degrees Fahrenheit (538 degrees Celsius) at 5 minutes, 1,700 degrees Fahrenheit (927 degrees Celsius) at 1 hour, and 2,300 degrees Fahrenheit (1,260 degrees Celsius) at 8 hours.) Normally concrete would not be exposed to jet or auxiliary power unit exhaust for extended periods of time, and any thermal damage will be concentrated in the upper surface concrete. Concrete exposed to high temperatures must be of high quality. It should have a low water/cement ratio, and it must be properly cured. Leaner concrete mixes perform better than richer mixes. Construction must also be of high quality. Proper consolidation and proper finishing are critical. Finishing techniques that cause a paste on the surface will result in scaling. Selection of the proper materials in the concrete also has a dramatic effect on heat resistance. Aggregate selection probably is the most important single materials-related factor; however, no standard specification has been developed for heat-resistant aggregate.

An aggregate with a low coefficient of thermal expansion is generally considered to be desirable, and one rating system roughly groups aggregates as follows in descending order of desirability for heat-resistant concrete, as shown in Table 1.

Aggregate Performances.

Lightweight aggregates such as expanded shale tend to perform better than conventional natural concrete aggregates when exposed to high temperatures. Good results have also been reported for air-cooled slag aggregates. Hydrated Portland cement that has lower calcium hydroxide content appears to be preferable to those with higher contents for high-temperature applications. Therefore, some benefit may be obtained by using Portland cement blended with slag cement. For temperatures of 1,500 degrees Fahrenheit (816 degrees Celsius) or more, high alumina cement will provide superior performance over conventional Portland cement. Repair of concrete that has suffered thermal damage is a difficult problem. Proper patching procedures for spalls and popouts must be meticulously followed, and the repair material should have similar thermal characteristics to the original concrete. Even so, the repairs may only be temporary. Overlays using heat resistant concrete are a potential repair for scaled areas or for areas with concrete of poor heat resistance. If scaling is due to a paste on a concrete surface that is otherwise acceptable, grinding the surface may be adequate. Joint sealant used in concrete pavements exposed to high temperatures should conform to Federal Specification, SS-S-200E. This specification does require testing of the material at 500 degrees Fahrenheit (260 degrees Celsius) for 2 minutes so that some resistance to high temperatures can be achieved. However, when high temperatures are combined with jet blast, the sealant may still be damaged or blown out of the joint. Under these circumstances, increased periodic resealing must be accepted as routine maintenance. Conventional concrete and joint sealants should provide reasonable service up to concrete temperatures of about 500 degrees Fahrenheit(260 degrees Celsius)

Table 1. Aggregate Desirability

Above this temperature, deterioration of concrete and increased loss of sealant can be expected. High-quality concrete with selected aggregates can reduce the amount of damage. Above 1,000 degrees Fahrenheit (538 degrees Celsius), severe deterioration can be expected, and refractory materials such as high alumina may be needed. Where possible, blast shields, diverters, or increased slope of pavements should be used to allow the maximum dissipation of the exhaust plumes temperature before it impinges on the concrete. Use of continuously reinforced concrete for areas such as power check pads removes the need for joints and joint sealants. In one installation, refractory brick was used to surface a test facility where high-temperature engines were tested and evaluated.