A review of foint sealant standards and specifications for aircraft pavements

Road & Transport Research, Jun 2000 by White, G, Allman, M

Hardness: A 'shore A' hardness reading is obtained from the sealants by a device known as a durometer. This test is considered to be very inconsistent depending on the durometer used and the temperature of the sealant during testing (Panek and Cook, 1989). Klosowski (1989) suggests that hitting a small aluminium object (reflecting a typical intrusion) with a hammer as it rests upon a sealant sample is a more appropriate assessment of a sealant material's resistance to intrusions.

Adhesion and Cohesion under cyclic movement: A sealant, cast between two concrete blocks, is subjected to 10 cycles of extension and compression following 7 days of heat exposure while compressed. Another 7 day period of compression and heat exposure follows. The samples are then extended while cooling from room temperature to -26 deg C. This extremely low temperature is not considered appropriate for Australian conditions given the difference in temperature ranges from that experienced in the US. Inspections of the sealant samples for adhesion and cohesion failure (which is not permitted to exceed a total of 9 cm^sup 2^ for three samples) are conducted at the various stages. The amount of change in sample width (extension and compression) is determined by the design requirements for the joint, and can be set at any amount of joint width.

Adhesion in peel: Adhesion in peel is assessed under this standard by the application of a shear force to the sealant cast between two substrate samples. A force of 2.3 kg shall not cause more than 25% of the sealant-substrate bond area to fail for the sealant to be considered acceptable. This test is not considered applicable for sealants in pavements, as there is no significant shear force present unless the joint is acting abnormally and causing a significant height difference between adjacent concrete slabs.

Accelerated weathering: The acceptability of a sealant material's resistance to weathering is determined by two criteria. First, sealant samples are cast and exposed to a combination of heat and moisture over 72 hours. The sealant samples are then compared to a number of photographs of other samples to determine the effect of the accelerated weathering. Second, the weathered samples are exposed to -26 deg C for 24 hours and then bent around a mandrel and inspected for cracks. The ability of a sealant sample to pass the Hockman cycle after artificial weathering is considered a more appropriate assessment of the sealant's resistance to such weathering.

Of the two standards studied, neither is considered to be stringent enough to ensure the long-term performance of sealants in rigid aircraft pavements or to be highly applicable to Australian conditions. Some industry experts suggest that, rather than joint sealant materials being designed and produced to meet the relevant standards, the standards and specifications are written to accommodate the commonly used materials (Panek and Cook, 1991). A set of performance tests that will assess sealant material quality, through close simulation of their service environment, is required. Only such a set of performance criteria can allow Defence and others to assess various sealant materials to determine their performance and compare a range of products on a value for money basis.