Permeability of Asphalt Surface Seals and Their Effect on Aging of Underlying Asphalt Concrete

Abstract

Asphalt surface seals are defined as slurry seals, micro-surfacings, and chip seals (seal coats). The relative aging abatement effects of surface seals on the upper 13 mm (½ in.) of an asphalt pavement were estimated. A laboratory aging experiment was devised wherein a surface seal was simulated by a membrane that could be easily removed after the artificial aging process without affecting the uppermost layer of asphalt in the asphalt concrete. By comparing laboratory aging data with existing field data on aged asphalt pavements it was possible to make inferences about the number of years an asphalt surface seal will delay hardening of an underlying asphalt concrete layer. Compacted asphalt concrete slab specimens (40 × 60 × 13 mm) were prepared in the laboratory. Half of each slab was covered with an impermeable membrane; then the specimens were exposed to hot air to accelerate oxidative aging. Following the heat exposure, asphalt was extracted and physical and chemical tests were performed to measure hardening of the covered and uncovered specimens. Permeabilities of laboratory prepared slurry seal, micro-surfacing, and seal coat specimens were measured. By knowing the amount of oxidative aging that occurred in uncovered and covered specimens of asphalt concrete and the permeability of the surface seals, the effects of the surface seals on aging of an underlying asphalt concrete layer were interpolated. Testing indicated that a surface seal can retard oxidative hardening of an underlying asphalt concrete layer by 0 to 2 years, depending on the situation. However, most of the oxidative aging in the upper stratum of an asphalt concrete pavement occurs during the first 4 years after construction. After this period, the asphalt aging rate decreases significantly. Therefore, for a surface seal to significantly delay oxidative hardening of the underlying pavement, it must be placed during the first 2 years (approximately) of the pavement's life. It was demonstrated that ultraviolet (actinic) light penetrates asphalt cement only a few microns and, therefore, does not contribute materially to hardening of the uppermost 13 mm of an asphalt concrete pavement. Permeability of a slurry seal or micro-surfacing after sufficient traffic to effect maximum compaction is less than 1 × 10⁻⁵ cm/sec. Permeability to water and air of an asphalt seal coat (chip seal) is essentially zero. For practical purposes, these three surface seals will protect the top 13 mm of an underlying pavement from oxidation as if they were impermeable to air and water.