Water/gas Permeability of Bituminous Mixtures and Involvement in Blistering Phenomenon

Moisture damage to bituminous pavement occurs via several mechanisms, and appears in diverse forms such as blistering, stripping, and disaggregation. Blistering is a fulminating phenomenon in which the trapped moisture evaporates and upheaves the pavement surface layer within a few hours as a result of so-called blistering heat. In contrast, stripping and disaggregation are types of internal chronic damage. Disaggregation is caused by the stripping and resolution of binders, and is considered to be the ultimate irreversible damage, whereas stripping is considered to be reversible by retacking (healing) of the binders on the aggregates. All these types of damage are caused by water accumulated in bituminous pavement layers. The mechanism of moisture intrusion into bituminous pavement may involve several factors, for example, air void content, cracking of the surface course, poor execution, inadequate subsurface drainage, and high groundwater level. Extensive research on stripping at the University of Idaho has established that air voids in bituminous mixtures are probably saturated with water from the subgrade or subbase. Groundwater beneath the pavement body and/or water penetrating from side sections and/or surface cracking may be the source of the water, and the moisture is aspirated up to just beneath the surface course. Drainage by evaporation from the road surface may also be important. Many investigations of the cause of blistering have simply examined the water impermeability of the surface layers. Although the ultimate cause of blistering certainly involves this factor, the mechanism of moisture intrusion into pavement layers is also important. Control of moisture movement in pavement structures is an important research target for practical solutions to deal with this phenomenon and also to develop adequate pavement design strategies. Blistering is unlikely to be related to hydrological fl ow of liquid water, because even vapor cannot escape from the boundary of the surface and base course. Therefore, where and how does water permeate the pavement? Primary blistering, which is observed just after the completion of the pavement, is usually caused by water trapped within the pavement layers during paving operations. However, secondary blistering, which probably occurs in the hot summer following paving completion, has no definitive evidence to sug57 Journal of the Japan Petroleum Institute, 49, (2), 57-64 (2006)