Abstract:
Surface dressings (chip seals) over unbound granular pavements are used extensively in most southern hemisphere countries with low population densities and therefore relatively low-volume roads in comparison to more dense European and northern hemisphere countries. It has been thought for many years that chip seals provide a waterproof barrier to the underlying unbound granular basecourse which is the main structural layer in unbound flexible pavements. The stimulus for this research is that some chip seal surfacings in New Zealand in recent years have not lasted their minimum expected design life for a combination of relatively high traffic volumes and wet climatic areas. Therefore, more regular and intensive maintenance is required thereby increasing the maintenance budget for maintaining minimum levels of service for road networks. A research project was conducted in the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) in Christchurch, New Zealand, to determine the suitability of the previously used prime coat in chip seal technology as a means to compare, for the same loading regime, chip seal surfaces that were applied with and without a prime coat. Prior to this research project, the CAPTIF facility excluded environmental factors including water in the accelerated pavement loading process. The relative change in moisture in unbound granular pavement layers as water seeps through the surface chip seals was monitored during the research project. In this research project, water was allowed to flow over the sealed pavement sections with accelerated loading, water entering only from the surface and not the sides of the pavement. The research results demonstrated very dramatically that water penetrated the underlying layers even under no loading. Furthermore, the sections with medium-graded basecourse with and without a prime coat lasted significantly longer under wet conditions with standard axle loading than the coarse-graded and fine-graded basecourses. Results also showed that the addition of a prime coat to the chip seal significantly extended the life of unbound pavements. With the recent development of emulsion seals that are much safer to construct than earlier ‘cut back’ bitumen prime coats, it is expected that prime coats will be able to be reintroduced where appropriate during sealing construction to better waterproof basecourse layers prior to chipsealing.