Aggregate breakdown as a cause of chip seal flushing

Flushing in chip seals is one of the main factors affecting seal lifetimes in New Zealand. This paper explores the hypothesis that a major cause of flushing in chip seals is the build- up of fine aggregate material in the seal layer, largely produced by surfacing aggregate breakdown and abrasion under traffic. The volume of fine aggregate particles and bitumen present, together eventually exceed the void volume available resulting in a flushed surface. Results of calculations based on measured void volumes in very dense aggregate particle packing configurations and ltextbookr bitumen application rates, showed that multiple seal layers seals should not in theory flush. Dense packing of various aggregate grades and combinations showed that void volume of approximately 40 per cent would be expected in heavily trafficked (compacted) seals- more than sufficient to accommodate the bitumen used in sealing. Data from volumetric measurements made on a large number of cores taken from flushed multiple layer seals showed the presence of significant quantities of aggregate material passing a 4.75 mm sieve (material which in theory should not be present). The volume of bitumen and fine material present was close to the 40 per cent value predicted at which flushing of well compacted seal layers should occur. Measurements made of two cores showed that the contribution of tyre rubber and other non-aggregate particulates to the fine material was negligible. Data from cores taken both in the wheel tracks and on the shoulder, at the same site, indicated that breakdown of aggregate due to over chipping during construction may be a significant contributor to fines generation. This contention is supported by preliminary laboratory experiments to measure breakdown under a loaded tyre.