Determination of Optimum Nominal Aggregate Size for Single Surface Dressings

Abstract

Surface dressings can be used as a successful road maintenance activity if carried out properly. One of the main decisions to be taken in designing a surface dressing in road maintenance is the selection of appropriate aggregate size. Improper selection could tarnish the performance of a surface dressing. The aim of this study is to find the optimum size of aggregate for a single surface dressing, especially in Sri Lankan Macadam roads. General size of aggregate for a particular surface dressing could be found using two parameters, commercial traffic volume on road and surface hardness of the road based on many available highway literature. But the aggregate sizes selected in above simple method have shown inconsistent results in Sri Lankan roads. Therefore tests were carried out to find the most appropriate aggregate size for the surface dressings in medium to low traffic Macadam roads in maintenance operations. Three most suitable nominal sizes of aggregates were selected using commercial traffic volume and the road surface hardness to begin the tests. These three sizes of aggregates were used to carry out three different surface dressings in the same road but in different stretches. Binder type were kept constant and binder rate was changed according to the aggregate size. The performances of these three surface dressings were evaluated by measuring aggregate removal rate and skid resistance. Digital photographs of demarcated locations in surface dressings of different aggregate sizes were taken at pre determined time intervals. .The numbers of aggregate were counted in each photo after certain time intervals up to 12 weeks and using this data, the behavior of each surface dressing over a period of time was studied. The aggregate size that could keep most of aggregate intact in its dressing after a certain time period would be a more durable chip size. The next aspect of checking performance of the dressing is the skid resistance. The techniques utilized to measure this value are Locked wheel test and Sand patch method. These tests were done after 12 weeks since the operation a sufficient enough to stabilize the dressing. The comparison of aggregate retention on road surface and the skid resistance after 12 weeks will give the short term performance of three aggregate sizes. It was found that 9.5mm aggregate size has better performance in aggregate retention and the 12.5mm size has better skid resistance.