The Use of Average Least Dimension in Surface Dressing Design

Surface dressing is the application of a thin layer of bituminous binder and single-sized aggregate chippings to the surface of a road, in one or more layers. The technique has been widely used on Irish roads for many years to... [ view full abstract ]

Surface dressing is the application of a thin layer of bituminous binder and single-sized aggregate chippings to the surface of a road, in one or more layers. The technique has been widely used on Irish roads for many years to improve skid resistance, seal the road surface and to arrest deterioration. The procedure has become increasingly important since the introduction of standards for skidding resistance on National roads by Transport Infrastructure Ireland (TII). TII are currently developing a new analytical design approach for surface dressing on National roads.

Based on best practice in other countries, an essential parameter used in the analytical design of surface dressing is the Average Least Dimension (ALD) of the aggregates used. The ALD is used to determine the optimum rates of spray of binder and rates of spread of chippings for surface dressing. The purpose of this research study carried out in 2015 was to determine the most appropriate way of calculating the ALD using surface dressing aggregates from Irish quarry sources. A total of five different methods were examined, two direct measurement methods and three computational methods. The direct measurement methods used included the use of a new device developed in South Africa to automatically measure ALD. The data for the study was collected by sampling and testing four different aggregate sizes from eight quarry sources nationwide. The testing included the determination of particle size distribution and flakiness index for each sample, in addition to determining the ALD by each of the five methods.

This paper will outline the findings of the research study undertaken including an analysis of the range of ALD values obtained for Irish quarry sources, and a comparison of the three computational methods of estimating ALD against the direct measurement methods. In addition, the paper will recommend a method for calculating the ALD of Irish aggregates to be used in surface dressing.