Asphalt mix design and analysis towards improving compactibility of wearing course mixes in the Western Cape

Abstract

Paper presented at the 21st Annual South African Transport Conference 15 - 18 July 2002 "Towards building capacity and accelerating delivery", CSIR International Convention Centre, Pretoria, South Africa.

The University of Stellenbosch in co-operation with Zebra Bituminous Surfacing CC. undertook an investigation into the compactibility of typical wearing course mixes used in the Western Cape. The harshness of these mixes without natural sand has several disadvantages including higher costs, inconsistent compaction quality and compaction failures. The study focussed on measures to improve the compactibility of asphalt wearing course mixes in the Western Cape. Factors investigated included: • Gradation, • Nature of the filler (particle size distribution), • Binder content, • Filler content, • Filler/binder ratio, and, • Compaction temperature The investigation was carried out by evaluating the volumetric properties of laboratory compacted wearing course specimens and selected variation of certain influencing factors. Mechanical tests were carried out to evaluate the performance properties of the laboratory mix. Special consideration was given to the characterisation of the filler and filler/binder system of the mixes. Fine aggregate angularity tests were performed. These were done to investigate the degree of internal friction and to evaluate the workability of the sand fraction. The influence of ageing was not investigated although all gyratory compacted mixes were aged at 150 ºC for one hour prior to compaction. The use of natural sand was only considered an option in the advanced stage of this study. This paper often makes mention of the term compactibility, which has been analysed as a volumetric property of an asphalt mixture directly related to the Voids In the Mix (VIM). Therefore, the compactibility of a mix was judged in terms of VIM at a specific compaction level. The compactibility of a number of mixes compacted at the same binder content and compaction level was ranked in terms of VIM.