The influence of filler content on the performance of unbound granular materials in pavement layers

The abundance of high quality unbound granular materials across most of South Africa has led to it being used within structurally significant pavement layers. In flexible thin-surface asphalt pavements the concept of the “upside down” pavement has been historically developed and utilises the principle of pavement balance. The theory typically includes a cement treated subbase followed by a crushed stone base course prior to the asphalt or surfacing seal. In order to mobilise the full stiffness of the crushed stone, it is necessary that aggregate fractions are packed to form a tight interlock. The role of the filler (minus 0.075mm fines) is to act as a lubricant towards assisting with the movement of particles to achieve a stone-on-stone packing. As part of the process towards stone-on-stone packing a construction activity called slushing is used in order to dispel excess fines. It has been reported in industry that target densities are being achieved even in the absence of applying slush-compaction for dispelling of fines. The question remains: to what degree do the filler influence the performance of the material when slushing has not occurred and the fines are left within the aggregate matrix. A study was undertaken to investigate the behaviour and performance of unbound materials in pavement layers, with filler content as a variable. Monotonic, resilient modulus and permanent deformation triaxial tests at various stress conditions were performed in order to correlate the behaviour with the change in filler percentages. Results from these tests do not indicate a clear trend in material behaviour for isolated changes in filler content only, however some findings are highlighted such as the importance of the moisture content and particle size distribution in combination with the percentage filler.