Factors Affecting the Stiffening Potential of Mineral Fillers

The workability and performance of bituminous mixes is known to be affected by the filler-asphalt mixture (or mastic) properties. The addition of fillers is known to stiffen asphalt. The degree of stiffening is a function of several filler and asphalt properties, which are not well understood. A fundamental approach to understanding the influence of such factors on the stiffening potential of filler in asphalt is discussed. This complex problem is simplified by introducing two intermediate measurable parameters, the maximum packing fraction, ϕ m , and the generalized Einstein coefficient, KE. This enables a better understanding of the influence of various factors such as average particle size, gradation, particle shape, presence of agglomerates, degree of dispersion, and the asphalt-filler interface on the stiffening potential of asphalt. First, the relationship between these two parameters and stiffening is thoroughly examined. The physical meaning of these parameters is discussed. Second, the influence of each factor on stiffening potential is addressed individually, supported by experimental data and relevent literature to derive a quantitative relationship. Finally, the relative advantages of the approach over the traditional treatment are discussed. It is further shown that, since the parameters φm and KE take into account the properties of an asphalt-filler system as a whole, they are likely to predict stiffening more accurately than Rigden’s fractional voids approach which is based on the properties of fillers alone.