The effect of material properties on the compactabillty of some untreated roadbuilding materials

Abstract

The proper densification of the separate pavement layers forms an integral part of road construction. Many problems, are, however, experienced in this area. Because of a lack of knowledge the compaction of untreated roadbuilding materials in problem situations is usually approached on a ''trial and error'' basis rather than basing possible solutions on scientific evidence of the collective influence of the material properties and site conditions. The purpose of the study was to place the compaction of untreated roadbuilding materials on a more scientific basis. An investigation was therefore launched to determine the effect of measured material properties on their compactability. A non-standard vibratory compaction test was used to compact the samples in one layer. New test parameters to quantify the shape and texture of the material were also developed, namely the shakedown bulk density and the shape factor. The CBR values of the materials at moulding moisture content were determined for each material for a range of densities and moisture contents. The maximum dry densities (MOD) (vibratory and mod. AASHTO) and optimum moisture content (OMC) (vibratory and mod. AASHTO) were also determined. The measured values were then evaluated in terms of the following physical properties of the materials: grading, Atterberg limits, linear shrinkage, shakedown bulk density (SBD), loose bulk density (LBO), shape factor (SF) and specific rugosity (Srv). In the extensive laboratory study of 21 different untreated roadbuilding materials, varying from TAB classes A-7-6 to A-1, it was found that both the maximum dry densities and moisture regimes can be quantified in terms of the grading, liquid limit and linear shnnkage of the materials. These relations were modelled by means of regression analysis. Besides this a general bearing capacity model was found for all these materials where the CBR is a function of the dry density and moisture content of the material. This model was further refined to take account of the influence of shape and texture of the particles so that it is possible to determine reasonable estimates of the bearing capacity for a range of densities and moisture contents from the grading, Atterberg limits, linear shrinkage, shakedown bulk density and shape factor. This investigation has shown that physical laws govern both the compactability and bearing capacity of untreated roadbuilding materials, irrespective of their composition or nature, making it possible to approach the compaction of untreated roadbuilding materials in a more generalised manner.