7. Influence of Aggregate Grading on Asphalt Mix Volumetrics
Date of Start:

April 2005
LEA Associates South Asia Pvt. Ltd., New Delhi (R,I)

Scope and Objectives
To study the effect of aggregate grading on the VMA of laboratory compacted asphalt paving mixes.

To determine an aggregate grading for bituminous concrete that gives rise to suitable VMA values for satisfactory performance of the mix.
In the standard marshall procedure for the design of asphalt paving mixes, the optimal asphalt content is determined by testing the mixes made with different asphalt contents from the selected grading. The selected grading is normally chosen to lie well within the grading band of the MOSRTH specifications. Mixes so designed do not perform satisfactorily, particularly when laid in hot climates and subjected to heavy traffic loadings even when stiffer asphalts like the polymer modified bitumens are used. In this case we require relatively higher asphalt contents and also higher initial air voids so that the mixes are stable for a long time during and after secondary compaction under traffic has taken place.

High asphalt content and high air voids content are contradictory requirements and cannot be achieved with dense aggregate gradings.

A series of Marshall tests have been conducted on a set of asphalt concrete mix specimens made with different aggregate gradings and changing the asphalt content and compactive effort and studying their Marshall properties.
Supporting Data
It has been observed that gradings giving rise to higher VMA values fulfill the requirement of having relatively higher asphalt contents with higher initial air voids content. Field behavior of the mixes collaborates these findings.

A method has been evolved to identify such gradings in relation to the 0.45 power grading which is the reference aggregate grading for Superpave mix design.
Significance / Utilisation Potential
In several on-going projects of the NHDP, it has been observed that the asphalt mixes in the wearing course undergo rutting within a few months of opening to traffic. The current method of Marshall mix design does not ensure failure against rutting when we use the stipulated minimum asphalt content in the MOSRTH specifications. The present investigation has brought out that by selecting a suitable aggregate grading, we can impart better resistance to rutting of asphalt mixes while complying the minimum asphalt requirements of the MOSRTH specifications. However, it was observed that the developed aggregate gradings deviate from the specified MOSRTH aggregate band to some degree.
Further Proposed Work
It is proposed to carry out uniaxial creep tests and wheel tracking tests to study the rutting pottential of asphalt mixes made with different aggregate gradings so as to validate the findings of the present study.