MIX DESIGN OF GAP GRADED BITUMINOUS MIXTURES

This type of mixture, widely used in Great Britain, is becoming increasingly utilized in Scandinavia, Belgium and South Africa. It consists of a mortar (sand, bitumen, filler) , in which there is a variable percentage of coated aggregates. It offers high qualities of workability, imperviousness, fatigue strength and skidding resistance, but care must be taken as regards its stability. The British standard BS 594 gives a large number of mix design formulae and possible materials but does not define the the mix design process, and, in particular, the elements which influence the stability of the mixture. Using results of tracking tests, the author puts forward the following recommendations 1) utilisation of a type of sand having the highest dry viscosity and adoption of a sand/ filler ratio equal to 6; 2) for mortar, selection of the bitumen content giving the highest density; 3) study of the mortar with different percentages of coated chippings (2% bitumen) and selection of the lowest voids content value in the mortar for a maximum number of chippings. For the latter it is advisable to choose crushed elements. 4) determination of th relation stability over Marshall creep. If it is higher than 150 kgf/mn, experience shows that the mixture selected will withstand heavy traffic in hot countries. /TRRL/