The Use Of Fly Ash As An Asphalt Extender In Asphalt Concrete Mixes.

Abstract

This study was initiated to investigate the feasibility of using fly ash, produced by coal burning plants for power generation, to replace part of asphalt cement in asphalt concrete mixes. An important aspect of this study was to study the effect of fly ash particle size. Three fly ash sizes, coarse (retained on sieve #270), medium (1-44(mu)), and fine (a 50/50 mix by weight of medium fly ash and silica fume which is 100% finer than 0.5(mu)), were used to study this effect. Two other variables were also used: aggregate gradation with three levels of fine, medium, and coarse; and binder content also with three levels 4, 5, and 6% by weight of total mix at zero replacement. The asphalt (74 pen.) was partially replaced by an equal volume of fly ash in five percentages, namely; 0, 10, 20, 30, and 40%. In this study a newly defined parameter, the adjusted voids in mineral aggregate (AVMA), was used as measurement of how well a given fly ash (or filler) replaces asphalt cement in asphalt concrete mixes. In addition, percent air voids, VMA, and unit weight were used to study asphalt-fly ash concrete mixes. Mechanical properties of various mixes, including dry and soaked resilient modulus, indirect tensile strength, and estimated rut depth were also investigated. It was hypothesized that the medium fly ash will be the best asphalt extender among the three fly ash sizes since both fine and coarse sizes were expected to create more voids in the mix. The above hypothesis was found to be true in all cases regardless of aggregate gradation or binder content used. Since most fly ashes are close to medium gradation, this is a finding of great practical importance. From the resilient modulus results before and after water immersion it was found that changing fly ash size did not affect the resistance of various mixes to water damage. There were some indications that increasing sand content in the mix increased the sensitivity of mix properties to changes in fly ash sizes. The AVMA was found to be very useful in judging how well a given fly ash (or filler) can replace asphalt cement in asphalt concrete mixes. All three aggregate gradations tested appeared suitable for asphalt replacement by fly ash which promises cost savings in the road.