Effects of Aggregate Degradation on Air Voids of Structural Asphalt Mixture in Florida

Abstract

In an effort to further improve the rut resistance of asphalt pavements in Florida, the Florida Department of Transportation implemented specifications requiring that the production of asphalt mixes be stopped when the air-void content falls below a critical level. To address the problem of low air voids and rutting in north Florida, a proposal was made to reduce the maximum amount of material allowed to pass the 75-μm sieve (P-75μm) at design for asphalt mixtures containing north Florida limestone aggregates. A field study was then undertaken to determine whether this proposal would adequately resolve the problem of low air voids during production due to high P-75μm. The purpose of the study was to determine the amount of degradation to a typical north Florida limestone material and the subsequent effects that degradation has on air voids. The results indicate that although the north Florida limestone aggregates used in this study did degrade significantly, the asphalt contractor was, in general, able to control the amount of P-75μm material in the mix by wasting the baghouse fines. During production, the air voids were low on a number of samples. The source of these low air voids appears to be related to a combination of a high asphalt content in the mix as well as a high P-75μm content. The findings do not support the proposal to reduce the P-75μm content at design at this time. An unexpected finding of this study was that the bulk specific gravities of the commercial aggregate products were less than expected. The impact of this finding is that the voids in the mineral aggregate (VMA) of the mix at design would not meet minimum specification requirements. Although this could make it difficult for an asphalt mixture to have adequate air voids during production, the primary impact of a low VMA is that the pavement would have poor durability and would potentially become brittle and crack prematurely.