This synthesis will be of interest to pavement designers, construction engineers, maintenance engineers, and others interested in avoiding or limiting moisture damage in asphalt concrete. Information is provided on physical and chemical explanations for moisture damage in asphalt concrete, along with a discussion of current practices and test methods for determining or reducing the susceptibility of various asphalt concrete components and mixtures to such damage. Moisture damage in asphalt concrete is a nationwide problem which often necessitates premature replacement of highway pavement surfaces. This report of the Transportation Research Board describes the underlying physical and chemical phenomena responsible for such damage. Current test methods used to determine the susceptibility of asphalt concretes, or their constituents, to moisture damage are described and evaluated. Additionally, current practices for minimizing the potential for moisture damage are examined.

The actual moisture susceptibility of 10 bituminous mixtures placed in the field was compared to the moisture susceptibility that was predicted during the laboratory evaluation of the same mix design. Laboratory mixtures were evaluated using the modified Lottman procedure and the Root-Tunnicliff procedure, in addition to routine moisture susceptibility testing which includes immersion compression testing. After two years, cores were taken from the field projects. The condition of the cores and their present susceptibility to moisture damage were determined by performing modified Lottman and Root-Tunnicliff testing. A feature of the evaluation process was the use of the ACMODAS program to predict the remaining service life of the plant mix cores.

This report summarizes research involving the determination of parameters causing water damage to asphalt concrete. The study was a cooperative effort between the Federal Highway Administration (FHWA) and NMERI. Bituminous mixtures with a history of water susceptibility were fabricated by FHWA. Compacted mixtures were evaluated for moisture damage at FHWA, then shipped to NMERI for chemical analysis of the binders. Extracted binders were analyzed by a modified Clay-Gel procedure and infrared spectroscopy to determine whether differences between moisture conditioned and control asphalts could be measured. An Index of Relative Severity (water damage) has been defined from analysis of the data. Changes in certain asphalt chemical constituents may relate to retained strength for mixtures without antistripping additives. Potential moisture damage is predicted equally with a least two mechanical tests used for mixtures antistripping compounds. Keywords: Clay-Gel compositional analysis, asphaltenes, saturates, aromatics, polars, asphalt generic composition, asphalt functional group concentrations, ketones, sulfoxides, carboxylic acids, IR spectroscopy, Lottman procedure, Immersion-Compression, percent retained strength, antistripping additives or agents, moisture-damage severity, and degree of relative severity.

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