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S U M M A R Y Moisture damage continues to be one of the major causes of premature failure of hot-mix asphalt (HMA) pavements. Moisture damage in HMA occurs due to a loss of adhesion and/or cohesion, resulting in reduced strength or stiffness of the HMA and the development of various forms of pavement distress. Numerous laboratory tests have been developed over the years to identify the moisture sen- sitivity of HMA. A general consensus in the industry is that laboratory tests performed on com- pacted HMA have the potential to be better indicators of moisture sensitivity than tests on loose mixture samples or the component asphalt binder or aggregate. Research documented by R. P. Lottman in NCHRP Report 192: Predicting Moisture-Induced Damage to Asphaltic Concrete (Transportation Research Board, National Research Council, Washington, D.C., 1978) with further modifications by D. G. Tunnicliff and R. Root (âAntistripping Additives in Asphalt ConcreteâState-of-the-Art 1981,â Proceedings, Association of Asphalt Paving Tech- nologists, Volume 51, 1982, pp. 265â293) resulted in the laboratory test that currently has the widest acceptance in the paving industry, AASHTO T283/ASTM D4867. Some state highway agencies have used alternatives such as the Hamburg Wheel Tracking Device (HWTD) and the Asphalt Pavement Analyzer (APA). Under NCHRP Project 9-34, research was undertaken to develop an improved laboratory test procedure for predicting asphalt concrete susceptibility to moisture damage through integrating the Environmental Conditioning System (ECS) and Superpave simple perfor- mance tests. The ECS was developed under the Strategic Highway Research Program (SHRP) for predicting the moisture sensitivity of asphalt concrete under conditions of temperature, moisture saturation, and dynamic loading similar to those found in pavements. Several modifications were made to the ECS based on Post-SHRP evaluations at the University of Texas at El Paso. Specific Superpave simple performance tests were researched and recommended under NCHRP Project 9-19. These tests included flow time (static creep), flow number (repeated load permanent deformation), and dynamic modulus. The objective of NCHRP Project 9-34 was to investigate whether an improved moisture sensitivity test could be developed by combining the ECS conditioning procedure with one of the NCHRP Project 9-19 simple performance tests. The scope of work for NCHRP Project 9-34 was divided into two Phases. Phase I focused on the development and execution of a limited experiment to evaluate the potential for combin- ing the NCHRP Project 9-19 simple performance tests with the ECS conditioning procedure to produce an improved moisture sensitivity test. Phase IA focused on further investigation of the ability of the selected simple performance test in combination with the ECS conditioning procedure to predict the moisture sensitivity of a larger number of mixes with documented field performance. Improved Conditioning and Testing Procedures for HMA Moisture Susceptibility 1
2The primary conclusion from the Phase I study was that the dynamic modulus test was the most suited of the three simple performance tests for possible use with the ECS in an im- proved moisture sensitivity test. In Phase IA, the dynamic modulus test was the only test used under the ECS. The combined system was referred to as ECS/dynamic modulus pro- cedure. Under Phase IA, the results from ECS/dynamic modulus tests were compared with those from the HWTD and ASTM D4867 tests. Two broad categories of mixes were included in the experiment: those mixes reported to perform poorly in the field and those mixes reported to perform well in the field. The degree of success of the developed ECS/dynamic modulus procedure was not meas- ured based on how well the results from this procedure compared with those from HWTD or ASTM D4867. Rather, the measure of success was based on the capability of the proce- dure to predict the reported field performance for each mix. The ECS/dynamic modulus procedure appeared to best match the reported field performance for each mix among the presented procedures. Procurement of detailed information regarding construction and performance of selected mixes for this study was a great challenge. In most cases, lack of detailed performance information prevented reliable ranking of the mixtures in terms of moisture damage resist- ance. Furthermore, construction data and in-place density information were not available for use in interpreting results. Three of the mixes selected for this study that were reported to perform poorly had been historically poor performers in regard to moisture damage, regardless of in-place air voids. However, for two of the mixes, reported moisture damage problems were associated with specific projects, and in-place density information for these projects could have been of value in interpreting results. In spite of the advantages of the ECS/dynamic modulus procedure, there are several short- comings that need to be addressed before the ECS/dynamic modulus procedure can be used as a routine mix design test to identify the moisture damage susceptibility of a mix. These problems are mostly associated with the duration of water/load conditioning, temperature at the time of conditioning, and the magnitude of the conditioning load. The developed ECS/dynamic modulus testing procedure has the potential to be used in the HMA design phase. A great benefit to this testing system is that it is focused on measur- ing a widely used engineering property, i.e., asphalt modulus. The technicians and engineers who will be conducting Superpave mix design will need to get familiar with the dynamic modulus test once it becomes part of the Superpave mix design system. As a result, it makes reasonable sense to utilize this testing with water/load conditioning to evaluate moisture damage. Another important application of the results from this testing system will be with the developed mechanistic empirical pavement design guide (MEPDG) used for pavement per- formance predictions. The pavement response models in the design guide utilize the dynamic modulus as one of the important input parameters. The modulus of the uncondi- tioned specimen as well as the retained modulus after the ECS/dynamic modulus testing could be used in the models to determine the impact of moisture damage on developed distresses (rutting and fatigue cracking). Distilled water was used for conditioning of specimens in this study. The researchers for this study are not aware of any study that has investigated mix behavior after the mix has been conditioned with different types of water containing various dissolved chemicals and salts. This may be important to research since rain water and underground water, major contributors to asphalt moisture damage, could contain different types of salts in different parts of the country.
