NCHRP 09-34 [Completed]
Improved Conditioning Procedure for Predicting the Moisture Susceptibility of HMA Pavements
| Project Data
||Pennsylvania Transportation Institute|
Background: Moisture susceptibility is a primary cause of distress in hot mix asphalt (HMA) pavements. There is good evidence that moisture susceptibility is influenced by aggregate mineralogy, aggregate surface texture, asphalt binder chemistry, and the interaction between asphalt and aggregate. The great number of different aggregate mineralogies and the numerous types of unmodified and modified asphalt binders used across the United States, coupled with varied environmental conditions, traffic, and construction practices, have made testing to accurately predict HMA moisture susceptibility a difficult task.
AASHTO Standard Method of Test T283, "Resistance of Compacted Bituminous Mixture to Moisture Induced Damage," is the most widely used method for determining HMA moisture susceptibility (ASTM D4867, "Standard Test Method for Effect of Moisture on Asphalt Concrete Paving Mixtures," is a comparable method). However, state highway agencies report mixed success with the method. Several research projects have dealt with the method's shortcomings, resulting in suggested "fixes," but the test remains empirical and liable to give either false positives or false negatives in the prediction of moisture susceptibility.
The SHRP Asphalt Research Program extensively investigated mechanisms of moisture susceptibility and developed new methods for its prediction. The Environmental Conditioning System (ECS; originally AASHTO TP34, "Determining Moisture Sensitivity of Compacted Bituminous Mixtures Subjected to Hot and Cold Climate Conditions") was designed to determine the moisture susceptibility of compacted HMA specimens under conditions of temperature, moisture saturation, and dynamic loading similar to those found in pavements.
The ECS test showed promise, but the visual stripping, permeability, and modulus procedures used in TP34 to evaluate moisture susceptibility gave results that were not significantly more precise or accurate than those of AASHTO T283. For this reason, AASHTO T283 was retained in the Superpave mix design method as the principal method for evaluating HMA moisture susceptibility. NCHRP Project 9-13, "Evaluation of Water Sensitivity Tests," completed in 1999, recommended changes to AASHTO T283 to better accommodate its use in Superpave volumetric mix design.
Research completed in Task C of NCHRP Project 9-19, "Superpave Support and Performance Models Management," has identified several simple performance tests (SPT) for HMA. Tests based on measurement of dynamic modulus (permanent deformation and fatigue cracking), flow time (permanent deformation), and flow number (permanent deformation) were selected for further field validation planned for completion early in 2002. NCHRP Project 9-34 will investigate whether combining the field-validated SPT with an improved ECS conditioning procedure offers an enhanced ability, compared to present methods, to predict HMA moisture susceptibility.
Objective: The objective of this research is to develop an improved conditioning procedure based on the environmental conditioning system (ECS) for evaluating the moisture susceptibility of compacted HMA in combination with the simple performance test (SPT) validated in NCHRP Project 9-19. When combined with the SPT for mix design, or with the SPT or other appropriate tests for field quality control and acceptance (QC/QA), the improved conditioning procedure will be sensitive to the effects of normal variability in material and mix properties expected during laboratory mix design and field construction, and will exhibit acceptable levels of repeatability and reproducibility.
Tasks: Accomplishment of the project objective will require the following tasks.
PHASE I: CONDUCT AN INITIAL ECS EVALUATION (1) Critically review the literature on basic and applied research conducted on HMA moisture susceptibility, including (1) work sponsored by Texas Department of Transportation on improvements to the original ECS, (2) the original ECS development work sponsored by SHRP, and (3) the fundamental work underway at Western Research Institute on the interaction of moisture with the asphalt-aggregate bond. From this review, identify (1) major gaps in current knowledge, (2) probable failure mechanisms contributing to HMA moisture susceptibility, and (3) the current status of the ECS conditioning procedure and possible modifications to improve the prediction of moisture susceptibility. Submit a letter report for NCHRP approval concisely describing the baseline (best currently available) ECS conditioning procedure proposed for use in Task 2 and the reasons for its selection. (2.) Conduct a limited laboratory experiment to statistically evaluate the sensitivity of SPT results to specimens conditioned with the baseline ECS procedure identified in Task 1. As a reference to current practice, compare the sensitivity of the baseline ECS/SPT method to that observed for replicate materials tested in accordance with ASTM D4867. (3) Submit within 6 months of the effective date of the contract an interim report presenting (1) the results of Tasks 1 and 2 and (2) a recommended experimental plan to statistically evaluate whether further proposed modifications to the ECS conditioning procedure (e.g., varying the loading and conditioning times or the conditioning temperature) can provide more accurate, repeatable moisture susceptibility predictions with a minimum of false positive and negative results for specimens tested with the SPT. In planning the experiment, consider controlled factors such as binder type, aggregate type and gradation (e.g., coarse and fine), specimen preparation and size, test temperature, air void content, presence of anti-strip agents, and Ndesign level. The research agency will be required to meet with the NCHRP approximately 1 month later to obtain approval of the experimental plan before beginning Phase II.
PHASE II: IDENTIFY A PRACTICAL, RELIABLE ECS/SPT METHOD (4.) Conduct the experimental plan approved in Phase I and analyze the results to identify the ECS conditioning procedure that, when used with the SPT, provides the most accurate, repeatable moisture susceptibility predictions with minimal false positive and negative results. (5.) Compare the moisture sensitivity predictions of the ECS/SPT method identified in Task 4 with those obtained on replicate materials tested according to (1) ASTM D4867 as a reference to current practice, (2) an ECS/split tensile test as a potential surrogate in field QC/QA applications, and (3) the Hamburg test as a "torture test" to identify false negatives from the Task 4 ECS/SPT test. (6) If the results of Tasks 4 and 5 warrant, develop a recommended, preliminary ECS/SPT test method in AASHTO standard format and suggest moisture susceptibility specification limits for the ranges of U.S. climate and traffic. Prepare a recommended work plan for a future project to (1) determine the ruggedness of the ECS/SPT test method and (2) adapt its use to QC/QA applications. (7.) Prepare a final report that summarizes findings, draws conclusions, and documents the research products, including the preliminary ECS/SPT test method and specification limits and the recommended work plan for future research.
Status: The project is complete.
Product Availability: The final report has been published as NCHRP Report 589: Improved Conditioning and Testing Procedures for HMA Moisture Susceptibility.