Moisture Sensitivity of Hot Mix Asphalt (HMA) Mixtures in Nebraska - Phase II
As a consequential effort to the previous NDOR research project (P-564) on moisture damage, this report presents outcomes from this project incorporated with the previous project. Performance changes and fundamental material characteristics associated with moisture damage due to various anti-stripping additives in asphalt mixtures are studied through various experimental approaches and a numerical simulation. Three additives (i.e., one reference additive, hydrated lime, and two alternative additives: fly ash and cement) are investigated by adding them into two types of mixes (SP2 for low-traffic-volume roadways and SP5 for high-traffic- volume roadways) where two different asphalt binders (PG 64-22 for the SP2 mix and PG 70-28 for the SP5) are used. Two asphalt concrete mixture scale performance tests, the AASHTO T-283 and the APA under water, and two local-scale mixture constituent tests, the boiling water test (ASTM D 3625) and the pull-off test, are conducted to characterize the effects of binder-specific anti-stripping additives on the binder-aggregate bonding potential in mixtures. The pull-off tensile strength tests are then numerically modeled through the finite element technique incorporated with the cohesive zone modeling approach to seek more fundamental scientific insights into the effect of each anti-stripping additive on the overall moisture damage resistance. Results from laboratory tests and numerical simulations indicate that the SP5 mixtures, where high-quality aggregates and polymer-modified binder are used, are fairly self-resistant to moisture damage without treating any anti-stripping additive and do not show any visible sensitivity among additives, whereas the effects of additives and their sensitivity are significant in the SP2 mixes that use the unmodified binder PG 64-22 and low-quality aggregates. With the limited amount of test data, hydrated lime seems to perform slightly better than other additives, particularly with longer moisture-conditioning time. Fly ash contributes to reducing moisture damage by improving binder-aggregate interfacial properties, which are validated from the integrated experimental-computational evaluation.
Evaluating Moisture Susceptibility of Cold-Mix Asphalt
Moisture damage–related distress is a primary concern limiting the application of cold-mix asphalt (CMA) as an alternative to hot-mix asphalt in the field. Thus, a simple and effective method is needed in the materials selection process to identify moisture susceptible materials. This study presents a modification to the boiling test procedure specified in ASTM D3625 based on the concept that a mix designer can limit moisture damage by controlling coating quality. The test was modified to present CMA-specific sample preparation guidelines and to remove subjectivity from the test by quantifying coating according to image analysis. Coating quality was evaluated with the coating loss ratio, defined as an index to compare coating before and after the boiling of cured loose mixtures. The relationship between coating quality and moisture resistance was verified with two separate tests: the AASHTO TP-91 binder bond strength test and the AASHTO T-283 tensile strength ratio test. The binder bond strength test provided a direct measure of the bond between binder and aggregate and how it was influenced by moisture. The effect of coating quality on bulk mixture performance was evaluated through comparison with the tensile strength ratio test. All compacted mixtures were produced with a newly developed CMA mix design method to maintain air void levels between samples. Results show that image analysis can be effectively applied to quantify the results of the boiling test, and rankings from all three tests are consistent; this efficacy suggests that the evaluation of coating quality with the modified boiling test can be used as an effective initial evaluation of the potential for CMA moisture susceptibility.
Effects of Water Saturation Level on Resistance of Compacted Hot-Mix Asphalt Samples to Moisture-Induced Damage
The Florida Department of Transportation (FDOT) initiated monitoring of its first Superpave section on I-75 in Columbia County for stripping potential using AASHTO T 283, which specifies that all conditioned test samples be saturated to between 55 and 80 percent based on 7 ± 1 percent air voids. A fairly large saturation range is allowed because it was thought that mixtures may have different percentages of permeable air voids. However, the potential interaction between air void content and the level of saturation has not been fully investigated. It is also not clear whether test results from samples saturated to 55 percent are comparable with those of the same mixture saturated to 80 percent for a similar air void content. Therefore, although FDOT adopted AASHTO T 283, it also initiated a parallel study on the effects of different degrees of saturation on moisture damage. Findings are reported of both the ongoing monitoring of the I-75 project for potential stripping and the investigation of the effects of different levels of saturation on moisture susceptibility test results as determined using AASHTO T 283.