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.
Ultrasonic Pulse Wave Velocity Test for Monitoring Changes in Hot-Mix Asphalt Mixture Integrity from Exposure to Moisture
The ultrasonic pulse wave velocity test is evaluated as a possible tool for monitoring changes in the integrity of mixtures due to moisture conditioning. Two sets of mixtures were prepared. The first group involved fine-grained (above the restricted zone) and coarse-grained (below the restricted zone) limestone mixtures commonly used by the Florida Department of Transportation (FDOT) that were produced with multiple void structure and permeability configurations by varying the gradations and proportions for a common set of aggregates and asphalt cement. The second set of mixtures consisted of three granite-based mixtures commonly used by FDOT. The results presented show that the small strain modulus obtained with the ultrasonic pulse wave velocity test appears to be sensitive to changes in mixture integrity due to moisture, and therefore it may be a possible tool for the characterization of damage in both laboratory and field specimens. Additional work remains to be performed to better define the meaning of the values obtained. However, the current results indicate that the ultrasonic pulse wave velocity test may be used as an indicator of damage in specimens as well as a quick monitoring device for detecting changes in the integrity of mixtures due to exposure to moisture.
PREMATURE FAILURE OF ASPHALT OVERLAYS FROM STRIPPING: CASE HISTORIES
Separation and removal of asphalt binder from aggregate surface due primarily to the action of moisture and/or moisture vapor is generally termed "stripping." In the identification of the cause of stripping practitioners have, historically, tended to focus their attention on the sensitivity of the aggregate and asphalt system in the presence of moisture. The authors classify this stripping as a physio-chemical incompatibility of the asphalt system, and the classical moisture sensitivity tests are relevant. The case histories in this paper document the effect of pavement saturation. The authors suggest that under saturated conditions all asphalt mixes may fail as a consequence of cyclical hydraulic stress physically scouring the asphalt binder from the aggregate. The authors classify this stripping as a mechanical failure of the asphalt pavement system, and the classical moisture sensitivity tests are irrelevant. While under saturated conditions a less moisture sensitive asphalt system may survive longer, it is probable that failure is deferred and not avoided. Stripping became a major problem in the United States in the late 1970s. Premature failures of asphalt overlays within two years of construction are not uncommon. This paper documents four such case histories from Pennsylvania, Oklahoma, and New South Wales in Australia. Case histories give the details of construction, visual observation of pavement distress, sampling and testing of pavement, and conclusions/recommendations. Moisture profile within the pavement structure was also determined by dry sampling with a jack hammer. The phenomenon of stripping was investigated from a global perspective, looking at the relative permeability of the pavement components, subsurface drainage system, and the interaction between different asphalt courses including open-graded friction courses. Hypotheses are presented to explain the mechanisms that will result in the pavement saturation observed. Recommendations have been made to minimize instances of such premature failures resulting from stripping.