Mechanical and surface free energy characterization of asphalt concrete for moisture damage detection

Moisture damage is defined as the degradation of the mechanical properties of asphalt concrete (AC) caused by moisture. Moisture damage causes a severe loss in the strength and durability of asphalt pavements, leading to a major decrease in pavement performance. Hence, many additives are used to mitigate the effect of moisture on AC. The moisture sensitivity of AC depends on many factors, including aggregates, binders, and AC properties, environmental conditions (e.g., temperature and humidity), traffic loading, quality of construction, and pavement design. Many laboratory tests have been developed to understand and evaluate moisture damage. Laboratory tests are conducted at two levels, AC mixture and its components (binder and aggregate), using various conditioning procedures to introduce the effects of moisture. Current AC moisture susceptibility identification laboratory tests have three main drawbacks: limited ability of simulating environmental conditions, dependence of results on test conditioning process, and limited correlation between laboratory and field results. These shortcomings have compelled the need for the development of new tools to study moisture damage; especially at the component level. The main objective of this study is to predict the impact of several additives/modifiers and recycled asphalt pavement (RAP) on moisture susceptibility of AC at the mixture- and component-level. The impact was evaluated under different aging and conditioning levels. In addition, an assessment of the impact was performed within mixture-level tests and between mixture- and component-level tests. The study is focused primarily on investigating the effects of moisture on AC mixtures. Two AC mixes (in addition to selected additives and modifiers), typically used in the State of Illinois, were considered: 19.0-mm nominal maximum aggregate size (NMAS) binder mix and 19.0-mm NMAS mix with 50% RAP. The additives and modifiers consist of liquid anti-strip (LAS), hydrated lime (HL), styrene butadiene styrene (SBS), SBS with LAS, and polyphosphoric acid (PPA) with HL. The effects of the additives and modifiers on the moisture susceptibility of the proposed AC mixtures were evaluated through several mixture- and component-level tests. The mixture-level tests are the modified AASHTO T-283 and dynamic modulus tests. The surface free energy (SFE), direct adhesion test (DAT) and blister test (BT) represent the component-level types. Samples for the mixture-level tests were prepared at different aging and conditioning durations. Aging durations include one hour and four hours after mixing and prior to compaction; conditioning durations comprise one thawing cycle, three freezing and thawing (FT) cycles, and five FT cycles. Aging was also being considered at the component level by using the rolling thin film oven (RTFO) and pressure aging vessel (PAV). Each binder was tested at the virgin, RTFO, and PAV aging status. The results from modified AASHTO T-283 test indicate that LAS and HL improved moisture resistance. RAP and modified mixes (mixes with SBS or with PPA+HL) were not efficient in reducing moisture susceptibility. Analyzing modified AASHTO T-283 results can be best evaluated using the modified Lottman test, if extensive conditioning is considered, at four hours aging with five FT cycles conditioning. Master curve obtained from the dynamic modulus test found to be not as effective as tensile strength for evaluating moisture damage in AC mixes. The improvement of AC with LAS and RAP is decreased with aging while AC with HL improvement increased with aging. Aggregate had higher SFE than asphalt binder. It was found that AC mixes with LAS, HL and RAP produced the best performance considering moisture resistance when added to asphalt binder. PPA and SBS modifiers produced insignificant alteration of control asphalt binder SFE. Results from the SFE test suggest that there is no unique relation between SFE parameters (SFE, work of adhesion, and work of cohesion) and aging. Direct adhesion test (DAT) appears to be incapable of capturing aging impact on adhesion performance and unreliable in ranking and classifying asphalt binders. This could be related to the drawbacks in sample preparation, repeatability, and sample failure. The current blister test (BT) setup needs improvement to be able to properly assess the adhesion characteristics of asphalt binder. The comparison of a component level parameter (SFE) with mixture-level parameter, tensile stress ration (TSR), showed that TSR and SFE are efficient in identifying the impact of additives on the moisture susceptibility of AC mixes and asphalt binders. However, dynamic modulus (E*) Ratio appears to have a potential for qualitatively evaluating moisture damage of AC mixes at excessive conditioning. This study found that adding LAS or HL are effective in reducing moisture susceptibility of AC mixtures