Physico-Chemical Characterization of Asphalt-Aggregate Interactions under the Influence of Freeze-Thaw Cycles

Experiments were conducted to investigate the impact of different freeze-thaw cycling on the mechanical strength of eight different types of asphalt-aggregate mixtures. In these experiments, cylindrical specimens prepared by gyratory compactor were subjected to different freeze-thaw cycling. The indirect tensile strength at 25°C (77°F) was determined after different numbers of freeze-thaw cycles. All broken samples were contacted with cyclohexane to elute asphalt from the mixtures. Polar organic fractions were isolated from each of the mixtures using a solvent extraction method. It was observed that mild oxidation of asphalt results in recovery of large amounts (compared with unaged asphalts) of polar organics from the aggregate surface of all mixtures. Presumably these strongly bonded materials are important factors in protecting mixtures against the action of moisture. Test results also indicated that mixtures prepared from asphalt, which had been pre-aged, resisted freeze-thaw action more than unaged asphalt, and showed higher retained tensile strength than the unaged mixtures. In addition, results showed that mixtures prepared from dodecanophenone-treated asphalt interact with aggregates differently. INTRODUCTION Although considerable work has been done to try to understand the mechanisms directing the behavior of asphaltaggregate mixtures in the presence of water, most of the emphasis has been placed either on empirical tests for measuring the physical strength (e.g., current Superpave® mixture design classified as TSR value) [1,2,3,4,5] or the influence of specific chemical components (functional groups) on the asphalt-aggregate bond [6,7,8,9,10,11,12]. A commonly reported difficulty is that tests are based on a pass-or-fail criterion. Unfortunately, the information obtained from the tests usually is too qualitative and field conditions are usually not simulated. Further, very little emphasis has been placed on the direct correlation of overall chemical composition with properties of asphaltaggregate interfaces and physical properties of asphalt-aggregate mixtures. Empirical models developed previously do not explain adequately asphalt-aggregate interactions in the presence of water because they are not based on fundamental physico-chemical principles. The current Superpave® mixture design criteria for water stripping are based on empirical correlations with field data without consideration of the chemical interactions of asphalts with aggregates which affect pavement stability under the action of water. Additionally, the ability of asphalt-aggregate interfacial bonds to withstand exposure or weathering over extended periods of time has been studied minimally. It has long been speculated that the adsorption and molecular orientation of asphalt molecules at the asphalt-aggregate interface region is a slow process and often continues for several hours or days even at high temperatures, depending on the composition of the asphalt binder and the nature of the aggregate surface. Obviously, the formation of the adsorbed layer may take longer at lower pavement temperatures because of the mobility effect. Therefore, it is important to understand how the loss of adhesive strength of mixes corresponds to the loss of certain chemical components that migrate to or from an aggregate surface as a function of time under the action of water. It is the authors’ speculation that the chemical and physical interactions that occur between asphalt and aggregate at the interface are influenced by freeze-thaw cycling over extended periods of time. Among these interactions affected by freeze-thaw cycling are those influencing adhesive strength. Water partially desorbs polar asphalt components from rock surfaces. The degree of desorption is a function of the asphalt-aggregate combination, the time in contact with water, the temperature of water, and the pH of the water. More specific and more speculative hypotheses proposed for this study are as follows. First, the greater the concentration of specific polar organics in asphalt (which in some cases can be determined by infrared spectroscopy), the more resistant asphalt-aggregate mixtures will be to water damage. It is speculated that bonding between polar organics in asphalts and rock surfaces is the major factor determining asphalt-aggregate bonding strength, and that moisture damage is related to the desorption of some of the polar organics. Second, water may disrupt binder cohesiveness as well as desorb polar organics from the rock surface. Third, the adhesive strength at the interface increases after mild oven aging. This is because the functional groups (especially ketones) formed during oxidation enhance adhesion and therefore mitigate moisture damage. The objectives of this study are: 1) to establish a correlation between chemical properties and interactions that influence adhesion at asphalt-aggregate interfaces in the presence of water over extended periods of time, 2) to determine why the adhesive strength at the interface increases (or decreases) after short-term and long-term oven aging, 3) to investigate the effect of model compounds at the asphalt-aggregate interface in the presence of water over extended periods of time, and 4) to determine whether there is a common set of asphalt components at aggregate surfaces at the time of moisture damage failure. TRB 2003 Annual Meeting CD-ROM Original paper submittal – not revised by author.