The effect of styrene-butadiene-styrene (SBS) additive percentage on the fatigue crack propagation behavior of AC-5 asphalt concrete mixture was studied. Beams were prepared from AC-5 asphalt binder containing 6, 10, and 15 percent SBS by weight. Flexural fatigue tests were conducted on three identical specimens at each additive percentage. Parameters controlling the crack propagation process were evaluated-namely, the energy release rate and the change in work expended on damage formation and history-dependent viscous dissipation processes. The modified crack layer model was used to extract the specific energy of damage-y'characteristic of the mixture's resistance to crack propagation and the dissipative coefficient J3'. It has been found that the 15 percent SBS mixture displayed superior fracture toughness as reflected in·-y'and 13'. As the additive percentage was increased, the fracture toughness of the mixture increased. Also, the ultimate strength and modulus increased. Within the range of additive percentage tested it appears that both the polystyrene endblocks and butadiene rubbery midblocks are working together to improve the ultimate strength and fracture toughness of the asphalt concrete mixture. Scanning electron microscope examination revealed an obvious change in the morphology of the fracture surface as the percentage of additive in the binder increased. This change is manifested in ridge formation in the binderrich areas of the mixture. This change is also indicative of better adhesion between the binder and the aggregate as well as better cohesion within the binder, which in turn contributes to the increased toughness of the asphalt concrete mixture.

Polymer modifiers vary in function and effectiveness. Elastomers, which are at least to some extent derived from a diene chemical structure, will toughen asphalt and improve temperature viscoelastic properties. Plastomers, which come from nondiene chemicals, improve the high-temperature viscoelastic properties of softer asphalt, which has good intrinsic low-temperature properties (1, p. 39). The properties of asphalt mixtures can be improved by selecting modifiers in the proper molecular weight range and mixing the modifiers with asphalt mixtures appropriately. In addition, these modifiers must have solubility parameters close to those of the asphalt mixtures. One of the critical factors that should be considered for better rubber modified asphalt is the air void percentage in the total mix. The performance of the rubber modified asphalt mixture will be improved as this percentage is reduced (2, 3). In general, the air void percentage depends on the load