Polymers are increasingly being used to modify asphalt and enhance highway pavement performance. This paper reports the development of a procedure to evaluate and optimize a polymer-modified asphalt (PMA). Two asphalt cements and two styrene-butadiene-styrene (SBS) copolymers were mixed at ten concentration levels. The engineering properties and morphologies of the binders were investigated using a dynamic shear rheometer, scanning electron microscopy (SEM), and other rheological techniques. The morphology of the PMA was characterized by the SBS concentration and the microstructure of the copolymer. Polymer modification increased the elastic responses and dynamic moduli of asphalt binders. As the SBS concentration increased, the copolymer gradually became the dominant phase, accompanied by a change in engineering properties. Results from SEM demonstrated that, up to 6% concentration, good compatibility exists between SBS and asphalt binder. The modified binders show either a continuous asphalt phase with dispersed SBS particles or a continuous polymer phase with dispersed asphalt globules, or two interlocked continuous phases. The optimum SBS content was determined based on the formation of a critical network between asphalt and polymer.
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