This work demonstrates the effectiveness of polymers in improving, especially, the high temperature properties of asphalt. The appropriate choice of asphalt, asphalt-grade, polymer type, polymer concentration, and the method of mixing determine if a network-like structure is formed. This morphology significantly improves the creep performance of the binder at elevated temperatures, i.e., the binder has the ability to store deformation energy with subsequent recoil. This is contrary to Newtonian fluids which transform the energy into viscous flow (no recoil). Within the context of dynamic mechanical measurements, the presence of a polymeric network is manifested through the appearance of a plateau modulus. In the case of binders containing block copolymers, we have repeatedly observed that such property improvement in the high-temperature range is generally accompanied by a reduction of the glassy modulus at the low-temperature range as well. It should be noted that by modifying low-viscosity asphalts (i.e., low AC-grades) with polymers, binders can be obtained which exhibit significantly lower moduli at low temperatures and higher moduli at elevated temperatures. This suggests that although using a high AC-grade asphalt may yield satisfactory results at a particular temperature (high temperature), one may instead optimize binders over the entire temperature range (high and low) by starting with a low AC-grade and adding polymer. These results indicate that careful Theological measurements can be a powerful tool in the characterization and design of viscoelastic blends.
Mechanical properties of asphalts containing styrenic block copolymers and properties of dense-graded asphalt concrete produced from these binders are presented. Materials studied include unmodified AC-5 and AC-20 asphalts and AC-5 containing 3 and 6% styrenic block copolymers. Dynamic rheology of the binders was studied as a function of temperature and deformation rate. Complex viscosity of the polymer-modified asphalts exhibits less temperature susceptibility than that of control asphalts from 0°C (32°F) to 93°C (200°F) and slightly higher temperature susceptibility above 93°C (200°F). The modified asphalts are viscoelastic throughout the pavement operating temperature range with a significant elastic component. However, the unmodified asphalts are essentially nonelastic above 38°C (100°F). Increases in polymer content increase viscosity, ductility, toughness, tenacity, and elasticity of the materials tested. However, shear-thinning characteristics of the polymer-modified asphalts allow handling by familiar techniques at conventional temperatures. Asphalt concrete was evaluated by resilient modulus and indirect tension over a range of temperatures. Results indicate tensile modulus is lowered at low temperatures and raised at high temperatures by addition of the polymer.
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