Cracking indices and effects of synthetic fibers on the mechanical properties of cold recycled asphalt mixtures

Despite their environmental and economic benefits, cold recycled asphalt mixtures are not widely used partly because of their inadequate mechanical properties. This study evaluates the validity and practicality of the flexibility index and CTIndex for emulsified asphalt cold recycled mixtures. Modifications in I-FIT analysis and a modified IDEAL-CT cracking index for mixtures without cement have been proposed and evaluated. It was found that the revised IDEAL-CT index can better capture the crack resistance of cold recycled mixtures with and without fibers. Polypropylene, A-glass, para-aramid and polyester synthetic fibers were added to emulsified asphalt cold recycled mixtures in different lengths and contents. IDEAL-CT and dry and wet ITS were used to determine the optimum fiber lengths and contents. The combined effects of the optimized fibers and cement were evaluated using IDEAL-CT, dry and wet indirect tensile and I-FIT testing. The performance tests showed that the addition of fibers generally increased the dry tensile strength and crack resistance, reduced the wet tensile strength and increased the moisture sensitivity. Except for the A-glass fibers, shorter lengths and lower fiber contents generally increased the strength, but decreased the crack resistance. In most cases, the combined use of fibers and cement retained the favorable properties of the mixtures having cement, but compensated for their inherent crack resistance defects. In some cases, the combined use of cement and fibers produced similar or better results than the control mixture having a higher emulsified asphalt content. The polypropylene and polyester fibers showed the most promising results overall.