Active Polypropylene Fibers for Controlling Shrinkage Cracks in Cement-Stabilized Materials

Conventional polypropylene fibers, characterized by their smooth surfaces, exhibit relatively weak bonding with cement-based materials, limiting their effectiveness in enhancing these materials’ mechanical properties. This study investigates a graft-modified approach to activating polypropylene fibers, introducing amide groups onto their surfaces to improve fiber–matrix interaction. The active polypropylene fibers were produced using an ultraviolet (UV) grafting technique, where maleic anhydride was first used to graft carboxyl groups onto the fiber surfaces, followed by acylation with diethylenetriamine to introduce amide bonds. The optimal experimental conditions were identified by using the degree of amidation as the response metric. Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) confirmed successful amination and surface activation, with a marked increase in specific surface area. The water contact angle of the active polypropylene fibers decreased significantly from 106.3° to 39.9°, indicating greatly improved wettability by the cement slurry and enhanced bonding strength between the fibers and the cement matrix. To evaluate the effects of the modified fibers, cement-stabilized macadam specimens incorporating various fiber contents were prepared and tested to determine their mechanical properties and shrinkage performance. The results indicated that, compared to conventional polypropylene fibers, the activated polypropylene fibers increased the 28-day compressive strength of CSM by 6.56%, enhanced tensile strength by 4.94%, reduced the rebound modulus by 7.42%, decreased the drying shrinkage coefficient by 25.55%, and lowered the thermal shrinkage coefficient by 13.16%. These findings demonstrate that the chemical bonding between the active polypropylene fibers and the cement matrix is significantly enhanced, leading to improved overall performance in crack resistance, material toughening, and shrinkage mitigation.