Experimental Investigation of Crack Resistance and Structural Behavior of Polypropylene-Fiber-Reinforced Concrete Box Girders

Crack control and deformation performance are critical to the serviceability of concrete box girders. This study investigates the structural behavior of concrete box girders reinforced with polypropylene fibers through four-point bending tests and clarifies the influence of fiber volume fraction on cracking, deformation, and strain responses. The results show that polypropylene fibers effectively reduce the number of cracks, maximum crack width, and equivalent crack area of the box girders, while improving crack closure. At the service-level load of 0.7P, increasing the fiber volume fraction from 0 to 0.3% reduced the number of cracks from 11 to 4, the maximum crack width from 0.16 mm to 0.12 mm, and the equivalent crack area from 653.97 mm² to 177.50 mm². The initial stiffness of the box girders did not exhibit a monotonic relationship with fiber volume fraction; however, at higher load levels, all fiber-reinforced specimens exhibited greater secant stiffness than the plain-concrete specimen. At a load of 315 kN, compared with the specimen without fibers, the specimen containing 0.2% polypropylene fibers showed an increase in secant stiffness from 15.021 kN/mm to 20.875 kN/mm, accompanied by a reduction in mid-span displacement from 20.971 mm to 15.090 mm. Fiber content also affected the transverse and longitudinal surface strains of the bottom slab at the mid-span section, as well as the longitudinal strains of the webs, with these effects becoming more evident at higher load levels. Overall, a fiber volume fraction of 0.2% provided a favorable balance between crack control and high-load deformation response, whereas 0.3% exhibited the strongest crack-control performance.