Polypropylene fibers can be classified as coarse and fine fibers. Fine polypropylene fibers are relatively small, can refine cracks after being mixed with concrete, improve the original defects of the concrete structure to a certain extent and inhibit cracks. The coarse polypropylene fiber diameter varies from 0.1 to 1.0 mm, and its surface is profiled by rolling. Compared with fine polypropylene fibers, coarse polypropylene fibers can bond tighter with concrete and the bearing capacity for each fiber is larger. In recent years, many scholars have researched the action mechanism of fiber concrete beams and many research results have been obtained. Thomas and Ramaswamy (
2006) studied the shear strength of prestressed steel–fiber-reinforced concrete T-beams, it is shown that the shear capacity of partially prestressed concrete beams can be significantly improved by providing fiber reinforcement. Khalid et al. (
2018) used recycled plastic waste as a fiber in concrete beams and found that a higher amount of fiber resulted in a higher concrete structure tensile strength. Aoude et al. (
2012) conducted tests on reinforced concrete and steel–fiber-reinforced concrete beams and studied the influence of steel fibers on the shear-bearing capacity, failure mechanism and crack control, it is found that the addition of fibers leads to improved shear resistance in shear-deficient beams. Ma et al. (
2018) studied the compressive strength, tensile strength and bending strength of concrete with mixed fibers, and found that a 1.5% volume fraction of long steel fibers and a 0.5% volume fraction of short steel fibers can provide the best bending strength. Hu et al. (
2004) studied the influence of fiber orientation, beam length and reinforcement ratio on the ultimate strength of the beams. The application of fiber-reinforced plastics can improve the stiffness and ultimate strength of the reinforced concrete beams significantly. Khan et al. (
2011) established a constitutive model for the elastic failure of concrete members, including concrete fracture, carbon-fiber-reinforced polymer fracture and concrete carbon-fiber-reinforced polymer interface failure. Lee (
2017) evaluated the influence of concrete strength and fiber content on the concrete proportional limit, residual bending strength and energy absorption capacity, it is shown that fiber-reinforced concrete beams with a strength of 45 MPa show a high increase in residual flexural strength immediately after concrete cracking, especially for 0.5% fiber volume fraction. Ferreira et al. (
2016) found that reinforced concrete with mixed fibers showed an excellent toughness because of the interaction between fibers. Nordin and Taljsten (
2004) studied the bending property of concrete beams with mixed fibers. Sahoo et al. (
2015) found that the displacement ductility of beam specimens increased by 120% with polypropylene fiber addition with a volume fraction of 1%. In all beam specimens, a better postpeak residual strength response resulted because of multi-place cracks that were caused by fiber bridging. Cardoso et al. (
2019) studied the bending characteristics of steel–fiber-reinforced concrete beams, it is found that compared with ordinary reinforced concrete beams, the bearing capacity of steel–fiber-reinforced concrete beams is increased by 21 to 109%, and the crack opening is significantly reduced under a given reinforcement stress. Asgari et al. (
2019) found that the glass–fiber-reinforced concrete layer can improve the bearing capacity and ultimate deflection of the beam significantly by strengthening the concrete beam by the section expansion method. Abdelrazik et al. (
2020) discussed the influence of fiber type and volume on properties of fiber-reinforced concrete. Younis et al. (
2020) reports on the results of an experimental study on the short-term flexural performance of seawater-mixed recycled-aggregate concrete beams, it is found that GFRP-RC beams show higher bearing capacity, but lower deformation characteristics compared with reinforced concrete beams. Liu et al. (
2016; Zhang et al.,
2018) studied the effects of fiber types and hybrid modes on the tensile behavior, flexural toughness and fracture mechanical properties of ultrahigh-performance concrete, it is shown that the critical value of substitution rate of coarse aggregate is 25%, and the effects of different fiber types on compressive strength are similar. Raja et al. (
2020) studied the effect of size and quantity of coarse aggregate on the fracture behavior of steel fiber-reinforced self-compacting concrete, it is argued that the fracture energy increases with the increase of the size and quantity of coarse aggregate. Meesala (
2019) discussed three types of fibers on the properties of recycled-aggregate concrete; it is found that the fiber significantly improves the mechanical properties of conventional and recycled-aggregate concrete. Saje et al. (
2011) found that the shrinkage of the fiber-reinforced concrete was considerably reduced by increasing the content of the fibers up to 0.5% of the volume of the composite. Koniki and Prasad (
2019) pointed out that short and fine fibers enhance the fresh property of concrete by controlling the growth of micro-cracks, and long and coarse fibers enhance the hardening property of concrete by arresting the propagation of macrocracks. Melian et al. (
2010) discussed the low fractions of polypropylene short fibers to increasing toughness of self-compacting concrete. Hameed et al. (
2020) found that the negative effect of recycled aggregates could be minimized by the addition of polypropylene fibers. Das et al. (
2018) revealed that the fibers play an important role in determining the split tensile and flexural strength of concrete, whose maximum increments are 12.01% and 17.15% for split and flexural strength values. Xu et al. (
2017) (Deng et al.,
2018; Huang et al.,
2019) studied bond strength of deformed reinforcement embedded in steel polypropylene hybrid fiber-reinforced concrete matrix and the interfacial bonding properties of steel fibers in steel polypropylene hybrid fiber-reinforced cement-based composites, it is found that compared to the specimen made with plain concrete, the introduction of hybrid fibers could exert obvious positive influences on the bond strength. Gail and Subramaniam (
2019) investigated the link between the fracture behavior and shear capacity of fiber-reinforced concrete composite. Spinella et al. (
2012) predicted the complete load-versus-displacement curves by suitably adapting a nonlinear finite element code for plain and reinforced concrete. Zhang et al. (
2014) studied shear behavior of polypropylene fiber-reinforced ECC beams with varying shear reinforcement ratios. Navas et al. (
2020) studied on the shear behavior of macrosynthetic fiber-reinforced concrete beams and compared them to steel fiber-reinforced concrete beams. Arslan et al. (
2017) found that both the shear strength and ductility of the beams were improved by adding polypropylene fibers, but the addition of polypropylene fibers even at 3% by volume was not able to change the failure mode for beams with shear span-to-effective depth ratios of 2.5 and 3.5.
This paper studies the reinforcement effect of polypropylene fibers on concrete beams. Three groups of beams, B0, B1 and B2, were designed and fabricated, with three beams in each group and nine test beams in total. B0 was a conventional reinforced-concrete beam and will serve as a control group. B1 was a concrete beam that is mixed only with coarse polypropylene fibers. B2 was a multisize polypropylene fiber concrete beam that was mixed with two fine polypropylene fiber types and one type of coarse polypropylene fiber. It is of great theoretical and practical significance to reveal the action mechanism of coarse and fine polypropylene fibers through inclined section shear tests.