The tensile and compressive properties of hybrid basalt‐polypropylene fiber‐reinforced concrete (HBPFRC) under uniaxial load were investigated in this study. First, the influence of mono‐basalt fibers, mono‐polypropylene fibers and hybrid fibers with different contents on the stress–strain curve, strength, and toughness of the concrete were analyzed. Second, the tensile and compressive constitutive equations proposed by the Code GB 50010‐2010 were used to fit the stress–strain curves, in which good fitting results are obtained. Finally, the tensile and compressive mechanism of the HBPFRC were discussed. The results indicated that the better positive hybrid effect for both tensile and compressive tests were achieved when the mass ratio of basalt fiber to polypropylene fiber is 1:2 with the total mass of 6 kg/m3. At this time, when compared with the concrete without fibers, the tensile strength and the tensile strain increased by 24 and 55% separately, while the compressive strength and the compressive strain increased by 14 and 36% separately. In addition, the tensile stress–strain curve of HBPFRC can be well fitted by peak tensile strength (ft), peak tensile strain (εt) and tensile shape factor (αt), while the compressive stress–strain curve of HBPFRC can be well matched by peak compressive strength (fc), peak compressive strain (εc) and compressive shape factor (αc).
To investigate the effects of mixing polypropylene fiber of different sizes and the effect of fiber size on the impact characteristics of concrete, two sizes of polypropylene fine fiber and one size of polypropylene coarse fiber were selected to design and fabricate nine groups of polypropylene fiber-reinforced concrete test specimens by controlling the fiber mixing ratio and conducting a split Hopkinson pressure bar test to obtain the stress-strain curves of the test specimens in various groups, and their parameters such as the elasticity modulus, peak strength, and peak strain. The incorporation of fiber improved the pre-peak impact properties of concrete to different extents. Strain hardening did not occur in the post-peak curves, and different types of fibers exhibited different characteristics. Thus, the fine fiber could significantly improve the peak strain, while the coarse fiber could more significantly improve the elasticity modulus and peak strength. The improving effects exerted by incorporating three types of fiber were better than those exerted by incorporating two types of fiber. Moreover, the statistical damage model was used to obtain the parameters by fitting and analyzing their variation rules based on the statistical damage constitutive model and the particle swarm optimization algorithm.
An experimental program consisting in producing and testing reinforced concrete pipes (RCPs) under the three‐edge bearing tests considering different types of reinforcement was carried out. Four types of RCPs were produced, these reinforced with: (1) polypropylene macrofibers; (2) basalt microfibers; (3) combination of both (hybrid reinforcement); and (4) plain concrete. The analysis of the crack patterns and both service and ultimate mechanical responses allowed concluding that the use of fibers do not lead to an effective increase of the first cracking load; however, both types of fibers allowed a better crack width control respect to the standard RCP. In this regard, basalt microfiber reinforced concrete led to a better response caused by concentrated loads (jacketing) whilst polypropylene macrofibers increased the concrete pipe performance in terms of bearing capacity and flexural crack control. The hybrid fiber reinforced concrete was found to be the most suitable alternative for increasing the load bearing capacity and the crack width control for service loads. These incipient experimental results permit to conclude that this type of hybrid basalt‐polypropylene fiber reinforced concretes are an interesting alternative to traditional steel‐cage RCPs.
To study the hybrid effects of polypropylene fiber and basalt fiber on the fracture toughness of concrete, 13 groups of notched concrete beam specimens with different fiber contents and mass ratios were prepared for the three-point bending test. Based on acoustic emission monitoring data, the initiation cracking load and instability load of each group of specimens were obtained, and the fracture toughness parameters were calculated according to the double-K fracture criterion. The test results show that the basalt fiber-reinforced concrete has a greater increase in initial fracture toughness, and the toughness of coarse polypropylene fiber-reinforced concrete is more unstable. Moreover, after the coarse polypropylene fiber content reaches 6 kg/m3 and the basalt fiber content reaches 3 kg/m3, increasing the content will not significantly improve the fracture toughness of the concrete. The polypropylene–basalt fiber will produce positive and negative effects when mixed, and the mass ratio of 2:1 was optimal. Finally, the fitting analysis revealed that the fracture process of polypropylene–basalt fiber-reinforced concrete (PBFRC) can be objectively described by the bilinear softening constitutive curve improved by Xu and Reinhardt.
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