Table 5 summarizes the experimental results in terms of compressive, tensile and flexural strength. TC-R and FC-R represent the ratios of the tensile strength to the compressive strength and the flexural strength to the compressive strength, respectively.
Effect of Volume Fraction on the HFRC Mechanical Properties
Effect of the Hybrid Fiber Volume Fraction on the Fluidity of the HFRC
Fig. 3 shows the properties of the HFRC with different volume fractions (0 ~ 0.60%) of hybrid fibers. It can be seen that with an increase in the volume fraction, the fluidity of the HFRC was noticeably reduced. When the volume fraction reached 0.48%, the HFRC exhibited a “dry” phenomenon. The working performances were reduced significantly in pumping concrete and shotcrete. The reasons can be summarized as follows: ① higher concrete strength is mainly dominated by a lower water–cement ratio. Hence, there is less water in the mixing process of C50. ② Carbon fibers are characterized by absorbing water. In the process of concrete mixing, fibers can absorb water from the concrete slurry to reduce the concrete fluidity (Song & Yin, 2016). ③ With increasing volume fraction of fibers, more water needs to be consumed, corresponding to an increase in the amount of the superplasticizer to supplement the water consumption. However, there is a limited effect on its water reduction under saturated superplasticizer, and the water reduction rate is not proportional to the amount of superplasticizer. ④ Fibers also provide additional internal friction, resulting in a reduction in the fluidity of fresh concrete. In other words, small amounts of cement slurry are adsorbed on the surface of the fibers while mixing the fibers into the HFRC. More cement slurry is adsorbed on the surface of the fibers corresponding to the increase in the fiber volume fraction. This phenomenon leads to less free mortar, which is not enough to wrap the coarse aggregate, thus affecting the workability of the concrete. In addition, the fluidity of the concrete is influenced by the viscous effect of a three-dimensional network structure composed of a large number of fibers in the concrete matrix.
Fig. 4 shows the failure modes of HFRC with different fiber volume fractions (0 ~ 0.60%). The results show that with increasing fiber volume fractions, small numbers of micro-fractures emerged on the specimen surface and alleviated the peeling failure. This phenomenon is mainly attributed to the fiber bridging effect (Yoo et al., 2017a, 2017b; Yoo et al., 2017a, 2017b). In other words, when the concrete cracks, fibers are embedded in the two cracked concrete matrixes to prevent further concrete cracking. Although fibers change the crack direction results in other minor cracks.
Effect of Hybrid Fiber Volume Fraction on Mechanical Properties
Fig. 5 shows the hybrid fiber volume fraction effectiveness on mechanical properties. The compressive strength of the HFRC decreased approximately linearly with an increase in the hybrid fiber volume fraction. With increasing volume fractions from 0.12 to 0.6%, the compressive strength of the HFRC was reduced by 4.6, 7.4, 9.6, 11.5, and 13.8% by contrast with that of normal concrete. During the mixing procedure of the fibers and concrete, air infiltration resulted in more pores in the concrete matrix (Balcikanli et al.,2020; Bolooki Poorsaheli et al., 2021; Cui et al., 2020; Scorza et al., 2021). This means that the loose construction around the fibers weakens the bonds of the HFRC. However, with the increasing volume fraction of hybrid fibers, the tensile strength and flexural strength of the HFRC show an unusual increase in contrast to the compressive strength. Moreover, the tensile strength and flexural strength increased first and remained almost unchanged with increasing fiber volume fraction. It can be concluded that the fiber content is a crucial influencing factor on the HFRC strength, while its effect is limited with increasing volume fraction, reaching a maximum at 0.12%.
Relationship of the Tensile and Flexural Strengths to the Compressive Strength
The tensile-to-compressive strength ratio (TC-R) is an important material property of concrete for representing the ultimate strain value in uniaxial tension. Furthermore, the flexural-to-compressive strength ratio (FC-R) is also an important parameter to indicate the brittle material mechanics of the concrete. Fig. 6 shows the effect of the volume fraction (VF) on the TC-R and FC-R of the HFRC. The increase in VF is accompanied by an increase in the TC-R and FC-R. Moreover, with a volume fraction in the range of 0–0.6%, the maximum differences in the TC-R and FC-R are 1.8 and 3.4%, respectively, corresponding to the average growth rates of the TC-R and FC-R compared with that of the normal concrete, which are 27.6 and 31.7%, respectively. This demonstrates that the volume fraction of hybrid fibers has a great effect on the TC-R and FC-R of HFRC. This phenomenon is mainly attributed to that the bond effect between fibers and concrete matrix can effectively resist part of the external force to avoid concrete creaking. After the concrete cracks, the fibers near the cracked part lose their bonding effect with the concrete matrix. Since the length of the fiber is generally larger than the width of micro-cracks, the remaining part of the fiber is embedded in the two parts of the cracked concrete matrix to limit the development of the crack (Yoo, et al., 2017a, 2017b; Yoo, et al., 2017a, 2017b).
Effect of Additional Ratio of Fibers on HFRC Mechanical Properties
Failure Modes of the HFRC Specimens
In the case of the failure modes of HFRC under the different additional ratios of fibers (Fig. 7), there is a significant difference from a single type of fiber concretes, especially in terms of the number of cracks. Fig. 7a, c shows that many cracks appeared on the cube surface under compressive loading. The cube remained whole, even though the corner surface of the cube was partially split. However, for Fig. 7b, the spalling was heavier on the surfaces, while there were fewer cracks on the surface compared with those of specimens (a) and (c). This reveals that the crack-resistance behavior of single CF-reinforced concrete and single AF-reinforced concrete is better than that of single PPF-reinforced concrete. This phenomenon is mainly due to the high elastic modulus of the CF and AF. When the concrete is creaked under pressure, those fibers are easily broken or pulled out of the concrete matrix to meet the compressive deformation or cracking of the concrete. As for the PPF with the low elastic modulus and large deformability, it is easily stretched as the concrete is deformed and creaked, thus inhibiting the development and extension of creaks.
After mixing the three types of fibers in the concrete matrix, although some cracks appeared on the surface of the HFRC cubes, the width of the cracks and the surface spalling level were minor compared with those of a single type of fiber-reinforced concrete. Moreover, Fig. 7d, e shows that the surfaces of the specimen (d) appeared only slightly peeling and bulging. This means that the crack resistance of the HFRC is alleviated by adding the hybrid fibers.
Effect of Hybrid Fiber Volume Fraction on Mechanical Properties
As summarized in Sect. 4.1.3, the optimum volume fraction of hybrid fibers is 0.12%. Therefore, under the different additional ratios of fibers in HFRC and the fixed volume fraction of 0.12%, the relationship of the fiber-to-volume fraction ratio and the HFRC-to-normal concrete strength ratio are investigated and exhibited in Fig. 8. In the case of CF-VF (Fig. 8a), under the same volume fractions of PPF and AF, the tensile strength decreased linearly with decreasing CF-VF and increasing PPF-VF and AF-VF. Meanwhile, the flexural strength decreased to a certain degree. It can be concluded that the strengthening effect of CF-VF on the tensile and flexural strength was weak. The phenomenon was mainly due to the CF adopted in this paper has a small diameter and strong surface adsorption, hence it was difficult to disperse in the concrete matrix, as well as to fully function (Huang et al., 2019; Scorza et al., 2021; Sujay et al., 2020). As PPF-VF (Fig. 8b) decreased and CF-VF and AF-VF increased, the tensile and flexural strengths reached maximums at a fiber ratio of 25:50:25 and subsequently decreased. In addition, the tensile and flexural strengths decreased first and then decreased with decreasing AF-VF and increasing CF-VF and PPF-VF, reaching a maximum at a fiber ratio of 0:0:100. In general, higher AF-VF with carbon fibers and aramid fibers prone to improve the flexural and tensile strengths of the HFRC, the variation of which is the same as PPF-VF. However, higher CF-VF is prone to negative effects with PPF and AF. The phenomenon is mainly attributed to that AF and PPF are easier to disperse than CF and can exert their role in the concrete matrix (Huang et al., 2019; Scorza et al., 2021; Sujay et al., 2020). Hence, as the PPF-VF and AF-VF change, the overall mechanical properties of HFRC change significantly.
Characteristics of the Stress–Strain Relationship
The uniaxial compression constitutive relationship is a fundamental property of concrete and an important basis for obtaining the bearing capacity, ductility, strain and stress of concrete structures. The stress–strain curve reflects multiple mechanical properties of concrete. Peak stress represents the compressive strength of concrete prisms. The tangent slope of the curve denotes the elastic modulus of concrete. The area enclosed by the curve indicates the elastoplasticity and toughness of the material.
The axial stress–strain relationship and their characteristic values are shown in Fig. 9 and Table 6, respectively. All the curves contained an ascending stage and two descending stages. The first descending stage refers the descending stage close to the ascending stage. However, the descending stage became relatively flat with the increase of the strain. In addition, there was a sharp decrease as the strain reached to 0.0035με of the normal concrete. The crack resistance of the normal concrete is mainly attributed to the bond between matrixes. Moreover, the bonding force among the concrete matrixes will disappear as the concrete matrix emerges creaks subjected to load. Although there are mechanical bite force and friction between matrixes, the crack resistance is still weak.
In addition, the overall performances of the single type of carbon fiber-reinforced concrete presented the best behavior in contrast to the others, corresponding to the maximum stress at 62.52Mpa. Moreover, the addition of high-elastic modulus fibers, including AF and CF, led to an increase in the concrete compressive strength. The low-elastic modulus fiber (PPF) generated the lowest stress in concrete compressive strength compared with other types of concrete. Meanwhile, the HFRC has the highest energy absorption, and the incorporation of CF in concrete also exerts a great influence on the energy absorption capacities. Nevertheless, the addition of PPF and AF not only cannot improve the energy dissipation capacity of the concrete but also weakens it, especially adding the PPF. Moreover, with the occurrence of cracks in the concrete matrix, the bonding force between the fibers and concrete matrix can consume part of the energy, as well as the bonding force between the fibers and concrete matrix on both sides of the crack. The bond behavior can alleviate crack propagation and mitigate the decreasing rate of stress. In conclusion, CF and hybrid fibers can increase the strength of concrete, as well as the ductility and energy absorption capacities.
Ratio of Peak Stress to the Cubic Compressive Strength
In general, the axial compressive strength is higher than the peak stress because of the higher loading speed and hoop effect in the testing. The peak stress and cubic compressive strengths of all the specimens are exhibited in Table 7.
The fck /fcu,k values of the five kinds of concrete are between 0.67 and 0.95. Meanwhile, the fck/fcu,k values of C50 and C60 concrete calculated according to GB50010-2010 are only 0.60 and 0.64, respectively, which are significantly lower than the experimental values. The phenomenon was mainly due to that the transverse expansion of the steel loading plate is smaller than that of the concrete specimen subjected to load. Therefore, the frictional force is emerged on the pressure surface of the concrete specimen, which restrains the transverse expansion of the specimen, thus enhancing the strength of specimens. Besides, the loading end of the axial compressive specimen is restrained to the surrounding and upper of the loading end, while the loading end of the compressive specimen only constrains the upper. Therefore, the transverse expansion constraint of the loading end for the axial compressive specimen is stronger than that of the compression specimen, so the measured concrete strength is also higher.
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