Flexural Performance of a New Hybrid Basalt-Polypropylene Fiber-Reinforced Concrete Oriented to Concrete Pipelines

: The bending performance of a basalt-polypropylene ﬁber-reinforced concrete (HBPFRC) was characterized by testing 24,400 × 100 × 100 mm 3 prismatic specimens in a four-point bending test JSCE-SF4 conﬁguration. The type and content of both ﬁbers were varied in order to guarantee different target levels of post-cracking ﬂexural performance. The results evidenced that mono-micro basalt ﬁber reinforced concrete (BFRC) allows the increase of the ﬂexural strength (pre-cracking stage), while macro polypropylene ﬁber reinforced concrete (PPFRC) can effectively improve both bearing capacity and ductility of the composite for a wide crack width range. Compared with the plain concrete specimens, ﬂexural toughness and equivalent ﬂexural strength of macro PPFRC and the hybrid ﬁber-reinforced concrete (HFRC) increased by 3.7–7.1 times and 10–42.5%, respectively. From both technical and economic points of view, the optimal mass ratio of basalt ﬁber (BF) to polypropylene ﬁber (PPF) resulted in being 1:2, with a total content of 6 kg/m 3 . This HFRC is seen as a suitable material to be used in sewerage pipes where cracking control (crack formation and crack width control) is of paramount importance to guarantee the durability and functionality of the pipeline as well as the ductility of the system in case of local failures.


Introduction
It is well known that concrete is one of the most widely used civil materials for various engineering applications, such as hydraulic engineering, architectural engineering, road and bridge engineering.However, the concrete used in engineering usually had a large number of cracks, thus making the concrete brittle and subject to varying degrees of damage under external loads [1].For the safety and reliability of concrete structures, higher requirements should be put forward for the energy dissipation capacity of concrete.FRC not only improves the brittleness of concrete but also significantly enhances the toughness and energy dissipation capacity of concrete [2][3][4][5].Thus, FRC has become a widely used composite building material [6,7].
At present, many countries in the world have successively established the standard test methods for testing the bending performance of fiber reinforced concrete, such as JSCE-SF4 [8], ASTM C1018 [9] and CECS 13:2009 [10].These standards provide a precise calculation method for the flexural strength, flexural toughness index and energy absorption of fiber reinforced concrete materials.Table 1 gathers relevant references related to the flexural characterization of FRCs with different types of fibers and amounts.Note: SF means steel fiber; MSF means macro-synthetic fibers; PFF means polyolefin fiber; PVAF means polyvinyl alcohol fiber; f c means compressive strength; / means lack of information; Φ f /λ f means aspect ratio.
Table 1 allows confirming that there exists extensive research on flexural performance of fiber reinforced concrete and that this topic is of interest from both scientific and industrial perspective.The general conclusion that can be extracted from the previous research is that the use of steel, polyolefin and/or polypropylene fibers, can largely improve the flexural performance of concrete.The mechanical properties of concrete reinforced with different types of hybrid fibers, which is termed hybrid FRC (HFRC), are usually superior to that of concrete reinforced with mono-fiber [22][23][24][25], HFRC is produced to achieve overall improvement both in energy absorption capacity and tensile strength.It is well known that coarse and long fibers control the propagation of macro-cracks and improve the toughness at the post-crack region [26,27], while micro and short fibers bridge the micro-cracks, thereby enhancing the peak strength [28].So, the combination of different lengths, diameters and elastic modulus of fibers are often adopted by researchers [29][30][31].An example of HFRC is the hybrid steel-polypropylene fibers (see Table 1), which are also used and confirmed to have obtained good hybrid effects [32][33][34][35].However, steel fiber is easy to corrode, which is not conducive to the long-term stability of the structure.Moreover, the incorporation of steel fiber will not only reduce the workability of concrete but also increase the weight of concrete [36,37].Therefore, under certain environmental conditions (such as an acidic environment), it is necessary to use corrosion-resistant material instead of steel fiber to improve the flexural performance of concrete.
A suitable material to substitute steel fiber is BF, which is characterized by good temperature stability, high tensile strength, strong corrosion-resistance, good deformation performance, low price, safe and environmental protection, etc. [38][39][40].In addition, it has been confirmed that HBPFRC can show excellent crack propagation inhibition, fire resistance and flexural performance [41][42][43][44][45].However, in the above studies, PPFs are all fine fibers, while the previous study [46] has shown that the improvement effect of macro PPFs on the bending properties of concrete is significantly superior to that of micro PPFs.
Therefore, micro BF and macro PPF are selected to be mixed into the concrete matrix to produce HBPFRC specimens.Furthermore, according to the Code CECS 13:2009 [10], the flexural performance of HBPFRC specimens was investigated by the four-point bending tests.This paper studies the flexural properties of HBPFRC, which indicates that this HFRC composite is a suitable material to be used in structures, such as sewerage pipes, where cracking control is of vital importance to guarantee the durability and functionality of the pipeline as well as the ductility of the system in case of local failures.

Test Material
The cement adopted in this test is Portland cement P.O.52.5.The coarse aggregate used were stones with grain sizes of 10-20 mm and 5-10 mm, while the fine aggregates were sands of dimensions less than 5 mm.In addition, a commercially available polycarboxylic acid superplasticizer was adopted to improve concrete workability, and the water reducing rate of it is 28%.The concrete mix proportion is shown in Table 2.The fibers used in this test are macro PPF with a wavy surface and micro BF with a smooth surface, as shown in Figure 1 [47].
pipes, where cracking control is of vital importance to guarantee the durability and functionality of the pipeline as well as the ductility of the system in case of local failures.

Test Material
The cement adopted in this test is Portland cement P.O.52.5.The coarse aggregate used were stones with grain sizes of 10-20 mm and 5-10 mm, while the fine aggregates were sands of dimensions less than 5 mm.In addition, a commercially available polycarboxylic acid superplasticizer was adopted to improve concrete workability, and the water reducing rate of it is 28%.The concrete mix proportion is shown in Table 2.The fibers used in this test are macro PPF with a wavy surface and micro BF with a smooth surface, as shown in Figure 1  In addition, the properties of the selected fibers are shown in Table 3.

Test Material Preparation
There are eight groups of HBPFRC with different fiber content, including two groups of mono-fiber, five groups of hybrid basalt-polypropylene fiber as well as one control group (no fiber).Details of each group specimen for the flexural bending test are shown in Table 4.In addition, the properties of the selected fibers are shown in Table 3.

Test Material Preparation
There are eight groups of HBPFRC with different fiber content, including two groups of mono-fiber, five groups of hybrid basalt-polypropylene fiber as well as one control group (no fiber).Details of each group specimen for the flexural bending test are shown in Table 4.As shown in Table 4, fiber contents ranged between 0 and 6 kg/m 3 .Amount of 4 kg/m 3 of PPFs was considered the lower bound to provide ductility of the composite respect the unreinforced concrete [48,49] whilst 6 kg/m 3 was fixed as an upper bound since higher amounts may compromise the workability and the finishing [50][51][52][53][54].The select of ratio between fiber content of BF and PPF referred to the previous research results [47,53,54].
The mixability of fiber is very important to improve the performance of fiber reinforced concrete.In order to make the fibers evenly distributed into the concrete, referring to relevant specification and literature [10,47], the mixing process adopted in this experiment is as follows: (1) Pour the pre-weighted coarse and fine aggregates into the forced mixer and mix the materials for 1 min.(2) Evenly scatter the macro PPF and BF into the mixer, and the mixing process still lasted for 2 min after the fibers are all put into the mixer.As shown in Table 4, fiber contents ranged between 0 and 6 kg/m 3 .Amount of 4 k of PPFs was considered the lower bound to provide ductility of the composite respec unreinforced concrete [48,49] whilst 6 kg/m 3 was fixed as an upper bound since hi amounts may compromise the workability and the finishing [50][51][52][53][54].The select of between fiber content of BF and PPF referred to the previous research results [47,53, The mixability of fiber is very important to improve the performance of reinforced concrete.In order to make the fibers evenly distributed into the conc referring to relevant specification and literature [10,47], the mixing process adopted in experiment is as follows: (1) Pour the pre-weighted coarse and fine aggregates int forced mixer and mix the materials for 1 min.( 2  In this test, each group contained three replicate specimens.According to CECS 2009 [10], a total of 24 concrete specimens with a length × width × height of 400 mm × 100 mm × 100 mm were produced, and the four-point bending test was conducted after 28 days of curing.

Experimental Test Method
The four-point bending test was conducted to measure the flexural bending performance of concrete.The test device was the INSTRON−1346 hydraulic servo testing machine system.YOKE method is used to measure the deflection of beams [8].Displacement control was adopted for continuous loading in the test, and the loading rate was 0.1 mm/min.After the deflection of the specimen reached 4 mm, the test was stopped.The test process was shown in Figure 3.In this test, each group contained three replicate specimens.According to CECS 2009 [10], a total of 24 concrete specimens with a length × width × height of 400 mm × 100 mm × 100 mm were produced, and the four-point bending test was conducted after 28 days of curing.

Experimental Test Method
The four-point bending test was conducted to measure the flexural bending performance of concrete.The test device was the INSTRON−1346 hydraulic servo testing machine system.YOKE method is used to measure the deflection of beams [8].Displacement control was adopted for continuous loading in the test, and the loading rate was 0.1 mm/min.After the deflection of the specimen reached 4 mm, the test was stopped.The test process was shown in Figure 3.

Failure Modes
As the failure modes of each group of HBPFRC specimens are similar, the failure modes of B2P4 are selected to represent that of the other HBPFRC specimens.The failure modes of the control group specimen, the mono FRC specimen and the HBPFRC specimen are shown in Figure 4a-d

Failure Modes
As the failure modes of each group of HBPFRC specimens are similar, the failure modes of B2P4 are selected to represent that of the other HBPFRC specimens.The failure modes of the control group specimen, the mono FRC specimen and the HBPFRC specimen are shown in Figure 4a-d.In this test, each group contained three replicate specimens.According to CECS 2009 [10], a total of 24 concrete specimens with a length × width × height of 400 mm × 100 mm × 100 mm were produced, and the four-point bending test was conducted after 28 days of curing.

Experimental Test Method
The four-point bending test was conducted to measure the flexural bending performance of concrete.The test device was the INSTRON−1346 hydraulic servo testing machine system.YOKE method is used to measure the deflection of beams [8].Displacement control was adopted for continuous loading in the test, and the loading rate was 0.1 mm/min.After the deflection of the specimen reached 4 mm, the test was stopped.The test process was shown in Figure 3.

Failure Modes
As the failure modes of each group of HBPFRC specimens are similar, the failure modes of B2P4 are selected to represent that of the other HBPFRC specimens.The failure modes of the control group specimen, the mono FRC specimen and the HBPFRC specimen are shown in Figure 4a-d   For the control group specimen B0P0 and the mono-basalt FRC specimen B6P0, the crack rapidly expands to the top of the specimen and runs through the entire section after it appears.Thus, the concrete was pulled apart.It shows that the failure modes of both B0P0 and B6P0 are characterized by brittle failure (see Figure 4a,c), although B6P0 shows a little higher flexural strength.It can be seen that the addition of mono-BF won't improve much of the brittleness of concrete.However, for the mono-polypropylene FRC specimen B0P6 and HBPFRC specimen B2P4, after the crack appears, it first rapidly expands to about 2/3 the height of the section.Then, it slowly expands towards the top of the specimen, but it did not run through the entire section during the whole test.The macro PPF in the specimen plays a significant role in bridging; thus, the beam can still show a certain bearing capacity and a long load-bearing time after the crack appears.It shows that the failure modes of both B0P6 and B2P4 change from the sudden brittle failure of plain concrete to ductile failure (see Figure 4b,d).It can be seen that the addition of macro PPF will largely improve the brittleness of concrete.

Load-Deflection Curve
The load-deflection curves of each group of HBPFRC specimens are shown in Figure 5a-g.According to Figure 5a-g, it can be seen that for the control group specimen B0P0 and the mono-basalt FRC specimen B6P0, the load of which immediately decreased to zero after it reached the peak value.B0P0 and B6P0 showed obvious brittleness, and they fractured into two parts in the middle.However, compared with B0P0, the descending stage of B6P0 was relatively slow, indicating the brittleness of concrete is somewhat improved due to the addition of mono-BF.
As for the specimens with macro PPF, although the bearing capacity of these specimens will drop instantly after reaching the peak load, they still maintain a certain residual strength and will not break during the whole test process.It was found that the macro PPF was continuously pulled out and broken during the loading process, which reflected the noticeable bridging effect of the fiber.The descending stage of the loaddeflection curves fluctuated to some extent locally due to the pulled out and broken of fibers.Still, the curves were generally gentle, and even the phenomenon of secondary peak appeared in different degrees.Among them, the secondary peak values of B0P6 and B2P4 were particularly apparent (see Figure 5b-e).It can be seen that the addition of macro propylene fiber significantly improves the bending performance of concrete specimens.For the control group specimen B0P0 and the mono-basalt FRC specimen B6P0, the crack rapidly expands to the top of the specimen and runs through the entire section after it appears.Thus, the concrete was pulled apart.It shows that the failure modes of both B0P0 and B6P0 are characterized by brittle failure (see Figure 4a,c), although B6P0 shows a little higher flexural strength.It can be seen that the addition of mono-BF won't improve much of the brittleness of concrete.However, for the mono-polypropylene FRC specimen B0P6 and HBPFRC specimen B2P4, after the crack appears, it first rapidly expands to about 2/3 the height of the section.Then, it slowly expands towards the top of the specimen, but it did not run through the entire section during the whole test.The macro PPF in the specimen plays a significant role in bridging; thus, the beam can still show a certain bearing capacity and a long load-bearing time after the crack appears.It shows that the failure modes of both B0P6 and B2P4 change from the sudden brittle failure of plain concrete to ductile failure (see Figure 4b,d).It can be seen that the addition of macro PPF will largely improve the brittleness of concrete.

Load-Deflection Curve
The load-deflection curves of each group of HBPFRC specimens are shown in Figure 5a-g.According to Figure 5a-g, it can be seen that for the control group specimen B0P0 and the mono-basalt FRC specimen B6P0, the load of which immediately decreased to zero after it reached the peak value.B0P0 and B6P0 showed obvious brittleness, and they fractured into two parts in the middle.However, compared with B0P0, the descending stage of B6P0 was relatively slow, indicating the brittleness of concrete is somewhat improved due to the addition of mono-BF.
As for the specimens with macro PPF, although the bearing capacity of these specimens will drop instantly after reaching the peak load, they still maintain a certain residual strength and will not break during the whole test process.It was found that the macro PPF was continuously pulled out and broken during the loading process, which reflected the noticeable bridging effect of the fiber.The descending stage of the load-deflection curves fluctuated to some extent locally due to the pulled out and broken of fibers.Still, the curves were generally gentle, and even the phenomenon of secondary peak appeared in different degrees.Among them, the secondary peak values of B0P6 and B2P4 were particularly apparent (see Figure 5b-e).It can be seen that the addition of macro propylene fiber significantly improves the bending performance of concrete specimens.

Flexural Strength
The peak load of each group specimen is extracted, and then the flexural strength of concrete is calculated by Equation (1): where fb is the flexural, MPa; P is peak load, N; L is the span of the specimen, mm; b is the section width of the specimen, mm; H is the height of the specimen section, mm.The flexural strengths of FRC specimens in this test are shown in Figure 6.As shown in Figure 6, the flexural strength of the control group specimen B0P0 is 4.0 MPa, while the flexural strength of the mono-basalt FRC specimen B6P0 is 4.4 MPa, increased by 10%.As for the flexural strength of the mono-macro polypropylene FRC specimen B0P6, it increases to 5.2 MPa with a growth rate of 30%.It can be seen that the improvement effect of macro PPF on the flexural strength of concrete specimens is more significant than that of BF.Among the HBPFRC specimens, when the mass ratio of BF to macro PPF is 1:2, the flexural strength of the specimen B2P4 reached 5.7 MPa, and the

Flexural Strength
The peak load of each group specimen is extracted, and then the flexural strength of concrete is calculated by Equation (1): where f b is the flexural, MPa; P is peak load, N; L is the span of the specimen, mm; b is the section width of the specimen, mm; H is the height of the specimen section, mm.The flexural strengths of FRC specimens in this test are shown in Figure 6.

Flexural Strength
The peak load of each group specimen is extracted, and then the flexural strength of concrete is calculated by Equation (1): where fb is the flexural, MPa; P is peak load, N; L is the span of the specimen, mm; b is the section width of the specimen, mm; H is the height of the specimen section, mm.The flexural strengths of FRC specimens in this test are shown in Figure 6.As shown in Figure 6, the flexural strength of the control group specimen B0P0 is 4.0 MPa, while the flexural strength of the mono-basalt FRC specimen B6P0 is 4.4 MPa, increased by 10%.As for the flexural strength of the mono-macro polypropylene FRC specimen B0P6, it increases to 5.2 MPa with a growth rate of 30%.It can be seen that the improvement effect of macro PPF on the flexural strength of concrete specimens is more significant than that of BF.Among the HBPFRC specimens, when the mass ratio of BF to macro PPF is 1:2, the flexural strength of the specimen B2P4 reached 5.7 MPa, and the As shown in Figure 6, the flexural strength of the control group specimen B0P0 is 4.0 MPa, while the flexural strength of the mono-basalt FRC specimen B6P0 is 4.4 MPa, increased by 10%.As for the flexural strength of the mono-macro polypropylene FRC specimen B0P6, it increases to 5.2 MPa with a growth rate of 30%.It can be seen that the improvement effect of macro PPF on the flexural strength of concrete specimens is more significant than that of BF.Among the HBPFRC specimens, when the mass ratio of BF to macro PPF is 1:2, the flexural strength of the specimen B2P4 reached 5.7 MPa, and the flexural strength increased the most, reaching 42.5%.This may be due to the good hybrid effect of BF and macro PPF with this hybrid ratio.

Flexural Toughness
Flexural toughness (T b ) is used to describe the energy absorption capacity of concrete quantitatively.The flexural toughness evaluation method suggested by JSCE-SF4 [8] is adopted in this paper, which does not need to determine the deflection of the initial crack point, and the unstable section of the curve has little influence on it.Flexural toughness is defined as the envelope area of the load-deflection curve under deflection from 0 mm to 1/150 L (2 mm).Because control group specimens and mono-basalt FRC specimens showed brittle failure during the loading process, the flexural toughness of which are calculated by the envelope area of all load-deflection curves according to the literature [55].Figure 7 shows the flexural toughness of different concrete specimens.flexural strength increased the most, reaching 42.5%.This may be due to the good hybrid effect of BF and macro PPF with this hybrid ratio.

Flexural Toughness
Flexural toughness (Tb) is used to describe the energy absorption capacity of concrete quantitatively.The flexural toughness evaluation method suggested by JSCE-SF4 [8] is adopted in this paper, which does not need to determine the deflection of the initial crack point, and the unstable section of the curve has little influence on it.Flexural toughness is defined as the envelope area of the load-deflection curve under deflection from 0 mm to 1/150 L (2 mm).Because control group specimens and mono-basalt FRC specimens showed brittle failure during the loading process, the flexural toughness of which are calculated by the envelope area of all load-deflection curves according to the literature [55].Figure 7 shows the flexural toughness of different concrete specimens.It can be seen from Figure 7 that the flexural toughness of the control group specimen B0P0 is only 2.0 J, while the flexural toughness of the mono-basalt FRC specimen B6P0 is 3.0 J, increasing by 50%.The flexural strength of mono-macro PPFRC (B0P6) is 13.3 J, which is 5.65 times higher than that of the control group.Compared with BF, the macro PPF has a more significant effect on improving the flexural strength of concrete.Among the HBPFRC specimens, the flexural toughness of B4.8P1.2 is the lowest, but it is still 4.25 times higher than that of the control group specimen B0P0.When the mass ratio of BF to PPF is 1:2, the flexural toughness of the specimen B2P4 reached 22.4 J, and it increased by 10.2 times.In conclusion, the flexural toughness of PPF reinforced concrete specimen is better than that of BF reinforced concrete specimen.Because of the fiber mixing effect, the specimens with mixed fiber reinforced concrete can show much better flexural performance than that of the specimens reinforced by mono-macro fiber when the mixing ratio of BF to PPF is 1:2.
Furthermore, the equivalent flexural strength and the percentage of equivalent flexural strength of each group were calculated according to the following Equations ( 2) and (3), respectively.It can be seen from Figure 7 that the flexural toughness of the control group specimen B0P0 is only 2.0 J, while the flexural toughness of the mono-basalt FRC specimen B6P0 is 3.0 J, increasing by 50%.The flexural strength of mono-macro PPFRC (B0P6) is 13.3 J, which is 5.65 times higher than that of the control group.Compared with BF, the macro PPF has a more significant effect on improving the flexural strength of concrete.Among the HBPFRC specimens, the flexural toughness of B4.8P1.2 is the lowest, but it is still 4.25 times higher than that of the control group specimen B0P0.When the mass ratio of BF to PPF is 1:2, the flexural toughness of the specimen B2P4 reached 22.4 J, and it increased by 10.2 times.In conclusion, the flexural toughness of PPF reinforced concrete specimen is better than that of BF reinforced concrete specimen.Because of the fiber mixing effect, the specimens with mixed fiber reinforced concrete can show much better flexural performance than that of the specimens reinforced by mono-macro fiber when the mixing ratio of BF to PPF is 1:2.
Furthermore, the equivalent flexural strength and the percentage of equivalent flexural strength of each group were calculated according to the following Equations ( 2) and (3), respectively.
where f e is equivalent flexural strength, MPa; δ tb is mid-span deflection, mm; λ e is the percentage of equivalent flexural strength.The equivalent flexural strength and Equivalent flexural strength ratio of each group are shown in Figures 8 and 9, respectively.of equivalent flexural strength ratio.For instance, the equivalent flexural strength of B6P0 is 10.2%, which is only 36% higher than that of the control group B0P0.However, the equivalent flexural strength ratios of specimens containing macro polypropylene were all above 30%, which were at least 3.3 times higher than that of the control group.Among the HBPFRC specimens, when the mass ratio of BF to PPF is 1:2, the equivalent flexural strength ratio of the specimen B2P4 is the largest, which is 7.1 times higher than that of the control group.   of equivalent flexural strength ratio.For instance, the equivalent flexural strength of B6P0 is 10.2%, which is only 36% higher than that of the control group B0P0.However, the equivalent flexural strength ratios of specimens containing macro polypropylene were all above 30%, which were at least 3.3 times higher than that of the control group.Among the HBPFRC specimens, when the mass ratio of BF to PPF is 1:2, the equivalent flexural strength ratio of the specimen B2P4 is the largest, which is 7.1 times higher than that of the control group.As shown in Figures 7 and 8, the equivalent flexural strength and flexural toughness of each group of specimens show a consistent variation trend.This is due to that the equivalent flexural strength is just the flexural toughness multiplied by 1 δ tb .L b.h 2 , and for the specimen of the same size, the value is constant [55].As shown in Figure 9, there is a certain difference between the variation trend of equivalent flexural strength and that of equivalent flexural strength ratio.For instance, the equivalent flexural strength of B6P0 is 10.2%, which is only 36% higher than that of the control group B0P0.However, the equivalent flexural strength ratios of specimens containing macro polypropylene were all above 30%, which were at least 3.3 times higher than that of the control group.Among the HBPFRC specimens, when the mass ratio of BF to PPF is 1:2, the equivalent flexural strength ratio of the specimen B2P4 is the largest, which is 7.1 times higher than that of the control group.

Analysis of the Mechanism of Hybrid Fibers
This study shows that the hybrid of fibers in concrete can effectively improve the brittleness of concrete, and the HBPFRC specimens show better flexural ductility than that of plain concrete specimens, which can be attributed to that the fiber can improve various original defects in the concrete matrix and inhibit the development of cracks in loading stage [46].The role of fiber in improving the bending performance of concrete specimens is played throughout the whole process, from the pouring of concrete and the failure of it.
In the casting stage, during the hardening process of the concrete matrix, the existence of fiber can restrain the micro-cracks caused by plastic shrinkage and temperature deformation, which not only reduces the number of cracks but also reduces the size of cracks.So, it is beneficial to reduce the stress intensity factor at the crack tip.At the loading stage of the specimens, the fiber dissipates the stress concentration at the crack tip, thus limiting the crack propagation.Micro BF and macro PPF play different roles in different stages.
BFs are randomly distributed in the concrete matrix, and these play a favorable role in connecting internal macrocracks.At the same time, due to the extremely high elastic modulus of BF, it can withstand greater tensile stress under smaller strain conditions.Before the appearance of macro-cracks, it requires a large amount of energy to break itself during the expansion process of micro-crack, so that the micro BF can improve the bending performance of concrete.However, due to the low fracture elongation rate of BF, the BF will be immediately broken or pulled out once the macro-crack appears.So the mono addition of BFs cannot effectively improve the brittleness of concrete.As for the macro PPF, it plays a bridging role and shares the load borne by the concrete matrix in the process of crack evolution from micro-crack to macro-crack.Figure 10  This study shows that the hybrid of fibers in concrete can effectively improve the brittleness of concrete, and the HBPFRC specimens show better flexural ductility than that of plain concrete specimens, which can be attributed to that the fiber can improve various original defects in the concrete matrix and inhibit the development of cracks in loading stage [46].The role of fiber in improving the bending performance of concrete specimens is played throughout the whole process, from the pouring of concrete and the failure of it.
In the casting stage, during the hardening process of the concrete matrix, the existence of fiber can restrain the micro-cracks caused by plastic shrinkage and temperature deformation, which not only reduces the number of cracks but also reduces the size of cracks.So, it is beneficial to reduce the stress intensity factor at the crack tip.At the loading stage of the specimens, the fiber dissipates the stress concentration at the crack tip, thus limiting the crack propagation.Micro BF and macro PPF play different roles in different stages.
BFs are randomly distributed in the concrete matrix, and these play a favorable role in connecting internal macrocracks.At the same time, due to the extremely high elastic modulus of BF, it can withstand greater tensile stress under smaller strain conditions.Before the appearance of macro-cracks, it requires a large amount of energy to break itself during the expansion process of micro-crack, so that the micro BF can improve the bending performance of concrete.However, due to the low fracture elongation rate of BF, the BF will be immediately broken or pulled out once the macro-crack appears.So the mono addition of BFs cannot effectively improve the brittleness of concrete.As for the macro PPF, it plays a bridging role and shares the load borne by the concrete matrix in the process of crack evolution from micro-crack to macro-crack.Figure 10 is   As shown in Figure 10, in the process of the crack extending upward from the bottom of the specimen, the fibers across the crack will continuously participate in sharing the stress of the concrete matrix.The lower PPFs were continuously pulled out or broken as the crack width increases.At the same time, the upper fiber will successively participate in bearing the load.With the increase of crack width, the load shared by the macro PPFs gradually increases until it is pulled out or broken, which is the reason why the concrete containing the coarse PPFs still has the residual strength for a long time after the peak load.Besides, due to the high elongation capacity of the macro PPF, these are capable to bridge wide cracks without breaking.During the process of crack propagating to the top of the specimen, the number of upper PPFs playing the role of fiber bridging increased, while the lower PPFs can still withstand the tensile force before it is broken.Thus, the total number of fibers playing the role of fiber bridging may increase when the crack propagates.Therefore, the cumulative effect of effective fiber bridging will lead to the second peak value of HBPFRC specimens (such as Figure 5b,e-g).As shown in Figure 10, in the process of the crack extending upward from the bottom of the specimen, the fibers across the crack will continuously participate in sharing the stress of the concrete matrix.The lower PPFs were continuously pulled out or broken as the crack width increases.At the same time, the upper fiber will successively participate in bearing the load.With the increase of crack width, the load shared by the macro PPFs gradually increases until it is pulled out or broken, which is the reason why the concrete containing the coarse PPFs still has the residual strength for a long time after the peak load.Besides, due to the high elongation capacity of the macro PPF, these are capable to bridge wide cracks without breaking.During the process of crack propagating to the top of the specimen, the number of upper PPFs playing the role of fiber bridging increased, while the lower PPFs can still withstand the tensile force before it is broken.Thus, the total number of fibers playing the role of fiber bridging may increase when the crack propagates.Therefore, the cumulative effect of effective fiber bridging will lead to the second peak value of HBPFRC specimens (such as Figure 5b,e-g).
From the above analysis, it can be seen that BF and coarse PPF cannot be substituted for each other in improving the bending performance of concrete.These two fibers play a role in different loading periods of concrete.The bending performance of concrete can be improved by adding two kinds of fibers into concrete collectivity in a certain proportion.Due to the positive hybrid effect of fiber, the B2P4 specimen obtained the optimal bending performance.

Technology and Economic Analysis
Compared with steel fiber, macro PPF has a lower price, less labor cost, strong corrosion resistance and less carbon dioxide emission in the production process [56][57][58].Therefore, macro PPF has been used to replace steel fiber in recent years.What is more, as BF is a more environmentally friendly material compared with traditional reinforced concrete materials, it is more environmentally friendly to mix with PPF to produce FRC.
In terms of price, the BF used is 25 China Yuan (CNY)/kg, while the macro PPF is 35 CNY/kg (2019 price).The cost per cubic meter of fiber for each group is shown in Table 5, where the cost-effectiveness is defined as the ratio between the improvement value of the test bending performance of each group compared with the control group and the corresponding fiber cost [59].As shown in Table 5, the total price is lower when the amount of BF is more significant since the price of BF is lower than that of PPF.Although the price of mono-basalt FRC specimen B6P0 is low, it shows the characteristics of brittle failure when subjected to bending force and the increase in flexural strength and toughness is relatively small compared with that of the control group specimen B0P0.Thus, the addition of BF alone cannot effectively improve the bending performance of concrete.Among the HBPFRC specimens, the price of B2P4 is not the lowest, but it is only increased by 17.2% compared with that of the HBPFRC specimens B4.8P1.2 with the lowest price.However, when compared with the B4.8P1.2 specimen, the flexural strength and flexural toughness of B2P4 increased by 16.3% and 113%, separately.Thus, the selection of B2P4 specimen will significantly improve the energy dissipation capacity of concrete material with a small increase in cost.As for the mono-polypropylene FRC specimen, although it shows high flexural strength and high flexural toughness, it has no advantages compared with the B2P4 group in terms of technology and economy.What is more, B2P4 obtained the highest cost-effectiveness of both flexural strength (8.9 kPa/CNY) and flexural toughness (107.4 × 10 −3 J/CNY).Therefore, based on comprehensive technical and economic analysis, it can be seen that the B2P4 group is the test group with the optimal ratio.

Conclusions
(1) The addition of mono-BF proved not to enhance the concrete ductility, while the macro PPF significantly improve the brittleness of concrete and makes the failure modes of PPFRC and HBPFRCs change from the sudden brittle failure of plain concrete to ductile failure.(2) The addition of hybrid BF and PPF can effectively improve the flexural strength of concrete.Further, the addition of macro PPF proved to increase the post-cracking flexural toughness of concrete.Compared with the control group, when the mass ratio of BF to PPF is 1:2, the flexural toughness and equivalent flexural strength of the HBPFRC specimen were increased by 10.2 times, and the percentage of equivalent flexural strength of it was increased by 7.1 times.(3) BF mainly improves the flexural performance of concrete before the occurrence of macro-cracks, while the macro PPF plays the bridging role and improves the flexural performance of concrete in the process of crack evolution from micro-crack to the macro-crack.(4) B2P4 specimen significantly improves the energy dissipation capacity of concrete material with just a small increase in cost.From the perspective of both technology and economy, when the mass ratio of BF to PPF is 1:2, the bending performance and economic benefits of FRC reach the optimal level, and this mix ratio is the optimal fiber mixing ratio in this test.
(3) Pour the cement into the mixer and start mixing for 1 min.(4) Pour the water and water reducer slowly and evenly into the mixer and keep mixing for minutes.(5) Pour out the mixture and then pour them into the mold to produce HBPFRC specimens.Part of the production and curing procedures of HBPFRC specimens are shown in Figure 2.
is a schematic diagram of the bridging action of macro PPF.
a schematic diagram of the bridging action of macro PPF.

Figure 10 .
Figure 10.Schematic diagram of the bridging action of macro PPFs.

Figure 10 .
Figure 10.Schematic diagram of the bridging action of macro PPFs.

Table 1 .
Studies on the flexural performance of fiber reinforced concrete.

Table 4 .
Details of each group specimen.

Table 4 .
Details of each group specimen.

Table 5 .
Price of fibers and cost-effectiveness for per cubic meter FRC.