Effect of Using Hybrid Polypropylene and Glass Fibre on the Mechanical Properties and Permeability of Concrete

A comprehensive program of experiments consisting of compression, uniaxial compression, direct shear, flexural as well as splitting tensile and air permeability tests were performed to analyse the effect of the level of fibre dosage and the water–cement ratio on the physical properties of hybrid fibre-reinforced concrete (HFRC). Two types of fibres were studied in terms of their effect on the properties of HFRC. The results indicated that the mechanical properties of concrete were significantly improved by increasing the fibre content. However, increasing the percentage fibre content past a certain peak performance limit (0.9% glass fibre (GF) and 0.45% polypropylene fibre (PPF)) led to a decrease in strength compared to reference mixes. Additionally, the incorporation of hybrid fibres yielded an increase in air permeability in the tested specimens. The results showed that the strength-related properties of HFRC were superior to the properties of single fibre-reinforced concrete.


Introduction
Unreinforced concrete is brittle in nature, and is characterized by low tensile strength but high compressive strength. Because of this property and a lack of bonding in the concrete matrix at the transition zone, the brittleness increases along with increasing concrete strength. [1], Additionally, concrete is susceptible to early age cracking caused by shrinkage due to the use of mineral additives such as silica fume [2]. The addition of small, short, randomly spaced fibres in the concrete improves its properties and can mitigate its poor resistance to crack growth and brittleness. Combining fibres with concrete can produce a range of materials which possess enhanced tensile strength, elasticity, toughness, and durability. This is accomplished by limiting or controlling the start, spread, or spread persistence of cracks [3][4][5][6][7]. Several types of fibre are available-the predominant types are natural, synthetic, steel, carbon and glass [8][9][10][11][12][13].
Banthia et al. [3] investigated the effect of using macro-steel fibres and micro-cellulose fibres on the flexure and direct shear strength of single and hybrid concrete mixes. Test results for the hybrid composites examined under flexure showed that the cellulose fibre made an active contribution to toughness. When examined under direct shear, the synergy was not uniformly positive across all fibre types and combinations. Gabriel et al. [14] developed a self-consolidating hybrid fibre-reinforced concrete (HFRC) composite with superior flowability and workability incorporating polyvinyl acetate (PVA) fibres and steel fibres. The composite possessed crack control and crack stabilization characteristics by virtue of its toughening mechanisms, and had higher residual strength capacity compared with non-fibrous concrete. Banthia and R. Gupta [15] investigated the effect of steel, polypropylene fibre, and carbon fibre hybridization on high-strength concrete. They found

Test Methods
To determine the strength performance, for each mixture, fifteen specimens (six 100 × 100 × 100 mm cubes, three 100 × 100 × 400 mm beams, six 100 × 100 × 300 mm beams, and three 150 × 300 mm cylinders) were prepared. These were standard test specimens prepared in accordance with the Chinese standard CECS13:2009 [31]. Three specimens were prepared for each test, and the mean value was used.
The compressive strength and uniaxial compressive strength, shear strength, flexural strength, and splitting tensile strength were tested after curing the concrete for 28 days.
The samples were cured in a curing chamber at 100% relative humidity and a temperature of 20 ± 2 • C.
The compressive test was conducted based on the Chinese standard GB/T50081-2016 [32]. Cubes of side 100 mm and prisms of dimensions 100 mm × 100 mm × 300 mm were produced to measure the compressive strength of concrete mixtures. A hydraulic universal testing machine (WE-30) was used to conduct the compression test with a loading control of 0.5-0.8 MPa/s. The Chinese standard JSCE-G 553-1999 [33] describes a method for generating a pure shear condition which can be used to test samples in the lab, as shown in Figure 1. A prism of dimensions 100 mm × 100 mm × 400 mm was used to estimate shear strength. A dry and clean local river sand was used as the fine aggregate, and a coarse aggregate with a maximum particle size of 20 mm was used in the concrete.
The properties of the glass and polypropylene fibres are shown in Table 1.

Test Methods
To determine the strength performance, for each mixture, fifteen specimens (six 100 × 100 × 100 mm cubes, three 100 × 100 × 400 mm beams, six 100 × 100 × 300 mm beams, and three 150 × 300 mm cylinders) were prepared. These were standard test specimens prepared in accordance with the Chinese standard CECS13:2009 [31]. Three specimens were prepared for each test, and the mean value was used.
The compressive strength and uniaxial compressive strength, shear strength, flexural strength, and splitting tensile strength were tested after curing the concrete for 28 days.
The samples were cured in a curing chamber at 100% relative humidity and a temperature of 20 ± 2 °C.
The compressive test was conducted based on the Chinese standard GB/T50081-2016 [32]. Cubes of side 100 mm and prisms of dimensions 100 mm × 100 mm × 300 mm were produced to measure the compressive strength of concrete mixtures. A hydraulic universal testing machine (WE-30) was used to conduct the compression test with a loading control of 0.5-0.8 MPa/s. The Chinese standard JSCE-G 553-1999 [33] describes a method for generating a pure shear condition which can be used to test samples in the lab, as shown in Figure 1. A prism of dimensions 100 mm × 100 mm × 400 mm was used to estimate shear strength. The bending test was conducted on prisms of dimensions 100 mm × 100 mm × 400 mm to estimate the flexural strength of each mixture according to GB/T50081-2016. The hydraulic universal testing machine (WE-30) was used to conduct the bending, and the flexural test was conducted with a loading control of 0.5-0.8 MPa/s. machine (WE-30) was used to conduct the bending, and the flexural test was conducted with a loading control of 0.5-0.8 MPa/s. Splitting tensile strength tests were performed on the cylindrical specimens of dimensions 150 × 300 mm following the method in ASTM C496 [34] using a loading rate of 0.8 MPa/min until the samples failed.
Air permeability was measured using a standardized air permeability measurement system (Autoclam, amphora NDT, Belfast, Northern Ireland, UK). To calculate the air permeability index (API), natural logarithms of air pressure were plotted with respect to time. The slope of the last 10 results was designated as the API [35].
The average permeability coefficients were calculated to obtain a representative value of permeability for each specimen.
A JSM-7500F scanning electron microscope (JEOL (BEIJING) CO. LTD., Beijing, China) was used to obtain scanning electron microscopy (SEM) images. At the microstructural level, the bonding surface between the fibres and cement paste can be observed in single or hybrid concrete. The capillary pores and microcracks of concrete can be better understood through the SEM images.

Mixture Proportions
Twenty mixes were designed with water-to-binder ratios of 0.35 and 0.30 and containing a total cementitious material of 370 kg/m 3 and 430 kg/m 3 , respectively. The control mixture included only cementitious materials (Ordinary Portland cement, fly ash, silica fume and mineral powder) and fine and coarse aggregate, while fibres were added to the remaining mixtures as shown in Table 2. The percentages of added fly ash, silica fume and mineral powder were 20%, 5% and 10%, respectively, by weight of cement. The concrete mixture proportions are summarized in Table 3.

Results
The results for the compressive strength, shear strength, flexural strengths, and splitting tensile strength for the cubes and prisms incorporating all mixtures are presented in Figures 2-11.

Compressive Strength
The largest improvement in compression strength for the cubes and prisms was 54.1% and 56.34%, respectively, compared with the reference mix. This occurred in the set C7, which contained 1.35% glass fibre (GF) and 0.0% polypropylene fibre (PPF) with a water-to-binder ratio (W/B) of 0.30. This is shown in Figures 2 and 3.
Upon examining the compressive strength results, it was noted that the strength of the concrete containing GF improved, compared with mixes containing PPF and with the reference mix. This result indicates that the addition of PPF had a small impact on the concrete's compressive strength. [36]. The maximum increase in prism uniaxial compressive strength was 34.14%, which occurred in group B6 containing 0.9% GF and 0.9% PPF.
Comparisons between the control mix and the hybrid fibre mix indicated that the use of a low volume fraction of PPF (0.45%) enhanced the compressive strength of the concrete. This was likely because hybrid fibres with different sizes and types offer different restraint conditions [37][38][39]. On the other hand, the compressive strength results showed that the use of more than 0.45% PPF in single fibre mixes reduced the compressive strength. This was due to the low stiffness of the PPF and the displacement of the mortar matrix of the mixes, leading to a reduction in compressive strength. The maximum increase in prism uniaxial compressive strength was 34.14%, which occurred in group B6 containing 0.9% GF and 0.9% PPF.
Comparisons between the control mix and the hybrid fibre mix indicated that the use of a low volume fraction of PPF (0.45%) enhanced the compressive strength of the concrete. This was likely because hybrid fibres with different sizes and types offer different restraint conditions [37][38][39]. On the other hand, the compressive strength results showed that the use of more than 0.45% PPF in single fibre mixes reduced the compressive strength. This was due to the low stiffness of the PPF and the displacement of the mortar matrix of the mixes, leading to a reduction in compressive strength.     The maximum increase in prism uniaxial compressive strength was 34.14%, which occurred in group B6 containing 0.9% GF and 0.9% PPF.
Comparisons between the control mix and the hybrid fibre mix indicated that the use of a low volume fraction of PPF (0.45%) enhanced the compressive strength of the concrete. This was likely because hybrid fibres with different sizes and types offer different restraint conditions [37][38][39]. On the other hand, the compressive strength results showed that the use of more than 0.45% PPF in single fibre mixes reduced the compressive strength. This was due to the low stiffness of the PPF and the displacement of the mortar matrix of the mixes, leading to a reduction in compressive strength.

Shear Strength
The shear strengths of all samples containing fibres were noted as being higher than the reference mix. This is shown in Figures 6 and 7. Unlike the small improvement seen in compressive strength, the maximum increase in direct shear strength was approximately 65%. The experimental results showed that group C3 as shown in Figure 6, which contained 0.45% GF and 0.45% PPF with a W/B of 0.30, possessed the maximum shear strength amongst all test samples. The increase in GF content resulted in considerable improvements in the shear strength. When the W/B was 0.35, the test results indicated the maximum increase in direct shear strength (approximately 34%) in the B3 group, which contained 0.45% GF and 0.45% PPF.
An increase in the added volume fraction of single and hybrid fibres resulted in this significant enhancement of the direct shear strength. However, the shear strength may decrease due to the volumetric expansion of PPF when it meets water. If the unit weight of PPF exceeds a reasonable range, the extra PPF may cause a nonuniform distribution of the fibres, and thus lead to a decrease in the direct shear strength.

Shear Strength
The shear strengths of all samples containing fibres were noted as being higher than the reference mix. This is shown in Figures 6 and 7. Unlike the small improvement seen in compressive strength, the maximum increase in direct shear strength was approximately 65%. The experimental results showed that group C3 as shown in Figure 6, which contained 0.45% GF and 0.45% PPF with a W/B of 0.30, possessed the maximum shear strength amongst all test samples. The increase in GF content resulted in considerable improvements in the shear strength. When the W/B was 0.35, the test results indicated the maximum increase in direct shear strength (approximately 34%) in the B3 group, which contained 0.45% GF and 0.45% PPF.
An increase in the added volume fraction of single and hybrid fibres resulted in this significant enhancement of the direct shear strength. However, the shear strength may decrease due to the volumetric expansion of PPF when it meets water. If the unit weight of PPF exceeds a reasonable range, the extra PPF may cause a nonuniform distribution of the fibres, and thus lead to a decrease in the direct shear strength.

Shear Strength
The shear strengths of all samples containing fibres were noted as being higher than the reference mix. This is shown in Figures 6 and 7. Unlike the small improvement seen in compressive strength, the maximum increase in direct shear strength was approximately 65%. The experimental results showed that group C3 as shown in Figure 6, which contained 0.45% GF and 0.45% PPF with a W/B of 0.30, possessed the maximum shear strength amongst all test samples. The increase in GF content resulted in considerable improvements in the shear strength. When the W/B was 0.35, the test results indicated the maximum increase in direct shear strength (approximately 34%) in the B3 group, which contained 0.45% GF and 0.45% PPF.
An increase in the added volume fraction of single and hybrid fibres resulted in this significant enhancement of the direct shear strength. However, the shear strength may decrease due to the volumetric expansion of PPF when it meets water. If the unit weight of PPF exceeds a reasonable range, the extra PPF may cause a nonuniform distribution of the fibres, and thus lead to a decrease in the direct shear strength.

Splitting Tensile Strength
As can be seen from Figures 8 and 9, the fibre concrete mixes demonstrated enhanced splitting tensile strength in the hybrid form containing GF and PPF, whilst demonstrating a splitting tensile strength reduction of over 45% when they contained only PPF.
The results were similar to the compressive strength tests discussed in the previous section, in that the GF additive yielded the highest splitting tensile strength, which was 44.73% higher than that of the unreinforced control concrete. This occurred in group B4 containing 0.9% GF and 0.0% PPF, while the PPF additive yielded the lowest splitting tensile strength of (−72.8%) compared with reference mix. This occurred in group B8 containing 0.0% GF and 1.35% PPF.
In the second group of mixes with W/B of 0.30, the maximum increase in direct shear strength was approximately 34.65% for group C4 compared with the reference mix. The PPF additive yielded the lowest splitting tensile strength of 44% compared with the reference mix. This occurred in group C8 containing 0.0% GF and 1.35% PPF.
It was clear that the GF additive increased the splitting tensile strength. This was very clear in group #9 (C9 and B9), which contained a larger volume of fibre (1.35% GF and 1.35% PPF). However,

Splitting Tensile Strength
As can be seen from Figures 8 and 9, the fibre concrete mixes demonstrated enhanced splitting tensile strength in the hybrid form containing GF and PPF, whilst demonstrating a splitting tensile strength reduction of over 45% when they contained only PPF.
The results were similar to the compressive strength tests discussed in the previous section, in that the GF additive yielded the highest splitting tensile strength, which was 44.73% higher than that of the unreinforced control concrete. This occurred in group B4 containing 0.9% GF and 0.0% PPF, while the PPF additive yielded the lowest splitting tensile strength of (−72.8%) compared with reference mix. This occurred in group B8 containing 0.0% GF and 1.35% PPF.
In the second group of mixes with W/B of 0.30, the maximum increase in direct shear strength was approximately 34.65% for group C4 compared with the reference mix. The PPF additive yielded the lowest splitting tensile strength of 44% compared with the reference mix. This occurred in group C8 containing 0.0% GF and 1.35% PPF.
It was clear that the GF additive increased the splitting tensile strength. This was very clear in group #9 (C9 and B9), which contained a larger volume of fibre (1.35% GF and 1.35% PPF). However,

Splitting Tensile Strength
As can be seen from Figures 8 and 9, the fibre concrete mixes demonstrated enhanced splitting tensile strength in the hybrid form containing GF and PPF, whilst demonstrating a splitting tensile strength reduction of over 45% when they contained only PPF.
The results were similar to the compressive strength tests discussed in the previous section, in that the GF additive yielded the highest splitting tensile strength, which was 44.73% higher than that of the unreinforced control concrete. This occurred in group B4 containing 0.9% GF and 0.0% PPF, while the PPF additive yielded the lowest splitting tensile strength of (−72.8%) compared with reference mix. This occurred in group B8 containing 0.0% GF and 1.35% PPF.
In the second group of mixes with W/B of 0.30, the maximum increase in direct shear strength was approximately 34.65% for group C4 compared with the reference mix. The PPF additive yielded the lowest splitting tensile strength of 44% compared with the reference mix. This occurred in group C8 containing 0.0% GF and 1.35% PPF.
It was clear that the GF additive increased the splitting tensile strength. This was very clear in group #9 (C9 and B9), which contained a larger volume of fibre (1.35% GF and 1.35% PPF). However, the splitting tensile strength of group #9 was higher than that of group #8 (C8 and B8), which had a smaller volume of fibre but did not contain GF (0% GF and 1.35% PPF).
The impact of the added fibres on tensile strength indicated that the interfacial bonding between the fibres and matrix had improved. the splitting tensile strength of group #9 was higher than that of group #8 (C8 and B8), which had a smaller volume of fibre but did not contain GF (0% GF and 1.35% PPF). The impact of the added fibres on tensile strength indicated that the interfacial bonding between the fibres and matrix had improved.

Modulus of Rupture (MOR)
Mixes containing single or double fibres showed improvements in flexural strength compared to the equivalent reference plain concrete mix.
As shown in Figures 10 and 11, GF concrete showed a 40.0% and 48% increase in flexural strength over the reference mix for samples C4 and B4, respectively, while the PPF concrete showed a decrease of −65% in both C9 and B9, respectively, which contained 1.35% GF and 1.35% PPF. The increase in the MOR resulted principally from the fibres which acted as lines or wires that resisted the micro-cracks in the tension zones of the samples.
As noted in the discussion of the compressive and splitting tensile strength tests, the GF and PPF additives improved the MOR for the hybrid fibre concrete compared to the control mixes. This could 0.00 0.50  the splitting tensile strength of group #9 was higher than that of group #8 (C8 and B8), which had a smaller volume of fibre but did not contain GF (0% GF and 1.35% PPF). The impact of the added fibres on tensile strength indicated that the interfacial bonding between the fibres and matrix had improved.

Modulus of Rupture (MOR)
Mixes containing single or double fibres showed improvements in flexural strength compared to the equivalent reference plain concrete mix.
As shown in Figures 10 and 11, GF concrete showed a 40.0% and 48% increase in flexural strength over the reference mix for samples C4 and B4, respectively, while the PPF concrete showed a decrease of −65% in both C9 and B9, respectively, which contained 1.35% GF and 1.35% PPF. The increase in the MOR resulted principally from the fibres which acted as lines or wires that resisted the micro-cracks in the tension zones of the samples.
As noted in the discussion of the compressive and splitting tensile strength tests, the GF and PPF additives improved the MOR for the hybrid fibre concrete compared to the control mixes. This could 0.00 0.50

Modulus of Rupture (MOR)
Mixes containing single or double fibres showed improvements in flexural strength compared to the equivalent reference plain concrete mix.
As shown in Figures 10 and 11, GF concrete showed a 40.0% and 48% increase in flexural strength over the reference mix for samples C4 and B4, respectively, while the PPF concrete showed a decrease of −65% in both C9 and B9, respectively, which contained 1.35% GF and 1.35% PPF. The increase in the MOR resulted principally from the fibres which acted as lines or wires that resisted the micro-cracks in the tension zones of the samples.
As noted in the discussion of the compressive and splitting tensile strength tests, the GF and PPF additives improved the MOR for the hybrid fibre concrete compared to the control mixes. This could be due to improvements in the toughness matrix, compactness and homogeneity of fibre distribution in hybrid fibre concrete.
Materials 2019, 12, x FOR PEER REVIEW 9 of 17 be due to improvements in the toughness matrix, compactness and homogeneity of fibre distribution in hybrid fibre concrete.

Mechanical Performance Percentage
To calculate the mechanical performance of concrete mixes, a certain weight was assumed for each strength parameter according to their importance in the design of concrete.
As compressive strength is an important parameter to determine the performance of the material during service conditions, added to the fact that concrete is known for its compressive strength, the highest weight (4 of 4) was considered for compressive strength, followed by flexural strength (2 of 4), and split tensile strength (1 of 4).
The mechanical performance percentage of plain and fibre concrete mixes was calculated using the equation proposed by Ali et al. [40].
The mechanical performance (MP) of each mixture was evaluated using Equation (1)

Mechanical Performance Percentage
To calculate the mechanical performance of concrete mixes, a certain weight was assumed for each strength parameter according to their importance in the design of concrete.
As compressive strength is an important parameter to determine the performance of the material during service conditions, added to the fact that concrete is known for its compressive strength, the highest weight (4 of 4) was considered for compressive strength, followed by flexural strength (2 of 4), and split tensile strength (1 of 4).
The mechanical performance percentage of plain and fibre concrete mixes was calculated using the equation proposed by Ali et al. [40].
The mechanical performance (MP) of each mixture was evaluated using Equation (1)

Mechanical Performance Percentage
To calculate the mechanical performance of concrete mixes, a certain weight was assumed for each strength parameter according to their importance in the design of concrete.
As compressive strength is an important parameter to determine the performance of the material during service conditions, added to the fact that concrete is known for its compressive strength, the highest weight (4 of 4) was considered for compressive strength, followed by flexural strength (2 of 4), and split tensile strength (1 of 4).
The mechanical performance percentage of plain and fibre concrete mixes was calculated using the equation proposed by Ali et al. [40].
The mechanical performance (MP) of each mixture was evaluated using Equation (1): where: "CON" is control or reference mix, and "mix" is the particular mix whose mechanical performance is being evaluated. As shown in Figures 12 and 13, the MP (%) for each mix was calculated by incorporating the results of the compressive strength, flexural strength, and tensile strength tests.

7
where: "CON" is control or reference mix, and "mix" is the particular mix whose mechanical performance is being evaluated.
As shown in Figures 12 and 13, the MP (%) for each mix was calculated by incorporating the results of the compressive strength, flexural strength, and tensile strength tests.
It was noted that all mixes containing fibre in single or hybrid form exceeded the reference mix MP until the fibre content reached 0.45%. When the fibre content increased over that volume fraction, the MP started to decrease to the minimum value noted in sample #9 (C9, B9).
The maximum MP was noted in sample #4, which contained 0.9% GF and 0% PPF. This was because mixes B4 and C4 demonstrated high compressive strength, flexural strength, and tensile strength.

Air Permeability of Fibre Concrete
The objective of this test was to examine the impact of hybrid fibre on air permeability volume by examining the permeability of both plain concrete and fibre concrete.
The permeability test results for the fibre concrete and the plain concrete are shown in Table 4 and Figure 14. The behaviour of samples during the permeability test is shown in Figures 15 and 16.  where: "CON" is control or reference mix, and "mix" is the particular mix whose mechanical performance is being evaluated. As shown in Figures 12 and 13, the MP (%) for each mix was calculated by incorporating the results of the compressive strength, flexural strength, and tensile strength tests.
It was noted that all mixes containing fibre in single or hybrid form exceeded the reference mix MP until the fibre content reached 0.45%. When the fibre content increased over that volume fraction, the MP started to decrease to the minimum value noted in sample #9 (C9, B9).
The maximum MP was noted in sample #4, which contained 0.9% GF and 0% PPF. This was because mixes B4 and C4 demonstrated high compressive strength, flexural strength, and tensile strength.

Air Permeability of Fibre Concrete
The objective of this test was to examine the impact of hybrid fibre on air permeability volume by examining the permeability of both plain concrete and fibre concrete.
The permeability test results for the fibre concrete and the plain concrete are shown in Table 4 and Figure 14. The behaviour of samples during the permeability test is shown in Figures 15 and 16. It was noted that all mixes containing fibre in single or hybrid form exceeded the reference mix MP until the fibre content reached 0.45%. When the fibre content increased over that volume fraction, the MP started to decrease to the minimum value noted in sample #9 (C9, B9).
The maximum MP was noted in sample #4, which contained 0.9% GF and 0% PPF. This was because mixes B4 and C4 demonstrated high compressive strength, flexural strength, and tensile strength.

Air Permeability of Fibre Concrete
The objective of this test was to examine the impact of hybrid fibre on air permeability volume by examining the permeability of both plain concrete and fibre concrete.
The permeability test results for the fibre concrete and the plain concrete are shown in Table 4 and Figure 14. The behaviour of samples during the permeability test is shown in Figures 15 and 16. Due to the large variability normally associated with water permeability results, it is often advisable to compare permeability data based on ratios [41].

Microstructure Observation
The microstructure of concrete is very important in terms of the concrete's strength and durability. Figure 17 shows scanning electron microscope (SEM) images of selected mixtures using different W/B ratios and fibre dosages after 28 days of concrete curing, and therefore arrest of hydration was not necessary.
The figure shows that the hybrid fibre and single fibre concrete possessed more compact internal structure compared to the control mix. This was due to the fibre having a great affinity for water. By restricting the evaporation of water required for cement hydration from the structure of the mortar samples during the curing period, the cement binder around the fibre could properly hydrate and fill part of the space between the fibre and the binder. This was very conducive to strength development in the concrete.
It could be plainly observed that there was coherence in the fibres and the binder. It could be concluded that the addition of fibres affected the mortar setting, since it could be seen that the mortar was set nearby the fibres without producing further notable porosity. Hence, the fibres were embedded in the set mass and demonstrated good adhesion between themselves and the mortar. This was verified by the mechanical behaviour. On the other hand, the concrete samples containing higher doses of single or hybrid fibre (B8 and B9) demonstrated low compactness, large porosity and some micro-cracks, which all affected the strength development of the concrete.
When the volume of fibre increased, the GF-being the fibres with the highest density-took up a larger volume of the composite material, which led to further deterioration of the mechanical   [42].
W/B ratio and the content and type of supplementary cementitious materials have a significant impact on air permeability. An improvement in the impermeability of concrete containing fly ash is due to pozzolanic reaction effects and changes in the phase composition of hardened cement paste [35].
When samples containing 1.35% GF and 1.35% PPF were used (i.e., when C9 and B9 samples were tested), the tests could not be completed because there was insufficient pressure in the samples. When samples containing 1.35% GF and 0% PPF (i.e., C7 and B7) were used, the permeability coefficient significantly decreased to maximum values of 57% and 50%, respectively. The reduction in the permeability of concrete due to fibre reinforcement is congruent with the findings of Sanjuan et al. [43]. The permeability coefficients of the concrete containing 0.45% and 1.35% PPF were higher than those of the concrete specimens containing 0.45% and 1.35% GF. Therefore, it could be concluded that GF additives can improve the impermeability of concrete. The reduction in permeability associated with fibre reinforcement is due to the fact that the fibres produce mixture stiffening, reduce the settlement of aggregates and decrease bleeding. These lead to a decrease in the ease with which flow can occur through the material. On the other hand, GF and PPF are expected to improve water retention in the cement binder and reduce shrinkage. This is expected to produce a more solid material with less internal cracking. The capability of fibre to reduce the permeability of concrete is affected by many factors, such as the mixture composition, fibre type and dimensions and the nature of the fibre [44]. Table 4 shows the protective quality of the air permeability of concrete. For structural concrete, the Autoclam air permeability index (API) in ln(pressure)/min can be expressed in terms of intrinsic air permeability, K a , which is given by using the formula in the AutoCLAM permeability system operating manual: K a m 2 = (API) 0.8754 × 8 .395 × 10 −16 (2) The intrinsic air permeability results for the fibre concrete and the plain concrete are shown in Table 4 and Figure 14.

Microstructure Observation
The microstructure of concrete is very important in terms of the concrete's strength and durability. Figure 17 shows scanning electron microscope (SEM) images of selected mixtures using different W/B ratios and fibre dosages after 28 days of concrete curing, and therefore arrest of hydration was not necessary.
The figure shows that the hybrid fibre and single fibre concrete possessed more compact internal structure compared to the control mix. This was due to the fibre having a great affinity for water. By restricting the evaporation of water required for cement hydration from the structure of the mortar samples during the curing period, the cement binder around the fibre could properly hydrate and fill part of the space between the fibre and the binder. This was very conducive to strength development in the concrete.
It could be plainly observed that there was coherence in the fibres and the binder. It could be concluded that the addition of fibres affected the mortar setting, since it could be seen that the mortar was set nearby the fibres without producing further notable porosity. Hence, the fibres were embedded in the set mass and demonstrated good adhesion between themselves and the mortar. This was verified by the mechanical behaviour. On the other hand, the concrete samples containing higher doses of single or hybrid fibre (B8 and B9) demonstrated low compactness, large porosity and some micro-cracks, which all affected the strength development of the concrete.
When the volume of fibre increased, the GF-being the fibres with the highest density-took up a larger volume of the composite material, which led to further deterioration of the mechanical properties. Based on the above analyses, to obtain superior strength and mechanical properties, the application ratios of GF and PPF should be designed so as not to exceed approximately 0.9% and 0.45% of the PC, respectively.
properties. Based on the above analyses, to obtain superior strength and mechanical properties, the application ratios of GF and PPF should be designed so as not to exceed approximately 0.9% and 0.45% of the PC, respectively.