Properties of Fibre-Reinforced High-Strength Concrete with Nano-Silica and Silica Fume

: This study intends to assess the inﬂuence of steel ﬁbres (SF) and polypropylene ﬁbres (PPF) on the hardened and fresh state properties of high-strength concrete (HSC). For this purpose, 99 concrete mixes were designed and applied. SF and PPF were used at six-volume replacement contents of 0%, 0.1%, 0.2%, 0.3%, 0.4% and 0.5%. Moreover, nano-silica (NS) was used at three contents, 0%, 1% and 2%, and silica fume powder (SP) was also used at three weight ratios (0%, 5% and 10%). The slump, compressive and tensile strength, elasticity modulus, water absorption and the electric resistivity of concrete specimens were examined. The results showed that using 1% NS and 10% SP together with 0.5% PPF improved the compressive strength of HSC by about 123%; however, the effect of SF on tensile strength is more signiﬁcant and adding 0.5% SF with both 2% NS and 10% SP increased the tensile strength by 104%. Moreover, increasing the SF content reduces the electric resistivity while using PPF improves this property especially when 1% NS was employed, and it was enhanced by about 68% when 0.5% SF and 1% NS were utilized with 10% SP.


Introduction
Cement is one of the most largely produced materials in the world. A high value of greenhouse gases is released into the environment within the process of manufacturing cement, affecting the health of humans, animals and plants [1]. The cement industry is one of the largest industrial manufacturers of carbon dioxide (CO 2 ), more than 5% of which 50% is released due to the chemical process and 40% from burning fuel. This industry, by producing around 7% of all CO 2 emissions, is the second-highest emitter of all industrial carbon dioxide [2]. Using waste materials is a useful way to reduce the amount of harmful emitted gases in industrial processes. In this area, many types of research have been carried out to study the influence of silica fume powder (SP) on the mechanical behaviour of concrete. Hemavathi et al. [3] employed SP and glass fibres to manufacture high-strength concrete (HSC). For this aim, SP replaced cement at 20% by weight. Moreover, 1% of glass fibres were used, and marble was utilized at three weight contents of 0%, 50% and 100%. In another study, Wu et al. [4] assessed the influence of SP (ranging from 0% to 25%) on the mechanical and rheological performance of HSC. Besides, 2% steel fibres (SF) were added to the concrete mixes. For this purpose, the compressive, flexural and tensile strengths of concrete were examined. The composite theory was also employed for numerical simulation. The results showed that the maximum compressive, flexural and tensile strengths could be obtained using 10% to 15% SP. On the other hand, the theoretical methods could predict the mechanical behaviour of HSC when SP was used cement, water, SP and aggregates, the maximum size of aggregate and curing age were the variables under analysis. It was demonstrated that the proposed formula could accurately predict the optimal value of SP in concrete. Mehta et al. [16] published a review article in the area of the influence of SP and waste glass on the mechanical performance of concrete. In addition, a microstructural analysis was employed. Khan and Ali [17] analysed the behaviour of concrete with fly ash, SP and coconut fibres. 15% SP replaced cement and fly ash was employed at four contents by weight: 0%, 5%, 10% and 15%. Besides, 2% of coconut fibres were added to the concrete mixes. The compressive strength, splitting tensile strength, stress-strain relationship, load-deflection and load-time curves of specimens were assessed in this investigation. According to the results, using both coconut fibres and 10% of fly ash significantly improved the mechanical properties of concrete.
In 2019, Sasanipour et al. [18] studied the impact of SP on the durability of recycled aggregates SCC. Specimens were manufactured in three families. In the first and second ones, coarse recycled aggregates replaced natural aggregates at four contents: 25%, 50%, 75% and 100%, with and without SP respectively. In the third family, 25% of fine recycled aggregates replaced fine natural aggregates. Both the fresh and hardened state properties of SCC were examined. SP significantly improved the fresh state properties of SCC. Conversely, increasing the recycled aggregates' content resulted in reducing the electric resistivity. SP also controlled the temperature of the solutions used during the test. Esfandiari and Loghmani [19] examined the impact of perlite powder and SP on the compressive strength and microstructural properties of SCC with a lime-cement binder. For this purpose, SP and perlite powder were employed at 0%, 5%, 10% 15% and 20%. It was demonstrated that the water absorption, dry density and compressive strength increased by increasing the SP content.

Research Significance
A review of the literature showed that considerable research work has been carried out to evaluate the influence of SP and NS separately on the mechanical and durability performance of concrete. It showed that the contents of SP and NS should be limited and using high-contest of these materials leads to reducing the mechanical properties of concrete [12][13][14][15]. On the other hand, previous investigations demonstrated that using different types of fibres plays a positive role on the mechanical behaviour of concrete especially in terms of compressive, tensile and flexural strength [20][21][22][23][24][25][26][27][28][29][30][31][32]. Guo et al. [33] evaluated the effect of PPF and SF on the static and dynamic characteristics of HSC. They found that adding SF resulted in brittle failure in the indirect tensile strength test, while the simultaneous incorporation of SF and PPF led to ductile failure. Besides, the resistance was substantially enhanced with increasing PPF and SF content due to the bridging role of the fibres and their higher tensile strength in comparison with the tensile strength of the concrete matrix. Mo et al. [34] investigated the mechanical properties of PPF-reinforced concrete with rubber aggregates. They showed that adding PPF led to reducing the dilapidation rate of the dynamic bending stiffness. Rashid [35] used SF and PPF to improve the durability performance of concrete under natural weathering. The specimens were exposed to natural conditions for 360 days. Additionally, the compressive strength, water absorption and porosity specimens containing various PPF content were studied. It was found that PPF significantly improved the durability and mechanical characteristics of the specimens. Therefore, finding a way to increase the contents of SP and NS in order to improve the mechanical and durability performance of concrete by combining these materials was the main aim of this study. Moreover, PPF and SF were used to prevent the reduction effect of using a high content of SP and NS on the properties of concrete.

Materials and Specimens' Specifications
In this research, ordinary Portland cement (type I) was used to manufacture concrete according to ASTM C150/C150M [36]. The physical and chemical characteristics of cement are represented in Tables 1 and 2. Moreover, the mechanical properties of aggregates are illustrated in Table 3, according to the requirement of ASTM D2419 [37]. The grading curves of the aggregates are also shown in Figure 1 and Table 4, as per ASTM C33 [38].

Materials and Specimens' Specifications
In this research, ordinary Portland cement (type I) was used to manufacture concrete according to ASTM C150/C150M [36]. The physical and chemical characteristics of cement are represented in Tables 1 and 2. Moreover, the mechanical properties of aggregates are illustrated in Table 3, according to the requirement of ASTM D2419 [37]. The grading curves of the aggregates are also shown in Figure 1 and Table 4, as per ASTM C33 [38].     To manufacture fibre concrete, SF and PPF were employed as illustrated in Figure 2. The properties of fibres are also presented in Table 5. In this study, the fibres were added Appl. Sci. 2021, 11, 9696 5 of 21 to the mix at three-volume contents of 0%, 1% and 2%. NS and SP were selected as the amorphous kind, F-110 grade A102, as a replacement for cement, respectively, as shown in Figure 3. Moreover, the physical and chemical properties of SP and NS are represented in Tables 6 and 7 To manufacture fibre concrete, SF and PPF were employed as illustrated in Figure 2. The properties of fibres are also presented in Table 5. In this study, the fibres were added to the mix at three-volume contents of 0%, 1% and 2%. NS and SP were selected as the amorphous kind, F-110 grade A102, as a replacement for cement, respectively, as shown in Figure 3. Moreover, the physical and chemical properties of SP and NS are represented in Tables 6 and 7.     To manufacture fibre concrete, SF and PPF were employed as illustrated in Figure 2. The properties of fibres are also presented in Table 5. In this study, the fibres were added to the mix at three-volume contents of 0%, 1% and 2%. NS and SP were selected as the amorphous kind, F-110 grade A102, as a replacement for cement, respectively, as shown in Figure 3. Moreover, the physical and chemical properties of SP and NS are represented in Tables 6 and 7.       A high-performance superplasticizer was added until the fibres were uniformly distributed in the concrete matrix. The total water/cement ratio of all samples was kept constant at 0.35. To assess the compressive and tensile strengths of each concrete mix, six cylinders with a diameter of 150 mm and a height of 300 mm were manufactured and tested under a hydraulic jack [39][40][41]. The average compressive and tensile strength of three of those cylinders was considered as the specimens' strength. The mix proportions of specimens are also seen in Table 8. According to Table 8, concrete mixes were categorized into three general groups based on fibres content: group 1 without fibres, group 2 includes specimens containing SF, and group 3 includes specimens produced by PPF. In addition, groups 2 and 3 are divided into 5 sub-groups based on fibres contents: 0.1%, 0.2%, 0.3%, 0.4% and 0.5%. In addition, SF, PPF, SP and NS in Table 8 indicates the steel fibres, polypropylene fibres, silica fume powder and nano-silica contents, respectively.  Regarding Table 8, 99 mixes were produced. Then, six tests were carried out: slump, compressive and splitting tensile strengths, modulus of elasticity, water absorption and electrical resistivity. In each test, three specimens were tested for each mix and the average of three samples was considered. Therefore, 1782 specimens were manufactured and tested at 28 days of curing age.

Methods
In this section, the methodology of performed tests in this study is discussed, as below:

Slump
To measure the properties of the fresh state of HSC, slump flow was utilized. This test was performed according to ASTM C33 [38] to determine the workability of concrete. To that end, a circular truncated cone of 100 mm, 200 mm and 300 mm high, bottom and top diameters was filled with 3-layer fresh concrete and compacted by 25 strikes each. Slowly the mould was then raised and the height of the concrete settlement was deemed to be the downside.

Compressive Strength
To assess the hardened state performance of HSC, the effect of NS, SP, SF and PPF on the compressive strength of HSC was examined according to ASTM C39 [42], For this aim, three 150 mm × 300 mm cylindrical specimens were subjected to a hydraulic jack, and the average of three samples was considered.

Splitting Tensile Strength
To measure, the splitting tensile strength three 150 mm × 300 mm cylindrical specimens were tested under a hydraulic jack from the lateral side, as per ASTM C496/C496M [43], the average of three tested specimens was considered.

Modulus of Elasticity
Moreover, 100 mm × 300 mm cylindrical samples were used to assess the modulus of elasticity according to ASTM C469/C469M [43]. Therefore, a steel ring with a strain gauge has been placed around the cylindrical sample and stress-strain data have been utilized to determine the initial and secant modulus of elasticity.

Water Absorption
After 28 curing days, water absorption was evaluated. The size of the sample shall not be less than 350 cm 2 and the specimens should be genuine without fractures or splitting on their surfaces, according to the standards defined by ASTM C642 [44].

Electric Resistivity Test
The electric resistivity test (ERT) was performed to evaluate the resistance and pore interconnections of concrete [45]. For this aim, three cylindrical specimens per mix, with a diameter of 100 mm and a height of 200 mm, were manufactured and tested under a voltage of 30 V at 28 days, as illustrated in Figure 4.
Appl. Sci. 2021, 11, x FOR PEER REVIEW aim, three 150 mm × 300 mm cylindrical specimens were subjected to a hydraulic j the average of three samples was considered.

Splitting Tensile Strength
To measure, the splitting tensile strength three 150 mm × 300 mm cylindric mens were tested under a hydraulic jack from the lateral side, as per ASTM C496 [43], the average of three tested specimens was considered.

Modulus of Elasticity
Moreover, 100 mm × 300 mm cylindrical samples were used to assess the mo elasticity according to ASTM C469/C469M [43]. Therefore, a steel ring with a strai has been placed around the cylindrical sample and stress-strain data have been to determine the initial and secant modulus of elasticity.

Water Absorption
After 28 curing days, water absorption was evaluated. The size of the samp not be less than 350 cm 2 and the specimens should be genuine without fractures ting on their surfaces, according to the standards defined by ASTM C642 [44].

Electric Resistivity Test
The electric resistivity test (ERT) was performed to evaluate the resistance a interconnections of concrete [45]. For this aim, three cylindrical specimens per m a diameter of 100 mm and a height of 200 mm, were manufactured and tested voltage of 30 V at 28 days, as illustrated in Figure 4.

Slump
The results of all concrete mixes are presented in Figure 5. According to F

Slump
The results of all concrete mixes are presented in Figure 5. According to Figure 5, increasing the value of NS and SP leads to decreasing the slump of concrete, and the impact of SP on that reduction is more significant than NS's. Therefore, the slump dropped by 3% and 6% by using 1% and 2% NS, respectively. Moreover, employing 5% and 10% SP reduces slump by about 4% and 9%, respectively, and using both 2% NS and 10% SP leads to a considerable reduction of about 14%. The previous investigations reported almost the same results thus confirm those presented in this study [46][47][48]. Jagadisha et al. [49] stated that adding NS unfavourably decreases the workability of concrete due to raising the viscosity of the mixes. The other previous studies showed a significant increase in the viscosity of concrete and a drop in slump value [50][51][52]. According to their results, the slump of concrete declined by 20% when 1.5% NS was used in the mixes. In another study, Duval and Kadri [53] performed a comprehensive evolution on the workability of concrete mixes produced with various SP contents. They showed that increasing SP led to reducing the slump value, so that they recommended increasing the superplasticizer in other to increase the slump. Additionally, increasing the SF content reduces the slump value ( Figure 5). Besides, NS and SP have a negative effect on fibre reinforced HSC. Therefore, the slump dropped by approximately 6%, 11%, 17%, 22% and 28% when 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF were used, respectively. The reduction in slump due to using SF could be attributed to the bridging role of SF that keeps particles together and makes concrete paste denser [54,55]. Karimipour and Ghalehnovi [56] showed that the slump of concrete declined by 20% when 1% SF were added. In addition, using 2% NS and 10% SP together with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF decreased the value of slump by about 19%, 24%, 30%, 36% and 41%, respectively. On the other hand, increasing the PPF content resulted in reducing the slump. Moreover, PPF reduces the slump flow diameter more than SF. This could be attributed to the higher water absorption of polymer fibres in comparison with SF [57]. In Figure 5, adding 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF resulted in decreasing the slump by about 7.5%, 14%, 20%, 27% and 34%, respectively. Furthermore, using 2% NS and 10% SP with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF reduced the slump by roughly 20%, 27%, 33%, 40% and 47%, respectively. According to the results reported by Gencel et al. [58] the workability of concrete substantially declined when PPF were added due to increasing the contact between concrete paste and fibres.
The influence of SF and PPF on the workability of HSC can also be seen in Figure 6. According to this figure, using both SF and PPF have a substantial impact on dropping the value of slump. To evaluate the impact of NS and SP on the workability of fibres reinforced HSC, the value of slump containing different NS and SP contents is illustrated in Figure 6. Additionally, increasing the SF content reduces the slump value ( Figure 5). Besides, NS and SP have a negative effect on fibre reinforced HSC. Therefore, the slump dropped by approximately 6%, 11%, 17%, 22% and 28% when 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF were used, respectively. The reduction in slump due to using SF could be attributed to the bridging role of SF that keeps particles together and makes concrete paste denser [54,55]. Karimipour and Ghalehnovi [56] showed that the slump of concrete declined by 20% when 1% SF were added. In addition, using 2% NS and 10% SP together with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF decreased the value of slump by about 19%, 24%, 30%, 36% and 41%, respectively. On the other hand, increasing the PPF content resulted in reducing the slump. Moreover, PPF reduces the slump flow diameter more than SF. This could be attributed to the higher water absorption of polymer fibres in comparison with SF [57]. In Figure 5, adding 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF resulted in decreasing the slump by about 7.5%, 14%, 20%, 27% and 34%, respectively. Furthermore, using 2% NS and 10% SP with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF reduced the slump by roughly 20%, 27%, 33%, 40% and 47%, respectively. According to the results reported by Gencel et al. [58] the workability of concrete substantially declined when PPF were added due to increasing the contact between concrete paste and fibres.
The influence of SF and PPF on the workability of HSC can also be seen in Figure 6. According to this figure, using both SF and PPF have a substantial impact on dropping the value of slump. To evaluate the impact of NS and SP on the workability of fibres reinforced HSC, the value of slump containing different NS and SP contents is illustrated in Figure 6. In SF reinforced HSC, a reduction trend occurred by raising the content of SN and SP. A similar tendency was observed in PPF reinforced HSC manufactured with NS and SP (Figure 7). By comparing Figure 7a,b, a greater impact of PPF on slump reduction is observed due to higher water demand and absorption of polymer fires (PPF in this study), in comparison with SF [32].  Figure 8 shows that increasing the SF content improves the compressive strength of HSC. Duval and Kadri [49] showed almost the same results thus confirming those in the current study. They showed that pozzolanic activity of SP led to improving the compressive strength of concrete by up to 20%. In another investigation, Jalal et al. [52] found that, by using 2% NS in the mix, the compressive strength of concrete substantially improved. Li et al. [59] measured the mechanical characteristics of NS mortars and found that the compressive strength of those specimens was greater than that of the control sample with no NS. They stated that, if the NS is uniformly distributed in the cement paste, the microstructure of the concrete matrix could be improved, which results in improving the compressive strength. Besides, Khanzadi et al. [60] revealed that the compressive strength of concrete was improved by the use of NS, particularly at an early age. In SF reinforced HSC, a reduction trend occurred by raising the content of SN and SP. A similar tendency was observed in PPF reinforced HSC manufactured with NS and SP (Figure 7). By comparing Figure 7a,b, a greater impact of PPF on slump reduction is observed due to higher water demand and absorption of polymer fires (PPF in this study), in comparison with SF [32]. In SF reinforced HSC, a reduction trend occurred by raising the content of SN and SP. A similar tendency was observed in PPF reinforced HSC manufactured with NS and SP (Figure 7). By comparing Figure 7a,b, a greater impact of PPF on slump reduction is observed due to higher water demand and absorption of polymer fires (PPF in this study), in comparison with SF [32].  Figure 8 shows that increasing the SF content improves the compressive strength of HSC. Duval and Kadri [49] showed almost the same results thus confirming those in the current study. They showed that pozzolanic activity of SP led to improving the compressive strength of concrete by up to 20%. In another investigation, Jalal et al. [52] found that, by using 2% NS in the mix, the compressive strength of concrete substantially improved. Li et al. [59] measured the mechanical characteristics of NS mortars and found that the compressive strength of those specimens was greater than that of the control sample with no NS. They stated that, if the NS is uniformly distributed in the cement paste, the microstructure of the concrete matrix could be improved, which results in improving the compressive strength. Besides, Khanzadi et al. [60] revealed that the compressive strength of concrete was improved by the use of NS, particularly at an early age.  Figure 8 shows that increasing the SF content improves the compressive strength of HSC. Duval and Kadri [49] showed almost the same results thus confirming those in the current study. They showed that pozzolanic activity of SP led to improving the compressive strength of concrete by up to 20%. In another investigation, Jalal et al. [52] found that, by using 2% NS in the mix, the compressive strength of concrete substantially improved. Li et al. [59] measured the mechanical characteristics of NS mortars and found that the compressive strength of those specimens was greater than that of the control sample with no NS. They stated that, if the NS is uniformly distributed in the cement paste, the microstructure of the concrete matrix could be improved, which results in improving the compressive strength. Besides, Khanzadi et al. [60] revealed that the compressive strength of concrete was improved by the use of NS, particularly at an early age. On the other hand, raising the amount of SP led to enhancing the compressive strength. Finally, increasing NS at up to 1% also enhances the compressive strength of HSC; however, using NS at more than 1% has a negative impact on this property. Furthermore, raising the SP content has a substantial influence on the improvement of HSC, especially when 10% SP is used. The compressive strength of HSC raised by almost 15%, 20%, 30%, 32% and 36% when 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF were employed, respectively. Moreover, adding 1% NS and 10% SP with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF has the greatest impact on the HSC compressive strength: about 82%, 90%, 105%, 109% and 115%, respectively.

Compressive Strength
In addition, the improvement effect of SF on the compressive strength decreased by more than 0.3% SF when NS was employed. Additionally, the substantial positive impact of PPF on the compressive strength of HSC with NS and SP is seen in Figure 8. The main reason for improving the compressive strength of concrete with the use of fibres could be attributed to the bridging role of fibres, which create a strong core in the mid-height of the specimens. Karimipour and de Brito [61] performed a comprehensive evaluation of the influence of PPF on improving the compressive strength of concrete. They stated that, when a specimen is loaded by compression, a lateral expansion happens at the mid-height of the cylindrical sample. The bridging role of PPF improved the tensile strength of the matrix and enhanced the cohesion between concrete paste and aggregates and that limited lateral deformation of the sample, which enhanced the compressive behaviour of the specimen. Therefore, using 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF improved the compressive strength by about 15%, 22%, 34%, 43% and 50%, respectively. By comparing Figure 8a,b, increasing the value of SP did not have a negative effect on the improved performance of compressive strength when the value of PPF increased; however, using NS at more than 1% has a negative impact on this property. Besides, employing 1% NS and 10% SP together with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF enhances the compressive strength of HSC by approximately 66%, 81%, 99%, 113% and 123%, respectively.
In Figure 9, the effect of fibres on the compressive strength of HSC is illustrated in a different manner. As seen in Figure 9a, there is no substantial impact on this property between 0.1% and 0.2% SF. In addition, a similar trend is observed when 0.4% and 0.5% SF are used. On the other hand, raising the amount of SP led to enhancing the compressive strength. Finally, increasing NS at up to 1% also enhances the compressive strength of HSC; however, using NS at more than 1% has a negative impact on this property. Furthermore, raising the SP content has a substantial influence on the improvement of HSC, especially when 10% SP is used. The compressive strength of HSC raised by almost 15%, 20%, 30%, 32% and 36% when 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF were employed, respectively. Moreover, adding 1% NS and 10% SP with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF has the greatest impact on the HSC compressive strength: about 82%, 90%, 105%, 109% and 115%, respectively.
In addition, the improvement effect of SF on the compressive strength decreased by more than 0.3% SF when NS was employed. Additionally, the substantial positive impact of PPF on the compressive strength of HSC with NS and SP is seen in Figure 8. The main reason for improving the compressive strength of concrete with the use of fibres could be attributed to the bridging role of fibres, which create a strong core in the mid-height of the specimens. Karimipour and de Brito [61] performed a comprehensive evaluation of the influence of PPF on improving the compressive strength of concrete. They stated that, when a specimen is loaded by compression, a lateral expansion happens at the mid-height of the cylindrical sample. The bridging role of PPF improved the tensile strength of the matrix and enhanced the cohesion between concrete paste and aggregates and that limited lateral deformation of the sample, which enhanced the compressive behaviour of the specimen. Therefore, using 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF improved the compressive strength by about 15%, 22%, 34%, 43% and 50%, respectively. By comparing Figure 8a,b, increasing the value of SP did not have a negative effect on the improved performance of compressive strength when the value of PPF increased; however, using NS at more than 1% has a negative impact on this property. Besides, employing 1% NS and 10% SP together with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF enhances the compressive strength of HSC by approximately 66%, 81%, 99%, 113% and 123%, respectively.
In Figure 9, the effect of fibres on the compressive strength of HSC is illustrated in a different manner. As seen in Figure 9a, there is no substantial impact on this property between 0.1% and 0.2% SF. In addition, a similar trend is observed when 0.4% and 0.5% SF are used. Figure 10 illustrates the splitting tensile strength of HSC containing various contents of fibres, NS and SP. The splitting tensile strength depends on the tensile strength of the matrix and the adhesion between cement and aggregates. As seen in Figure 10, increasing the value of SF resulted in raising the tensile strength of HSC. Using SP improves the splitting tensile strength of HSC; however, the positive effect of SF on this property dropped by adding SP at more than 5%. Moreover, using 10% SP with no NS has a negative impact on the splitting tensile strength of HSC. On the other hand, using NS enhances the tensile strength of HSC. Almost the same improvement influence of SP and NS on the splitting tensile strength of concrete was reported by previous studies [10][11][12][13]. Moha and Hayat [62] showed that the splitting tensile strength of concrete substantially improved with the use of low SP content, while by increasing SP content, the splitting tensile strength gradually declined. The pozzolanic activity of SP is the main reason for improving the splitting tensile strength of HSC [63,64]. Moreover, the negative impact of using 10% SP dropped by raising the SF content. As seen in Figure 10, the tensile strength of HSC raised by nearly 15%, 20%, 26%, 30% and 37% by adding 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF, respectively. Besides, the tensile strength increased by about 65.5%, 86.4%, 88%, 92.6% and 104% when 0.1%, 0.2%, 0.3%, 0.4%, respectively, and 0.5% SF with 2% NS and 10% SP were employed. Duval and Kadri [53] showed that the SP particles size is an important property to improve the mechanical characteristics of concrete due to filling porous and creating a strong ITZ.  Figure 10 illustrates the splitting tensile strength of HSC containing various contents of fibres, NS and SP. The splitting tensile strength depends on the tensile strength of the matrix and the adhesion between cement and aggregates. As seen in Figure 10, increasing the value of SF resulted in raising the tensile strength of HSC. Using SP improves the splitting tensile strength of HSC; however, the positive effect of SF on this property dropped by adding SP at more than 5%. Moreover, using 10% SP with no NS has a negative impact on the splitting tensile strength of HSC. On the other hand, using NS enhances the tensile strength of HSC. Almost the same improvement influence of SP and NS on the splitting tensile strength of concrete was reported by previous studies [10][11][12][13]. Moha and Hayat [62] showed that the splitting tensile strength of concrete substantially improved with the use of low SP content, while by increasing SP content, the splitting tensile strength gradually declined. The pozzolanic activity of SP is the main reason for improving the splitting tensile strength of HSC [63,64]. Moreover, the negative impact of using 10% SP dropped by raising the SF content. As seen in Figure 10, the tensile strength of HSC raised by nearly 15%, 20%, 26%, 30% and 37% by adding 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF, respectively. Besides, the tensile strength increased by about 65.5%, 86.4%, 88%, 92.6% and 104% when 0.1%, 0.2%, 0.3%, 0.4%, respectively, and 0.5% SF with 2% NS and 10% SP were employed. Duval and Kadri [53] showed that the SP particles size is an important property to improve the mechanical characteristics of concrete due to filling porous and creating a strong ITZ. In addition, using PPF resulted in improving the tensile strength of HSC. Furthermore, using PPF compensates for the negative influence of using SP at more than 5%. Furthermore, the impact of SF on the improvement of the tensile strength of HSC was greater than that of PPF. The higher tensile strength of fibres, in comparison with concrete paste and the bridging role of fibres, are the main reasons why SF and PPF improve the splitting tensile strength of HSC [61]. Therefore, this property was improved by 7%, 10%, 18%, 25% and 31% by adding 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF, respectively. Moreo- In addition, using PPF resulted in improving the tensile strength of HSC. Furthermore, using PPF compensates for the negative influence of using SP at more than 5%. Furthermore, the impact of SF on the improvement of the tensile strength of HSC was greater than that of PPF. The higher tensile strength of fibres, in comparison with concrete paste and the bridging role of fibres, are the main reasons why SF and PPF improve the splitting tensile strength of HSC [61]. Therefore, this property was improved by 7%, 10%, 18%, 25% and 31% by adding 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF, respectively. Moreover, using both 2% NS and 10% SP with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF enhanced the splitting tensile strength of HSC by approximately 56.8%, 66.8%, 79.6%, 87% and 102%, respectively. Besides, Jalal et al. [52] stated that the use of low SP content improved the splitting tensile strength of concrete and it was further enhanced when SF was also added.

Splitting Tensile Strength
The effect of fibres on the splitting tensile strength of HSC can also be observed in Figure 11. Using either SF or PPF considerably improves the tensile strength of HSC. In addition, using PPF resulted in improving the tensile strength of HSC. Furthermore, using PPF compensates for the negative influence of using SP at more than 5%. Furthermore, the impact of SF on the improvement of the tensile strength of HSC was greater than that of PPF. The higher tensile strength of fibres, in comparison with concrete paste and the bridging role of fibres, are the main reasons why SF and PPF improve the splitting tensile strength of HSC [61]. Therefore, this property was improved by 7%, 10%, 18%, 25% and 31% by adding 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF, respectively. Moreover, using both 2% NS and 10% SP with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF enhanced the splitting tensile strength of HSC by approximately 56.8%, 66.8%, 79.6%, 87% and 102%, respectively. Besides, Jalal et al. [52] stated that the use of low SP content improved the splitting tensile strength of concrete and it was further enhanced when SF was also added.
The effect of fibres on the splitting tensile strength of HSC can also be observed in Figure 11. Using either SF or PPF considerably improves the tensile strength of HSC.

Modulus of Elasticity
The initial and secant moduli of elasticity of all concrete mixes are presented in Figures 14 and 15, where the influence of NS, SP, PPF and SP is illustrated.

Modulus of Elasticity
The initial and secant moduli of elasticity of all concrete mixes are presented in Figures 14 and 15, where the influence of NS, SP, PPF and SP is illustrated.

Modulus of Elasticity
The initial and secant moduli of elasticity of all concrete mixes are presented in Figures 14 and 15, where the influence of NS, SP, PPF and SP is illustrated.  There, the modulus elasticity was raised by increasing the SF content as well as that of NS and SP. This could be attributed to the improvement of compressive strength due to the bridging role of the fibres. Hannawi et al. [65] performed SEM of the interfacial transition zone (ITZ) and showed a strong crystal bond between concrete paste and aggregate when nanoparticles were incorporated. Besides, it showed that the modulus of elasticity of concrete improved by 6% and 11% when 1% and 1.5% NS was added. It also There, the modulus elasticity was raised by increasing the SF content as well as that of NS and SP. This could be attributed to the improvement of compressive strength due to the bridging role of the fibres. Hannawi et al. [65] performed SEM of the interfacial transition zone (ITZ) and showed a strong crystal bond between concrete paste and aggregate when nanoparticles were incorporated. Besides, it showed that the modulus of elasticity of concrete improved by 6% and 11% when 1% and 1.5% NS was added. It also showed that the use of a large NS content adversely affected the modulus of elasticity of concrete. Also, the improvement effect of NS on the initial elasticity modulus raised by increasing the amount of SP There, this property improved by 15%, 20%, 36%, 42% and 50% when only 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF were added, respectively. Furthermore, using a combination of 2% NS and 10% SP with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF led to enhancing this property by approximately 55%, 62%, 82%, 92% and 102%, respectively. On the other hand, a similar trend can be observed for the secant modulus of elasticity; naturally, the value of the secant modulus of elasticity is lower than that of the initial modulus of elasticity in each concrete mix.
Using PPF improves the value of the initial modulus of elasticity. Almost the same improvement in the modulus of elasticity due to PPF incorporation was reported by previous studies [61]. Małek et al. [66] measured the effect of PPF on the mechanical characteristics of conventional concrete. They showed that the use of 0.15% and 0.3% PPF improved the modulus of elasticity of plain concrete respectively by 15% and 20%, in comparison with the control sample. They also argued that PPF bridges the cracks and transfer stress over cracking regions. In addition, PPF creates a strong concrete core at the middles of cylindrical specimens, which led to increasing the stress resistance and reducing the lateral and axial strain of samples that led to improving the modulus of elasticity of fibres-reinforced HSC [61]. Moreover, employing NS and SP resulted in improving the modulus of elasticity. As seen in these figures, this property raised around 7%, 10%, 15%, 21% and 27% by using 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF, respectively. Besides, incorporating 2% NS with 10% SP and 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF led to enhancing this property by 44%, 48%, 55%, 63% and 71%, respectively. According to Table 8, the influence of SF on increasing the content of the moduli of elasticity of HSC is greater than that of PPF.
A high accuracy relationship between the moduli of elasticity and compressive strength was obtained, as illustrated in Figures 16 and 17. The proposed equations were established according to the type of fibres with R2 above 0.79 for the initial modulus of elasticity and 0.45 for the secant elasticity modulus. As seen in Figure 16, the slope of the relationship between the initial modulus of elasticity and compressive strength of PPF reinforced HSC was higher than that of SF reinforced HSC; however, an inverse trend is found for the secant modulus of elasticity ( Figure 17).
Appl. Sci. 2021, 11, x FOR PEER REVIEW 17 of 23 reinforced HSC was higher than that of SF reinforced HSC; however, an inverse trend is found for the secant modulus of elasticity ( Figure 17).

Water Absorption
The results of water absorption are represented in Figures 18 and 19. According to Figure 18, using SF resulted in raising the water absorption. Moreover, adding 10% SP significantly improved the water absorption of SF reinforced HSC. On the other hand, 5% SP reduces this property while using 1% NS considerably increased it when less than 5% SP was used. Using 1% NA dropped the water absorption when using 10% SP, and using a combination of 2% NS and 10% SP with 0.5% SF had a significant impact on increasing the water absorption. This could be attributed to the high surface area of NS. As shown by Mahdikhani et al. [67], the water absorption increased by adding NS, due to incremented specimens destruction, so that increasing the concrete specimens capillary porosity and permeability resulted by raising the acidity of the solution, while SP could act as a filler and reduce the porosity and permeability of HSC mixes. Additionally, Ali et al. [68] found that using a high SP content (20%) declined the workability by up to 20 mm. So, the incorporation of 0.5% SF and 10% SP with 0% and 2% NS raised the water absorption by about 56% and 68%, respectively. A similar trend was observed when PPF was used; however, the influence of PPF on the water absorption was more significant than

Water Absorption
The results of water absorption are represented in Figures 18 and 19. According to Figure 18, using SF resulted in raising the water absorption. Moreover, adding 10% SP significantly improved the water absorption of SF reinforced HSC. On the other hand, 5% SP reduces this property while using 1% NS considerably increased it when less than 5% SP was used. Using 1% NA dropped the water absorption when using 10% SP, and using a combination of 2% NS and 10% SP with 0.5% SF had a significant impact on increasing the water absorption. This could be attributed to the high surface area of NS. As shown by Mahdikhani et al. [67], the water absorption increased by adding NS, due to incremented specimens destruction, so that increasing the concrete specimens capillary porosity and permeability resulted by raising the acidity of the solution, while SP could act as a filler and reduce the porosity and permeability of HSC mixes. Additionally, Ali et al. [68] found that using a high SP content (20%) declined the workability by up to 20 mm. So, the incorporation of 0.5% SF and 10% SP with 0% and 2% NS raised the water absorption by about 56% and 68%, respectively. A similar trend was observed when PPF was used; however, the influence of PPF on the water absorption was more significant than that of SF. Therefore, using 10% SP and 0.5% PPF together with 0% and 2% NS increased this property by 70% and 82%, respectively (Figure 18b).
Appl. Sci. 2021, 11, x FOR PEER REVIEW 18 of 23 that of SF. Therefore, using 10% SP and 0.5% PPF together with 0% and 2% NS increased this property by 70% and 82%, respectively (Figure 18b).    Figure 19 shows the impact of a combination of NS, SP, SF and PPF on the porosity of HSC. SF increased the water absorption of SF reinforced HSC, and further using 10% SP considerably it. Conversely, using 5% SP decreases the value of this property, whereas using 1% NS substantially enhanced it when less than 5% SP was employed. Besides, using 1% NA led to reducing the water absorption when using 10% SP and using 2% NS and 10% SP with 0.5% SF has a far-reaching influence on raising the water absorption. Therefore, a combination of 0.5% SF and 10% SP with 0% and 2% NS increased this property by nearly 103% and 115%, respectively. The same trend was experienced when PPF was used; however, the impact of PPF on water absorption was greater than that of SF. Consequently, using 0.5% PPF and 10% SP together with 0% and 2% NS raised this property by 147% and 173%, respectively (Figure 19b).  Figure 19 shows the impact of a combination of NS, SP, SF and PPF on the porosity of HSC. SF increased the water absorption of SF reinforced HSC, and further using 10% SP considerably it. Conversely, using 5% SP decreases the value of this property, whereas using 1% NS substantially enhanced it when less than 5% SP was employed. Besides, using 1% NA led to reducing the water absorption when using 10% SP and using 2% NS and 10% SP with 0.5% SF has a far-reaching influence on raising the water absorption. Therefore, a combination of 0.5% SF and 10% SP with 0% and 2% NS increased this property by nearly 103% and 115%, respectively. The same trend was experienced when PPF was used; however, the impact of PPF on water absorption was greater than that of SF. Consequently, using 0.5% PPF and 10% SP together with 0% and 2% NS raised this property by 147% and 173%, respectively (Figure 19b). Figure 20 shows the ERT results of specimens manufactured with different contents of NS, SP, SF and PPF. Increasing SF content resulted in a fall in ERT. Moreover, using 1% NS had a significant effect on improving ERT, and this property was enhanced by using NS at up to 2% while employing NS at more than 1% resulted in decreasing the ERT value. Alternatively, increasing the SP content did not have a significant effect on this property. Conversely, increasing the PPF content considerably improved the electric resistivity. Also, the ERT was significantly improved when PPF and 1% NS were combined.

Electric Resistivity
Appl. Sci. 2021, 11, x FOR PEER REVIEW 19 of 23 Figure 20 shows the ERT results of specimens manufactured with different contents of NS, SP, SF and PPF. Increasing SF content resulted in a fall in ERT. Moreover, using 1% NS had a significant effect on improving ERT, and this property was enhanced by using NS at up to 2% while employing NS at more than 1% resulted in decreasing the ERT value. Alternatively, increasing the SP content did not have a significant effect on this property. Conversely, increasing the PPF content considerably improved the electric resistivity. Also, the ERT was significantly improved when PPF and 1% NS were combined. Therefore, this property was enhanced by about 57%, 62% and 68% when 0.5% SF and 1% NS were used with 0%, 5% and 10% SP, respectively. Previous investigations showed the same influence of PPF and SF on ERT thus confirming the results of our study. Recently, Karimipour [57] performed a study on the influence of different fibre types on the ERT. It showed that the main reason for reducing ERT results due to SF incorporation could be associated with the easier transfer of electricity across steel fibres while electricity transfer by polymer fibres is negligible.

Conclusions
In this study, 99 concrete mixes were designed and applied to assess the mechanical and durability performance of fibres reinforced high-strength concrete containing NS and SP. For this aim, SF and PPF were used at six-volume contents: 0%, 0.1%, 0.2%, 0.3%, 0.4% Therefore, this property was enhanced by about 57%, 62% and 68% when 0.5% SF and 1% NS were used with 0%, 5% and 10% SP, respectively. Previous investigations showed the same influence of PPF and SF on ERT thus confirming the results of our study. Recently, Karimipour [57] performed a study on the influence of different fibre types on the ERT. It showed that the main reason for reducing ERT results due to SF incorporation could be associated with the easier transfer of electricity across steel fibres while electricity transfer by polymer fibres is negligible.

Conclusions
In this study, 99 concrete mixes were designed and applied to assess the mechanical and durability performance of fibres reinforced high-strength concrete containing NS and SP. For this aim, SF and PPF were used at six-volume contents: 0%, 0.1%, 0.2%, 0.3%, 0.4% and 0.5%. Furthermore, NS were used at four contents: 0%, 1% and 2%. In addition, SP was used at three ratios by weight (0%, 5% and 10%). Therefore, the slump, compressive and splitting tensile strength, modulus of elasticity, water absorption and electric resistivity of the specimens were measured. Based on the experimental results, the following conclusions could be drawn:

1.
Slump decreased by increasing the NS and SP contents. In addition, the influence of SP on decreasing the slump is greater than that of NS. In addition, adding fibres had a negative influence on the workability of HSC mixes and the reduction influence of PPF on the slump was greater than SF; 2.
Adding SF and PPF improved the compressive and splitting tensile strengths of HSC and strengths were further enhanced with increasing fibres contents. Conversely, the optimal content of NS and SP was obtained by 1% NS and 10% SP in terms of maximum compressive and splitting tensile strengths; 3.
The initial modulus of elasticity is raised by increasing the SF content, as well as that of NS and SP. Additionally, using PPF improved the initial modulus of elasticity. Using NS and SP resulted in raising the modulus of elasticity by around 7%, 10%, 15%, 21% and 27% by using 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF, respectively; 4.
Using SF increased the water absorption. Adding 10% SP significantly increased the water absorption of SF reinforced HSC. Oppositely, 5% SP reduced the value of this property while using 1% NS increased water absorption considerably when less than 5% SP was used; 5.
Using 1% NS had a substantial impact on ERT results: this property was enhanced by using NS at up to 2% while employing it at more than 1% resulted in a drop. On the other hand, increasing the value of SP did not have a significant influence on this property. Increasing PPF content considerably enhanced the electric resistivity. ERT results were also significantly improved when both PPF and 1% NS were used. Informed Consent Statement: Not applicable.

Data Availability Statement:
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.