The Durability of High-Strength Concrete Containing Waste Tire Steel Fiber and Coal Fly Ash

Department of Civil Engineering (CVE), COMSATS University Islamabad (CUI)-Sahiwal Campus, Sahiwal 57000, Pakistan Department of Civil Engineering, Recep Tayyip Erdoğan University, Rize, Turkey Department of Mechanical Engineering, College of Engineering, King Khalid University, Abha 61421, Saudi Arabia Laboratory of Mechanics of Sousse, National Engineering School of Sousse, University of Sousse, Sousse 4054, Tunisia Department of Civil Engineering, College of Engineering, King Khalid University, P.O. Box 394, Abha 61411, Saudi Arabia Applied Mechanics and Systems Research Laboratory, Tunisia Polytechnic School, University of Carthage, La Marsa, Tunis 2078, Tunisia


Introduction
e impacts of cement concrete manufacturing and its uses are quite complex to comprehend. Some impacts are positive, and others are negative, depending on the situation. Portland cement is the vital ingredient of concrete that has various environmental, economic, and social impacts. e impacts of cement also contribute to those of the concrete. e cement industry alone releases about 7% of the total greenhouse emissions produced all over the world [1]. Rapid growth in urban populations and the requirement of modern infrastructure have increased the demand for cement, which consequently reflects badly on the quality of the environment and social life. About 0.7-1 kilogram of global warming gases is produced due to 1-kilogram production of Portland cement, depending on the type of energy source and technology employed to manufacture cement [2]. While the other ingredients of concrete such as sand and gravels have a small CO 2 footprint, their cost and CO 2 footprint largely depend on their transportation distances between the quarry and concrete batching plant. erefore, about 85% of emissions of cement concrete are dependent on the binder constituent of concrete [3,4]. e most effective way to control the environmental impact of concrete is to minimize its cement consumption. is can be achieved by the utilization of industrial waste powders that possess pozzolanicity and hydraulicity. e substitution of a small and medium volume of cement with these waste powders can drastically reduce the CO 2 footprint of concrete. However, the efficacy of potential cement substitution materials should be assessed properly in terms of their contribution towards durability and mechanical performance. e cost to strength [5,6] or CO 2 footprint to strength ratio analysis [7] should always be performed to judge the technical performance of cementitious materials.
Fly ash (FA) is a common waste mineral powder, which is a product of pulverized fuel ash burning for electric power generation. FA incorporation into cement concrete cannot only decrease the CO 2 footprint of concrete, but it can also resolve waste disposal problems associated with high volumes of coal ashes. FA can help in gaining a circular economy in modern-day concrete manufacturing. Pakistan relies heavily on nonrenewable supplies of energy, such as coal-fed power plants. erefore, abundant supplies of FA are available in this country. FA is rich in alumina-silica, and it has minor amounts of calcium and iron oxides. It qualifies as a potential cement replacement material [8]. e effects of FA on the properties of concrete have been studied properly. It helps in the slow consumption of residual portlandite Ca(OH) 2 and positively affects the resistance of concrete against water absorption, chloride attack, and drying shrinkage [9][10][11]. Low levels of FA can cause minor improvements in the mechanical performance of concrete [12], but its high levels drastically reduce mechanical strength [7,13]. e degree of the effectiveness of FA depends on its fineness, chemical composition, and unburnt carbon content [14][15][16][17]. Generally, FA with high fineness and low carbon content is considered suitable for concrete applications.
Due to the increasing information gained related to material availability, design, and construction techniques, the practical scope of high-strength concrete (HSC) applications has been expanded dramatically. Rising inclination towards lightweight elements, large spans in buildings and bridges have increased the demands for HSC. However, there is a faction among designers unwilling to use HSC owing to its some drawbacks compared to conventional normal strength concrete (NSC) [18]. To begin with, HSC has a low f CTM compared to its f CM . e increase in the strength class of concrete reduces the ratio between f CTM and f CM [19]. is means that the gain in f CTM achieved due to the low water-cement ratio is not proportional to the gain in f CM . Due to low ductility, HSC is extremely brittle in fire temperatures. Due to a dense microstructure, the fire resistance of HSC is incredibly lower than the NSC [20]. e brittleness issue of HSC can be addressed by using fibers. Various options for fiber reinforcements are available, such as steel, carbon, polypropylene, polyvinyl, and glass fibers [21][22][23]. e use of fibers substantially enhances the tensile and fracture toughness of HSC. e selection of fiber type varies depending upon the application of HSC. Fiber addition is highly useful in enhancing f CTM and flexural strength f CRM of HSC [22]. In short, fibers can overcome the inherent issue of brittleness associated with both plain HSC and NSC. Research has shown the negative effects of fiber addition on the economy and the environmental impact of concrete [24]. High transportation distances significantly increase the cost and CO 2 footprint of fibers [24]. eir small doses can noticeably increase the cost and CO 2 footprint of concrete. Fiber addition also requires technical supervision; it creates workability issues when used in HSC. erefore, the use of additional measures and materials to control the quality of concrete can increase the final cost. erefore, the selection of fiber type should be made based on a comprehensive cost to benefit ratio analysis. e development of ductile, cheap, and environmentfriendly HSC is not possible without considering the less energy-intensive fibers compared to industrially manufactured fibers available at distant locations. Currently, researchers are investigating the potential of waste tire steel fibers (WSF) as the fiber reinforcement in cement concrete. Since WSF is composed of ultra-high-strength steel wires, which are designed for good fatigue resistance, it can become a potential fiber reinforcement material. WSF behaves similar to virgin steel fiber to a great extent, considering the properties of ultra-high performance concrete [25]. Considering the wider availability of old waste tires, WSF can become a local fiber-reinforcement material in all regions of the world, so high transportation costs can be avoided by adopting WSF instead of industrial fibers.
New steel fiber and WSF behave similarly as fiber reinforcement due to the same material. Small doses of WSF can be useful to f CM of concrete, whereas using a large dose of WSF can lessen the f CM of concrete [26]. A high dose of WSF increases the porosity of concrete due to workability issues that reflect badly in terms of compression stiffness of concrete [26]. WSF can postpone the collapse of concrete under compression; it can ensure ductile and slow cracking with a significant warning before collapse [27]. e effect of WSF on the properties of concrete significantly depends on the length, shape, texture, and residual rubber content of fiber [28]. e incorporation of 0.46% volume of irregularshaped WSF in concrete improved its f CM by 25% [29]. At the same volume of 0.75% WSF and new steel fibers, f CTM of concrete increased by 28% and 26%, respectively, whereas by the use of WSF carrying mixed filament lengths, f CTM of plain concrete was increased by about 50% [30,31]. Similar to compressive behavior, the postpeak flexure response of concrete depends on the type, dose, and the number of filaments per unit volume. f CRM of concrete significantly increases with the rise in WSF dosage [32,33]. WSF provides a crack-arresting mechanism that helps in delaying the onset of rupture failure of concrete [31,[34][35][36][37]. e incorporation of FA and WSF into HSC can integrate the benefits of ductility, durability, and ecofriendliness. e simultaneous use of waste mineral admixtures and fibers proves beneficial in three different ways: (1) fine particles of mineral admixtures can improve the distribution of filaments throughout the matrix of concrete [38], [39]; (2) mineral admixtures increase the interface between filaments and binder matrix that improves the bond performance of fibers [40,41]; and (3) some mineral admixtures reduce the water demand of concrete due to their slow hydration and filling action, hence reducing the requirement of water-reducing agents to maintain the workability of fiber-reinforced concretes [42][43][44][45]. Due to these benefits, fibers and mineral admixture addition show some synergistic results on the performance of concrete [9]. ere are very few studies that investigate the combined behavior of FA and WSF. However, many studies are available related to the combined behavior of industrial fibers and waste mineral admixtures (i.e., FA, silica fume, slag, etc.) [46][47][48].
e combined behavior of WSF and mineral admixture was studied by Mastali and Dalvand [27]. ey found that the combined addition of silica fume and WSF improves the overall toughness and strength of concrete.
Very little information is available on the durability behavior of HSC made with the combined incorporation of WSF and FA. Moreover, information on mechanical behavior is also deficient. Due to already explained environmental, economic, and ductility benefits, the combined effect of WSF and FA should be properly investigated on the properties of HSC. erefore, this research aimed to evaluate the effects of different combinations of FA (0, 10, 15, 25, and 35%) and WSF (0, 0.5, and 1%) on the properties of HSC. e examined durability parameters involve RCP and CPD by immersion method (at 28 and 120 days) and AAR (at 28 and 120 days). Various basic mechanical properties of HSC were also studied experimentally, such as f CM , f CTM , f CRM , and E CM . e findings of this research fill an important research gap related to the durability of waste tire steel fiberreinforced concrete. Moreover, the combined effect of FA and WSF on both durability and mechanical properties of HSC has never been studied before.

Constituent Materials
2.1.1. Binding Materials. HSC mixes were prepared with 53grade cement, which was used as the major binder. It is qualified as "Type-I cement" per ASTM C150 [49]. FA containing a low percentage of lime was acquired from a local coal power plant. It was a by-product of bituminous coal. e generation of FA was estimated to be 10% of the annual cement production of Pakistan. e composition of the FA is Class F type, known for pozzolanicity potential but low hydraulicity. Important properties of FA and 53-grade cement are given in Table 1.

2.1.2.
Aggregates. Quarry sand of "Lawrencepur" was used as fine aggregate to manufacture HSC.
is sand is recommended for good-quality concrete production in Punjab, Pakistan. is coarse-grained sand has a good distribution of particle sizes and has a "fineness moduli" of 2.92. Crushed dolomite sandstone was used as coarse aggregate. is aggregate was derived from Kirana Hills of Sargodha, Punjab, Pakistan. Engineering properties of fine and coarse aggregates are given in Table 2, which were important inputs in the mix design procedure of HSC. e diversity in particle sizes of both "quarry sand" and "crushed coarse aggregate" is shown in Figure 1. e maximum aggregate sizes for "fine" and "coarse" aggregates are 4.75 and 12.5 mm, respectively.
2.1.3. WSF. WSF used in this research was derived from old waste tires of truck vehicles. e tire-bead wires, when removed from waste tires, contained residual rubber. erefore, heat treatment was applied to remove the rubber particles from steel wires. Removing rubber particles is necessary to ensure a good bond between fibers and the matrix of the concrete. Moreover, rubber reduces f CM of concrete owing to its low density [28]. Finally, clean steel wires were chopped into lengths of about 30 mm. WSF consists of spun filaments as can be noticed in Figure 2. e diameter of each filament was about 1.2 mm. WSF also contains microsteel wires that were twined around the main filament in the tire bead. ese small wires may provide hybrid (mixed-length and mixed-diameter effects) of fibers on the properties of HSC.

Water-Reducing Agent and Water.
e desired workability of an HSC mixture was achieved by using a commercial third-generation water-reducing agent "Vis-coCrete 3110." It also helped in controlling the drop in workability due to the addition of WSF. Tap water from the concrete laboratory was used in the preparation and curing of HSC mixes. It has a pH of 7.9 and total dissolved solid content of 170 mg/L.

Design HSC Concrete Mixes with Different
Combinations of WSF and FA. In this research control or reference, HSC was designed for a cubical f CM of 70 MPa. is strength class was achieved by employing a water-binder ratio of 0.30. In order to achieve good workability (a slump value of 190-210 mm), the "ViscoCrete 3110" water-reducing agent was used at 0.75% by weight of binder in reference HSC. Details about the composition of a reference mix are given in Table 3. In HSC, FA was used at five different levels, 0%, 10%, 15%, 25%, and 35%, by volumetric replacement of cement. Since FA is lighter than cement, the replacement of cement with FA should be done by volume. en, with each incorporation level of FA, three different doses of WSF, 0%, 0.5%, and 1%, by a volumetric fraction of concrete were used. erefore, the experimental campaign studied a total of 15 concrete mixtures. Details of all 15 mixes are given in Table 3. It is worth mentioning here that plain HSC mixtures containing FA achieved the required range of workability at 0.75% dose of water-reducing agent, while all fiber-reinforced HSC mixes required a 1% dose of water-reducing agent to achieve desirable workability. e workability was because the use of WSF increased the stiffness of fresh concrete. erefore, the loss in workability due to the fiber addition was compensated with a high dose of plasticizer. All plain mixtures (without fibers) (serial nos. 1, 4, 7, 10, 13) Advances in Materials Science and Engineering attained a slump value of 180-210 mm, while WSF-reinforced mixes (serial nos. 2, 3, 5, 6, 8, 9, 11, 12, 14, and 15) attained slump values of 125-145 mm.

Mixing
Method. HSC mixes containing WSF (serial nos. 2, 3, 5, 6, 8, 9, 11, 12, 14, and 15) were mixed in four continuous stages: (1) in the first step, cement, FA, and aggregates were mixed in machine mixer for 2 mins at a speed of 40 rev/min (revolutions per minute); (2) in the second step, half the amount of water and water-reducing agents were added to mix and blend in machine mixer continued for 2 mins at a speed of 40 revs/min; (3) in the third step, the remaining quantities of water-reducing agent and water were added and mixed, and blending was done at high speed of 60 rev/min for 2 mins; and (4) in the    fourth stage, measured quantities of WSF were charged into the mixer, and machine mixing was done for 4 mins at 80 revs/min to ensure proper dispersion of cement particles and fiber filaments. Plain mixes (serial nos. 1, 4, 7, 10, 13) were mixed in the first three stages which were used for the mixing of WSF-HSC mixes.
After completion of mixing, fresh concrete mixtures were tested for the slump, and mixes qualifying the workability requirements were proceeded for casting. During the casting stage, the machine mixer kept running at a low speed of 40 rev/min. Casting was completed within 4-5 mins of completion of the mixing stage.

Sample Details and Testing Methods.
Six cubes of 100 mm dimensions were prepared for each mixture. e f CM was determined at 28 and 120 days; three cubes of each mix were tested at one age. e standard of testing was followed as per ASTM C39 [51]. e testing setup is shown in Figure 3(a). Cylindrical samples of 100 mm diameter and 200 mm height were cast for E CM testing, as shown in Figure 3(b). ree cylinders per mix were cast, of which were tested at 28 days and the remaining three 120 days. e standard of testing was adopted from ASTM C469 [52]. is test was performed to estimate the effect of WSF on the ductility of samples. For this purpose, cylindrical specimens of "100 mm diameter and 200 mm" height were prepared. ree replica samples of each mix were tested at 28 days. e standard of testing was adopted from ASTM C496 [53]. e testing overview is shown in Figure 3(c). "Flexural strength" or "modulus of rupture f CRM " is a tensile property of concrete that is employed in the design equations of flexural elements, such as slabs and pavements. For each mix, three specimens having dimensions of 100 mm × 100 mm × 350 mm were tested for the calculation of f CRM . f CRM was determined under the thirdpoint loading method adopted from ASTM C1609 [54]. e test setup is shown in Figure 3(d). e chloride durability is an important characteristic of concrete that tells about the life of a reinforced structural element. RCP test was conducted on 100 mm diameter × 50 mm height disc specimens of each mix. e test method was adopted from ASTM C1202 [55]. RCP test was performed by maintaining a potential difference of 60 volts for the duration of 6 hrs. e overview of RCP testing is shown in Figure 4.
Chloride-ion penetration depth (CPD) by immersion method was also measured to understand the effects of WSF and FA on the permeability of chloride ions in the absence of applied voltage. Since WSF addition highly increases the electrical conductivity of HSC [43], it becomes more convenient to adopt the natural process of measuring chloride penetrability of HSC rather than the RCP test, to avoid wrong interpretation of RCP test results of steel fiberreinforced concretes. For the immersion test method, six cylindrical specimens (100 mm height × 100 mm diameter) of each mix were cured in tap water for 28 days, and then airdried for 3 days at room temperature. ese air-dried specimens were then soaked in a 10% solution of sodium chloride (NaCl) salt. CPD was then measured by spraying 0.1 normality solution of silver nitrate (AgNO 3 ) salt, on the split surfaces of NaCl conditioned samples after periods of 28 and 120 days. ree replica samples were tested at each age to determine the average CPD value of each HSC mix.
Measuring the AAR of HSC is very important, as there are some applications where concrete experiences harsh acidic environments, such as in components of sewerage networks. Method of AAR testing was adopted from a previous study [9], where AAR was measured by quantifying the differences between fresh samples and acid exposed samples. For AAR testing, three replica specimens (100 mm cubes) of each mix were cured in tap water for 28 days. ese specimens were then air-dried at room temperature for 3 days. en, the samples of all mixes were exposed to sulfuric acidic (H 2 SO 4 ) solution of 5% concentration. e change in

Compressive Properties
. f CM of all fifteen mixes at the ages of 28 and 120 days is shown in Figure 5. Both FA and WSF addition showed mixed effects of f CM depending on their percentage in an HSC mix. e 28 days f CM of HSC was increased by 6% at 10% FA addition. is improvement in f CM was credited to the effective filling effect and development of pozzolanic products [12,15]. While f CM of HSC decreased notably compared to the reference mix, with the rising level of FA. is could be blamed on a reduction in the overall lime content of the binder. Although smaller particles of FA provided a filling effect, at a high level, FA fails to develop necessary reactions responsible for strength.
As FA particles reacted slowly with available portlandite,   Advances in Materials Science and Engineering HSC mixtures containing 10-15% developed noticeably (i.e., 7-11%) higher f CM than the reference mix. While mix containing 25% FA showed f CM similar to the reference mix at 120 days. 0.5% WSF improved f CM by about 6.5% and 10% at 28 and 120 days, respectively. While 1% WSF did not show a significant effect on f CM generally on all five types of plain mixes (1,4,7,10,13). At a high-volume fraction, the negative effect of WSF can be blamed on an increase in porosity and heterogeneity in the HSC matrix. A high volume of fibers produces small void pockets that decrease the efficiency of fibers contributing to the compression stiffness of concrete. Similar behavior was observed with WSF in another study [37]. Moreover, new steel fiber also showed mixed effects on f CM of HSC with varying doses [56,57]. High volume doses of fibers were not effective in increasing the peak-load capacity, but these were beneficial to the postpeak response. e high volume of WSF restricted the sharp failure and helped in sustaining a noticeable residual strength after peak load. e failure patterns of plain HSC and WSF-HSC are shown in Figure 6. e combined effect of WSF and FA on f CM of HSC at 28 and 120 days can be observed in Figure 7. Maximum f CM was shown by HSC containing 0.5% WSF and 10-15% FA because both 0.5% WSF and 10-15% FA were individually helpful to f CM ; therefore, their combined addition significantly improved f CM . Mix no. 5 (F10WF0.5) showed 19% and 21% greater f CM than reference mix at 28 and 120 days, respectively. It is also worth mentioning that, due to the increase in age and hardening of the binder with 10-15% FA, 1% WSF showed a similar effect on f CM compared to 0.5% WSF at 120 days. An increase in age may improve the bond of fibers with HSC's matrix. erefore, F10WF1 showed performance similar to FA10WF0.5 at 120 days.
At 120 days, maximum f CM was shown by FA15WF0.5, which was 22% higher than that of the reference mix. At 120 days, mixes incorporating 0.5-1% WSF and 10-25% FA showed noticeably higher f CM than the reference mix. ese results showed the usefulness of ecofriendly FA and WSF in improving f CM . e combined addition of FA and WSF not only improves the mechanical and ductility performance of HSC but also can substantially decrease cost and carbon footprint due to a reduction in cement quantity.

Modulus of Elasticity (ECM)
. E CM is a measure of compression stiffness of concrete significantly within the elastic limit state of the material.
e results of all HSC mixtures with different combinations of FA and WSF are shown in Figure 8. e relationship between WSF, FA, and relative E CM of HSC is shown in Figure 9. e addition of 10% FA showed a small improvement in E CM of HSC. In contrast, the 28-day E CM of HSC decreased considerably with the rising FA percentage. While, at 120 days, E CM of HSC containing 10-15% FA was comparable with that of the reference HSC. Small percentages of FA can increase the compression stiffness by decreasing the pore size. e filling effect of small FA particles can cause a small increase in the density of concrete that improves the compressive stiffness, while pozzolanic activity also contributes to strength at lower levels of FA [7,58]. In contrast, high-volume FA addition decreases the pozzolanic activity and inactive filler content of concrete increases. is slows the strength development and reduces E CM [59].
Literature has shown insignificant effects of new steel fibers on E CM [60] because fibers do not activate when the loading is well within the elastic limit of concrete. erefore, E CM of HSC entirely depends on the development level of basic ingredients of concrete. e addition of 1% WSF proved detrimental to E CM as it caused a small reduction of 2-5% in E CM because, at a high-volume fraction, the density of concrete might decrease due to the poor dispersion of fibers. 1% new steel fiber addition also showed a negative effect on E CM [60]. Very few studies investigated the axial stress-strain characteristics of WSF-HSC mixes. A study showed a small increase (1-8%) in E CM of ultra-high  Advances in Materials Science and Engineering performance concrete due to 2-3% volume of WSF [61]. In contrast, a lot of efforts are still required towards understanding the effects of WSF on E CM of concrete as the deficiency on this topic has been highlighted in a recent review study [62]. However, from this study, it can be concluded that 0.5% WSF proved to be useful to E CM , while 1% WSF slightly lowered the E CM . e maximum E CM , 4-5% higher than reference HSC, was shown by HSC containing 10% FA and 0.5% WSF at both 28 and 120 days. e negative effect of 1% WSF was more pronounced in HSCs containing a high volume of FA because increasing FA content in the binder decreased the matrix strength and eventually the bond performance of WSF. e low strength of the matrix weakened the grip over fibers; hence, it decreased the efficiency of WSF. In contrast, the negative effect of 1% WSF on E CM was more pronounced at 28 days in the HSC mix with a high volume of FA compared to the E CM of these mixtures at 120 days. As the age of HSC increased, the negative effect of 1% WSF on E CM reduced because the hardening of binder paste improves the bond performance of fibers [41].

Splitting-Tensile Strength (f CTM )
. f CTM was found out by conducting a split-tensile test on standard cylindrical samples of all mixes. e average f CTM of each HSC mix with standard deviation value is shown in Figure 10. Brittleness is a major issue with the application of HSC because it has a small f CTM value compared to the respective f CM . erefore, the fiber addition becomes a viable option to increase the ductility and fire resistance of HSC. As can be seen from the      [63]. is improvement, as observed with industrial fibers, is credited to the crack-arresting behavior of WSF [27]. FA addition showed a negative effect on f CTM of HSC. FA higher than 10% caused a drastic decline in tensile strength, whereas 10% replacement of cement with FA caused a small 2% increase in f CTM . In contrast, 15, 25, and 35% FA addition reduce f CTM of HSC by 8, 22, and 36%, respectively, because the decrease in strength development with the rising FA percentage in a binder and the filler effect of FA particles does not contribute to tensile strength.
e combined addition of 1% WSF and 10% FA showed an increase of 49% in f CTM of HSC compared to the reference HSC mix. is showed using a smaller percentage of FA as cement replacement can synergize the benefits of using FA and WSF together. ere was a certain improvement in the bond strength of WSF with the addition of 10% FA in the binder matrix. is has been observed with the combined use of FA (at a small level) and industrial steel [9] and glass fibers [58]. FA particles being spherical and smaller than cement particles can improve the particle packing in the binder matrix of HSC, in addition to the pozzolanicity potential of FA. ese positive effects of a small percentage of FA reflect the improvement of the bond performance of fibers.
WSF has shown some positive effects on high-volume FA-HSC mixes. As we know, FA is ecofriendly and the cost and carbon footprint of concrete significantly drops when it replaces cement in the binder [7], but it badly affects the tensile performance of HSC. erefore, WSF can help in controlling the drop of f CTM due to the addition of a high volume of FA. e more important observation here is that HSC made with 25% FA and 1% WSF showed higher f CTM than the reference HSC mix. e mix made with both FA and WSF was not just better in f CTM than the reference mix, but it was also ecofriendly and cheap compared to the reference mix. WSF addition was not just useful in increasing the load at which HSC failed under splitting action. It was also very beneficial in containing the crack-width after the peak load, as shown in Figure 11. After the peak load, WSF-reinforced HSC possessed high residual strength than the plain HSC mix. e net effect of WSF addition on f CTM of HSC was reduced with the rising FA level in the binder. As FA decreased the strength of HSC, the grip of the binder over WSF weakens. erefore, the net increase in f CTM due to WSF addition was significantly influenced by the binder composition.
e ratio between f CTM and f CM of each HSC mix is shown in Figure 12. is ratio can be used to assess the ductility of a particular concrete mix. A higher value of the f CTM /f CM ratio indicates high ductility, while a lower value shows low ductility. It can be observed that the use of fibers increased the f CTM /f CM of HSC. e increase in f CTM /f CM of HSC was proportional to the reinforcement index. Figure 12 also showed that f CTM of HSC increased from 6.7% to 9.3% of f CM as WSF content increased from 0 to 1%. f CTM /f CM of FA mixes went on decreasing with the rise in FA content.
is showed that the brittleness of concrete increased when a high volume of FA was used. e relationship surface between FA and WSF contents and relative or normalized f CTM is shown in Figure 13. According to this surface, maximum relative f CTM of HSC was achieved within 0-15% FA and 0.5-1% WSF contents. On the contrary, the lowest relative f CTM was shown by mixes incorporating 35% FA for a given content of WSF.

Modulus of Rupture (f CRM )
. f CRM is another indirect measure of the tensile capacity of cement-based composites. e effect of different combinations of FA and WSF on f CRM of HSC is shown in Figure 14. e variation in f CRM results with the changing FA and WSF contents was similar to that observed in the case of f CTM results. However, f CRM was highly sensitive to WSF addition compared to f CTM .
Comparing the results of f CM , E CM , f CTM , and f CRM revealed that WSF addition yielded maximum benefits to f CRM . Similar to WSF, other industrial fibers (steel, coconut, glass, polypropylene, etc.) [43,56,58,64] have also shown maximum utilization in flexural behavior.
f CRM of HSC increased by 56% and 78% due to the addition of 0.5% and 1% WSF, respectively. is showed a significant increment in the flexural toughness and ductility of HSC with WSF addition. On the contrary, a significant reduction in f CRM was noticed at 15-35% FA addition. f CRM of HSC dropped by 37% at 35% replacement of cement with FA. At high volume incorporation, FA is generally known to aggravate the mechanical properties of HSC similarly [5,13,65]. e common effect of FA addition was observed on the reduction level of all tested mechanical strength properties, that is, f CM , E CM , f CTM , and f CRM . It is because the change in microstructural growth is similarly reflected in compression and tensile properties. e combination of 10% FA and 0.5-1% WSF showed superior results among all mixes. F10WF1 and F10WF0.5 showed 83% and 65% higher rupture strength than the reference mix. e synergistic effect of FA and WSF was noticed in F10WF1 and F10WF0.5. For example, the net increase in f CRM due to 1% WSF addition in 0% FA-HSC was 70%, while 1% WSF showed an improvement of about 80% in 10% FA-HSC. e high net effect of WSF was achieved because of the synergistic behavior of 10% FA and 1% WSF. Improvement in the particle size distribution within HSC's matrix increases the pullout strength of WSF. However, the efficiency of WSF decreased significantly in HSC mixes containing a high volume of FA because the binder matrix containing high FA volume showed slow development and incomplete growth of microstructure at an early age.
It is generally known that FA incorporation shows some detrimental effects on the tensile properties of concrete. However, it is an ecofriendly and durable substitution of cement. e results of this study show that mixes with up to 25% FA can show better f CRM than reference mix if 0.5-1% WSF is used as the reinforcement. Although choosing industrial fibers, to overcome the f CRM strength loss of HSC due to FA, is an expensive and non-ecofriendly option, WSF, which is a waste product with minimum energy involved in its processing, can be used with FA to produce ductile and ecofriendly HSC. In addition to economic benefits, the HSC composite built with FA and WSF offers high flexural toughness and residual strength than reference HSC.
e f CRM /f CM ratio of each HSC mix is shown in Figure 15. It is noticed that plain HSC mixes made with or without FA showed f CRM /f CM ratio between 0.06 and 0.1. It means for HSC, f CRM is about 6-10% of the corresponding f CM . Fiber-reinforced HSC mixes yielded f CRM about 16-18% of the f CM value. High f CRM /f CM was shown by HSCs made with 0.5-1% WSF and 0-10% FA. e use of high-volume FA decreased the f CRM /f CM value significantly even for WSFreinforced mixes. F35WF0, F35F0.5, and F35F1 showed f CRM /f CM values notably lower than the plain-reference mix (F0WF0). ese findings suggested that the strength and development of the binder matrix substantially influence the efficiency of WSF. A 3-dimensional plot between relative f CRM , WSF, and FA percentage is shown in Figure 16. e highest plateau (the maximum values of relative f CRM ) on this surface belongs to 0.5-1% WSF contents and 10% FA, 16 Flexural strength, while the lowest valley on the surface (the minimum values of relative f CRM ) is related to HSC mixes with 25-35% FA and 0-0.5% WSF contents.

Rapid Chloride Permeability (RCP). RCP test is used
for rapid assessment of chloride-ion permeability resistance of cementitious concretes. e representative RCP value of a mix largely depends on the microstructural density and development. Moreover, it is also affected by the ability of concrete to conduct electrical charges. e effect of WSF and FA was determined on RCP resistance of HSC at 28 and 120 days. e results are presented in Figure 17.
e RCP values of all HSC mixes are lower than 2000 coulombs, which means RCP of all mixes comes under the category of "low" chloride-ion permeability, according to Table 4. Moreover, plain mixes made with and without FA showed RCP values significantly lower than those noticed for WSF-reinforced mixes. RCP values of plain HSC mixes are lower than 1000, which indicates that plain mixes have "very low" chloride-ion permeability.
Very low RCP values of plain mixes are mainly because of a small water-binder ratio (i.e., 0.3). RCP of concrete further decreased with the rise in FA content; see Figure 18. e use of 10, 15, 25, and 35 FA decreased the RCP by 15%, 34%, 42%, and 40%, respectively. e decrease in RCP with FA inclusion in the binder is mainly credited to the filling effect of FA particles. Moreover, an increase in the tricalcium aluminate content of concrete increases the chloride binding capacity of concrete [10]. e inverse relationship between FA and RCP has also been observed in past research [67]. A decrease in the electrical conductivity of concrete has also been reported with the addition of slag in the binder of HSC [43]. e high electrical conductivity of WSF increased the RCP of HSC. About 50% and 100% increase of RCP was observed at the addition of 0.5 and 1% volume fraction of WSF in HSC. is immense increase in RCP indicates that WSF-HSC is more vulnerable to corrosion attacks. Not only are the main steel bars vulnerable to corrosion attack but also the filaments of WSF can degrade over time in HSC. Degradation of both WSF and main reinforcement will eventually lead to a decrease in the capacity of structural elements. e small increase in the permeability of HSC due to fiber addition may also contribute to an increase in RCP since permeability also favors the rapid penetration of chloride ions. A significant increase in the electrical conductivity of HSC has been observed due to the incorporation of industrial steel fiber [57]. e directly-proportional relationship between RCP and WSF contents is shown in Figure 19. e use of FA significantly decreased the RCP of WSFreinforced HSC. is means that FA can be used to compensate for the loss in chloride durability of HSC due to WSF addition. To obtain RCP values of WSF-reinforced HSC in the range of the "very low" category, the use of 10-15% FA is necessary. e use of the high volume of FA showed high RCP resistance, but it reduced the strength, which would probably be the main drawback considering the application of high-volume FA in HSC. However, it is not possible to achieve an RCP value similar to the reference mix when using 1% WSF without considering the use of the high volume of FA.

Chloride Penetration Depth (CPD).
e CPD is a reliable and more realistic measurement of chloride penetrability of cementitious material than RCP, which is not affected by the electrical conductivity of the material. It largely depends on the porosity, saturation of pore-solution, and microstructural growth of concrete. e CPD of each HSC mix after immersion in chloride solution for 28 and 120 days is shown in Figure 20.
e incorporation of FA leads to a drop in CPD, indicating that chloride durability of concrete increases with FA addition. As already explained in Section 3.3.1, FA improves the distribution of particle sizes in the matrix of HSC. e decrease in pore size slows the movement of the chloridebearing medium into the concrete. Moreover, alumina silicate particles of FA have chloride binding capacity. e minimum CPD was shown by HSC containing 25% FA. e relationship between FA (%) and CPD of HSC is shown in Figure 21. e CPD value of HSC kept decreasing until 25% FA, while a small increase in CPD was observed at 35% FA. is can be explained by a decline in the microstructural growth of the binder matrix at a high volume of FA addition. However, still, all FA mixes showed notably lower CPD values than the reference mix.
Contrary to RCP test results, CPD test results show a small effect of WSF on the chloride permeability of concrete; see Figure 22 because the high conductivity of WSF-reinforced concrete does not affect the penetration of chloride ions in natural conditions. CPD was increased only by 2-7% at 0.5-1% addition of WSF. e small increase in porosity due to WSF addition is responsible for a slight rise in the CPD. A study has shown that chloride diffusion of concrete is not significantly affected by the electrical conductivity of the material [68]. erefore, steel fiber addition did not increase the chloride diffusion in the concrete. HSC mixes made with the combined FA and WSF addition show CPD values significantly lower than the reference mix. A small increase in CPD caused by WSF is substantially suppressed by FA addition.

Relationship between CPD and RCP.
e relationship between CPD and RCP values of all HSC mixes is shown in Figure 23. Since RCP is significantly influenced by electrical conductivity, it does not find a strong correlation with CPD, as CPD is not affected by the conductivity of the material. For plain mixes (made with or without FA), it is possible to find a strong correlation between CPD and RCP values, but for WSF-reinforced mixes, it is difficult to correlate these two parameters without considering the effect of fiber. Figure 24 shows the relationship between CPD, RCP, and WSF content of HSC at each level of FA. e relationship is derived in the format of equation (1), where CPD is calculated as a function of RCP and volume of WSF (V WSF ). A and B are constants of the linear equation. RCP is taken as an independent variable because it is determined quickly compared to CPD. erefore, CPD can be estimated from  RCP for WSF-reinforced concretes. As shown in Figure 24, CPD and RCP can be correlated strongly if V WSF is considered. ese relationships have important implications for applications of steel fiber-reinforced concretes, as CPD values, which are found after a long time, can be accurately estimated from RCP as a function of V WSF :   the concrete, the total alumina-silica content of the binder increased. e rise in FA causes a reduction in the calcium oxide content of the binder. e reduction in calcium hydrates and unhydrates makes the HSC resistant to acid attack. Similar observations were observed due to the addition of rice husk ash into the binder [68]. e increase in silica content and decline in lime content is the major reason responsible for the high durability of HSC in the acid attack. Another reason is that the low permeability of FA mixtures slows the movement of acidic solution into the matrix of the concrete.
e relationship between FA, WSF contents, and relative loss in mass (LIM) under acid attack are shown in Figure 26.
is relationship shows that high AAR or low LIM under acid attack is achieved with high-level incorporation of WSF and FA. WSF increases AAR by its capability to hold crack propagation. Sulfate attack is accompanied by the formation of expansive salts and water. It exerts the pressure inside the binder matrix to disintegrate the microstructure of concrete and finally deteriorate the mechanical strength of HSC. WSF controls such disintegration and degradation of mechanical strength; as a result, low LIM was observed in mixes with fibers.   Improvement in the AAR of concretes with industrial steel and glass fibers has been reported in previous studies due to their crack-bridging effects [9,58,69]. Mixes made with 10-35% FA and 0.5-1% WSF showed almost half disintegration under acid attack compared to the reference mixture. WSF also improved the residual strength of HSC exposed to an acidic solution.

Conclusions
In the present study, the combined effect of fly ash (FA) and waste tire steel fiber (WSF) was investigated on the durability and mechanical performance of high-strength concrete (HSC). e main theme of this research is the advancement in knowledge about ecofriendly and ductile cementitious composites. e following are the key findings of the present research: (1) HSC made with low-to-medium levels of FA (10-15%) and WSF (0.5%) showed optimum f CM . (4) e CPD results are not affected by the electrical conductivity. erefore, WSF addition did not show a drastic increase in CPD. All WSF-reinforced mixes incorporating FA showed lower CPD than the reference HSC mix. HSC made with 25% FA and 0.5% WSF showed 50% lower CPD than the reference mix. A strong mathematical correlation between CPD and RCP can be derived if the volume of WSF is considered.
(5) Both FA and WSF were extremely useful in AAR. Increased integrity of concrete due to the fibers and decline in calcium content of the binder is accountable for excellent acid attack durability of HSC incorporating FA and WSF.

Data Availability
Data will be provided upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.

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Advances in Materials Science and Engineering