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

Abstract

The demands for high-strength concrete (HSC) have been increasing rapidly in the construction industry due to the requirements of thin and durable structural elements. HSC is highly brittle. Therefore, to augment its ductility behavior, expensive fibers are used. These negative drawbacks of HSC can be controlled by incorporating waste materials into its manufacturing instead of conventional ones. Therefore, this study assessed the performance of HSC produced with different quantities of waste tire steel fiber (WSF) and fly ash (FA). WSF was used at two doses, namely, 0.5% and 1%, by volume in HSC, with low-to-medium volumes of FA, that is, 10%–35%. The studied durability parameters included rapid chloride permeability (RCP) and chloride penetration depth (CPD) by immersion method (28 and 120 days) and acid attack resistance (AAR) (28 and 120 days). Various basic mechanical properties of HSC were also analyzed, such as compressive strength (f_CM), modulus of elasticity (E_CM), splitting-tensile strength (f_CTM), and modulus of rupture (f_CRM). The results revealed that the damaging effect of WSF on the RCP resistance of HSC is probably due to the high conductivity of steel fibers. However, test results of CPD showed that WSF produced insignificant changes in chloride permeability of HSC. Furthermore, when made with FA, WSF-reinforced HSC yielded very low chloride permeability. Both WSF and FA contributed to the improvement in the AAR of HSC. WSF was highly useful to tensile properties while it showed minor effects on compressive properties (f_CM and E_CM). Optimum ductility and durability can be achieved with HSC incorporating 1% WSF and 10%–15% FA.

1. Introduction

The 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. The impacts of cement also contribute to those of the concrete. The 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 CO₂ footprint, their cost and CO₂ footprint largely depend on their transportation distances between the quarry and concrete batching plant. Therefore, about 85% of emissions of cement concrete are dependent on the binder constituent of concrete [3, 4]. The most effective way to control the environmental impact of concrete is to minimize its cement consumption. This can be achieved by the utilization of industrial waste powders that possess pozzolanicity and hydraulicity. The substitution of a small and medium volume of cement with these waste powders can drastically reduce the CO₂ 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. The cost to strength [5, 6] or CO₂ 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 CO₂ 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. Therefore, 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]. The 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 [9–11]. 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]. The degree of the effectiveness of FA depends on its fineness, chemical composition, and unburnt carbon content [14–17]. 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. The increase in the strength class of concrete reduces the ratio between f_CTM and f_CM [19]. This 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].

The 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 [21–23]. The use of fibers substantially enhances the tensile and fracture toughness of HSC. The 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 CO₂ footprint of fibers [24]. Their small doses can noticeably increase the cost and CO₂ footprint of concrete. Fiber addition also requires technical supervision; it creates workability issues when used in HSC. Therefore, the use of additional measures and materials to control the quality of concrete can increase the final cost. Therefore, the selection of fiber type should be made based on a comprehensive cost to benefit ratio analysis.

The development of ductile, cheap, and environment-friendly 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]. The effect of WSF on the properties of concrete significantly depends on the length, shape, texture, and residual rubber content of fiber [28]. The incorporation of 0.46% volume of irregular-shaped 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, 34–37].

The incorporation of FA and WSF into HSC can integrate the benefits of ductility, durability, and ecofriendliness. The 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 [42–45]. Due to these benefits, fibers and mineral admixture addition show some synergistic results on the performance of concrete [9]. There 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.) [46–48]. The combined behavior of WSF and mineral admixture was studied by Mastali and Dalvand [27]. They 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. Therefore, 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. The 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. The findings of this research fill an important research gap related to the durability of waste tire steel fiber-reinforced concrete. Moreover, the combined effect of FA and WSF on both durability and mechanical properties of HSC has never been studied before.

2. Materials and Methods

2.1. Constituent Materials

2.1.1. Binding Materials

HSC mixes were prepared with 53-grade 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. The generation of FA was estimated to be 10% of the annual cement production of Pakistan. The 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.

Table 1 

Engineering characteristics of binders used in this study.

2.1.2. Aggregates

Quarry sand of “Lawrencepur” was used as fine aggregate to manufacture HSC. This sand is recommended for good-quality concrete production in Punjab, Pakistan. This 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. This 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. The diversity in particle sizes of both “quarry sand” and “crushed coarse aggregate” is shown in Figure 1. The maximum aggregate sizes for “fine” and “coarse” aggregates are 4.75 and 12.5 mm, respectively.

Table 2 

Engineering characteristics of aggregates used in this research.

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(b)

Figure 1 

The aggregate size gradation curves of (a) quarry sand and (b) coarse aggregate following ASTM C33 [50].

2.1.3. WSF

WSF used in this research was derived from old waste tires of truck vehicles. The tire-bead wires, when removed from waste tires, contained residual rubber. Therefore, 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. The 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. These small wires may provide hybrid (mixed-length and mixed-diameter effects) of fibers on the properties of HSC.

Figure 2 

An overview of a random sample of WSF.

2.1.4. Water-Reducing Agent and Water

The desired workability of an HSC mixture was achieved by using a commercial third-generation water-reducing agent “ViscoCrete 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.

2.2. 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. This 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. Then, with each incorporation level of FA, three different doses of WSF, 0%, 0.5%, and 1%, by a volumetric fraction of concrete were used. Therefore, 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. The workability was because the use of WSF increased the stiffness of fresh concrete. Therefore, 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) 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.

Table 3 

Information about the composition of concrete mixtures.

2.3. 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.

2.4. Sample Details and Testing Methods

Six cubes of 100 mm dimensions were prepared for each mixture. The f_CM was determined at 28 and 120 days; three cubes of each mix were tested at one age. The standard of testing was followed as per ASTM C39 [51]. The 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). Three cylinders per mix were cast, of which were tested at 28 days and the remaining three 120 days. The standard of testing was adopted from ASTM C469 [52]. This 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. Three replica samples of each mix were tested at 28 days. The standard of testing was adopted from ASTM C496 [53]. The 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 third-point loading method adopted from ASTM C1609 [54]. The test setup is shown in Figure 3(d).

(a)

(b)

(c)

(d)

Figure 3 

Overview of mechanical testing. (a) Compression test on 100 mm cubes. (b) Overview of axial testing for ECM determination. (c) Splitting-tensile test on “100 mm × 200 mm” cylindrical samples. (d) Overview of flexural testing on “100 mm × 100 mm × 350 mm” prismatic samples.

The 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. The 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. The overview of RCP testing is shown in Figure 4.

Figure 4 

Overview of RCP testing.

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 fiber-reinforced 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 air-dried for 3 days at room temperature. These 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 (AgNO₃) salt, on the split surfaces of NaCl conditioned samples after periods of 28 and 120 days. Three 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. These specimens were then air-dried at room temperature for 3 days. Then, the samples of all mixes were exposed to sulfuric acidic (H₂SO₄) solution of 5% concentration. The change in the mass of specimens was measured after exposure periods of 14, 28, 56, and 120 days.

3. Results and Discussion

3.1. Compressive Properties

3.1.1. Compressive Strength (f_CM)

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. The 28 days f_CM of HSC was increased by 6% at 10% FA addition. This 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. This 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, 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.

Figure 5 

Effect of FA and WSF combinations on compression testing results of concrete.

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. The high volume of WSF restricted the sharp failure and helped in sustaining a noticeable residual strength after peak load. The failure patterns of plain HSC and WSF-HSC are shown in Figure 6.

(a)

(b)

Figure 6 

Compression failure. (a) Cube with 0% WSF. (b) Cube with 1% WSF.

The 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. Therefore, F10WF1 showed performance similar to FA10WF0.5 at 120 days.

(a)

(b)

Figure 7 

Relationship between FA, WSF, and relative f_CM values of HSC: (a) 28 days and (b) 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. These results showed the usefulness of ecofriendly FA and WSF in improving f_CM. The 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.

3.1.2. Modulus of Elasticity (ECM)

E_CM is a measure of compression stiffness of concrete significantly within the elastic limit state of the material. The results of all HSC mixtures with different combinations of FA and WSF are shown in Figure 8. The relationship between WSF, FA, and relative E_CM of HSC is shown in Figure 9.

Figure 8 

Effect of different combinations of WSF and FA on the (E)_CM of HSC.

(a)

(b)

Figure 9 

Relationship between FA, WSF, and relative (E)_CM values of HSC: (a) 28 days and (b) 120 days.

The 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. The 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. This 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. Therefore, E_CM of HSC entirely depends on the development level of basic ingredients of concrete. The 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 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.

The 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. The 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. The 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].

3.2. Tensile Properties

3.2.1. Splitting-Tensile Strength (f_CTM)

f_CTM was found out by conducting a split-tensile test on standard cylindrical samples of all mixes. The 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. Therefore, the fiber addition becomes a viable option to increase the ductility and fire resistance of HSC. As can be seen from the results, WSF proved very useful in advancing the tensile strength of HSC. An overall improvement of 32% and 42% was noticed in f_CTM of HSC (containing 0% FA) at 0.5% and 1% WSF addition. Studies have shown positive effects of WSF on f_CTM [63]. This 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.

Figure 10 

The f_CTM value of HSC with different incorporation levels of FA and WSF.

The combined addition of 1% WSF and 10% FA showed an increase of 49% in f_CTM of HSC compared to the reference HSC mix. This showed using a smaller percentage of FA as cement replacement can synergize the benefits of using FA and WSF together. There was a certain improvement in the bond strength of WSF with the addition of 10% FA in the binder matrix. This 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. These 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. Therefore, WSF can help in controlling the drop of f_CTM due to the addition of a high volume of FA. The more important observation here is that HSC made with 25% FA and 1% WSF showed higher f_CTM than the reference HSC mix. The 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.

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(b)

(c)

Figure 11 

Splitting-tensile failure modes of HSC with (a) 0% WSF, (b) 0.5% WSF, and (c) 1% WSF.

The 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. Therefore, the net increase in f_CTM due to WSF addition was significantly influenced by the binder composition.

The ratio between f_CTM and f_CM of each HSC mix is shown in Figure 12. This 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. The 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. This showed that the brittleness of concrete increased when a high volume of FA was used. The 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.

Figure 12 

The ratio between f_CTM and f_CM of all HSC mixes.

Figure 13 

Relationship between WSF and FA contents and relative f_CTM.

3.2.2. Modulus of Rupture (f_CRM)

f_CRM is another indirect measure of the tensile capacity of cement-based composites. The effect of different combinations of FA and WSF on f_CRM of HSC is shown in Figure 14. The 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.

Figure 14 

The f_CRM of each HSC mix with different incorporation levels of FA and WSF.

f_CRM of HSC increased by 56% and 78% due to the addition of 0.5% and 1% WSF, respectively. This 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]. The 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.

The 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. The 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. The 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. The 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.

The 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. The use of high-volume FA decreased the f_CRM/f_CM value significantly even for WSF-reinforced mixes. F35WF0, F35F0.5, and F35F1 showed f_CRM/f_CM values notably lower than the plain-reference mix (F0WF0). These 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. The highest plateau (the maximum values of relative f_CRM) on this surface belongs to 0.5–1% WSF contents and 10% FA, 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.

Figure 15 

The ratio between f_CRM and f_CM of all HSC mixes.

Figure 16 

Relationship between WSF and FA contents and relative f_CRM.

3.3. Chloride Permeability Test Results

3.3.1. Rapid Chloride Permeability (RCP)

RCP test is used for rapid assessment of chloride-ion permeability resistance of cementitious concretes. The 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. The effect of WSF and FA was determined on RCP resistance of HSC at 28 and 120 days. The results are presented in Figure 17.

Figure 17 

RCP of each HSC mix with different incorporation levels of FA and WSF.

The 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.

Table 4 

Electrical indication of chloride durability [66].

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. The use of 10, 15, 25, and 35 FA decreased the RCP by 15%, 34%, 42%, and 40%, respectively. The 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]. The 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].

Figure 18 

Relationship between FA content in binder and corresponding RCP values of HSC.

The 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. This 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. The 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]. The directly-proportional relationship between RCP and WSF contents is shown in Figure 19.

Figure 19 

Relationship between WSF content and corresponding RCP values of HSC.

The use of FA significantly decreased the RCP of WSF-reinforced HSC. This 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. The 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.

3.3.2. Chloride Penetration Depth (CPD)

The 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. The CPD of each HSC mix after immersion in chloride solution for 28 and 120 days is shown in Figure 20.

Figure 20 

CPD of each HSC mix with different incorporation levels of FA and WSF.

The 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. The decrease in pore size slows the movement of the chloride-bearing medium into the concrete. Moreover, alumina silicate particles of FA have chloride binding capacity. The minimum CPD was shown by HSC containing 25% FA. The relationship between FA (%) and CPD of HSC is shown in Figure 21. The CPD value of HSC kept decreasing until 25% FA, while a small increase in CPD was observed at 35% FA. This 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.

Figure 21 

Relationship between FA content in the binder and corresponding CPD values of HSC.

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. The 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]. Therefore, 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.

Figure 22 

Relationship between WSF content and corresponding CPD values of HSC.

3.3.3. Relationship between CPD and RCP

The 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 23 

Relationship between CPD and RCP.

Figure 24 shows the relationship between CPD, RCP, and WSF content of HSC at each level of FA. The 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. Therefore, 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. These 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:

Figure 24 

Relationship between CPD, RCP, and WSF content of WSF for each level of FA.

3.4. Sulfuric Acid Attack Resistance (AAR)

Cementitious concretes often experience acidic environments in their application. In most cases, acidic solution dissolves both hydrated and unhydrated cementitious compounds, leading to the deterioration of the mechanical performance of concrete. If calcareous aggregates are used, they are also dissolved by acidic action. Therefore, the measurement of AAR becomes a related durability parameter. In this study, the durability of HSC mixes was measured in an artificial acidic medium created by a 5% solution of sulfuric acid in tap water. The results of all HSC mixtures are shown in Figure 25.

Figure 25 

Loss in mass of each HSC mix with different incorporation levels of FA and WSF as a result of acid attack.

The plain HSC or reference mix showed the maximum loss in mass compared to all other HSC mixtures at both ages of testing. The AAR of HSC substantially increased due to the addition of both FA and WSF. As FA content increased in the concrete, the total alumina-silica content of the binder increased. The rise in FA causes a reduction in the calcium oxide content of the binder. The 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]. The 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.

The relationship between FA, WSF contents, and relative loss in mass (LIM) under acid attack are shown in Figure 26. This 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.

(a)

(b)

Figure 26 

Relationship between FA, WSF, and relative loss in mass-LIM values of HSC: (a) 28 days and (b) 120 days.

4. 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). The main theme of this research is the advancement in knowledge about ecofriendly and ductile cementitious composites. The 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. High-level incorporation of FA lessened the efficacy of WSF on f_CM. For all incorporation levels of FA, the optimum dosage of WSF is 1%. E_CM of HSC did not change considerably due to WSF. Lessening in E_CM was observed with the rising FA and WSF contents in HSC. HSC made with 0.5% WSF and 10% FA showed higher E_CM than the reference mix.(2)f_CTM of HSC was considerably sensitive to WSF addition. The HSC made with 10% FA and 1% WSF showed maximum f_CTM, about 50% higher than the reference HSC mix. The splitting-tensile efficiency of WSF decreased with the rising FA percentage in the binder. Similar to f_CTM, f_CRM was also sensitive to WSF addition. Maximum f_CRM was shown by HSC incorporating 10% FA and 1% WSF, which was 83% higher than the reference mix.(3)The RCP of HSC was increased drastically with WSF addition. 1% WSF addition caused about a 100% increase in the RCP of HSC mainly because of the high electrical conductivity. FA caused a decrease in RCP with each rising level. FA minimized the negative effect of WSF on RCP. However, RCP should not be taken as a good measure of chloride permeability capacity of steel fiber-reinforced concretes. On the contrary, CPD test results gave realistic values of chloride permeability capability of WSF-reinforced concretes.(4)The CPD results are not affected by the electrical conductivity. Therefore, 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

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors would like to extend their appreciation to the Deanship of Scientific Research at King Khalid University, Saudi Arabia, for funding this work through the Research Group Program under Grant no. RGP. 1/14/42.