**Babar Ali 1, Rawaz Kurda 2,3,4,\*, Bengin Herki 5,6, Rayed Alyousef 7, Rasheed Mustafa 8, Ahmed Mohammed 9, Ali Raza 10, Hawreen Ahmed 2,3,4 and Muhammad Fayyaz Ul-Haq <sup>1</sup>**


Received: 26 November 2020; Accepted: 14 December 2020; Published: 16 December 2020

**Abstract:** For the efficient and durable design of concrete, the role of fiber-reinforcements with mineral admixtures needs to be properly investigated considering various factors such as contents of fibers and potential supplementary cementitious material. Interactive effects of fibers and mineral admixtures are also needed to be appropriately studied. In this paper, properties of concrete were investigated with individual and combined incorporation of steel fiber (SF) and micro-silica (MS). SF was used at six different levels i.e., low fiber volume (0.05% and 0.1%), medium fiber volume (0.25% and 0.5%) and high fiber volume (1% and 2%). Each volume fraction of SF was investigated with 0%, 5% and 10% MS as by volume of binder. All concrete mixtures were assessed based on the results of important mechanical and permeability tests. The results revealed that varying fiber dosage showed mixed effects on the compressive (compressive strength and elastic modulus) and permeability (water absorption and chloride ion penetration) properties of concrete. Generally, low to medium volume fractions of fibers were useful in advancing the compressive strength and elastic modulus of concrete, whereas high fiber fractions showed detrimental effects on compressive strength and permeability resistance. The addition of MS with SF is not only beneficial to boost the strength properties, but it also improves the interaction between fibers and binder matrix. MS minimizes the negative effects of high fiber doses on the properties of concrete.

**Keywords:** mechanical properties; fiber-reinforced concrete; permeability; durability; tensile strength; micro-silica/silica fume; steel fiber

#### **1. Introduction**

Plain cement concrete (PCC) is the most versatile construction material owing to its multiple benefits i.e., high compressive strength, cost-efficient, in-situ formability, thermal and electrical insulation, imperviousness, etc. Ingredients and mix design of PCC can be changed to obtain different types of concrete that suit different structural loadings and environments. To further raise the importance of conventional concrete, several performance-related issues need to be resolved. PCC generally performs weakly under tensile loadings. Its splitting tensile strength (STS) is very low compared to its compressive strength (CS). According to Ali and Qureshi [1,2] and Koushkbaghi et al. [3] PCC has a STS/CS ratio of about 7–9.5%. Zain et al. [4] showed that the STS/CS ratio decreases as the strength class of concrete is upgraded, therefore, high strength PCCs are more vulnerable to brittle failure than normal strength PCCs. PCC has low energy absorption capacity (or toughness) under both tensile and compressive loadings [5,6]. It undergoes a sudden failure after carrying the load beyond its peak capacity and it has very low residual strength (almost negligible compared to fiber-reinforced concrete) [7]. Mechanical performance of PCC undergoes significant degradation with time when subjected to environmental stresses (freeze-thaw) [8] and weathering actions of water and acid attack [9]. Due to the inherent weakness of PCC under tensile loadings, large structural dimensions cannot be avoided unless it is reinforced with some high strength material i.e., steel rebars, glass fiber-reinforced polymers (GFRP) bars, carbon fiber-reinforced polymer (CFRP) bars, etc.

Nowadays, fibers are being used as a discrete 3-dimensional reinforcement to overcome the deficiency of PCC in tensile strength. With the addition of proper fiber-type in concrete, initiation and proliferation cracks under both tensile and compressive loads can be controlled or delayed. Many types of reinforcements are available commercially that own their application-specific characteristics e.g., carbon fiber, steel fiber (SF), glass fiber, polypropylene fiber [10] organic fibers [11], carbon nano-tubes [12,13] etc. Tensile reinforcements disperse throughout the PCC matrix, bridge the microcracking [12,13]. Considering the mechanical performance of concrete, SF is by far more superior fiber compared to other industrial fibers [10]. SF has a very high tensile strength of over 1200 MPa and an elastic modulus of about 200 GPa. The literature confirms its vitality as a superior reinforcement material that ensures satisfying tensile, compressive, flexural and shear strength properties [14–17]. By improving the strength per unit quantity of material, SF-reinforced concrete (SFRC) shows lower economic and environmental impact compared to PCC [14].

There are several issues relevant to the underutilization of SF in fiber-reinforced concrete that must be addressed. According to Lee et al. [18], the primary reason for the failure of SF-Reinforced Concrete (SFRC) is the failure linked to the interface between fiber and binder matrix of the concrete. Two different types of failures are linked to SFRCs when the underutilized fiber is separated from the binder matrix [19]. The first type of failure occurs at the interface between the fiber and binder matrix, whereas another type of failure occurs in the adhering binder matrix. Both of these failures lead to under-utilization of matrix and full tensile strength potential of fibers, and consequently leading to cracking of concrete. Therefore, bond strength between fibers and binder matrix plays a significant role in defining the tensile capacity of fibrous composites.

The effect of SF on the properties of the fibrous composite also depends on the dosage of fiber. There is a consensus among researchers that the tensile and bending capacity of concrete improves the increasing fiber dosage (0 to 2% by volume fraction) [3,9,20–23], but the literature has shown the mixed effects of varying SF dosage on the compressive behavior of concrete. Some studies [23,24] report that SF induces porosity into concrete therefore, compressive strength and elastic modulus of concrete undergoes degradation with the rising fiber volume. Whereas, there are studies [3,9,21] which have shown positive effects of SF on compressive behavior of concrete. The improvements in compressive strength were attributed to the increased stiffness and confinement of concrete [3,9,21]. However, a consensus is found among findings of the researchers that SF is beneficial to compression toughness of concrete [5,23,25,26].

There are some issues related to application of SFRC that are vital to be understood and resolved. The reasons behind the mixed effects (positive, negative or inconsiderable) of SF dose on compressive strength of concrete are still needed to be understood for the effective use of fibers under compression loadings. Moreover, poor bond strength between fiber and binder matrix reduces the utilization of full potential of SF. It is essential to investigate the influence of such materials (i.e., MS) on SFRC that help in strengthening binder matrix and improve the dispersion of fibers [2,10]. This study is designed to evaluate the effects of varying SF dosage on the properties of a high strength concrete. To investigate the role of bond strength at interfacial zone between fiber and matrix, the effect of SF dosage was also explored with and without micro-silica (MS). SF was used at six different levels i.e., low fiber volume (Vf = 0.05% and 0.1%), medium fiber volume (Vf = 0.25% and 0.5%) and high fiber volume (Vf = 1% and 2%). Each dose of SF was investigated with 0%, 5% and 10% MS as by volume of binder. MS is an excellent consumer of portlandite CH (in pozzolanic reaction, that strengthens the binder matrix and improves the bond strength of fibers [27]. Concrete mixtures were assessed based on the results of basic mechanical and permeability tests. The results of this study provide a useful information on the selection of SF dose for the optimum mechanical and durability performance. Moreover, experimental results favor the use of MS to maximize the utilization of SF under compressive and tensile forces.

### **2. Materials and Methods**

#### *2.1. Materials*

Conventional and supplementary materials used for the production of concrete mixtures are explained in this section. Type I general purpose cement (Bestway 53 Grade, Haripur, Pakistan) was used as the main binder as per specifications of ASTM C150 [28]. Micro-silica (also known as silica fume obtained from Jaza Minerals, Karachi, Pakistan) containing 90–94% silica was used as a partial cement substitute. Properties of cement and micro-silica can be assessed from the study of Ali et al. [2]. Siliceous sand from the Lawrancepur quarry (Attock, Pakistan) was used as fine aggregate. Dolomite sandstone of Kirana-hills of Sargodha was used as the coarse aggregate. The maximum aggregate size of coarse and fine aggregate is 12.5 mm and 4.75 mm. The distribution of different particle sizes in both coarse and fine aggregates is shown in Figure 1. Characteristics of aggregates are given in Table 1.

**Figure 1.** Gradation of (**a**) Siliceous sand/fine aggregate and (**b**) crushed dolomite limestone/coarse aggregate.


**Table 1.** Characteristics of aggregates.

Hooked steel fiber (SF) was studied as a discrete reinforcement in concrete. It has tensile strength of 750 MPa and a density of 7750 kg/m3. It was sourced from Dramix© (Zwevegem, Belgium). SF has a length of 30 mm and a diameter of 0.38 mm. Its overview is shown in Figure 2. To control the loss of workability with a rising dose of SF, Viscorete 3110 (Sika Chemicals, Lahore, Pakistan) was used as an ultra-high range plasticizer. Tap water free from organic/inorganic impurities was used during the mixing and for curing as well.

**Figure 2.** Overview of SF.

#### *2.2. Design of Concrete Mixtures*

A total of 21 concrete mixes were studied in this research. Six different doses (Vf = 0.05, 0.1, 0.25, 0.5, 1 and 2%) of SF were used to produce fiber-reinforced concretes. SF volumes were selected to observe the effects of a wide range of fiber doses on the properties of concrete. Fibers were used as by volume fraction of concrete (i.e., 1% Vf of SF = 78 kg of SF). Each fiber dosage is investigated with 0%, 5% and 10% micro-silica (MS). MS serves as the matrix-strengthening agent by producing calcium silicate hydrate gels from the free portlandite-CH a by-product from the hydration of cement. Plain concretes were also produced with and without MS and served as reference mixes. Composition and mix proportioning of all concrete mixtures are provided in Table 2.

Mixing of all concretes was done in a mechanical mixer having adjustable rotation speed. In the first stage, aggregates and binder were dry mixed for 3 min at speed of 40 rpm. In the second stage, half of the mixer and water reducer were added to the mix and blending continued for 3 min at speed of 40 rpm. In the third stage, the remaining water was added to the mix, and mixing was done a high speed of 80 rpm for 2 min. In the last stage, SF was added to the concrete mix, and blending continued for the next 4 min at 80 rpm. After mixing, a slump test was performed to check the desired workability of mixes (i.e., slump between 80 and 110 mm). Mixer continued to run at a slower speed of 20 rpm until the casting of specimens was completed.


**Table 2.** Design of concrete mixes.

MS: Micro-Silica; SF: Steel Fiber; HWR: High-range Water Reducer.

#### *2.3. Sample Preparation and Testing Techniques*

All specimens were cast in standard steel molds and protected with a waterproof membrane for 24 h setting immediately after casting. After setting, specimens were cured in tap water for 28-days at room temperature conditions. All mixes were tested for three important mechanical parameters i.e., compressive strength (CS), modulus of elasticity (MOE), and splitting tensile strength (STS). For CS, 100 ϕ mm × 200 mm cylindrical specimens were tested as per ASTM C39 [29]. To determine CS, specimens were tested under compressive-hydraulic press at the rate of 0.3 MPa/s. The static MOE of each mix was determined according to ASTM C469 [30]. MOE test was conducted on the specimens of 150 ϕ mm × 300 mm at the stress-rate of 0.15 MPa/s. The strain data (deformation characteristics) was recorded using compressometer-extensometer. To evaluate STS, 100 ϕ mm × 200 mm specimens were tested following ASTM C496 [31]. The splitting-load was applied at the stress rate of 0.015 MPa/s on the specimen to determine STS. All mechanical tests were performed in a controls compression testing machine with a loading capacity of 3000 kN. To understand the effects of varying SF and MS contents on the durability of concrete, two permeability-related durability indicators were evaluated i.e., water absorption and chloride ion penetration. To test for water absorption (WA) capacity, 100 ϕ mm × 50 mm concrete disc specimen of each mix was tested following ASTM C642 [32]. To determine chloride ion penetration (CIP) resistance of each mix, an immersion technique was adopted as explained by the authors [2]. For the CIP test, a 100 mm × 100 mm cylindrical specimen was first cured in normal water for 28 days. Then the specimen was immersed in a 10% NaCl solution for 56 days. After conditioning in chloride solution, the specimen was split, and the failed surface of the specimen was sprayed with 0.1 N AgNO3 solution to observe the depth of CIP. The further detailed procedure for CIP testing can be assessed from studies [2,33]. All the results presented in this research are the mean values of the three results of each concrete mixture. The schedule of casting and testing is shown in Table 3.


**Table 3.** Overview of mechanical and permeability testing methods and schedule.

#### **3. Results and Discussion**

#### *3.1. Compressive Strength (CS)*

Figure 3 shows the effect of varying SF dose on the CS of concrete. Figure 3b shows the net age change in CS with the varying SF dose. These results show a mixed effect of SF on CS at different doses. CS goes on increasing when the SF dose changed from 0 to 0.25%. Further increasing SF beyond 0.25%, CS starts reducing, and at 2% SF, CS of fibrous concrete is lesser than that of the plain concrete. Three different causes contribute to CS property due to the inclusion of fibers. The first cause is related to the confinement effect of fibers that increases the stiffness of concrete and it is known to positively affect the CS [10,34,35]. The second phenomenon is related to the entrainment of additional ITZs in concrete that has a detrimental effect on the CS. The introduction of a high number of ITZs contributes to porosity and permeable channels into the concrete and ITZs act as a weak link in the fibrous composite. The third phenomenon pertains to the resistance of cracking to the propagation of micro and macro-cracks; thus, it is known to improve the compressive stiffness of concrete. The first and third phenomenon prevails at 0.1–0.25% dose of SF, therefore, CS shows improvement due to fiber addition, whereas, at high fiber doses, a high number of ITZs introduction facilitate crack propagation and it adds to the total porosity of concrete.

**Figure 3.** Compressive strength (CS) results (**a**) Variation of CS with SF dosage (**b**) Net change in CS with varying SF dosage.

Figure 4 shows the effect of MS on CS results of high strength concrete at different doses of SF. Figure 4b shows the effect of varying SF dose on CS at the levels of 0%, 5% and 10% MS. MS shows a positive effect on compressive strength concrete due to its ability to produce calcium silicate hydrate gels in pozzolanic reactive with free portlandite. The strengthening of the binder leads to improvement in the bond strength of fibers and matrix, that is why a clear difference (Figure 4b) between "net change" of SF mixes with and without MS. For example, at 0.25% SF the net changes in CS at 0%,

5% and 10% MS are 5.4%, 7.03% and 10.37%, respectively. MS also minimizes the negative effect of high fiber volumes (i.e., 2% SF) on CS. This can be credited to the strengthening of the bond at ITZ, which enhances the utilization of fibers in compression. It is confirmed from the results that the combined incorporation of 10%MS and 0.5%SF can increase the CS by more than 20%. It is verified by the literature that MS addition does not only contribute to the bond strength of fibers but it also improves the dispersion of fibers [27,36,37]. Therefore, it can be said that SF and MS have synergistic effects on the properties of concrete.

**Figure 4.** Compressive strength results (**a**) effect of MS on CS with varying SF dosage (**b**) effect of MS on the net change in CS with varying SF dosage.

#### *3.2. Modulus of Elasticity (MOE)*

Figure 5 shows the effect of SF on the MOE of concrete. MOE linearly increases when the SF dose changes from 0 to 0.5%. Figure 5b shows the effect of SF dose on the net change in MOE. The improvements in MOE at 0.05–0.5% can be ascribed to increment in the confinement of specimen under compression that helps in the utilization of the full potential of the concrete matrix. Beyond 0.5%SF, MOE starts degrading similar to CS. This shows that for given high-strength concrete the optimum dose of SF for optimum MOE is 0.5%. As already explained, high fiber doses can increase the number of ITZs in concrete which leads to the reduction in compression stiffness of concrete. A slight increase in porosity of concrete due to fibers (higher than 0.5%) can also damage the MOE considerably [38]. This finding is in line with the study of Xie et al. [24]. It was observed that during compression testing, mixes with high fiber doses showed more ductile failure before collapsing completely unlike the mixes with smaller doses. A linear increase in energy absorption capacity was observed with the rise in SF dose. Ou et al. [39] reported that the main role of SF is prominent in compression toughness of concrete (post-peak load behavior) because, before peak load in the determination of MOE, fibers are not activated.

In Figure 6, the effect of MS content is shown on the MOE of concrete. Figure 6b shows the net change in MOE due to varying dose of SF at 0, 5 and 10% replacement levels of MS. A clear improvement is noticed in the MOE of concrete due to MS addition. This is credited to (1) the improved packing density of binder particles and (2) the pozzolanic reaction that consumes free lime. MOE concrete with 10% MS is 11% higher than that of the plain concrete without MS. Figure 6b shows that MS enhances the utilization of fibers. Moreover, MS minimizes the negative effect of high SF volume on MOE. The combined incorporation of MS and SF shows synergistic behavior. For example, 0.5%SF and 10% MS individually leads to improvement of 3.4% and 7%, respectively. But simultaneous incorporation of both MS and SF improves the MOE by 14.8%. This is true for all mixes made with both MS and SF.

**Figure 5.** Modulus of elasticity (MOE) results (**a**) Variation of MOE with SF dosage (**b**) Net change in MOE with varying SF dosage.

**Figure 6.** Modulus of elasticity (MOE) results (**a**) effect of MS on MOE with varying SF dosage (**b**) effect of MS on the net change in MOE with varying SF dosage.

From the results of CS and MOE, it is quite clear that both of these mechanical properties show a similar response to varying SF and MS contents. Therefore, both parameters can predict each with great accuracy. Since MOE is difficult to determine in the laboratory; therefore, it is predicted usually from CS. The relationship between MOE and half power of CS is shown in Figure 7. This relationship (Equation (1)) is drawn without considering the impact of SF or MS content:

$$\text{MOE} = 7007\sqrt{\text{CS}} - 18500 \tag{1}$$

where MOE = modulus of elasticity (MPa); CS = compressive strength (MPa).

**Figure 7.** Relationship between MOE and CS1/2.
