*3.3. Splitting Tensile Strength (STS)*

STS in an estimate of true tensile strength of concrete. Due to the complexity of measuring the true tensile strength under the direct tension test, STS provides a simpler measurement of the tensile strength of cementitious materials. Figure 8 shows the effect of varying SF content on STS. Unlike results of CS and MOE, STS does not show a mixed response to increasing the dose of SF. This is because, under tensile load, fibers become active way before the failure at peak load; therefore, stretching action on concrete is resisted by both concrete matrix and fibers. Figure 8 shows that the net change in STS due to SF addition is very huge compared to that observed in the results of MOE and CS. STS achieves more than 3 times positive gain compared to CS and MOE at each dose of SF. This confirms that fibers are more useful in tensile stiffness than they are in the compressive stiffness of concrete.

**Figure 8.** Splitting-tensile strength (STS) results (**a**) Variation of STS with SF dosage (**b**) Net change in STS with varying SF dosage.

MS addition provides a little advancement in the tensile strength, see Figure 9. Since MS strengthens the binder matrix, some small improvements can be anticipated in the STS. The filling effect of MS particles cannot contribute to STS, only the pozzolanic reaction between portlandite and silica strengthens the concrete matrix against tensile stresses [40]. A clear view of the synergistic effect of MS and SF on the STS can be seen in Figure 9. MS addition improves the net gain due to fibers by more than 30%. Densification of the matrix leads to an efficient transfer of tensile stresses to fiber-filaments; thus, MS addition improves the utilization of fibers. The results show that using MS along with SF can help in yielding 20% more STS than that could be achieved without MS. These results have important implications for fiber-reinforced concrete/composites. Since fibers are very expensive materials, their full utilization is very necessary to design cost and performance efficient structures. Therefore, MS and other high-performance mineral admixtures can help in enhancing the utilization of fibers.

**Figure 9.** Splitting-tensile strength (STS) results (**a**) effect of MS on STS with varying SF dosage (**b**) effect of MS on the net change in STS with varying SF dosage.

For plain concrete, STS can be fairly correlated with CS or MOE. But for fibrous concrete STS cannot be correlated with CS or MOE, see Figure 10. As, activation of fibers during compression mostly starts near or after the peak load loads; therefore, fibers do not contribute a great deal towards the advancement of CS or MOE. Whereas, under tension, fibers activate way before peak load; therefore, concretes show a huge STS change with fiber addition. Under tension, fibers do not only contribute to the peak strength of concrete, but they are also useful in the post-peak load resistance. CS, MOE, and STS of each mix are correlated in Figure 10, without considering the role of SF dose. This surface plot shows a general trend that each mechanical parameter is directly proportional to each other but with a huge scatter (R2 < 0.6).

#### *3.4. Water Absorption (WA)*

WA capacity of concrete represents its water-permeable volume of voids. High WA generally indicates high porosity. The effect of SF on the WA capacity of each mix is shown in Figure 11. WA undergoes mixed changes with the rising dose of SF. Small fractions of SF cause minor reductions in the WA capacity of concrete, whereas, at high doses, SF, WA absorption of fibrous concrete is slightly higher than that of the plain concrete. Both positive [21,41] and negative [42] effects of SF on WA has been reported in the literature. No study in the literature has examined the permeability characteristics of SF-reinforced concretes considering a wide range of fiber dosage. Fibers can control micro-cracking during the evolution of cementitious compounds in concrete. These can restrict the

plastic and temperature shrinkage cracking which ultimately improves the permeability resistance of concrete. At the same time, fiber addition increases the number of ITZs in concrete. Poor bond at ITZs favor permeability, hence it increases of WA capacity. Apparently, at low fiber volumes, controlled shrinkage leads to reduction in WA capacity and the role of ITZs is not very dominant at low fiber volumes. But as the fiber volume increases, the number of weak ITZs favor permeability and increase the WA. The minimum WA is observed at 0.1% SF, whereas maximum WA is noticed at 2% SF.

**Figure 10.** Correlation between mechanical properties (MOE, STS and CS).

**Figure 11.** Water-absorption (WA) results (**a**) Variation of WA with SF dosage (**b**) Net change in WA with varying SF dosage.

Figure 12 shows the effect of MS on WA capacity of concrete. MS brings down the WA capacity of concrete significantly. As extremely fine particles of MS fill the gaps left between cement particles, the overall density of matrix undergoes improvement. MS can reduce the pore-size at the ITZ between fiber and matrix. MS can nullify the negative effect of SF on WA. These results implicate an important role of MS in fibrous concretes. Since, fibers at medium to high volumes (0.5–2%), increase the permeability which may favor the corrosion of SF. Corrosion of SF will significantly lower the

performance of fibrous concrete over time. Therefore, the conjunctive use of fibers and MS can increase the durability life of fibrous concrete composites.

**Figure 12.** Water-absorption (WA) results (**a**) effect of MS on WA with varying SF dosage (**b**) effect of MS on the net change in WA with varying SF dosage.

#### *3.5. Chloride Ion Penetration (CIP)*

Figure 13 shows the effect of SF content on the CIP of concrete. CIP results also experience changes similar to WA with the variation of SF content. CIP undergoes reduction when fiber dose changes from 0 to 0.1%. Since there is no involvement of forced electrical transfer of chloride ions in the immersion technique high conductivity of SF does not play any role in determining the CIP resistance of concrete. CIP resistance improvement at low fiber volumes can be ascribed to a reduction in the WA capacity of concrete. On the other hand, reduction in CIP resistance at high fiber volumes (1% and 2%) can be blamed to an increase in porosity or absorption capacity of the matrix. At 2%SF, CIP of concrete is about 18% higher than that of the plain concrete. Since chloride-induced corrosion is usually experienced in most concrete structures, low chloride permeability resistance of fibrous concretes (especially with a high volume of fibers) can create durability issues which must be considered while designing a concrete mix.

**Figure 13.** Chloride-ion penetration (CIP) results (**a**) Variation of CIP with SF dosage (**b**) Net change in CIP with varying SF dosage.

Figure 14 shows the effect of MS content on the CIP. The addition of 5%MS and 10%MS brings down the CIP by 23% and 33%, respectively w.r.t plain concrete (without MS). The behavior of WA and CIP with the addition of MS is very similar because MS substantially reduces the volume of permeable voids [2]. As fibrous concretes struggle with the issue of low CIP resistance at high fiber doses, MS can be a befitting addition to enhance the imperviousness of concrete. With 5 or 10% MS, high fiber volume concretes show lower CIP than control concrete (see Figure 14a).

**Figure 14.** Chloride-ion penetration (CIP) results (**a**) effect of MS on CIP with varying SF dosage (**b**) effect of MS on the net change in CIP with varying SF dosage.

By constricting the microchannels across the ITZs at fibers, MS can efficiently minimize the degrading effect of fibers on CIP. Almost all engineering properties of concrete depend on the growth and density of microstructure i.e., strength and permeability characteristics. CS, CIP, and WA are correlated with each other in Figure 15. The surface plot shows a general trend that CS is inversely related to both WA and CIP. All data points in Figure 15, congregate near-surface plot which means CS, CIP and WA are strongly correlated (R2 > 0.8) and models developed to predict these parameters from each other can be formulated for design purposes.

**Figure 15.** Correlation between CS, CIP and WA.

#### **4. Conclusions**

This study evaluates the influence of a wide range of SF doses on the basic engineering characteristics of high strength concrete. It also explores the modifications of SF-reinforced concrete properties with micro-silica (MS). Following important conclusions can be taken from this research:

