Coupled effect of waste tire rubber and steel fibers on the mechanical properties of concrete

Ordinary Portland type I cement with a specific gravity of 3.15 was used in this research. Natural river sand with fineness modulus of 2.40 was used as fine aggregate. Crushed aggregate from the local source with a maximum aggregate size of 19 mm was used as coarse aggregate. The properties of fineness modulus, saturated surface dry (SSD) specific gravity, moisture content, and bulk density of fine and coarse aggregates are listed in Table 1. High range water reducer Sika ViscoCrete-20 HE superplasticizer was used to enhance the workability of the concrete mixtures.

Tire rubber and steel fibers, as shown in Figure 1, were used in this study to evaluate their coupled effect on mechanical performance of concrete. Tire rubber fibers were obtained from the waste tires. For this purpose, the central portion between the tire steel fibers and protrusions was obtained, and cut into strips of length 25 mm and width 2–3 mm. Commercial straight steel fibers with length of 25 mm, diameter of 0.50 mm, specific gravity of 7.85, and tensile strength of 1,100 MPa were used. Properties of fibers are enlisted in Table 2.

Fig. 1

In this study, nine mixes were prepared for better understanding of the coupled effect of tire rubber and steel fibers on mechanical properties of concrete. The control mix was prepared according to ACI 211.1 [22] to achieve a target compressive strength of 45 MPa at 28 days. Each mix is designated as RxSy, where R represents the tire rubber fibers, x is the percentage of tire rubber fibers, S represents the steel fibers, and y is the percentage of steel fibers. To evaluate the performance of the various mixes against that of plain concrete, the control mix has been prepared without any fiber. Concrete mixtures were prepared with different percentages of tire rubber and steel fibers. The tire rubber fibers were added as 1%, 2%, and 3% by volume of concrete while steel fibers were added as 0.1%, 0.2%, and 0.3% by volume of concrete. For all mixes, the water/cement ratio was fixed at 0.36 and a nominal mix of 1:1:2 was used. Compositions of concrete mixes are presented in Table 3.

In this research, batching is done by weight. The order that was followed in adding the ingredients was: coarse aggregate, fine aggregate, and then binder; after that, the mixer was started to mix the dry ingredients. Then, water was added, and almost after adding half amount of water the admixture (superplasticizer) was mixed in the remaining half water. Now the remaining water that has been mixed with admixture was added to the concrete within 2–3 min. After adding the whole amount of water and admixture, steel fibers and tire rubber fibers were added to the mixer. This whole procedure of mixing was done within 5–6 min. The material after mixing was homogenous in color, non-bleeding, and non-segregated. All the remaining batches were mixed in the same way. At the end of mixing, prepared concrete was poured into the steel molds. After removing from the molds at the age of 1 day, specimens were moist cured using jute bags up to the age of 28 days at ambient conditions.

The slump test was performed for each batch to verify the uniformity or homogeneity of the concrete during construction as per ASTM C143 [23]. Tests were performed on hardened concrete at the age of 28 days to evaluate mechanical properties, including compressive strength, splitting tensile strength, and flexural strength. The compressive strength test was performed on cylindrical specimens of Φ 150 mm × 300 mm as per the ASTM C39 [24]. Two specimens were prepared for each type of concrete mix to determine the compressive strength. The compressive strength test was performed on 1,000 kN capacity displacement controlled universal testing machine (UTM) at a platen displacement rate of 1 mm/min.

The splitting tensile strength test was performed on cylindrical specimens of Φ 150 mm × 300 mm as per the ASTM C496 [25]. Two specimens were prepared for each type of concrete mix to determine the splitting tensile strength. The same UTM that was employed in compressive strength determination was used at a platen displacement rate of 0.5 mm/min to determine the splitting tensile strength. Split tensile strength was calculated using Eq. (1). (1) ft=2PπDL f_t = {{2P} \over {\pi DL}} where f_t represents the splitting tensile strength (MPa), P the load at failure (N), D the diameter of the specimen (mm), and L the length of the specimen (mm).

Four-point bending tests were performed on tire rubber and steel fiber beams to investigate the effect of the combination of rubber fibers and steel fibers on the bending performance of concrete. Three beams of 100 mm × 100 mm × 350 mm for each type of mixture were prepared for this test. The test was performed using a Shimadzu 1,000 kN UTM. The concrete specimens were loaded to final failure at a constant loading rate. Loads were recorded against first crack and the modulus of rupture (MOR) was calculated. The area under the load-deflection curve corresponding to deflection equal to span divided by 150 (Figure 2A) was calculated to assess the flexural toughness for all types of concrete mixes according to ASTM C1609 [26]. Equivalent flexural strength ( fe, 150D f_{e,\,150}^D ) and equivalent flexural strength ratio ( RT, 150D R_{T,\,150}^D ) were also calculated to evaluate the post-peak behavior as per ASTM C1609 using Eqs (2) and (3), respectively. (2) fe, 150D=150T150Dbd2 f_{e,\,150}^D = {{150T_{150}^D } \over {bd^2 }} where fe, 150D f_{e,\,150}^D represents equivalent flexural strength (MPa), T150D T_{150}^D the area under load-deflection curve from 0 to L/150, b the width of the specimen (mm), and d the depth of the specimen (mm).

Fig. 2

Further, (3) RT,150D=fe,150Df1×100% R_{T,150}^D = {{f_{e,150}^D } \over {f_1 }} \times 100\% where RT,150D R_{T,150}^D represents the equivalent flexural strength ratio, fe, 150D f_{e,\,\,150}^D the equivalent flexural strength (MPa), and f₁ the first peak strength (MPa).

Two more flexural toughness indices I5 and I10 are calculated according to ASTM C1018 [27]. For calculation of index I5, we calculate the area between 0δ to 3δ (3.0 times the first crack deflection) of the load-deflection curve, and divide this area by the area up to the first crack (δ), as shown in Figure 2B. For calculation of index I10, we calculate the area between 0δ and 5.5δ (5.5 times the first crack deflection) of the load-deflection curve, and divide this area by the area up to the first crack (δ). Load-deflection data were automatically calculated by the machine.

To evaluate the fresh state behavior of concrete, slump tests were performed according to ASTM C143 [23]. Slump test results for all mixes are represented in Figure 3. The slump of all concretes was between 25 mm and 65 mm. For a steel fiber volumetric content of 0.1%, by increasing the value of tire rubber fiber percentage, the slump decreases by 23%, 30.8%, and 38.5% than control concrete for 1%, 2%, and 3% rubber fiber content, respectively. This decrease is further pronounced for tire rubber and steel fiber concrete mixes when steel fiber volumetric content is increased to 0.2%. The slump values for 0.2% steel fibers are decreased by 30.8%, 53.8%, and 61.5% than control concrete for 1%, 2%, and 3% tire rubber fiber content, respectively. In general, with the addition of tire rubber fibers, percentage slump value decreases. Also, this decrease in workability was more pronounced when steel fibers were added. The reason of this decrease is the increased friction between the fibers and other constituents of the concrete mix, resulting in the mobility barrier of concrete. The decrease in workability is in line with past studies, showing that fibers reduced the workability when their volume fraction (V_f) is increased [28, 29].

Fig. 3

The compressive strength results of all mixes are presented in Figure 4. For a steel fiber volumetric content of 0.1%, by increasing the value of tire rubber fiber content, compressive strength was decreased by 8.1%, 10.3%, and 12.9% than control concrete for 1%, 2%, and 3% tire rubber fiber content, respectively. Similarly, for a steel fiber volumetric content of 0.2%, by increasing the value of tire rubber fiber content, compressive strength was decreased by 2.4%, 5.3%, and 11% than control concrete for 1%, 2%, and 3% tire rubber fiber content, respectively. Compressive strength for all the mixes was reduced by increasing the tire rubber fiber content.

Fig. 4

Comparing the concrete mixes with same tire rubber fibers content, an increase in compressive strength was observed with the increase in steel fiber content. For rubber fiber volumetric content of 1%, 2%, and 3%, by increasing the value of steel fiber content from 0.1% to 0.2%, compressive strength was increased by 6.2%, 5.6%, and 2.2%, respectively. This shows the positive effect of steel fibers on strength and stiffness of concrete mixes with tire rubber fibers. In general, with the addition of tire rubber fiber content, compressive strength values were decreased. However, when steel fiber percentage was increased, compressive strength values showed an improvement. The loss in the compressive strength pursuant to an increase in the percentage of tire rubber fibers is due to the deformability of rubber fibers and a weak interfacial bond between rubber fibers and surrounding concrete. On other hand, when steel fiber was incorporated into the concrete that contained rubber fiber content, it exhibited an increased compressive strength, which is due to the bridging action of the former fiber. Similar results of increased compressive strength pursuant to the addition of steel fibers to rubberized concrete have been reported in the literature [30].

The split tensile strength of all mixes is represented in Figure 5. Concrete mix R3S0, in which 3% tire rubber fibers were added, showed 4.2% less split cylinder strength than control concrete. However, concrete mix R0S0.3, in which only 0.3% steel fibers were added, showed a 5.6% increase in split cylinder strength in comparison with control concrete. It was observed that split tensile strengths of mixes R2S0.1 and R3S0.1 are decreased in comparison with mix R1S0.1 by 7.7% and 11.5%, respectively. Also, split tensile strengths of mixes R2S0.2 and R3S0.2 are decreased in comparison with mix R1S0.2 by 3.7% and 11.1%, respectively. In coupled tire rubber and steel fiber concrete mixes, for a given steel fiber content, split tensile strength decreased corresponding to an increase in the tire rubber fiber content.

Fig. 5

For tire rubber fiber volumetric content of 1%, 2%, and 3%, by increasing the value of steel fiber percentage from 0.1% to 0.2%, split tensile strengths were increased by 3.8%, 8.3%, and 4.3%, respectively. Hybrid fiber concrete mix R1S0.2 showed a relatively better performance with respect to split tensile strength among coupled tire rubber and steel fiber concrete mixes. In general, with an increase in the tire rubber fiber percentage, split tensile strength values were decreased. However, when steel fiber percentage was increased, split tensile strength values were improved. The reduction in the split tensile strength corresponding to an increase in the percentage of tire rubber fibers is due to lower stiffness and a weak interfacial bond between rubber fibers and surrounding concrete. Using steel fibers has a positive effect on the splitting tensile strength. Concrete is a brittle material and cannot withstand tensile force, and the initiation of cracks within concrete is still controlled by the quality of the cement matrix. However, after crack initiation, the fiber bridging effect played an important role in controlling the crack growth. Therefore, steel fibers improved the splitting tensile strength and controlled crack propagation effectively. The results obtained in the present research, indicating an increase in tensile strength in rubberized concrete resultant to the addition of steel fibers, are in line with the findings reported in the literature, which showed that the tensile strength reduced pursuant to increasing the content of rubber in concrete and increased for each dosage when steel fibers are introduced in the concrete [31, 32].