General-purpose type-I OPC (Ordinary Portland Cement) per ASTM C150 (ASTM, 2015) was used for the current research. The physical and chemical characteristics of cement are provided in Table 1. The complete mix proportion of concrete samples in three stages is provided in Table 2. The mix IDs of all samples are designed in such a way that number next to “RFA” denotes the rate of substitution of recycled fine aggregate, and the number next to “CF” represents the rate of carbon fibers that are included. In the first stage, the concrete was made by substitution of natural sand with recycled fine aggregate with varying proportions such as (0, 35, 75, and 100%). Due to the difference in absorption values of natural and recycled fine aggregate, corrected water values were included in a concrete mix. In the second stage of research, discarded carbon fibers were included in normal concrete by (2, 4, and 6%) weight. In the third stage, 2, 4, and 6% of discarded carbon fibers were added by volume fraction with 35% replacement of natural sand with recycled fine aggregate, which was recognized as an optimum ratio in the first stage. To make a concrete flowable minimum amount of superplasticizer was also included in the concrete mixes.

Particle size distribution for fine and coarse aggregates is presented in Figure 1A,B. The maximum size for coarse aggregate was 25 mm. Waste pieces of carbon fibers were obtained from the leftover sheets of other researchers, which were then chopped to 3.5 cm in length as presented in Figure 2. The characteristics of carbon fibers are displayed in Table 3.

During the process of mixing, the first fine and coarse aggregates were mixed per ACI 211.1 (ACI 211.1, 1991). After that superplasticizer and cement were added, the mixture was blended for 10 min. To avoid balling phenomenon, the waste carbon fibers were added slowly to the mixture. Then slump test was conducted on all concrete specimens as per ASTM C 143 (ASTM C143/C143M-15a, 2015). All the concrete specimens with recycled aggregates, waste fibers, were prepared and cast following ASTM C31 (ASTM C31/C31M-17. A-C, 2017). The size of the cube mold was (100 mm × 100 mm x 100 mm), cylinder molds (200 mm × 100 mm), and the beam mold (100 mm × 100 mm x 500 mm). The samples were removed from the molds after 24 hours, and then they were kept in water at a temperature of 24°C for 28 days for its curing.

The compressive test was performed following ASTM C 39 (ASTM C39/C39M-17. A-C, 2017), thirty concrete cubes with a size of (100 × 100 × 100 mm³) were prepared for the compressive test at 28 days of curing. For every mix proportion, three concrete cubes were made. For the first stage the impact of the substitution of recycled aggregates, and as far as the second and the third stages, the influence of including fibers by (2, 4, and 6%) by volume fraction were examined. Control concrete compressive strength came out to be 53 MPa since recycled concrete with 35% of substitution didn’t display a significant reduction in compression capacity, it was specified as a primary mixture proportion utilized in the third stage of the study for including varying proportions of discarded carbon fibers.

The splitting tensile test was performed following ASTM C 496 (ASTM C496/C496M-17. A-C, 2017), thirty concrete cylinders with a size of (100 × 200 mm) were prepared for the splitting tensile test at 28 days of curing. For every mixture proportion, three concrete cylinders were prepared, the average value of those three samples was taken as the final tensile capacity of concrete.

To evaluate concrete bending behavior, a three-point bending test (see Figure 3A,B) was performed as per ASTM C 293 (293 AC, 2016). Thirty beams which had the dimension of (100 × 100 × 500 mm³) were prepared, the deflection at mid-span was recorded using (LVDT) displacement data logger. The load was applied on concrete beams at a rate of 0.6 mm/min.

A slump test was performed to assess the workability of concrete, its values are provided in Table 4 and Figure 4. From the slump test result, it was revealed that the inclusion of discarded carbon fibers reduces slump and hence reducing the concrete workability. These fibers could make a network structure in concrete that restrain concrete mix from flow and segregation. Moreover, because of the large surface area and high content of discarded carbon fibers, they could effortlessly absorb water from cement paste to wrap around, consequently enhancing recycled concrete aggregate viscosity. In the first stage, as the quantity of absorbed water for RAC raised, the slump also enhanced with it.

The compressive strength of all 3 stages of samples is presented in Figure 5. The compressive strength of the control sample was 53 MPa at 28 days of curing and when 100% of natural sand was substituted with recycled fine aggregate, the compressive strength was reduced by 25%, see Figure 5. The compressive strength of waste carbon fiber concrete with recycled fine aggregate and only waste carbon fiber concrete was 62 and 69 MPa, respectively. Likewise, the compressive strength was improved as we increased the dosage of fibers in concrete, see figure. When 6% of waste carbon fibers were added, the compressive strength touched 69 MPa, which is 36% higher than the same mix without fibers.

To validate and verify our research data with the present literature, comparative analysis was performed and statistical model was made, from Table 5 we can see that the current research falls between the present literature for the compressive strength at 28 days, and from Figure 6 it can be observed that the value of R square is close to unity which shows consistency in the values, therefore validating and verifying the results for the present research.

The splitting tensile strength of all three stages of samples is presented in a Figure 7. The splitting tensile strength of the control sample was 3.9 MPa and when 100% of natural fine aggregate was substituted with RFA, the splitting tensile strength came out to be 3.4 MPa. As shown in Figure 7, the highest splitting tensile strength that was obtained was 5.6 and 5.3 MPa for the RFA-0-CF-6 and RFA-35-CF-6 mix design, respectively. It is noticeable (Ferdosian and Camões, 2021) that with the addition of fibers the split tensile strength of the concrete improved significantly and prevents crack propagation. By adding more amount of waste carbon fibers to the concrete mix, the length and depth of the cracks will be reduced.

To validate and verify our research data with the present literature, comparative analysis was performed and statistical model was made, from Table 6 we can see that the current research falls between the present literature for the splitting tensile strength at 28 days, and from Figure 8 it can be observed that the value of R square is close to unity which shows consistency in the values, therefore validating and verifying the results for the present research.

The values for the bending point test of all three stages of samples are presented in Figure 9. When more quantity of natural fine aggregate was swapped with RFA, the flexural strength of concrete specimens was reduced, with 35, 70, and 100% substitution of natural fine aggregate was replaced with RFA, the flexural strength reduced by 4.1, 8.3, and 14.9%, respectively. Though, with increasing the percentage of fibers in concrete, the flexural strength raised to 7.1 MPa for mix design of RFA-0-CF-6. Figure 10 shows the load-displacement curve for the second and the third stages of concrete beams made of waste carbon concrete and waste carbon concrete with recycled sand. From Figure 10, it could be noticed that inclusion of discarded carbon fibers enhances the capacity of energy absorption and the ductility of samples. This was also (Kazmi et al., 2018) proved because of the sample’s elastic fractures. Control concrete samples and concrete samples with only recycled fine aggregate made for the split tensile and flexural test were all fractured in a brittle fashion, but when the waste carbon fibers were added to the concrete, they made the concrete more ductile and the fibers also helped by restraining the cracks propagation in concrete.

To validate and verify our research data with the present literature, comparative analysis was performed and statistical model was made, from Table 7 we can see that the current research falls between the present literature for the flexural strength at 28 days, and from Figure 11 it can be observed that the value of R square is close to unity which shows consistency in the values, therefore validating and verifying the results for the present research.

From the concrete SEM images, it may be observed that the discarded carbon fibers are uniformly dispersed in the concrete matrix when the sample broke, the bond amid the mortar and the fibers made the fibers to be stretched and scratched. The micro openings that are formed when the concrete failed are presented in Figure 12, which show (Baena et al., 2016) the parting of the carbon fiber from the sample and indicates that the fibers’ mechanical strength is not utilized completely. Though, the existence of carbon fibers in samples enhances the split tensile, compression, and flexural capacity. When the concrete sample with waste carbon fibers was broken during the test see Figure 13, the role of crack bridging of carbon fibers was easily visible to the naked eye.