Kazim Turk

Hybrid fiber‐reinforced self‐compacting concrete (SCC) is gaining widespread popularity in construction engineering applications owing to its superior hardened concrete properties, while it isn't valid for workability, especially when it includes a high volume of fibers. While the incorporation of fibers can enhance the performance of SCC, it often challenges key fresh properties such as flowability and passing ability, thereby necessitating mix adjustments that result in additional costs. Furthermore, the high cost of fibers requires careful determination of the optimal fiber type combination and content. In this context, the aim and objective of this work is to comprehensively investigate the effects of different types (steel and synthetic) and combination (single, binary and ternary) of fibers on the workability (slump‐flow, T500, J‐ring and V‐funnel) and mechanical (compressive strength, splitting tensile strength, elastic modulus, shear strength, and flexural tensile strength) properties of SCC mixes with low hybrid fiber content (total by volume 0.75%). After defining the workability properties of all mixes, mechanical property tests were performed on samples with curing periods of 7‐, 28‐, and 56‐days. According to the results obtained, all fiber‐reinforced SCC mixes exhibit high performance in terms of both workability and mechanical properties. In particular, binary hybrid fiber systems containing long hooked‐end steel fibers and short straight steel fibers provided the best overall performance, yielding more effective outcomes compared to other mixes. In conclusion, this work has demonstrated that SCC mixes with low hybrid fiber content can successfully meet a wide range of engineering requirements, from workability to mechanical performance. This also implies that these mixes offer significant advantages in terms of both cost‐effectiveness and ease of application due to workability superior of SCC mixes with low hybrid fiber content. These outcomes emphasize that SCC mixes with low hybrid fiber content can be safely used in structural concrete members subjected to complex loads such as punching, shear, and bending, and may offer more economical and sustainable alternatives to high‐fiber content systems.

Ultra‐fine copper slag (CS), a byproduct of the copper industry, is al waste material that is produced in large volumes annually, and its disposal and management are a major environmental concern. Therefore, the utilization of CS in various sectors, especially in the production of construction and building materials, offers enormous potential for both environmental sustainability and economic benefits. In this research, the feasibility of using CS at a ratio of 0%, 25%, 50%, 75%, and 100% by replacing fine aggregate in steel fiber‐reinforced self‐compacting concrete (SFR‐SCC) with high‐volume steel fiber (1.50% by volume) has been explored for the first time in the literature. To identify the workability properties of SFR‐SCC mixes incorporating 0%, 25%, 50%, 75%, and 100% ultra‐fine CS by replacing the fine aggregate, slump‐flow, flow times (T500) and J‐ring tests were performed, whereas compressive strength ( f _c ), splitting tensile strength ( f _ct ), and modulus of elasticity ( E _c ) tests were applied to the samples for different curing days to evaluate the mechanical properties. Additionally, electrical resistivity/conductivity tests were conducted to determine the electrical properties as well. The experimental results revealed that the inclusion of ultra‐fine CS into SFR‐SCC mixes with high‐volume fiber improved the slump‐flow and T500 values, whereas the use of ultra‐fine CS above 50% induced a remarkable increase in the T500 value. In addition, the addition of ultra‐fine CS caused significant decreases in J ‐ring height difference (Δ H ) values, and a 4‐times decrease in Δ H value was observed when the CS ratio was raised from 0% to 100%. In this context, in terms of workability, all mixes exhibited acceptable stability with minimal segregation tendency. On the other hand, the mechanical performance of SFR‐SCC samples with different ultra‐fine CS ratios was found to be better than those without CS; specifically, the samples incorporating 25% ultra‐fine CS reached outstanding values such as f _c , f _ct , and E _c with 83.4, 9.1, and 40.4 GPa, respectively. Furthermore, increasing the CS content in SFR‐SCC samples led to considerable improvements in electrical properties, with the CS content raising the electrical conductivity values by an average of 60%. Consequently, it has been proven that the use of ultra‐fine CS by replacing fine aggregate in SFR‐SCC mixes having high‐volume steel fiber improves both the workability and mechanical properties as well as its electrical performance, resulting in high‐performance, eco‐friendly composites. Thus, it also contributes to the protection of natural resources and the sustainable utilization of industrial waste as well as providing an innovative solution that improves the performance of building materials.

In this article, the effect of hooked‐end fibers with different lengths on the structural performance of RC‐FPs fabricated from hybrid fiber‐reinforced self‐compacting concrete (HFR‐SCC) was investigated. For this purpose, a total of 15 full‐scale test samples having different plate thicknesses (60, 70, and 80 mm) were produced and tested under bending after a 90‐day curing period. Subsequently, load‐carrying capacity (P_p), flexural toughness (Fth), and deflection ductility index (μ_u) of all RC‐FPs were found using load‐deflection curves obtained from bending tests, while crack patterns were drawn from the samples tested. Besides, high‐precision contour plots are proposed to estimate the structural performance values of RC‐FPs depending on plate thickness and fiber reinforcing index. As a result, the best structural performance in RC‐FPs was obtained from the use of a longer hooked‐end steel fiber together with micro steel fiber as a hybrid, followed by the lower length hooked‐end steel fibers as singles. Specifically, irrespective of the plate thickness, the hybrid use of longer hooked steel fibers in combination with micro fibers increased the P_p, Fth, and μ_u values of RC‐FPs on average 1.67, 1.76, and 1.57 times, respectively, compared to the control specimens. As for when using the lower length hooked‐end fiber as single, the values of P_p, Fth, and μ_u increased on average 1.57, 1.69, and 1.30 times. Lastly, whereas plate thickness has little effect on improving the structural performance of thin‐walled carrier elements such as RC‐FPs, adding fibers in different lengths, aspect ratios, and combinations is much more effective. The collective test results demonstrate that using RC‐FPs made of HFR‐SCC in the roof carrier system of large span structures could improve structural performance, aesthetics, erection time, and earthquake behavior thanks to reduced dead load.