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Article

Low-Carbon Concrete Reinforced with Waste Steel Rivet Fibers Utilizing Steel Slag Powder, and Processed Recycled Concrete Aggregate—Engineering Insights

by
Dilan Dh. Awla
1,*,
Bengin M. A. Herki
1 and
Aryan Far H. Sherwani
2
1
Civil and Environmental Engineering Department, Faculty of Engineering, Soran University, Soran 44008, Iraq
2
Scientific Research Center, Soran University, Kurdistan Region, Soran 44008, Iraq
*
Author to whom correspondence should be addressed.
Fibers 2025, 13(8), 109; https://doi.org/10.3390/fib13080109
Submission received: 11 May 2025 / Revised: 27 July 2025 / Accepted: 7 August 2025 / Published: 14 August 2025
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Abstract

The construction industry is a major source of environmental degradation as it is responsible for a significant share of global CO2 emissions, especially from cement and aggregate consumption. This study fills the need for sustainable construction materials by developing and evaluating a low-carbon fiber-reinforced concrete (FRC) made of steel slag powder (SSP), processed recycled concrete aggregates (PRCAs), and waste steel rivet fibers (WSRFs) derived from industrial waste. The research seeks to reduce dependency on virgin materials while maintaining high values of mechanical performance and durability in structural applications. Sixteen concrete mixes were used in the experimental investigations with control, SSP, SSP+RCA, and RCA, reinforced with various fiber dosages (0%, 0.2%, 0.8%, 1.4%) by concrete volume. Workability, density, compressive strength, tensile strength, and water absorption were measured according to the appropriate standards. Compressive and tensile strength increased in all mixes and the 1.4% WSRF mix had the best performance. However, it was found that a fiber content of 0.8% was optimal, which balanced the improvement in strength, durability, and workability by sustainable reuse of recycled materials and demolition waste. It was found by failure mode analysis that the transition was from brittle to ductile behavior as the fiber content increased. The relationship between compressive, tensile strength, and fiber content was visualized as a 3D response surface in order to support these mechanical trends. It is concluded in this study that 15% SSP, 40% PRCA, and 0.8% WSRF are feasible, specific solutions to improve concrete performance and advance the circular economy.

1. Introduction

The global development depends on the construction industry, which is also a main driving force of environmental degradation, thus contributing 8 per cent of worldwide CO2 emissions by cement production alone [1]. Concrete is by far the most common construction material and uses energy and resource-intensive natural aggregate extraction and cement production [2].
In light of these impacts, the use of sustainable alternatives such as low-carbon fiber-reinforced concrete (FRC) with off-the-shelf industrial by-products such as waste and recycled steel fibers, steel slag powder (SSP), and recycled concrete aggregates (RCA) has come to mitigate dependency on natural materials [3]. Plizzari & Mindess (2019) [4] stated that FRC improves mechanical properties such as tensile strength and crack resistance. Nevertheless, conventional FRC relies on nonrenewable resources, and this necessitates the development of innovative solutions based on recycled and waste materials, especially in areas with plenty of industrial by-products. FRC is steel, synthetic, or natural fibers added into the concrete matrix to improve durability and engineering performance [5]. Micro-cracks are bridged by these fibers to improve ductility and reduce the need for traditional steel reinforcement [6]. Waste steel fibers (such as those from discarded rivets in the steel industry) have been recently explored as a sustainable alternative to virgin fibers [7].
In addition, the use of RCA, which is processed from construction, renovation, and demolition waste, is another sustainable option. Although RCA is known to have higher porosity and water absorption because of old cement paste and residual mortar, optimized processing methods have been promising. For instance, Papachristoforou et al. (2020) [8] showed that RCA can maintain up to 90% of the compressive strength of conventional concrete if it is cleaned and graded properly. The European example of reaching over 90% recycling rates for construction and demolition waste demonstrates the possibility of RCA incorporation into normal concrete use [9]. RCA obtained from construction and demolition waste is similar to an eco-friendly substitute for natural aggregates, which helps in reducing landfill use (waste materials are often dumped in open spaces in some developing countries), conserving natural resources [10]. RCA adoption is limited in developing countries such as Iraq/Kurdistan region because recycling infrastructure is not developed [11].
Another promising material is the steel slag (SS), a steel manufacturing by-product, used in the applications of road construction and production of cement due to the high strength and skid resistance capabilities of this material [12]. However, its use in the construction sector of developing countries is minimal, which is a missed opportunity for the development of the country [13]. SSP has been found to have pozzolanic properties and high levels of calcium and iron oxides and thus has been found to enhance mechanical performance and durability [12,14].
From these innovations, FRC has become an attractive solution, particularly for applications needing increased crack resistance, tensile strength, and overall durability. Steel fibers are especially useful because of their high stiffness and crack-bridging abilities [4,15]. Nevertheless, conventional fibers are produced from virgin resources, which cancels out the sustainability advantages. Research conducted by Grover & Kumar (2022) [7] and Paul et al. (2020) [6] demonstrated that the use of waste steel fibers (WSFs) obtained from industrial by-products, such as rivets, can offer similar mechanical reinforcement at a reduced environmental impact and a solution to the problem of metal waste disposal.
According to Chiang and Pan (2017) [16], steel slag aggregates increase abrasion resistance and structural strength in pavement applications, but volumetric instability poses a problem that requires attention to quality control. Other studies have also started to examine the effects of using RCA, SSP, and steel fibers separately. Kryeziu et al. (2023) [17] noted that slag may reduce the strength loss that is usually experienced in RCA, while the angularity of RCA promotes improved particle interlock and crack resistance. Similarly, Papachristoforou et al. (2020) [8] reported that slag aggregates enhanced high-temperature resistance in FRC.
Although these promising results exist, gaps still exist in assessing combined systems with multiple recycled components, especially in developing countries such as Iraq/Kurdistan, where recycling infrastructure is underdeveloped [11]. The results reported in previous studies, for example, from Liu et al. (2016) [18], who showed that type of aggregate and fiber interaction plays a significant role in tensile stress transfer, and Mohamed and Zuaiter (2024) [19], who indicated that slag-based FRC needs a delicate dosage of fibers to prevent heterogeneity. By using locally available waste materials, this study adds a scalable, environmentally friendly solution for structural concrete production that is in line with circular economy principles and climate resilience objectives.
The next sections describe the experimental methodology, results, comparative analysis with literature, and practical recommendations for applying in construction systems of developing regions. Although there is growing interest in sustainable concrete, critical gaps remain in the research on slag-based low-carbon FRC with single or multi-source RCA and waste steel fibers [10,20]. Gap filling done in this study entails investigation of the mechanical properties, durability, and sustainability of steel slag-based FRC with PRCA, local low-quality natural aggregate reinforced with WSRF. Therefore, this work introduces a novel low-carbon FRC that incorporates various industrial and construction waste streams, thereby addressing the persistent need for sustainable building materials. In particular, it creates and tests a novel FRC reinforced with WSRF, utilising processed RCA and SSP, which have not been mixed together in this manner for structural uses before. In order to lessen reliance on natural resources while maintaining excellent mechanical performance and durability, the research innovation consists of combining these three different waste components into a single optimized concrete mix represents a feasible and innovative solution for sustainable construction.

2. Methodology

The aim of this research was to develop and evaluate the performance of sustainable low-carbon FRC made with processed RCA, WSRF, and SSP. The methodology was material selection, formulating the mix design, specimen preparation, mechanical and durability testing, and result analyses.
SSP, as shown in Figure 1, was used in this study as a partial replacement for cement in concrete mix designs to evaluate its effects on the mechanical and durability properties of concrete. Steel slag is a by-product of the steel-making process and some types are rich in calcium, silica, and iron compounds; thus, it is a potential supplementary cementitious material. The steel slag was oven dried and ground into a fine powder that made it through a No. 200 (75 µm) sieve was utilized as a powder and cement substitute prior to use to improve its reactivity and to ensure uniform blending with other concrete ingredients. To maintain a balance between mechanical properties, workability, overall performance, and environmental issues that determine the replacement level (15%) of this type of steel slag, it was necessary to review the published literature [21] on steel slag powder with similar chemical and physical properties and to perform some trial mixtures. Chemical composition of SSP is presented in Table 1.
Ordinary Portland cement (OPC) conforming to ASTM C150 (2024) [22] and having a specific gravity of 3.15 was the primary binder used. The cement had a high calcium oxide (CaO) content of 66.2% and relatively lower contents of silicon dioxide (SiO2—14.6%), iron oxide (Fe2O3—3.8%), sulfate (SO3—4.1%), and potassium oxide (K2O—2.5%). Incorporation of SSP as a partial cement replacement at 15% by weight of cement was carried out to reduce the environmental impact of cement use. Chemical composition of cement is presented in Table 1.
Natural river sand with maximum particle size of 4.75 mm, specific gravity of 2.6, and water absorption capacity of 1.99% was used as the fine aggregate. Crushed stone with a nominal maximum size of 12.5 mm, specific gravity of 2.638, and water absorption of 0.62% was used as coarse aggregate (Figure 2). Natural coarse aggregate was obtained at a low price from a local construction materials market in Soran, Iraqi Kurdistan. It was dirty, not cleaned, and not well graded. RCA (Figure 2) was single-sourced from a demolition building and processed (crushed in Soran/Kurdistan/Iraq crusher plant as shown in Figure 3), cleaned, old cement paste removed or reduced, and graded. To make concrete a sustainable construction material, the natural coarse aggregate was replaced at 40% (25% 6 mm aggregate + 25% 8 mm aggregate + 50% 12.5 mm aggregate) by RCA to evaluate its effect on the concrete’s fresh, durability, and mechanical performance. Physical properties of natural fine, natural coarse, and recycled concrete aggregates are presented in Table 2.
The concrete mixes were prepared with the WSRF at different replacement levels by concrete volume fraction (0%, 0.2%, 0.8%, and 1.4%). Reviewing literature [23] justifies the choice of 0.8% WSRF content as optimal for performance and workability. Preparations were made for each mix according to standards’ procedures [24] and keeping a constant water-to-binder (W/B) ratio of 0.49 based on the trial mixtures made before conducting the main mixtures. To achieve homogeneity, the materials were mixed thoroughly using a mechanical mixer. Workability of the fresh concrete was tested, hardened samples were cured and tested to determine the effect of SSP, PRCA, and WSRF on overall concrete performance. To improve the tensile properties and crack resistance of the concrete WSRF shown in Figure 4, which are obtained from discarded rivets and waste products from the local door and window manufacturing industries in Soran, Kurdistan, Iraq. The average length and diameter of fibers measured 22.54 mm and 1.63 mm, respectively; the density was 7750 kg/m3 and other properties are presented in Table 3.
A polycarboxylate-based high-range water-reducing admixture conforming to ASTM C494 (2024) [25] was used to ensure adequate workability with a low water-to-cement ratio. A reduced water-to-binder ratio (W/B) of 0.49 was used with the superplasticizer at 0.5–1% of the cement weight, to achieve slumps between 35 and 65 mm without compromising workability or compaction. Each mix was evaluated for its workability with a standard slump test [26]. The mix proportions, by weight, were 1:2:3 (binder:NFA:NCA). Variations in the materials’ specific gravities were taken into consideration in the mix design actual calculations to guarantee precise batching by mass. Table 4 presents the weight of ingredients in all sixteen concrete mixtures.
For each test, three samples were prepared for each mixture and tested after 28 days of curing in water. The average of the three samples was calculated to provide the final result. Specimens were demolded after 24 h and cured in water at 20 ± 2 °C shown in Figure 5, until the testing age after casting. The development of strength was assessed through mechanical tests conducted at 28 days. A standardized mixing sequence was used to create concrete specimens that would be consistent. Compressive strength according to BS EN-12390 (2019) [27] and tensile strength according to ASTM C496 (2024) [28] were tested on 100 mm cubes, and 200 mm length and 100 mm diameter cylinder molds (Figure 6) [29].
For water absorption test of concrete specimens, first, the concrete specimens are oven dried at 105 ± 5 °C for 24 h or to a constant mass presented in the study [30]. After completely dried, the specimens are cooled to room temperature in a dry environment and weighed to get the dry weight (W1). The specimens are then fully immersed in clean water for 24 h. After the immersion, they are surface dried with a damp cloth and weighed immediately to find the saturated weight (W2) complies with ASTM C642 (2024) [31]. The formula for calculating water absorption is WA (%) = [(W2 − W1)/W1] × 100. This value represents the percentage of water absorbed by the concrete, which is a measure of porosity of the material and indirectly its durability. The Excel program is used as the basic statistical analysis for the obtained results.
The volume of the concrete specimen is determined (usually calculated from the known dimensions of the standard mold) and the dry weight is used to calculate dry density. This gives the density of the concrete in a dry state and gives insight into its compaction quality. Water absorption [30] and density [32] tests are both essential to the evaluation of concrete mixes containing alternative materials such as SSP and RCA.

3. Results

3.1. Compressive Strength

Results of the 28-day compressive strength are presented for concrete mixes containing varying volumes of steel fiber (0%, 0.2%, 0.8%, and 1.4%). The mixes are classified as Control, SSP, Slag+RA, and RA, shown in Figure 7. Strength increases from 42.5 MPa at 0% fiber content to 44.0 MPa at 0.2%, 44.1 MPa at 0.8% and reaches the highest value of 46.6 MPa at 1.4% fiber in the Control group (no slag or recycled aggregates). The steady increase in fiber reinforcement indicates the beneficial effect of fiber reinforcement on conventional concrete. The compressive strength at 28 days for the Slag mixes begins at 32.9 MPa without fibers. Strength is greatly improved upon with fibers, particularly to 40.1 MPa at 0.8% fiber content. Nevertheless, the strength falls slightly to 38.4 MPa at 1.4%, indicating a possible optimal fiber content of around 0.8% for this mix. Strength increases monotonically with fiber addition in the Slag/RA mixes: from 37.9 to 41.0 MPa at 0.2%, 41.5 MPa at 0.8%, and 41.9 MPa at 1.4%. This increase is more stable here, indicating that the combined use of slag and recycled aggregates still benefits from fiber reinforcement. All mixes had an increase in the compressive strength by 28 days with increasing fiber volume, although the amount of increase and the fiber content that provides optimum strength are slightly dependent on mix composition.
Figure 8 shows the net change in compressive strength of all concrete mixes compared to the control mix. The trends in 28-day compressive strength for all mixes are observed and show that steel fibers play a reinforcing role and that mix composition plays a role. This is supported by Yoshinaka et al. (2016) [33], who suggest that the fibers do not contain disruptive additives, have the ability to bridge microcracks, delay crack propagation, and enhance the post-crack load distribution utilized in a homogeneous matrix. The sharp improvement up to 40.1 MPa at 0.8% is attributed to the pozzolanic reaction of slag improving matrix densification and the synergy with fibers at optimal dosage as shown in Figure 7 and Figure 8, while a slight decline at 1.4% suggests that excessive fibers may disrupt the matrix or reduce workability, which is in agreement with Mohamed and Zuaiter (2024) [19] on slag’s sensitivity to fiber induced heterogeneity. The gradual fiber reinforcement of the strength seems to stabilize the distribution of internal stress and works with the reactivity of slag to compensate for RCA’s inherent weakness, which seems to be in line with Park (2008) [34]. Here, values are not listed, but similar studies show that RCA surface roughness enhances mechanical interlock with fibers and sustained strength gains in the RA mix. Overall, the compressive strength improvement is more pronounced in mixes where the matrix quality and fiber dispersion are in balance, and 0.8% fiber dosage is optimal in slag-based mixes, and higher dosages (1.4%) are more effective in conventional concrete. When compared to concrete made with unprocessed natural aggregates (NCAs), concrete made with well-processed RCA—that is, aggregates that have been crushed, cleaned, excess old cement paste reduced/removed, and well graded—achieved equivalent or even greater compressive strength. Unprocessed NCA frequently contains contaminants, including dirt, clay, and organic content, which can result in poor compaction, weaker interfacial bonding, and uneven performance. However, because of its rough texture and angularity, properly prepared RCA offers better paste–aggregate interaction.

3.2. Splitting Tensile Strength

Results from the experiments show the evolution of indirect tensile strength in sixteen concrete mixtures with increasing exposure levels, as shown in Figure 9. Figure 10 shows the net change in tensile strength of concrete mixes compared with the control mix. Tensile strength increases steadily as well from baseline (0% fiber) to maximum level (1.4% fiber volume) for the control mix (conventional concrete without additives): 3.68 MPa, 3.75 MPa, 3.89 MPa, 3.99 MPa. The 8.4% improvement in this case represents the expected maturation of ordinary concrete, where the cementitious matrix is continually hydrated to strengthen, as observed by Felekoğlu Tosun and Baradan (2009) [35]. However, the behavior for slag-modified concrete starts with a reduced baseline tensile strength (2.89 MPa), which is likely caused by the slower initial reactivity of the supplementary cementitious material. Nevertheless, the progressive pozzolanic contribution of steel slag is confirmed by the more pronounced 12.5% increase in tensile strength to 3.25 MPa at 1.4% exposure. It is characteristic of slag blended systems that have long-term performance benefits over initial strength reductions. This is in line with Varghese et al.’s (2019) [36] findings.
At peak exposure, the hybrid slag/RA mixture has intermediate characteristics, starting at 3.20 MPa and increasing 15.9% to 3.71 MPa. The recycled aggregate mix performs better than plain slag concrete, achieving high tensile strengths; however, remaining approximately 7% lower than the control mix at maximum level; thus, while some compensation is provided by the recycled aggregates for the initial weakness of plain slag concrete, they cannot provide the same tensile performance as conventional concrete as shown in Figure 10.
Notably, the RA exclusive mix possesses the highest initial (3.93 MPa) and final (4.32 MPa) strengths, which represent a 9.9% improvement over the RCA inclusive mix. Based on this performance, recycled aggregate is thought to act more advantageously with the cement matrix than when combined with slag, because better stress transfer to the aggregate occurs. If proper recycling aggregate processing yields concrete with improved tensile properties, then the outperformance of the RA mix over control and modified mixes is consistent. The practical implications of these findings are for projects emphasizing early age tensile strength, conventional or RCA exclusive mixes should be applied; slag containing mixes require longer curing time but are comparable to performance wise when initial strength is not critical; and slag/RA combination lies in between and may be appropriate for applications that require a compromise between sustainability and performance. In terms of performance, RCA concrete has a superior tensile performance, which supports the use of RCA concrete in structural elements under flexural stresses, as well as advancing circular economy objectives of construction. This is because the processed RCA mix outperforms all others, including unprocessed NA, from 3.93 MPa to 4.32 MPa, which indicates the beneficial interaction between coarse recycled aggregates and cement paste, which improves tensile stress transfer and fiber anchorage, in particular at moderate fiber levels [18].
It should be noted here that a direct comparison of the mechanical performance of the type of fibers used in the present study with hooked and deformed steel fibers is not the aim of the present study. These waste steel fibers used in the present study were collected and provided free of charge. Therefore, the use of this type of plane or straight waste steel fibers is environmentally and economically beneficial, but these types of fibers have limited mechanical anchorage in concrete because their bond relies only on friction and adhesion, not mechanical interlocking. Also, according to the literature [37,38,39,40,41], under load, they tend to be pulled out more easily, which leads to a decrease in the effectiveness of bridging cracks compared to embedded and deformed fibers. Therefore, plane and straight fibers are less effective unless high volumes are used, which may affect workability and cause agglomeration/clumping. However, according to the results obtained in the present study, an increase in compressive and tensile strengths can still be observed.

3.2.1. Analysis of Splitting Tensile Failure Modes

In this section, the failure behavior of cylindrical concrete specimens tested according to ASTM C496 (2024) [28] in splitting tensile strength testing is discussed. Crack pattern, post-crack behavior and general failure mechanism are studied as a function of different dosages of steel fibers (0%, 0.2%, 0.8%, 1.4%). The observed failure modes for each fiber content level are shown in the figures.
Plain Concrete—0% WSRF
The specimen with 0% steel fiber represents the baseline behavior of unreinforced concrete under tensile stress. As shown in Figure 11a, the failure mode is purely brittle. A vertical, clean, sharp crack forms across the diameter, splitting the specimen into two completely detached halves. Once the tensile capacity is exceeded, the crack propagates rapidly with little resistance to crack initiation and no post-peak load-carrying capacity. No bridging mechanism is visible across the crack, and no energy absorption occurs beyond the peak tensile stress. Such failure is expected in conventional concrete, where tensile strength is intrinsically poor, and failure occurs in a catastrophic manner. The 0% fiber specimen fails catastrophically because concrete has inherently low tensile strength and lacks crack-bridging mechanisms. Once cracking starts, plain concrete cannot resist tensile stress and will fracture rapidly with no energy absorption and post-peak load capacity, as reported by Carpinteri and Brighenti (2009) [42].
Low Fiber Content—0.2% WSRF
The addition of 0.2% steel fibers results in a subtle but meaningful change in the failure mode. As illustrated in Figure 11b the internal resistance to propagation of the crack pattern remains vertically aligned only with a slight increase in vertical pattern jaggedness. Crack opening fibers engage some fibers that provide minimal bridging action. The specimen still splits into two main halves, but the fracture is less severe than the 0% fiber specimen. Fibers help in distributing tensile stress slightly beyond the initial crack and delay complete failure. The fact that this is a transitional behavior implies that low fiber volumes can still improve crack control and delay brittleness. The 0.2% fiber mix exhibits transitional failure behavior with less severe cracking. Fibers start to bridge microcracks and slightly delay fracture and decrease brittleness. This is in according with Khan (2019) [43], who reported that even low fiber volume improves tensile stress distribution and crack resistance at early stages of concrete.
Moderate Fiber Content—0.8% WSRF
The specimen with 0.8% fiber content exhibits a significant change in failure mode. Figure 11c clearly shows a more ductile response. A central vertical crack is still visible, but the two halves are more or less intact, with many fibers bridging across the fracture, holding the crack together. This dosage of the fiber reinforcement arrests crack widening and prevents complete structural separation. There is a transition from brittle to quasi-ductile failure mode with higher energy absorption and better toughness. This dosage is a threshold above which the fibers dominate post-crack load transfer and structural integrity. The quasi-ductile failure of the 0.8% fiber specimen is due to fibers that effectively bridge cracks and prevent separation. This is in agreement with the findings of Carpinteri and Brighenti (2009) [42], who demonstrate that moderate fiber content leads to a significant increase in energy absorption, arrest of crack propagation, and ductile behavior in FRC.
High Fiber Content—1.4% WSRF
Figure 11d presents the most advanced behavior. The specimen with 1.4% steel fiber is observed. The failure is highly controlled here. The specimen shows almost no physical separation, but a fracture line exists, which is very narrow. The tensile stresses are distributed widely and uniformly throughout the internal fiber network, which is dense enough to do so. The fibers still carry loads after matrix cracking, improving the residual tensile capacity. The specimen remains cylindrical, and fibers provide strong pullout resistance. This failure mode signals a material with excellent post-peak toughness and crack resistance, a highly ductile material, which is ideal for structural and high-performance concrete applications. The specimen has a high fiber content of 1.4%, and the failure is very ductile with little separation and narrow cracking. Tensile stress is distributed across tens of thousands of fiber networks to maintain integrity post-cracking. Liu et al. (2016) [16] agree that high fiber volumes improve pullout resistance, residual strength, and toughness, which are good for structural applications.
Table 5 summarizes the progressive enhancement in failure characteristics with increasing fiber dosage. At higher fiber contents, the failure transitions from sudden and brittle to ductile and energy absorbing. Minimal crack opening and full post-crack structural retention were achieved in the 1.4% fiber sample.
Visual inspection and photographic documentation of the failure modes show that waste steel rivet fibers play a major role in improving the tensile behavior of concrete. On the other hand, plain concrete fails catastrophically when it reaches its tensile limit, but fiber-reinforced concrete, especially at higher dosages, fails in a much more desirable way. Fibers contribute to delayed cracking, reduced separation, and post-peak load redistribution beginning at 0.2%. The transition to ductile behavior is clearly at 0.8%, and the specimen has advanced mechanical resistance with good crack control at 1.4%. Based on these findings the use of WSRF is compatible with structural applications where tensile performance, resistance to crack formation, and ductility are features.

3.3. Relationship Between Compressive and Tensile Strengths, and the Fiber Content

As shown in Figure 12 the 3D plot shows the relationship between compressive strength, tensile strength, and the WSRF content in concrete mixtures. The blue spheres represent the individual experimental data points from different fiber content percentages (0%, 0.2%, 0.8%, and 1.4%) and four types of concrete mixes (Control, Slag, Slag+RA, and RA). Each point accurately reflects the measured compressive and splitting tensile strength at the fiber dosage in each case.
A brown curved surface was constructed to visually guide the understanding of how strength properties change as a function of changing fiber content. The compressive and tensile strengths are lower in the lower right corner and the surface begins there and slopes upward to the upper left corner, where the strengths and fiber contents are higher. This indicates a positive relationship between the fiber content and mechanical properties as the dosage increases.
The compressive and tensile axes are boldly labeled, and the fiber content is indicated externally on the right side of the plot by a clear, vertical label, which keeps the style clean and modern and avoids repetitive labeling. What we find out is that this visualization also tells us the general trend and the trend is that as WSRF content increases, compressive and tensile strengths increase, but at different rates based on the mix type. Though there are small local variations at the points, the overall behavior is captured by the carefully curved surface. The compressive and tensile strengths are positively correlated with steel fiber content, as shown in the 3D plot. According to Plagué, et al.(2017) [44], higher fiber dosage increases crack resistance and load transfer. This trend is then captured in the curved surface of which shows a clear visualization of this fiber’s positive gain to strength across mix types.

3.4. Dry Density

Figure 13 presents the bulk density (dry) of various concrete mixtures, Control, Slag, Slag+RA, and RA, in kg/m3. Bulk density is defined as the mass per unit volume of dry concrete and is an important property that indicates the material’s compactness, porosity, and structural integrity. The Control mix starts at a bulk density of 2410 kg/m3 at 0% content and continues to increase to 2494 kg/m3 at the maximum replacement level (1.4%). This trend upward suggests that the compactness is improving with increasing replacement, possibly due to better particle packing or hydration effects. Once again, the dead load disadvantages of steel fibers are shown. Bulk densities of the Slag mix are consistently greater than those of the control mix at all replacement levels. At 2417 kg/m3 it starts, and at the 1.4% level it reaches 2508 kg/m3. The increase is gradual and gradual, implying that slag with a finer particle size and a higher slag specific gravity increases the overall concrete density. This denser structure suggests a lower porosity, and better long-term durability and environmental resistance. The Slag/RA mix has an even stronger trend. It increases continuously from 2410 kg/m3 to 2502 kg/m3 at 1.4% replacement. This shows that recycled aggregates and slag are showing a synergistic effect, resulting in high bulk density, which may indicate that the slag helps with better particle interlocking and lower void content. This combination makes the concrete more compact and potentially more durable. The bulk density of the RA mix also increases from 2405 kg/m3 at 0% to 2485 kg/m3 at 1.4%. Although slightly lower than the slag-containing mixtures, it still exhibits a positive trend, which supports the notion that recycled aggregates do contribute to density enhancement, albeit to a lesser extent when used without slag. Overall, the bulk densities for the Slag/RA mix are consistently lower than the Slag mix, which in turn is higher than the RA mix, which is higher than the Control mix at all replacement levels. The data clearly shows that among those aggregates, the addition of slag and recycled aggregates increases the dry bulk density of concrete, which thereby corresponds to the mechanical properties and durability of concrete.
The compactness of concrete mixes is increased by slag and recycled aggregates. Vijayaraghavan, et al., (2017) [45] confirm that slag’s fine particles and high specific gravity increase density, and RCA’s angularity increases interlocking. Optimal packing and reduced porosity are indicated by the highest bulk density in the Slag/RA mix.

3.5. Absorption Characteristics

Figure 14 shows the water absorption (%) after 24 h immersion for all concrete mixes, including Control, Slag, Slag/RA, and RA mixes reinforced with different volumes of waste steel rivet fibers (WSRFs), 0% to 1.4%. Water absorption is an important parameter of porosity and, therefore, durability and resistance of concrete in wet and freeze-thaw conditions. The Slag mix showed the highest absorption among the tested mixes, 5.25% (0% fiber) and 5.59% at 1.4% fiber. This goes against standard expectations that slag should decrease porosity, implying that under the current mix design, slag may induce higher micro-capillarity or incomplete densification—possibly because of excessive fiber dosage inhibiting the pozzolanic activity [46]. The Slag/RA mix demonstrated a slightly lower but stable absorption tendency, increasing from 5.01% to 5.12%. The interaction of slag’s fine particles with the rough texture of recycled aggregates probably enhanced particle packing and reduced pore connectivity. This intermediate behavior is a compromise between the porosity increase from RCA and the densification effects of slag [47]. The Control mix demonstrated moderate absorption values that increased from 4.89% to 5.02% with higher fiber content. The trend implies that fiber clustering at high volumes can cause internal void development, slightly increasing porosity [48]. Interestingly, the RA mix showed the lowest absorption values overall, beginning at 4.84% and decreasing to 4.76% at 0.8% fiber content before rising again. This was surprising because recycled aggregates are usually more porous. However, the processed RA used in this study probably had a denser structure and better surface quality, which would reduce water uptake, as reported by Al-Rekabi et al. (2023) [49]. In summary, water absorption was ranked as follows from highest to lowest: Slag > Slag/RA > Control > RA. Moderate fiber content (especially 0.8%) seems to enhance compaction and minimize absorption in most mixes but excessive fiber dosage (1.4%) may compromise matrix integrity. Such results emphasize the significance of optimized mix design in incorporating alternative materials for durability in sustainable concrete applications.

4. Conclusions

“The development of sustainable low-carbon concrete reinforced with WSRF, utilizing PRCA, and SSP was examined in this study. The results show that using these elements together can result in concrete that meets durability and mechanical requirements while supporting the goals of the circular economy. Adding 0.8% WSRF offered the best compromise between compressive and tensile strengths, while the fibers successfully improved resistance to cracking. The processed RCA increased pore structure and decreased absorption, even though slag mixes showed somewhat higher water absorption. A total of 15% SSP, 40% PRCA, and 0.8% WSRF make up the recommended ideal mix design, which has favorable mechanical qualities, durability, and environmental advantages. All things considered, optimizing fiber dose and processing recycled materials correctly are essential to getting the intended results. To support large-scale practical applications, future research should concentrate on flexural performance, including deformation behavior, and long-term durability under actual environmental conditions.

Author Contributions

Conceptualization, D.D.A. and B.M.A.H.; Data curation, A.F.H.S.; Formal analysis, B.M.A.H.; Methodology, D.D.A.; Project administration, B.M.A.H.; Resources, D.D.A.; Supervision, B.M.A.H.; Writing—original draft, D.D.A.; Writing—review and editing, B.M.A.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The datasets used and/or analysed during the present study are available from the corresponding author on reasonable request.

Acknowledgments

The authors would like to thank the Civil and Environmental Engineering Department, Faculty of Engineering, Soran University, for their assistance and laboratory facilities. The authors express their deep gratitude to the Hend Steel factory and local industries for providing the materials. We are also grateful to the local Soran/Barzewa quarry.

Conflicts of Interest

The authors declare no conflict of interest.

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Fibers 13 00109 g001
Figure 1. (a) Cement powder. (b) Steel slag powder.
Figure 1. (a) Cement powder. (b) Steel slag powder.
Fibers 13 00109 g001
Fibers 13 00109 g002aFibers 13 00109 g002b
Figure 2. (a) NCA 12.5 mm, (b) RCA 6 mm, (c) RCA 8 mm, (d) RCA 12.5 mm.
Figure 2. (a) NCA 12.5 mm, (b) RCA 6 mm, (c) RCA 8 mm, (d) RCA 12.5 mm.
Fibers 13 00109 g002aFibers 13 00109 g002b
Fibers 13 00109 g003
Figure 3. Crusher plant.
Figure 3. Crusher plant.
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Fibers 13 00109 g004
Figure 4. WSRF.
Figure 4. WSRF.
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Figure 5. Curing concrete specimens.
Figure 5. Curing concrete specimens.
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Fibers 13 00109 g006
Figure 6. (a) Splitting tensile strength and (b) compressive strength tests.
Figure 6. (a) Splitting tensile strength and (b) compressive strength tests.
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Figure 7. Compressive strength of concrete mixes.
Figure 7. Compressive strength of concrete mixes.
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Fibers 13 00109 g008
Figure 8. Compressive strength net change of control, SSP, Slag+RA, and RA concrete mixes.
Figure 8. Compressive strength net change of control, SSP, Slag+RA, and RA concrete mixes.
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Figure 9. Tensile strength of concrete mixes.
Figure 9. Tensile strength of concrete mixes.
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Figure 10. Splitting tensile strength net change of control, SSP, Slag+RA, and RA concrete mixes.
Figure 10. Splitting tensile strength net change of control, SSP, Slag+RA, and RA concrete mixes.
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Fibers 13 00109 g011
Figure 11. Failure mode of (a) 0%, (b) 0.2%, (c) 0.8% and (d) 1.4% WSRF concrete samples.
Figure 11. Failure mode of (a) 0%, (b) 0.2%, (c) 0.8% and (d) 1.4% WSRF concrete samples.
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Figure 12. STS and CS curve corresponding to WSRF content.
Figure 12. STS and CS curve corresponding to WSRF content.
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Figure 13. Bulk density of concrete mixes.
Figure 13. Bulk density of concrete mixes.
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Figure 14. Water absorption of concrete mixes.
Figure 14. Water absorption of concrete mixes.
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Table 1. Chemical composition comparison between cement and steel slag powder.
Table 1. Chemical composition comparison between cement and steel slag powder.
Oxide/ElementCement (%)Steel Slag Powder (%)
CaO (Calcium Oxide)66.236.2
SiO2 (Silicon Dioxide)14.619.7
MgO (Magnesium Oxide)4.13.6
SO3 (Sulfur Trioxide)4.10.1
Al2O3 (Aluminum Oxide)4.05.2
Fe2O3 (Ferric Oxide)3.731.9
K2O (Potassium Oxide)2.50.1
Others0.22.6
Table 2. Physical properties of NFA, NCA, and RCA.
Table 2. Physical properties of NFA, NCA, and RCA.
PropertyNFANCARCA
TypeNatural river sandNatural crushed stoneProcessed demolished concrete
Maximum size (mm)4.7512.512.5
Specific gravity (ssd)2.602.6382.42–2.55
Water absorption (%)1.990.623.5–6.0
ShapeRoundedAngularAngular with low-adhered paste
Surface textureSmoothRoughRougher due to residual paste
Cleanliness (IMPURITY CONTENT)LowHighClean
GradationWell-gradedLow-gradedWell-graded (after processing)
SourceLocal river Local quarry Soran city demolition waste
Table 3. Mechanical and geometrical properties of WSRF. (Source: Soran local industries and Ali et al., 2020).
Table 3. Mechanical and geometrical properties of WSRF. (Source: Soran local industries and Ali et al., 2020).
PropertyValueUnit/Description
Fiber typeWaste steel rivet fibersFrom discarded rivets and metal scraps
ShapeStraight, low ribbedAs collected from waste
Length (ave)22.54mm
Diameter (ave)1.63mm
Aspect ratio (l/d)13.83—
Density7750kg/m3
SourceLocal manufacturersSoran, Kurdistan Region
Table 4. Concrete mixture proportions.
Table 4. Concrete mixture proportions.
Mix NoMix CodeWSRF (%)SSP
(%)		Cement (kg/m3)NFA (kg/m3)NCA (kg/m3)RCA (%)Water (kg/m3)SP (%)
1C00380760114001850.5
2C0.20.20380760114001850.5
3C0.80.80380760114001850.75
4C1.41.40380760114001851.0
5S015323760114001850.5
6S0.20.215323760114001850.5
7S0.80.815323760114001850.75
8S1.41.415323760114001851.0
9SRA015323760684401850.5
10SRA0.20.215323760684401850.5
11SRA0.80.815323760684401850.75
12SRA1.41.415323760684401851.0
13RA00380760684401850.5
14RA0.20.20380760684401850.5
15RA0.80.80380760684401850.75
16RA1.41.40380760684401851.0
WSRF: waste steel rivet fiber; SSP: steel slag powder; NFA: natural fine aggregate; NCA: natural coarse aggregate; RCA: recycled concrete aggregate; SP: superplasticizer.
Table 5. Comparative evaluation of specimen failure mode.
Table 5. Comparative evaluation of specimen failure mode.
Fiber ContentCrack PatternSeparationCrack WidthDuctilityPost-Crack Integrity
0.0%Clean, straightFullWideBrittleNone
0.2%Jagged, verticalPartialMediumSlightLow
0.8%Stable, bridgedMinorNarrowDuctileModerate to high
1.4%Bridged, stableLowVery narrowHighly ductileExcellent
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Awla, D.D.; Herki, B.M.A.; Sherwani, A.F.H. Low-Carbon Concrete Reinforced with Waste Steel Rivet Fibers Utilizing Steel Slag Powder, and Processed Recycled Concrete Aggregate—Engineering Insights. Fibers 2025, 13, 109. https://doi.org/10.3390/fib13080109

AMA Style

Awla DD, Herki BMA, Sherwani AFH. Low-Carbon Concrete Reinforced with Waste Steel Rivet Fibers Utilizing Steel Slag Powder, and Processed Recycled Concrete Aggregate—Engineering Insights. Fibers. 2025; 13(8):109. https://doi.org/10.3390/fib13080109

Chicago/Turabian Style

Awla, Dilan Dh., Bengin M. A. Herki, and Aryan Far H. Sherwani. 2025. "Low-Carbon Concrete Reinforced with Waste Steel Rivet Fibers Utilizing Steel Slag Powder, and Processed Recycled Concrete Aggregate—Engineering Insights" Fibers 13, no. 8: 109. https://doi.org/10.3390/fib13080109

APA Style

Awla, D. D., Herki, B. M. A., & Sherwani, A. F. H. (2025). Low-Carbon Concrete Reinforced with Waste Steel Rivet Fibers Utilizing Steel Slag Powder, and Processed Recycled Concrete Aggregate—Engineering Insights. Fibers, 13(8), 109. https://doi.org/10.3390/fib13080109

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