Behavior of Lightweight Self-Compacting Concrete with Recycled Tire Steel Fibers

Abstract

The utilization of recycled materials in concrete technology has gained significant attention in recent years, promoting sustainability and resource conservation. This paper investigates the behavior of lightweight self-compacting concrete (LWSCC) with recycled tire steel fibers (RTSFs). The effects of RTSFs on the flowability of the composite material and its density were assessed. The mechanical properties of the developed material were examined and beam tests were performed, aiming to assess its feasibility for structural applications. The compressive and tensile strengths were determined to evaluate the mechanical properties of the developed concrete mixtures. The beam tests were conducted to assess the flexural behavior of the beam specimens. Three different steel fiber contents of 0, 0.5, and 1% volumetric fractions of concrete were used in this study. The test results indicate that incorporating the fibers did not negatively impact the flowability and density of the LWSCC mixtures. In addition, the use of RTSFs enhanced the tensile strength of the developed concrete mixtures, where fibrous concrete showed increases in the splitting tensile strength in the range of 38 to 76% over that of non-fibrous concrete. On the other hand, the compressive strength of the mixtures was not affected. The test beams with RTSFs exhibited improved flexural performance in terms of delaying and controlling cracking, enhancing ultimate load, and increasing ductility. Compared with the control non-fibrous beam, the increases in the cracking load, ultimate load, and ductility index were up to 63.8, 9.3, and 16%, respectively. The test results of the beams were compared with theoretical predictions, and good agreement was found.

1. Introduction

Lightweight concrete (LWC) has been used in the construction industry for many years [1,2,3,4,5]. The development of this type of concrete was motivated by the necessity to decrease the structural dead load in buildings and structures made of concrete. While traditional concrete is robust and adaptable, its weight can pose challenges in terms of transportation, handling, and overall structural efficiency. In response to these challenges, LWC was developed with reduced density without compromising its mechanical and structural properties. On the other hand, self-compacting concrete (SCC) has emerged as a construction material with the ability to flow and compact under its weight, eliminating the need for external compaction [6]. SCC improves construction efficiency, minimizes labor needs, and ensures uniformity in complex structures. These benefits have led to an increased adoption of SCC in diverse applications within the construction industry [7,8,9,10,11,12]. However, the use of SCC is limited in some applications where concrete with a lighter weight is required. This motivated the development of lightweight self-compacting concrete (LWSCC).

The combination of LWC and SCC in LWSCC results in a material that is not only lighter but also capable of offering improved workability and durability. LWSCC presents a promising solution for various construction applications demanding both lightweight properties and superior performance [13,14,15]. In addition, the use of LWSCC can improve the thermal and sound insulation properties. Research efforts have been undertaken to advance the development of LWSCC [13,14,15,16,17,18,19]. This speeds up its adoption in a variety of applications within the construction industry [20,21,22,23]. However, there are some concerns about the performance of structural elements fabricated with LWSCC. These concerns have arisen from the fact that lightweight aggregate (LWA) is relatively weak compared with the normal-weight aggregate (NWA) typically employed in normal-weight concrete (NWC). Furthermore, the formulation of SCC necessitates reduced size and quantity of coarse aggregate. These features related to the development of LWSCC may adversely affect its mechanical characteristics, thereby influencing the structural performance of reinforced concrete members constructed with this type of concrete. Recent studies have addressed these concerns. The experimental study reported by Ranjbar et al. [24] showed that LWSCC with expanded polystyrene polymeric beads as a partial replacement of NWA exhibited lower compressive and tensile strength in comparison to normal-weight SCC (NWSCC). Similar results were obtained by Wan et al. [25], in whose study different types of LWAs, including perlite, scoria, and polystyrene, were used for developing LWSCC. Chen et al. [26] reported that the modulus of elasticity of LWSCC was about 60–85% of that of NWSCC. The flexural behavior of reinforced concrete beams fabricated using LWSCC was observed to be inferior to that of counterpart beams made with NWSCC, as reported by Omar and Hassan [27]. LWSCC beams showed larger deflection, lower cracking and ultimate capacities, and lower ductility. Similar deficiencies were observed concerning the shear behavior of LWSCC beams. The shear strength of LWSCC beams was 15 to 25% lower than that of NWSCC beams [28,29,30].

One of the practical solutions to overcome the structural deficiencies that may be associated with LWSCC is the addition of discrete fibers to the LWSCC mixture. The inclusion of fibers is known to enhance the tensile strength, flexural behavior, shear strength, and toughness of concrete, making it more resistant to cracking and improving its overall performance. Recent experimental investigations have validated the beneficial impacts of incorporating fibers into LWSCC mixtures [31,32,33,34]. With the growing emphasis on sustainable practices in the construction industry, researchers have started to explore the incorporation of recycled materials into concrete, reducing the environmental impact associated with traditional concrete production [35,36,37]. Recycled tire steel fibers (RTSFs), derived from discarded tires, present a promising opportunity to enhance the properties of LWSCC while addressing the issue of waste management. Furthermore, the use of RTSFs aligns with the principles of circular economy and sustainable development by repurposing waste materials. This not only reduces the environmental footprint linked with tire disposal but also contributes to resource conservation by incorporating recycled materials into the construction industry. In fact, RTSFs have been utilized in conventional concrete, demonstrating similar enhanced performance to that observed in the case of engineered steel fibers [38,39,40,41]. Yet, the use of RTSFs in LWSCC has not been explored.

The aim of this study is to examine the feasibility of using RTSFs within LWSCC. The fresh and hardened performances of RTSF-reinforced LWSCC (RTSF-LWSCC) mixtures are characterized. The study also includes the testing of beams constructed by using RTSF-LWSCC. The flexural behaviors of the beams is investigated considering the effects of RTSFs on the cracking, deflection, ductility, and load carrying capacity of the beams. The cracking and ultimate load capacities of the beams were predicted by using ACI provisions and compared with the experimental results.

2. Experimental Investigation

Three LWSCC mixtures were developed in this investigation. One of the mixtures (RF–00) was without RTSFs, serving as a control, whereas the other two mixtures (RF–0.5 and RF–1.0) included RTSFs with volumetric contents of 0.5 and 1.0%, respectively. These mixtures were also used in fabricating the beams tested in this investigation.

2.1. Materials

2.1.1. Cement Matrix

The cementitious matrix formulated in this study was composed of ordinary Portland cement (PC), class F fly ash (FA), silica fume (SF), and limestone powder as an eco-friendly alternative (LP). PC, FA, and SF satisfied ASTM specifications C150 [42], C618 [43], and C1240 [44], respectively. The median sizes of the powders were calculated by using a laser particle size distribution (PSD) analyzer, Horiba model LA-950. The PSD analyses of all powders are shown in Figure 1. The median sizes of PC, FA, SF, and LP were 11, 11, 0.22, and 19 µm, respectively. The chemical analyses of the fine powders are shown in

Table 1. Chemical compositions of cement and powders used in this study.

	CaO	MgO	SiO2	Al2O3	Fe2O3	SO3	P2O5	TiO2	MnO	Na2O	K2O	Cl	L.O.I
PC	64.14	0.71	20.41	5.32	4.1	2.44	0.04	0.3	0.07	0.1	0.17	0.01	2.18
FA	0.9	0.6	53.2	27.3	4.03	0.2	0.4	1.52	0.03	0.22	1.22	0.05	10.02
SF	0.15	0.75	91.3	0.35	0.05	0.86	0.06	-	0.03	0.4	0.8	0.1	5.1
LP	54.41	0.45	1.8	0.45	0.66	0.46	0.01	0.04	-	0.06	0.03	0.03	41.6

.

2.1.2. Aggregates

The LWSCC developed in this investigation was produced by utilizing both fine and coarse lightweight scoria aggregates derived from natural volcanic rock found along the coast in the western part of Saudi Arabia. The lightweight characteristic of the scoria aggregates is attributed to their porous structure, featuring pore sizes reaching up to 450 µm. The optical microscopic analysis of the scoria rock grains is shown in Figure 2. The sieve analysis of the combined fine and coarse scoria aggregates is shown in Figure 3, with a nominal maximum size of 9.5 mm and about 75% of the sizes being below 4.75 mm.

Table 2. Comparison of the actual grading and ASTM C330/C330M requirements [45] for combined fine and coarse lightweight aggregates.

		Passing %
Nominal Size Designation		9.5 mm	4.75 mm	2.36 mm	300 μm	150 μm	75 μm
Combined fine and coarse aggregates 9.5 mm to 0	ASTM C330 requirements	90–100	65–90	35–65	10–25	5–15	0–10
	Actual	96.67	74.75	60.74	31.94	15	9.64

gives a comparison between the actual grading of the combined fine and coarse lightweight aggregates shown in Figure 3 and the grading requirements of the ASTM C330 standard [45]. The actual grading of the used aggregate satisfies all the requirements of the ASTM C330 standard [45] except for the 300 μm sieve size. The actual passing percentage for this size is slightly higher than the standard requirement.

Table 3. Physical properties of scoria aggregates.

Aggregate	Dry Loose Bulk Density (kg/m3)	Rodded Bulk Density (kg/m3)	Particle Density (kg/m3)	Water Absorption (%)
Fine lightweight scoria aggregate [0–4.75 mm]	790	1028	1710	14
Coarse lightweight scoria aggregate [4.75–9.5 mm]	416	506	1260	22

details the physical properties of the scoria aggregates, including their water absorption capacity measured in a saturated surface dry (SSD) state.

2.1.3. Recycled Tire Steel Fibers (RTSFs)

The RTSFs utilized in this investigation were procured from a local company, employing a multi-step procedure involving tire recycling and steel fiber extraction. Upon the collection of the discarded tires, they are subjected to shredding, where they are broken down into smaller pieces by using specialized machinery. Subsequently, the shredded tire material undergoes separation techniques to isolate the steel fibers from the rubber and other constituents. Once the steel fibers are extracted, they undergo a cleaning process to eliminate any remaining impurities or contaminants. Figure 4 displays the RTSFs used in this study, and the physical properties of the fibers are given in

Table 4. Physical properties of RTSFs.

Properties	RTSFs
Diameter (μm)	195–380
Length (mm)	10–35
Aspect ratio	26–180
Density (gm/cm3)	7.85

. The lengths of the fibers varied between 10 and 35 mm.

2.1.4. Steel Bars

Deformed steel bars were used for reinforcing the test beams. The main tension steel had a diameter of 10 mm, whereas the diameters of the compression and transverse steel bars were 8 mm. The tensile properties of the bars were determined as per the ASTM A370 standard [46]. The obtained results, based on average values, are presented in

Table 5. Steel bars’ tensile properties.

Bar Diameter (mm)	Yield Stress (MPa)	Ultimate Tensile Stress (MPa)	Elastic Modulus (GPa)
8	460	523	200
10	503	572	200

.

2.2. Concrete Mixture Composition

The composition of the three LWSCC mixtures developed in this study is shown in

Table 6. Concrete mixture proportions.

Concrete Mix	Material Quantities (kg/m3)
	Cement	Fly Ash	Silica Fume	Limestone Dust	Fine Aggregate	Coarse Aggregate	Superplasticizer	Water	Steel Fibers
RF–00	442	149	57	158	644	94	17.5	253	0
RF–0.5	442	149	57	158	644	94	17.5	253	39
RF–1.0	442	149	57	158	644	94	17.5	253	78

. A water–binder ratio of 0.31 was used in the mixtures. The content of PC represented 55% of the total binder, whereas the contents of FA, SF, and LP were 18.5, 7, and 19.5%, respectively. The fine-to-coarse aggregate ratio of 6.85 was applied to ensure optimal distribution of recycled fibers. A polycarboxylate-based superplasticizer served as a high-range water-reducing agent in the concrete mixtures. RTSFs were incorporated into concrete mixtures FR–0.5 and RF–1.0 at the volumetric ratios of 0.5 and 1.0%, respectively, while concrete mixture RF–00 acted as a control, devoid of steel fibers.

The mixing procedure began by homogenizing the dry powders, starting with PC, followed by the additions of FA, SF, and LP. Subsequently, water containing the optimal dosage of superplasticizer was added and mixed, followed by the addition and mixing of aggregates. Finally, the steel fibers were added by using a custom-made sieve with dimensions of 800 mm × 800 mm and a mesh opening of 1.2 mm × 1.2 mm. Figure 5 illustrates different stages during the mixing process.

2.3. Concrete Tests

The concrete properties of the three mixtures were determined in the fresh and hardened states. The tests conducted in the fresh state included slump flow, slump flow time (T50), J-ring flow, and fresh concrete density. The flow tests provide insights into the workability and flow characteristics of the concrete mixtures upon initial placement. In the slump flow test, both the flow diameter and the time taken to reach a diameter of 500 mm were recorded. In the J-ring test, the flow diameter in the presence of steel bars was measured. The flow tests were carried out in accordance with the ASTM C1611 [47] and C1621 [48] standards, whereas the fresh concrete density was carried out as per the ASTM C138 [49] standard.

The concrete hardened properties were determined by testing the compressive and splitting tensile strengths of concrete. For each mixture, three cylindrical specimens measuring 100 mm in diameter and 200 mm in height were prepared for either the compressive (ASTM C39) [50] or splitting test (ASTM C496) [51]. The cylindrical concrete specimens were tested at the age of 28 days.

2.4. Beam Tests

A total of six beams with dimensions of 120 mm in width, 180 mm in depth, and 1500 mm in length were constructed by using the three concrete mixtures developed in this study. Two replicate beams were cast for each concrete mixture. The beams were reinforced in the longitudinal direction with two bars of 10 mm in diameter along the tension side and with two bars of 8 mm in diameter along the compression side. Additionally, 8 mm diameter stirrups were spaced at 75 mm throughout the beams to prevent shear failure. The typical dimensions and reinforcement configurations are depicted in Figure 6.

All beams were simply supported and tested in flexure under a four-point loading setup as shown in Figure 6. A pure moment zone 400 mm long between the two concentrated loads was kept constant for all beams. Linear variable differential transducers (LVDTs) were used to measure the displacements at the midspan of the beams and beneath each concentrated load. In the midspan region, electrical resistance strain gauges with a gauge length of 6 mm were used to measure the tensile strains in the longitudinal steel bars, while the strain gauges with a gauge length of 60 mm were used to measure the compressive strains on the top concrete surface. Crack openings resulting from the applied load were measured by using small LVDTs. The load was applied monotonically at a displacement rate of 0.5 mm/min by using an AMSLER testing machine (Figure 7). All measurement data were continuously recorded throughout the entire testing period by using a data acquisition system.

3. Results of Concrete Tests

3.1. Fresh Properties

Table 7. Concrete fresh-state characteristics.

Concrete Mix	Slump Flow (mm)	T50 (sec)	J-Ring Flow (mm)	Density (kg/m3)
RF–00	830	7.6	820	1706
RF–0.5	810	8	790	1785
RF–1.0	780	11	695	1813

provides the test results of the fresh properties, including slump flow, T50, J-ring flow, and concrete density. These findings are also illustrated in Figure 8 and Figure 9. It can be noted that as the fiber content increases, the slump flow slightly decreases. The slump flow of concrete mixture RF–0.5 exhibited a 2.4% decrease compared with the control mixture, RF–00. With the further increase in fiber content, a reduction of 6.0% in the slump flow was observed for concrete mixture RF–1.0. Nevertheless, the reported slump flow values exceeded the lower limit of 650 mm specified in the EFNARC document [52] for SCC. Table 7 also indicates that with the increase in fiber content, there was a corresponding increase in T50. The J-ring flow result shows similar performance to that of slump flow for mixture RF–0.5, experiencing a 3.6% decrease compared with the control mixture, RF–00. On the other hand, when the fiber content was increased to 1%, a more noticeable reduction of 15% was obtained (see Table 7 and Figure 8).

The addition of RTSFs was also found to slightly affect the fresh concrete density of the developed mixtures as given in Table 6 and illustrated in Figure 9. Incorporating 0.5 or 1.0% RTSFs led to density increases of 4.6 and 6.3%, respectively. Despite this increase, the densities of the concrete mixtures remained within the range of the unit weight of structural LWC specified by both the ACI 318 code [53] and Eurocode 2 [54].

3.2. Compressive Strength

Table 8. Test results of concrete compressive and splitting tensile strength.

Concrete Mix	Average Compressive Strength (MPa)	Average Splitting Tensile Strength (MPa)
RF–00	34.2	2.9
RF–0.5	35.3	4.0
RF–1.0	34.7	5.1

summarizes the test results of the compressive strength of the concrete mixtures, expressed in terms of their average values. The results are also graphically presented in Figure 10. It can be noted that the addition of steel fibers did not affect the compressive strength of the tested mixtures significantly. Mixtures containing steel fibers exhibited only a marginal enhancement in compressive strength compared with the control mixture.

3.3. Splitting Tensile Strength

Unlike the compressive strength, the inclusion of steel fibers appeared to have a substantial impact on the splitting tensile strength of the concrete mixtures, evident from both Table 8 and Figure 10. Introducing a fiber content of 0.5% resulted in a 38% enhancement in the tensile strength compared with the control mixture. Further increasing the fiber content to 1.0% correspondingly yielded a 76% increase. The improved tensile strength is attributed to both the high tensile strength of the steel fibers and the effective bonding between these fibers and the matrix. The level of tensile strength enhancement achieved with RTSFs in this study was similar to the results reported by Alkhattat and Al-Ramahee [31] using engineered steel fibers in LWSCC. This finding indicates that RTSFs perform similarly to engineered steel fibers in improving the tensile strength of LWSCC.

4. Beam Test Results

4.1. Load Capacity and Mode of Failure

All tested beams experienced a typical flexural failure mode characterized by the yielding of the main steel reinforcement, followed by the crushing of concrete in the constant moment zone, where the bending moment is the highest. Additionally, the control beams experienced rupture of the longitudinal tension steel bars, which occurred simultaneously with concrete crushing. Figure 11 illustrates the failure modes of the tested beams.

The experimental load capacities of the beams are given in

Table 9. Cracking, yielding, and ultimate load capacities of tested beams.

Beam	Cracking Load (kN)		Yielding Load (kN)		Ultimate Load (kN)
	Each	Average	Each	Average	Each	Average
B–0–1	8.6	8.0	45.1	44.5	57.6	55.9
B–0–2	7.4		43.8		54.1
B–0.5–1	12.6	12.2	52.6	50.8	58.3	59.1
B–0.5–2	11.8		48.9		59.9
B–1.0–1	12.6	13.1	54.5	54.1	58.8	61.1
B–1.0–2	13.6		53.7		63.4

, including cracking, yielding, and ultimate load capacities. Also, the effect of RTSFs on the load capacities of the beams is presented in Figure 12. The beams exhibited cracking initiation in the early stage of loading, with the control beam (B–0) without fibers experiencing an average cracking load of 8.0 kN. The addition of fibers delayed the onset of cracking in beams B–0.5 and B–1.0 with 0.5 and 1.0% fiber contents, respectively. Beam B–0.5 showed an increase of 52.5% in the cracking load compared with the control beam, while beam B–1.0 showed a corresponding increase of 63.8%. The higher tensile strength of fibers allows for strengthening the concrete matrix, thereby preventing cracks from appearing early. Once the cracks appear, the fibers function to restrict the opening of the cracks and effectively transfer tensile stresses across the crack faces. This beneficial effect of the fibers contributes to increasing the yielding and ultimate loads of the beams, as can be noted from Table 9 and Figure 12. The yielding loads of the two fibrous beams B–0.5 and B–1.0 increased by 14.2 and 21.6%, respectively, in comparison to the control beam, B–0. On the other hand, beam B–0.5 showed an increase in the ultimate load of 5.7% over that of the control beam, while the corresponding increase in the ultimate load for beam B–1.0 was 9.3%. This result indicates that the increase rate in the yielding load is more pronounced than that of the ultimate load. This is because before the yielding of the steel bars, the fibers are more efficient in restricting the opening of the cracks and transferring the tensile stresses. Once the steel bars begin to yield, the strain in the bars increases at a significant rate, leading to a dramatic widening of the cracks. This excessive widening of the cracks results in the fibers being pulled out, thereby reducing their contribution to the ultimate capacity of the beams. The test results reported by Alkhattat and Al-Ramahee [31] indicated that the ultimate load increased by 11% when a 1% volumetric fraction of engineered steel fibers was used in LWSCC beams. This outcome closely matches the results obtained in the present study using RTSFs.

4.2. Deflection and Ductility

Figure 13 presents the load–deflection curves of the tested beams. Each curve represents the average of the two curves of replicate beams. It can be observed that the load–deflection curves of all beams followed similar patterns and consisted of four distinct behavioral stages. The first stage extends from applying the load to the appearance of the first flexural crack, reflecting the behavior of an uncracked beam. In this stage, the increase in the load is accompanied by a relatively low deflection rate. After the formation of the first flexural crack, additional cracks develop along the beam, causing a reduction in its stiffness. This characterizes the deflection behavior of the second stage, from the initiation of cracks up to the yielding of the steel bars. Then, the third stage begins and continues until the peak load is reached. In this stage, the deflection increases at a relatively higher rate as the applied load increases. This behavior of reduced stiffness is evident from the decrease in the slope of the deflection curve compared with the preceding stage. The fourth stage represents the post-peak deflection behavior, where a drop in the applied load is associated with the increase in the deflection.

For comparing the deflection behavior of the tested beams, Figure 13 indicates that all beams showed similar behaviors in the uncracked stage, where the behavior is dependent on the gross moment of inertia of the cross-section of the beams. In the post-cracking stage, the fiber-reinforced beams exhibited higher stiffness in comparison to the non-fibrous control beam. This can be attributed to the role of fibers in delaying the formation of cracks and controlling crack opening. Figure 13 also indicates that the fiber-reinforced beams showed improved post-peak behaviors. The beams showed more gentle slopes of the descending parts of the deflection curves compared with the steeper slope experienced by the non-fibrous control beam. The figure also indicates that as the fiber content increases, the stiffness of the post-cracking part of the deflection curve increases, and the slope of the descending part of the curve becomes more gradual.

Table 10. Deflection and ductility of tested beams.

Beam	Deflection at Service Load, Δser (mm)	Deflection at Yielding Load, Δy (mm)	Deflection at Ultimate Load, Δu (mm)	Deflection at 0.85 of Ultimate Load, Δ0.85u (mm)	Ductility Index, Δ0.85u/Δy
B–0	2.45	5.97	16.8	18.03	3.02
B–0.5	2.2	5.58	14.03	18.89	3.39
B–1.0	1.92	6.4	15.2	22.44	3.51

gives the deflection values experienced by the tested beams at different load levels obtained from the average curves of Figure 13. For the comparison at the service load level (taken as 60% of the yielding load of the control beam), beams B–0.5 and B–1.0 with the fiber contents of 0.5 and 1%, respectively, exhibited reductions in their deflection of 10 and 22% compared with the control beam (B–0) without fibers.

In this study, deflection was used as an indicator of ductility in the beams, where it is expressed in terms of a ductility index as given in Table 10 for the tested beams. This index is calculated as the ratio of the deflection measured at 85% of the ultimate load, Δ_0.85u, on the descending part of the deflection curve to the deflection measured at the yield load, Δ_y. Table 10 indicates that the addition of 0.5 and 1.0% recycled steel fibers resulted in increases in the ductility index of 12 and 16%, respectively, over that of the control beam without fibers. The table also indicates that all beams achieved a ductility index within the minimum range of 3 to 5 required for sufficient ductility [55].

4.3. Cracking Behavior and Strains

Crack formation and appearance were observed in the tested beams during the early stage of loading. The first cracks were vertical flexural ones that appeared in the constant moment zone. As loading increased, additional cracks developed in the shear spans, and the existing cracks extended more deeply along the beam height and widened further. Finally, the beams failed in a typical flexural mode, characterized by the yielding of the reinforcing steel bars, followed by the crushing of the top concrete in the constant moment zone, as illustrated in Figure 11.

The inclusion of RTSFs contributed to delaying the initiation of flexural cracks, as previously discussed, and helped control the widening of the cracks, as shown in Figure 14. The figure presents the development of the maximum crack width measured in the beams during testing. It can be noticed that the curves follow similar trends to the load–deflection curves presented in Figure 13.

Table 11. Maximum crack widths of tested beams.

Beam	Crack Width at Service Load (mm)	Crack Width at Yielding Load (mm)	Crack Width at Ultimate Load (mm)	Crack Width at 0.85 of Ultimate Load (mm)
B–0	0.2	0.53	2.08	2.68
B–0.5	0.15	0.47	2.33	4.78
B–1.0	0.11	0.48	2.81	4.52

provides the crack width values for the tested beams at various load levels. For comparison at the service load level, beams B–0.5 and B–1.0 with the fiber contents of 0.5 and 1%, respectively, exhibited reductions of 25 and 45% in the measured crack width in comparison to the non-fibrous control beam, B–0. The existence of fibers allows for bridging the cracks, resisting the load, and restricting the opening of the cracks. This beneficial effect enabled the fibrous beams to sustain higher flexural loads and exhibit more ductile behaviors. This can be noticed from Figure 14 and Table 11, where the fibrous beams showed wider crack widths at higher ultimate loads compared with the control beam. Additionally, in Figure 14, the descending branches of the curves of the fibrous beams are more gradual compared with that of the control beam.

The tested beams experienced higher strain values either in the tension steel or in the concrete at failure. For the control beam, B–0, the maximum tensile strain measured in the steel bars was 14,000 micro-strains at the 50 kN load level, beyond which the strain gauges malfunctioned. The maximum compressive strain measured in the concrete for the same beam at failure was 3400 micro-strains. On the other hand, fibrous beams B–0.5 and B–1.0 experienced tensile strains of 7500 and 11,500 micro-strains, respectively, in the steel bars at failure, while the corresponding concrete compressive strains were 3500 and 3900 micro-strains.

5. Comparison of Predictions and Experimental Results

As reported in this section, the experimental cracking load (P_cr,exp) and the experimental ultimate load (P_u,exp) of the beams were compared with the corresponding predicted values. For the cracking load predictions (P_cr,pred), ACI 318 code provisions [53] were used, whereas ACI 544.4R code provisions [56] were used for predicting the ultimate load (P_u,pred).

5.1. Cracking Load

The cracking moment (M_cr,pred) was calculated according to the following:

$$M_{cr,pred}=f_r{\displaystyle \frac{I_g}{y_t}}$$

(1)

where I_g is the gross moment of inertia of the cross-section of the beam, y_t is the distance from the centroidal axis to the extreme tension fiber of the cross section of the beam, and f_r is the modulus of rupture of concrete, which can be calculated according to ACI 318 [53] as follows:

$$f_r=0.62\lambda \sqrt{f_c^{\prime }}$$

(2)

where $f_c^{\prime }$ is the specified compressive strength of concrete in MPa and λ is a modification factor that accounts for the density of concrete. The code recommends a value of 1 for λ in the case of NWC, whereas a value of 0.85 is recommended for λ in the case of concrete with a combination of lightweight coarse aggregate and normal-weight fine aggregate. For concrete where all aggregates are lightweight, a value of 0.75 is recommended by the code.

For the test setup of this study (Figure 6), the cracking load, P_cr,pred, can be calculated as

$$P_{cr,pred}={\displaystyle \frac{{2M}_{cr,pred}}{0.45}}$$

(3)

In Equation (2), a value of 0.75 for λ was taken, since all-lightweight concrete was used in preparing the beams of this study. The predicted values of the cracking loads are given in

Table 12. Comparison of predictions and experimental results.

Beam	Cracking Load			Ultimate Load
	Pcr,exp (kN)	Pcr,pred (kN)	Pcr,exp/Pcr,pred	Pu,exp (kN)	Pu,pred (kN)	Pu,exp/Pu,pred
B–0	8.0	7.83	1.02	55.9	50.4	1.11
B–0.5	12.2	7.96	1.53	59.1	53.6	1.10
B–1.0	13.1	7.89	1.66	61.1	56.7	1.08

for the tested beams along with the comparison with the experimental ones. The comparison is also presented in Figure 15. It is evident that the predicted cracking load matched well with the experimental one for the control beam, as the ratio P_cr,exp/P_cr,pred was 1.02. On the other hand, Table 12 and Figure 15 indicate that the ACI 318 method provided conservative predictions for the cracking load of the tested fibrous beams with an increasing level of conservatism as the fiber content increased. The comparison gave the P_cr,exp/P_cr,pred ratios of 1.53 and 1.66 for beams B–0.5 and B–1.0, respectively. These conservative predictions for the fibrous beams are attributed to the calculation of the modulus of rupture, f_r, of concrete, which is based on the compressive strength as given in Equation (2). On the other hand, the hardened properties of the fibrous concrete revealed that although the splitting tensile strength increased with the increase in fiber content, the compressive strength did not show much increase.

5.2. Ultimate Load

The ACI 544.4R code provisions [56] account for the contribution of the residual tensile strength in fibrous concrete beams to the moment capacity of the beams. With reference to Figure 16, which illustrates the stress and strain distributions across the beam section at the ultimate limit state, Equation (4) is provided by the code for calculating the moment capacity of the beams, M_u,pred.

$$M_{u,pred}=A_s{f}_y\left(d-{\displaystyle \frac{a}{2}}\right)+A_s^{\prime }{f}_y^{\text{'}}\left({\displaystyle \frac{a}{2}}-d^{\prime }\right)+{\sigma }_{t}b\left(h-e\right)\left({\displaystyle \frac{h}{2}}+{\displaystyle \frac{e}{2}}-{\displaystyle \frac{a}{2}}\right)$$

(4)

where A_s and f_y are the area and yield strength of the tensile steel, respectively; $A_s^{\prime }$ and $f_y^{\prime }$ are the area and yield strength of the compressive steel, respectively; d is the distance from the extreme compression fiber to the centroid of the tensile steel; d’ is the distance from the extreme compression fiber to the centroid of the compressive steel; a is the depth of rectangular stress block; σ_t is the residual tensile strength of fibrous concrete; b is the beam width; h is the overall depth of the beam; and e is the distance from the extreme compression fiber to the top of the tensile stress block of fibrous concrete.

The residual tensile strength, σ_t, can be calculated as follows [56]:

$${\sigma }_t=0.00772{\displaystyle \frac{L_f}{d_f}}{V}_f{F}_{be} (\mathrm{MPa})$$

(5)

where L_f and d_f are the fiber length and diameter, respectively; V_f is the volume percentage of fibers; and F_be is the bond coefficient of the fibers, varying from 1.0 to 1.2.

As per the test setup used in this study (Figure 6), the ultimate load, P_u,pred, can be calculated as

$$P_{u,pred}={\displaystyle \frac{{2M}_{u,pred}}{0.45}}$$

(6)

Equations (4)–(6) were used for predicting the ultimate load, P_u,pred, of the tested beams. The predicted values of the ultimate load are given in Table 12 along with the comparison with the experimental ones. Figure 17 also presents this comparison. Table 12 and Figure 17 indicate that the ultimate loads of the beams were accurately predicted. The P_u,exp/P_u,pred ratios were 1.11, 1.1, and 1.08 for beams B–0, B–0.5, and B–1.0, respectively. It is clear that the level of accuracy is approximately the same for the non-fibrous and fibrous beams. This comparison result reveals that the ACI 544.4R method [56] can capture the beneficial effect of recycled fibers on enhancing the ultimate capacity of the beams in an accurate and consistent manner.

6. Conclusions

The feasibility of developing LWSCC mixtures with RTSFs was examined in this paper. Three mixtures with recycled fiber contents of 0, 0.5, and 1.0% were produced. The fresh and hardened properties of the concrete mixtures were evaluated. Additionally, the flexural behaviors of beams fabricated by using the developed mixtures were investigated. Based on the findings of this study, the following conclusions can be made:

• The addition of RTSFs to the LWSCC mixtures resulted in slight decreases in the slump flow of the mixtures. However, the obtained slump flow values for the concrete mixtures remained well above the EFNARC-specified lower limit of 650 mm for SCC with sufficient flowability.
• The obtained fresh densities of the developed concrete mixtures fell within the range specified for structural lightweight concrete by both the ACI 318 code and Eurocode 2, despite the fibers’ effect on increasing the density of the fibrous concrete mixtures.
• The inclusion of RTSFs was found to significantly enhance the splitting tensile strength of the concrete mixtures. However, the compressive strength results show only a marginal improvement.
• The cracking load of the beams with 0.5 and 1.0% fiber contents increased by 52.5 and 63.8%, respectively, compared with the non-fibrous control beam. However, the corresponding increases in the ultimate load were 5.7 and 9.3%.
• The inclusion of RTSFs reduced the midspan deflection and crack width of the beams at service load level. Also, the RTSFs enhanced the ductility index by 12 and 16% for the beams with 0.5 and 1.0% fiber contents, respectively, over that of the non-fibrous control beam.
• The ACI 318 method accurately predicted the cracking load of the control beam without fibers. On the other hand, it provided conservative predictions for the beams containing RTSFs.
• The ACI 544.4R method was able to capture the beneficial effect of the RTSFs of enhancing the ultimate load of the beams, providing accurate and consistent predictions.

The above results and conclusions are encouraging and demonstrate the feasibility of using RTSFs in LWSCC. However, future research focusing on long-term studies to assess durability and performance under various environmental conditions is needed. Additionally, field application studies are essential to validating laboratory findings and understanding real-world performances, including load carrying capacity, crack resistance, and overall structural integrity. This research is crucial to developing standardized guidelines and ensuring the reliable use of RTSFs along with LWSCC in sustainable construction practices.