Introduction

Slab-on-grade are widely used in a range of applications, including industrial slabs, footpaths, highways, and decorative floorboards. Industrial concrete slabs must be capable of supporting heavy loads resulting from operational movements of vehicles and stored materials. The ability of reinforced slab-on-grade to withstand moments and shears caused by applied loads depends on the interaction between the concrete slab and the supporting materials. The characteristics and geometry of the reinforced concrete slab and the supporting materials are important in the design of a slab-on-grade. The support system should be of acceptable uniform capacity and not easily susceptible to being affected by climatic changes. Failures of slab-on-grade might occur because a proper support system was not achieved [1, 2]. Recent studies [1, 3, 4] have predicted the structural response of slabs-on-ground. Several experimental tests on slab-on-grade with different reinforcing normal concrete cases under static loads have been performed [3,4,5]. These normal concrete slabs are reinforced with steel mesh, polypropylene, and steel fibers reinforcement. Experimental tests found that the results under numerous loading points (at the middle, edge, and corner) were significantly varied. Additionally, slab-on-grade are one of the main applications for fiber reinforced concrete (FRC). In these structures, fibers can completely replace traditional reinforcement (rebars or welded mesh), improving toughness and strength under static and dynamic stresses [6, 7]. The incorporation of fibers minimizes heat or shrinkage-induced cracking. FRC has become more popular over the past few decades as a reliable substitute for traditional reinforcement in store floors, roadways, parking-lots, and runways [6,7,8]. A distributed fiber reinforcement may enhance the structural behavior due to strong, focused loads from industrial gear and shelves, which create extensive cracking and excessive deformation of slabs. Several of these slabs are supported slab-on-grade that have statically undetermined structures. Therefore, steel fibers can be utilized to partially (or completely) replace traditional reinforcement of slabs on the ground, even at very low volume fractions [5,6,7,8]. This effectively increases the ultimate load capacity of the prepared FRC. Fiber reinforcement offers improved fracture management, enhances structural durability, and reduces joint requirements. Additionally, fiber reinforcement increases the impact and fatigue resistance of different buildings and lowers labor expenses because the reinforcement deployment process takes less time [5,6,7,8].

Up to now, the design of FRC slab-on-grade was based on the same traditional theories that were applied to the design of airport/highway pavements ACI 360R [3], while their analysis under static loads continued to be based on the same traditional theories created by Westergaard [10]. Overli [11] described an experimental test program in which focused applied load on a slab-on-grade in the middle and corners. The analytical responses for the maximum load capacity were in good agreement with the test findings. These also suggested that slabs cracked severely due to drying shrinkage. It was also reported that the slab rested on elastic bearing, and the reaction of the slab was the formation of circular fractures at the top surface. The results of experiments were reasonably in line with the design code criteria for punching resistance [11] and conventional yield line fixes for bending failures. Recently, several design standards dealing exclusively with the design of slabs-on-ground were published. Aboutalebi et al. [2] investigated the behavior of a slab-on-grade subject to different loading sites (at center, edge, and corner). It was stated that the failure loads of this study were far lower than the test levels. Additionally, to investigate punching shear failure, Sucharda et al. [9] investigated the performance of slab on ground with dimensions (2.0 m × 1.95 m) with a 12 cm thickness. The tested slab was deteriorated at a stress of 344 kN, and it was reported that the shear strength value was higher than what was predicted by ACI Committee 318 [3].

Tang et al. [13] also investigated the effect of geogrids in slabs-on-ground. Large slab-on-grade were constructed under monotonic static loads. The findings demonstrated that the geogrids carried extra weight once cracking started and postponed the failure of the concrete beams to collapse. The geogrids permitted for the formation of extra fractures following the initial cracks, which led to a high failure load. Shaaban et al. [14] carried out a parametric analysis utilizing nonlinear finite element analysis (NLFEM) to use the behavior of slab-on-ground to loads from automobiles. The load location in respect to slab geometric, fiber content, mesh steel reinforcement, and Winkler soil (k-response or soil subgrade reaction) were all factors that were examined. To replicate the properties of soil resistance, k was represented using boundary-spring components of a compression model. The investigation demonstrated that the slab thickness and k-response had a significant impact on the load-carrying capacity of slab panels. A reinforced concrete and FRC slab with a thickness of 0.15 m and a size of 2.0 m was modeled experimentally using NLFEM in three dimensions [12,13,14]. Normal concrete and FRC were both made of the model's fracture-plastic substance. The findings demonstrated that the outcomes produced by theoretical method were in strongly agreed with the experiment’s outcomes. Rizzuto et al. [1] examined the experimental and theoretical studies of the structural behavior of loaded, slab-on-grade of steel mesh reinforced concrete (with dimensions 6.0 m × 6.0 m, 150 mm thick) that were supported by the ground. Static loading experiments were conducted at the middle of the slabs, the edges, and corners. In experimental center loading, load failure predominated at 417 kN. Circumferential and radial fractures caused bending and a load failure for the 300 mm edge loading at 369 kN. For the center loading position and the 300 mm edge loading point, respectively, the experimental values were 51.0 and 53.2% higher compared to the corner loading.

Recycling solid waste materials as aggregate offers a solution to the problems encountered with the quarrying of natural aggregates and supports the sustainability. As these substitutes require extensive studies about their effect on the properties of concrete, several research studies were performed [15,16,17,18,19,20]. On the other hand, rapid automobile industry growth has led to a rise in waste tires [20,21,22]. As waste tire rubber is non-degradable, burning, or landfilling creates environmental, economic, and health issues [20,21,22,23]. Recycling crumb rubber (CR) as a replacement for fine aggregates produces rubberized concrete to lessen the ecological impact of waste tires [24,25,26,27]. Numerous research [20, 23,24,25] have examined rubberized concrete's fresh and hardened characteristics. Several studies [27,28,29,30] tested the flexural strength of rubberized concrete beams. It was reported that the addition of CR in concrete diminishes beams' flexural strength and stiffness. It was also reported that the addition of CR as a partial substitution for fine aggregates reduces the shear strength of rubberized concrete beams, according to Hosseini et al [30]. The researchers [31,32,33,34,35,36] evaluated rubberized concrete's mechanical and dynamic properties. They found that CR improves rubberized concrete structural members' cracking, ductility, toughness, and energy dissipation. As industrial fibers are highly expensive and possess a high carbon footprint, the current study is geared toward the use of eco-friendly fibers to increase the ductility of slab-on-grade. The available literature focused on the assessment of the fracture of the slab-on-grade with industrial steel fibers. While RSF is an inexpensive and eco-friendly fiber processed from scraped steel wires or chords extracted from discarded tires, steel fibers are stronger and more beneficial to the mechanical performance of concrete as compared to synthetic fibers. Like industrial steel fiber, RSF is resistant to weather changes and possesses high tensile strength and toughness, as tire wires are processed from high-grade steel. The addition of RSF has shown improvements in the tensile strength and flexural behavior of concrete [31, 33]. Fiber activation occurs after the concrete matrix begins to crack (invisible microcracks). However, a hybrid of short and long steel fibers may increase the concrete toughness at tiny crack opening displacements because fibers of different diameters become effective at different stages of the cracking process (Fig. 1) [7]. Additionally, shorter fibers lower material permeability due to improved control of the cracking process, and hybrid FRC shows to be a viable application for industrial structures exposed to harsh conditions.

Fig. 1
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Role of various fibers in reduction of crack width during a tensile test [7]

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Research significance

This study focuses on investigating the fracture performance of an eco-friendly rubberized concrete slab on ground. Rubberized reinforced concrete (RRC) slab-on-grade measuring 1000 m × 1000 mm with a thickness of 60 mm were tested experimentally, and the soil was simulated with a steel model. The primary variables were the addition of hybrid fibers (different lengths of raw steel fibers from old tires) and the volume fraction of CR (0%, 10%, and 20%) used as fine aggregate. The fresh and mechanical properties of prepared RRC samples were investigated. After that, RRC slabs (S0, S1, S2, S4, and S5) on grade are experimentally investigated in this study. The prepared slab-on-grade were cast and put through testing with concentrated static loading in the middle of the slab. The load–deflection responses, crack patterns, failure loads, deflection responses, and fracture toughness were analyzed. Finally, the findings of the current study were also compared with those of previous studies.

Experimental program

Materials

In the present study, Portland cement (CEM I 42.5 N) complied with EN 197-1:2011 with a specific gravity of 3.15, an initial setting time of 96 min, and a final setting time of 174 minutes. The chemical composition of the PC is shown in Table 1. Locally crushed dolomite with a particle diameter with type 4/12 was used. It has a specific gravity of 2.66 (EN 12620:2013) and an apparent density of 1739.0 kg/m3 (EN 12620:2013). As natural fine aggregates, river sand (RS) type 0/4 was used. RS had an apparent density of 1629.0 kg/m3, a specific gravity of 2.55, and a fineness modulus of 2.89. The river sand's characteristics were assessed in line with EN 12620:2013.

Table 1 Chemical oxide of PC (%)
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The products of waste tire rubber employed in this current study were supplied by a local company. In the current study, fine crumb rubber (CR) was used as a partial substitution for river sand extracted from waste tire rubber. The CR fine aggregates were subjected to a chemical treatment to roughen the rubber particle surface and increase the adhesion between the CR and the paste in concrete. The chemical composition of CR is listed in Table 1. To increase the ability of CR to adhere to PC paste and to roughen the rubber's surface, a chemical treatment approach was applied. The CR was immersed in a 5% sodium hydroxide (NaOH) solution for 2 hours to increase the roughness of the rubber's surface [31, 33]. The rubber particle was washed with water to remove the sodium hydroxide solution and then dried in the air, as shown in Fig. 2. After treatment processing, the CR's characteristics were assessed according to ASTM C128. CR had a particle size distribution of 0.69 mm to 3.10 mm, a specific gravity of 1.03, and a fineness modulus of 3.21. The gradation of the utilized raw materials and aggregates, as determined in accordance with ASTM C136, is shown in Fig. 3. The CR was examined by scanning electron microscopy (SEM) as shown in Fig. 4. EDS analysis was undertaken on the treated rubber particles that were soaked in a 5% NaOH solution for 2 hours and then dried in the air. The EDS analysis of white particles deposited on the surface of the rubber particles soaked in NaOH solution clearly shows sodium, S, and O elements, indicating the deposition of sodium particles.

Fig. 2
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Chemical treatment process of crumb rubber

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Fig. 3
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PSD curves of raw materials

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Fig. 4
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SEM–EDS test of CR

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The tire-recycled steel fibers (RSF) used in this study were recovered by shredding waste tires [32, 34]. The CR was successively separated from the steel by an electro-magnetic process performed by the supplier. The geometric and mechanical characteristics of the steel fibers are presented in Table 2. The RSFs are characterized by different diameters, lengths, and shapes and present irregular wrinkles. The RSF properties were tested according to EN 14889-1. Different lengths of RSF were used in the current study, according to the previous studies. In this research, hybrid steel fiber was used (40% of the 15 mm steel fiber length, 40% of the 10 mm length, and 20% of the 5 mm steel fiber length) as shown in Fig. 5. To attain the desired workability of the fresh concrete, a locally available superplasticizer type F was used.

Table 2 Geometric and mechanical properties of RSF
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Fig. 5
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Recycled steel fiber used

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Rubberized slab-on-grade preparation and testing setup

In the current study, the rubberized concrete slabs-on-ground containing RSF were exposed to static loading applied at the center of the slab. The slab-on-grade had a square geometry (1000 mm × 1000 mm), and its thickness was 60 mm. The crack patterns, deflections, and failure loads under experimental load were recorded. The loading on the slab was via hydraulic jack capacity with a displacement rate (0.30 mm/min) to simulate the loading of site loads. Figure 6 shows the test and monitoring locations. In the current study, seven concrete mixtures with fine CR (10%, 20%, and 30%) and RSF (0.5% RSF) were performed and tested, and the mix proportions are listed in Table 3. Then, five concrete slabs (S0, S1, S2, S4, and S5) were designed and prepared. Two slabs (S1 and S2) prepared with a volume fraction of fine CR (10% and 20%) as a partial substitution of RS without steel fiber. Another two slabs (S4 and S5) prepared with a volume fraction of fine CR (10% and 20%) as a partial substitution of RS with a volume fraction of 0.5% RSF, and a reference slab free of rubber or steel fiber (S0) are performed. It is worth noting that the mixtures with 30% CR were excluded from slabs casting due to the deterioration of the concrete's mechanical properties, as explained later.

Fig. 6
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Slab loading and monitoring locations

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Table 3 Mix proportions of concrete (kg/m3)
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The target compressive strength of the control mixture (S0) after 28 days was 40 MPa. All the combinations were carried out with a constant water-to-binder ratio of 0.47, and the dosage of the superplasticizer was 1.75 kg/m3. The tensile strength (fct) of cylinders (diameter = 150 mm, length = 300 mm) and the cube compressive strength (fc) (150 × 150 × 150 mm3) were evaluated. The Young's modulus of the prepared cylinder samples (diameter = 150 mm, length = 300 mm) was examined (Fig. 7). Three prisms (100 × 100 × 500 mm) evaluated for each mixture under three-point bending were used to assess the modulus of rupture properties, as shown in Fig. 7.

Fig. 7
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Test setup for mechanical properties of the prepared concrete mixtures

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Ground conditions

To simulate a soil subgrade reaction, 100 steel supports (springs) were designed and strategically positioned beneath the rubberized panel at centers of 500 mm in both ways (Fig. 8a). These supports are steel springs rested on a square base. Previous simulations have demonstrated that the experimental subgrade offers a reliable representation of a continuous soil subgrade reaction [1, 8]. The average spring stiffness (K) was established by compression tests conducted on each individual spring, with findings approximately equivalent to 7.58 kN/mm. The average subgrade reaction or Winkler constant (k) was evaluated for the used steel frame varied from 0.0442 to 0.0541 N/mm3 based on the consideration of the effect area of each spring, which measures 100 × 100 mm. This value indicates that the soil may be classified as uniformly fine-grained according to the ACI classification.

Fig. 8
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Test setup for slab-on-grade

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Slab loading and monitoring

In the current investigation, five rubberized concrete slab-on-grade resting at soil spring constants as simulating fine-grained soil were conducted and tested. 4 mm in diameter @ 200 mm mesh mild steel rebar was used as the tension reinforcement of the slab. As shown in Table 4 and Fig. 8b, the utilized steel mesh rebars' characteristics were defined in line with ASTM A370. The experimental load was applied to the central region of the panel using a hydraulic jack that was connected to a steel top plate. To address any irregularities on the surface, a plywood spreader plate of 100 × 100 mm was inserted between the machine loading plate and the prepared slabs. A steel plate measuring 100 × 100 mm was affixed to the steel loading plate. Three LVDTs with 0.15 sensitivity, labeled as 1, 2, and 3, were positioned on the plate in the center and corners, respectively, as shown in Fig. 6. The load pressure cell, positioned between the hydraulic jack and the 100 mm square steel loading plate, was used to measure and record the applied load. The displacements of the slab under loading were recorded by LVDTs positioned close to the loading plate as well as at several additional locations [3, 4, 8]. The sampling of observation locations was conducted using LVDT displacement transducer sensors, as shown in Fig. 8c. The configuration of these sensors is illustrated in Fig. 8c. The loading rate exhibited a gradual increase, with each slab being loaded over an average duration of 35 min. The load increments for the center loading were monitored. The recorded data consisted of the output values from each sensor during the incremental loading process. The identified cracking inside the slab was analyzed in relation to the matching displacement value that was recorded. The experimental test machines at Tanta and Mansoura University, Egypt, were used for studying the panel properties, testing and monitoring slab failure.

Table 4 Characteristics of steel reinforcement
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Experimental results and discussion

Workability of rubberized concrete

The workability of the various prepared mixtures is shown in Fig. 9. As the CR content rises, the rubberized concrete loses workability. The workability of the concrete mixtures (S1, S2, and S3) incorporating 10%, 20%, and 30% CR is, respectively, 2.6%, 15.4%, and 28.2% less than those of the reference mixture (S0). Due to the rubber particles' irregular form and rougher surface as compared to sand particles, rubberized concrete's workability has decreased. Similar findings were observed by the authors of [8, 35,36,37,38,39]. Tahwia et al. reported that the crumb rubber decreases the workability of concrete [21]. On the other hand, the workability also decreased with addition of the recycled steel fiber, so, the control mixture reduced in ranges of 12–18% with addition of RSF and crumb rubber. The density of concrete also decreased with the increase of the CR percentage as replacement of sand. It was probably this reduction due to the specific gravity of CR less than the specific gravity of sand. These results are agreed with the previous studies [21, 24, 39].

Fig. 9
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Properties of RRC a Slump value of RRC and b unit weight of prepared concrete

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Compressive strength rubberized concrete

Figure 10 shows the effect of the addition of CR and RSF on the compressive strength of rubberized concrete mixtures. The compressive strength declines as the CR content increases, which is expected and consistent with the findings of earlier investigations [25, 33, 40,41,42,43,44]. The compressive strength of S1, S2, and S3 containing 10%, 20%, and 30% CR is, respectively, 30.1% (27.61 MPa), 33.2% (26.80 MPa), and 42.2% (23.3 MPa) less than that of the S0. The decrease in compressive strength in concrete is caused by a lack of adhesion between the paste and the CR. Therefore, it is anticipated that cracks will initially appear around the rubber during loading. On the other hand, the compressive strength increased with addition of the 0.5% RSF. The incorporation of RSF fibers has positive effects on the compressive strength of concrete. The positive effect corresponds to the increased crack resistance and confinement of concrete, which delay the failure of RRC under the compressive load. Agrawal et al. [38] reported that the compressive strength of rubberized concrete was 26.59 MPa with 20% CR, which is 37.89% less than the control mix. The enhancement in compressive strength can be noted using the pre-treatment of rubber particles, and a similar trend of enhancement of compressive strength was observed by Chen et al. [45] after surface modification of rubber particles, as followed in the current study. Similarly, the addition of RSF results in ductile failure and can delay the collapse of concrete samples. It was found that the addition of 0.5% RSF ultimately helped in increasing the compressive strength by 59%, 26%, and 16% compared to the mixtures without RSF at 10%, 20%, and 30%, respectively. Similar results were reported by Shah et al. [37], who reported an increase in the compressive strength of RRC prepared with RSF.

Fig. 10
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Effect of CR and RSF on compressive strength of rubberized concrete

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Splitting tensile strength

The splitting tensile strength (STS) of the concrete mixtures is shown in Fig. 11. As would be expected, the rubberized concrete's STS declines as the CR concentration increases [27, 39, 46]. The reference mixture (S0) has a splitting tensile strength that is approximately 5.7%, 17.1%, and 31.4% more than that of the concrete mixtures S1, S2, and S3, respectively. Based on the experimental results, the percentage of declination in compressive strength is lower than the percentage reduction in STS. The weak tension of rubber particles is the cause of the loss in concrete STS that occurs when CR % increases. Similar findings were observed by the authors of [27, 46,47,48], namely that rubberized concrete loses tensile strength as CR is increased. The introduction of RSF has resulted in an improvement in the tensile strength behavior of concrete. Mastali et al. [49] reported improvements in the mechanical properties and impact toughness of self-compacting concrete due to RSF incorporation. The ultimate crack toughness of plain concrete increased by more than 450% at a 1.5% volume fraction of RSF. Ali et al. [50] showed that the effectiveness of RSF is between 50 and 76% compared to industrial steel fibers in updating the compressive and tensile properties of concrete. RSF can be utilized as a promising fiber without substantially increasing the cost and environmental impact of concrete [51,52,53].

Fig. 11
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Effect of CR and RSF on STS of rubberized concrete

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Flexural strength and modulus of elasticity

The 28-day flexural strength of concrete mixtures with various CR amounts is shown in Fig. 12. When CR content rises, the flexural strength declines. The flexure strength of S1, S2, and S3 was reduced by around 3.8%, 13.2%, and 22.6%, respectively, compared to the reference mixture S0. This reduction may be attribute to the smooth surface of CR which may have a detrimental impact on concrete by weakening the interface. As a result, while loading, cracks are easily propagated. Additionally, the flexural strength was improved with the addition of RSF. The same findings, namely that flexural strength diminishes with increasing CR concentration in concrete, were reported by the authors of [17, 54,55,56]. Similar results were reported that an optimum dose of RSF of 0.5% for the maximum gain in flexural strength. Owing to the addition of fibers, like conventional concrete systems [42, 50], RRC also experienced a higher improvement in its flexural strength as compared to its compressive strength. The modulus of elasticity follows the trend of concrete compressive strength, where the higher the substitution of crumb rubber, the lower the modulus of elasticity of concrete. Theoretically, the modulus of elasticity can be calculated based on concrete weight and compressive strength. The results show that the modulus of elasticity from experimental and theoretical is almost the same, as presented in Table 5.

Fig. 12
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Effect of CR and RSF on flexural strength of rubberized concrete

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Table 5 Modulus of elasticity of rubberized concrete
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Results of rubberized reinforced concrete slab-on-grade

Crack patterns and failure Loads

The crack and failure fracture patterns of the prepared slab-on-grade (S0, S1, S2, S4, and S5) are displayed in Fig. 13. Each sample failed in the flexural mode under axial compression load in the center of the slab. After the slab collapsed, flexural failure was confirmed when a quick fall was seen when the load was applied to the slab. As the applied load grew, the first crack developed on the tension side of the slabs. The cracks number and size grew, and they spread radially from the slab center to the slab's margins. Circumferential cracks began to emerge around the slab center after this occurrence. The presence of CR, which probably acted as springs and maintained a portion of the applied load, caused the cracks in the samples included with CR to grow in comparison with those in the slab without rubber, increasing the area of the surface failure. For the samples with recycled steel fiber, cracks were reduced, and the failure load also increased. These reactions agree with the previous studies [1, 8, 39]. These cracks in the slab containing RSF on grade began from the center of the slab in radial directions and spread toward the slab's margins. Additionally, large cracks with an approximate 37–43° angle formed. The presence of crumb rubber converted the slab's two-way shear failure mechanism into a one-way one. The incorporation of CR and RSF improves the ductility of the slab-on-grade, delays the failure of slabs, and avoids sudden failure. It was also found that S5 had more cracks but not as many large cracks as all the others slab-on-grade, as presented in Fig. 13e. This finding resulted in the presence of 20% fine rubber and 0.5 hybrid RSF, which increase the ductility of the concrete. These results agree with the investigations reported by Rizzuto [1] and Overli [11], which investigated the slab-on-grade under static loading. These slabs subjected to concentrated loading at the center, the edges, and the corners. The soil is modeled as a no-tension bedding, and a smeared crack approach is employed for the concrete.

Fig. 13
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Crack patterns and failure of the rubberized concrete slabs for center load

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Load–deflection curves

The observed response of the slab-on-grade (S0, S1, S2, S4, and S5) exposed to a center load is supported by the load–defection curve shown in Fig. 14. The experimental curves resembled one another; the slabs achieved their maximum load before failing brittlely and abruptly losing their flexural capacity, which indicates low residual strength and ductility. These curves agree with previous studies [5, 8, 9, 33,34,35,36]. There are three stages to the load–deflection curves. The load–deflection curves are almost linear in the first stage (uncracked stage). Between the cracking load and the yield load is the second stage, often known as the pre-yield, cracked stage. Flexural cracking develops at the tension side of a slab at this stage, gradually reducing the slope of the curve. Between the yield load and the flexural failure load is the final stage, often known as the post-yield, cracked stage. The slope of the bends on the slab-on-grade was lower than that of the slabs of normal concrete. The curve slope was significantly influenced by the CR content and the presence of the steel fiber. The test samples' ductility and final deflection improved as the CR content rose and the presence of the steel fiber. The final deflection of the slab-on-grade that contained 10%(S1) and 20% CR (S2) was higher than that of the reference slab by around 23.61% and 18.2%, respectively. Additionally, the final deflection of the rubberized concrete slabs that contained 10% and 20% CR and the presence of the RSF (S4 and S5) was higher than that of the control slab by around 18.2% and 27.3%, respectively [8, 33].

Fig. 14
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Experimental load ver. displacement curves of center and 2-point corner loading for all slab-on-grade at loading center

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First cracking load

All samples were inspected up until the first crack appeared due to the increasing of the applied load, and the matching first crack load and deflection were recorded as shown in Table 6. The initial load crack denotes the formation of round or close-shaped cracks beneath the patch load at the bottom surfaces of the slab-on-grade. As presented in Table 7, with 0%, 10%, and 20% CR, the initial crack loads of the slabs (S0, S1, and S2) are roughly 38.25%, 35.8%, and 33.25% of the failure load, respectively. The initial crack loads of the prepared samples (S4 and S5) with recycled steel fiber and combined with 10% and 20% CR are roughly 34.96% and 35.31% of the failure load, respectively. The first crack load and deflection of the slab-on-grade of the samples typically reduce as CR content rises. This result is explained by a weak interlock between rubber and cement paste, which lowers flexural strength. Additionally, with increased CR content and the presence of steel fiber, the first crack load is increased compared to the control slab-on-grade, which is associated with the increased ductility of the concrete under flexural loading due to the presence of the CR and RSF. Elsayed et al. [42] investigated the failure load of a loaded rubberized reinforced slab. In the reinforced concrete slab, the optimum soil ratio is 10% of CR, with a loss in failure load of the reinforced slab being acceptable. Additionally, Elsayed et al. [42] studied the failure load of the reinforced panels with openings next to columns. The failure load of reinforced slabs declined by increasing the incorporation of CR and opening size. Furthermore, several studies have been performed to estimate the effectiveness of adding CR to concrete slabs and mixtures.

Table 6 Experimental results of test samples (S0, S1, S2, S4, and S5)
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Table 7 Comparison between the current results with previous studies [1, 5, 8, 8, 47]
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Toughness

As shown in Table 6 and Fig. 15, the overall area under the curve of load versus deflection up to the ultimate load was used to determine the toughness of the prepared slabs with/without crumb rubber and RSF. The test samples' toughness increased as the CR content increases to 10% (S1) before declining at 20%CR values (S2). A CR addition of up to 10% (S1) enhanced toughness for the test samples by 21.36% and increasing to 23.33% with the presence of steel fiber (S4). A significant CR content, such as 20%, had a negative impact on toughness without steel fiber. The test samples with recycled steel fiber showed significant increasing in the toughness which achieve increasing up to 87.98% for S4 and 91.92% for S5 compared to the reference slab (S0). When the inclusion of RSF and presence of CR, it increased the toughness of the test samples by 87.98% for S4 and 91.92% for S5 compared to the reference slab (S0) [8, 33].

Fig. 15
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Results of the toughness of the rubberized slabs

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Comparison between the current experimental results with previous studies

According to the experimental tests of concrete mixtures, rubberized concrete slab-on-grade were performed by incorporating the RSF. As previously stated, before the slabs testing, a ground condition and fabrication of the slabs were carried out. The main aim of the current study was to define the load curve, crack patterns, and failure loads for the case of an eco-friendly slab-on-grade. Therefore, the monitoring works show that the change in stiffness and the first crack opening at the bottom surface occurred with the load in the range of 12.80–17.0 kN after 12 min. Moreover, it was observed that a slightly increase in load and crack growth from the bottom surface upward. The cracks may be accurately identified since they exhibit a vertical orientation and progressively widen [3, 8, 33]. Furthermore, it is worth noting that there is a common occurrence of the borders of the slab being elevated. The presence of crumb rubber and recycled steel fiber changes the mechanism of the fracture of the slab-on-grade. To support the current study and examine the accuracy of the experimental work, more than 60 slab test results from recent studies were collected and analyzed. The comparing factors, including the process of loading, the fiber type and size used, the Ec, fc, and modulus of the subgrade, the geometry of the prepared slab-on-grade, and the failure loads, are listed in Table 7. Numerous research have been conducted to investigate the lab behavior of slab-on-grade, as well as the role of fibers in influencing the overall behavior of these slabs. Several studies have documented the implementation of comprehensive tests conducted on slab-on-grade incorporating with steel fiber [1,2,3, 8, 9]. The experiments were carried out using three primary loading positions in relation to the edges of the slab: middle, edges, and corners. The occurrence of slab failure in these experiments was considered when fractures manifested at the upper surface of the slab, mostly because of the presence of negative bending moment. According to the current study, the inclusion of recycled steel fibers from tires was found to enhance the flexural strength of the slab-on-grade and increase the failure load. The magnitude of this increase is contingent upon several factors, including the strength of the concrete, the dispersion and orientation of the fibers, and most significantly, the percentage of fibers and their geometry [1, 5, 8, 8, 47, 55,56,57].

Conclusion

The present investigation aimed to investigate the behavior of reinforced rubberized concrete slab-on-grade incorporated with recycled tire steel fiber. Five rubberized concrete slab-on-grade were performed under static loading the center of slab with dimensions 1.0 m × 1.0 m × 0.06 m thickness. The main parameters were the addition of hybrid fibers and the volume fraction of CR (0%, 10%, and 20%) used as fine aggregate. The findings were compared with previous studies. The following conclusions are obtained from this research work:

  1. 1.

    For the loaded control slab (S0), the slab-on-grade failed under static load. The failure load was 43.0 kN at 8.64 mm of slab deflection, and wide crack was obvious on the top surface of the panel. Meanwhile, slab with 10% and 20% crumb rubber had a failure load of 43 kN and 38.70 kN, respectively. It was noting that the defection of the slab increased by 12.28% and 20.13%, respectively, compared to the reference slab.

  2. 2.

    For the center loading of a rubberized slab-on-grade with RSF, the vertical cracks and crack width were reduced compared to other slabs, which appeared with increasing loads and progressively enlarged, followed by peripheral and radial cracks. Additionally, new microcracks formed closer to the loaded area and increased the ductility of the slab-on-grade, leading to a failure load of 48.50–52.03 kN for S4 and S5, respectively.

  3. 3.

    A low content of hybrid recycled tire steel fibers (0.50%) with a medium content of fine crumb rubber (20% by volume of sand) effectively enhances the ductility, toughness, and load-carrying capacity of slab-on-grade.

  4. 4.

    Preliminary results revealed higher energy dissipation at small crack openings and increased toughness of the prepared slab with hybrid RSF, which encourages further research on this topic. Therefore, the authors are working on designing and predicting the collapse of these environmentally friendly rubber slab-on-grade in the near future using the FEM program and working on a comparison between that experimental study as well as the numerical study and comparing it with previous studies as well.