1 Introduction
Hollow-core slabs (HCSs), traditionally constructed using prestressed concrete are a popular choice for multi-storey buildings due to their reduced self-weight and efficient material use [
1–
3]. They also offer the advantage of spanning longer distances with fewer intermediate supports, enhancing design flexibility and construction efficiency [
4–
6]. These advantages, particularly in terms of reduced material consumption, make HCSs a favorable alternative to conventional reinforced concrete (RC) solid slabs (SSs) [
7–
9]. A recent study conducted by Kıpçak et al. [
10], further highlights that the presence of voids could enhance structural efficiency while maintaining flexural performance. Their voided design facilitates efficient load transfer and supports longer spans, making them a viable option in sustainable construction practices [
11,
12].
However, the requirement for prestressing in traditional HCS systems adds complexity and increases construction costs, prompting the exploration of alternative non-prestressed systems that leverage conventional reinforcement methods. Studies have explored several approaches to enhance HCSs’ performance while reducing construction complexity and costs. For instance, the addition of high-strength concrete (HSC) and supplementary layers has been found to significantly improve the flexural and shear capacities of HCSs. Experimental research by Ghamry et al. [
13] demonstrated that a layered hollow-core slab design with a 90 mm top layer of HSC and a lightweight aggregate concrete bottom layer resulted in substantial improvements in ultimate strength, particularly when optimized with lower shear span-to-depth ratios and higher reinforcement ratios. Similarly, strengthening with Ultra-High-Performance Concrete (UHPC) led to a 21.4% increase in ultimate load capacity and a 45.8% reduction in deflection, highlighting the enhanced composite action achievable when UHPC is integrated into prestressed systems [
14].
Furthermore, using silica fume and polypropylene fibers in HCSs has shown to optimize compressive and flexural strengths [
15]. Recent studies have highlighted the pivotal role of innovative reinforcement strategies in enhancing the structural performance, ductility, and serviceability of HCSs. Cho et al. [
16] investigated the influence of hollow-core ratios on shear capacity in prestressed HCSs, revealing that increased void ratios compromise peak shear resistance. Sahoo et al. [
17] demonstrated that incorporating macro-synthetic fibers improved serviceability and strain energy absorption, though peak load capacity remained largely unaffected. Complementarily, Naser et al. [
18] observed significant enhancements in compressive and flexural strengths when utilizing steel fibers, steel wire mesh, and carbon fiber-reinforced polymer bars, particularly with steel fiber reinforcement. Al-Fakher et al. [
19] introduced a Composite Reinforcing System with four external flanges, which improved mechanical interlock and stability while reducing the dependence on prestressing and simplifying construction processes.
Beyond conventional fibers and flanged systems, alternative and composite reinforcement methods have shown substantial promise. Khalil et al. [
20] reported that using Ultra-High-Performance Strain-Hardening Cementitious Composites tubes to form voids in HCSs enhanced ultimate load capacity by 54% and reduced deflection. Awad and Al-Ahmed [
21], embedded steel tubes into one-way slabs, achieving a 59.02% increase in ultimate load capacity and a 12.75% rise in initial crack resistance. Similarly, Lee et al. [
22] developed the TUBEDECK system-composite slabs incorporating paper tube voids, which demonstrated superior ductility and load-bearing capacity compared to traditional solid and standard HCSs. Among these advancements, the incorporation of steel sheets as reinforcement has emerged as a particularly effective method for enhancing ductility and resilience in HCSs. This approach leverages the tensile strength and energy dissipation capacity of steel to delay brittle failure and support greater post-cracking deformation. For instance, Lee et al. [
22] showed that composite steel deck slabs with circular voids exhibited improved ductile behavior and energy dissipation. Awad and Al-Ahmed [
21] similarly reported more gradual failure modes and higher deflection capacities with embedded steel tubes. Li et al. [
6] further confirmed these benefits using steel bars and prestressed steel wire ropes, which significantly improved ultimate deformation and load capacity.
These findings are reinforced by Jin et al. [
23] and Khalil et al. [
20], who emphasized the mechanical synergy between concrete and steel in composite systems. Collectively, these studies affirm the effectiveness of steel-based reinforcement in extending the service life, improving seismic performance, and ensuring the overall structural integrity of HCSs.
Despite the growing body of research on reinforcement, limited attention has been given to the use of waste plastic fibers in slabs. While studies on fiber reinforcement in beams have explored the potential of waste Polyethylene Terephthalate (PET) fibers [
24,
25], there remains a gap in research specifically addressing the use of plastic fibers in concrete slabs. This gap in literature offers a significant opportunity for innovation. Mohammed et al. [
26] explored the use of PET bottles with steel meshes to fabricate one-way RC voided slabs. Load–displacement responses of voided and SSs were comparable up to the ultimate load, beyond which voided slabs with larger void depths exhibited higher load capacities and reduced deflections. Recent studies by Suchorab et al. [
27], Safayenikoo [
28], and Suksiripattanapong et al. [
29], on other types of fibers, including metalized plastic waste fibers and polypropylene fibers, have demonstrated improvements in flexural performance and toughness. These findings affirm the potential of fiber reinforcement yet highlight a clear need for deeper investigation into the behavior and optimization of waste plastic fibers in structural slab applications.
Considering the existing gaps in current research, as highlighted earlier, this study introduces a novel composite voided slab system incorporating steel sheets as tension reinforcement and spiral waste plastic fibers to enhance performance and sustainability. While voided slab systems offer benefits such as reduced weight and improved structural efficiency, their construction complexity, particularly when prestressing is required, remains a challenge. The proposed system addresses this issue by integrating steel sheets, which serve both as formwork and reinforcement, simplifying the construction process, reducing costs, and improving ease of construction. Additionally, the use of spiral waste plastic fibers as reinforcement in slabs represents a pioneering approach, marking, to the best of the authors’ knowledge, the first experimental and analytical investigation into their potential. This research fills a critical gap by not only exploring the use of innovative materials but also by providing a practical solution that merges sustainability with structural efficiency. By combining these advanced materials, the proposed system aims to offer a lightweight, high-performance, and eco-friendly solution, paving the way for the future development of more sustainable and efficient construction practices.
2 Methodology
2.1 Finite element analysis (FEA)
The methodology consists of two phases; phase A, where ANSYS Workbench Software was used to model and analyze 13 slabs, and phase B where the FEA was verified experimentally. Conducting FEA prior to the experimental investigation was proposed to reduce the costs related to casting and testing of huge number of slabs experimentally, to reduce the time required to accomplish satisfying and reliable results, and to maximize the use of the benefits of FEA software. Indeed, ANSYS software has been used extensively in the last decades and proved to produce reliable results [
25,
30,
31].
2.2 Specimens design
Thirteen slabs shared the dimensions of 1500 mm × 500 mm × 150 mm and were modeled and analyzed to assess their load carrying capacity, deformation, modes of failure, ductility and stiffness. A SS and a hollow core slab (HO) with normal reinforcement of 10 mm, and 50 mm diameter voids, was considered as control specimens. A steel sheet of 1 mm thickness was added to the HO with three different projections welded to the top and sides of the steel sheets’ flanges. These projections contribute to the improvement of the bond between the steel sheet and concrete. The details and dimensions of the steel sheets and slabs are illustrated in Figs. 1–12. The main purpose of adding the steel sheet is to function as tension reinforcement at the bottom of the slab. However, the role of the steel sheet is not limited to resisting forces during the service life of the structure, but also to perform as formwork during construction as concrete is poured on top of it immediately with no need of any prior preparation. This effectively reduces the costs of propping and unpropping and at the same time improves the structural integrity and performance of the slab.
Due to the special geometry of the slab, as the longitudinal voids are still considered openings within the body of slab, stress concentration is highly expected near and surrounding the circular voids. Therefore, two new types of shear connectors were suggested named as Crossed-Bar (CB) shear connectors and Hat-Section (HS) Shear connectors. The novel shear connectors surround the hollow cores and reinforce them to reduce the stresses. They are welded to the steel sheets at equal intervals of 300 mm in compliance with IS 3935-1966 [
33]. The newly proposed shear connecters were compared to the commercially used welded shear studs. The main aim of the new shear connectors is to reduce the stress concentration and to strengthen the weak concrete areas around the circular voids. However, they still contribute to the bond enhancement between the concrete and steel sheets. Details of shear connectors are illustrated in Figs. 13–16. The specifications of the slabs are illustrated in Table 1.
Composite voided slabs with no shear connectors were first analyzed to identify the slab with the steel sheet configuration that yields optimal load carrying capacity. Subsequently, shear connectors were incorporated to evaluate their influence on structural performance. Finally, the slab combining the optimal steel sheet configuration and shear connector was further investigated with the addition of spiral waste plastic fibers.
2.3 Materials’ properties
Material’s properties for both FEA and experimental investigations were determined based on primary tests on both concrete and plastic fibers, with the results proposed in Ref. [
33]. Following IS 10262-2019 [
34], the study was conducted on M25 conventional concrete as well as spiral plastic waste fiber RC to determine the mechanical properties. The spiral shape of plastic fibers was chosen as they showed the optimum results, in terms of the compressive, tensile, flexural and impact strengths, compared to the other two shapes under investigation (straight and channel). The compressive strength of spiral waste fiber RC was found to be comparable to that of conventional concrete. This result represents a significant advancement, as the incorporation of waste fibers has typically been associated with a reduction in compressive strength of concrete. Additionally, the tensile strength exhibited an improvement of approximately 3%. The fabrication of spiral waste plastic fibers was done manually, post collecting and cleaning of waste single use polypropylene food trays. The trays were cut into strips of 3 mm width and 0.3 mm thickness, and then fabricated into the required shape of 30 mm length (Fig. 17). The aspect ratio is 100, the tensile strength is 540 MPa, the elongation ranges between 7% to 10% and the aspect ratio is 10. The optimum fibers dosage was 0.6% as stated in Ref. [
33]. Concrete was designed as per IS 10262-2019 [
34] with the mixing proportions as illustrated in Table 2. The polycarboxylic ether superplasticizer was added to concrete with fibers to improve its workability.
The yielding strength of the steel sheets is 250 MPa. The top reinforcing mesh is of 8 mm, with a yielding strength of 250 MPa, which is particularly added to resist temperature effects. The spacing and distribution of the bars is illustrated in Figs. 1–12. The normal reinforcing that is located at the bottom of the control SS and HO is of 10 mm diameter and a yielding strength of 500 MPa. The spacing between the bars is 200 mm. Moreover, the novel shear connectors (crossed bars and hat section shear connectors) are made of 8 mm steel bars with a yield strength of 250 MPa. The construction method is illustrated in Fig. 18.
2.4 Materials modeling and meshing
Twenty nodes Solid 186 finite element (FE), which is a high-order 3D element, was chosen to model concrete for its ability to accurately model concrete, including significant deflections, plastic deformation, creep, and other relevant mechanical behaviors. Each node of the element has three degrees of freedom: translation in X, Y, and Z directions. Drucker–Prager concrete model was adopted to accurately capture the nonlinear and asymmetric behavior of concrete in compression and tension.
The same element was used to model the steel sheet, while REINF265 FE was used to model the shear connectors and reinforcing bars. The number of nodes and their degrees of freedom are identical to the base element which is the Solid 186 in this case. REINF265 is a specialized ANSYS element formulated to represent reinforcement layers (reinforcement) or discrete bar clusters (Hat section and crossed bar shear connectors) embedded within a host solid element, without the need to explicitly mesh the steel as separate solids or beams. This approach ensures perfect bond between the reinforcement and the surrounding concrete, while significantly reducing the model’s degrees of freedom compared to discrete three dimensional (3D) or beam elements.
To optimize computational efficiency, a half-slab model was developed and utilized throughout the analysis, by taking advantage of the symmetry within the central section as shown in Figs. 19–21. A compatible mesh was adopted to ensure accurate interaction between the REINF265 reinforcement and the surrounding SOLID186 concrete elements. The mesh was refined along the reinforcement and shear connector paths so that each REINF265 element remained fully embedded within individual solid elements, enabling proper force transfer. Local refinement was introduced around the HS and CB to capture stress concentrations and improve numerical accuracy, while a coarser mesh (20 mm) was applied in less critical regions to optimize computational efficiency. The convergence was insured by trial tests.
The slab is simply supported at 100 mm from the edges and subjected to loads at L/4 = 375 mm at two points. The load was increased gradually until the failure of the slab model, which was indicated by the software when the model reached the point of divergence.
The debonding at the interface was modeled using nonlinear frictional contact with a 0.5 frictional constant. This method allows for realistic simulation of sliding and separation during debonding since the stress transfer across interfaces relied only on the frictional resistance between the contacting surfaces.
2.5 Test setup and instrumentation
For the experimental validation, the specimens were subjected to a load-controlled test with a loading rate of 0.2 kN/s, using a loading frame of 1000 kN, with the concentrated load applied through a 200 t hydraulic jack. A distribution girder was used to achieve load transfer. Simple supports were provided using pin and roller supports. Three linear variable differential transformers (LVDT) were positioned under the point load, above the support on top of the slab, and at the mid-span to measure the slabs’ deflection as shown in Fig. 22. Strain gauges were placed on the steel sheets, shear connectors as well as mid span of concrete and were connected to a digital acquisition system with 8 channels.
3 Results and discussion
3.1 Load vs. deflection behavior
Figures 23–25 show the stresses of rectangular, trapezoidal and triangular projections, and Table 2 depicts a summary of the FEA. Results showed that the behavior of composite voided slabs tends to show higher load carrying capacities as well as deflections compared to SS and normally reinforced hollow core specimens (HO). Adding steel sheets considerably increased the load carrying capacity by 44.6%, 44.2%, and 43.7% for RC, TZ, and TR in relative to SS, while the improvement was 49.50%, 49%, and 48.6% for RC, TZ, and TR, respectively, compared to HO. It can be noticed that the configuration of the proposed projections did not cause significant variation, though composite voided slabs with rectangular and trapezoidal projection recorded very close load capacities compared to the slab with triangular projection. This is explained by the stress transforming mechanism between the projections and the steel sheets which is governed by the number of projection legs and their respective angels as shown in Figs. 23–25. It is seen, from Table 3, that despite the close load carrying capacities of both RC and TZ, the deflections considerably vary. The deflection of RC was 11.60 mm while it was 40 mm for TZ, which is 71% higher. This observation underscores the potential adaptability of each slab for diverse applications contingent upon specific structural demands, where TZ may be more suitable to structures where higher ductility is required. Therefore, both slabs were investigated with the three shear connectors to get a clear vision on their behavior.
Results showed that adding shear connectors to the slabs improved their load capacities as well as deflections. In the case of RC, it was noticed that the ultimate loads of RC-W, and RC-CB were nearly identical with an approximate 11% increment compared to RC. However, RC-HS recorded a notable drop of 7% as shown in Figs. 26–28. The deflections were almost identical which is attributed to the stress distribution due to the different shapes of shear connectors as well as the stress transferred from the steel sheets to the shear connector as shown in Figs. 29–34.
On the other hand, for composite voided slabs with trapezoidal projection, the slab with HS exhibited the highest load carrying capacity of 370.40 kN, while the lowest was 321.70 kN for the slab with the commercial welded studs shear connectors that failed much earlier. The Hat-Section configuration likely promoted more efficient shear transfer at the slab–concrete interface, resulting in improved composite action and delayed failure mechanisms (Figs. 29–34). Interestingly, the deflections of the three slabs were approximately equal, suggesting that the global flexural stiffness of the systems was governed more by the slab geometry and steel sheets’ configuration than by the type of shear connector. This suggests that while the shear connectors played a critical role in the ultimate load resistance, their influence on serviceability performance reflected in deflection behavior was marginal under the tested conditions.
Spiral waste plastic fibers were added to RC-CB-F and TZ-HS-F, as they recorded the highest load carrying capacities. Interesting results were noticed as the load capacities in both cases did not increase, on the contrary in the case of TZ-HS-F, it dropped by 3%. This reduction comes in compliance with findings of Refs. [
35,
36], as the same sort of decrement in loads was noticed when PET fibers were added to beams. The deflection values in both cases were similar to slabs with conventional concrete with a marginal variation of no more than 2%. This similarity in deflection values suggests that the spiral fibers may help in controlling the cracks which increases the post-crack stiffness and delay deflections.
3.2 Failure modes
The failure mode of both SS and HS was flexural failure with ultimate loads of 184 and 168 kN, respectively. In the case of composite voided slabs, the failure was longitudinal shear failure, where combined vertical and horizontal debonding were noticed at the concrete–steel interface, indicating a loss of composite action. The debonding occurs due to longitudinal and vertical shear stresses, and their values refer to the bonding strength between the steel sheets and concrete. In the case of composite voided slabs with no shear connectors, the projections serve as mechanical interlocks, and the difference in their configurations account for the variation in debonding behavior. Vertical debonding ranged between 3.61 mm for TR to 7.17 mm for TZ, while horizontal slippage varied from 6.18 mm for TR and 10 mm for TZ. This indicates that the triangular projections are able to maintain better bonding. Adding shear connectors notably improved the bonding behavior of the slabs, as the debonding values reduced in a consistent range. Composite voided slab RC-CB attained the lower thresholds of 2.41 mm for vertical gap and 3.82 mm for horizontal slippage, marking a reduction of 63.7% and 59.8% compared to RC. On the other hand, the TZ-HS, achieved an optimal value of 2.36 mm for vertical gap and 3.77 mm for horizontal slippage. These values are 67% and 62.6% less than those of TZ. Figures 35–38 shows the vertical and horizontal debonding for RC-CB and TZ-HS.
Moreover, RC-CB-F and TZ-HS-F displayed values akin to those of RC-CB and TZ-HS, suggesting that the presence of waste plastic fibers does not compromise the bonding strength, as the fibers primarily interact within the concrete matrix alone.
3.3 Ductility
Ductility values, shown in Table 3, are assessed based on the ductility ratio and the EAI, with the latter obtained based on the equation:
where
A1 is the area below the elastic domain and
A2 is the area below the plastic domain as shown in Fig. 39. Fazaa et al. [
37] have discussed the mentioned methods in their research on voided slabs. A notable improvement in the ductility was observed for the composite voided slabs with welded projections, with an increase of approximately three times for the TR and four times for TZ, when compared to HO. The trapezoidal projection configuration achieved the highest ductility ratio surpassing the rectangular and triangular projections by 41.2% and 40.2%, respectively. Furthermore, the EAI for TZ was found to be 59.5% and 47% higher than that of RC and TR, respectively, indicating a significant enhancement in the slab’s post-yield energy dissipation and superior deformation capacity.
The introduction of shear connectors further enhanced the ductility of the composite voided slabs, with the CB demonstrating the most significant ductility for both RC-CB and TZ-CB configurations. Specifically, when applied to RC, the crossed-bar shear connector in RC-CB achieved a ductility ratio exceeding those of the of RC-W and RC-HS by 3% and 1.6%, respectively. In case of TZ, the application of crossed-bar shear connector resulted in an excellent ductility ratio of 6.61, which is 14.8% and 13% higher than TZ-W and TZ-HS.
The addition of waste plastic fibers led to a significant improvement of ductility properties for the composite voided slabs. RC-CB-F achieved a ductility ratio of 8.2 which falls under the excellent rating category and showed a 25.4% increase compared to RC-CB. RC-CB-F displayed an enhanced EAI with a value of 30.95 that exceeded the EAI of RC-CB by 44.23%. Similarly, TZ-HS-F exhibited a ductility ratio of 6.92, also classified as excellent, recording a 17% enhancement compared to TZ-HS. The EAI for TZ-HS-F reached 23.83, recording a 26.5% increase over TZ-HS. The significant improvement in ductility is mainly attributed to the reinforcing action of the spiral waste plastic fibers, which delays crack propagation and improve both the tensile and flexural capacities of the concrete matrix.
3.4 Stiffness
Ki, which is the yield load divided by the deformation at the same point, as well as the Ks, which is identified as the failure load divided by the corresponding deformation, were calculated in Table 3. The stiffness of the composite voided slabs exhibited a reduction in comparison to the control slabs SS and HO. However, RC achieved the most substantial Ki of 94.31 kN/mm, which is 61.6% and 61.7% higher than that of TZ and TR. The improved stiffness of RC is attribute to the greater mechanical interlock provided by the geometry of the projection, that also enhances the resistance to early-stage deformation.
For RC, the introductions of shear connectors led to an overall reduction in stiffness compared RC. Nevertheless, among the studied shear connectors, the cross-bar connector recorded the highest stiffness of 54.04 kN/mm for Ki and 21.82 kN/mm for Ks. This suggests that the introduction of shear connectors may have disrupted the stress distribution, when the RC provided sufficient mechanical interlock on its own as shown in Fig. 40. In contrast, for TZ, the addition of shear connectors improved the stiffness, with the cross-bar connector again achieving the highest stiffness value. The presence of shear connectors added a complementary effect along with the TZ, which together improved force transfer across the slab interface and provided better resistance against deformation. Based on the previous discussion, it can be concluded that the Ki of the composite voided slabs depends significantly on the projections’ geometry. Generally, cross-bar connectors consistently recorded the highest stiffness despite the variation of projections configurations. This divergence in the properties of slabs with the same shear connector emphasizes the importance of the interaction between the projection geometry and connector design in optimizing composite voided slab performance.
Since the plastic material (Polypropylene) of the spiral waste plastic fibers is softer and more flexible than the concrete matrix, and as the elastic modulus of concrete decreased, the slight reduction of the slabs’ stiffness when fibers were added was expected. The stiffness of RC-CB-F reduced by 7.8% compared to the same with conventional concrete RC-CB. On the other hand, for TZ-HS-F slab, the stiffness reduced by 4.4% compared to the same with conventional concrete and by 13.6% compared to TZ-CB.
4 Experimental validation
For experimental validation, four slabs were casted and tested to validate the FE analysis results. Solid and normally reinforced HOs, and two composite voided slabs. Composite voided slabs RC and RC-CB with conventional and spiral plastic waste fibers RC were chosen to be tested based on their optimum load carrying capacity compared to other specimens. Steel sheets were manufactured with the projections being welded to the top and sided of the flanges as shown in Figs. 41–43, and they were considered as the bottom face of the mold that holds concrete on top of it during casting. Crossed bar shear connectors were also fabricated and welded to the steel sheets on equal intervals of 300 mm as shown in Fig. 43, and a special wooden mold was made to host the Polyvinyl Chloride (PVC) pipes (Fig. 44). The slabs were casted and cured for 28 d as shown in Figs. 45 and 46. Table 4 shows the experimental test results.
4.1 Load vs. deflection behavior
Table 4 presents the results of the experimental investigation. The observed structural response closely aligned with the trends predicted by the FEA by Ansys. Composite voided slabs recorded higher load carrying capacities and deflections compared to the control specimens. Specifically, composite voided slab incorporating rectangular projections, crossed bars shear connectors and conventional concrete (Experimental) (RC-CB-EX) achieved a load carrying capacity of 370.31 kN, which is approximately 1% lower than the prediction of FEA. On the other hand, RC-CB-F exhibited a load carrying capacity nearly 1% greater than composite voided slab incorporating spiral waste plastic fiber reinforced concrete (Experimental) (RC-CB-F-EX). Interestingly, on the contrary to the FEA results, the deflection of RC-CB-F-EX was 3.2% lower than that of RC-CB-EX. This unexpected reduction was attributed to fiber balling noticed during the concrete placement. When fibers are not uniformly distributed, they tend to form localized zones of high stiffness and low workability, resulting in heterogeneous internal structure. This field observation suggests the need for improved mixing method that ensures uniform fibers distribution.
However, both composite voided slabs showed significantly enhanced structural performance. The ultimate load of RC-CB-EX improved by 50.3% and 59.5% compared to Solid slab (Experimental) SS-EX, and normally reinforced hollow core slab (Experimental) (HO-EX), respectively. Similarly, the ultimate load of RC-CB-F-EX enhanced by 51.7% and 59.7% in comparison with SS-EX and HO-EX.
The load–deflection curves shown in Figs. 47 and 48 reveal that, unlike the behavior of SS-EX and HO-EX, the composite voided slabs exhibited a ductile response characterized by a softening trend, demonstrating improved energy absorption and deformation capacity. The comparison between experimental and FE analysis results demonstrated strong agreement, with the deflection remains within acceptable margins. For the control specimens, the difference between measured and simulated deflections did not exceed 10%, while for the composite voided slabs, the difference was limited to 5.7%. This consistency emphasizes the reliability of the adopted analytical model.
4.2 Failure modes
The tested slabs exhibited different failure modes influenced by their structural configurations. The control specimens, SS-EX and HO-EX, showed classic flexural failure. In both slabs, failure process started with yielding of tensile reinforcement, followed by progressive concrete crushing in the compression zone.
In contrast, the composite voided slab RC-CB-EX, experienced debonding between the steel sheet and the concrete matrix. The progressive debonding, that is a result of the increment of longitudinal and vertical shear stress, led to the gradual loss of composite action. As the load increased, separation intensified leading to rupture due to weld failure between the steel projections and the steel as shown in Figs. 49 and 50. The rupture occurred without significant deflection in the slab.
The RC-CB-F-EX exhibited an improved failure response. Although the dominant failure mode remained shear debonding, the inclusion of spiral waste plastic fibers delayed the separation phenomena and enhanced post-cracking resistance. The fibers helped maintain integrity across the steel-concrete interface by reducing stress concentration and absorbing part of the applied energy. Therefore, weld failures were not noticed, and the rupture stage involved more controlled separation with reduced horizontal and vertical slippage. The failure of RC-CB-F-EX was slower indicating improved ductility and energy dissipation capacity. The horizontal and vertical debonding were less than 10 mm in both RC-CB-EX and RC-CB-F-EX, with the horizontal slippage and vertical separations of RC-CB-F-EX being 2.5 and 4 mm, respectively, which were less than those of RC-CB-EX by 1.26 and 1.2 times.
These results align with findings by Ref. [
33], where spiral fibers demonstrated superior mechanical performance due to their complex geometry, which improved anchorage and stress transfer efficiency up to fiber fracture.
4.3 Crack widths and distribution
The observed crack widths and distributions varied significantly among the slabs, indicating differences in stress transfer mechanisms and structural behavior. In SS-EX, three major flexural cracks developed at mid-span as shown in Figs. 51–54, with the first crack being observed when the load reached about 27% of the ultimate load. The maximum crack width increased progressively from initial of 3 mm at the onset of cracking to 26 mm at the failure load (Figs. 53 and 54). In contrast, HO-EX demonstrated a more intense cracking pattern, with greater number and reduced widths of flexural cracks forming along the span (Fig. 57). This behavior is characteristic of HO, where stress redistribution across multiple cracking reduces local cracks openings. In contrast, the monolithic nature of SS concentrated stresses into fewer location, leading to wider crack formations.
In RC-CB-EX, multiple narrow flexural cracks were observed near the mid-span, none exceeding 2 mm in width (Figs. 55–56). Though, shear crack developed along the side of the slab, particularly near the edges of the steel sheet and the voids. The most significant shear crack reached a width of 7 mm and extended longitudinally near the bottom edge, terminating above the support region. This crack pattern suggests a combination of flexural and shear stresses, with the shear stresses amplifying crack growth along the horizontal path defined by tensile stress concentrations. Moreover, the shear cracks observed in areas surrounding the voids, indicated localized debonding along both horizontal and vertical planes due to weak interfacial bonding in these zones.
The RC-CB-F-EX demonstrated improved cracking response. The number and width of crack significantly reduced in comparison to RC-CB-EX. While flexural cracks remained below 1 mm in width, the widest shear cracks reached about 3 mm as shown in Fig. 58. This behavioral improvement is directly attributed to the incorporation of spiral waste plastic fibers, that enhanced the crack-bridging capacity of the concrete matrix. The fibers, when under loading, effectively redistributed tensile stresses and limited crack propagation by engaging progressively and resisting micro-crack widening. In result, the overall structural integrity of RC-CB-F-EX was maintained during loading, as evidenced by the Ks values illustrated in Table 4.
4.4 Validation
Results of load carrying capacities and deflections for both FEA and experimental results are illustrated in Figs. 47 and 48. Based on the similarity of the outcomes of both experimental tests and the FE analysis, it can be stated that the FE model is verified and the methodology followed may be appropriate for other research. The experimental and FE outcomes were within 10% agreement across all cases, and their structural behavior exhibited strong similarity (Fig. 59–61), where the horizontal slippage and vertical debonding were almost identical in experiment and simulation for composite voided slab with rectangular projections, crossed bar shear connectors and conventional concrete.
5 Conclusions
The aim of the current study was to investigate a novel type of slabs called composite voided slabs with different welded projections configurations (rectangular, triangular, trapezoidal) and specially designed shear connectors (crossed-bar and hat section shear connectors). Spiral waste plastic fibers were used to reinforce concrete, which is a first-time attempt. Thirteen slabs were analyzed using ANSYS software and the best performing configuration was experimentally tested using conventional and spiral waste plastic fiber concrete. The key conclusions are as follows.
1) Composite voided slabs reinforced with steel sheets achieved an ultimate load of 370 kN, improving capacity by 50.3% over SSs and 59.5% over conventional HCSs.
2) Projection geometry strongly influenced deflection: rectangular projections = 11.6 mm, while TZ = 40 mm (71% increase), suggesting different application potentials.
3) Novel shear connectors enhanced bond strength: RC-CB and TZ-HS reduced vertical and horizontal debonding by more than 60% compared to slabs without connectors.
4) Spiral waste plastic fibers improved ductility, raising ductility ratios by up to 25% and energy absorption indices by over 40%, while maintaining similar load capacity.
5) FE predictions were validated experimentally, with maximum differences of 1% in load capacity and 5.7% in deflection, confirming the robustness of the FE model.
6) The proposed slab system demonstrates a sustainable and efficient alternative for future construction, combining enhanced strength, ductility, and crack control with the environmental benefits of recycled fibers.
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