Experimental Study on the Flexural Performance of Grooved-Connected Truss-Reinforced Concrete Composite Slabs

Abstract

To address the conflicts between traditional composite slab reinforcement layouts and supports—which adversely affect construction quality and efficiency—and to fill the theoretical gap regarding end connections without projecting bars in terms of interface shear transfer, staged flexural behavior, and anchorage reliability, a grooved end-connection configuration for composite slabs is proposed. In this configuration, the longitudinal bars of the precast slab do not extend beyond the slab end. The precast slab end is formed with a recessed–protruding profile; the longitudinal bars are exposed within the groove, where additional reinforcement is pre-embedded (with a diameter not less than the area-equivalent of the longitudinal bars that would otherwise extend into the support). After erection, the additional bars are extended using straight-thread sleeves; short longitudinal bars within the groove are tied to the bottom longitudinal bars. Both the extended additional bars and the short longitudinal bars are anchored into the support by at least 5d and pass the support centerline. To evaluate the global flexural behavior of slabs with grooved end-connections, a two-span, full-scale specimen was tested under static loading. Failure characteristics, crack initiation and propagation, ultimate capacity, deflection, and ductility were investigated. The results indicate that, in the full-scale two-span test, the service load was 11.35 kN/m² (approximately 13.5% higher than the design value of 10.0 kN/m²); the midspan deflection was about L/110 (smaller than the L/50 limit); the first cracking and the pronounced nonlinearity inflection point occurred at approximately 4.25 kN/m² and ≥9.35 kN/m², respectively; and the maximum crack width was 1.66 mm. The test was terminated prior to reaching the durability and deformation limits, after which the load was increased to 22.20 kN/m². The specimen exhibited a ductile flexural failure governed by tensile reinforcement yielding; the top concrete did not crush, no shear failure was observed at the ends, and no delamination occurred at the composite interface, demonstrating favorable global flexural performance.

1. Introduction

Prefabricated construction, enabled by stable component production environments and assembly-line manufacturing, effectively ensures component quality while reducing formwork costs and improving production efficiency. By meeting part of the demands of building industrialization, it has attracted increasing attention in contemporary architecture [1,2,3,4,5,6].

As a critical type of precast concrete element, composite slabs are widely employed in prefabricated concrete structures. In current engineering practice, truss-reinforced concrete composite slabs with longitudinal reinforcement extending from slab ends and sides are predominantly used. To ensure diaphragm action and the integrity of the floor system, as well as to satisfy horizontal force transfer requirements, the longitudinal tensile reinforcement within the precast portion at slab supports must be extended out of the slab end (hereinafter “projecting bars”) and anchored into the cast-in-place concrete of the supporting beam or wall by not less than 5d (where d is the diameter of the longitudinal tensile bar) and must pass beyond the support center line [7].

However, during hoisting and installation, these projecting bars frequently conflict with the reinforcement already placed in the supporting beams, leading to reinforcement congestion and physical interference (see Figure 1). This problem is particularly acute in moment-resisting frame systems located in high seismic intensity regions: wider frame beams demand longer bar extensions, which exacerbate interference and complicate placement. The difficulty becomes more pronounced when installing composite slabs at locations where both transverse and longitudinal frame beams are adjacent, necessitating repeated adjustments to achieve final positioning and thereby significantly reducing construction efficiency.

Recent years have seen extensive domestic and international research on mitigating the interference caused by projecting reinforcement at precast slab ends. Two principal solution strategies have emerged—(i) non-projecting reinforcement details at precast slab ends for the bottom (precast) layer [8,9,10,11] and (ii) separated or discontinuous joint details (“segmented” or “split” joints) [12,13,14,15,16,17,18,19,20,21,22]—with numerous studies evaluating their structural performance. Zhang Jiajun [23], drawing on relevant Chinese and U.S. standards, established possible shear failure modes at the composite slab end and proposed corresponding shear capacity formulations. Parametric case studies indicated that the slab-end shear capacity can be ensured by adjusting the cast-in-place topping thickness, the overall reinforcement ratio of the composite slab, and by providing shear-resisting reinforcement across the interface. Provided that the slab end possesses adequate shear capacity, the longitudinal reinforcement at the slab end need not project beyond the precast unit. Zhang Xuefeng [24] proposed a closely butted composite slab system with non-projecting reinforcement on all four sides. Through in situ loading tests benchmarked against cast-in-place slabs, the study compared global and sectional responses. The results showed that the four-edge, non-projecting, closely butted composite slabs exhibited higher cracking load and superior flexural stiffness relative to conventional cast-in-place composite slabs. Liu Wei et al. [25] introduced an L-shaped groove, non-projecting composite slab detail, in which L-shaped recesses are formed at slab ends to accommodate edge and inter-slab connection reinforcement, thereby eliminating projecting bars. Full-scale load tests demonstrated that the overall mechanical performance of the L-groove non-projecting composite slab is comparable to that of traditional composite slabs and satisfies code requirements.

Relevant codes and standards [26,27,28] have also permitted indirect lap-type mechanical connections for precast slabs without projecting bars. However, these provisions typically require a cast-in-place topping thickness not less than 100 mm and not less than 1.5 times the precast slab thickness. Such requirements substantially increase floor dead load, which in turn amplifies seismic actions on the overall structure and necessitates additional reinforcement in slabs and beams. The resulting material and structural penalties render these solutions uneconomical and hinder their widespread adoption in practice.

To address these challenges, this paper proposes a grooved connection for truss-reinforced concrete composite slabs (see Figure 2). In this configuration, the longitudinal bars of the precast slab do not extend beyond the slab end. Instead, the slab end is cast in the factory with a profiled, recess–protrusion geometry; the longitudinal bars remain exposed within the recess. Additional stub “strengthening” bars are pre-embedded within the recess region, with a total area not less than that of the longitudinal bars located over the protruding portion at the precast bottom slab end that would otherwise not extend into the support. After hoisting and alignment, the additional bars are extended using parallel-thread couplers (straight-thread sleeves). Concurrently, short longitudinal bars are arranged within the recess and are tied/lapped to the bottom slab longitudinal bars. Both the extended additional bars and the short longitudinal bars are anchored into the support by a length not less than 5d (d = diameter of the respective longitudinal bar) and pass beyond the support centerline. To enhance force transfer reliability, short transverse bars are provided within the recess as construction detailing. All other fabrication and erection procedures remain consistent with those of conventional truss-reinforced concrete composite slabs. Because no bars project from the slab ends, on-site hoisting efficiency is markedly improved.

This study targets the following scientific issues: (i) the interface shear transfer and its post-cracking degradation for end connections without projecting bars; (ii) the force-sharing mechanism and ductility contribution of supplementary reinforcement and short bottom bars across different flexural stages; (iii) the effects of internal force redistribution in a two-span system on crack development and deflection evolution; and (iv) the safety reserve arising from the coupled shear–flexure resistance at the ends. To this end, the flexural performance under vertical loading of truss-reinforced concrete composite slabs employing grooved end-connections at the precast panel ends was investigated. Following prior studies on the flexural behavior of composite slabs [29,30,31,32,33,34,35], a full-scale two-span specimen was fabricated. By applying vertical loads on the slab surface, the failure mode, crack progression, load-carrying capacity, deflection, and reinforcement strains were systematically monitored to assess conformity with code-based calculations and to elucidate the end-force transfer path, thereby providing a basis for implementing this novel connection configuration in engineering practice.

2. Test Overview

2.1. Specimen Design

This study focuses on the global flexural performance of truss-reinforced concrete composite slabs employing grooved connections. A full-scale two-span test under static uniformly distributed surface loading was conducted to evaluate the load-carrying capacity, cracking behavior, and ductility of the connection under conventional vertical loading. As shown in Figure 3, the specimen consists of two equal-span composite slabs, a middle (interior) beam, and an edge (exterior) beam, and is designed as a one-way slab. The support at the middle beam simulates a fixed (encastre) restraint for the slab, while the support at the edge beam simulates a pinned restraint. Both the precast slab and the cast-in-place topping are 60 mm thick, giving a total composite slab thickness of 120 mm. Each individual composite slab has a span of 3300 mm and a width of 1500 mm. The overall span of the specimen is 7400 mm, with a width of 1500 mm. The bottom longitudinal reinforcement and the distribution reinforcement in the precast slab, as well as the distribution reinforcement in the topping, are C8@200. The topping’s support (negative-moment) reinforcement is C8@120. Additional strengthening bars are C16. The short longitudinal bars are of the same specification as the precast slab’s main reinforcement, and the short transverse construction bars are C6.

2.2. Material Properties

The specimens were produced using C30 ready-mixed concrete. From the same batch, two sets of twelve 100 mm cubic concrete specimens were reserved—six from the cast-in-place topping and six from the precast slab—and cured under the same conditions as the test specimens. Tests on concrete material properties were carried out in accordance with the Standard for Test Methods of Physical and Mechanical Properties of Concrete (GB/T 50081-2019) [36]. The measured values were converted as specified, and the results are reported in

Table 1. Mechanical properties of concrete.

Concrete Location	Compressive Strength (MPa)
Precast slab concrete	32.6
Cast-in-place topping concrete	35.5

.

HRB400 reinforcing bars were employed. Tensile tests on the reinforcing steel were conducted in accordance with Metallic Materials—Tensile Testing—Part 1: Method of Test at Room Temperature (GB/T 228.1-2021) [37]. The corresponding material property results are presented in

Table 2. Mechanical properties of reinforcing steel.

Rebar Specification	Yield Strength (MPa)	Ultimate Tensile Strength (MPa)
C8	421	665
C16	443	630

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2.3. Specimen Fabrication

The precast slabs were fabricated first, then transported to the test site for assembly and casting of the topping. The specimens were cured periodically until the concrete reached 100% of the design strength. The fabrication process of the specimen is shown in Figure 4.

Two rolling pinned supports were arranged beneath the edge beam of the specimen, as shown in Figure 5. Each support consisted of two steel plates measuring 1500 mm × 200 mm and a steel tube of Φ75 mm filled with concrete; two C16 shear dowels were welded to the upper steel plate of the support.

2.4. Test Setup and Loading Protocol

The loading scheme followed the Standard for Test Methods of Concrete Structures (GB/T 50152-2012) [38]. Sandbags were used to simulate the vertical surface load on the slab, with each bag weighing 0.35 kN. After partitioning the slab surface, the sandbags were uniformly distributed across the entire surface of the two spans. The sandbags were placed with a planned spacing of 500 mm × 500 mm, leaving gaps between bags to prevent anti-arching, and stacked layer by layer until the target load was achieved, as shown in Figure 6. Prior to formal loading, preloading was conducted to check the condition of the instruments. During formal loading, the two spans were loaded synchronously. The loading stages and corresponding load values are given in

Table 3. Loading stages and corresponding loads.

	Loading Stage	Load (kN/m2)	Loading Stage	Load (kN/m2)
Preloading	1	0.60	2	0.80
Formal loading	1	0.85	8	6.80
	2	1.70	9	7.65
	3	2.55	10	8.50
	4	3.40	11	9.35
	5	4.25	12	14.4
	6	5.10	13	17.68
	7	5.95	14	22.20

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Loading was terminated when any of the following conditions were met: (1) crushing of concrete in the compression zone within the span; (2) maximum soffit deflection reaching 66.6 mm (L/50); (3) maximum crack width at the slab soffit reaching 1.5 mm; or (4) fracture of the main tensile reinforcement or tensile strain reaching 0.01.

2.5. Instrumentation Layout

Reinforcement strains were recorded using an XL2118BX static strain indicator, with strain gauges on the rebars arranged as shown in Figure 7. Two dial gauges were installed along the short-span direction of the specimen at midspan (L/2) and at 3/8 span at the slab soffit, as shown in Figure 8.

3. Results

3.1. Crack Patterns

The crack patterns of the specimens are shown in Figure 9 and are predominantly flexural in nature. Crack development proceeded through three stages:

Stage I: Prior to reaching the cracking load, no visible cracks appeared, and the slabs behaved elastically.

Stage II: At the cracking load, initial cracks penetrated the tensile zone and became through-cracks. With further loading, both the number and width of cracks increased, and cracks propagated upward along the slab sides, eventually reaching the vicinity of the composite interface.

Stage III: The number of cracks stabilized, while crack widths continued to increase, with a noticeably higher growth rate than in Stage II.

The longest through-crack on the slab surface was approximately 75 mm, and the maximum crack width reached 1.66 mm. Cracks were relatively uniformly distributed near the junctions of the central rib and in regions between 1/2 and 3/8 of the span along the soffit and slab sides. These observations indicate a typical flexural cracking pattern for composite slabs, with crack initiation at regions of maximum tensile stress and subsequent stable crack propagation governed by tension stiffening and interface restraint.

Additional Remarks for Clarity

The transition from Stage II to Stage III coincided with the formation of a stable crack pattern and the onset of more pronounced stiffness degradation.

The tendency of cracks to approach the composite interface suggests effective load transfer through the interface; however, increased crack width in Stage III implies reduced tension-stiffening contribution of concrete as steel strain grows.

3.2. Load–Deflection Response

As shown in Figure 10, the load–deflection curves exhibit a characteristic tri-linear trend.

Stage I: Before Load Level 5 (4.25 kN/m²), deflection increased linearly with load. No cracking was observed at either the slab ends or midspan; the composite slab behaved elastically.

Stage II: Between Load Levels 5 and 10 (4.25–8.5 kN/m²), an inflection point appeared, and the response became nonlinear with a reduced slope, indicating the onset and progression of cracking.

Stage III: Beyond Load Level 11 (≥9.35 kN/m²), deflection increased nonlinearly with a new inflection point, corresponding to service-stage behavior with cracking of the soffit concrete. Relative to Stage II, both the deflection growth rate and the crack development rate increased significantly.

Loading was terminated upon reaching the predefined serviceability limit; at that time, the specimen had not fully entered the plastic stage. Hence, both ultimate deflection and ultimate load retained a reserve margin, evidencing pronounced deformation capacity. Overall, the grooved connection composite slab displayed favorable global flexural performance and ductility.

3.3. Reinforcement Strain Analysis

Strain gauges were installed at critical sections with adverse bending effects and at locations with bar geometry transitions near the slab ends. The load–strain curves are presented in Figure 11, allowing assessment of the reinforcement stress state throughout loading.

Figure 11a–c, top and bottom reinforcement: The responses are broadly tri-linear. In the initial loading range, steel strains increased linearly with load. After cracking, strain growth accelerated due to reduced tension stiffening of concrete. Once yielding was initiated, the apparent strain growth rate with respect to load diminished, reflecting stress redistribution and localization of plasticity in yielded zones.

Figure 11d–e, bottom slab end bars and additional reinforcement: According to the plane-sections assumption, at the member end under bending, fibers above the neutral axis are in tension, and those below are in compression. The bottom-end bars and the additional bars were positioned in the compressive zone. After concrete cracking, the height of the compression zone decreased, causing these bars to engage in compression. Their load–strain responses remained approximately linear within the measured range. At Load Level 13, the microstrain in the additional bar was approximately −500 με, compared with −175 με in the bottom-end bar, indicating that the additional reinforcement carried the predominant compressive force.

Comparative note: The load–strain evolution of the top and bottom bars is broadly consistent with the trends reported by Liu Zengcheng [39] for cast-in-place slabs, supporting the similarity in fundamental flexural behavior. The end bends provided for the additional bars effectively limited relative slip at the composite interface, preventing bar pullout and premature failure. As the load increased and the bottom slab transitioned from elastic to plastic response, the additional bars contributed significantly to compressive force transfer and overall rotational capacity. These findings elucidate the force transfer mechanism of the grooved connection, highlighting the roles of interface restraint and additional reinforcement in sustaining composite action after cracking.

3.4. Comparative Analysis of Load Values

Considering the self-weight and related conditions of the grooved connection composite slab, the load-carrying capacity was calculated using the design equations of the Code for Design of Concrete Structures [40] (GB 50010-2010); see

Table 4. Comparison of load-carrying capacities.

Design Load (KN/m2)	Cracking Load (KN/m2)	Yield Load (KN/m2)
10	7.25	12.35

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Treating the slab as a one-stage composite member, the cracking loads at the top surface over the supports and at the soffit were computed per the same code; see

Table 5. Comparison of cracking loads.

Location	Calculated Cracking Load (KN/m2)	Measured Cracking Load (KN/m2)
Top (slab surface) at support	4.21	4.25
Bottom of slab	12.20	9.35

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The measured cracking load at the top surface over the supports agrees well with the calculated value, whereas the measured soffit cracking load is lower than the prediction. From the standpoint of elastic analysis, once cracking occurs at the intermediate support during testing, internal forces are redistributed, causing the tension zone at the soffit to reach the cracking condition earlier; consequently, the actual soffit cracking load is lower than the theoretical value.

4. Discussion

4.1. Failure Mechanism Analysis

Under vertical loading, the specimens exhibited a typical flexural response that can be divided into three stages. In the elastic stage (prior to approximately 4.25 kN/m², applied via sandbags), both concrete and reinforcement strains were small, and the load and deflection remained limited. The onset of concrete cracking marked the beginning of the elastic–plastic stage (about 4.25–8.5 kN/m²). With increasing load, flexural cracks initiated at the soffit, propagated along the spanwise direction, increased in number, and extended toward both side edges. The plastic stage (beyond about 9.35 kN/m²) was characterized by a slow increase in load accompanied by rapid growth in deformation. In this stage, the number and width of soffit cracks increased markedly, the slab underwent large deflections, and loading was stopped when the preset deflection limit was reached. At termination, the applied load was 17.68 kN/m². The slope of the load–deflection curve had not begun to degrade, indicating that additional load-carrying capacity remained.

To elucidate the ultimate failure mode, after reaching the termination criterion, the specimen was subjected to a one-step heavy ballast load (22.20 kN/m²) until complete loss of load-carrying capacity. The specimen failed in a ductile flexural manner governed by tensile yielding of the bottom reinforcement (Figure 12). The top concrete did not crush, the specimen experienced large deformations without rupture, and no shear failure occurred at the slab ends. Side cracks were predominantly vertical, initiating at the soffit and extending upward to the top surface. Cracks were continuous across the composite interface, which maintained a good bond with no separation between the precast base slab and the cast-in-place topping. These observations indicate that the roughened interface provided sufficient shear transfer to meet load-carrying requirements.

Based on the test phenomena and crack evolution, the crack development pattern and failure mode of the two-span grooved connection composite slab are similar to those of a conventional cast-in-place one-way slab. The internal force distribution is also analogous to that of a cast-in-place slab, with negative moment zones over the interior support and positive moment zones within the spans behaving in a comparable manner.

4.2. Risks and Limitations

Loading-type limitations: The present study considers only static, uniformly distributed vertical loading; cyclic loading and in-plane shear coupling due to seismic actions are not covered. Low-cycle fatigue in the grooved connection region and the hysteretic energy dissipation capacity of the coupler connections require dedicated testing.

Insufficient parameter coverage: Systematic parametric analyses were not conducted for material grade, reinforcement ratio, interface roughness, topping thickness, or anchorage length. The conclusions are therefore applicable only within the parameter domain adopted in this study.

Constructability sensitivity: The performance is sensitive to the anchorage quality of straight-thread couplers and short bars. Field deviations—such as insufficient coupler tightening torque and bar misalignment—may affect interface composite action and crack control.

Scale and boundary effects: Although the boundary conditions of the two-span one-way slab approximate engineering practice, they remain a laboratory equivalent. Caution is advised when extrapolating to complex frames, slab openings, or layouts with dense secondary beams.

4.3. Comparative Analysis with Existing Studies

Addressing the long-standing issue of projecting reinforcement in conventional composite slabs, this section contrasts two prevalent solutions reported in the literature—L-shaped grooves without projecting bars and separated (split) joints—with the proposed groove-type connection. The advantages and limitations are summarized in

Table 6. Scheme Comparison.

Scheme	Advantages	Limitations
L-shaped groove without projecting bars	Eliminates bar projection and offers improved constructability; overall mechanical performance comparable to conventional practice	Requires precise fabrication and installation of the groove; interface shear resistance and local stress concentration must be controlled
Separated (split) joint	Rapid assembly; global integrity can be ensured through detailing and enhanced interface treatment	Codes often require a thicker topping layer, increasing self-weight and amplifying seismic demand
Groove-type connection	Simple fabrication with reduced lifting interference; flexural performance comparable to cast-in-place/conventional solutions	Local combined compression–shear effects at the groove must be controlled; performance is sensitive to the construction quality of mechanical couplers

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5. Conclusions

Addressing limitations in existing composite slab technologies, a channel-type end-connection detail for truss-reinforced composite slabs was proposed and evaluated through flexural testing. The following conclusions were drawn:

(1)

Compared with conventional protruding-rebar (dowel) composite slabs, the proposed grooved connection system optimizes slab geometry and the lapping/anchorage of additional reinforcement. The experimental results verify the feasibility of the grooved connection composite slab configuration.

(2)

The new configuration satisfies relevant code requirements for load-carrying capacity. Its failure mode and crack evolution are comparable to those of conventional cast-in-place one-way slabs, demonstrating favorable global performance and deformation capacity.

(3)

Under the design load, the measured midspan deflection was L/110, which is less than the code limit of L/50. The measured serviceability load capacity was 11.35 kN/m², exceeding the design value of 10 kN/m² and indicating a margin of safety.

Overall, the truss-reinforced concrete composite slabs employing grooved connection between precast panels exhibit safe and reliable flexural behavior under vertical loading. The connection detail is also straightforward for on-site construction, facilitating practical implementation. Future work should focus on the in-plane shear performance of this connection under seismic actions to clarify its behavior and to inform design provisions for earthquake-resistant applications.