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Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

71880
10.1038/s41598-024-71880-8
Article
Experimental study on the flexural performance of concrete hollow composite slabs with tightly connected panel sides
Chen Xudong xdchen_ah@163.com

123
Ma Qinyong 13
1 https://ror.org/00q9atg80 grid.440648.a 0000 0001 0477 188X School of Civil Engineering and Architecture, Anhui University of Science and Technology, Huainan, 232001 Anhui China
2 https://ror.org/0108wjw08 grid.440647.5 0000 0004 1757 4764 School of Civil Engineering, Anhui Jianzhu University, Hefei, 230601 Anhui China
3 National-Local Joint Engineering Laboratory of Building Health Monitoring and Disaster Prevention Technology, Hefei, 230601 Anhui China
6 9 2024
6 9 2024
2024
14 2078429 4 2024
2 9 2024
© The Author(s) 2024
2024
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The performance of joint connections in composite slabs is crucial for ensuring their bidirectional load-bearing capacity and overall structural integrity. To effectively address the challenge of protruding rebar in precast components, a new structural form of a tightly connected hollow concrete composite slab without protruding rebar on the slab side is proposed. To investigate the mechanical performance and failure modes of this slab-side tight-joint connected hollow concrete composite slab, bending performance tests under monotonic load were conducted on three tightly connected hollow composite slabs and one seamless hollow composite slab. The analysis focused on the failure modes, crack distribution, bending capacity, and bending stiffness of additional steel bars at the joints of the slabs. The results indicate that, under normal operating conditions, the flexural performance development of concrete hollow composite slabs with tightly connected slab sides is generally consistent with that of the non-spliced cast hollow composite slab. Under ultimate conditions, tearing or brittle fracture failure at the joint interface is likely to occur in the composite slabs, leading to a reduction in flexural bearing capacity. With an increase in joints, the bending capacity of the hollow composite slab decreases by 17.1%, Conversely, when joints are positioned away from the most stressed section, the flexural stiffness of the section increases by 13.5%. A comparison of experimental data and theoretical calculations indicates that the additional rebar at the joints effectively contributes to the lateral load transfer in the hollow composite slab, achieving bidirectional load-bearing capacity. This further validates that the design of the single-joint, tight-joint connected hollow concrete composite slab without protruding rebar meets the requirements for bidirectional load-bearing performance.

Keywords

Hollow composite slab
Tightly connected
Steel truss
Static test
Flexural performance
Subject terms

Engineering
Civil engineering
National-local joint Engineering Laboratory of Building Health Monitoring and Disaster Prevention TechnologyGG22KF001 GG22KF001 Chen Xudong Ma Qinyong Anhui Province Department of Housing and Urban-Rural Development Construction Science and Technology Plan Project2022-YF083 Chen Xudong issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

With the rapid evolution of industrialized construction, prefabricated concrete double-layer slab structures offer several advantages, including good integrity, high stiffness, short construction periods, and enhanced construction efficiency. As a result, their application in engineering projects is becoming increasingly widespread1–3. These composite slabs typically entail the assembly and pouring of multiple prefabricated bottom slabs, where the performance of joint load transfer becomes pivotal in ensuring bidirectional load-bearing capabilities and, consequently, influences the overall performance of the composite panel. There are two primary forms of joint connections: monolithic joints with post-poured strips and monolithic joints with tightly connected slabs. While the former exhibits superior overall performance, the presence of "whiskers" adversely impacts production and construction efficiency. In contrast, the latter type, characterized by efficient production and construction with a straightforward joint structure, demonstrates inferior bidirectional load-bearing performance compared to the former.

In recent years, domestic and foreign scholars have engaged in extensive experimental studies and theoretical analyses concerning the flexural performance of tightly connected composite slabs. Foreign scholars have conducted more analyses on the shear performance of the hollow composite slab interfaces4–8, with less research on the flexural performance of the hollow composite slabs. Yardim and others9 proposed a composite floor system filled with precast wire mesh cement slabs and conducted full-scale tests on precast slabs, with the test results showing that the SBS connection system improved the structural performance of the slab. Park and others10, through pull-out tests on composite interfaces under different roughness and embedded steel mesh conditions, determined the optimal composite arrangement for prestressed hollow composite slab interfaces. They also conducted bending tests on prestressed hollow composite slabs, demonstrating that the interfaces can meet the horizontal shear strength of the composite layer by incorporating rough surfaces or using steel mesh. Domestic scholars have been actively exploring and developing various new types of composite slabs and joint connection forms, and have conducted a significant amount of experimental research in this area. Ye and others11 conducted comparative analyses on the flexural performance of various composite slabs by manipulating the number, form, and location of joint connections. They complemented their experimental findings with nonlinear numerical simulations using ANSYS, thereby providing rational overall joint construction forms and calculation methods. Liu and colleagues12 introduced a reinforced overall joint connection form, conducting multiple experiments and revealing improved stiffness and load-bearing capacity. Cui and his team13 explored the impact of joint quantity, location, and the thickness of the cast-in-place layer on the bending performance of separated joint composite slabs. They proposed calculation formulas for the reduction coefficients of bending stiffness in two-joint and three-joint slabs, validated through numerical simulations. Yu and others14 delved into the flexural performance of single-seam tightly connected reinforced concrete composite slabs, offering insights into the load transfer mechanism of truss reinforcement joints and the quantitative relationship between truss reinforcement and longitudinal load-bearing reinforcement. Practical design recommendations and construction requirements for bidirectionally connected tightly joined composite slabs were presented. Lin and his team15 conducted a comparative analysis of three enhanced tightly connected composite slabs with additional reinforcement joints, integral groove joints, and spaced groove joints. Their findings affirmed the effectiveness of these joint configurations in improving ultimate bearing capacity and stiffness. He and his colleagues16 employed stirrups as shear reinforcement at separated joint connections in composite slabs, effectively improving tightly connected steel reinforcement bonding slip behavior and restricting crack development. Yang and his colleagues17 investigated the load-bearing performance of tightly connected concrete composite slabs, analyzing the load transfer mechanism of additional steel reinforcement in the slots and presenting calculation methods for the slabs' bearing capacity and stiffness. They validated the structural practicality through typical engineering cases.

The guidelines outlined in Specification JGJ1-2014, titled "Technical Specification for Assembly Concrete Structures"18, recommend the use of concrete hollow slabs for composite slabs with a thickness greater than 180 mm. However, limited information is available regarding the construction and design of tightly connected concrete hollow composite slabs, and the effective load transfer of joints in concrete hollow composite slabs is integral to ensuring bidirectional load-bearing and overall structural integrity. Combining the advantages of existing concrete hollow composite slabs, the research team proposed a steel truss concrete composite slab with embedded hollow thin-walled boxes19. On the basis of the study of the mechanical properties of hollow composite slabs, this paper conducts experimental research on the bending performance of the side non-protruding reinforcement tight joint connection of hollow composite slabs, in order to study the influence of joints on the stress performance of specimens. A total of three tight joint hollow composite slab specimens and one seamless hollow composite slab specimen were designed for comparison. The experimental results were compared with theoretical calculations to verify that the additional reinforcement at the joints played an effective role in the lateral non protruding reinforcement tight joint connection stress performance of hollow composite slabs, achieving bidirectional stress of hollow composite slabs and solving the problem of unidirectional stress of hollow composite slabs. This provides a reference for studying the mechanical properties and design methods of large-span precast concrete hollow composite floor slabs. The reference has further promoted the application of hollow composite slabs in engineering.

Experimental overview

Sample design and production

The experiment involved the design of four concrete hollow composite slab specimens, which are as follows: a seamless standard composite hollow slab (Specimen P1), a single-joint tightly fitted hollow composite slab (Specimens P2 and P3), and a double-joint tightly fitted hollow composite slab (Specimen P4). All specimens shared identical dimensions, measuring 4200 mm in length, 1400 mm in width, and 180 mm in thickness. The prefabricated bottom panel had a uniform thickness of 50 mm, and the dimensions of the hollow thin-walled box were 450 mm × 450 mm × 80 mm. Reinforcement mesh made of HRB400 steel with a diameter of 8 mm and a spacing of 100 mm was arranged in both the precast and cast-in-place layers of the slabs. The top and bottom chords of the trusses were made of HRB400 steel with a diameter of 8 mm, while the web bars of the trusses were made of HPB300 steel with a diameter of 6 mm. The composite surface was intentionally roughened along the direction of the truss reinforcement, with a roughness depth of no less than 4 mm. The joint structures of each specimen are shown in Fig. 1, and the main parameters are listed in Table 1.Fig. 1 Joint construction measures of specimens.

Table 1 Main parameters of specimens.

Specimen code	(L × B × H)/mm × mm × mm	Prefabricated panel joint construction features and arrangement of additional reinforcement	N/↑	
P1	4200 × 1400 × 180	The truss reinforcement aligns with the long direction of the specimen.	0	
P2	The load-bearing steel bars do not extend beyond the joint side; the truss reinforcement is aligned with the joint direction, and throughout the joint area, longitudinal load-bearing is arranged with continuous steel mesh, spaced at 120 mm intervals; transverse structural steel mesh, featuring 8mm diameter bars, is strategically placed across the joint.	1	
P3	The load-bearing steel bars do not extend beyond the joint side. The truss reinforcement aligns with the joint direction, and at the joint location, steel bars with a length of 1600 mm, diameter of 8 mm, and spaced at 120 mm intervals are strategically arranged.	1	
P4	The load-bearing steel bars do not extend beyond the joint side. The truss reinforcement aligns with the joint direction. At each of the two joints, steel bars with a length of 1600 mm, diameter of 8mm, and spaced at 120 mm intervals are strategically arranged.	2	
L represents the span of the panel, B represents the width of the panel, H represents the height of the panel, N represents the number of joints.

Mechanical properties of materials

The experiments utilized C35 commercial concrete. During the slab casting process, cubic test blocks with a side length of 100 mm were reserved, and they were cured under the same conditions as the specimens. The measured compressive strength of the concrete cubes for the prefabricated bottom slab and the cast-in-place top slab were 36.30 MPa and 44.80 MPa, respectively, which, when converted to concrete axial compressive strength, were 22.40 MPa and 28.46 MPa. The measured mechanical properties of the steel bars used in the experiments are shown in Table 2. Table 2 Mechanical properties of reinforcement.

d/mm	fy/MPa	fu/MPa	Es/MPa	
6	305	413	2.1 × 105	
8	500	611	2.0 × 105	
d represents the diameter of the steel bar, fy represents the yield strength of the steel bar, fu represents the ultimate strength of the steel bar, Es represents the elastic modulus of the steel bar.

Loading apparatus and loading scheme

For static load tests on thick slabs, the method of stacking counterweights is commonly used to simulate uniform load. However, due to experimental constraints, this approach is challenging to implement. Refer to the test methods in the specifications20 and related literature21,22, a three-point loading method was adopted for the bending test. The experimental loading apparatus is illustrated in Fig. 2.Fig. 2 Schematic diagram of loading device (unit: mm).

A 500 kN pressure sensor and a hydraulic jack are mounted on the reaction frame, which is anchored to the floor. The hydraulic jack applies force to the distribution beam, transmitting the force to the loading beam. The loading beam imparts a line load in the width direction of the panel, achieving three-point loading. The two ends of the panel are supported by two circular steel supports and 10 mm-thick steel plates forming a simply supported bearing.

To observe the working condition of the data acquisition instruments, pre-loading was performed before the formal test. During the formal loading, the specimen was loaded in five stages until the initiation of the cracking load. After cracking, the load was increased by 10% of the calculated value of the ultimate load. The test was terminated when the deflection of the specimen exceeded 1/50 of the span or when the maximum crack width exceeded 1.5 mm26, indicating specimen failure.

Layout of measurement points

The measurement content mainly includes overall deformation of the slab, steel reinforcement strain, concrete strain, and crack distribution. The specific content is as follows:Displacement gauges L1 to L7 were arranged at the bottom of the slab and at the supports to measure the deflection of the slab at mid-span relative to the supports, as well as the distribution of deflection along the span of the slab. Additionally, displacement gauges H1 to H4 were arranged at the middle positions of both ends of the specimen, both on the precast and cast-in-place layers, to measure relative slip between the layers, as shown in Fig. 3a.

Strain gauges were installed on the longitudinal reinforcement in the bottom slab within the pure bending section, on the additional reinforcement at the joints, and on the additional reinforcement mesh, as shown in Fig. 1. These gauges were used to measure the strain in the longitudinal reinforcement, additional reinforcement, and reinforcement mesh at the joints. Concrete strain gauges were installed on the top and bottom surfaces at mid-span of specimens P1 to P4, and at the 1/4 span on the top and bottom surfaces of specimen P4, as shown in Fig. 3b.

The appearance, development, and width of cracks were measured using a combination of visual inspection, magnifying glass observation, and an HC-F800 concrete crack and defect comprehensive tester. The crack patterns were sketched, and the corresponding load values were recorded.

Fig. 3 Strain and deflection measuring locations (unit: mm).

Experimental results and analysis

Failure process and modes of hollow composite slabs

The failure processes and modes of the specimens varied significantly during the experiments, as depicted in Fig. 4. Notably, specimen P1, a seamless hollow composite slab, exhibited a well-reinforced failure mode. Uniform, full-length cracks appeared on the bottom surface, extending upwards along the side until the compressed concrete was crushed. Specimen P2, featuring a longitudinally stressed steel mesh with a single joint in a tightly connected hollow composite panel, exhibited extensive tensile deformation at the joint. The transverse connecting steel mesh fractured, resulting in a sudden failure without apparent tearing on the joint surface. This led to a brittle failure mode, with no additional cracks on the bottom or sides. Specimens P3 and P4, which were single- and double-joint tightly fitted hollow composite slabs with additional reinforcement, exhibited similar failure modes. Cracks first appeared at the joints and then propagated uniformly across the mid-span of the specimen, extending from the bottom of the slab to the composite interface and further to the slab surface. The cracks at the composite interface were not prominent. Before failure, the mid-span deflection and the width of the bottom cracks increased rapidly, with the maximum crack width exceeding 1.5 mm. However, the concrete in the compression zone was not crushed, and the load-deflection curve showed a descending phase, indicating that failure had occurred.Fig. 4 Mid span failure diagram of the specimen.

The test results indicate that the single- and double-joint tightly fitted hollow composite slabs with additional reinforcement (P3 and P4) tend to fail at the joints, with more uniformly distributed cracks. The horizontal cracks at the composite interface were not significant, suggesting that the truss reinforcement effectively restrained cracking at the composite interface. In Specimen P2, the additional reinforcement mesh at the joint failed to effectively transfer the load to the longitudinal reinforcement of the precast base slab due to insufficient load-bearing capacity. This led to a significant reduction in the concrete thickness in the compression zone at the joint, ultimately resulting in brittle failure. Future research should focus on optimizing the design of the longitudinal reinforcement mesh at the joint.

Figure 5 illustrates the crack distribution observed at the point of failure for each specimen. The crack distribution in Specimen P1 is typical of flexural failure, with large mid-span deflection, similar to the failure characteristics of cast-in-place concrete slabs. Specimen P2 exhibited no cracks other than those at the joint, indicating that the additional reinforcement mesh at the joint lacked sufficient load-bearing capacity and failed to effectively transfer the load, leading to stress concentration. In Specimens P3 and P4, cracks were concentrated at the joints and distributed evenly on both sides of the mid-span, indicating that the specimens were in a state of overall bending. The more uniform crack distribution in Specimens P3 and P4 compared to P2 suggests that the load transfer provided by the additional reinforcement was more effective than that of the additional reinforcement mesh, leading to better overall integrity.Fig. 5 Crack distribution diagram of the specimen.

Mid-span deflection of hollow composite slabs

The load-deflection curves for each specimen are depicted in Fig. 6, and the corresponding mid-span deflections under loads of 9 kN, 18 kN, and 27 kN are listed in Table 3. From Fig. 6 and Table 3, it can be observed that the load-bearing capacity and deformation ability of Specimen P1 are significantly greater than those of the other specimens, indicating that the presence of joints reduces the overall load-bearing performance of the specimen. Specimen P3 demonstrated higher load-carrying capacity and stiffness than that of specimen P2, suggesting that, in single-joint tight connection, the effect of additional connection reinforcement surpasses that of longitudinal force steel mesh. This further underscores the potential for enhancing overall bending performance through optimized joint connection design. Additionally, the load-bearing capacity and stiffness of Specimen P3 are greater than those of Specimen P4, with a load-bearing capacity increase of 17.13%, indicating that a single-joint tightly fitted slab of the same size performs better under load than a double-joint tightly fitted slab. Similar to cast-in-place concrete slabs, the load-midspan deflection curves of the specimens exhibit a three-segment development pattern. Except for Specimen P2, which experienced brittle failure, all other specimens exhibited ductile failure, indicating that the configuration of tightly fitted joints facilitates effective load transfer between slabs.Fig. 6 Load-mid-span deflection curves.

Table 3 Comparison of mid span deflection of hollow composite slabs.

F/kN	∆/mm	
P1	P2	P3	P4	
9	0.02	7.79	0.85	2.23	
18	0.4	–	2.27	5.79	
27	1.02	–	15.53	19.74	

Reinforcement strain

The load-strain curves for additional steel reinforcement at joints in each specimen are presented in Fig. 7, where specimens P4-1 and P4-2 represent the strain of additional reinforcement in double-joint tight connection hollow composite slabs. From the graph, it is evident that the additional reinforcement at mid-span reached the yield strain value, indicating that the additional steel reinforcement at joints in the tight connection hollow composite slabs effectively exhibits tensile resistance, and the anchoring force transmission was efficient. Some of the additional reinforcement in the middle of the specimens did not reach the yielding state. This is attributed to the influence of the hollow position of the thin-walled hollow box, where the bond force between concrete and additional reinforcement cannot be guaranteed. Consequently, the load on the additional reinforcement gradually transfers to its ends, resulting in a slipping phenomenon, and the yielding of additional reinforcement lags behind the entire specimen.Fig. 7 Load-strain curves.

Concrete strain and deformation in hollow composite slabs

The variation of concrete strain with load is illustrated in Fig. 8. From the figure, it can be seen that the concrete strain at the bottom of the slab is primarily tensile, while the strain at the top of the slab is primarily compressive. In the initial loading stage, the strain exhibits a linear growth trend, increasing as the concrete pressure increases. The strain values on the same side are relatively consistent. However, when the concrete of the bottom slab cracks (specimen P1) or cracks extend to the cast-in-place concrete at the joint (specimens P2 to P4), the strain experiences a sudden increase. The strain values on both sides differ significantly. The strain on the tension side of the slab fails first, but the overall trend remains consistent. Specifically, in specimen P4, the concrete strain on the top and bottom surfaces at mid-span is smaller than that at 1/4 of the joint, indicating that the primary failure location for double-joint composite hollow slabs is concentrated at the joint rather than at the mid-span section.Fig. 8 Strain curve of concrete.

The vertical displacement distribution at different points during the loading process of specimens under various loads is depicted in Fig. 9, where "P" represents the ultimate load value for each specimen. Specimen P1 exhibits superior overall integrity, with deformation that significantly exceeds that of other specimens. For single-joint specimens P2 and P3, the displacement shows a two-segment linear shape during loading, while double-joint specimen P4 displays a three-segment linear shape. The presence of joints causes the specimen to yield first at the joints, with the other cross-sections of the specimen not reaching a yielding state. Consequently, after yielding, the deformation is concentrated primarily at the joints. Comparing specimens P2 and P3, it is evident that the bearing capacity and stiffness of specimen P3 are significantly greater than those of P2, indicating that the joint design of specimen P3 is more reasonable, while the joint design of specimen P2 requires further optimization. Comparing specimens P3 and P4, it is noted that the deformation of specimen P4 is more uniform than that of P3, but the bearing capacity is slightly smaller. This suggests that the increase in the number of joints in specimen P4 improves the non-uniformity of concrete strain in the tension zone, avoiding excessively concentrated crack development and providing some assistance in controlling the width of cracks in the bottom slab concrete.Fig. 9 Deformation curves of the specimens.

It is noteworthy that in the experiments, the joints of the specimens were strategically located in the least favorable stress positions. The effective force section height at the joint, where new and old concrete meet, is relatively small, making it prone to stress concentration and potential failure. This directly impacts the effective transfer of forces between the additional reinforcement at the joint and the longitudinal reinforcement of the prefabricated base slab. Moreover, certain additional reinforcement experiences bond slip in the specimens, not fully realizing their tensile strength. In practical engineering applications, it is advisable to position joints in areas with lower stress, and additional reinforcement should be arranged with proper anchorage at both its ends.

Theoretical calculations

Load-carrying capacity

Specimen P1, a seamless concrete hollow composite slab, exhibited typical characteristics of a well-reinforced flexural member during the experiment. It did not show any signs of separation at the composite interface during the normal service stage, and its bending deformation capacity far exceeded that of the other specimens. The mid-span concrete satisfied the plane section assumption, and the cracking load and ultimate load of the section can be calculated using the following formulas23–25,1 Mcr,kdc=γftkW0

2 γ=0.7+120hγm

3 Mu,kde=αfcbxh0-xn2+fy′As′h0-as′

4 fyAs=fy′Ay′+afcbxn

where: Mcr,kdc is the design cracking moment of the concrete hollow composite slab. Mu,kde is the design ultimate moment of the concrete hollow composite slab. γ is the plastic modification factor of the section resistance moment. γm is the basic value of the section resistance moment plasticity coefficient, taken as 1.45 for I-shaped or box-shaped sections. αct is the concrete tensile stress reduction factor, taken as 0.85. ftk is the axial tensile strength of concrete, taken as 2.2 MPa. W0 is the section resistance moment, W0=I0/(h0-xn). I0 I s the converted section moment of inertia.fc is the average measured compressive strength of concrete. fy is the measured tensile strength of the reinforcement. fy′ is the measured compressive strength of the reinforcement. AS is the cross-sectional area of the longitudinal tensile reinforcement. As′ is the cross-sectional area of the longitudinal compressive reinforcement. h is the section height. h0 is the height of the post pouring layer. b for board width. xn is the actual height of the compressed zone of concrete. as′ is the distance from the force point of the longitudinal tensile reinforcement to the edge of the compressed zone of the concrete section. α is a coefficient related to the concrete strength grade, taken as 1.0 when the concrete strength grade does not exceed C50.

The experiment showed that Specimen P1 had the typical characteristics of a well-reinforced beam failure. Considering the cracking moment test value derived from the ultimate load, structure, and equipment self-weight, the test values for the cracking moment and ultimate moment were 26.87 kN m and 93.98 kN m, respectively. The calculated theoretical values for Specimen P1’s cracking moment and ultimate moment were 20.67 kN m and 96.46 kN m, respectively. The close match between the theoretical and test values for the bending capacity of Specimen P1 indicates that the calculation methods in GB50010-2010 Code for Design of Concrete Structures23 are applicable to the bending capacity calculation of concrete hollow composite slabs.

Specimens P2 and P3 adopt a single-joint dense assembly, while specimen P4 adopts a double-joint dense assembly. After the connection reinforcement reaches the yield limit, cracks continue to develop. Specimen P2 experiences brittle failure at the joint, exhibiting lower flexural deformation capacity than specimens P3 and P4. Specimens P3 and P4 show more uniform crack development in the tensile zone of concrete, with no failure observed in the compressed zone. Therefore, when calculating the bending capacity of dense assembly concrete hollow composite slabs, the rectangular stress diagram in the compressed zone of concrete under the ultimate state can be optimized to a triangular stress distribution23, as shown in Fig. 10. Here, Mu,kd is the ultimate bending moment of the concrete hollow composite slab section, εc is the compressive strain at the top of the concrete in the compressed zone under the action of the ultimate load, εy′ is the strain at the top of the reinforcement in the compressed zone under the action of the ultimate load, and εy is the yield strain of the tensile reinforcement in the tensile zone.5 εc+εy′εy=xnh0-xn

6 fyAs=12Ecεcbxn

7 Mu,kd=fyAsh0-13xn

Fig. 10 Cross-section strain distribution in ultimate strength condition.

The theoretical values of the ultimate moments for specimens P3 and P4 were calculated based on Eqs. (5) to (7) and compared with the experimental values, as presented in Table 4. It is observed from the table that the theoretical value for specimen P2 significantly deviates from the experimental value, primarily due to its brittle failure. The ratio of the experimental cracking load to the theoretical value for specimens P3 and P4 is 0.97 and 0.71, respectively, while the ratio of the experimental flexural capacity to the theoretical value is 0.90 and 0.76, respectively. Specimen P3 exhibits a closer alignment between the experimental and theoretical values, albeit lower than the theoretical values. This indicates the presence of some bond slip in the additional steel reinforcement at the joint, and the lateral single-joint dense connection formula for calculating the flexural capacity of hollow composite slabs in this study is reasonably accurate. Compared to specimen P3, specimen P4, with an increased number of joints, shows a 14.6% decrease in its overall bending performance. Table 4 Comparison of experimental and theoretical values of flexural bearing capacity.

Specimen code	Mcrc/kN m	Mcre/kN m	Mcre/Mcrc	Mu,kdc/kN m	Mu,kde/kN m	Mu,kde/Mu,kdc	
P1	20.67	26.87	1.30	96.46	93.98	0.97	
P3	9.76	9.49	0.97	26.58	23.78	0.90	
P4	9.76	6.91	0.71	26.58	20.30	0.76	

Bending stiffness

Based on the experimental observations and results, it is evident that hollow core slabs exhibit deformation characteristics similar to those of ordinary floor slabs. In calculating the flexural stiffness of hollow core slabs, various standards23,26 and literature27 are referenced to conduct a comparative analysis of the feasibility and applicability of three methods for determining the flexural stiffness of side-stitched concrete hollow core slabs.

The bottom slab of the densely jointed hollow composite slab exhibits a distinct two-stage characteristic, delineated by the onset of cracking. The calculation method for the flexural stiffness of reinforced concrete members is provided in GB50010-2010 "Code for Design of Concrete Structures"23 and can be expressed as follows:

Before cracking:8 Bs1-1=EcI0

After cracking:9 Bs1-2=EsAsh021.15ψ+0.2+6aEρ

10 ψ=1.1-0.65ftkρteσs

In the formula, Bs1-1 represents the short-term stiffness of the uncracked bending member, Bs1-2 represents the short-term stiffness of the bending member with cracks, Es represents the elastic modulus of the steel, I0 represents the moment of inertia of the converted section, ψ represents the unevenness coefficient of longitudinal tensile steel strain between cracks, aE represents the ratio of the elastic modulus of steel to concrete, ρ represents the longitudinal tensile steel reinforcement ratio,the value is As/bh0, ρte taken as the reinforcement ratio calculated based on the effective tensile concrete section area,the value is As/Ate, and σs represents the stress in ordinary steel bars in steel-reinforced concrete members calculated for the quasi-permanent load combination. Ate taken as the effective area calculated for the quasi-permanent load combination.

The calculation formula for the short-term flexural stiffness of reinforced concrete members under bending, as provided by the Design Code for Assembled Integral Concrete Residential Building Systems (DG/TJ08-2071-2010)26, is given by Eq. (11).11 Bs2=EsAsh020.7+0.6h1h+4.5aEρ

where: Bs2 is the short-term flexural stiffness of the bent member, h1 is the thickness of the prefabricated layer, h is the total thickness of the member. The meanings and values of other parameters are consistent with Eq. (9).

Reference27 proposes a calculation method for the short-term stiffness of prefabricated members of reinforced concrete composite slabs with steel trusses based on three assumptions related to different constructions. The three assumptions are as follows: Considering only the contribution of longitudinal chord reinforcement to the stiffness of the prefabricated section, without considering the contribution of the concrete prefabricated layer to the stiffness of the prefabricated section. Considering the contribution of longitudinal reinforcement and the concrete prefabricated layer to the stiffness of the prefabricated member section, without considering the impact of section concrete cracking. Considering the contribution of longitudinal reinforcement and the concrete prefabricated layer to the stiffness of the prefabricated member section, taking into account the influence of crack occurrence.

The calculation methods are as follows:12 Bs3=h102·γ(1+α)·(1+β)

13 α=h10y10-c-D22-1

14 β=1h10h1+y102-c-D22-1

15 γ=αAs′Es+βAsEs

where: Bs3 is the short-term stiffness of prefabricated members of the reinforced concrete composite slab with steel trusses, h10 is the distance between the upper and lower chord axes of the truss reinforcement, y10 is the distance from the lower edge to the centroid of the transformed section, c is the thickness of the concrete protective layer, D2 is the diameter of the lower chord reinforcement, h1 is the thickness of the prefabricated bottom plate.

Table 5 shows the flexural stiffness values of each specimen calculated using the different methods, compared with the experimental values, where By is the bending stiffness of the additional reinforcement when it reaches yield after the specimen cracks. The values calculated using Formula (12) for Specimen P1 closely align with the experimental values, with a ratio of 0.97, suggesting that the stiffness calculation method proposed in the literature27, based on three assumptions for the reinforced concrete composite slab with steel trusses, is suitable. For Specimen P3, the calculated values using Formula (9) closely match the theoretical values, with a ratio of 1.11, indicating that the stiffness formula provided in the "Code for Design of Concrete Structures" (GB 50010-2010) is appropriate for the calculation of stiffness in the working stage with cracks for the single-joint densely assembled reinforced concrete composite slab with steel trusses. In the case of Specimen P4, the experimental values calculated using Formulas (9), (11), and (12) show differences exceeding 25% compared to the theoretical values, with ratios of 1.25, 1.30, and 1.48, respectively. This indicates that directly applying the mentioned formulas for the double-joint densely assembled reinforced concrete composite slab with steel trusses is not suitable. The value of specimen P4 is 13.5% higher than that of specimen P3, indicating that joints located away from the most critical stress section help improve the yield bending stiffness of the specimen. The calculated flexural stiffness values using Formula (9) are consistently greater than those using Formulas (11) and (12) but are smaller than By, indicating that after cracking, if the influence of tensile reinforcement stress is not considered, the stiffness calculation values are slightly lower and more conservative. Table 5 Comparison of bending stiffness (/kN m2).

Specimen code	Bs1-1	By	Bs1 - 2	By/Bs1-2	Bs2	By/Bs2	Bs3	By/Bs3	
P1	1914.15	1851.01	2254.86	0.82	2181.74	0.85	1909.05	0.97	
P3	2493.82	1.11	1.14	1.31	
P4	2829.54	1.25	1.30	1.48	

Conclusions

During normal usage, the bending performance of the laterally tightly fitted hollow composite slab is generally consistent with that of a seamless hollow composite slab, indicating that the lateral joints can effectively transfer internal forces. However, under ultimate conditions, before the additional reinforcement yields, some bond slippage occurs at the joints, leading to a significant reduction in the load-bearing capacity and stiffness of the tightly fitted hollow composite slab.

Load-Bearing Capacity Calculation: When using the triangular stress distribution method to calculate the bending load-bearing capacity of the laterally single-joint tightly fitted hollow composite slab (Specimen P3), the results are within 10% of the experimental values. However, there is a significant difference when compared to the bending load-bearing capacity of the double-joint tightly fitted hollow composite slab.

Impact of Joint Quantity and Position: The number and position of joints have a significant impact on the bending stiffness of laterally tightly fitted hollow composite slabs. Joints should not be placed at the least favorable cross-section for loading, as specimens located away from the mid-span section show an increase in bending stiffness. The yield stiffness of seamless concrete hollow composite slabs is consistent with the calculation results in the literature27, with a difference of less than 5%. The yield stiffness of single-joint tightly connected concrete hollow composite slabs is close to the results calculated using the "Code for Design of Concrete Structures" and consistent with a difference of approximately 10%. However, the yield stiffness of double-joint tightly connected concrete hollow composite slabs deviates by more than 25% from the three aforementioned calculation methods.

The tested laterally tightly fitted hollow composite slabs did not exhibit horizontal cracks or shear failure along the composite interface. This indicates that the traditional joint construction method for composite slabs can meet the design requirements for bidirectional load-bearing, providing a reference for further optimization of the theoretical model and the study of the mechanical performance and design methods for large-span precast concrete hollow composite floors.

Author contributions

XC: Investigation, Data curation, Funding acquisition, Project administration, Supervision, Methodology, Writing—original draft, Writing—review & editing. QM: Conceptualization, Funding acquisition, Project administration, Supervision, Visualization, Writing review, editing.

Funding

The authors greatly appreciate the financial support provided by the National-local joint Engineering Laboratory of Building Health Monitoring and Disaster Prevention Technology (No. GG22KF001). Anhui Province Department of Housing and Urban-Rural Development Construction Science and Technology Plan Project (No. 2022-YF083).

Data availability

The datasets generated and/or analyzed during the current study are not publicly available because all data are presented in the article and therefore, there is no need to include raw data but they are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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