MECHANICAL PROPERTIES OF A NEW FULLY PREFABRICATED STAGGERED FLIP-DOWN SLAB

. Prefabricated slab has been widely used in the global construction industry due to energy saving, environmental protection, and good economic advantages. In this paper, a new type of fully prefabricated staggered flip-down slab without cast-in-situ operation has been proposed. First, the experiments were carried out on the new slab. The structural performance of the new slab was compared with the cast-in-situ slabs and composite slabs of the same specification. The experimental results showed that the ultimate bearing capacity of the new slab meets the requirements for practical utilization. On this basis, an additional CFRP sheet could be pasted on the bottom initial seam between prefabricated slabs to enhance the integrity and prevent cracks. Then, the whole loading process of the slab was simulated, and the results were consistent with the experimental results. Finally, through experiments and parametric analysis, recommendations for improvement were put forward to enhance the mechanical properties of this kind of slab.


Introduction
Prefabricated building systems are often referred to as precast constructions in which the structural components are manufactured in factories based on standardization, then transported to the sites, and finally assembled into the final product on the sites (Goodier & Gibb, 2007;Hassan et al., 2021).Because of prefabrication, the traditional execution of architectural design and construction is changed.Prefabricated buildings have been widely used and developed in the global construction industry due to their fast construction speed, site disruption reduction, sound economic benefits, better construction quality control, and improved material efficiency (Tam & Hao, 2014;Tam et al., 2007).In countries like Germany, New Zealand, and England, prefabrication accounts for 20%, almost 20% and around 10% of the total constructions, respectively (Heaton, 2017).Moreover, the proportion of new prefabricated dwellings in Japan has remained steady at 12-16% over the past decade (Steinhardt & Manley, 2016).In Sweden's housing industry, the market share of prefabricated construction has exceeded 80% (Navarat-nam et al., 2019).In China, the government mandated that by 2020, 15% (by building slab area) of the nation's annual new construction should be prefabricated, and by 2025, this target is increased to 30% (State Council of the People's Republic of China, 2016).
A prefabricated slab is an important part of assembled residential buildings.It adopts the form of a precast panel plus a cast-in-situ layer.The precast panel can be used as a template to replace the bottom mold required by the cast-in-situ concrete floor (Xu et al., 2022).Although prefabricated slabs have the advantages of prefabricated structures, the inferior integral performance leads to poor seismic performance (Wang et al., 2019).In order to make the prefabricated slabs better meet the actual needs of the engineering project, experts and researchers have done many studies, and most have developed new types of slabs by improving the slab form.The ultimate bearing capacity of prefabricated steel-concrete composite slabs with an interlocking connection system proposed by Hassan et al. (2021) is 40% higher than that of the traditional simply supported non-composite slabs.Lu et al. (2022) designed a prefabricated concrete composite slab that uses the profiled steel sheeting as prefabricated formwork and shear connectors, and experimentally evaluated its flexural performance in terms of reinforcement yield strength.Al-Fakher et al. (2021) demonstrated that the bearing capacity of a prefabricated concrete composite slab made with steel and composite reinforcing systems (CRS) reinforcement was increased by 112% when compared with the attachment of carbon fiber reinforced polymer sheets applied by Meng et al. (2016).Jiang et al. (2003) and Liu and Jiang (2004) proposed a composite slab with a precast inverted T-type bottom panel, which could improve the slab stiffness and integrity.Huang et al. (2015) concluded that the T-type ribs have almost no influence on the flexural performance of concrete composite slab with precast prestressed ribbed panel and suggested the calculation formulas for the cracking moment and ultimate bending moment.On this basis, Liu et al. (2020) presented a new prestressed concrete composite slab with precast inverted T-shaped ribbed panels (CSPRPs) and showed that different rib forms have little influence on the flexural behavior of CSPRPs.Chen et al. ( 2022) investigated a novel precast concrete slab with crossed bent-up rebar and showed that when the bend-up end angle of the slab was 60°, its flexural performance could be significantly improved.Nguyen et al. (2021) suggested hidden boundary one-way rib precast concrete slabs (HRS) and studied in detail the effects of strand wires and rebars on the bending performance of HRS.
The existing research studies show that the structural performance of slabs can be improved by changing the material while reducing the cost (Lukaszewska et al., 2010).By combining thin-walled cold-formed steel elements (CFS) and panels made of wood, cement, or plaster, de Seixas Leal and de Miranda Batista (2020) showed that the thin-walled channel shear connectors had excellent performance and could improve the bending capacity of the slab system.Mansour et al. (2015) found that adding a thin layer of steel fiber concrete into precast concrete slabs could increase its flexural performance.Dal Lago et al. ( 2017), Lima et al. (2018), and Crocetti et al. (2014, 2015) used fiber reinforcement in prefabricated concrete slabs and found that it has clear benefits on the post-cracking flexural capacity.May et al. (2019) studied the required load-bearing capacity and deflection stiffness of precast slabs made of carbon reinforced concrete at conventional construction height and demonstrated that the slab concrete could be reduced continuously by optimizing the load-bearing structure.Rochman et al. (2021) proved that lightweight foamed precast concrete slabs have good flexural behavior and can be used as a good prefabricated structural slab in the future.It can be seen from the literature review that the new slabs are mostly obtained by improving the form of composite slabs or changing materials, and the mechanical properties of slabs are explored through experiments and numerical simulation.However, the efficient assembly approach of slabs is rarely proposed.
In this paper, based on the survey of currently assembled slabs, a new fully prefabricated staggered flip-down slab with a high degree of prefabrication, good integrity, simple and reliable joint connections, no cast-in-situ operation, and low cost is developed.First, the bending test was carried out on the new slab.Then, its structural performance was compared with cast-in-situ and composite slabs of the same specification.The experimental results illustrate that the ultimate bearing capacity of the new slab meets the requirements for practical utilization.Further, the new slab was strengthened with the CFRP sheet to enhance the integrity of the initial seam between prefabricated slabs.Then, the whole loading process of the slab was simulated, and the results were found to be in good agreement with the experimental findings.Finally, through experiments and parametric analysis, some suggestions were presented to facilitate the construction and improve the mechanical properties of the slab.

Conceptual design
A fully prefabricated staggered flip-down slab is a new type of slab developed based on cast-in-situ and composite slabs.Figure 1 shows the specific form of the slab.After the plate is cut according to the modulus, the precast components with concave-convex grid-shaped tooth grooves are first produced in factories.Then, they form the prefabricated slab through staggered joints and buckles.Tensile and distribution reinforcements are embedded in the components, and the overlapping area between the top and bottom components is a quarter of the total area.In addition, the joint of the new slab is to put the two ends of the bottom prefabricated component on the steel beam, and the top part is staggered with the bottom.At the same time, the CFRP sheet can also be pasted on the bottom initial seam between prefabricated slabs to improve the integrity and prevent cracks.
Compared with other slabs, the new type of slab proposed in this paper has the following main advantages: 1) The presence of interlocking tooth grooves can improve the shear capacity and structural integrity; 2) The slab size has strong adaptability, so it can be transported conveniently; 3) Simple construction can improve assembly efficiency and advance the construction schedule; and 4) Compared with the traditional profiled steel plate composite slab, the flat slab surface can avoid suspended ceilings in decoration, leading to huge cost savings.
Figure 1.Slab specific form

Specimen design
In order to study the bending performance of the full prefabricated staggered flip-down slabs, six (6) slabs were designed in this experiment.The serial number and size of each specimen are shown in Table 1.The control group includes cast-in-situ slabs and composite slabs.Considering that the mechanical properties of the full prefabricated staggered flip-down slabs are smaller than those of castin-situ slabs and composite slabs in the same specification, additional control slabs with the thickness of 120 mm are set.The calculated span of each specimen is L 0 = 2200 mm.The HRB400 tensile and distribution reinforcements in all specimens adopt 4C8@150 and C6@200, respectively.
The measured yield strength and tensile strength are f y = 421 MPa and f u = 647 MPa, while the elastic modulus is E s = 2×10 5 MPa.Moreover, each specimen uses C30 concrete.Figure 2 shows the design of the cast-in-situ slab and composite slab, while Figure 3 represents the design of the new fully prefabricated slab.Slab ZPB consists of three components, i.e., one ZP-1 and two ZP-2.Figure 4 shows the production of ZP-1 and ZP-2.Considering that the new slab is fully assembled, one or two workers can carry out simple material transportation and slab installation.Their tooth grooves are designed as 150×150 mm with 40 mm height.The height of the bottom component is 55 mm.Slabs ZPB and ZPB-CFRP are equipped with the same reinforcement and concrete.CFRP can provide passive suppression for concrete and restrain the initial seam between prefabricated slabs, so the flexural capacity and stiffness of the slab can be improved.The position of the CFRP sheet is shown in Figure 5.The 200 mm CFRP sheet was pasted on longitudinal seam and extended to the edge of the supports.The tensile strength, elastic modulus, elongation at break, and thickness of CFRP sheet are f u = 3870 MPa, E s = 2.45×10 5 MPa, 1.74%, and 0.167 mm, respectively.

Experimental scheme
As shown in Figure 6, the specimen was loaded at three points.The preloading was divided into two steps.The load in each step was 2 kN, i.e., 50% of the cracking load.Each step lasted for 10 minutes for observation.Similarly, there were also two unloading stages.In formal loading, the stepped load was 2 kN until 8 kN.Then, it decreased to 1 kN when the cracking load reached and lasted until the first crack occurred.Next, the load increased by 2 kN every step before tensile reinforcement yielded.Finally, a 1 kN load lasted for the rest.The duration of every stage was 15 min to observe the experiment phenomenon and record it in detail.During the whole process, mid-span deflection, the strain in concrete, reinforcement, and CFRP sheet, and crack development should be concerned.Corresponding measuring points arranged are demonstrated in Figure 7 (Deflection: CD-1~CD-5, the strain of concrete: CS-1~CS-4, the strain of reinforcement: CX-1~CX-8, the strain of CFRP sheet: CFX-1~CFX~3).

Bearing capacity analysis
Figure 8 shows the mid-span cracks of each specimen.The experimental phenomena, corresponding bearing capacity, and mid-span deflection in each stage are shown in Table 2, where P and u represent the load and deflection (the subscripts cr, y, and u represent the appearance of the first crack, and reaching yield and ultimate failure, respectively).
The crack development of all specimens was similar.The representative bending test is illustrated in Figure 9. Taking ZPB as an example, the first tiny crack appeared when the load was 3 kN.The cracking load of ZPB was less than that of XJB or DHB because initial cracks existed at weak positions during installation.Then, new cracks developed in the mid-span at 12 kN, and some small cracks appeared near the supports at 20 kN.Next, the cracks extended upward and widened continuously until 32 kN when the maximum crack width of 1.5 mm reached the limit state, and the specimen failed.Compared with ZPB, due to the reinforcement effect of CFRP, the first crack, midspan cracks, and cracks near the supports of ZPB-CFRP all appeared later.The slabs XJB, DHB, and ZPB, failed because the maximum crack width reached the limit state.
There is no obvious cracking at the mid-span of the bottom of ZPB-CFRP, and the CFRP tensile fracture is taken as the limit state of ZPB-CFRP.However, as the load continued to increase, the reinforcement in the slab still had ductility and could continue to bear the tension force.
Figure 10 illustrates the relationship between the midspan deflection of the slab and the external load.The initial stiffness and stiffness at the working stage (with cracks) of the composite slab are greater than that of the cast-in-situ slab due to a steel truss.However, because of inferior integrity, the ultimate bearing capacity of the composite slab is lower than that of the cast-in-situ slab.For ZPB and ZPB-CFRP, the tooth groove does not transfer internal force at the beginning of loading, and only the bottom component bears the load.Then, with increasing load, the mid-span deflection increases, and the top and bottom components slide, causing the tooth groove to squeeze each other and eliminate the gap between the tooth grooves.Because of the participation of the tooth groove, the bearing capacity of the slab will increase until the reinforcement yields.Further, the deflection of ZPB before the yield stage is larger than DHB-150, so its stiffness is lower than that of DHB-150.However, the ultimate bearing capacity of ZPB is about 86.6% of that of DHB-150, which meets the requirements for practical utilization.Also, the yield strength of ZPB-CFRP is the highest among all specimens.The reason is that CFRP bears tensile force and shows good anchorage and cementation effects in addition to reinforcement.Based on this, the ultimate bearing capacity of ZPB-CFRP increases by 17.2% and 35.4%, higher than that of DHB-150 and ZPB, respectively.

Load-strain curves analysis
As shown in Figure 11a, the load-strain curve of ZPB tensile reinforcement has the same trend as those of XJB and DHB, but the elastic stage for ZPB is short.It indicates that the concrete cracks when the load is not large, and its tensile part exits from work, thereby increasing stress in tensile reinforcement.Then, the strain of tensile reinforcement increases significantly with increasing load.In addition, it is worth mentioning that since the truss height adopted by DHB-150 is 90 mm, the top reinforcement is compressed first, and then the compressed reinforcement changes from compression to tension with the expansion of cracks.At the initial loading stage, due to the gaps between the tooth grooves, the top and bottom components of ZPB-CFRP work independently, so the strain of tensile reinforcement is small.However, as the load gradually increases, the top and bottom components slide and cause tooth grooves to squeeze each other and eliminate the gap, making ZPB-CFRP play a role as a whole.Therefore, the strain of tensile reinforcement in ZPB-CFRP is the largest under a large load.Figure 11b describes the load-strain curve of concrete in the compressive zone.The maximum strain of concrete in the compressive zone for six specimens was all within 3×10 -3 , which did not reach the ultimate compressive strain of 3.3×10 -3 .It is consistent with the phenomenon that the concrete was not crushed in the experiment.

Model development
To carry out numerical simulations for verifying experimental results and parametric analysis of fully prefabricated staggered flip-down slab, ABAQUS was used for modelling and simulating the loading process of ZPB and ZPB-CFRP.In the simulation process, the following assumptions were made in this study: 1) Reinforcement and concrete are closely integrated without relative slip, which accords with the coordination of displacement; 2) According to the experimental phenomena, there is no obvious slip between CFRP and slab, so it is assumed that CFRP and slab are closely combined without relative slip; 3) Although CFRP is an anisotropic material, the slab mainly relies on the CFRP sheet in a single direction in experiments.Hence, it is assumed that CFRP is a linear elastic material.The mesh of slab components is shown in Figure 12.The basic mesh size was 50 mm, and local mesh refinement was conducted in the CFRP sheet.Brick element (C3D8R), truss element (T3D2), and membrane element (M3D4R) were used for concrete, reinforcement, and CFRP, respectively.The gap between the tooth grooves was 0.3 mm, and general contact was applied between the concrete components.The friction formula of the tangential behavior was the penalty function, and the friction coefficient was 0.55.The default setting was utilized for normal behavior.In order to facilitate the computation, the bilinear model and plastic damage model were utilized for constitutive relations by reinforcement and concrete, respectively.Poisson's ratios of concrete and steel were taken as 0.2 and 0.3.One end of the slab was considered fixed hinge support, while the other was the sliding hinge support.

Comparison of results
In order to validate the feasibility and accuracy of the above finite element simulation method, the loading process of ZPB-CFRP was simulated and compared with the experimental results.As shown in Figure 13 and Table 3, the simulation results are consistent with the experimental results.The errors for yield deflection, ultimate strength, and ultimate deflection are all within 3%, and only the error for yield strength is 8.8%.The relative error may be caused due to the following reasons: 1) Ignoring the relative slip between reinforcement and concrete will lead to a larger calculation stiffness of the slab in simulations; 2) The plastic damage model is employed in simulations, simplifying the softening phenomenon of concrete after cracking; 3) The initial cracks caused during installation are not considered in the simulation process.Figure 14a shows the longitudinal stress of the bottom concrete.The tensile stress of concrete in the mid-span is the largest, so the slab is prone to crack in the mid-span.The high tensile stress zone extends from the mid-span to the supports, which corresponds to the experimental phenomenon of crack development.Moreover, it can be seen from Figure 14b that the high damage zone of the bottom concrete is distributed from mid-span to the loading point, and the maximum damage reaches 0.872.In the bending process, due to the mutual occlusion and extrusion between tooth grooves, it is easy to appear tensile damage in the tooth groove sides.Further, the longitu-

Analysis of key parameters
In order to identify the key factors affecting the mechanical properties of the slab, parametric analysis was carried out.The test conditions, including the slab thickness and CFRP width, are given in Table 4. Figure 15 shows the load-deflection curves under different test condi-tions.Test conditions 1, 2, and 3 illustrate that the slab thickness significantly influences the yield strength and ultimate strength because the effective height of the slab increases with increasing thickness.However, due to the influence of concrete strength and tensile reinforcement strength, thickness has little effect on cracking load.The ultimate strength of the slab with a thickness of 175 mm is 34.64% higher than that with a thickness of 150 mm.Compared to the 175 mm thick slab, the 200 mm thick slab has a 16.40% higher ultimate strength.It can be seen that the increase rate gradually decreases.Considering the principle of economy, the applicability of the structure, and construction standardization, after meeting the requirements of the bearing capacity of the slab, the slab thickness should be appropriately reduced to lower its dead weight and save materials.Therefore, under the same working conditions, for example, when the slab span is 3 m, it is recommended that the thickness of the new fully prefabricated slab be slightly greater than that of the castin-situ slab in practical applications, i.e., about 150 mm.As shown in test conditions 1, 4, 5, and 6, the increase of CFRP width has little effect on the cracking load because concrete, reinforcement, and CFRP sheet bear the force together.In addition, since the CFRP sheet bears the tensile force, the neutral axis moves up slowly during the loading process, so the bearing capacity will become larger.The ultimate strength of the slab with 100 mm wide CFRP is 13.23% higher than that of the slab without CFRP pasting.From 100 mm to 200 mm, for every 50 mm increase in CFRP width, the ultimate strength of the slab rises by 8.44% and 8.59%, respectively, compared with the previous grade.The rate of increase gradually declines.To prevent the member from shear failure before flexural failure, the improvement of the flexural bearing capacity of the slab by CFRP reinforcement should not be too high.In practical application, it is recommended that the CFRP bonding area at the seam should account for 10-15% of the slab area.16.The tooth groove size is related to the difficulty of template production and installation.It is of great significance to study the tooth groove size to improve construction efficiency and reduce resource waste.Moreover, the appropriate tooth groove height can reduce the structural weight.Test conditions 1, 7, 8, 9 and 1, 10, 11, 12 show that the size and height of the tooth groove have little effect on the bearing capacity of the slab.However, as the tooth groove size decreases, the corresponding deflection becomes smaller.Therefore, smaller tooth grooves can be selected in practical engineering.
Based on the experimental results and parametric analysis, the new slab can be optimized using the following approach.Since the stiffness of the new fully prefab-ricated slab is still inferior to that of the composite slab of the same specification after pasting the CFRP sheet on it, the tooth grooves can be improved by designing the contact surface between components as the inclined plane, thereby avoiding loose occlusion and enhancing the integrity of the fully prefabricated staggered flip-down slab.Furthermore, additional steel mesh can be configured on the side of tooth grooves to control the crack propagation.

Conclusions
In this paper, the investigations on the mechanical properties of a new type of fully prefabricated staggered flipdown slab were conducted through experimental analysis and simulations.The following conclusions are drawn from the obtained results: 1.A new, fully prefabricated, staggered flip-down slab was proposed for conceptual design.In production, there are no cast-in-situ operations for the new slab, and the bearing capacity is enhanced by tooth groove occlusion and staggered joint buckle.Furthermore, the CFRP sheet can be attached to the bottom initial seam between prefabricated slabs to improve the integrity and prevent cracks.2. The stiffness of the new fully prefabricated slab before the yield stage is lower than that of a composite slab under the same thickness.However, its ultimate

Figure 2 .Figure 4 .
Figure 2. Design of the control group: a -cast-in-situ slab; b -composite slab

Figure
Figure 9. Bending test Figure 10.The relationship between mid-span deflection and external load

Figure 12 .
Figure 12.Modelling of the fully prefabricated staggered flip-down slab: a -component; b -overall model; c -CFRP

Figure 14 .Figure 15 .
Figure 14.Simulation results of ZPB-CFRP: a -longitudinal stress of bottom concrete; b -tensile damage of bottom concrete; c -longitudinal stress of CFRP sheet

Figure 16 .
Figure 16.Load deflection curves with different parameters: a -tooth groove size; b -tooth groove height

Table 3 .
Comparison of experimental and simulation results stress of the CFRP sheet is described in Figure14c.In addition to reinforcement, the CFRP sheet al.o bears tensile force, which shows good anchorage and cementation effects.At the end of loading, the tensile stress of the CFRP sheet reaches the largest value of 1518 MPa. dinal

Table 4 .
Test conditions for slab thickness and CFRP width Table 5 describes the test conditions for different sizes and heights of the tooth groove.The load-deflection curves of different test conditions are presented in Figure

Table 5 .
Test conditions for tooth groove size and height