Abstract

Composite slabs, consisting of a precast plank and a cast-in-situ concrete topping, are the most commonly used horizontal structural components in prefabricated buildings. In order to solve the problem that the precast plank of composite slab is easy to crack and a lot of vertical supports are needed in its construction, this study proposes a new type of composite slab with a joint. The precast plank of this novel composite slab is fully prefabricated in the midspan area, and concrete is poured on-site only at the surrounding joints. In this paper, the bending performance of three composite slabs with a joint in different shapes of concrete topping and one cast-in-situ slab was investigated. The test results showed that the novel composite slabs with a joint in T-shape and trapezoidal concrete topping had similar properties as the cast-in-situ slab, including bearing capacity and cracking load, which can meet practical requirement. The use of bent-up steel bars and roughening of the interface between the precast plank and topping concrete can ensure effective transmission of internal forces at joint. The bearing capacity of the composite slabs with a joint in T-shape and trapezoidal concrete topping was evaluated using the formula in GB50010–۲۰۱۰, and the experimental values were 1.06 and 1.1 times the theoretical values, respectively, which provides a foundation for the design of two types of composite slabs with a joint. Meanwhile, based on the FEA software ABAQUS, numerical simulation was conducted on the composite slab with a joint in T-shaped concrete topping. The numerical finding exhibit close agreement with experimental results in terms of load-midspan deflection curve and the crack distribution.

Introduction

Precast concrete (PC) structures can be seen in buildings throughout the world due to its advantage of better construction quality, lower labor costs, and shorter construction times [1]. Composite slab, as one of the most commonly used elements in floor systems of PC residential buildings, is formed by bottom precast planks and a cast-in-situ concrete topping. In order to improve the flexural stiffness of the bottom precast plank and increase the bonding performance between the precast plank and the cast-in-situ concrete topping, lattice girders are usually installed in the precast plank. However, due to the limitation of the thickness of the composite slab with lattice girders used in China, the height of the lattice girder is usually small [2]. As a result, the improvement in stiffness of the precast plank by the lattice girder is limited, which requires the erection of vertical supports during the construction process, increasing the construction cost and prolonging the construction period [3]. Therefore, it is very necessary to propose a composite slab that can achieve unsupported construction.

Some improvement measures have been proposed to reduce the use of vertical supports in the construction process of composite slabs. Wu et al. [4] set a detachable truss rib on the bottom of the precast plank to replace the vertical support, realizing the construction without vertical support and improving the crack resistance of the slab. The truss rib that was placed at the bottom of the precast plank can be reused, but it is inconvenient to remove it. Moreover, based on the concept of a steel-concrete composite structure, measures such as concrete ribs and steel ribs were set on top of the precast plank can effectively improve its bending performance [5], [6], [7]. In addition to the above measures, good results have also been achieved by using grout steel tube, special materials, or components. Hou et al. [8] proposed a new type of composite slab with grouted-round-steel tube truss and conducted bending tests on it. The test results indicate that this type of composite slab has good ductility and can meet construction requirements. Ou et al. [9] investigated the flexural performance of composite slabs made of crumb rubber concrete or conventional concrete. They discovered that the bearing capacity and end-slippage of composite slabs made of crumb rubber were equivalent or even better than those made of the conventional concrete. Erfan et al. [10] conducted numerical simulations on concrete beams strength with FRP materials and found that using CFPR sheets to reinforce concrete beams can effectively improve their bearing capacity and prevent crack propagation. Maedeh et al. [11] conducted bending tests on concrete beams with steel reinforcement and a hydrid usage of GFRP reinforcement with varying fiber ratios, respectively. They concluded that concrete beams using 1.5% polypropylene fibers and GFRP rebars can reach the same bending strength as concrete beams using steel bars. Wang et al. [12] proposed a novel bamboo-concrete composite beam with precast lightweight concrete slab. Shear test results indicated that this new type composite slab had a similar shear strength but a higher stiffness that the cast-in-situ bamboo-lightweight concrete composite slab. However, the composite slabs mentioned above have a large amount of concrete poured on site and a low assembly rate.

Conclusions

In this study, the flexural behaviors of composite slabs and three different forms of cast in-situ slabs with a joint were compared, and the specimen SS-1 was taken as an example to conduct finite element analysis. the conclusions are as follow:

The failure mode of cast-in-place slabs and composite slabs with a joint in shape of T-shaped and rectangular concrete topping is that the crack width reaches the standard limit, while the failure mode of trapezoidal composite slabs is that the midspan deflection reaches the limit. Overall, all four slabs are ductile failure.

The ultimate bearing capacity of composite slabs with a joint in the shape of T-shaped and trapezoidal concrete topping are equivalent to or even higher than that of cast-in-situ slabs, and 12.9% and 19.7% higher than that of composite slab with a joint in shape of rectangular concrete topping.

According to the calculation formula for the bearing capacity of cast-in-situ slabs provided by GB50010–۲۰۱۰, the measured bearing capacity of composite slabs was evaluated. The ratio of actual measured values to theoretical values of CS, SS-1, SS-2, and SS-3 was 1.09, 1.09, 1.16, and 0.97 respectively. The reasonable agreement indicating a good potential for the code in the design of the composite slabs with a joint in shape of T-shaped and trapezoidal concrete topping.