# The Behaviour of Half-Slabs and Hollow-Core Slab in Four-Edge Supported Conditions
## 1 Introduction
Previous tests on full-size slabs with more than one precast unit have occasionally been performed [40; 41; 42; 43]. They are mainly concerned with shaping and reinforcing the longitudinal joint in the bidirectional behaviour. The effect of reinforcement passing through inverted multi-rib panels [42] and prestressing [43] on cracking resistance and load capacity of the models of single-span strips also appear to be an intriguing aspect. The analyses concerning the effect of a rib head on the static behaviour of the unit, including delamination [44], show no impact of the inclination angle of rib planes on the load capacity of the members. Some tests focused on the connections of panels without in situ reinforcement, but only with the specifically shaped interface and reinforcement passing through sidewalls of the members [45]. They show that, inter alia, cracking resistance and load capacity at an increasing deflection of the slab made from panels with structural joints and two-way bent rebars is reduced when referring to the solid slab. Other tests examined the effect of half-slabs' longitudinal joint without rebar reinforcement or other connection methods only under short-term loading. The tests were usually performed on hollow-core slabs used in bridge structures [46; 47]. Transmission of shearing forces is safe even for relatively small longitudinal joints (concerning dimensions of the members). Numerical analyses and shape engineering of the slab interface are presented in the papers [48; 49]. The paper [50] describes tests on the model slabs consisting of a few panels supported on walls. The authors demonstrated that longitudinal reinforcement does not increase resistance to joint shearing but only increases the potential of plastic strain. The same situation is observed for transverse reinforcement at the interface. The transverse joint resulting in interlock between slabs impacts the distribution of loads, the knowledge of which is used in the design procedures. Based on tests [51], the observed transverse load distribution (Figure 1) in slabs connected only by unreinforced longitudinal joints were found to correspond to hinged connections between slabs that are recommended in, inter alia, the standard EN 1168:2011 [52] for prestressed HC slabs. Slabs with panels connected with solid sections of longitudinal joints have a decreasing distribution coefficient, whereas the directly loaded slab takes the majority of loading. Despite the fact that precast panels have been used for many years and designed in a manner that only considers one-way behaviour, no tests have been conducted under long-term loading for the common support condition that enforces the two-way behaviour. This gap is also evident in the standards for precast slabs [53; 54]. The standard [52] on hollow-core slabs contains limited information on this subject. It only specifies the equation for shear strength at the shear key and introduces the computational model for load distribution among separate panels. In the research analyses to date, the longitudinal and transverse interaction properties have been clearly revealed, and the behaviour at the concrete-concrete interface under short-term loading has been thoroughly examined. By contrast, long-term loading, which is crucial considering the ultimate limit state (USL) and serviceability limit state (SLS), is insufficiently studied. The scope of previous long-term research [55; 56; 57] has been focused on beam elements. The majority of long-term effects (mainly displacements) were observed within the first 100 days of loading. After the first year of loading, deflection reached 95% of the final displacements observed after seven years. For the purpose of filling the cognitive gap, tests were carried out on precast slabs. They mainly focused on the long-term effect on cracking morphology, transverse behaviour of the precast units, and changes in displacements. Four full-scale slabs were examined by exerting uniformly distributed load and non-uniform long-term loads.