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In typical precast construction practice of floor slabs using precast concrete hollow core unit (HCU), in-situ concrete is cast on top of the HCU to obtain smooth and even floor finish. The surface of the HCU is seldom given proper treatment prior to casting the concrete topping. The texture and surface moisture condition of the HCU just before receiving concrete topping may affect the overall strength of the slabs when the concrete topping and the HCU act compositely during service. This paper presents the experimental study on shear-flexure capacity of composite slabs using HCU and concrete topping. Full scale three point load test are carried out on 14 composite slab specimens with different surface roughness and surface condition of the HCU before casting the concrete topping. The surface roughness considered is smooth and rough, while the moisture conditions are dry, ponded and optimum wet. The effect of the longitudinal joint between the HCU panels is also considered. The experimental results are also compared with predicted values using the available equation in Eurocode 2 and an equation published by a previous researcher. The results of the experiment show that the HCU surface condition and longitudinal joint affect the stiffness and shear-flexure strength of the slabs. The optimum HCU surface condition which can produce highest stiffness and shear strength is rough and wet conditions, while the longitudinal joint between HCU panels reduces the slab shear strength. The interfacial horizontal shear is not the factor that governs the strength and behaviour of the slabs. The equation available in Eurocode 2 gives non-conservative prediction of the shear strength. In contrary, the equation published by the previous researcher gives conservative prediction of the shear strength. [ABSTRACT FROM AUTHOR]

• The comparison between shallow recess and deep recess beam. • The substitution of bar reinforcement with the steel fibre reinforcement. • The influence of a v / d ratio in the failure mode of the beam half-joints. • Two semi-empirical equations were proposed based on the failure mode of beam. A complex geometry of precast reinforced concrete half-joint beams has resulted from the high shear force at the recessed ends of such beams. Immense levels of reinforcements are provided close to the re-entrant corner of these beams to prevent the propagation of diagonal cracks and sudden failure. However, this condition leads to reinforcement congestion and affects the bond between concrete and reinforcing steel due to improper concrete filling around the congested bars. Hence, the use of steel fibre self-compacting concrete (SFRSCC) was found to combine the advantages of eliminating the vibration works, and reduction of secondary reinforcement. An experimental study was conducted to examine the ability of 1.0% by volume of steel fibre to partially replace the traditional reinforcements on two types of half-joint beams (short recess (SR) and deep recess (DR) beams). Other factors taken into account is the variance in shear span values and nib height, as well as the ratio of shear-span-to-depth, a v / d. Observations showed that the replacement of 1.0% by volume of steel fibre volume fraction is possible only when substituting at least 50% of the vertical or horizontal reinforcement in SR beams. Meanwhile, the replacement over diagonal reinforcement is seemed to be not enough and the replacement is only promising in substituting part of stirrups in DR beams. The variance in the shear-span-to-depth a v / d ratio affects the shear failure mode of the beam half-joints. Two semi-empirical equations for predicting the shear strength of precast SCC and SFSCC beam-half joints are developed based on the analysis of failure modes. The first equation, which relies on the crushing of the concrete strut, was found to suit the SR beams, while the equation for DR beams suits the theoretical of reinforcement yielding. The two equations give a good correlation with the experimental results. [ABSTRACT FROM AUTHOR]