Punching shear of self-consolidated two-way slabs

Abstract

An experimental program was conducted to investigate the structural behaviour of two-way slabs made with Self-consolidating concrete (SCC). Four different SCC mixtures were developed with targeted compressive strength of 30 MPa. Mixtures A and B contained maximum coarse aggregate size of 10 mm, and coarse to fine aggregate ratio (C/F) of 0.70 and 1.20, respectively. Mixtures C and D contained a larger coarse aggregate size of 20 mm, and (C/F) of 0.70 and 1.20, respectively. The properties of the fresh and hardened concrete for each mixture were measured. Each concrete mixture was used to construct three slabs with different thicknesses of 150 mm, 200 mm, and 250 mm. Thus, a total of twelve slabs were tested in the experimental program. All test slabs had a reinforcement ratio of approximately 1.0%. Hence, the main parameters in the experimental program were the coarse aggregate size, coarse to fine aggregate ratio, and slab depth. The structural behaviour of the slabs was examined under static monotonic load with regard to the deformations, strains in the reinforcement and concrete, ultimate capacity, modes of failure, and crack development. The C/F ratio and maximum aggregate size do not show significant influence on the slab deformation characteristics such as deflection, stiffness, ductility and energy absorption, steel and concrete strains and cracking characteristics. The slab thickness has the most significant effect among the test parameters on the behaviour of the test slabs. The depth and aggregate size are the most influential parameters on the capacity of the slab; increasing the slab thickness lead to a decrease in the normalized shear strength of the slab while increasing the aggregate size lead to an increase in the normalized shear strength of the slab. The Critical Shear Crack Theory (CSCT) by Muttoni (2008) is able to reasonably predict the structural behaviour of the test slabs. However, the predictions of the capacity by the CSCT had a high scatter. In addition, the test results did not show any clear trend in the relationship between the aggregate size and the slab rotation. The Canadian Code (CSA A23.3-04), the American Code (ACI 318-11) and the British Code (BS8110-97) give safe predictions of the capacity of the SCC test slabs. The predictions of those codes are more conservative and have less scatter when applied to SCC slabs with 20 mm coarse aggregate size compared to those with 10 mm coarse aggregate size. Therefore, these codes can be safely used to check the punching shear capacity of SCC slabs without the need of any modification to the equations used for such shear check. The predictions of the Eurocode (EC2) are unsafe for most of the slabs with thicknesses of 200 mm and 250 mm. Hence, further research is needed to examine the use of EC2 in the design of SCC slabs for punching shear.