Concrete with strengths above 50 MPa is currently used due to an increasing requirement for higher strengths and improved long-term properties. HSC is being utilised in many projects. High-strength concrete members exhibit, in some instances, different failure mechanisms and simply extrapolating models and equations meant for normal strength to high-strength concrete may lead to unsafe designs. One of the reasons why some structural engineers are reluctant to use high-strength concrete is due to the lack of provisions in the Concrete Structures Standards.
The reinforced concrete flat slab system is a widely used structural system. Its formwork is very simple as no beams or drop panels are used. However, the catastrophic nature of the failure exhibited at the connection between the slab and the column has concerned engineers. This area becomes the most critical area as far as the strength of flat slabs is concerned due to the concentration of high bending moments and shear forces. The failure load may be considerably lower than the unrestrained flexural capacity of the slab. A typical punching shear failure of a bridge deck during testing is shown in. The use of high-strength concrete improves the punching shear resistance allowing higher forces to be transferred through the slab-column connection. In spite of the wide use, only a few research projects have been conducted on the punching shear resistance of high-strength concrete slabs. The empirical expressions given in design codes are based on the experimental results from slabs with concrete strengths between 15-35 MPa. Hence it is necessary to re-examine the applicability of the present punching shear design methods for HSC slabs, using the published data.
Fig. 2 – A typical punching shear failure of a bridge deck