Ductility in concentrically loaded reinforced concrete columns

In recent years, the use of high-strength concrete materials has been regulated into Australian design standards. The use of high-strength concrete is desirable in many cases. For instance, in reinforced concrete columns of high rise buildings, the columns can carry more load with a smaller cross section compared to reinforced concrete columns built of normal strength material. However, there are some disadvantages, one being the reduction of ductility. The Australian Concrete Standard AS3600 deals with this issue by changing the tie arrangement in reinforced columns for different concrete strength grades. This study reviews the ductility index used to measure the ductility of reinforced concrete columns and uses an analytical model to predict the ductility index of several practical example columns. These columns are designed and detailed using the requirements of the Australian Concrete Standard. The outcome of a parametric study shows that the columns designed and detailed using the rules in the Australian Concrete Standard may not necessarily have the ductility index which the code assumes. Another well-known deficiency observed in the behaviour of reinforced high-strength concrete columns is premature spalling of the cover concrete. The Australian Concrete Standard addresses premature cover spalling by modifying a reduction factor which is applied to the strength of the concrete when the squash load of a reinforced concrete column is calculated. This reduction factor accounts for many issues not only premature cover spalling. Using an analytical model, it is shown that the code formula for estimating the squash load is too conservative and needs adjustment for very large columns with small cover to core ratio.