Early Cover Spalling in High-Strength Concrete Columns

The American Concrete Institute (ACI) 318-02 provisions for stress block parameters were derived using normal-strength concrete column tests. Azizinamini et al. in 1994, Ibrahim and MacGregor in 1996, Bayrak in 1999 have reported that the use of current ACI stress block parameters results in estimations that are not conservative for high-strength concrete column sections. This was mainly attributed to the early cover spalling in high-strength concrete columns. In this paper, a numerical procedure for simulating premature cover spalling in high-strength concrete columns $\left(66\mathrm{MPa}<f_c^{\prime }<140\mathrm{MPa}\right)$ is presented. The proposed method for nominal moment capacity calculation employs rectangular stress block parameters and an analytically derived strength reduction parameter to integrate the effect of early cover spalling into the design process. Although the proposed method for moment capacity calculation is analytically derived, its results are validated with the experimental data from 224 column tests reported in the literature. Twenty-four of the 224 tests were conducted by Bayrak as part of a comprehensive research program in which the behavior of reinforced concrete columns was studied and provided the motivation for the research reported here. ACI 318-02 provisions for stress block parameters are critically examined using the experimental data from 224 tests. It is concluded that the ACI stress block parameters do not result in conservative estimations for the moment capacity of high-strength concrete column sections failing in compression. However, the proposed technique results in conservative estimations of strength for high-strength concrete column designs as well as normal strength concrete column designs.