Structural Behavior of Reinforced High-Strength Concrete Columns

Doctoral thesis, 1998

The high compressive strength of high-strength concrete is especially advantageous in compressed members such as columns, which can be made more slender and, consequently, make economic benefits possible. However, the behavior of high-strength concrete columns is not yet fully understood. This thesis deals with the behavior of reinforced normal and high-strength concrete columns under compressive loading. Numerical results from non-linear finite element analyses were compared with results from columns tested. In the present study, thirty reinforced short stub concrete columns and sixteen reinforced long slender concrete columns have been tested under axial compressive short-term loading to failure. In addition, two long slender columns were subjected to sustained compressive loading. The parameters varied in the study were the concrete strength, stirrup spacing, reinforcement strength, slenderness of the columns, and eccentricity of the axial load applied. The test results for the short stub columns show that the load capacity increased in proportion to the increased compressive cylinder strength. The short stub columns of high-strength concrete exhibited a sudden, explosive type of failure. When the concrete strength of the long slender columns was increased, the maximum load capacity became greater. Although closer stirrup spacing did not provide an increase in load bearing capacity, it did give the columns a more ductile behavior in the post-peak region. The most important parameters for obtaining a ductile behavior were the spacing of the stirrups and the reinforcement configuration. Furthermore, it was observed that the stirrups in the high-strength concrete columns did not necessarily yield at maximum load. Therefore, to estimate the strength correctly it is necessary to use the actual stirrup strain or to design the reinforcement configuration so that yielding is reached at maximum load. Tests showed that the structural behavior of a reinforced high-strength concrete columns is favorable for sustained loading, i.e., the column exhibited less tendency to creep and could sustain the axial load without much increase of deformation for a longer period of time. The nonlinear finite element analyses show good agreement with the test results. The analyses have been performed with two types of elements, beam elements and three-dimensional solid elements; each type has its advantages. This study has shown that the non-linear finite element method, together with non-linear fracture mechanics, provides a useful tool for the detailed analysis of reinforced concrete structures and contributes to a better understanding of the structural behavior of reinforced concrete columns subjected to axial loading.