Modeling cover spalling behavior in high strength reinforced concrete columns using a plasticity-fracture model

The cover spalling behavior in reinforced concrete (RC) columns can greatly affect the axial peak load carrying capacity. For RC columns with normal-strength concrete, the concrete cover still carries some load in the post-peak regime. However, for high-strength concrete, the concrete cover can spall prematurely, eliminating the cover capacity from the peak axial capacity of the column. The strength reduction due to premature cover spalling in high strength concrete columns may be recovered once confinement of the core increases and results in a second peak axial load. Hence, in nonlinear analysis a proper strategy is required to quantify any premature cover spalling for high- or very high-strength reinforced concrete columns. This paper discusses the strategies in modeling the concrete cover region spalling at the constitutive level. These strategies involve: (1) Adjusting the material properties for the concrete cover elements because of restrained shrinkage caused by the steel reinforcement; (2) Extending the plastic dilation rate into the tensile pressure region and adjusting the concrete dilatancy in presence of the tensile pressure; (3) Adding a tension cut-off failure surface to distinguish cracks caused by pure tensile stresses and concrete crushing (tensile splitting cracks) and (4) Assigning the cover spalling material properties only for cover elements outside the reinforcement cage or weak planes. To verify the proposed strategies, numerical validations against experimental published results are undertaken using a 3D-NLFEA finite element package. From the comparisons, it is shown that the proposed model is in good agreement with the experimental results and can accurately predict the location of the cover spalling in the load-deflection behavior.