Cover Spalling in HSC Columns Loaded in Concentric Compression

Abstract

Experimental studies on concentrically loaded high-strength reinforced-concrete columns have indicated that the cover spalls prior to the section reaching its squash load, if the squash load is calculated from the gross sectional area. To investigate the mechanics of cover spalling, specifically for high-strength reinforced-concrete columns but also for conventional-strength columns, a finite-element model is developed. Cover spalling is simulated by setting the elastic modulus of the cover elements to a low value once a threshold tension strain is reached at the cover-core interface, with the threshold tension strain chosen to match experimentally recorded axial strain data. Radial tension stresses exist at the cover-core interface due to restraint of the Poisson growth of the core provided by the ties. Increasing the volume of ties increases the restraint and increases the radial tension stress at the cover-core interface. Postanalysis revealed that the stress at the point of cover spalling matches well with the triaxial tension strength of the unreinforced cover. Analysis of experimental data reinforces the findings of the finite-element model. It is recommended that for reinforced columns cast from concrete with cylinder strengths greater than 60 MPa, the maximum load be taken as 0.85 times the capacity of the concrete section (based on the in-situ concrete strength) or the capacity of the confined core, whichever is greater, plus the capacity of the longitudinal reinforcement.