On Behavior of High-Strength Concrete Columns: Cover Spalling, Steel Fibers, and Ductility

It has been shown that the cracking at the cover-core interface, which proceeds cover spalling in high-strength concrete (HSC) columns, is a direct result of a triaxial stress condition. The stress state is a fundamental consequence of the confinement provided to the core by the ties. This condition is explored further in this paper, and it is shown that the mechanism of cover spalling is the same for HSC columns as for conventionalstrength concrete columns. Discussed are the driving mechanisms needed to cause spalling of the cover once the cover-core interface crack has been established. Steel fibers in the concrete mixture have been shown to prevent early spalling of the cover and improve ductility. In this paper, a model is developed to determine the volume of fibers needed for ductility in conventional and HSC columns.