Effect of predamage on the stress-strain relationship of confined concrete under monotonic loading

Jacketing concrete columns with steel or fiber-reinforced polymer (FRP) has been identified as a highly effective way of restoring and enhancing the strength and ductility of columns damaged by catastrophic events such as earthquakes, blasts, and vehicle impact. However, a stress-strain relationship for damaged and subsequently repaired concrete is not available for design of the repair works. An extensive experimental and analytical investigation was undertaken in this work to study the mechanical properties of repaired concrete columns. Tests were conducted on 102 concrete cylinders involving variations of damage degree, concrete grade, and confinement pressure. The effect of damage on the compressive strength, strain capacity, initial elastic modulus, and strain-hardening modulus of repaired concrete is quantified in terms of a damage index. A stress-strain model is developed for the design of structural repair works. Evaluation of the test data also exhibits that the classic model of confined concrete proposed by a previous study and the classic internal friction theory are qualitatively applicable to failure of concrete materials not only in its original form, but also in the heavily disturbed state. The Hoek-Brown failure criterion is unconservative for heavily damaged concret