Monitoring of Bond in FRP Retrofitted Concrete Structures

Abstract

Debonding failure of the concrete cover in reinforced concrete beams retrofitted with fiber reinforced polymer (FRP) composites is a brittle phenomenon which in most cases occurs in an abrupt manner. A complete understanding of bond requires information about the relationship between the local bond stress and slip. Bond-slip defines the constitutive relationship of the interface, and it provides means for computation of ultimate strength and distribution of the bond stress. For FRP composites interface slip is very small even at the ultimate stage prior to failure. For this reason, the study of local bond in the earlier studies did not include slip. This article presents a fiber optic based method for measurement of local slip at the interface between concrete and FRP as well as for prediction of bond failure in reinforced concrete members. Since bond failure in FRP retrofitted concrete is a brittle phenomenon, development of effective means to predict the failure prior to occurrence plays an important role in structural health monitoring of such structures. Therefore, the scope of the study includes two types of tests, namely pull out tests and beam flexure tests. In general, the measured interface slip between concrete substrate and FRP at ultimate load stage is <60 μm. The fiber optic based system is capable of measuring the interface slip with a resolution of 1 μm. In flexure, the long gauge distributed sensors are able to predict the debonding and peeling of the FRP fabric from the concrete beam through deformation reversals. A numerical model based on finite element analysis of the beams is developed to verify the capabilities of the fiber optic sensor through computation of principal stresses at the interface.