Digital-image-correlation-based experimental stress analysis of reinforced concrete beams strengthened using carbon composites

Abstract

The strengthening of reinforced concrete beams through the use of epoxy-bonded carbon composites has been widely researched in the United States since 1991. Despite the widespread attention of researchers, however, there are no reliable methods of predicting the failure of the repaired and strengthened beams by delamination of the carbon composite from the parent concrete. To better understand delamination, several investigators have presented analytical work to predict the distribution of stresses along the interface between the carbon composite and the concrete. Several closed-form solutions can be found in the literature to predict the levels of shear stress present between the bonded composite plate and the parent concrete beam. However, there has been very little experimental verification of these analytical predictions. The few experiments that have been conducted have used numerous electrical resistance strain gages, adhered to the surface of the carbon composite at various intervals along the length of the test section, in order to deduce the interfacial shear stress using first differences. This method, though very crude, demonstrated that there are substantial differences between the distributions of interfacial shear stresses in actual repaired beams versus the analytical predictions. This paper presents a new test program in which large-scale (2.4 m long), carbon-fiber-strengthened reinforced concrete beams are load-tested to failure, while employing digital image correlation (DIC) to record the three-dimensional displacements of the surface of the carbon fiber plate. Three-dimensional digital image correlation is a two-camera, stereoscopic technique for measuring true, 3D full-field surface displacements. The technique uses a subset-based correlation method to determine the correspondence between images from the two cameras and between images at different load levels. From each load’s full-field surface displacements, a surface strain map can be generated. The resulting strain maps allow the investigation of the load transfer from the carbon fiber to the concrete beam with a level of detail not achievable with standard strain gages. The focus of this paper is the application of the three-dimensional digital image correlation technique to the investigation of FRP reinforced concrete beams. The paper presents: 1) the results of the experimental testing; 2) an overview the three-dimensional digital image correlation technique; 3) the adaptations required to utilize the 3D correlation method on the large, 0.1m x 2.0m, imaged area of the beam; and 4) the effect of the discontinuous failure mechanisms, inherent in reinforced concrete structures, on the analysis of the data.