Tensile stress-strain relaxation as a failure precursor for FRP-strengthened RC beams

The paper presents findings from an experimental investigation and finite element (FE) analysis of RC beams strengthened in flexure and shear with externally bonded carbon FRP composites. The work initially focuses on the brittle plate-end concrete cover failures of RC beams strengthened in flexure only. The strain measurements reveal that the 'plate-end debonding' is preceded by tensile relaxation in the anchorage zone of the FRP plate as a result of tension softening within the concrete cover layer. Experimental work then dealt with the role of the side-bonded 'U'-shaped FRP strips in preventing the plate-end concrete cover failure and in increasing shear capacity of flexurally strengthened concrete beams. For the two strengthening configurations, with and without FRP strips, FE models developed with ADINA and ABAQUS were used to predict brittle failure through concrete. The non-linear FE analysis based on the smeared-crack constitutive model for concrete in tension produced the load-history of principal tensile stresses in concrete at the plate-end. This load-history was then used to define a design criterion for predicting the safe load levels against the plate-end concrete failure for FRP-plated RC beams.