Interfacial stresses in reinforced concrete beams bonded with a soffit plate: a finite element study

This paper presents a careful finite element investigation into interfacial stresses in reinforced concrete (RC) beams strengthened with a bonded soffit plate, with the aims of assessing the accuracy of existing approximate closed-form analytical solutions based on simplifying assumptions and highlighting aspects which are omitted by them. Finite element modelling issues are first discussed, with particular attention on stress singularities in such beams and appropriate finite element meshes for the accurate determination of interfacial stresses. Finite element stresses are then carefully examined and compared with the predictions from a closed-form solution recently derived by the authors. The finite element results show that stresses vary strongly across the adhesive layer, with the stresses along the adhesive-to-concrete (AC) interface being very different from those along the plate-to-adhesive (PA) interface. In particular, near the end of the plate, the interfacial normal stress is tensile along the AC interface but compressive along the PA interface, offering a plausible explanation for the fact that PA interfacial failure in tests has rarely been if at all reported. The closed-form solution, being based on the assumption of uniform stresses in the thickness direction in the adhesive layer, is incapable of predicting such complex stresses, but nevertheless provides a reasonably close prediction of stresses along the middle-thickness section of the adhesive layer, particularly when the adhesive layer is not very thin. This and other similar closed-form solutions are therefore believed to still form a useful basis on which design rules against debonding failures can be built, with appropriate tuning using experimental data. Finally, the effect of spew fillets on interfacial stresses is examined.