Refined approach for modelling strength enhancement of FRP-confined concrete

• A surface fitting function is used for formulating refined confined-concrete model.
• Nonlinear relations between confining parameters influence confinement levels.
• Confinement level may be over or under estimated by ignoring nonlinear influences.

The application of FRP-confined concrete is recognized to be an effective solution to increase the axial strength of many new and existing concrete columns. A number of experimental investigations and developments for numerical models have been established to understand the strength enhancement behaviour of the FRP-concrete confined section. This paper aims to investigate the influence of the confinement parameters including the concrete strength and the confining pressure on the strength enhancement of FRP-confined concrete specimens. Investigated data were collected from experimental works performed by the authors as well as results surveyed from state-of-the-art papers carefully selected and reviewed to provide an overview and in-depth knowledge in the development of this strengthening technique. Utilizing 927 experimental data from 127 papers from the literature, a 3-dimensional graph presentation (surface chart) have been utilized to investigate the strength enhancement of the FRP-confined concrete. This new graphing format assisted in demonstrating the non-linear influences for each of the parameters investigated on the confinement parameters on the strength enhancement of FRP-confined section. Unlike the conventional 2-dimensional plot of the confined concrete strength versus the confining pressure (both normalized by the unconfined concrete strength), the 3-dimensional graphing approach introduced in this paper is found to provide more information about the strength enhancement. Utilizing a Gauss surface fitting function, a refined enhancement ratio expression incorporating the individual influence of the confinement parameters is proposed in the paper and experimentally evaluated.