Influence of bond on the hinge rotation of FRP plated beams

Fibre reinforced polymer (FRP) plate reinforcement is a brittle material which has a brittle interfacial bond with concrete. This can lead to the misconception that all FRP retrofitting techniques provide brittle members and, hence, limited rotational capacity which has severe limitations for structural applications. This paper shows that the FRP reinforcement behaviour is but one of three components that govern the rotational capacity of plated reinforced concrete beam hinges. It is shown that FRP retrofitted beams and slabs can achieve ductile behaviour and provide rotational capacity and, furthermore, that the rotational capacity of FRP plated members depends very importantly on the interface bond characteristics.M. Haskett, M.S. Mohamed Ali, D.J. Oehlers and C. W

The hunt for the elusive concept

Structural engineers, whether they are designers, contractors or academics, are frequently faced with the problem of introducing innovation into practice through research, such as for the development of new structures or the use of new materials such as fibre reinforced polymer. Hence, research is pivotal to the introduction of innovation. How research is applied is invariably a very complex and often costly problem and consequently needs careful examination to be effective. All engineers do research but by the very nature of research we all do research in different ways. This paper is the author's reflections on research procedures and approaches often used in structural engineering. These approaches are then illustrated using the author's research particularly on developing a generic failure model for reinforced concrete which can be applied to the seemingly disparate reinforced concrete problems of moment-rotation, shear capacity and the effect of confinement. Numerous concepts are introduced in this paper such as: the use of empirical modelling in plugging the gaps in the structural mechanics models; the elusive concept that is pivotal to understanding and should not be confused with the illusive concept; and categorisation of researchers to help sort out the mass of information available.Deric John Oehler

Strain-gradient-dependent stress-strain curve for normal-strength concrete

The stress-strain distribution of concrete in the compression zone of reinforced concrete (RC) flexural members is the most important parameter for assessing the ultimate flexural strength and ductility. Currently, the stress-strain curve of concrete developed in flexure is taken as the uni-axial compressive curve incorporating a scale-down factor k3, which is the ratio of the maximum concrete stress developed under flexure to the concrete cylinder strength. In current RC design codes, the ratio of the equivalent concrete stress to cylinder strength is taken as constant equal to 0.85 for normal-strength concrete but reduces as concrete strength increases. However, in a recent study carried out by the authors, it was found that the maximum concrete stress developed in the flexure increases significantly as the strain gradient (ratio of extreme concrete strain to neutral axis depth) increases, until reaching a maximum limit. Therefore, the value of k3 should not be taken as a constant for flexural RC members. In this study, the authors will adopt the results obtained in the previous experimental tests on concentrically, eccentrically and horizontally loaded RC columns to derive the stress-strain curve of concrete under different extents of strain gradient. The derived values of k3 are then correlated to the strain gradient using empirical equation. The applicability of the proposed equation is checked by comparing the flexural strengths of NSC beams and columns so calculated with those experimentally measured by different researchers, in which good agreement has been obtained