Deformability and ductility in over-reinforced concrete structures

Abstract

It is often useful to use a high percentage of steel reinforcement in a concrete flexural member in order to minimise structural depth and still provide adequate stiffness. This paper shows that, by placing a steel helix in the compression zone of a heavily over-reinforced concrete beam, considerable strength, stiffness and ductility may be achieved, even when the steel percentage is as high as 7%. This is because the concrete in compression becomes triaxially stressed and confined during loading, leading to a high uniaxial strain capacity in the concrete at an enhanced stress level. It is shown that, even though the serviceability limit states of deflection and cracking are critically important, they are just satisfied under normal conditions. The testing programme described here consisted of seven tests on rectangular steel-reinforced concrete beams. Each helically reinforced specimen contained a single helix with a 20 mm pitch. The helices were formed from either 3 mm or 4·8 mm diameter mild steel wire. To show the full benefit of the presence of a circular helix, control specimens (no helix) and specimens containing a similar volume by weight of rectangular links were tested for direct comparison purposes. This research is exciting, as it demonstrates that it is possible to heavily over-reinforce concrete structures and nonetheless expect a ductile response. This is potentially very useful to clients, consultants and contractors who wish to use shallower concrete members. Further, two analytical approaches are developed here for predictive assessment. A full load–displacement approach is presented, followed by a simpler design-oriented approach. Both methods predict behaviour reasonably accurately.