Strengthening reinforced concrete columns by high performance ferrocement

Abstract

n Hong Kong, over 90% of our buildings are reinforced concrete structures with majority facing different degrees of aging. Proper maintenance and rehabilitation is required to preserve their life span. Further, those completed prior to the enforcement of the current Code of Practice do not fulfil the current detailing requirement. One main area of concern is related to the strengthening of reinforced concrete columns, which is the subject matter of this study. With transverse reinforcement at 300 mm spacing and using 90° hooks, existing columns fall short of providing sufficient lateral confinement and ductility in compliance with the current Code of Practice. It follows that strengthening of the existing columns is inevitable. This study is mainly focused on assessing the behaviour of circular reinforced concrete columns strengthened with high performance ferrocement ("HPF"). By applying different types of rendering material, HPF has improved properties as compared with traditional type of ferrocement. Direct tensile tests were carried out on dog bone shaped HPF specimens. Two equations were proposed for the prediction of tensile strength of cracked type and uncracked type HPF. Confinement action provided by these two types of HPF was studied by performing compression tests on 19 plain concrete cylinders. Equations for the prediction of confined compressive strength and stress-strain relationship of concrete confined by HPF were proposed. The predictions agree well with the test data. A method of strengthening circular columns using HPF is proposed. This is achieved by replacing the concrete cover by HPF. Full-scale circular plain or reinforced concrete columns were strengthened by HPF and tested under monotonic compression. 4 were plain concrete columns. 8 were detailed to the old Code of Practice with volumetric ratio of transverse reinforcement at 0.230 %. HPF having three different rendering materials and with 1 or 3 layers of wire mesh was used for strengthening. A set of empirical equations for predicting the peak strength of plain or reinforced concrete columns strengthened with HPF is derived. Prediction using this set of equations shows good agreement with the test data. A design procedure for strengthening reinforced concrete columns using HPF is proposed. All in all, the proposed equations provide a useful means to quantify the axial load carrying capacity of existing circular reinforced concrete columns. Applying a performance based strategy, it is possible to strengthen existing columns using HPF with appropriate choice of reinforcing mesh and rendering material.