Ductility Enhancement of R.C. Columns Using FRP Composites

Abstract

Ductility in the structures is very important parameter when structures have to deal with earthquake and other forces. Structural elements must be ductile to resist earthquake forces. An attempt here is made to understand the improvement in behavior of the Fiber Reinforced Polymer wrapped R.C. columns. This major project focuses on ductility enhancement of R.C. column using FRP composites. Based on extensive literature review it has been revealed that very few attempts are made by researchers to study eccentrically and axial cyclic loaded columns.

Hence behavior of the column is studied here under axial, eccentric and axial cyclic loading respectively. This report also presents an extensive literature review on R.C. columns wrapped using FRP composites.

Comparison is reported on important parameters i.e. column geometry, suitable wrapping material and technique, type and positioning of reinforcement, type and combination of loading etc. Experimental work has been carried out on total 36 RC columns. Square and circular columns of 18 nos. each are cast. Out of 18 square columns, six columns each are tested for axial, eccentric and axial cyclic loading respectively. For every load case, two columns are unwrapped, two are wrapped with GFRP composites and remaining two are wrapped by CFRP composites respectively. Same kind of variations in loading conditions and wrapping materials are selected for circular column also. In experimental work, variation of parameters in terms of shape of column, wrapping material and loading pattern. Columns have been tested for support condition of free at one end and fix at other end. To simulate actual fix support conditions, fix supports are fabricated and they are kept remaining attached with the loading frame during load application.

For all columns axial and eccentric load is applied through knife edge plate system. Eccentricity of 20 mm kept constant throughout whole height of the column. Axial cyclic load is applied through strain controlled approach. Displacement of 1 mm is applied on the column and corresponding load is measured during axial cyclic loading. Different structural parameters i.e., failure load, displacement and strains at various heights have been measured. Failure modes for all RC columns have been discussed in details. For axial, eccentric and axial cyclic loading cases ductility improvement in columns has been evaluated.

Energy dissipation, Specific Damping Capacity (SDC), energy applied and stiffness degradation have been measured and cyclic loading. CFRP wrapped columns perform better compared to GFRP and un-wrapped columns respectively for load carrying capacity, ductility and stiffness. It is found that axial load carrying capacity of circular columns increase by 113% using CFRP whereas GFRP enhances load carrying capacity of columns by 70%. In square columns also CFRP increases load carrying capacity by 80% whereas 61% improvement in axial load carrying capacity is observed using GFRP composites. Similar kind of benefits of incorporation of FRP composites has been observed in eccentrically loaded columns as well as cyclic loaded columns respectively.

Evaluation of load carrying capacity for columns subjected to axial load has been carried out using confinement model available in literature and analytical results have been compared with experimental results to validate the confinement model. An attempt has been made to formulate Pu-Mu interaction curve analytically for FRP wrapped columns and results are compared with unwrapped columns.