STRENGTHENING OF RC COLUMNS USING FRP WRAPS

Abstract

• Fibre Reinforced Polymers (FRP) composites material have been widely used for various applications in aerospace and automobile industries since 1940's. However, from last twenty years this composites material has found its way in the construction industry, initially for strengthening of the degraded structures. The need to strengthen the existing old and dilapidated structures has been observed in the recent earthquakes in which many of the buildings and bridges were collapsed. The need arises because of the inadequate lateral strength and ductility of these old structures which has been primarily designed to resist gravity loads. To arrest this problem the interest of civil industry and the researchers has grown rapidly for use of FRP composites as a strengthening material and nowadays as a material for new structural components. This is because of the unique properties of FRP composites, such as the high strength-to-weight ratio and stiffness-to-weight ratio, corrosion and fatigue resistance, tailorability, and ease of installation. It is an established fact that confinement provided to the reinforced concrete (RC) columns increases its load-carrying capacity and the ductility. FRP composites have been widely used for strengthening existing RC columns for improving both strength and ductility. Researchers have reported that depending on the properties of FRP composites the enhancement in ultimate strength and corresponding strain have found to be 2 to 3 times and 5 to 15 times greater than that of unconfined concrete strength and strain, respectively. However, the behaviour of FRP wrapped RC columns is not yet fully understood, and the design codes are still in their primary stages. The models to predict the accurate behaviour of FRP wrapped columns are required for reliable and cost-effective design. Based on an extensive review of the available literature it has been observed that most of the work has been done on low to medium strength concrete cylinders tested under concentric axial load, mainly to establish stress-strain relationship of FRP-confined concrete. The models proposed in the open literature for FRP-confined RC columns mostly deals with the strength and strain enhancement without considering the contribution of transverse confining steel. Enhancement in ductility is typically required in existing RC columns subjected to a combined axial load and bending moment. In such loading conditions the material strains begin to go into the nonlinear range and therefore, the section behaviour becomes iii nonlinear. For the design of such sections it is necessary to obtain the moment-curvature relationship. In the existing literature it has been reported that the non-linear behaviour of RC columns can best be addressed by employing a finite element (FE) approach. FE method is particularly advantageous in the modeling of non-uniformly confined concrete as it is capable of capturing complex stress variations in the concrete. However, the extent of work done has been found to be limited in all these areas. The present study aims to deal with the above mentioned shortcomings. The main objective of the present work is to study the behaviour of FRP-confined plain and RC columns and to develop stress-strain models. Experiments were conducted on FRP-confined low and high strength concrete cylinders subjected to axial compression to gain a good understanding of the stress-strain behaviour of confined concrete. The effects of various parameters (i.e. concrete grade - Low and High, type of FRP material - GFRP and CFRP, number of FRP wraps - One and Two) on structural behaviour of confined concrete has been studied. It has been observed from the obtained results that the loadcarrying capacity and the ductility of plain concrete columns increased with the increase in confinement provided by the FRP wraps. For low strength concrete specimens, the confinement effectiveness coefficient varied from 2.14 to 4.44, while for high strength it ranged from 1.08 to 2.25. This clearly shows the dependence of confinement effectiveness coefficient on unconfined concrete strength. Based on the present study test data, a simple but accurate design-oriented and an analysis-oriented stress-strain model for predicting the behaviour of FRP-confined concrete has been developed and validated with the test data. Experiments were also conducted on RC short columns subjected to axial compressive load to study the effects of the internal transverse steel reinforcement, section geometry (Circular and Square) and the parameters mentioned above for FRP wrapped plain concrete. Similar to the plain concrete columns, the RC columns has shown increase in strength and ductility due the confinement provided by the FRP wraps. The contribution of internal transverse steel reinforcement in presence of FRP wraps in enhancing the strength and ductility of concrete columns has been clearly identified. The FRP wrapped RC columns with high amount of internal confining steel reinforcement achieved higher ultimate stress and corresponding strain compared to those with little or no internal confining steel reinforcement in the columns. The dependence of strength and ductility of the confined concrete on the cross-section of the RC columns has been observed. The IV increase in strength and ductility for circular cross-section RC columns has been found to be more than the square cross-section columns. All FRP-confined RC column specimens failed by sudden rupture of FRP wrap followed by buckling of steel reinforcement. The rupture of FRP occurred near the mid height portion of specimen and out side the overlapping zone. Based on the experimental results a stress-strain model to predict the response of FRP-steel-confined concrete columns has been developed and validated with the test results. The moment-curvature analysis has been performed to serve as a basis to develop design equations. An attempt has been made to evaluate the special confining reinforcement requirements of the Indian Standard Code IS 13920 : 1993. The models developed for predicting the stress-strain response of FRP and FRP-steel confined concrete has been incorporated into the proposed analysis procedure and moment-curvature interaction diagrams for short FRP-confined RC columns were generated. To validate the proposed procedure, a comparison of generated momentcurvature curves with the experimental moment-curvature curves from the existing literature has been carried out. A close correlation has been obtained between them. Based on this analysis, design equations and design charts have been developed. Finite Element (FE) Method has been employed to analyze the behaviour of FRPconfined concrete columns as it remains the most convenient method of treating the complex nonlinear problems. Using general purpose FE analysis software (ANSYS), 3-D FE models of FRP and FRP-steel confined concrete columns has been developed and analyzed to study the behaviour of confined concrete. The elasto-plastic behaviour of concrete has been modeled using Willam-Warnke (1975) five parameter model. The results from the FE analysis showed a bilinear stress-strain curve for the FRP-confined, and bilinear ascending with a third descending branch for the FRP-steel-confined concrete as it has been observed by experimental results. The developed FE model has been validated with the present study test data. Despite the increasing popularity of the FRP strengthening technique, relevant design provisions in most of the existing design guidelines are found to be in their primary stages. This study is therefore, intended as a step towards improving this status and contributes to the available database. The present study data may also contribute to formulate design provisions for the Indian Code for the Structural Use of FRP Composites in Construction which is not yet prepared.