Abstract: This research work presents an innovative approach to retrofit the buckling of longitudinal reinforcement at the plastic hinge region of the column by using an external bolted mechanical coupler with and without FRP wrapping. Cyclic testing on full scale retrofitted columns is carried out to examine the effect of efficiency of epoxy filled mechanical coupler connection with and without external FRP confinement. Four column specimens in original position are tested under cyclic loading and after that retrofitted with the different techniques. The hysteretic behavior, energy dissipation, stiffness behavior and failure pattern are compared with original specimens. Enhanced ultimate strength and energy dissipation of retrofitted specimen using epoxy filled mechanical coupler and FRP confinement authenticate the effectiveness of employed technique in retrofitting work at field.
The past reconnaissance reports manifest that the number of moment resisting frame buildings have failed due to formation of plastic hinges in columns. It is mainly due to lack of confinement, large tie spacing, insufficient splice length, inadequate splicing at the same section, hook configurations, poor concrete quality etc. This is not only catastrophic consequences for a structure but it is very difficult to retrofit due the buckling of the longitudinal reinforcement of column in plastic hinge region. As a result, the column shortens and loses its ability to bear even the gravity load. This type of damage is also more sensitive to the cyclic moments generated during an earthquake and the increasing intensity of axial load. The prevailing practice for the repair and retrofitting under this situation is only to remove the damaged portion i.e. portion including the buckling of longitudinal reinforcement thoroughly and to replace with new concrete. The yielded or buckled reinforcement is to be cut and replaced by new reinforcement since it is extremely difficult to straighten the buckled reinforcement with the help of any mechanical or hydraulic device under the axial load. Replacement of buckled reinforcement or providing additional new reinforcement, welding is the major source till date. The main difficulty in welding at site is that it requires removal of large concrete at both the sides of damaged column to expose the required length of original reinforcement for welding that restricts its practical applicability. Moreover, in some critical situations, particularly in congested sections/ regions welding may not be possible. Therefore, the main objective of this experimental investigation is to develop a suitable effective technique to connect the two ends of the main reinforcement arising due to cutting of buckled reinforcement. The study is mainly focused on retrofitting of column using an innovative approach using epoxy filled mechanical coupler with and without external FRP confinement. The test results reveal that the mechanical coupler along-with FRP wrapping may be an effective alternative solution for the retrofitting of this kind of damage and can be easily applied in the field.
Many past earthquake damage surveys across the world show that the main reinforcement of columns have buckled in the plastic hinge region due to inadequate shear reinforcement, improper confinement of core concrete and large tie spacing as shown in Figure 1. These columns of building or bridges, when subjected to seismic motion, concrete begins to disintegrate and the load carried by the concrete shifts to its longitudinal reinforcement. As a result, the column shortens and looses its ability to carry even the gravity load. The repair and retrofitting of buckled reinforcement type of failure in columns is a challenging task and the studies are also very limited1-3.
The complete experimental program for the evaluation of seismic efficiency of retrofitted column has been divided in two phases. In phase I, the four column specimens with and without confinement under two grade of TMT reinforcing bars are tested under cyclic load. In phase II, these tested columns are retrofitted with the help of four techniques namely (a) epoxy filled mechanical couplers (b) by welding (c) epoxy filled mechanical couplers with Basalt Fiber Reinforcement Polymers (BFRP) confinement and (d) epoxy filled mechanical couplers with Carbon Fiber Reinforcement Polymers (CFRP) confinement. These retrofitted specimens are again tested in the similar conditions as in phase I. The load- deformation (hysteresis ) behavior of the columns in original condition as well as in retrofitted conditions are compared. The complete details of the test program in phase I and II are describe below;
Phase I: Cyclic Behavior of RC columns with and without Confinement
Four columns of size 275mm x 275mm with an effective height of 1.94m, reinforced with TMT type of reinforcements (Grade G1 and G2) under confined and unconfined condition are tested under cyclic loading. The mechanical stress-stain characteristics of both the type of TMT reinforcement is given in Table 1. The complete details of reinforcement of column specimen are shown in Figure 2. The systematic test set-up for the testing of columns is shown in Figure 3. The column specimens are tested under displacement control sinusoidal loading. The displacement increases from +/- 5mm to +/- 50mm at an interval of 5mm afterwards displacement increases from +/-50 mm to +/-150 mm at an interval of 10mm. The top displacement and load have been measured by the inbuilt load cell and LVDT of the actuator while the three external LVDTs are fixed to capture displacement profile at the plastic hinge location of the column specimens. Figures 4 shows the hysteretic behavior on column specimens under unconfined and confined conditions respectively.
Phase II: Cyclic Behavior of Retrofitted RC Columns with Different Techniques
The tested columns are retrofitted with the in-house made epoxy filled mechanical coupler with and without FRP. The cross sectional size and reinforcement ratio of retrofitted specimen remains the same to compare the effectiveness of the employed retrofitting technique. Table 2 provides the complete details of column specimens used in the experimental work.
The previously tested damaged columns under the cyclic loading as shown in Figure 5, are retrofitted by providing the additional support with the help of hydraulic jack before removing the cracked concrete in the damaged portion of the column. The cracked concrete is completely removed in the affected region and the buckled reinforcement is cut and removed. After the reinforcement curtailment, the remaining buckled reinforcement is straightened manually. Before retrofitting, the loose concrete surface is sizzled and removed from the bottom foundation surface also. The mechanical coupler is inserted into the bottom main reinforcement and the other end of curtailed reinforcement of the column section is gradually inserted into the coupler, as shown in Figure 6.
An eight bolted mechanical coupler with a length of 160 mm and 6.5 mm wall thickness is used to connect the two discontinuous ends of the reinforcing bar in the retrofitting work as shown in Figure 7. The inner surface of coupler is threaded to create friction resistance during loading. After connecting the broken reinforcement the vacuum space is filled with epoxy. This epoxy filler creates better bond and increases the efficiency of frictional resistance.
After inserting, both the curtailed ends of reinforcement into the coupler, the bolts are tightened properly. In order to increase stability of the connection, the vacuum space in the coupler is infiltrated with low viscosity steel wool mixed epoxy binder. Now the external support is released and the transverse reinforcement is inserted at the same spacing as in original column member as shown in Figure 8. To fill the concrete at the retrofitted portion, letter box type shuttering is used in order to avoid the contact problem between the existing concrete cut surfaces at both the ends with new concrete, as shown in Figure 9. Now, the specimens are properly cured for 28 days in laboratory. Among the two retrofitted columns one is additionally confined with external two layers of Carbon Fiber Reinforced Polymer (CFRP) is used for the wrapping of damaged portion of the column. CFRP sheet of 0.15mm thickness with an average ultimate tensile strength of 160 MPa and 13.18GPa elastic modulus is used in wrapping. Other specimens are confined with Basalt Fiber Reinforced Polymer (BFRP).
Figure 10 shows the hysteretic behavior of all the column specimens. The envelope curve of all the retrofitted specimens is presented in Figure 11. The test results are compared with the original un-confined column specimen CF 1. The retrofitted column specimen CF2 with coupler shows entirely distinguished response over original specimen CF1. The test results show higher yield strength as compared to original specimen but the post yield behavior is completely brittle because of coupler connection. In specimen CF2, two curtailed reinforcing bars are connected using coupler and tightened with bolts. This mechanical anchorage connection could not behave like the conventional reinforcing bar under tension due to rebar slipping rather than rebar elongation. Also the coupler is filled with epoxy to increase the bond strength. The load-deformation behavior of specimen after the yielding entirely depends on the frictional force between the threaded surface and the rebar. The observed yield point at 80mm deflection shows that the connection has lesser yield stiffness than original specimen but the maximum load is 20% higher than the original specimen CF1. Figure 10 (b) shows a sudden drop in load at 85mm deflection and a brittle failure is observed due to the sudden failure of coupler into two half at the connection region. It is also observed that loop area of specimen CF2 in each cycle is comparatively lesser than the original specimen CF1. The measured envelope energy dissipation shows 50% lesser dissipation capacity over controlled specimen CF1.
The specimen CF3 retrofitted with welding technique shows better deformation behavior than the specimen CF2. The connected region effectively transfers the applied load and allows the specimens to deform better than control specimen CF1. At the deflection of 140mm, the welded region breaks and the applied load drop suddenly. The specimen retrofitted by using coupler and BFRP wrapped specimen behaves different than the retrofitted specimen without FRP. In specimen CF 4, sudden degradation in load at 80 mm deflection and complete failure at 110 mm deformation is observed from the hysteresis loop. It shows the performance enhancement of rebar connector activeness because of external confinement. Also the loop proves that the specimen encounters coupler failure but not sudden as specimen CF2. The specimen CF 5, strengthened with CFRP wrapping shows enhanced post-yield performance over other specimens. The higher tensile modulus of CFRP holds the column top and bottom portion effectively and allows the coupler to transfer the load to the foundation. There is no sudden degradation in load as well as rebar failure in specimen.
The proposed study is focused on the behavior of coupler in the seismic retrofitting of buckled reinforcing bars in the plastic hinge region of building column. Four columns under unconfined and confined conditioned are tested in cyclic loading and again retrofitted with the four different techniques. The main conclusions of the study based on the experimental test results are as follows;
- The coupler may be a very practical and viable solution for the retrofitting of damaged structures under earthquake or other environmental reasons to restrain the load carrying capacity of structural member. The couplers are more effective in lower diameter of reinforcing bars and the efficacies of the coupler may be restricted up to a certain diameter of connecting two cut/broken longitudinal bars.
- The proposed coupler configuration is more effective and practical as compared to conventional type of couplers used in the field. The external threaded bolts are able to restrict the slippage as well as apply the confining pressure on existing reinforcing bar which is the partial substitution of bond length. The efficiency of the coupler is further improved by filling the space between the coupler and reinforcement with a high strength epoxy.
- The application study of couplers in retrofitting of damaged columns with buckled reinforcement clarifies that the efficacy of coupler may be further improved by the external confinement by the FRP jacketing. This combination is one of effective possible retrofitting solutions for buckled reinforcement in axial members.
- This study paves the path for external bolted coupler in retrofitting of reinforced concrete structural components but for its versatility and reliability, more experiment tests are required by using different wall thickness to length ratio under various types of load applications particularly vertical/axial loads.
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