Seismic strengthening of low-strength RC concrete columns using low-cost glass fiber reinforced polymers (GFRPs)

The main objective of this study is to investigate the performance of low-cost glass fiber reinforced polymer (GFRP) in strengthening the extremely low strength and poorly-detailed concrete columns typically found in a rural area of Thailand. These structures were usually constructed by local people without following any standard guidelines. A detailed survey of these structures revealed that longitudinal reinforcement in reinforced concrete columns is comprised of round bars along with widely spaced stirrups. Further, the compressive strength of concrete is observed very low i.e., 5.0 MPa. This type of column is referred as “Non-engineered Reinforced Concrete Column” or NRCC. To achieve the research objectives, two reinforced concrete columns representative of existing NRCC columns were constructed and subjected to cyclic loadings. One column was tested as built to serve as a control column, whereas second column was strengthened by using locally available low cost bi-directional GFRP. Both columns were subjected to lateral loading along with constant axial load to simulate the effect of earthquake forces. Test results indicate that the ultimate failure mode of the control column is very brittle. Severe crushing of concrete and spalling of concrete cover along with buckling of the longitudinal steel bars was observed at very low drift levels. The maximum lateral strength (14.73 kN) was observed at the drift level of 0.75 %. Following this, lateral load carrying capacity was rapidly dropped and gravity collapse was occurred at 1.75 % lateral drift. In contrast, the counterpart GFRP column performed very well under cyclic loading. The maximum lateral strength (19.47 kN) was observed at the drift level of 1.22 %. The damage was almost invisible up to 3% drift and the lateral strength degradation occurred gradually up to 8% drift. Finally, the well-known Paulay and Priestley’s model based on plastic hinge theory together with the method essentially based on ACI440 equations were used to calculate the lateral resistance and drift of both control and GFRP strengthened columns. The predicted results in terms of lateral forces are found in well agreement with the experimental results, however, predicted results in terms of lateral displacements are found on conservative side especially for lateral displacements at the onset of ultimate lateral load and 20 % drop in lateral load carrying capacity.