Seismic Retrofit of Bridge Joints in Central U.S. with Carbon Fiber-Reinforced Polymer Composites

ACI Structural Journal, Mar/Apr 2007 by Silva, Pedro F, Ereckson, Nicholas J, Chen, Genda D

The objectives of this study were 1) to investigate failure modes of reinforced concrete bridge beam-column subassemblies under seismic loads; and 2) to propose an economic and efficient retrofit scheme using composites. To fulfill these objectives, two test units, designated as Units 1 and 2, were retrofitted and tested. Unit 1 was retrofitted at different damage levels and Unit 2 was strengthened in its virgin condition according to nearly the same retrofit scheme. Test results indicate that the repair and retrofit schemes employed in the damaged or undamaged columns were equally efficient in preventing shear failure of the column. On the other hand, whereas repairing the damaged beam-column joint in Unit 1 was inefficient, retrofitting the undamaged joint of Unit 2 was successful. An analytical model was also developed to include the contribution of joint flexibility into the lateral response of the units. Test results along with the recommended procedures for seismic design and implementation of the retrofit schemes are presented and discussed in this paper.

Keywords: fiber-reinforced polymer; joint; reinforced concrete.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

INTRODUCTION

Many bridges built near the New Madrid seismic zone (NMSZ) are likely to suffer extensive damage resulting from insufficient ductility capacity in reinforced concrete (RC) beam-column subassemblies. As such, a research program was initiated to: 1) investigate potential failure modes of typical bridge RC beam-column subassemblies built in the NMSZ; and 2) propose economic and efficient seismic retrofit measures. Currently, extensive research have dealt with developing tools to investigate the vulnerability of existing RC beam-column subassemblies (Priestley et al. 1995; Mazzoni and Moehle 2001) and retrofit measures to ensure appropriate ductile behavior (Seible et al. 1997; Pantelides et al. 1999; Gergely et al. 2000).

These and many more citations report on many assessment and retrofit measures that were used to retrofit existing RC beam-column subassemblies built with improper seismic detailing and the improvements in their seismic performance after retrofit measures were implemented in construction. Although not investigated in this research program, many other assessment tools have been developed that can be used to trace failure modes in other regions of bridge structures (Yashinsky and Karshenas 2003).

Using well established assessment tools (Priestley et al. 1995), 13 bridges built in the state of Missouri were investigated within the RC beam-column subassemblies leading to the general conclusion that there is a high probability for extensive damage resulting from inadequate column transverse reinforcement, bent cap longitudinal and transverse reinforcement, and joint shear reinforcement.

As part of this research program, two 4/5-scale beam-column assemblage specimens, designated as Units 1 and 2, were retrofitted and tested under simulated seismic loads. Prior to undergoing any strengthening, Unit 1 was tested up to onset of column shear failure. At this performance level, the column was strengthened using carbon fiber-reinforced polymer (CFRP) sheets to increase its confinement and shear capacity. After retrofit, testing proceeded up to the onset of joint shear failure. At this performance level, the bent cap was strengthened with CFRP sheets. Testing continued until the next failure mode was observed. Unit 1 test results indicate that the confinement and shear capacity of the column were enhanced by strengthening of the column with CFRP sheets in the hoop direction. Although reinforcing bar fracture was observed under low-cycle fatigue, the failure mode was best characterized by anchorage failure of the column longitudinal reinforcement due to joint shear failure beyond a displacement ductility of 4.

Unit 2 was strengthened according to nearly the same retrofit scheme as in Unit 1, but strengthening was fully completed prior to testing. Unit 2 test results indicate that: 1) as in Unit 1, shear and confinement capacity of the column were enhanced by the CFRP sheets; and (2) unlike Unit 1, if the retrofit scheme is undertaken before failure of the joints have initiated, strengthening of the joint region with CFRP sheets was adequate in preventing strength degradation resulting from joint shear failure. The failure of Unit 2 was characterized by low-cycle fatigue fracture of the column longitudinal reinforcement with minimum strength degradation under repeated reversal cyclic loading up to the displacement ductility of 6. In both of these test units, post-test investigation clearly showed that bars located in the extreme faces of the column fractured under low-cycle fatigue.

An analytical model was also developed to include the contribution of joint flexibility and damage into the lateral response of the test units. Priestley (1993) and Mazzoni and Moehle (2001) have successfully used joint shear stress-strain models to closely predict the seismic response of unconfined and confined joints, respectively. These joint shear stress-strain models were modified in this research program to account for the flexibility of joints retrofitted with CFRP composites in predicting the load deformation response of the test units, and, as described in this paper, the analytical results matched well the experimental results. Test results, along with recommended procedures for seismic design and implementation of the retrofit schemes, are presented and discussed in this paper.