Rapid Repair of Severely Damaged RC Columns with Different Damage Conditions: An Experimental Study
© The Author(s) 2013
Received: 31 December 2012
Accepted: 13 February 2013
Published: 29 March 2013
Rapid and effective repair methods are desired to enable quick reopening of damaged bridges after an earthquake occurs, especially for those bridges that are critical for emergency response and other essential functions. This paper presents results of tests conducted as a proof-of-concept in the effectiveness of a proposed method using externally bonded carbon fiber reinforced polymer (CFRP) composites to rapidly repair severely damaged RC columns with different damage conditions. The experimental work included five large-scale severely damaged square RC columns with the same geometry and material properties but with different damage conditions due to different loading combinations of bending, shear, and torsion in the previous tests. Over a three-day period, each column was repaired and retested under the same loading combination as the corresponding original column. Quickset repair mortar was used to replace the removed loose concrete. Without any treatment to damaged reinforcing bars, longitudinal and transverse CFRP sheets were externally bonded to the prepared surface to restore the column strength. Measured data were analyzed to investigate the performance of the repaired columns compared to the corresponding original column responses. It was concluded that the technique can be successful for severely damaged columns with damage to the concrete and transverse reinforcement. For severely damaged columns with damaged longitudinal reinforcement, the technique was found to be successful if the damaged longitudinal reinforcement is able to provide tensile resistance, or if the damage is located at a section where longitudinal CFRP strength can be developed.
Damage to bridge structures during an earthquake can have devastating social and economic consequences, particularly for bridges located along key routes that are critical for emergency response and other essential functions. Such bridges are defined as “important” by ATC-18 (1997), which stipulates that full access to “important” bridges should be possible within three days after an earthquake. In order to restore access to essential traffic in affected areas, rapid and effective repair methods are desired for varying levels of damage to minimize the impact on the community.
Decades of study have demonstrated the effectiveness of externally bonded fiber reinforced polymer (FRP) in strengthening and repairing reinforced concrete (RC) columns. Most studies have focused on flexural or shear strengthening or repair application of various types of members or providing confinement in case of columns. Among the studies on repair, most have focused on columns with slight or moderate damage in which concrete, steel, or FRP jacketing was used to restore the strength and displacement capacity (Elkin et al. 1999; Stoppenhagen et al. 1995; Chai et al. 1991; Saadatmanesh et al. 1997; Cheng et al. 2003). Few studies, however, have focused on repairing severely damaged ductile RC bridge columns, especially those with buckled or fractured longitudinal reinforcing bars (Elkin et al. 1999; Cheng et al. 2003). Although these techniques have been shown to be effective in restoring the strength and displacement capacity, rapid repair was not emphasized, and timely reopening of the bridge was not a consideration. To address this issue, Vosooghi and Saiidi (Vosooghi and Saiidi 2012) recently developed guidelines for rapid repair of damaged bridge columns with carbon FRP (CFRP). Their studies focused on circular RC bridge columns under flexural and shear loading conditions without ruptured longitudinal reinforcing bars.
Bridge columns may experience complex axial, shear, bending, and torsional loading during an earthquake. As shown by Prakash et al. (2012), interaction between loading actions influences the location and type of damage. Therefore, it is of interest to develop a repair technique for damaged columns with different damage conditions resulting from combined loading effects.
The present study was conducted as a proof-of-concept with the objective of determining the feasibility and effectiveness of a proposed technique to rapidly repair severely damaged RC bridge columns with different damage conditions using externally-bonded CFRP for emergency service use after an earthquake. The term “rapid” in the context of this study refers to a three-day time period as defined by ATC-18 (1997) and other researchers (Vosooghi et al. 2008). This research will fill in critical gaps in the literature with respect to the severe damage level and inclusion of torsional loading effects and will help guide future research efforts in this area. This experimental study included five half-scale square bridge columns that were tested to complete failure under different combined loading effects of axial, shear, bending, and torsion in a previous study (Prakash et al. 2012). After the previous tests, the columns were severely damaged with different damage conditions. Each column was repaired within a three-day period and retested on the fourth day under the same combined loading as the corresponding original column. The performance of the repaired columns was evaluated by comparing the response with that of the corresponding original columns. The large scale nature of the test specimens in this study allowed for evaluation of the constructability of the developed repair technique in practice.
2 Original Columns
The previous research studied the seismic performance of square RC bridge columns under combined loading effects including torsion. The study was focused on the interaction between bending and torsion, and the primary variable was the torque-to-moment ratio (T/M). All five columns were tested to failure under cyclic lateral loading and a constant axial load of approximately 150 kips (667 kN) to simulate the dead load from the superstructure. Column 1 was subjected to cyclic uniaxial cantilever bending and shear (T/M = 0) in addition to the constant axial load. Columns 2, 3, and 4 were subjected to the constant axial load and a combined cyclic loading effect of uniaxial cantilever bending, shear, and torsion, with torque-to-moment ratios (T/M) of 0.2, 0.4, and 0.6, respectively. Column 5 was tested under pure torsion (T/M = ∞) in addition to the constant axial load.
3 Column Damage Conditions
After the original tests, the columns were severely damaged with different damage conditions due to the different combined cyclic loading effects (T/M). The overall damage conditions were classified based on both visual observations and measured response data. According to previous work (Lehman et al. 2001), any visible evidence of core concrete crushing, longitudinal bar buckling, or longitudinal/transverse reinforcement fracture is classified as severe damage. Damage is classified as significant according to ATC 32 criteria if a permanent offset is apparent, if the reinforcement has yielded, or if major concrete spalling has occurred (Rojahn et al. 1997). The terms “significant” and “severe” are used interchangeably in this paper when referring to the column damage.
Measured data acquired during testing were used to monitor changes in load–displacement response and determine locations at which the reinforcement yielded. At completion of testing, the load–displacement responses showed that the stiffness of each column decreased significantly, and the residual strength was less than 50 % of the peak load. Some of the columns were completely damaged without any resistance to the applied loading (Prakash et al. 2012).
Summary of damage to original columns.
Reinforcing bar damage
25 in. (635 mm) above column base
10 in. (260 mm) above column base
All bars, 10 in. (260 mm) above column base
2 bars; 10 in. (260 mm) above column base (see Fig. 1)
37 in. (950 mm) above column base
20 in. (500 mm) above column base
10 bars, 20 in. (500 mm) above column base
58 in. (1,470 mm) above column base
30 in. (760 mm) above column base
10 bars, 30 in. (760 mm) above column base
94 in. (2,380 mm) above column base
40 in. (1,020 mm) above column base
10 bars, 40 in. (1,020 mm) above column base
120 in. (3,050 mm) above column base
64 in. (1,620 mm) above column base
4 Rapid Repair of Damaged Columns
4.1 Repair Materials
In view of the short time frame for the rapid repair, the repair materials used were selected for ease of installation, compatibility with the other materials, and capability of achieving their desired strengths within the timeframe. A quickset repair mortar and unidirectional CFRP strengthening system were used in this study. The repair mortar was used to replace the removed damaged concrete, while the CFRP strengthening system was used to compensate for the loss in strength due to material degradation during the previous column tests.
Repair mortar properties (provided by the manufacturer).
Fresh wet density, lb/ft3 (kg/m3)
ASTM C 138
Compressive strength, psi (MPa); 2 in. (51 mm) cubes
ASTM C 109
Compressive strength, psi (MPa); 3 by 6 in. (76 by 152 mm) cylinders, at 28 days
ASTM C 39
Flexural strength, psi (MPa), at 28 days
ASTM C 348
Slant shear bond strength, psi (MPa), at 28 days
ASTM C 882 (modified)
Splitting tensile strength, psi (MPa), at 28 days
ASTM C 496
The CFRP strengthening system consisted of unidirectional carbon fiber sheets. Putty was used to fill the voids on the column surface, while primer was use to facilitate the bond between the concrete and the CFRP system. The properties of the dry carbon fiber fabric provided by manufacturer were: tensile strength of 550 ksi (3,800 MPa); tensile modulus of 33,000 ksi (227 GPa); ultimate rupture strain of 0.0167; and nominal thickness of 0.0065 in. (0.165 mm) per ply. The carbon fiber was linear elastic.
Bond between the host concrete and externally applied CFRP is critical for flexural, shear, and torsional strengthening, so bond strength testing of the CFRP-to-concrete bond was performed in accordance with ASTM D7234 (2005). A representative sample of CFRP was bonded to the concrete surface that was prepared using the same techniques and at the same time as the CFRP application. The test was performed at the time of testing of the repaired column. For each column, the bond strength test results met the CFRP system manufacturer’s and ACI 440.2R (2008) minimum specified bond strength of 200 psi (1,380 kPa).
4.2 Repair Procedure
The entire repair process took approximately 30 man-hours over three days and involved the following seven steps: (1) straightening the column; (2) removing loose concrete; (3) placing repair mortar; (4) preparing the column surface; (5) installing longitudinal and transverse CFRP; (6) arranging instrumentation; and (7) retesting repaired columns. The axial load was not applied during the repair procedure considering that shoring systems can be used to support the self-weight of the superstructure in practice during the repair. Straightening of the column was challenging and time-consuming due to limited equipment available in the lab; therefore the time for straightening was not included in the three-day period here. On the first day, the damaged loose concrete was removed and formwork erected, then quickset mortar was placed. The mortar was allowed to set approximately 12 h before the formwork was removed on the second day. Then the column surface was prepared for installation of the CFRP system. The surface was smoothed and corners were rounded with a hand grinder, and then putty and primer were applied. The longitudinal CFRP was applied, followed by transverse CFRP. The transverse CFRP was applied after the longitudinal CFRP to help preventing the debonding of the longitudinal CFRP from the host concrete. For the longitudinal CFRP, fibers were aligned along the longitudinal axis of the column. For the transverse CFRP, fibers were oriented transverse to the longitudinal axis of the column. Detailing of the CFRP systems is discussed in a subsequent section. No special technique was used to cure the CFRP system except for Columns 1 and 2 in which a plastic sheet and a small heater were used to facilitate curing because the temperature in the laboratory was unusually low. Cracks on the concrete surface outside the region with CFRP were not repaired. An unexpected delay occurred during the repair of Column 1, which resulted in testing on the 5th day.
4.3 Test Setup and Loading Protocol
The repaired columns were tested under the same initial combined loading effects as the original columns. Similar to the procedure used for testing the original columns, the testing procedure for repaired columns was initiated in force control and then continued in displacement control. In testing the original columns, testing shifted to displacement control when first yield of the reinforcing steel occurred (Prakash et al. 2012). For the repaired columns, yielding of the steel had occurred during the previous test, and monitoring the strain was not always possible due to damage to the strain gages mounted to the steel reinforcement. Therefore, testing was shifted to displacement control when significant reduction of the stiffness was observed. In addition, different procedures were used to maintain the torque-to-moment ratio (T/M) during the displacement control testing. In the original tests, an iterative feedback system was used to control the torque-to-moment ratio (Prakash et al. 2012), whereas in the present program, a trial-and-error method was used based on values recorded from the previous cycles. As a result, some differences existed in the loading protocol details.
5 CFRP Layouts
The CFRP layouts are summarized in this section. The CFRP design procedures will be described in detail elsewhere by the authors. In general, the externally bonded CFRP strengthening system for each damaged column was designed to restore the column strength in terms of shear, bending, and torsion associated with the peak load in the original test. It should be noted that in the case of a permanent repair, the repair system should also be capable of restoring the ductility, although this aspect was not explicitly accounted for in the design due to the inclusion of torsion. The transverse CFRP wrap was designed to provide confinement to the concrete and to restore the strength in terms of torsion and shear, in which the CALTRANS provisions for RC column retrofit were used (2006, 2007). The longitudinal CFRP was designed to compensate for the flexural and torsional strength loss due to the damaged reinforcement and softened concrete. Interaction between bending and torsion was considered in the design (Park and Paulay 1975).
For Columns 4-R and 5-R, the repair regions extended along most of the column length. Column 4 was repaired along most of its height except for the top 12 in. (305 mm) because of lack of damage in the top region as well as difficulty of applying formwork and placing the repair mortar along the full height of the column. However, shifting of the plastic hinge location of Column 4-R prompted the full height repair of Column 5-R.
6 Test Results
6.1 Summary of Failure Modes
Summary of failure modes of repaired columns.
Column 1-R (T/M = 0)
Premature failure related to the detailing of the longitudinal CFRP anchorage system, followed by fracture of two additional longitudinal reinforcing steel bars
Column 2-R (T/M = 0.2)
Rupture of CFRP (flexure), crushing of concrete in the repaired region
Column 3-R (T/M = 0.4)
Testing terminated due to limitations of the actuators
Column 4-R (T/M = 0.6)
Crushing of concrete in the unrepaired region (torsion) followed by CFRP rupture next to the unrepaired region
Column 5-R (T/M = ∞)
Rupture of CFRP, crushing of concrete (torsion)
6.2 General Behavior of Repaired Columns
The measured lateral load and displacement in Column 1-R did not reach that of Column 1, which is due to premature failure associated with longitudinal CFRP anchorage as discussed in the previous section. A moment–curvature analysis of the repaired cross-section confirms that the lateral load associated with the predicted moment capacity after failure of the longitudinal CFRP was close to the peak lateral load measured during the test. It must be noted that anchorage of externally bonded longitudinal CFRP sheets is a crucial issue to ensure that the tensile force can be developed at the critical section. When the plastic hinge is located near a joint, the situation is even more complicated by the interaction between the column and the anchorage system, which was the situation of Column 1-R. Therefore careful attention must be paid to detailing of both the FRP and its anchorage system.
Comparison of the applied torque-twist envelopes of Column 5 and Column 5-R in Fig. 14 indicates that the torsional strength and twist at maximum torque were enhanced by the repair. For Column 5, the torsional strength reduced rapidly after the maximum torque was achieved because the core concrete crushed and thus could not provide further torsional resistance. The post-peak response of Column 5-R was characterized by a reduction in torsional strength with increasing applied torque, but not as rapidly as that of Column 5. This phenomenon can be explained in part by the confinement provided by the transverse CFRP wrap.
In general, Figs. 10 to 14 also show that the rate of stiffness deterioration of the repaired columns under large reversed cyclic loading was lower than that of the corresponding original columns. However, the initial stiffness of repaired columns was lower than that of corresponding original columns.
6.3 Evaluation of the Repair Technique
Comparison of the repaired column performances in this study is complicated by the different damage conditions of the corresponding original columns and the different repair profiles. Thus non-dimensional response indices were developed to compare the repaired column to the corresponding original column in terms of strength, stiffness, and ductility, which were the extension of previous work by Vosooghi and Saiidi (2013). The indices were then used to compare the performance of the repaired columns.
6.3.1 Strength Index
V r (T r ) and V o (T o ) in Eq. (1) represent the maximum lateral load (torque moment) measured in the repaired and original columns, respectively.
6.3.2 Stiffness Index
In Eq. (2), V p1 (T p1 ) is the measured positive peak lateral load (torque moment) during the first cycle, and D p1 (TW p1 ) is the corresponding lateral displacement (twist). V n1 (T n1 ) is the absolute value of measured negative peak lateral load (torque), and D n1 (TW n1 ) is the absolute value of the corresponding lateral displacement (twist).
It should be noted that the general service stiffness indices for the repaired columns are dependent on the idealization of the measured envelopes of both original and repaired columns. Results are sensitive to assumputions used in developing the idealized curves. Thus these index values are presented herein to compare the global behaviors of the repaired and corresponding original columns. Also, the torque-bending interaction should be kept in mind in evaluating these indices. In general, the general service stiffness was restored more effectively than the initial stiffness.
6.3.3 Ductility Index
Similar to the general service stiffness indices, the ductility indices for the repaired columns are dependent on the idealization of the measured envelopes of both original and repaired columns. However, results are encouraging and suggest that the ductility can be restored to an extent that can meet the needs of a temporary repair and allow emergency service use after an earthquake. More work is needed to determine whether this method can be used for permanent repair, in which case the ductility should be considered in design and should be fully restored.
The developed repair procedure was practical and achievable as an emergency repair;
The repair method is effective in restoring the bending and/or torsional strength. Factors such as bending-torque interaction, failure mode, and repair detailing played a role in the level of strength restored;
Results suggest that the repair method can restore the stiffness and ductility capacity of the columns to levels that can meet the needs of a temporary repair and allow emergency use after an earthquake;
In this study, for the flexural dominant columns with damage concentrated near the base, only the portion of the columns with severe damage, and the region immediately adjacent to it, were repaired. Results confirmed that the strength can be restored or even enhanced for columns without fractured longitudinal bars. These findings are significant in terms of time that can be saved in completing a temporary emergency repair;
The rapid repair method used in this study did not include repair of fractured longitudinal reinforcing bars. When fractured longitudinal bars (and critical section) are located near the base of the column, as was the case for Column 1 in this study, a large force demand is required of the CFRP strengthening system, as well as a substantial anchorage system to develop it. The method utilized in this study was found to be only partial unsuccessful in this case, since premature failure of the strengthening system limited the strength restoration; and
Though initial stiffnesses of the repaired columns were lower than that of original columns due to the unrepaired cracked portions, the general service stiffnesses were restored to a higher level. Also, the rate of stiffness deterioration under large reversal cyclic loading was lower for the repaired columns than that of the corresponding original columns.
The research was performed at Missouri S&T. The authors would like to express their appreciation to the University of Missouri Research Board for the financial support for this project. BASF is gratefully acknowledged for providing the repair materials. Thanks are also due to research specialist, Jason Cox, research/lab technician, John Bullock, electronics technicians, Brian Swift and Gary Abbott, and the group members, Stephen Grelle, Corey Grace, Qian Li, and Yang Yang, for their help throughout the repair and testing processes.
This article is published under license to BioMed Central Ltd.Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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