- Open Access
Behavior of Laterally Damaged Prestressed Concrete Bridge Girders Repaired with CFRP Laminates Under Static and Fatigue Loading
- Adel ElSafty^{1}Email author,
- Matthew K. Graeff^{2} and
- Sam Fallaha^{3}
https://doi.org/10.1007/s40069-013-0053-0
© The Author(s) 2013
- Received: 27 May 2013
- Accepted: 14 August 2013
- Published: 7 March 2014
Abstract
Many bridges are subject to lateral damage for their girders due to impact by over-height vehicles collision. In this study, the optimum configurations of carbon fiber reinforced polymers (CFRP) laminates were investigated to repair the laterally damaged prestressed concrete (PS) bridge girders. Experimental and analytical investigations were conducted to study the flexural behavior of 13 half-scale AASHTO type II PS girders under both static and fatigue loading. Lateral impact damage due to vehicle collision was simulated by sawing through the concrete of the bottom flange and slicing through one of the prestressing strands. The damaged concrete was repaired and CFRP systems (longitudinal soffit laminates and evenly spaced transverse U-wraps) were applied to restore the original flexural capacity and mitigate debonding of soffit CFRP longitudinal laminates. In addition to the static load tests for ten girders, three more girders were tested under fatigue loading cycles to investigate the behavior under simulated traffic conditions. Measurements of the applied load, the deflection at five different locations, strains along the cross-section height at mid-span, and multiple strains longitudinally along the bottom soffit were recorded. The study investigated and recommended the proper CFRP repair design in terms of the CFRP longitudinal layers and U-wrapping spacing to obtain flexural capacity improvement and desired failure modes for the repaired girders. Test results showed that with proper detailing, CFRP systems can be designed to restore the lost flexural capacity, sustain the fatigue load cycles, and maintain the desired failure mode.
Keywords
- CFRP
- repair
- prestressed concrete
- girder
- lateral damage
1 Introduction
Many bridges have been struck by overheight vehicle collisions that may result in bridge failure. In the United States, between 25 and 35 bridges are damaged by colliding overheight vehicles every year, in each state (Fu et al. 2003). Classifications for degrees of damage and applicable repair methods are presented in some literature (Kasan 2009). Also, that reference was updated from NCHRP Project 12-21 (Shanafelt and Horn 1980, 1985). Previous research addressed flexural and shear strengthening of reinforced and prestressed concrete (PS) beams using FRP (Choi et al. 2011; Ibrahim Ary and Kang 2012; Kang and Ibrahim Ary 2012). Several field studies indicated that FRP materials can be used to repair impacted PS bridge girders, after large losses of concrete cross-section and rupture of a small number of prestressing strands (Di Ludovico 2003; Schiebel et al. 2001; Stallings et al. 2000; Tumialan et al. 2001). From the previously conducted research, issues were reported related to premature debonding failures due to either inadequate transverse carbon fiber reinforced polymers (CFRP) anchors or inadequate development lengths (Rosenboom and Rizkalla 2007; Green et al. 2004). The American Concrete Institute reference ACI 440.2R-08 for designing externally bonded CFRP laminate repairs, addresses some debonding behaviors as “areas that still require research” (ACI Committee 440 2008). In spite of the information available on reinforced concrete (RC) repair, data on the behavior of PS girders strengthened with CFRP laminates is limited. Also, few studies address PS members with pre-existing damaged repaired with CFRP (Kasan and Harries 2009; Klaiber et al. 2003; Nanni et al. 2001). The CFRP repair can be used for both flexural and shear strengthening (ACI Committee 440 2008; Grace et al. 2003; Razaqpur and Isgor 2006; ElSafty and Fallaha 2013; Shin and Lee 2003; NCHRP R-655 2010; NCHRP R-514 2004). Thirty-four laterally damage RC beams were tested after being repaired with various CFRP configurations to investigate their behavior (ElSafty and Graeff 2011). The performance of repaired beams is limited by possible early debonding and the inability of the CFRP system to transfer stresses into the concrete substrate through bond. The debonding problem associated with FRP sheets hinders the ability to utilize the full tensile strength of the FRP, thus decreasing the efficiency of the repair. Therefore, there is a great need to investigate the effectiveness of using CFRP systems that mitigate the debonding problems in the repair of PS girders damaged due to the impact a vehicle collision.
This research addresses specific points of investigation including the effect of using discrete U-wraps on the strain developed in the longitudinal soffit laminates under static and fatigue loading, the optimum configuration of the discrete U-wraps to mitigate debonding strains, the most beneficial level of strengthening (number of CFRP layers), and any design criteria needed for efficient repair systems. This study presents an analysis of the behavior of thirteen half-scale AASHTO type II PS girders under both static and fatigue loading. The laterally damaged PS girders were repaired with different configurations of CFRP repair systems after the concrete integrity was restored. In this research, ten PS girders were tested in static loading and three more girders were tested in fatigue loading to evaluate residual strengths and longevity.
2 Experimental Study
In this research, experimental and analytical study was conducted to investigate the feasibility, performance, and most efficient configuration for repairing laterally damaged PC bridge girders using bonded CFRP laminates under both fatigue and static loading. In this study, the experimental work included testing a total of thirteen half-scale AASHTO type II PS girders. Ten girders were tested in static flexure loading and three were tested in fatigue. Two of the ten PS girders represented the control damaged and undamaged samples, without any CFRP. Thirteen PS girders had simulated impact damage imposed on them, concrete repair, two to three layers of CFRP, and U-wraps at various spacing to constitute the repair. Regarding the concrete repair of the cut and damaged area of the girders, the surfaces exposed by cutting were first roughened with chisels to improve bonding quality. These surfaces were then thoroughly cleaned with a water jet and pressurized air, as specified in both NCHRP 514 (2004) and ACI 440.2R-08 (2008). The cleaned cut was filled with a high-strength cementitious repair mortar, and a high-pressure epoxy injection procedure was performed after the mortar set. The procedure resulted in a perfect repair of the concrete cross-section. The spacing between U-wrappings of CFRP was set at a distance of 12 in. (304.8 mm), 20 in. (508 mm), or 36 in. (914.4 mm). Therefore, the repaired girders varied in both CFRP configurations and levels of strengthening. Ten of the PS girders were tested in flexure until failure under a four point static loading arrangement. Another three repaired PS girders were tested in flexure under a three point fatigue loading. The first girder (PS-1) was a control girder that represents an undamaged and unrepaired specimen. Similarly, the second girder (PS-2) was a damaged specimen which had received no CFRP repair (only concrete repair) representing the lower bound of the tested samples. The remaining girders (PS-3 to PS-5) had both simulated impact damage imposed on them, concrete repair, and two layers of CFRP at various spacing to constitute the repair. The spacing between U-wrappings was set at a distance of 12 in. (304.8 mm), 20 in. (508 mm), or 36 in. (914.4 mm). The three girders (PS-6 through PS-8) were damaged and repaired with three layers of CFRP at the girder soffit and U-wrappings at spacing of 12 in. (304.8 mm), 20 in. (508 mm), or 36 in. (914.4 mm). The final two girders (PS-9 and PS-10) were fully wrapped girders (U-wrappings cover entire girder) using two layers of CFRP for the repairs (soffit and U-wrapping). However, the U-wrappings applied to PS-10 were overlapped by inch (25.4 mm), whereas those applied to PS-9 were not overlapped. This was intended to investigate the effect of continuity in the direction opposite to that of the fibers.
Upon the completion of testing the ten half-scale AASHTO Type II girders under static loading and analyzing the results, the three top performing repair configurations from this set were duplicated and applied to the remaining three half-scale girders for dynamic loading tests (PS-11 to PS-13) to investigate fatigue properties of the repairs. The three best performing repairs from the initial ten half-scale girders that were chosen for fatigue testing were the two-layer and three-layer repairs with 20-in. (508 mm) spacing and the two layer with 36-in. (914.4 mm) spacing. These configurations were recreated exactly, maintaining the 8-in. (203-mm) wide longitudinal laminates which started at a length of 17 ft (5,181.6 mm) while reduced six in (150 mm) per each additional layer applied. Also, the 12-in. (304.8-mm) wide transverse U-wrappings extended to the top of the web of each girder. Loads, deflection, strains developed along the height of the girder, and strains developed along the span of the girders’ extreme bottom fiber were recorded for all girders during their testing. In addition, the modes of failure were also recorded.
2.1 Test Specimens
2.1.1 Materials
Properties of the CFRP materials.
CFRP material properties | Tensile strength | Tensile modulus | Ultimate elongation (%) | Density | Weight | Nominal thickness |
---|---|---|---|---|---|---|
Typical dry fiber properties | 550 ksi (3.79 GPa) | 33.4 × 10^{6} psi (230 GPa) | 1.70 | 0.063 lbs/in.^{3} (1.74 g/cm^{3}) | 19 oz./yd^{2} (644 g/m^{2}) | N/A |
Composite gross laminate properties^{a} | 121 ksi (834 MPa) | 11.9 × 10^{6} psi (82 GPa) | 0.85 | N/A | N/A | 0.04 in. (1.0 mm) |
Properties of used steel reinforcements.
Steel reinforcements | Diameter | Bar area | Grade | Young’s modulus | Weight | Yield strength | Ultimate strength |
---|---|---|---|---|---|---|---|
PS strand | 0.4375 in. (11.1 mm) | 0.115 in. ^{2}(96.9 mm^{2}) | 270 ksi (1,862 MPa) | 27.5 × 10^{6 }psi (189.61 GPa) | 0.367 lbs/ft (0.5 N/m) | 243,000 psi (1,676 MPa) | 270,000 psi (1,862 MPa) |
#3 bars | 0.375 in. (9.53 mm) | 0.11 in.^{2} (71.3 mm^{2}) | 60 ksi (413.7 MPa) | 29 × 10^{6} psi (200 MPa) | 0.376 lbs/ft (0.5 N/m) | 60,000 psi (345 N/mm^{2}) | 90,000 psi (621 N/mm^{2}) |
#4 bars | 0.5 in. (12.7 mm) | 0.2 in.^{2} (126 mm^{2}) | 60 ksi (413.7 MPa) | 29 × 10^{6} psi (200 MPa) | 0.683 lbs/ft (0.93 N/m) | 60,000 psi (345 N/mm^{2}) | 90,000 psi (621 N/mm^{2}) |
2.1.2 Girder Design
2.2 Application of CFRP
As shown in Fig. 5, girder (PS-1) is a control girder that represents an undamaged and unrepaired specimen. Also, girder (PS-2) is a damaged specimen with sawing through the concrete and slicing through one of the prestressing strands. The girder did not receive any CFRP repair and only had concrete repair, thus representing the lower bound of the tested girders. The remaining girders had both simulated impact damage imposed on them and two layers of CFRP at various spacing to constitute the repair. The spacing between U-wrappings was set at a distance of 12 in. (304.8 mm), 20 in. (508 mm), or 36 in. (914.4 mm). Similarly, Fig. 6 displays the CFRP configurations for the remaining girders tested. The girders (PS-6 through PS-8) were damaged and repaired with three layers of CFRP at the girder soffit and U-wrappings at spacings of 12 in. (304.8 mm), 20 in. (508 mm), or 36 in. (914.4 mm). The two girders (PS-9 and PS-10) were fully wrapped girders (U-wrappings covered entire girder) using two layers of CFRP for the repairs (soffit and U-wrapping). However, the U-wrappings applied to PS-10 were overlapped by an inch (25.4 mm), whereas those applied to PS-9 were not overlapped.
A visual inspection of the CFRP surface was performed for any swelling, bubbles, voids, or delamination, after a day for the initial curing of the resin. An acoustic tap test was performed at the whole covered CFRP surface to identify air pockets and delaminated areas by sound. Defects were repaired as per specifications and most defects were repaired using low-pressure epoxy injection.
2.3 Test Setup and Instrumentation
2.4 Test Results and Analysis
2.4.1 Load and Deflection
Flexure test results for PS girders.
Girder | Max load, kips (kN) | Corresponding deflection, in. (cm) | % increase compared to damaged girder PS-2 | % increase compared to un-damaged girder PS-1 |
---|---|---|---|---|
PS-1 | 75.87 (337.62) | 6.94 (17.63) | 22.60^{a} | N/A |
PS-2 | 61.88 (275.37) | 5.38 (13.67) | 0.00 | −18.44^{b} |
PS-3 | 90.14 (401.12) | 2.44 (6.20) | 45.66 | 18.81 |
PS-4 | 84.75 (377.14) | 2.14 (5.44) | 36.94 | 11.70 |
PS-5 | 78.92 (351.19) | 1.61 (4.09) | 27.53 | 4.02 |
PS-6 | 100.91 (449.05) | 2.39 (6.07) | 63.07 | 33.01 |
PS-7 | 104.42 (464.67) | 2.74 (6.96) | 68.74 | 37.63 |
PS-8 | 99.16 (441.26) | 2.29 (5.82) | 60.24 | 30.70 |
PS-9 | 77.26 (343.81) | 1.58 (4.01) | 24.85 | 1.83 |
PS-10 | 87.68 (390.18) | 2.14 (5.44) | 41.69 | 15.57 |
The results show that the damage of cutting one of the prestressing strands (girder PS-2) resulted in 18.44 % loss in flexural capacity compared to that of the undamaged control girder (PS-1). The CFRP repair of damaged girders (PS-3 to PS-10) restored their capacity and exceeded the capacity of the undamaged control girder PS-1 by up to 37.63 %. The results also show that U-shaped discrete strips or wraps of CFRP sheets of girders PS-3 to PS-8 enhanced the flexural capacity even if the girders were not fully wrapped with continuous wrapping covering the entire girder sides. U-wrappings covered the entire girders (PS-9 and PS-10) using 2 layers of CFRP (soffit and U-wraps). However, the U-wraps applied to PS-10 were overlapped by an inch (25.4 mm), whereas the U-wraps applied to PS-9 were not overlapped. By comparing the two fully wrapped girders (PS-9 and PS-10), an increase in the flexural capacity was observed for the girder with an overlap of its wraps (PS-10). That overlapping of transverse U-wraps is needed to develop proper continuity; even in a direction perpendicular to the direction of the fibers. That is in addition to overlapping the fibers along their length for lap splices, as indicated in ACI 440.2R-08.
Fatigue testing results for the half-scale AASHTO type II girders.
Half-scale girder designations | Loading level ranges | Loading rates | Number of loading cycles completed |
---|---|---|---|
PS-11 | 10–35 kips (44.48–155.69 kN) | Started at 4 Hz then to 3 Hz after 2,000 cycles, then 2 Hz after 214,000 cycles | 322,000 |
PS-12 | 10–35 kips (44.48–155.69 kN) | Started at 4 Hz then to 3 Hz after 6,000 cycles, then 2 Hz after 69,000 cycles | 296,000 |
PS-13 | 10–35 kips (44.48–155.69 kN) | 2 Hz | 635,000 |
2.4.2 Strain Characteristics
Strain values measured at various load levels for half-scale girders.
Girder designation | Maximum strain values recorded at various loads | |||||
---|---|---|---|---|---|---|
At 5 kip (22.25 kN) | At 15 kip (66.75 kN) | At 25 kip (111.3 kN) | At 40 kip (178 kN) | At 60 kip (267 kN) | At 70 kip (311.5 kN) | |
PS-1 | 52.58 | 158.51 | 280.33 | 291.40^{a} | Broke | Broke |
PS-2 | 61.32 | 200.39 | 1837.30 | Broke | Broke | Broke |
PS-3 | 51.03 | 167.19 | 314.76 | 1,295.52 | 2,984.16 | 4,075.28 |
PS-4 | 55.16 | 172.14 | 341.49 | 1,332.85 | 3,197.49 | 4,146.04 |
PS-5 | 53.03 | 146.52 | 316.97 | 1,270.22 | 5,213.27 | 8,939.73 |
PS-6 | 51.57 | 160.54 | 292.03 | 1,048.55 | 2,646.34 | 3,393.13 |
PS-7 | 49.05 | 150.30 | 266.07 | 835.90 | 2,415.59 | 3,203.85 |
PS-8 | 52.59 | 161.94 | 281.84 | 942.62 | 2,647.17 | 3,616.50 |
PS-9 | 58.40 | 180.76 | 368.50 | 1,357.88 | 3,433.54 | 5.409.16 |
2.4.3 Failure Modes
3 Design Model and Predictions
Tested values, predictions, and comparisons.
Girder designation | Tested max load, kips (kN) | Predicted max load, kips (kN) | % increase or decrease compared to prediction |
---|---|---|---|
PS-1 | 75.87 (337.62) | 81.9 (364.46) | Decreased 7.3 |
PS-2 | 61.88 (275.37) | 66.5 (295.92) | Decreased 6.9 |
PS-3 | 90.14 (401.12) | 79.7 (354.67) | Increased 13 |
PS-4 | 84.75 (377.14) | 79.7 (354.67) | Increased 6.3 |
PS-5 | 78.92 (351.19) | 79.7 (354.67) | Decreased 0.9 |
PS-6 | 100.91 (449.05) | 85.6 (380.92) | Increased 17.8 |
PS-7 | 104.42 (464.67) | 85.6 (380.92) | Increased 21.9 |
PS-8 | 99.16 (441.26) | 85.6 (380.92) | Increased 15.8 |
PS-9 | 77.26 (343.81) | 79.7 (354.67) | Decreased 3.1 |
PS-10 | 87.68 (390.18) | 79.7 (354.67) | Increased 10.0 |
As shown in Table 6, significant enhancements for the capacity of the repaired girders were recorded. Also, the analytical model predicted the maximum loads relatively close to the test values of failure loads in several repaired and control girders.
4 Conclusions
- 1.
CFRP repair systems can be applied in different configurations, discrete U-wrap, or full wrapping of the girder web. Evenly spaced transverse U-wrappings provide an efficient configuration for CFRP flexural enhancement repairs that mitigate debonding. When repairing laterally damaged girders having a loss of prestressing steel reinforcements it is necessary to cover the damaged section with longitudinal and transverse strips to reduce the crack propagation in the critical region which initiates early debonding.
- 2.
Externally bonded FRP U-wrapping could debond and result in premature failure if there is no proper anchorage system. Anchorage for the U-wraps prevents premature debonding of the FRP wraps, resulting in a greater increase of the ultimate flexural capacity.
- 3.
If CFRP shear enhancements are not needed, the configuration of discrete transverse U-wraps with spacing between them provides comparable flexural benefits to a fully wrapped girder. If shear improvement is not needed, spacing for discrete transverse wraps can be between a distance of one half to one times the depth of girder to constitute a safe CFRP repair. However, without consideration for shear enhancements, a more conservative spacing for transverse anchoring is recommended to be around one half the height of the girder.
- 4.
The damage and cutting of one of the prestressing strands (Girder PS-2) resulted in 18.44 % loss in flexural capacity compared to the undamaged control girder. The CFRP repair of the damaged girder restored its capacity and exceeded the capacity of the undamaged intact control girder with no cut strand by up to 37.63 %.
- 5.
A comparison between the failure load of control girder (with cut strand and un-strengthened with CFRP) and repaired girders with 2 layers of CFRP shows that CFRP repair enhanced the flexural capacity by 27.53–45.66 % compared to control girder (with cut strand and un-strengthened with CFRP). For repaired girders with three layers of CFRP, increases in the flexural capacity were reported to range from 60.24 to 68.74 % compared to control girder (with cut strand and un-strengthened with CFRP). An increase in the failure load of 24.85–41.69 % was observed for the two-layered fully CFRP wrapped repaired girders compared to the un-strengthened control girder. The CFRP repaired girders fail prematurely at <1 million cycles under overload fatigue conditions and improper CFRP anchoring.
- 6.
Proper CFRP repair design in terms of the number of CFRP longitudinal layers and U-wrapping spacing could result in obtaining significant enhancement for the capacity and desired failure modes for the repaired girders. Favorable failure modes of the repaired girders can be maintained using a CFRP repair configuration utilizing spacing between the U-wrappings to prevent undesirable modes of failure such as debonding of the longitudinal CFRP strips from the girder concrete soffit.
Declarations
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.
Authors’ Affiliations
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