The performance of the epoxy-injected specimens was evaluated through the shear-midspan deflection and shear- diagonal displacement responses, flexural and shear rein-forcement strains, and crack deformations. The data collected from the three phases of testing were compared to assess the local and global responses before and after epoxy injection. Results were also compared with an otherwise similar un-injected specimen. All of the specimens exhibited shear-compression failures and the applied shear at failure for each specimen is summarized in Table 2.
Shear-midspan displacement response
Applied shear-midspan displacement responses are shown in Fig. 3 and demonstrate the overall specimen behavior of the initial, baseline, and post-injection tests. The post-injec-tion response of each injected specimen showed decreased residual deformations and greater stiffness during the first two or three load steps as compared to the baseline response.
As the applied shear magnitudes increased, the specimens began to soften due to the development of new cracks, often adjacent to the repaired diagonal cracks. Specimens 3-ED and 4-EL were similar, especially in the service load range indicated in Fig. 3, and both had greater stiffness than Spec-imen 2-EC.
Table 2—Specimen experimental summary
Specimen VINITIAL, kN (kip) VAPP, kN (kip) VDL, kN (kip) VEXP, kN (kip) VR2K, kN (kip) VEXP/VR2K Failure mode
1-C N/A 902 (203) 16.9 (3.80) 919 (207) 952 (214) 0.97 Shear-compression
2-EC 723 (162) 983 (221) 20.3 (4.56) 1001 (225) 983 (221) 1.02 Shear-compression
3-ED 778 (175) 992 (223) 17.2 (3.87) 1009 (227) 965 (217) 1.05 Shear-compression
4-EL 778 (175) 1046 (235) 16.7 (3.75) 1063 (239) 947 (213) 1.12 Shear-compression
5-EA 778 (175) 1112 (250) 18.4 (4.14) 1130 (254) 943 (212) 1.20 Shear-compression
Mean 1.07
Coefficient of variation 0.08
The axially loaded specimen exhibited a unique shear-mid-span displacement response. The curve has a slender S-shape resulting from the specimen stiffening and then softening during each load cycle. The axial load as a function of the applied shear for Specimen 5-EA is displayed in Fig. 4. As the applied shear increased, the specimen length increased at the level of the axial apparatus, thereby reducing the hydraulic pressure in the axial load actuators, and thus reducing the externally applied axial tension. As the applied
shear decreased, the specimen shortened along the axis of the axial loading apparatus and the axial tension increased again. Like Specimen 4-EL, Specimen 5-EA had decreased residual deformations for many of the load cycles and did not begin to soften until near failure.
Shear-diagonal displacement response
The post-injection diagonal displacement data are shown in Fig. 5. All of the post-injection tests for the epoxy speci-mens had greater stiffness and smaller permanent deforma-tions during the initial load steps than the control specimen.
Specimens 3-ED and 4-EL were stiffer than Specimen 2-EC, with Specimen 4-EL performing slightly better than 3-ED.
Specimen 5-EA showed significantly improved stiffness and reduced permanent deformations after unloading following epoxy injection. For all the specimens except 4-EL and 5-EA, the north and south diagonal deformations were essentially identical, showing similar stiffness and exhib-iting increasing diagonal deformation at approximately the same shear magnitude. For the remaining two specimens, however, one end of the specimen produced a substantially larger diagonal deformation than the other side.
Diagonal cracking behavior
The orientation and location of the diagonal cracks produced during precrack and post-injection loading Fig. 3—Applied shear-midspan displacement response.
Fig. 4—Applied shear-axial load variability of Specimen 5-EA.
sequences are shown in Fig. 6. The locations of diagonal cracks that were epoxy-injected are also shown in Fig. 6.
Diagonal cracks that were injected did not reopen during post-injection tests. Instead, new cracks formed adjacent to the injected cracks and propagated at similar angles. Non-re-paired cracks propagated along the original paths.
The applied loads required to reinitiate diagonal cracking are shown in Fig. 7. The load required to reinitiate diagonal cracking was determined from the applied shear magnitude at the moment the stirrup strain showed an abrupt increase.
The pre- and post-injection diagonal cracking shears were compared with the precrack diagonal cracking shear on the abscissa and the post-injection cracking shear serving on the ordinate. Solid symbols represent stirrup strain gauges located near injected diagonal cracks, while hollow symbols represent stirrup strain gauges located away from injected diagonal cracks, as described in the next section. Injected diagonal cracks required higher applied shear than the orig-inal specimen to produce strains in the stirrups due to new diagonal cracking. Data above the reference line show that larger shear loads were required, while points below the line required smaller loads to propagate or reinitiate diagonal cracking. Non-injected cracks typically behaved similarly to the baseline tests, where stirrup strains began increasing immediately upon application of applied shear.
Reinforcement strains
The largest relative influences of epoxy injection were seen in the individual stirrup strains, but these effects were highly influenced by the proximity of injected diagonal cracks to the embedded strain gauge locations. Strain gauges located near diagonal cracks that were injected had lower strains after injection at similar shear magnitude. Strain gauges located between diagonal cracks or far from cracks that were not injected displayed relatively little change. This behavior was observed for all injected specimens. Diagonal cracks were considered to be “near” the stirrup strain gauge if the vertical distance that the crack crossed the stirrup with the strain gauge was within the AASHTO-LRFD (AASHTO 2005) calculated development length of the Grade 300 No. 13 (Grade 40, No. 4) stirrup (203 mm [8 in.]). An example of strains measured for a stirrup located near a diagonal crack and a stirrup located at a distance greater than the develop-ment length from a diagonal crack is shown in Fig. 8.
The strain behavior depicted in Fig. 9 shows the baseline and post-injection stirrup strains at the maximum service load range. In the figure, the baseline strains serving as the abscissa are plotted against the post-injection strains on the ordinate. Solid symbols represent stirrup strain gauges located near injected diagonal cracks, while hollow symbols represent stirrup strain gauges located away from injected diagonal cracks, as defined previously. The dashed refer-Fig. 5—Control and post-injection shear-diagonal displacement response recorded near centerline of specimens.
ence line marks the boundary between improved and unim-proved behavior. Points above the line had higher strains at the same service load after injection, whereas the points below the line had lower strains after injection. Most stirrup strains near injected diagonal cracks showed significantly reduced strains after injection, whereas uninjected regions were generally unaffected.
Interaction of epoxy curing process and cyclic live load
Data were collected continuously for Specimen 4-EL during curing of the epoxy with cyclic service-level live load being applied. An example of the diagonal deformation throughout the first 3 days of curing is shown in Fig. 10. The deformation range of a representative displacement sensor crossing the injected diagonal crack is shown in Fig. 11.
As seen in Fig. 10, within the first several hours of curing, Fig. 6—Crack pattern locations on east face of specimens. Figure includes precracking, epoxy-injected cracks, post-injection cracks, and final failure crack.
the average diagonal crack deformation reduced from 1.39 to 1.27 mm (0.0547 to 0.0500 in.) and remained relatively constant for the remainder of the curing process. However, the diagonal deformation range continued to reduce over the period of about 3 days, during which the deformation range decreased by nearly 75% (Fig. 11). An example of the stirrup strain throughout the curing period and the internal and external temperature recordings is shown in Fig. 12. The
stirrup is located in the same section as the example diagonal deformation shown in Fig. 11, and the strain range decreased by over 50% within the first 18 hours, while little addi-tional effects were observed for the remainder of the curing process. The curing time reported by the epoxy manufac-turer is 7 days at 4°C (40°F) and 2 days at 25°C (77°F). The average curing temperature for Specimen 4-EL was 10°C (50°F), which correlates to a required curing time of approx-imately 6 days. The stirrup strain was also observed to fluc-tuate with the external temperature with significant time lag between surface temperature and strain changes (Fig. 12).
DISCUSSION
The results of this study indicate that epoxy injec-tion affected the structural behavior of the RC specimens in several ways. Overall, the most dramatic effects were observed for Specimens 3-ED, 4-EL, and 5-EA, as described in the following discussion.
Shear capacity of specimens
In this study, a computer program call Response 2000 (R2K) (Bentz 2000) was used to estimate the strength of the specimens (neglecting any influence of the epoxy injection) as well as the strength of the control specimen. In a previous study, R2K, which uses the Modified Compression Field Theory (MCFT) (Vecchio and Collins 1986), was used to predict shear capacity for a series of 31 similar full-size RC specimens within 0.98 of the actual capacity with a coeffi-Fig. 7—Applied shear at diagonal cracking initiation before
and after injection.
Fig. 8—Applied shear-stirrup strain behavior for Specimen 3-ED. Strain gauges located near: (a) injected diagonal cracks;
and (b) uninjected diagonal cracks.
cient of variation under 8% (Higgins et al. 2004). The R2K predicted member capacities are shown in Table 2. The experimental shear strength is the applied actuator force at failure combined with the self-weight of the specimen acting at the failed section. Except for two specimens, the epoxy- injected specimens exhibited slightly larger shear capacities than predicted, ranging from 1.02 to 1.12. However, all but 4-EL and 5-EA fall within a standard deviation (68% predic-tion interval) of the expected shear strength ignoring the effects of epoxy injection. This is within the expected vari-ability of shear testing results. Specimens 4-EL and 5-EA exhibited shear strengths significantly above the predicted unaltered shear capacity. The shear strength of Specimen 4-EL was above the 95% prediction interval, and Specimen 5-EA was above the 99% prediction interval for the expected shear strength without epoxy injection and indicates the increase is not likely attributed to the inherent variability of shear strength testing of large reinforced concrete girders of the type studied herein. These two specimens received the least amount of epoxy injection in comparison to the other specimens, because they only exhibited three major diagonal crack systems injected on each specimen. Axial load was accounted for in the R2K capacity prediction of Specimen 5-EA but not 4-EL, and the differences in shear capacity for each of the specimens is described subsequently.
Interaction of epoxy repair and superimposed dead load
Specimen 3-ED exhibited a higher capacity than the predicted baseline capacity. Specimen 3-ED also achieved higher load prior to reinitiation of nonlinear response compared to the epoxy-injected control specimen, 2-EC. The dead load serves to keep the diagonal cracks open, allowing for more penetration of the epoxy at the crack tips. It further allows the epoxy to only carry superimposed live loads and leaves dead load stresses locked into the reinforcement and concrete. This tends to further delay crack reinitiation. This was observed by the reduced stirrup steel demand at injected diagonal cracks for otherwise similar load levels.
Cured epoxy characteristics following cyclic live load
The live load magnitudes, rates, and curing conditions considered in this program for Specimen 4-EL did not reduce the effectiveness of the epoxy injection compared to the specimen with dead load alone and, in fact, resulted in higher observed shear capacity. The cyclic loading acted as an internal pumping mechanism that enabled the epoxy to enter and fill finer cracks than in either Specimen 3-ED or Specimen 2-EC. It was observed during the injection process of Specimen 4-EL that the epoxy pump pressures built and dissipated in-phase with the actuator loading cycle, which is Fig. 9—Stirrup strains at service level shear before and after
injection. Shear magnitude taken as 311 kN (70 kip).
Fig. 10—Diagonal deformations reduced during curing of Specimen 4-EL.
Fig. 11—Diagonal deformation range during curing of Specimen 4-EL. (Note: Zero was taken as a reference value.)
Fig. 12—Stirrup strain during curing of Specimen 4-EL.
(Note: °C = [°F × 1.8] + 32.)
consistent with the epoxy being pushed out of the diagonal cracks as the cracks closed upon unloading.
Concrete cores measuring 102 mm (4 in.) in diameter were taken from epoxy-injected diagonal cracks for both Specimens 3-ED and 4-EL after failure. Both cores showed that the epoxy was well distributed through the cracks and even filled hairline subcracks within the cored region. The core taken from Specimen 4-EL had small visible voids, which were evidence of bubble formations likely caused by the cyclic loading noted previously. Examples of the porous epoxy matrix observed in Specimen 4-EL compared to the solid epoxy matrix observed in Specimen 3-ED are shown in Fig. 13. The development of bubbles within the epoxy did not diminish the performance of Specimen 4-EL compared to Specimen 3-ED. It is important to note that the cyclic live loading was representative of loads moving across a typical 15.2 m (50 ft) span continuous bridge at an approximate speed of 33.8 kph (21 mph). Additionally, due to setting of the epoxy at a larger average crack width (dead load plus average live load range), as compared to Specimens 2-EC and 3-ED, the cross section was effectively induced with compressive stresses that must be overcome before addi-tional cracking may occur. This post-tensioning effect also accounts for the increased shear strength observed for Spec-imen 4-EL. Further research is needed to study the effects of higher loading rates and other load magnitudes applied simultaneously during epoxy injection, as well was possible lower-range curing temperatures. However, based on these observations, it may be possible to inject cracks on existing bridges while positioning static superimposed live loads on the bridge (such as loaded maintenance trucks) to effectively open the cracks to their maximum service-level width. After curing the epoxy and removing the live load, compressive stresses would be induced in the cross section. If the live load used during repair is above the maximum expected service loads, this could effectively prevent or significantly delay future cracking.
Interaction of epoxy repair and axial load
The axially loaded specimen, 5-EA, exhibited the most dramatic change between the pre- and post-injection response. The specimen was injected with a constant exter-nally applied axial tension force of 645 kN (145 kip), which coincided with the load magnitude at the end of the precrack test. The applied axial force maintained larger diagonal and vertical cracks during injection, allowing for increased
pene-tration of the epoxy into microcracks within the concrete matrix, similar to Specimens 3-ED and 4-EL. More impor-tantly, as the vertical loading was applied, the axial force decreased during the failure test to a magnitude of 267 kN (60 kip) at ultimate load, resulting in a net compressive force of 378 kN (85 kip) induced into the epoxy-injected section.
Had the axial load not diminished with increasing transverse load, the specimen may have failed at lower load levels.
R2K predicted a capacity of 853 kN (192 kip) for a similar specimen with 890 kN (200 kip) total axial tension force.
Reducing the steel yield stress by an amount equivalent to the 267 kN (60 kip) axial tension and applying a 378 kN (85 kip) axial compression force on the section, R2K estimated a shear capacity of 987 kN (222 kip), which is closer to the observed shear capacity. This situation of loading and curing would represent a structure with shrinkage and/or tempera-ture-induced tensile strains that are recovered after epoxy injection. The strain recovery (release of restraints at supports or temperature change, for example) produces a post-ten-sioning effect for the injected cross section. However, the beneficial temperature effect could not be relied upon in the field and would vary during daily and seasonal changes.
CONCLUSIONS
Five RC deck girder specimens were fabricated to reflect the design and construction materials of the 1950s for RCDG bridges lightly reinforced for shear. The specimen tests were designed to study the effects of epoxy on diag-onal-tension, shear-dominated cracked girders. Specimens were precracked to similar levels observed in the field, subjected to baseline tests in the cracked condition, injected with epoxy resin at varying levels of applied dead and/or live load, and then, after curing the epoxy, were tested to failure. The results of the initial cracking, baseline, and post- injection responses were compared. Factors included in the study were superimposed dead load, service live load plus dead load, and externally applied axial tension during epoxy curing. Based on the experimental observations and analyt-ically predicted shear strengths, the following conclusions are presented:
• Most of the epoxy-injected specimens exhibited marginal increases in shear strength compared to well-correlated analytically predicted strengths of unrepaired specimens. The largest capacity increases were observed for specimens subjected to superim-posed cyclic live load and externally applied static axial tension during epoxy curing (which was due principally to an unintended post-tensioning effect). These showed that the epoxy injection did not effectively strengthen the specimens, as they would have failed very close to the observed capacity with or without epoxy injected cracks.
• Superimposed cyclic live loading during injection and curing of Specimen 4-EL produced dynamic pressure during injection and pumping of the epoxy within the diagonal cracks. Fine bubbles were formed within the epoxy matrix but did not reduce structural perfor-mance at service or ultimate states. In addition, the curing of the epoxy at larger average diagonal crack Fig. 13—Photographs of cores taken from Specimens 3-ED
and 4-EL: (a) core taken from Specimen 3-ED; and (b) core taken from Specimen 4-EL with small voids.
widths introduced compressive stresses in the cross section that increased the threshold load for recracking.
The compressive stresses in the stem also resulted in increased shear capacity. Consequently, this finding may permit strategic placement of loaded maintenance trucks to widen cracks at certain locations during the injection and curing process.
• Initial stiffness after injection was improved and devel-opment of residual deformations was delayed prior to recracking by epoxy injection compared with the cracked performance prior to injection.
• Epoxy injection increased the threshold load level required to form additional diagonal cracks or extend the existing cracks within the stem.
• Injected diagonal cracks did not reopen. Instead, new cracks formed adjacent to the original injected cracks.
• Epoxy injection reduced service-level stirrup strains compared to uninjected diagonal cracks prior to recracking. This may reduce bond fatigue, thereby slowing or preventing additional diagonal crack growth and help maintain force transfer across diagonal cracks.
• Careful and methodical epoxy installation proce-dures following industry best practices enabled epoxy penetration through the depth of the web. Based on the observed penetration and uniformity of the epoxy within the diagonal cracks, it is likely that the epoxy would restrict access of moisture and chlorides to the embedded stirrups and thereby delay or diminish corro-sion potential at the injected crack locations.
ACKNOWLEDGMENTS
This research was funded by the Oregon Department of Transportation.
The findings and conclusions are those of the authors and do not necessarily reflect those of the project sponsors.
AUTHOR BIOS
Matthew T. Smith is an Associate Engineer at CH2M-Hill, Corvallis, OR. He received his BS and MS from Oregon State University, Corvallis, OR. His research interests include the design of hydraulic and building
Matthew T. Smith is an Associate Engineer at CH2M-Hill, Corvallis, OR. He received his BS and MS from Oregon State University, Corvallis, OR. His research interests include the design of hydraulic and building