DURABILITY PERFORMANCE OF EPOXY INJECTED REINFORCED CONCRETE BEAMS WITH AND WITHOUT FRP FABRICS

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1 DURABILITY PERFORMANCE OF EPOXY INJECTED REINFORCED CONCRETE BEAMS WITH AND WITHOUT FRP FABRICS Prof. John J. Myers Associate Professor CIES / Department of Civil, Arch., & Env. Engineering University of Missouri-Rolla, Rolla, MO 6549, USA jmyers@umr.edu Mahmut Ekenel Post Doctoral Research Fellow CIES / Department of Civil, Arch., & Env. Engineering University of Missouri-Rolla, Rolla, MO 6549, USA KEYWORDS: Composite strengthening, crack repair, epoxy injection, environmental conditioning, durability performance. ABSTRACT Cracks in reinforced concrete (RC) should be repaired if they present the potential for durability related problems such as corrosion of reinforcing steel. One way to repair extensive cracks is the use of epoxy injection. Another repair technique to enhance shear or flexural strength in deficient RC members is the utilization of externally bonded carbon fiber reinforced polymer (CFRP) fabrics. The affect of environmental conditioning on crack injection with or without CFRP strengthening is investigated in this paper. Test results showed that the crack injection provided an increase in initial stiffness for unstrengthened RC beams. An increase in initial stiffness and ultimate strength was achieved in CFRP strengthened RC beams. Surface roughness combined with crack injection significantly increased the flexural capacity of the specimens. Environmental conditioning significantly affected the bond performance of the epoxy injection. The presence of sustained load during environmental conditioning resulted in reduced section capacity and ductility. INTRODUCTION RC structures are designed to work under compression as well as tension when subjected to bending stresses. It s been proven that concrete can only resist tension between 1/1 and 1/14 of its compressive strength; hence the cracking of concrete is inevitable. Apart from tensile cracks, concrete may crack because of drying shrinkage. Excessive cracking in concrete members due to either physical attack (e.g. corrosion, ASR, etc.), which can be caused by ingression of detrimental chemical gases or liquids, or overload can result in serviceability problems. Cracks can be categorized in three groups: cracks due to inadequate structural performance, cracks due to inadequate material performance, and acceptable cracks [Tsiatas, 22]. Structural cracks are caused primarily by overloading; material related cracks are due to shrinkage and chemical reaction; and acceptable cracks are required by current design procedures because cracks must be developed for tensile stresses to be distributed properly along the length of the material [Tsiatas, 22]. Cracks in structural elements can also be classified as dormant or active. Active cracks, such as cracks caused by foundation settlement, cannot be fully repaired, whereas dormant cracks can be successfully repaired. A recent development in the repair and rehabilitation of RC systems is the use of carbon fiber reinforced polymers (CFRP). They have a wide range of applications. CFRP fabrics offer superior performance such as resistance to corrosion, and a high stiffness-to-weight ratio [ACI 44.2R-2]. Because the manual wet-layup CFRP strengthening technique provides additional flexural and/or shear reinforcement by adhesively bonding to RC beams, the reliability of this technique is highly dependant on its bond performance. This study was intended to investigate the effectiveness of epoxy injection with and without bonded fabrics subjected to environmental conditioning under stress. American Concrete Institute (ACI) 44.2R-2 document reports that cracks wider than.25 mm (.1 in.) can move and may affect the performance of externally bonded FRP system through delamination or fiber

2 crushing. Small cracks exposed to aggressive environments may also require resin injection to improve durability performance and delay corrosion of existing steel reinforcement before FRP strengthening. EXPERIMENTAL PLAN Material Properties ' The compressive strength ( f c ) of the concrete was 29.3 MPa (425 psi) at 28-days [ASTM C 39-1]. The compressive strength was 34.5 MPa (5 psi) at the time of testing. The modulus of elasticity of the concrete ( ) was measured as 25,855 MPa (375 ksi) at 28-days [ASTM C ]. The yield E c strength of the steel ( f y ) was determined to be 414 MPa (6 ksi) [ASTM A 37-2]. The ultimate strength ( f ) and the elastic modulus ( E ) of the CFRP fabric were 379 MPa (55 ksi) and 227,5 fu f MPa (33, ksi), respectively. The ultimate tensile strain ( ε ) was.167. The epoxy resin properties as reported by the manufacturer are presented in Table 1. fu Material Low Viscosity Resin High Viscosity Resin N/A: Not available. Shear Strength, MPa (psi) 24.1 (3,5) N/A Table 1. Epoxy Resins Properties. Bond Strength Compressive Tensile to Concrete, Strength, Strength, MPa (psi) MPa (psi) MPa (psi) (5-6) >13.8 (>2,) 72.4 (1,5) 86.2 (12,5) Tensile Elongation at Failure 35.1 (4,5) 1. % 27.6 (4,) 1. % Compressive Modulus, MPa (psi) 1,396 (22,43) 3,13 (45,) Sample Preparation Twenty-three (23) RC specimens were fabricated to study the behavior of epoxy injected RC specimens with and without CFRP and environmental conditioning under flexural loading. The test matrix is presented in Table 2. Table 2. Test Matrix. Sample Condition No Strengthening & Injection Laboratory Conditions 3 (Control) Number of Samples (Specimen Code) Environmental Chamber Epoxy Injection 2 (C1) 2 (C2) CFRP Strengthening 4 (C3) 2 (C4) * Environmental Chamber (Under Sustained Load) Epoxy Injection & CFRP 4 (C5) Strengthening 2 (C6) * 2 (C7) 2 (C8) * These samples subjected to surface roughening by sand blasting prior to CFRP application. As illustrated in Table 2, no CFRP strengthening or crack injection were applied to three specimens, which were maintained under laboratory conditions at 21 o C ± 3 o C (7 o F ± 3 o F) to serve as control specimens. Epoxy injection without CFRP strengthening was performed on four specimens; two of them were maintained under laboratory conditions, while the remaining two were conditioned in an environmental chamber for one environmental cycle. One environmental conditioning cycle consisted of 5 freeze and thaw cycles between 18 o C below zero and 4 o C ( o F and 4 o F), 12 (4x3) extreme temperature cycles between 27 o C and 49 o C (8 o F and 12 o F), 6 (2x3) relative humidity cycles between 6% and 1%, and UV light exposure during high to low temperature cycles. CFRP strengthening without crack injection application was performed on six specimens to serve as a reference to investigate the impact of the epoxy injection on the CFRP strengthening. No surface

preparation was performed on these specimens in order to simulate the minimal surface preparation (worst case bond condition) except for two of them, which were roughened according to ACI Committee

3 preparation was performed on these specimens in order to simulate the minimal surface preparation (worst case bond condition) except for two of them, which were roughened according to ACI Committee 44.2R-2 recommendations. Previous work conducted at the University of Missouri-Rolla (UMR) reported improved bond performance through surface preparation by applications of sand blasting and water jetting [Shen et al., 22]. Ten specimens were strengthened with CFRP fabrics and epoxy injection. Eight of these specimens were strengthened with CFRP fabrics without surface preparation. Surface roughening was applied on two series of specimens (C4 and C6) according to ACI Committee 44.2R-2 recommendations. Four specimens were conditioned in an environmental chamber for one environmental cycle. Two of these conditioned specimens were subjected to a sustained loading level of 4 % of predicted ultimate moment capacity (22.2 kn or 5 psi) throughout conditioning. This load level corresponds to a service level loading. The specimens were mm x mm (6 in. x 6 in.) in cross-section, with a length of mm (36 in.). The specimen size was selected to adequately fit in the existing environmental chamber at UMR. The tension reinforcement consisted of one 9.5 mm diameter (#3) steel reinforcing bar. The number and size of the reinforcement were kept low in order to obtain an adequate injectible crack size. The cover depth was also selected as 5.8 mm (2 in.) to obtain an injectible crack size. No shear reinforcement was used. The reinforcement ratio was calculated as.45, which is between the minimum (.33) and maximum (.21) ACI code specified reinforcement levels. Table 3 presents the predicted cracking, yielding and ultimate design loads of test specimens. The procedure in designing the CFRP strengthened specimens followed the current ACI 44.2R-2 guidelines. Test Set-up The test specimens were pre-cracked prior to CFRP strengthening and flexural testing by loading the specimens beyond the cracking load to simulate the conditions of a typical RC specimen prior to repair/strengthening. All specimens were pre-cracked over a simply supported span of 813 mm (32 in.) (Fig. 1). The specimens were loaded with two concentrated loads placed at a distance of 254 mm (1 in.) from each other. The supports were placed 5.8 mm (2 in.) away from the end points. The flexural testing was also applied as pre-cracking test set-up. Loading was applied at a rate of.22 kn/sec (5 lbs/sec) during pre-cracking and flexural testing. Table 3. Predicted Design Properties. Cracking Load kn (lbs.) Yielding Load kn (lbs.) Expected Failure Loads, kn (lbs.) No Strengthening 1.67 (2,4) (4,35) (4,75) CFRP Strengthening * 53.6 (12,5) 57.4 (12,9) * These specimens were already cracked. RC Beam Springs W-Beam Fig. 1 Test set-up for pre-cracking and flexural testing of beams Crack Injection and CFRP Application Fig. 2 Sketch of crack injection set-up (all dimensions are in inches, 1 in. = 25.4 mm.)

4 The specimens were loaded to 4% of the ultimate moment capacity prior to the injection application using two springs with a known stiffness. The loading opened up the cracks to enabled the injection process (Fig. 2). The springs used in this test set-up were compressed in a testing machine in the linear range prior to assembling them in the frame; 25.4mm (1 in.) displacement in the two springs corresponded to a load level of 22.2 kn (5 lbs). Two different injection epoxy resins were used for the injection process. One type of epoxy resin with high viscosity was used for sealing the outer perimeter of crack; another type with very low viscosity was used for crack penetration to ensure proper sealing. Both of the epoxy resins had high modulus and high range of application temperatures of 1.7 o C and 43.3 o C (35 o F - 11 o F). They were also moisture insensitive. Initially, crack widths were determined by using a non-destructive crack inspection device (crack comparator). The crack measurements of all specimens were in a range from.38 mm (1/66 in.) to.86 mm (1/3 in.). EXPERIMENTAL RESULTS Specimens without CFRP Strengthening Three control and two crack-injected samples were tested under flexural loading. These samples did not include CFRP strengthening. The mid-span displacement and corresponding load reading were monitored via a data acquisition system (DAS). Load versus mid-span displacement readings are presented in Fig. 3. Each curve represents the average test results of specimens in each group. As illustrated in Fig. 3, all three groups exhibited an average ultimate load capacity of 35.4 kn (5,7 lbs.) with a standard variation (SV) of 51 lbs (.22 kpa) and mid-span displacements of.45 in. (11.4 mm), on average, with a SV of.1 in. (.29 mm), respectively. However, the initial slope of the curves up to the proportional limit exhibited major differences between test samples. The epoxy injected (C1) samples exhibited a slope value which was 1.25 times and 3.5 times higher than epoxy injectedenvironmental conditioned (C2) and control samples, respectively (see Fig. 3). All samples failed by concrete crushing. C2 specimens had lower stiffness than C1 specimens and degraded at a higher rate suggesting that the epoxy injection was less effective, in terms of stiffness replacement, when subjected to environmental conditioning. Fig. 4 shows the load versus extensometer readings (crack opening displacement measurements) curves. As illustrated in Fig. 4, the injected cracks in samples C1 showed almost no opening displacement readings up to failure load; whereas, the samples of C2 and control presented large crack opening readings with similar slopes Displacement (in.) C1 C2 Control Fig. 3 Load vs. mid-span displacement curves of control and crack injected samples.6 2 C1 C2 Control Extensometer Readings (in.) Fig. 4 Load vs. extensometer readings curves of control and crack injected samples (1 lbs =.44 kn; 1 in. = 25.4 mm.)

A linear increase in crack opening readings with very minimal increase was presented by samples of C1. This growth may be attributed to section elongation due to strain.

6 Crack-injected sample (Envir. chamber) INJECTED CRACKS Fig. 5 shows one of the C1 specimens and Fig. 6 represents one of the C2 specimens.

5 A linear increase in crack opening readings with very minimal increase was presented by samples of C1. This growth may be attributed to section elongation due to strain. The average ultimate crack opening displacement reading for the control and C2 specimens was 15 times higher than C1 specimens. INJECTED CRACK Fig. 5 Crack-injected sample (Lab. condition) Fig. 6 Crack-injected sample (Envir. chamber) INJECTED CRACKS Fig. 5 shows one of the C1 specimens and Fig. 6 represents one of the C2 specimens. The injected crack in specimen C1 did not show a visual opening during loading and two new cracks formed next to injected one at load levels close to ultimate; however, the injected crack in C2 exhibited large deformations during loading as shown in Figs. 5 and 6, respectively. Specimens with CFRP Strengthening and Maintained in Laboratory Conditions Twelve specimens with CFRP strengthening were tested under flexural loading (see Table 2). These specimens were maintained under laboratory conditions. Average load versus mid-span displacement readings for all specimens tested in this grouping are presented in Fig. 7. As illustrated in Fig. 7, the surface roughened specimens with crack injection (C6) exhibited higher ultimate strength and deflection as compared to the non-surface roughened non-injected specimen (C3), surface roughened non-injected specimen (C4), and non-surface roughened injected specimen (C5) C3 C4 C5 C Displacement (in.) Fig. 7 Load vs. displacement curves of CFRP strengthened samples 4 C3 C4 C5 C Extensometer Readings (in.) Fig. 8 Load vs. extensometer readings curves of CFRP strengthened samples (1 lbs =.44 kn; 1 in. = 25.4 mm.) Specimen C4 showed failure load and standard deviation (SV) of 56.5 and 1.5 kn (12,7 and 336 lbs.). Specimen C6 showed failure load and standard deviation of and 3.7 kn (14,1 and 826 lbs.), on average. These load values correlate to the predicted design load of 57.4 kn (12,9 lbs.). However, C3 and C5 specimens exhibited lower failure loads. C3 exhibited an average failure load and SV of 46.7 and 2.8 kn (1,5 and 636 lbs.); C5 exhibited an average failure load and SV of 52. and 9.3 kn (11,7 and 292 lbs.). The initial slope of the curves up to the proportional limit exhibited

differences between test samples. The C6 samples exhibited a slope value which is 1.27, 1.46, and 2.15 times higher than C5, C4, and C3, respectively (see Fig. 7).

After further examination of the failed specimens, it was observed that the surface roughened specimens (C4 and C6) appeared to debond in the substrate region while the non-surface preparation

6 differences between test samples. The C6 samples exhibited a slope value which is 1.27, 1.46, and 2.15 times higher than C5, C4, and C3, respectively (see Fig. 7). All CFRP fabric applied samples failed by concrete crushing followed by a complete CFRP delamination. After further examination of the failed specimens, it was observed that the surface roughened specimens (C4 and C6) appeared to debond in the substrate region while the non-surface preparation applied specimens (C3 and C5) debonded at the FRP-concrete interface. Fig. 8 illustrates the load versus extensometer readings (crack opening displacement measurements) curves. As illustrated in Fig. 8, C6 specimens did not show any crack opening displacement readings at the lower load levels and slightly higher opening readings at the levels close to ultimate load; whereas specimens of C3, C4, and C5 presented large crack opening readings which started soon after initial loading. Fig. 9 shows one of the CFRP strengthened specimens and Fig. 1 represents one of the CFRP strengthened and crack injected specimens. The crack injected specimen did not show a visual opening during loading and a new crack formed next to injected one (Fig. 1); however, the specimen without crack injection exhibited large deformations during loading (Fig. 9). It can be clearly interpreted from the data presented that the injection aids in limiting crack opening; moreover injection with surface roughening is the best case scenario for structural repair. Pre-crack Injected-cracks Fig. 9 CFRP strengthened sample (lab. condition) Fig. 1 CFRP strengthened and crack-injected sample (lab. condition) Specimens with CFRP Strengthening Maintained in an Environmental Chamber Fig. 11 shows the load versus mid-span displacement curves of the CFRP strengthened and crack injected specimens C7 and C8. These specimens were maintained in an environmental chamber for one environmental conditioning cycle. Specimens of C8 were also maintained under a sustained loading of 4% of ultimate capacity during cycling. Even though both specimens showed similar initial slope, the failure loads and failure deflections varied. C7 specimens exhibited a failure load and SV of 5.7 and 5. kn (11,4 and 1131 lbs.), which was 16% higher than the sustained load applied C8 specimen (42.7 and 1.5 kn [96 and 346 lbs.]). Fig. 12 exhibits the load versus extensometer readings for specimens C7 and C8. As illustrated in this figure, the injected crack on specimen C8 showed 5% higher opening displacement as compared to C7 specimens. Sustained loading during conditioning resulted in reduced ultimate strength and higher crack width. It may be noted that the initial stiffness was higher compared to the laboratory conditioned specimens. It is speculated that the conditioning cycles from the high temperature cycling caused an increase in cure rate of the FRP matrix and thereby an improvement in bond strength. More study is warranted to verify this behavior.

7 Displacement (in.) C7 C8.4 C7 C Extensometer Readings (in.) Fig. 11 Load vs. displacement curves of CFRP Fig. 12 Load vs. extensometer readings curves of strengthened samples (Envir. condition) CFRP strengthened samples (Envir. condition) (1 lbs =.44 kn; 1 in. = 25.4 mm.) CONCLUSIONS The research presented herein was conducted to address the durability performance and behavior of crack injection on RC beams with and without CFRP strengthening. Based on the research performed, following conclusions can be drawn: Crack injection provided an increase in stiffness in the linear region of the load-displacement curves for all of the RC beams without CFRP strengthening; the increase was as high as 3.5 times the control specimens. However, no increase in flexural capacity was observed; The control samples under laboratory conditions and injected specimens under environmental conditioning (C2) showed crack opening displacements significantly higher (15% more) than injected specimens under laboratory conditions (C1). The higher crack opening readings and reformation of cracks at injected locations exhibited by specimen C2 suggest that the epoxy injection is less effective when subjected to environmental conditioning; An increase in ultimate strength and initial stiffness of load versus deflection curves is achieved by CFRP strengthened RC specimens with crack injection as compared to CFRP strengthened specimens without crack injection. Injected cracks in CFRP strengthened specimens showed minimal crack opening displacement; specimens without crack injection showed crack opening displacements to some extend but not as high as the un-strengthened samples. This can be explained by CFRP application, which caused a reduction of stress in the reinforcement steel and reduced crack propagation. Hence crack injection prior to CFRP strengthening is recommended in cases where severe cracking has occurred and durability is a major concern; Surface roughness combined with crack injection increased the flexural capacity of specimens significantly and reduced the crack width opening as compared to other CFRP strengthened specimens; Environmental conditioning and sustained loading significantly affected the ultimate strength (16% lower) and crack width (5% higher) of specimens (C8) as compared to environmental conditioned specimens (C7). Even though environmentally conditioned specimens (C7) and laboratory conditioned specimens (C5) exhibited very close ultimate loads and crack opening displacements, C8 specimens exhibited 18% lower failure load and 5% higher crack opening readings as compared to C5 at ultimate loads. ACKNOWLEDGEMENTS The authors wish to express their gratitude and sincere appreciation to the Federal Highway Administration (FHwA) and Center for Infrastructure Engineering Studies (CIES) at the University of Missouri-Rolla for supporting this research study.

8 REFERENCES ACI Committee 224R-8. (198), Control of cracking in concrete structures. American Concrete Institute. Michigan. ACI Committee 224.1R-93. (1993), Causes, evaluation, and repair of cracks in concrete structures. American Concrete Institute. Michigan. ACI Committee (25), Building code requirements for structural concrete and commentary. American Concrete Institute. Farmington Hills, Michigan. ACI Committee 44.2R-2. (22), Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures. American Concrete Institute. Michigan. ACI Committee 546R-96. (1996), Concrete repair guide. American Concrete Institute. Michigan. ASTM A 37. (22), Standard test methods and definitions for mechanical testing of steel products. 22; Vol. 4-2, Philadelphia, PA. ASTM C 39. (22), Standard test method for compressive strength of cylindrical concrete specimens. 21; Vol. 4-2, Philadelphia, PA. ASTM C 469. (1994), Standard test method for static modulus of elasticity and poisson s ratio of concrete in compression. 1994; Vol. 4-2, Philadelphia, PA. Calder A.J.J., Thompson DM. (1998), Repair of cracked reinforced concrete: assessment of corrosion protection. Transport and Road Research Laboratory, Department of Transport, Research Report 15. Macgregor J.G. (1997) Reinforced concrete, mechanics and design Third edition, Prentice-Hall Inc., New Jersey. Minoru, K., Toshiro, K., Yuichi, U., and Keitetsu, R. (21), Evaluation of bond properties in concrete repair materials. Journal of Materials in Civil Engineering, Vol. 13, No. 2, pp Shen X., Myers J.J., Maerz N., and Galecki G. (22), Effect of roughness on the bond performance between FRP Laminates and Concrete. Proceedings of CDCC, Montreal, CAN. Tsiatas, G., Robinson, J. (1994), Durability evaluation of concrete crack repair systems. Transportation Research Record 1795, Paper No , pp

STRENGTHENING OF UNBONDED POST-TENSIONED CONCRETE SLABS USING EXTERNAL FRP COMPOSITES

STRENGTHENING OF UNBONDED POST-TENSIONED CONCRETE SLABS USING EXTERNAL FRP COMPOSITES F. El M e s k i 1 ; M. Harajli 2 1 PhD student, Dept. of Civil and Environmental Engineering, American Univ. of Beirut;