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Durability performance of RC beams strengthened with epoxy injection and CFRP fabrics
Construction and Building
MATERIALS
Construction and Building Materials 21 (2007) 1182–1190
www.elsevier.com/locate/conbuildmat
Durability performance of RC beams strengthened with epoxy injection and CFRP fabrics Mahmut Ekenel, John J. Myers
*
Center for Infrastructure Engineering Studies, Department of Civil, Architectural, and Environmental Engineering, University of Missouri-Rolla, Rolla, MO 65409, USA Received 22 August 2005; received in revised form 12 June 2006; accepted 19 June 2006 Available online 7 September 2006
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 effect of environmental conditioning on crack injection with or without CFRP strengthening is of interest in this investigation. Test results showed that the crack injection provided an increase in initial stiffness for un-strengthened 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. 2006 Elsevier Ltd. All rights reserved. Keywords: Composite strengthening; Crack repair; Epoxy injection; Environmental conditioning; Durability performance
1. Introduction RC structures are designed to work under compression as well as tension when subjected to bending stresses. It is been proven that concrete can only resist tension between 1/10 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 [15]. Structural cracks are caused primarily by overloading; material related cracks are due to shrinkage and chemical *
Corresponding author. Tel.: +1 573 341 6618; fax: +1573 341 4729. E-mail address: [email protected] (J.J. Myers).
0950-0618/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.06.020
reaction; and acceptable cracks are those that develop due to service level loading for tensile stresses to be distributed properly along the length of the material ([15]). 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. Crack repair systems have been used for many years. The most common crack repair materials are cementitious and polymer products. The most common methods include epoxy injection and grouting. ACI Publication 546R-96 ‘‘Concrete Repair Guide’’ documents standard techniques for concrete repair with cementitious materials and polymer materials. Two other ACI publications which are directly related to crack repair include 224R-80 ‘‘Control of Cracking in Concrete Structures’’ [1], and ACI 224.1R-93 ‘‘Causes, Evaluation, and Repair of Cracks in Concrete Structures’’ [2]. According to ACI 224.1R-93 [2], any crack repair material and method must not only address the cause of the cracking, but also repair the crack
M. Ekenel, J.J. Myers / Construction and Building Materials 21 (2007) 1182–1190
itself. According to ACI 546R-96 [5], epoxy resins are commonly used repair materials that generally have very good bonding and durability characteristics. Calder and Thompson [9] reported that the overall structural performance of RC slabs repaired using epoxy resin injection performed best compared to other materials such as polyester and methyl methacrylate resins. The stiffness of the cracked slabs in this study was about one quarter of that of the un-cracked slabs and the repairs reinstated only about half of the stiffness loss. According to Minoru et al. [12], the bond between concrete and the injection material is very critical; a good bond may restore the original stiffness of the repaired material and prevent further penetration of chloride ions and water. The crack should also be clean and dry prior to injection. Epoxy injection is not applicable if the cracks are actively leaking or cannot be dried out, unless moisture tolerant epoxies are used which can flush the moisture from the inner crack surfaces. A new development in the repair and rehabilitation of RC systems is the use of carbon fiber reinforced polymers (CFRP). These materials have received great attention and their applications to structural repair and retrofit have grown significantly in recent years. CFRP fabrics offer superior performance such as resistance to corrosion, and a high stiffness-to-weight ratio [ACI 440.2R-02] [4]. Because CFRP strengthening 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) document 440.2R-02 [4] reports that the cracks wider than 0.25 mm (0.01 in.) can move and may affect the performance of externally bonded FRP system through delamination or fiber 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. A research study conducted at Kansas State University examined the effectiveness of epoxy injection on several reinforced concrete T-shaped beams which were strengthened with carbon fiber reinforced polymer (CFRP) [13]. The cracks were injected with epoxy prior to CFRP strengthening to evaluate the ability to effectively seal existing cracks. These specimens were not exposed to any envi-
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ronmental conditioning. It was reported that in all but one case, the epoxy injection of the initial cracks restored the stiffness of the beams to their original stiffness. Haroun et al. [10] repaired a RC column by injecting concrete grout prior to strengthening with carbon/epoxy composite jacketing. They reported that there was a satisfactory increase in structural performance with the grout injection [10]. 2. Research objectives This research is undertaken to enhance the database of information on the effects of aggressive environment on the behavior of pre-cracked RC specimens after treatment with and without epoxy injection and strengthening with and without FRP fabrics to flexural loading. The findings of this research will enhance the understanding of how epoxy injection affects the behavior of RC specimens with and without FRP fabrics subjected to aggressive environmental conditioning. 3. Experimental plan 3.1. Material properties The compressive strength ðfc0 Þ of the concrete was 29.30 MPa (4250 psi) at 28-days [ASTM C 39-01] [7]. The compressive strengths was 34.50 MPa (5000 psi) at the time of testing. The modulus of elasticity of the concrete (Ec) was 25,855 MPa (3750 ksi) at 28 days [ASTM C 469-94] [8]. The yield strength of the reinforcing steel (fy) was found to be 414 MPa (60 ksi) [ASTM A 370-02] [6]. The ultimate strength (ffu) and the elastic modulus (Ef) of the CFRP fabric were 3790 MPa (550 ksi) and 227,500 MPa (33,000 ksi), respectively. The ultimate tensile strain (efu) of the CFRP fabric was 0.0167. The epoxy resin properties, which were reported by manufacturer, are presented in Table 1. 3.2. 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. As illustrated in this table, no CFRP strengthening or crack injection were applied to three specimens, which were maintained under laboratory conditions at 21 ± 3 C
Table 1 Epoxy resins properties Material
Shear strength, MPa (psi)
Bond strength to concrete, MPa (psi)
Compressive strength, MPa (psi)
Tensile strength, MPa (psi)
Low Viscosity resin High Viscosity resin
24.10 (3500)
3.40–4.10 (500–600) >13.80 (>2000)
72.40 (10,500) 86.20 (12,500)
35.10 (4500) 27.60 (4000)
N/A: Not available.
N/A
Tensile elongation at failure (%) 1.0 1.0
Compressive modulus, MPa (psi) 1396 (202,430) 3,103 (450,000)
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Table 2 Test matrix Sample condition
Number of samples (specimen code) Laboratory conditions
No strengthening and Injection Epoxy injection CFRP strengthening
3 2 4 2 4 2
Epoxy injection and CFRP Strengthening a
Environmental chamber
(control) (C1) (C3) (C4)a (C5) (C6)a
Environmental chamber (under sustained load)
2 (C2)
2 (C7)
2 (C8)
These samples subjected to surface roughening by sand blasting prior to CFRP application.
(70 ± 3 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 50 freeze and thaw cycles between 18 C below zero and 4 C (0 F and 40F), 120 (40 · 3) extreme temperature cycles between 27 C and 49 C (80 F and 120 F), 60 (20 · 3) relative humidity cycles between 60% and 100%, 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 440.2R-02 recommendations [4]. 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 [14]. 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 440.2R-02 recommendations [4]. 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 40% of predicted ultimate moment capacity (22.2 kN or 5000 psi) throughout conditioning. This load level corresponds to a service level loading. Fig. 1 exhibits the longitudinal and cross-section of the test specimens. The specimens were 152.4 mm · 152.4 mm (6 in. · 6 in.) in cross-section, with a length of 914.4 mm
10
#3 Tension Reinforcement
6
2
CFRP Fabric 30 32
(a)
36
#3 Tension Reinforcement
6 2
(b)
6
Fig. 1. Test specimen’s longitudinal and cross-sections (all dimensions are in inches, 1 in. = 25.4 mm): (a) longitudinal cross-section (in.) and (b) cross-section (in.).
(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 50.8 mm (2 in.) to obtain an injectible crack size. No shear reinforcement was used. The reinforcement ratio was calculated as 0.0045, which is between the minimum (0.0033) and maximum (0.021) 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 440.2R-02 guidelines [4].
Table 3 Predicted design properties
No strengthening CFRP Strengthening a
Cracking load kN (lbs)
Yielding load kN (lbs)
Expected failure loads, kN (lbs)
10.67 (2400)
19.35 (4350) 53.60 (12,050)
21.13 (4750) 57.40 (12,900)
a
These specimens were already cracked prior to CFRP application.
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3.3. Crack width estimate ACI 318-05 [3] reports that the reinforcing steel carries tensile loads as well as helps to obtain a uniform crack distribution and a reasonable crack width. Increases in crack width occur due to increases in any one of the following: the stress level in the steel, the cover thickness, and/or the area of concrete surrounding each reinforcing bar increases [2]. Therefore, using a larger amount of steel with a reduced cover will result in reduced the crack width. The maximum estimated crack width can be calculated using the Gergely and Lutz equation as follows [11]: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi w ¼ 0:076 b f s 3 ðd c AÞ 103 where x = estimated maximum crack width (in.), b = ratio of distance between neutral axis and tension face to distance between neutral axis and centroid of reinforcing steel, fs = service load stress in the reinforcing steel (ksi), dc = distance from the extreme tension fiber to the center of the reinforcing bar located closest to it (in.), A = effective tension area of concrete surrounding the tension reinforcement, and having the same centroid as the reinforcement, divided by the number of bars, (in.2). Calculating the b as 1.571, fs as 248 MPa (36 ksi), dc as 50.8 mm (2.0 in.) and A as 21,306 mm2 (33.024 in.2) for the test specimen used in this experimental program, the maximum estimated crack width under service load stress in the reinforcing steel can be estimated as 0.43 mm (1/59 in.). ACI 224.1R [2] document reports that cracks as narrow as 0.05 mm (1/500 in.) can be bonded by the injection of epoxy. ACI 318-02 [3] limits crack widths as 0.4 mm (1/ 62 in.) for interior exposure and 0.33 mm (1/77 in.) for exterior exposure.
Fig. 2. Test set-up for pre-cracking and flexural testing of beams.
RC Beam
Springs
W-Beam
11
10
11
Fig. 3. Sketch of crack injection set-up (all dimensions are in inches, 1 in. = 25.4 mm).
3.4. 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. 2). The specimens were loaded with two concentrated loads placed at a distance of 254 mm (10 in.) from each other. The supports were placed 50.8 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 0.22 kN/s (50 lbs/s) during precracking and flexural testing. 3.5. Crack injection and CFRP application The specimens were loaded to 40% of the ultimate moment capacity prior to the injection application using two springs with a known stiffness. The loading opened up the cracks to enable the injection process. The injection frame set-up and picture are shown in Figs. 3 and 4, respectively. The springs used in this test set-up were compressed
Fig. 4. View of injection set-up.
in a universal tinius-olsen testing machine in the linear range prior to assembling them in the frame; 25.4 mm (1 in.) displacement in the two springs corresponded to a load level of 22.2 kN (5000 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
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temperatures of 1.7 C and 43.3 C (35–110 F). They were also moisture insensitive. The mechanical properties of epoxy resins were previously presented in Table 1 as reported by the manufacturer. Initially, crack widths were determined by using a nondestructive crack inspection device (crack comparator) as illustrated in Fig. 5. The crack comparator was a small, hand-held microscope with a scale in millimeters on the lens. The crack measurements of all specimens were in a range from 0.38 mm (1/66 in.) to 0.86 mm (1/30 in.). The following steps were taken for epoxy injection and CFRP application: The first step was placement of specimen on the test setup as upside down position and load up to service load levels via springs so the cracks open up (Fig. 4); Cracks were cleaned using a wire brush, pressurized air, and vacuum in order to remove dirt which may prevent epoxy penetration and adhesion; The ports were mounted on the crack at a spacing approximately the same as the thickness (depth) of the cracked member (Fig. 6). The method used for affixing the ports was placing a small amount of epoxy paste at the feet of the port and sticking it over the crack. Extra care was taken not to plug the port by epoxy paste which may prevent or limit injection material to pass through;
The crack surfaces were sealed using a high viscosity epoxy to prevent the injection epoxy from leaking out prior to set. Twenty-four hours cure time was used (Fig. 6); Cracks were injected via ports by using an air-actuated pump. First, the top port was used for injecting low viscosity epoxy and this continued until the injection epoxy started leaking out from side ports. Finally the ports were plugged. A cure time of 48 h was used as per manufacturers recommendation; After the injected epoxy had cured, the load was released and the specimen was removed from injection set-up. Finally, the surface seal was removed by grinding (Fig. 7); Following the grinding process, the CFRP strengthening was applied on ten crack-injected samples according to the test matrix (Table 2). Surface preparation was performed on two out of ten specimens according to ACI 440.2R-02 [4]. The width of the CFRP sheets was selected as 76.2 mm (3 in.) in order to yield a desired ductile failure by steel yielding followed by concrete crushing. The CFRP sheets were applied over 762 mm (30 in.) of the span length; Four of the specimens were moved into an environmental chamber for one environmental cycle. Two of these specimens were maintained under sustained loading of 40% of the predicted ultimate moment capacity of the specimens. The specimens were stacked one above the other and two springs were placed between two specimens to maintain the load during environmental conditioning in chamber.
4. Experimental results 4.1. Specimens without CFRP strengthening
Fig. 5. Crack comparator.
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
Fig. 6. Epoxy injection.
Fig. 7. Grinding the epoxy seal.
M. Ekenel, J.J. Myers / Construction and Building Materials 21 (2007) 1182–1190
are presented in Fig. 8. Each curve represents the average test results of specimens in each group. As illustrated in Fig. 8, all three groups exhibited an average ultimate load capacity of 35.40 kN (5700 lbs) with a standard variation (SV) of 51 lbs (0.22 kPa) and mid-span displacements of 0.45 in. (11.40 mm), on average, with a SV of 0.01 in. (0.29 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 injected-environmental conditioned (C2) and control samples, respectively (Fig. 8). 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. 9 shows the load versus extensometer readings (crack opening displacement measurements) curves. As illustrated in Fig. 9, the injected cracks in samples C1 showed almost no opening displacement readings up to failure load; whereas, the sam-
Twelve specimens with CFRP strengthening were tested under flexural loading (Table 2). These specimens were maintained under laboratory conditions. Average load ver-
6000 Load (Lbs.)
ples of C2 and control presented large crack opening readings with similar slopes. 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. Fig. 10 shows one of the C1 specimens and Fig. 11 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 the injected one at load levels close to ultimate; however, the injected crack in C2 exhibited large deformations during loading as shown in Figs. 10 and 11, respectively. This could be explained by the bond-strength-to-concrete of the low-viscosity injection material (between 500 and 600 psi [3.4 and 4.1 MPa] which was higher than the tensile strength of concrete (roughly estimated between 300 and 420 psi [2.1 and 2.9 MPa]); hence new cracks were formed next to the injected ones at weakened locations. 4.2. Specimens with CFRP strengthening and maintained in laboratory conditions
8000
4000
2000
C1 C2 Control
0 0
0.2 0.4 Displacement (in.)
0.6
Fig. 8. Load versus mid-span displacement curves of control and crack injected samples (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
8000 Fig. 10. Crack-injected sample (laboratory conditions).
6000 Load (Lbs.)
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4000
2000
C1 C2 Control
0 0
0.04 0.08 0.12 0.16 Extensometer Readings (in.)
0.2
Fig. 9. Load versus extensometer readings curves of control and crack injected samples (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
Fig. 11. Crack-injected sample (environmental chamber).
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sus mid-span displacement readings for all specimens tested in this grouping are presented in Fig. 12. As illustrated in Fig. 12, the surface roughened specimens with crack injection (C6) exhibited higher ultimate strength and deflection as compared to the non-surface roughened non-injection specimens (C3), surface roughened non-injection specimens (C4), and non-surface roughened injected specimens (C5). Specimen C6 exhibited a failure load and standard deviation of 62.72 and 3.7 kN (14,100 and 826 lbs), on average. These load values correlate to the predicted design load of 57.40 kN (12,900 lbs) However, C3 and C5 specimens exhibited lower failure loads. C3 exhibited an average failure load and SV of 46.70 and 2.80 kN (10,500 and 636 lbs); C5 exhibited an average failure load and SV of 52.0 and 9.3 kN (11,700 and 2092 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
16000
Load (Lbs.)
12000
8000
C3 C4 C5 C6
4000
0 0
0.1 0.2 0.3 Displacement (in.)
is 1.27, 1.46, and 2.15 times higher than C5, C4, and C3, respectively (Fig. 12). Specimens with crack injection exhibited higher initial stiffness than their counterpart specimens without injection as the epoxy injection helped to ‘‘restore’’ their initial pre-cracked stiffness. 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. 13). Fig. 14 illustrates the load versus extensometer readings (crack opening displacement measurements) curves. As illustrated in Fig. 14, 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. 15 shows one of the CFRP strengthened specimens and Fig. 16 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. 16); however, the specimen without crack injection exhibited large deformations during loading (Fig. 15). 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. 4.3. Specimens with CFRP strengthening maintained in an environmental chamber
0.4
Fig. 12. Load versus displacement curves of CFRP strengthened samples (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
Fig. 17 shows the load versus mid-span displacement curves of the CFRP strengthened and crack injected specimens C7 and C8. These specimens were maintained in
Fig. 13. Specimens after failure: (a) non-surface roughened and (b) surface roughened.
16000
16000
12000
12000 Load (Lbs.)
Load (Lbs.)
M. Ekenel, J.J. Myers / Construction and Building Materials 21 (2007) 1182–1190
8000
C3 C4 C5 C6
4000
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8000
4000 C7 C8
0
0 0
0.01 0.02 Extensometer Readings (in.)
0.03
Fig. 14. Load versus extensometer readings curves of CFRP strengthened samples (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
0
0.1
0.2
0.3
0.4
Displacement (in.) Fig. 17. Load versus displacement curves of CFRP strengthened samples (environmental conditions) (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
16000
Load (Lbs.)
12000
8000
4000 C7 C8 0 0
Fig. 15. CFRP strengthened sample (laboratory conditions).
0.01 0.02 Extensometer Readings (in.)
0.03
Fig. 18. Load versus extensometer readings curves of CFRP strengthened samples (environmental conditions) (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
Fig. 16. CFRP strengthened and crack-injected sample (laboratory conditions).
an environmental chamber for one environmental conditioning cycle. Specimens of C8 were also maintained under a sustained loading of 40% 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 50.70 and 5.0 kN (11,400 and 1131 lbs), which was 16% higher than
the sustained load applied C8 specimens (42.7 and 1.5 kN [9600 and 346 lbs]). Fig. 18 exhibits the load versus extensometer readings for specimens C7 and C8. As illustrated in this figure, the injected crack on specimen C8 showed 50% 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. 5. Conclusions The research presented herein was conducted to investigate 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:
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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 injected cracks for specimen C1 showed insignificant linear opening readings. Minimal opening readings were attributed to section elongation due to strain. However, new small cracks formed around the injected ones. 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 (50% 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 50% higher crack opening readings as compared to C5 at ultimate loads.
Acknowledgements The authors wish to express their gratitude and sincere appreciation to the authority of Federal Highway Administration (FHwA) and Center for Infrastructure Engineering Studies (CIES) at the University of Missouri-Rolla for supporting this research study. The authors would also like to acknowledge Mr. Nathan Marshall for his effort on this project as an undergraduate research assistant. References [1] ACI Committee 224R-80. Control of cracking in concrete structures. MI: American Concrete Institute; 1980. [2] ACI Committee 224.1R-93. Causes, evaluation, and repair of cracks in concrete structures. MI: American Concrete Institute; 1993. [3] ACI Committee 318-05. Building code requirements for structural concrete and commentary. Farmington Hills, MI: American Concrete Institute; 2005. [4] ACI Committee 440.2R-02. Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures. Michigan: American Concrete Institute; 2002. [5] ACI Committee 546R-96. Concrete repair guide. Michigan: American Concrete Institute; 1996. [6] ASTM A370. Standard test methods and definitions for mechanical testing of steel products. 2002; vol. 04-02, Philadelphia, PA. [7] ASTM C 39. Standard test method for compressive strength of cylindrical concrete specimens. 2001; vol. 04-02, Philadelphia, PA. [8] ASTM C 469. Standard test method for static modulus of elasticity and poisson’s ratio of concrete in compression. 1994; vol. 04-02, Philadelphia, PA. [9] Calder A.J.J., Thompson DM. Repair of cracked reinforced concrete: assessment of corrosion protection. Transport and Road Research Laboratory, Department of Transport, Research Report 150, 1998. [10] Haroun MA, Mosallam AS, Feng MQ, Elsanadedy HM. Experimental investigation of seismic repair and retrofit of bridge columns by composite jackets. J Reinf Plast Compos 2003;22(14):1243–68. [11] Macgregor JG. Reinforced concrete mechanics and design. Third ed. New Jersey: Prentice-Hall Inc.; 1997. [12] Minoru K, Toshiro K, Yuichi U, Keitetsu R. Evaluation of bond properties in concrete repair materials. J Mater Civ Eng 2001;13(2): 98–105. [13] Reed CE, Peterman RJ, Rasheed H, and Meggers D. Adhesive applications used during the repair and strengthening of 30-year-old Prestressed concrete girders (input to TRB). Transportation Research Board Annual Meeting CD-ROM, 2003. [14] Shen X., Myers J.J., Maerz N., and Galecki G. Effect of roughness on the bond performance between FRP laminates and concrete. Proceedings of CDCC, 2002, Montreal. [15] Tsiatas, G., Robinson, J., Durability evaluation of concrete crack repair systems. Transportation Research Record 1795, 1994, Paper No 02-3596, p. 82–7.
MATERIALS
Construction and Building Materials 21 (2007) 1182–1190
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Durability performance of RC beams strengthened with epoxy injection and CFRP fabrics Mahmut Ekenel, John J. Myers
*
Center for Infrastructure Engineering Studies, Department of Civil, Architectural, and Environmental Engineering, University of Missouri-Rolla, Rolla, MO 65409, USA Received 22 August 2005; received in revised form 12 June 2006; accepted 19 June 2006 Available online 7 September 2006
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 effect of environmental conditioning on crack injection with or without CFRP strengthening is of interest in this investigation. Test results showed that the crack injection provided an increase in initial stiffness for un-strengthened 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. 2006 Elsevier Ltd. All rights reserved. Keywords: Composite strengthening; Crack repair; Epoxy injection; Environmental conditioning; Durability performance
1. Introduction RC structures are designed to work under compression as well as tension when subjected to bending stresses. It is been proven that concrete can only resist tension between 1/10 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 [15]. Structural cracks are caused primarily by overloading; material related cracks are due to shrinkage and chemical *
Corresponding author. Tel.: +1 573 341 6618; fax: +1573 341 4729. E-mail address: [email protected] (J.J. Myers).
0950-0618/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.06.020
reaction; and acceptable cracks are those that develop due to service level loading for tensile stresses to be distributed properly along the length of the material ([15]). 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. Crack repair systems have been used for many years. The most common crack repair materials are cementitious and polymer products. The most common methods include epoxy injection and grouting. ACI Publication 546R-96 ‘‘Concrete Repair Guide’’ documents standard techniques for concrete repair with cementitious materials and polymer materials. Two other ACI publications which are directly related to crack repair include 224R-80 ‘‘Control of Cracking in Concrete Structures’’ [1], and ACI 224.1R-93 ‘‘Causes, Evaluation, and Repair of Cracks in Concrete Structures’’ [2]. According to ACI 224.1R-93 [2], any crack repair material and method must not only address the cause of the cracking, but also repair the crack
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itself. According to ACI 546R-96 [5], epoxy resins are commonly used repair materials that generally have very good bonding and durability characteristics. Calder and Thompson [9] reported that the overall structural performance of RC slabs repaired using epoxy resin injection performed best compared to other materials such as polyester and methyl methacrylate resins. The stiffness of the cracked slabs in this study was about one quarter of that of the un-cracked slabs and the repairs reinstated only about half of the stiffness loss. According to Minoru et al. [12], the bond between concrete and the injection material is very critical; a good bond may restore the original stiffness of the repaired material and prevent further penetration of chloride ions and water. The crack should also be clean and dry prior to injection. Epoxy injection is not applicable if the cracks are actively leaking or cannot be dried out, unless moisture tolerant epoxies are used which can flush the moisture from the inner crack surfaces. A new development in the repair and rehabilitation of RC systems is the use of carbon fiber reinforced polymers (CFRP). These materials have received great attention and their applications to structural repair and retrofit have grown significantly in recent years. CFRP fabrics offer superior performance such as resistance to corrosion, and a high stiffness-to-weight ratio [ACI 440.2R-02] [4]. Because CFRP strengthening 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) document 440.2R-02 [4] reports that the cracks wider than 0.25 mm (0.01 in.) can move and may affect the performance of externally bonded FRP system through delamination or fiber 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. A research study conducted at Kansas State University examined the effectiveness of epoxy injection on several reinforced concrete T-shaped beams which were strengthened with carbon fiber reinforced polymer (CFRP) [13]. The cracks were injected with epoxy prior to CFRP strengthening to evaluate the ability to effectively seal existing cracks. These specimens were not exposed to any envi-
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ronmental conditioning. It was reported that in all but one case, the epoxy injection of the initial cracks restored the stiffness of the beams to their original stiffness. Haroun et al. [10] repaired a RC column by injecting concrete grout prior to strengthening with carbon/epoxy composite jacketing. They reported that there was a satisfactory increase in structural performance with the grout injection [10]. 2. Research objectives This research is undertaken to enhance the database of information on the effects of aggressive environment on the behavior of pre-cracked RC specimens after treatment with and without epoxy injection and strengthening with and without FRP fabrics to flexural loading. The findings of this research will enhance the understanding of how epoxy injection affects the behavior of RC specimens with and without FRP fabrics subjected to aggressive environmental conditioning. 3. Experimental plan 3.1. Material properties The compressive strength ðfc0 Þ of the concrete was 29.30 MPa (4250 psi) at 28-days [ASTM C 39-01] [7]. The compressive strengths was 34.50 MPa (5000 psi) at the time of testing. The modulus of elasticity of the concrete (Ec) was 25,855 MPa (3750 ksi) at 28 days [ASTM C 469-94] [8]. The yield strength of the reinforcing steel (fy) was found to be 414 MPa (60 ksi) [ASTM A 370-02] [6]. The ultimate strength (ffu) and the elastic modulus (Ef) of the CFRP fabric were 3790 MPa (550 ksi) and 227,500 MPa (33,000 ksi), respectively. The ultimate tensile strain (efu) of the CFRP fabric was 0.0167. The epoxy resin properties, which were reported by manufacturer, are presented in Table 1. 3.2. 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. As illustrated in this table, no CFRP strengthening or crack injection were applied to three specimens, which were maintained under laboratory conditions at 21 ± 3 C
Table 1 Epoxy resins properties Material
Shear strength, MPa (psi)
Bond strength to concrete, MPa (psi)
Compressive strength, MPa (psi)
Tensile strength, MPa (psi)
Low Viscosity resin High Viscosity resin
24.10 (3500)
3.40–4.10 (500–600) >13.80 (>2000)
72.40 (10,500) 86.20 (12,500)
35.10 (4500) 27.60 (4000)
N/A: Not available.
N/A
Tensile elongation at failure (%) 1.0 1.0
Compressive modulus, MPa (psi) 1396 (202,430) 3,103 (450,000)
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Table 2 Test matrix Sample condition
Number of samples (specimen code) Laboratory conditions
No strengthening and Injection Epoxy injection CFRP strengthening
3 2 4 2 4 2
Epoxy injection and CFRP Strengthening a
Environmental chamber
(control) (C1) (C3) (C4)a (C5) (C6)a
Environmental chamber (under sustained load)
2 (C2)
2 (C7)
2 (C8)
These samples subjected to surface roughening by sand blasting prior to CFRP application.
(70 ± 3 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 50 freeze and thaw cycles between 18 C below zero and 4 C (0 F and 40F), 120 (40 · 3) extreme temperature cycles between 27 C and 49 C (80 F and 120 F), 60 (20 · 3) relative humidity cycles between 60% and 100%, 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 440.2R-02 recommendations [4]. 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 [14]. 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 440.2R-02 recommendations [4]. 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 40% of predicted ultimate moment capacity (22.2 kN or 5000 psi) throughout conditioning. This load level corresponds to a service level loading. Fig. 1 exhibits the longitudinal and cross-section of the test specimens. The specimens were 152.4 mm · 152.4 mm (6 in. · 6 in.) in cross-section, with a length of 914.4 mm
10
#3 Tension Reinforcement
6
2
CFRP Fabric 30 32
(a)
36
#3 Tension Reinforcement
6 2
(b)
6
Fig. 1. Test specimen’s longitudinal and cross-sections (all dimensions are in inches, 1 in. = 25.4 mm): (a) longitudinal cross-section (in.) and (b) cross-section (in.).
(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 50.8 mm (2 in.) to obtain an injectible crack size. No shear reinforcement was used. The reinforcement ratio was calculated as 0.0045, which is between the minimum (0.0033) and maximum (0.021) 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 440.2R-02 guidelines [4].
Table 3 Predicted design properties
No strengthening CFRP Strengthening a
Cracking load kN (lbs)
Yielding load kN (lbs)
Expected failure loads, kN (lbs)
10.67 (2400)
19.35 (4350) 53.60 (12,050)
21.13 (4750) 57.40 (12,900)
a
These specimens were already cracked prior to CFRP application.
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3.3. Crack width estimate ACI 318-05 [3] reports that the reinforcing steel carries tensile loads as well as helps to obtain a uniform crack distribution and a reasonable crack width. Increases in crack width occur due to increases in any one of the following: the stress level in the steel, the cover thickness, and/or the area of concrete surrounding each reinforcing bar increases [2]. Therefore, using a larger amount of steel with a reduced cover will result in reduced the crack width. The maximum estimated crack width can be calculated using the Gergely and Lutz equation as follows [11]: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi w ¼ 0:076 b f s 3 ðd c AÞ 103 where x = estimated maximum crack width (in.), b = ratio of distance between neutral axis and tension face to distance between neutral axis and centroid of reinforcing steel, fs = service load stress in the reinforcing steel (ksi), dc = distance from the extreme tension fiber to the center of the reinforcing bar located closest to it (in.), A = effective tension area of concrete surrounding the tension reinforcement, and having the same centroid as the reinforcement, divided by the number of bars, (in.2). Calculating the b as 1.571, fs as 248 MPa (36 ksi), dc as 50.8 mm (2.0 in.) and A as 21,306 mm2 (33.024 in.2) for the test specimen used in this experimental program, the maximum estimated crack width under service load stress in the reinforcing steel can be estimated as 0.43 mm (1/59 in.). ACI 224.1R [2] document reports that cracks as narrow as 0.05 mm (1/500 in.) can be bonded by the injection of epoxy. ACI 318-02 [3] limits crack widths as 0.4 mm (1/ 62 in.) for interior exposure and 0.33 mm (1/77 in.) for exterior exposure.
Fig. 2. Test set-up for pre-cracking and flexural testing of beams.
RC Beam
Springs
W-Beam
11
10
11
Fig. 3. Sketch of crack injection set-up (all dimensions are in inches, 1 in. = 25.4 mm).
3.4. 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. 2). The specimens were loaded with two concentrated loads placed at a distance of 254 mm (10 in.) from each other. The supports were placed 50.8 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 0.22 kN/s (50 lbs/s) during precracking and flexural testing. 3.5. Crack injection and CFRP application The specimens were loaded to 40% of the ultimate moment capacity prior to the injection application using two springs with a known stiffness. The loading opened up the cracks to enable the injection process. The injection frame set-up and picture are shown in Figs. 3 and 4, respectively. The springs used in this test set-up were compressed
Fig. 4. View of injection set-up.
in a universal tinius-olsen testing machine in the linear range prior to assembling them in the frame; 25.4 mm (1 in.) displacement in the two springs corresponded to a load level of 22.2 kN (5000 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
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temperatures of 1.7 C and 43.3 C (35–110 F). They were also moisture insensitive. The mechanical properties of epoxy resins were previously presented in Table 1 as reported by the manufacturer. Initially, crack widths were determined by using a nondestructive crack inspection device (crack comparator) as illustrated in Fig. 5. The crack comparator was a small, hand-held microscope with a scale in millimeters on the lens. The crack measurements of all specimens were in a range from 0.38 mm (1/66 in.) to 0.86 mm (1/30 in.). The following steps were taken for epoxy injection and CFRP application: The first step was placement of specimen on the test setup as upside down position and load up to service load levels via springs so the cracks open up (Fig. 4); Cracks were cleaned using a wire brush, pressurized air, and vacuum in order to remove dirt which may prevent epoxy penetration and adhesion; The ports were mounted on the crack at a spacing approximately the same as the thickness (depth) of the cracked member (Fig. 6). The method used for affixing the ports was placing a small amount of epoxy paste at the feet of the port and sticking it over the crack. Extra care was taken not to plug the port by epoxy paste which may prevent or limit injection material to pass through;
The crack surfaces were sealed using a high viscosity epoxy to prevent the injection epoxy from leaking out prior to set. Twenty-four hours cure time was used (Fig. 6); Cracks were injected via ports by using an air-actuated pump. First, the top port was used for injecting low viscosity epoxy and this continued until the injection epoxy started leaking out from side ports. Finally the ports were plugged. A cure time of 48 h was used as per manufacturers recommendation; After the injected epoxy had cured, the load was released and the specimen was removed from injection set-up. Finally, the surface seal was removed by grinding (Fig. 7); Following the grinding process, the CFRP strengthening was applied on ten crack-injected samples according to the test matrix (Table 2). Surface preparation was performed on two out of ten specimens according to ACI 440.2R-02 [4]. The width of the CFRP sheets was selected as 76.2 mm (3 in.) in order to yield a desired ductile failure by steel yielding followed by concrete crushing. The CFRP sheets were applied over 762 mm (30 in.) of the span length; Four of the specimens were moved into an environmental chamber for one environmental cycle. Two of these specimens were maintained under sustained loading of 40% of the predicted ultimate moment capacity of the specimens. The specimens were stacked one above the other and two springs were placed between two specimens to maintain the load during environmental conditioning in chamber.
4. Experimental results 4.1. Specimens without CFRP strengthening
Fig. 5. Crack comparator.
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
Fig. 6. Epoxy injection.
Fig. 7. Grinding the epoxy seal.
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are presented in Fig. 8. Each curve represents the average test results of specimens in each group. As illustrated in Fig. 8, all three groups exhibited an average ultimate load capacity of 35.40 kN (5700 lbs) with a standard variation (SV) of 51 lbs (0.22 kPa) and mid-span displacements of 0.45 in. (11.40 mm), on average, with a SV of 0.01 in. (0.29 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 injected-environmental conditioned (C2) and control samples, respectively (Fig. 8). 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. 9 shows the load versus extensometer readings (crack opening displacement measurements) curves. As illustrated in Fig. 9, the injected cracks in samples C1 showed almost no opening displacement readings up to failure load; whereas, the sam-
Twelve specimens with CFRP strengthening were tested under flexural loading (Table 2). These specimens were maintained under laboratory conditions. Average load ver-
6000 Load (Lbs.)
ples of C2 and control presented large crack opening readings with similar slopes. 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. Fig. 10 shows one of the C1 specimens and Fig. 11 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 the injected one at load levels close to ultimate; however, the injected crack in C2 exhibited large deformations during loading as shown in Figs. 10 and 11, respectively. This could be explained by the bond-strength-to-concrete of the low-viscosity injection material (between 500 and 600 psi [3.4 and 4.1 MPa] which was higher than the tensile strength of concrete (roughly estimated between 300 and 420 psi [2.1 and 2.9 MPa]); hence new cracks were formed next to the injected ones at weakened locations. 4.2. Specimens with CFRP strengthening and maintained in laboratory conditions
8000
4000
2000
C1 C2 Control
0 0
0.2 0.4 Displacement (in.)
0.6
Fig. 8. Load versus mid-span displacement curves of control and crack injected samples (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
8000 Fig. 10. Crack-injected sample (laboratory conditions).
6000 Load (Lbs.)
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4000
2000
C1 C2 Control
0 0
0.04 0.08 0.12 0.16 Extensometer Readings (in.)
0.2
Fig. 9. Load versus extensometer readings curves of control and crack injected samples (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
Fig. 11. Crack-injected sample (environmental chamber).
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sus mid-span displacement readings for all specimens tested in this grouping are presented in Fig. 12. As illustrated in Fig. 12, the surface roughened specimens with crack injection (C6) exhibited higher ultimate strength and deflection as compared to the non-surface roughened non-injection specimens (C3), surface roughened non-injection specimens (C4), and non-surface roughened injected specimens (C5). Specimen C6 exhibited a failure load and standard deviation of 62.72 and 3.7 kN (14,100 and 826 lbs), on average. These load values correlate to the predicted design load of 57.40 kN (12,900 lbs) However, C3 and C5 specimens exhibited lower failure loads. C3 exhibited an average failure load and SV of 46.70 and 2.80 kN (10,500 and 636 lbs); C5 exhibited an average failure load and SV of 52.0 and 9.3 kN (11,700 and 2092 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
16000
Load (Lbs.)
12000
8000
C3 C4 C5 C6
4000
0 0
0.1 0.2 0.3 Displacement (in.)
is 1.27, 1.46, and 2.15 times higher than C5, C4, and C3, respectively (Fig. 12). Specimens with crack injection exhibited higher initial stiffness than their counterpart specimens without injection as the epoxy injection helped to ‘‘restore’’ their initial pre-cracked stiffness. 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. 13). Fig. 14 illustrates the load versus extensometer readings (crack opening displacement measurements) curves. As illustrated in Fig. 14, 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. 15 shows one of the CFRP strengthened specimens and Fig. 16 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. 16); however, the specimen without crack injection exhibited large deformations during loading (Fig. 15). 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. 4.3. Specimens with CFRP strengthening maintained in an environmental chamber
0.4
Fig. 12. Load versus displacement curves of CFRP strengthened samples (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
Fig. 17 shows the load versus mid-span displacement curves of the CFRP strengthened and crack injected specimens C7 and C8. These specimens were maintained in
Fig. 13. Specimens after failure: (a) non-surface roughened and (b) surface roughened.
16000
16000
12000
12000 Load (Lbs.)
Load (Lbs.)
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8000
C3 C4 C5 C6
4000
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8000
4000 C7 C8
0
0 0
0.01 0.02 Extensometer Readings (in.)
0.03
Fig. 14. Load versus extensometer readings curves of CFRP strengthened samples (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
0
0.1
0.2
0.3
0.4
Displacement (in.) Fig. 17. Load versus displacement curves of CFRP strengthened samples (environmental conditions) (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
16000
Load (Lbs.)
12000
8000
4000 C7 C8 0 0
Fig. 15. CFRP strengthened sample (laboratory conditions).
0.01 0.02 Extensometer Readings (in.)
0.03
Fig. 18. Load versus extensometer readings curves of CFRP strengthened samples (environmental conditions) (1 lbs = 0.0044 kN; 1 in. = 25.4 mm).
Fig. 16. CFRP strengthened and crack-injected sample (laboratory conditions).
an environmental chamber for one environmental conditioning cycle. Specimens of C8 were also maintained under a sustained loading of 40% 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 50.70 and 5.0 kN (11,400 and 1131 lbs), which was 16% higher than
the sustained load applied C8 specimens (42.7 and 1.5 kN [9600 and 346 lbs]). Fig. 18 exhibits the load versus extensometer readings for specimens C7 and C8. As illustrated in this figure, the injected crack on specimen C8 showed 50% 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. 5. Conclusions The research presented herein was conducted to investigate 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:
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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 injected cracks for specimen C1 showed insignificant linear opening readings. Minimal opening readings were attributed to section elongation due to strain. However, new small cracks formed around the injected ones. 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 (50% 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 50% higher crack opening readings as compared to C5 at ultimate loads.
Acknowledgements The authors wish to express their gratitude and sincere appreciation to the authority of Federal Highway Administration (FHwA) and Center for Infrastructure Engineering Studies (CIES) at the University of Missouri-Rolla for supporting this research study. The authors would also like to acknowledge Mr. Nathan Marshall for his effort on this project as an undergraduate research assistant. References [1] ACI Committee 224R-80. Control of cracking in concrete structures. MI: American Concrete Institute; 1980. [2] ACI Committee 224.1R-93. Causes, evaluation, and repair of cracks in concrete structures. MI: American Concrete Institute; 1993. [3] ACI Committee 318-05. Building code requirements for structural concrete and commentary. Farmington Hills, MI: American Concrete Institute; 2005. [4] ACI Committee 440.2R-02. Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures. Michigan: American Concrete Institute; 2002. [5] ACI Committee 546R-96. Concrete repair guide. Michigan: American Concrete Institute; 1996. [6] ASTM A370. Standard test methods and definitions for mechanical testing of steel products. 2002; vol. 04-02, Philadelphia, PA. [7] ASTM C 39. Standard test method for compressive strength of cylindrical concrete specimens. 2001; vol. 04-02, Philadelphia, PA. [8] ASTM C 469. Standard test method for static modulus of elasticity and poisson’s ratio of concrete in compression. 1994; vol. 04-02, Philadelphia, PA. [9] Calder A.J.J., Thompson DM. Repair of cracked reinforced concrete: assessment of corrosion protection. Transport and Road Research Laboratory, Department of Transport, Research Report 150, 1998. [10] Haroun MA, Mosallam AS, Feng MQ, Elsanadedy HM. Experimental investigation of seismic repair and retrofit of bridge columns by composite jackets. J Reinf Plast Compos 2003;22(14):1243–68. [11] Macgregor JG. Reinforced concrete mechanics and design. Third ed. New Jersey: Prentice-Hall Inc.; 1997. [12] Minoru K, Toshiro K, Yuichi U, Keitetsu R. Evaluation of bond properties in concrete repair materials. J Mater Civ Eng 2001;13(2): 98–105. [13] Reed CE, Peterman RJ, Rasheed H, and Meggers D. Adhesive applications used during the repair and strengthening of 30-year-old Prestressed concrete girders (input to TRB). Transportation Research Board Annual Meeting CD-ROM, 2003. [14] Shen X., Myers J.J., Maerz N., and Galecki G. Effect of roughness on the bond performance between FRP laminates and concrete. Proceedings of CDCC, 2002, Montreal. [15] Tsiatas, G., Robinson, J., Durability evaluation of concrete crack repair systems. Transportation Research Record 1795, 1994, Paper No 02-3596, p. 82–7.