Chapter 2. Strengthening of Reinforced Concrete Structures with FRPs
2.9 Durability o f FRP Strengthening Techniques
Although FRP composites perform extremely well in practice, there are heightened concerns related to their durability in the field as related to civil infrastructure applications. In these cases, FRP composites are exposed to harsh environmental conditions ranging from wide temperature fluctuations and humidity levels to rain and
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snow. It is therefore imperative to examine the effects o f these conditions, which could influence the performance o f FRP materials and the bond between FRP and concrete.
This section summarizes recent representative studies on durability o f FRP rehabilitation techniques.
2.9.2 Wet— Dry Exposure
Chajes et al. (1995) tested 24 reinforced concrete beams strengthened w ith externally bonded AFRP, GFRP and CFRP sheets. The beams were 38 mm wide by 28 m m deep with a total length o f 330 mm. A wet-dry cycle was completed by immersing the beams into a 4 percent CaCl solution for 18 hours, followed by 8 hours o f drying at room temperature. Test results showed that AFRP and GFRP strengthened beams lost 36 percent o f the unexposed strength, while CFRP strengthened beams lost 19 percent after 100 wet-dry cycles. Toutanji and Ortiz (1997) tested reinforced concrete beams strengthened with externally bonded GFRP and CFRP plates in a wet-dry environment.
The beams were 50 mm wide by 50 mm deep with a total length o f 300 mm. A wet-dry cycle involved immersion o f the specimens into a 3.5 percent salt solution for 4 hours followed by 2 hours o f drying at 35°C and 90 percent relative humidity. After 300 wet- dry cycles, failure was due to debonding o f the FRP plates. The strength o f the exposed specimens ranged from 3 to 33 percent o f the unexposed strength. Ferrier et al. (1998) concluded that the shear modulus o f the adhesive at the FRP-concrete interface is more critical for obtaining durable FRP retrofitted concrete members. W ith an exposure to 20°C dry environment, 80 percent decrease in shear modulus was observed.
Mukhopadhyaya et al. (1998) reported that wet-dry cycles exposures had a significant
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effect on the bond length, shear stress distribution and differential strain between the FRP and concrete.
Adhesives are generally sensitive to water [Mays and Hutchinson, 1992], D eterioration o f bonded joints is characterized by absorption o f water by the adhesive and by moisture diffusion to the adhered interface. Voids can be created in the adhesive layer and at the interface. The presence o f voids implies less area o f contact. W ater can also replace the adhesive by capillary transmission and awakens the bond [Leung et al., 2001].
2.9.3 Freeze—Thaw Exposure
Kaiser (1989) performed a series o f freeze-thaw cycles on concrete beams strengthened with CFRP plates. Kaiser found no detrimental effects on the overall structural performance o f the beams after 100 cycles from -25°C to +25°C. Baumert et al. (1996) investigated the influence o f extreme cold on the structural performance o f FRP plated beams. Test results showed that, for CFRP plated beams exposed to a temperature range o f -27°C to +21°C, there were no adverse effects on the structural behaviour o f the beam when subjected to static tests. Green et al. (1998 and 2000) conducted a series o f tests to investigate the freeze-thaw durability o f concrete beams strengthened with CFRP sheets.
The beams were subjected to 50, 150 and 300 freeze-thaw cycles from -18°C to +15°C. It was concluded that the freeze-thaw action did not degrade the bond o f CFRP strengthened beams. Tysl et al. (1998) studied the effect o f surface delam ination on the freeze-thaw durability o f CFRP plated reinforced concrete beams. It was found that neither the freeze-thaw cycling nor partial surface delamination had a diminishing effect
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on the overall load-deflection response o f the strengthened beams. Very limited research has been reported on the effects o f freeze-thaw cycling on FRPs. Tests conducted by Dutta (1988) where the FRPs were subjected to 150 freeze-thaw cycles from -40°C to +23°C showed that the tensile strength o f glass-epoxy FRP was reduced by about 10 percent. Deterioration due to freeze-thaw cycles in concrete is caused primarily by freezing o f pore water inside the concrete. If the pores are too small, the expansion caused by freezing can exert tensile stresses on the concrete and cause cracking and further deterioration [Neville, 1995].
2.9.4 Thermal Exposure
High temperatures showed detrimental effects on the bond characteristics o f FRP rebars [Honma and Maruyama 1989]. Tsuji et al. (1991) conducted pull-out tests on CFRP, AFRP and GFRP bars exposed to 80, 140, 200 and 260°C. The reported residual bond strength was in the range o f 56 to 86 percent o f the initial values. Kumahara et al. (1993) studied the effects o f high temperatures on the mechanical properties o f FRP rebars. A reduction o f 20 percent in the tensile strength o f GFRP and CFRP bars was observed at 250°C. Matthys et al. (1996) conducted numerical simulations to predict thermally induced transverse cracking in concrete reinforced with AFRP bars and strips. Katz et al.
(1999) studied the bond properties o f FRP rebars with different surface treatments at temperatures ranging from 20 to 250°C. Test results showed a reduction o f 80 to 90 percent in the bond strength. In comparison, steel rebars showed a reduction o f 38 percent. Mosallam and Dutta (2001) examined the strength o f various adhesives under extreme temperature environments. Test results showed that the strength o f adhesives
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were entirely dependent on cure time and temperature with temperature playing the significant role. A temperature-strength relationship was established for various adhesives. In general, as the temperature increased, the epoxy bond strength increased.
2.9.5 Fatigue
Fatigue o f concrete beams strengthened with externally bonded CFRP sheets was first studied by Kaiser (1989). Fatigue failure was observed to occur in the internal steel bars at 4 .8 x l0 5 cycles. The first external damage occurred at 7.5x10s cycles and complete failure o f the strengthened beams occurred at 8.05x105 cycles. Grace and Abdel-sayed (1996) tested a T-bridge girder externally prestressed with CFRP cables and internally reinforced with GFRP bars. The girder was loaded up to 60 percent o f the ultimate load.
No significant changes were observed in the static or dynamic characteristics o f the specimens after 7 million cycles. Shahawy and Beitlman (1998) investigated the fatigue behaviour o f concrete beams strengthened with externally bonded CFRP sheets. All the specimens were subjected to four point bending and the fatigue load ranged from 25 to 50 percent o f the ultimate capacity o f the control sample, with a frequency o f .l Hz. The increase in fatigue life o f the strengthened specimens was considerable. Ferrier et al.
(1999) studied the fatigue behaviour o f various adhesives. Test results showed that the average shear strength was 0.8 M Pa for a double lap joint with a fatigue life o f one million cycles.
Based on the above discussion, there are limited theoretical and experimental studies on the durability o f bond between FRPs and concrete. Investigations by different researchers
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are focusing on the durability of externally bonded FRP reinforcement. N o literature is currently available on the durability o f near surface mounted FRP reinforcement. The author expects that the performance o f near surface mounted FRP reinforcement will be superior under severe environmental conditions as the reinforcement is protected inside the concrete. However, the durability o f bonding adhesives needs to be investigated.
Further studies are still needed to establish accurate reduction factors to be used in bond strength models for design purposes.