WET-BONDING BETWEEN FRP LAMINATES AND CAST-IN-PLACE CONCRETE
Y. Shao1, Z. S. Wu 2 and J. Bian 3 1Deparment of Civil Engineering, McGill University, Canada
2 Department of Urban and Civil Engineering, Ibaraki University, Japan 3 Department of Architecture Engineering, Qinghai University, China (Formally, Visiting Scholar, Department
of Urban and Civil Engineering, Ibaraki University, Japan)
ABSTRACT The bond of FRP laminates to the cast-in-place concrete is investigated. The system is comprised of prefabricated FRP profiles, made by either hand lay-up or pultrusion, and cast-in-place concrete as a core. The bond between the dry FRP and wet concrete is realized through an epoxy adhesive which is coated to the inner surface of the FRP profiles. The wet-bond behavior is characterized by JSCE pullout tests and compared to conventional dry-bond using matured dry concrete strengthened by FRP. It was found that the load-displacement curves, the strain distributions in FRP and the interfacial fracture energy of dry-bond and wet-bond specimens were closely comparable, suggesting that an appropriate bond was achieved by casting concrete directly to composite profile via an epoxy adhesive. The wet-bonding technology is crucial to the development of cast-in-place concrete structures using FRP composite profiles as externally bonded reinforcements. KEYWORDS FRP, concrete, bond property, wet-bonding, pullout test, interfacial fracture energy. INTRODUCTION Interfacial bond between FRP and concrete plays a critical role in structural renovation and upgrade using FRP composites as external reinforcements. A good bond can be achieved with a well-prepared concrete surface which is dry, clean and sound. Pullout tests are extensively used to evaluate the bond behaviour as well as the efficiency of various adhesives (Teng et al. 2004, Ueda and Dai, 2004, Wu and Yin, 2003, Wu et al. 2001). The external reinforcement concept is applied to the development of FRP-concrete composite structure with or without internal steel reinforcement (Wu, et al, 2004). A preliminary study has demonstrated that, with a hybrid carbon and T-glass fiber system as an external flexural reinforcement and a T-glass fiber system as an external shear reinforcement, FRP-concrete beams with steel ratio of 0.4% could have a flexural stiffness, moment capacity and postpeak ductility equivalent to RC beams with a steel reinforcement ratio of 1.5% (Wu et al. 2004). The initial flexural stiffness and postpeak ductility were attributed to the use of high modulus carbon fibers and ductile E- or T-glass fibres. FRP-concrete composite structures can find wide applications in waterfront and marine areas where corrosion resistance becomes crucial for long term performance. The challenge is whether the bond between the prefabricated FRP and the cast-in-place concrete can be secured after the setting of the concrete. Mechanical anchors which are exclusively used in steel-concrete composite structures are not appropriate for FRP-concrete structures. Since fiber reinforced composites are usually designed as thin-walled structural members, stress concentration introduced by the mechanical anchors could be high enough to cause premature failure. Hence the adhesive bond presents an alternative. Nevertheless the bond between prefabricated laminate and cast-in-place concrete has not been explored before. This paper is to examine the possibility of using epoxy adhesive technology to achieve a wet-bond between FRP laminates and cast-in-place concrete. The JSCE pullout tests were conducted to evaluate the bond properties. The prefabricated laminate plates coated with epoxy resin were placed in a mold which was later filled with normal strength concrete. The wet-bond behaviour was compared with conventional dry-bond which was accomplished with matured concrete strengthened by FRP sheets using the same epoxy adhesive. Their pullout load-displacement curves, strain distributions, and interfacial fracture energies were evaluated.
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EXPERIMENTS Two groups of pullout specimens were prepared following JSCE CL.101 guideline for continuous fiber strengthened concrete; one was made by dry-bond as reference and one by wet-bond. Fig. 1a shows a standard mold. The specimen was composed of two 300-mm long concrete prisms of a cross section of 100 x100 mm2. The two 300-mm long concrete prisms were each reinforced by a 500-mm long steel rod with three nuts on each and separated by a 3-mm thick wood plate serving as a separation line. The two steel rods were also supported by the 3-mm thick wood plates to keep them aligned during casting. For reference specimens, concrete was cast directly in the mold without laminates and was demolded within 48 hours. After 14 days curing under a plastic sheet, a resin based primer was applied to the two sides of the dry concrete, followed by one-ply high modulus carbon fiber (C7) and four-ply E-glass fibers (EG) using an epoxy adhesive of E=2.5 GPa and Ft=29 MPa (FR-E3P, Nippon Steel). The test area for bonding was 50 mm wide by 200 mm long. For wet-bond specimens, the 50-mm wide and 500-mm long FRP laminates were prefabricated by hand lay-up with one layer of high modulus carbon fiber (C7) and four layers of E-glass fibers (EG). Two laminates were placed on each side of the mold with carbon fiber exposed and bonded directly to concrete, as is shown in Fig. 1a. The two laminates were then coated by the same epoxy adhesive as used in the reference specimens on the entire length. Concrete was poured immediately after the coating. The wet-bond specimens had the same test area as the dry-bond specimens (50 mm wide and 200 mm long). To assure the bond failure to occur inside the test area, the prism was wrapped transversely by a continuous carbon sheet at the anchor side. The wet-bond specimens are displayed in Fig. 1b. The mechanical properties of fiber materials used in the project are summarized in Table 1. The pullout tests were carried out after 14 days of concrete casting. Three specimens for each group were tested to obtain averaged properties. The concrete compressive strength at 14 days was 34 MPa and the concrete elastic modulus was 26 GPa.
(a) Mold for wet-bond specimens (b) Wet-bond specimens
Fig. 1: Preparation of wet-bond specimens RESULTS To measure the strain distribution in FRP along the 200-mm bonded length, six strain gauges were glued along the centreline of the FRP plate at varied spacing. The first strain gauge was placed at the separation line provided by the 3-mm thick wood plate. The remaining gauges were positioned at a distance of 10 mm, 50 mm, 90 mm, 140 mm and 200 mm from the separation line, respectively. Both sides of the FRP plates were instrumented with strain gauges to monitor the bond failure. Two clip gauges were also used to record the pullout displacements at the separation line. Typical load – displacement curves of dry-bond and wet-bond are compared in Fig. 2. They were the average of two clip gauge readings for each specimen. Except the way the epoxy adhesive was applied, the two specimens were identical. Their similar responses suggested that the wet-bonding behaviour is comparable to the conventional dry-bond. It was interesting to notice that the load-displacement curve appeared to be bilinear up to the peak. This was probably attributed to carbon-glass hybrid design. The fracture of high modulus carbon layer led to a reduction of laminate stiffness and resulted in an extensive elongation.
Table 1: Mechanical properties of fiber sheets Thickness
Fig. 2: Typical load – displacement curves Fig. 3 presents typical load – strain curves obtained at different locations along the FRP laminate plates. The maximum strain in FRP was assumed to occur at the separation line where FRP was loaded without restraint from concrete. This maximum strain provided an upper bound value of strain distribution in the entire plate. The P-εp relation can be expressed as (Seracino, 2001):
ppEAP ε)(= (1) where (EA)p = EcAc + EgAg, A is the cross sectional area, E is elastic modulus, the subscript p refers to FRP plate, subscript c refers to carbon fiber layer and subscript g refers to glass fiber layer. The minimum strain in FRP plate happened at the free end of the plate where FRP had a perfect bond with concrete and represented the lower bound of the strain distribution. The expression of P vs. εp for the lower bound strain takes the form:
pconcreteEAP ε)(= (2) The upper bound and lower bound for load-strain curves are also shown in Fig. 3. The strain at the loaded end (0 mm) did not follow the upper bond curve. It started with higher stiffness and bent over later towards the maximum value, indicating that the strain at the loaded end was not completely free from the restraint of concrete. Only after the concrete crack did the strain at the loaded end reach its maximum value.
(a) Dry bond specimen (b) Wet bond specimen Fig. 3: Comparison of typical load-strain curves of dry-bond and wet-bond specimens
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The lower bound strain coincided with the strain at a distance of 140 mm when the load reached 35 kN in specimen #2 and when the load reached 22 kN in specimen #6. This implied the effective bond length was larger than 140 mm at failure. The resemblance of Fig. 3b to Fig. 3a revealed that the load-strain curves of wet-bond specimen were comparable to that of dry-bond specimen, both macroscopically and microscopically. Typical strain distributions along FRP plates are given in Fig. 4. Although they were not exactly the same, the trend of distribution was similar. There were no apparent plateaus in strain distributions, indicating that no complete debonding had initiated at the loaded end before ultimate failure. The local strains, developed in the wet-bond specimen, appeared to be slightly greater than that in the dry-bond specimen, as well as the effective bond length.
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Distance from loaded end, (mm)
Stra
in, x
10-6
P/Pu=1
P/Pu=0.94
P/Pu=0.9
P/Pu=0.8
P/Pu=0.7
P/Pu=0.5
Specimen #2 (dry-bond)
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0 20 40 60 80 100 120 140 160 180 200
Distance from loaded end, (mm)
Stra
in, x
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P/Pu=0.87
P/Pu=0.77
P/Pu=0.75
P/Pu=0.5
Specimen #6 (wet-bond)
(a) Dry-bond specimen (b) Wet-bond specimen
Fig. 4: Strain distributions along FRP laminates Typical debonding failures of a dry-bond specimen and a wet-bond specimen are shown in Fig. 5 and Fig. 6, respectively. Debonding occurred in the concrete adjacent to concrete-adhesive interface. A crack in the concrete oriented about 45o to the load was observed in every test. The crack was responsible for the initiation of debonding at about 40 mm from the separation line and the propagation through the entire bond length at ultimate failure.
Fig. 5: Debonding failure in dry-bond specimen
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Fig. 6: Debonding failure in wet-bond specimen Interfacial fracture energy (Gf) can be estimated using the following equation (Wu et al, 2001):
fff tEb
PG 2
2max
2= (3)
where Pmax is pullout force of one lap FRP laminate, b is the laminate width and Eftf is the stiffness of the laminate. Since the two material systems are identical, except for the application methods of the epoxy adhesive, the interfacial fracture energy is proportional to 2
maxP . Table 2 summarizes the results using b = 50 mm and Eftf = 115 kN/mm. The averaged interfacial fracture energies calculated from dry-bond and wet-bond specimens are almost the same. This suggests that the shear stiffness of an adhesive developed in wet-bond is close to that developed in dry bond since Gf is solely dependent on the shear stiffness of adhesive and the tensile strength of concrete.
CONCLUSION The possibility of using wet-bonding technology to join prefabricated FRP with fresh concrete to form an FRP-concrete composite structure was studied through a pullout test. The preliminary experimental results have shown that wet-bond can have a similar load capacity as dry-bond. The pullout load – displacement curves, the load – strain curves, the strain distributions and the interfacial fracture energy are closely comparable in the two systems. Epoxy adhesive wet-bonding technology provides an economic alternative to the traditional mechanical shear key bond and introduces opportunities to develop new FRP-concrete structures with unlimited designability.
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It is believed that epoxy adhesive can not develop sufficient bond strength if there is water present on the surface of the substrate since the curing of the epoxy will be hindered by water. The bond of fresh concrete to dry prefabricated FRP profiles through an in-situ epoxy adhesive is challenged by the same principle. A study on the durability of wet-bond will be carried out by examining the degree of curing of epoxy adhesive at the presence of wet concrete and its effect on the long term performance of the bond. REFERENCES Dai, J. G. And Ueda, T. (2003). ”Local Bond stress Slip relations for FRP Sheets-Concrete Interfaces”,
FRPRCS-6 Singapore, 141-152. JSCE (2001). ”Recommendations for Upgrading of Concrete Structures with Use of Continuous Fiber Sheets”,
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FRP and Concrete”, FRP Composites in Civil Engineering, CICE 2004, Seracino ed. 55-68. Teng, J.G., Chen, J.F., Smith, S.T. and Lam, L. (2002). FRP strengthened RC Structures, Wiley, Chichester,
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