INFLUENCE OF WATER IMMERSION ON THE BOND BEHAVIOR BETWEEN CFRP AND CONCRETE SUBSTRATE

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1 Singapore, st July INFLUENCE OF WATER IMMERSION ON THE BOND BEHAVIOR BETWEEN CFRP AND CONCRETE SUBSTRATE Yunfeng Pan 1, Guijun Xian 1*, Jian-Fei Chen 2 and Hui Li 1 1 School of Civil Engineering, Harbin Institute of Technology, Harbin, , China * gjxian@hit.edu.cn 2 School of Natural and Built Environment, Queen s University Belfast, UK Keywords: Water immersion, CFRP, Concrete, Interface Abstract This paper presents an experimental study on the effect of water immersion on the behavior of bond between a CFRP plate and concrete substrate. The results show that the fracture energy was reduced significantly after 6 months immersion in water but it remained close to constant when the immersion time further increased. They also show that the higher the concrete compressive strength, the more significant the deterioration. The failure mode of the CFRP-to-concrete bonded joint in a single shear pull-off test shifted from a concrete cohesion failure to debonding at the primer/concrete interface. Under the microscope, the failure mode in the aged specimens was a combination of mortar cohesion failure and primer-mortar de-bonding failure. 1. Introduction Strengthening existing concrete structures with externally bonded FRP relies on the integrity of the bond between FRP and concrete. Many civil structures are designed to have a long service life under various harsh environment (e.g., water immersion, freeze-thaw cycles), the lack of the long-term durability data for the FRP-concrete bond has become a major concern. The effects of water immersion on the FRP-to-concrete bond properties and the constituent materials are significant [1-4]. This is especially the case for the deterioration of the FRP-to-concrete interfacial bond [4]. It has been reported that the CFRP-concrete bonded system showed a reduction in the fracture toughness of the interface by 62.8% after exposure to moisture at 50 C for 8 weeks [2]. An exposure to 100% humidity for 10,000 hours was shown to reduce the bond strength by 37% for a CFRP plate strengthened concrete beam, and by 10% for a wet layup CFRP strengthened concrete beam [5]. The reduction in the fracture toughness and strength of the FRP-concrete bond has been attributed to the presence of water molecules in the bond zone due to water ingress [2, 6]. The water molecules in the interfacial zone is mainly penetrated from the concrete owing to its larger diffusivity coefficient [6, 7]. There are mainly two mechanisms leading to the deterioration of the bond due to water molecules. First, the presence of moisture in the primer results in its plasticization, hydrolysis and cracking [8]. Second, the presence of moisture weakens the bond between SiO 2 components of the concrete substrate and the primer [2]. Premature de-bonding between concrete and adhesive interface may occur when the system has a high moisture content, and the failure mode may shift from concrete fracture to interfacial de-bonding [4]. It is understood that the bond strength includes mainly two components: mechanical interaction and chemical bond. The interlocking resistance depends on the surface roughness and the pore structure in the concrete. A higher water/cement ratio would result in a lower pore structure and roughness on the concrete surface. It has been reported that 18 months immersion in water reduced the fracture energy by 68% for a CFRP-to-high strength concrete with f c= 88.6 MPa, while a reduction in the fracture

2 Singapore, st July energy for CFRP-to-normal strength concrete (f c= 39.4 MPa) was 7%. The high compressive concrete is associated with a smoother surface and lower porosity [4]. The present study investigates the influence of water immersion on the bond behaviors of a CFRP adhesive concrete system and the primer. The effects of moisture uptake, concrete compressive strength, and concrete surface roughness were considered. 2. Experimental Program The experimental study includes the test of: a) material properties; b) the effects of water immersion on the properties of the primer; and c) the effects of water immersion on the CFRP plate-concrete bond behavior Raw Materials CFRP plates with a thickness of 1.4 mm and a width of 25 mm were used in this study. Their average tensile strength, modulus of elasticity and the rupture strain were tested following ASTM D3039 to be 1.8 GPa, GPa and 1.07% respectively. Three types of concrete were used. Standard 150 x 150 x 150 mm 3 cubes cast from the same batches of concrete were cured in the standard curing room with 20 C and RH 98% for one month. The strength of the cubes were tested at 0, 6, 12, 18 months. The mix details and mean values of the strength at different ages are listed in Table 1. Table 1. Concrete mixes and strengths Water Cement Sand Gravel, kg w/c Cube compressive strength, MPa kg kg kg D=20 mm D=5 mm 0 Month C NA NA NA C40BG C40SB Dog-bone specimens of both epoxy adhesive and epoxy primer were prepared. They were tested following ASTM D 638 [9] after curing for one month at room temperature. The tensile strength, modulus of elasticity and ultimate strain were respectively 48.5 MPa, 3.46 GPa and 1.92% for the adhesive, and 49.3 MPa, 3.18 GPa and 2.92% for the primer. The glass transition temperature (T g) of the adhesive and primer were respectively 65.7 C and 78.4 C Single Shear Pull-off Test The dimensions of the concrete block were shown in Figure 1. The bonding surface of the concrete was ground to remove the top layer of cement paste, and then cleaned with acetone. After drying, the cleaned surface was coated with a low viscosity epoxy primer. A layer of adhesive with about 1 mm thickness was then applied on the surface, and the CFRP plate was carefully placed on the adhesive layer, parallel to the longitudinal axis of the concrete block. The specimens were kept in the laboratory at room temperature (around 20 C) for one month to cure the adhesive. All specimens had a FRP bond length of 280 mm. The FRP axial strains were measured using 11 electrical resistance foil strain gauges along the centerline of the CFRP plate. A 100 kn universal test machine was used for all the single shear pulloff test at a displacement rate of 0.1 mm/min.

Singapore, 19-21 st July 2017 3 3. Results and Discussion 3.1. Mechanical behavior of the primer Figure 1.

immersion time increased. This phenomenon may be attributed to two factors.

Figure 2b shows the IR spectra of the primer before and after immersion in water.

Second, the epoxy continues to cure with the increase of the immersion time. Figure 2.

3 Singapore, st July Results and Discussion 3.1. Mechanical behavior of the primer Figure 1. Single shear pull-off test setup (all units in mm) Figure 2a shows that the modulus of elastic of the primer was reduced first and then increased as the water immersion time increased. This phenomenon may be attributed to two factors. First, the moisture penetration into the epoxy primer results in the hydrolysis of OH and plasticization. Figure 2b shows the IR spectra of the primer before and after immersion in water. The characteristic absorption bands at 3421 cm -1 and 2965 cm -1 are reduced in intensity, representating the hydrolysis of OH. Second, the epoxy continues to cure with the increase of the immersion time. Figure 2. The effect of water immersion on the a) modulus of elasticity and the IR spectra of the primer 3.2. Failure modes Figure 3 shows the failure modes of the single shear pull-off test specimnes. For the reference specimens without water immersion, the failure was a cohesive failure in the concrete substrate (Figure 3a, c and e), due to the low cohesive strength of concrete [2, 6]. After water immersion, the failure mode was shifted from the fracture of concrete to de-bonding between the adhesive and the concrete owing to moisture ingress. (c) (d)

Singapore, 19-21 st July 2017 4 (e) (f) Figure 3.

(6 months) Figure 4. shows two SEM images of the CFRP plate after pulled off from the concrete. The content of Si was 24% by weight (Figure 4a) when the failure was in the concrete.

For the specimen tested after water immersion, the failure mode changed to debonding at the adhesive/concrete interface which led to higher C content (due to carbon backbone in the primer and the

$SEM images of CFRP plate pulled off from the concrete: reference specimen, specimen after 6 months immersion in water Figure 5 shows the effect of water ingress on the fracture energy.$

4 Singapore, st July (e) (f) Figure 3. Typical failure modes of CFRP-concrete single shear pull-off test specimens: C25 (0 month water immersion), C25 (6 months), (c) C40SG (0 month), (d) C40SG (6 months), (e) C40BG (0 month), (f) C40BG (6 months) Figure 4. shows two SEM images of the CFRP plate after pulled off from the concrete. The content of Si was 24% by weight (Figure 4a) when the failure was in the concrete. The presence of C is attributed to the carbonation of concrete. For the specimen tested after water immersion, the failure mode changed to debonding at the adhesive/concrete interface which led to higher C content (due to carbon backbone in the primer and the carbonation of concrete) and lower Si content (as there is no Si in epoxy primer) (Figure 4b). At the micro level, the failure involved a combination of mortar-primer interface failure and mortar failure. Figure 4. SEM images of CFRP plate pulled off from the concrete: reference specimen, specimen after 6 months immersion in water Figure 5 shows the effect of water ingress on the fracture energy. For the reference specimens without water immersion, the fracture energy of C40SG was higher than that of C25 and C40BG as C40SG had the highest concrete compressive strength. The fracture energy was reduced rapidly after 6 months water immersion, but levelled off as the immersion time further increased. The deterioration of the fracture energy for C40BG and C40SG was more significant than that of C25, and that of specimen C40BG was smaller than that of C40SG. The fracture energy of the reference specimens is dominated by the tensile strength of the concrete, while that of the specimens with water immersion depends on the properties of the primer/concrete interface due to failure mode shift. The mechanical interaction due to the higher roughness and more porosity of the concrete for C25 is stronger than that of the C40BG and C40SG. The porosity of C40BG is less than that of C40SB owing to the coarse aggregates. Figure 5. Effect of water ingress on the interfacial fracture energy

5 Singapore, st July Conclusions The paper has presented an experimental study on the effect of water ingress on the bond behavior between CFRP plate and concrete. Based on the results, the following conclusions can be drawn: (1) The effects of water uptake in the primer on the chemical bond is more significant than that of the mechanical behavior; (2) The CFRP-to-concrete failure mode shifts from concrete cohesion to primer-concrete interface debonding. Under the microscope, the failure mode in the aged specimens is a mix of mortar cohesion and the primer/mortar debonding; (3) The fracture energy decreases rapidly after 6 months immersion in water. The higher the concrete compressive strength, the more the deterioration. Acknowledgments The financial support provided by the National Natural Science Foundation of China (Project No ) and the National Key Research and Development Program of China (Project No. 2017YFC ) is gratefully acknowledged. References [1] J.G. Dai, H. Yokota, M. Iwanami and E. Kato, Experimental investigation of the influence of moisture on the bond behavior of FRP to concrete interfaces, Journal of Composites for Construction, 14(6): , [2] C. Au and O. Büyüköztürk, Peel and shear fracture characterization of debonding in FRP plated concrete affected by moisture, Journal of Composites for Construction, 10(1): 35-47, [3] M.A. Silva, H.C. Biscaia and R. Marreiros, Bond-slip on CFRP/GFRP-to-concrete joints subjected to moisture, salt fog and temperature cycles, Composites Part B: Engineering, , [4] J. Shrestha, T. Ueda and D.W. Zhang, Durability of FRP concrete bonds and its constituent properties under the influence of moisture conditions, Journal of Materials in Civil Engineering, 27(2): A , [5] N.F. Grace and M. Grace, Effect of repeated loading and long term humidity exposure on flexural response of CFRP strengthened concrete beams, Proceedings of the International Symposium on Bonded behavior of FRP in Structures (BBFS 2005), Chen and Teng (eds), pp , [6] Y.F. Pan, G.J. Xian and M.A. Silva, Effects of water immersion on the bond behavior between CFRP plates and concrete substrate, Construction And Building Materials, , [7] Z.Y. Ouyang and B.L. Wan, Modeling of moisture diffusion in FRP strengthened concrete specimens, Journal of Composites for Construction, 12(4): , [8] G.J. Xian and V.M. Karbhari, DMTA based investigation of hygrothermal ageing of an epoxy system used in rehabilitation, Journal Of Applied Polymer Science, 104(2): , [9] ASTM, Standard Test Method for Tensile Properties of Plastics, West Conshohocken, Pennsylvania, USA, 2014.

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