EFFECT OF ADHESIVE THICKNESS ON BOND BEHAVIOUR OF CARBON FIBER SHEET UNDER STATIC AND FATIGUE LOADING

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1 EFFECT OF ADHESIVE THICKNESS ON BOND BEHAVIOUR OF CARBON FIBER SHEET UNDER STATIC AND FATIGUE LOADING Hiroya Tamura 1, Ueda Tamon 2, Furuuchi Hitoshi 3 and Seiji Fujima 4 1 Hokkaido Univ., Sapporo , Japan. lionvskitsune@yahoo.co.jp 2 Hokkaido Univ., Sapporo , Japan. ueda@eng.hokudai.ac.jp 3 Hokkaido Univ., Sapporo , Japan. jin@eng.hokudai.ac.jp 4 Denki Kagakukogyo Kabushiki Kaisha, Tokyo , Japan. seiji-ujima@denka.co.jp ABSTRACT Carbon Fiber Sheet (CFS) is used or reinorcing concrete structures. CFS is useul because it is light, durable, and corrosion resistant. A typical type o racture in concrete that has been reinorced with Carbon Fiber Reinorced Polymer (CFRP) is caused by sheet debonding, so it is very important to understand the debonding mechanism. In execution, the thickness o the adhesive layer between CFRP and concrete is controlled by the condition o the substrate concrete interace and the expertise o the workorce. However, there have so ar been ew studies o the eect o dierences in bond thickness on debonding. This study ocuses on the eects o varying adhesive thickness rom the point o view o static and atigue debonding strength, taking adhesive thickness and atigue loading level as test parameters. The atigue test with adhesive thickness as a parameter seems to be the irst o its type in the world. In the static test, the eects o adhesive thickness are observed in pull-out bond strength and eective bonding length, observed by measuring CFS strain, as predicted by a model in the literature 1). In the atigue test, however, the eects o the adhesive layer seem to be dierent. Proposals are made as to how to consider the eect o adhesive layer thickness in design. KEYWORDS Carbon Fiber Sheet (CFS), adhesion, atigue loading, resin, eective adhesive length INTRODUCTION The demand or concrete reinorced with Carbon Fiber Reinorced Polymer (CFRP) sheet is increasing. As a consequence, research into the adhesion o Carbon Fiber Sheet (CFS) has been rapidly increasing, leading to clariication o the bond stress-slip (τ-s) relationship and the racture energy in static loading. On the other hand, the ew studies o adhesion characteristics in cyclic loading have not been enough to clariy the mechanism. Cyclic loading has a greater inluence in bridge applications, so rapid unravelling o the mechanism is desired. Past research 1) 2) has indicated that the inluence o adhesion characteristics increases with adhesive thickness. Adhesive thickness is controlled by conditions at the surace o the substrate concrete. However, there is no reliable and ull consideration o the eect o adhesive thickness and i the eect is ound to be large, it will be necessary to restrict thickness variation in design. This paper aims at deeper insight into the eect o varying adhesive thickness under static and atigue loading. Variations in adhesive thickness are intentionally made large, so specimens are prepared with three thicknesses, 0.2mm, 1mm, and 3mm. It is also investigated whether pull-out bond strength with rather thick adhesive under static loading matches the prediction by Dai et al s model 1). EXPERIMENTAL OUTLINE Test equipment: experiments were carried out on equipment (see Fig. 1) prepared according to Osaka University s bending machine test setup. The equipment has a centre hinge connecting two wide langed beams. The specimen is attached to the resulting beam assembly. A load-distributing beam, distributing o loading is connecting to an actuator, applies loading to both o the distribution on the both side wide langed beams, 241

2 causing the central point at the join to open and close around the hinge. This rotation causes a shear stress at the interace between the substrate concrete and the CFS in the CFRP-concrete. The distance between loading points on the upper surace o the beam assembly is 1200mm. The distance between the ixing points below the beam assembly is 600mm. The testing machine is 200mm high. Specimens: two concrete prisms (length 600mm, section mm) are aligned longitudinally with a small gap between their end aces. A single-lap CFS is bonded across the top suraces o the slabs. The CFS is 60mm wide and the area o attachment to each concrete prism is mm. In order to avoid local damage to the concrete block, an unbonded section (50mm) is ensured by using vinylon tape to separate the concrete surace rom the CFS. Specim Loading Beam Wide lange Hinge Unbonde d area Fig. 1 Loading Machine Fig. 2 Specimen Dimension In order to ensure debonding on the intended side, the bonded CFRP was covered and glued with a channel steel plate. The gluing resin was the same as the adhesive or CFRP-concrete interace. Moreover, to prevent damage concentrating at the ends o the slabs, the ends o the specimen were reinorced with a L-shaped steel plate. And, the specimen was reinorced with D10 reinorcing bar or longitudinal direction to prevent lexural ailure beore CFRP debonding and D6 reinorcing bar as additional reinorcement to prevent unexpected cracking. The ready-mixed concrete used or experiment was made with early strength Portland cement and had a slump o 12cm and a design strength o 30N/mm 2. The curing period was two weeks. The resin adhesive was applied as ollows. First, a divider was used to encircle the area to be bonded to the intended adhesive thickness. Resin was then poured into the encircled area. Finally, the CFS was impregnated with resin and laid down over the bonding area. The characteristic values o the CFS and the resin are shown in Table 1 and Table 2, respectively. The specimen is shown in Fig. 2. Measurements: the values measured were CFS strain, acting load, and atigue lie. Strain gauges o length 5mm were attached at intervals o 20mm to the surace o the CFS in the longitudinal direction. Measurement method: in static loading, the load was adjusted by hand under load control. The above measurements were made. In atigue loading, the load was adjusted as in the static test up to ten cycles. Beyond ten cycles, the load was controlled automatically, but at measurement points the load was once again hand operated. Table 1 Properties o CFS Thickness Elastic modulus Tensile strength Density t (mm) E (GPa) (MPa) (g/m 2 ) Table 2 Adhesive properties Elastic modulus Tensile strength Compressive strength Ga (GPa) (MPa) (MPa)

3 ANALYSIS OF TEST RESULTS STATIC TEST The relationship between pull-out strength and adhesive thickness is shown in Fig. 3. It can be seen that pull-out strength increases with adhesive thickness. The solid line in the igure represents Dai et al. s model 2). A prediction equation is proposed as ollows: ( G / t ) ( E t ) G = a a c P b 2E t G max = G = Fracture Energy b = Width o CFS The experimental pull-out strengths are larger than the model predicts. However, the results are similar in terms o shape and rate o increase. In act, it is ound that the dierence between 0.2mm and 1mm o adhesive is bigger than the dierence between 1mm and 3mm in both experimental results and the prediction. The main reason or this relation between adhesive thickness and pull-out strength is the eect o eective bond length. The eective bond length is the axial distance rom the loading end over which the bond stress is borne and it increases with adhesive thickness. A greater eective bond length means that bond stress is carried over a greater length o the CFS and consequently the local bond stress is reduced, making it less likely to reach the local bond strength. The variation in eective bond length will be discussed later in connection with the strain distribution. The scatter in pull-out strength can be considered a result o variations in concrete strength, concrete surace conditions, errors, and torsion. Fig. 3 Relation between pull-out strength and adhesive thickness The strain distribution over the length o the CFS is shown in Fig. 4. The strains are determined as simple mean values o measured CFS strains at three locations nearby. The strain distribution gradient corresponds to bond stress. That is to say, bond stress is not borne equally over the ull CFS. Where the distribution is lat, there is no bond stress. The CFS has already debonded at the load end and the bond stress does not extend that ar toward the ree end. In early loading with ST-01, the strain distribution curve orms a parabolic curve and the strain rises gradually. However, the strain stops growing at certain value, which is the local bond strength, and eective bonding extends away rom the load end. Finally, eective bonding reaches the ree end and the CFS debonds completely rom the substrate concrete. This behaviour conorms generally with that clariied in previous research. Comparing the three specimens with dierent o adhesive thicknesses in Fig. 4, it is seen that a thicker adhesive layer results in a gentler gradient and greater eective bond length. This veriies the earlier discussion on the increase in pull-out bond strength shown in Fig. 3. Moreover, it can be seen that the growth in eective bond length between 0.2mm and the other adhesive thicknesses (1mm and 3mm) is more obvious. That is to say, eective bond length increases signiicantly when the adhesive is increased rom 0.2mm to 1mm in thickness, but thereater the increase is smaller. It can be understood that this is relected in the changes in pull-out bond strength. 243

4 80m 120m ST-01 (ta=0.2mm) ST-11 (ta=1mm) 120m ST-31 (ta=3mm) Fig.4. Strain distribution FATIGUE TEST In the atigue test, adhesive thickness and maximum loading level were set up as experimental parameters. The nine specimens and their atigue lie results are shown in Table 3. The maximum loading level and minimum loading level are based on the ratio o static pull-out bond strength. Table 3 Properties o specimens and experimental results Thickness (mm) Pull-out strength (kn) Minimum loading (kn) Minimum loading level (%) Maximum loading (kn) Maximum loading level (%) Fatigue lie (cycle) FA FA FA FA FA FA FA FA FA The relation between atigue lie and maximum loading level is shown in Fig. 5. It can be seen that there is a linear relationship between atigue lie and loading level or each adhesive thickness, while atigue lie decreases with adhesive thickness. In contrast with the results or static pull-out bond strength, atigue lie decreases with adhesive thickness or a particular loading level. In particular, the atigue lie or an adhesive thickness o 0.2mm 244

5 is much greater than or the other thicknesses. In the case o maximum loading levels rom 80% up, the atigue lie exceeds one hundred thousand cycles. On the other hand, the atigue lie or adhesive thicknesses o 1mm and 3mm reaches one hundred thousand cycles when the maximum loading level is 50% or less. Fig.5. Fatigue lie normalized maximum load relationship The relation between atigue lie and actual atigue loading is shown Fig. 6. The curves or adhesive thicknesses o 1mm and 3mm are similar. In the case o atigue loading o 15kN, atigue lie can reach eight hundred thousand cycles. In the case o an adhesive thickness o 0.2mm, atigue lie barely decreases with maximum loading. With a atigue loading o 22kN, the specimen with an adhesive thickness o 0.2mm has a slightly longer atigue lie than those with 1mm and 3mm thickness. Consequently the specimens with 0.2mm, 1mm and 3mm thickness show similar atigue lie or loadings around 20kN. Fig.6. Fatigue lie maximum load relationship The strain distribution during atigue testing is shown in Fig. 7. In the same way as in the static test, the strais are calculated as simple mean values o CFS strain given by the strain gauges. These strain distributions are shown or certain numbers o cycles at the maximum load. Under atigue loading, eective bond length does not depend on adhesive thickness. In other words, dierent eective bond lengths during the stages o cyclic loading converge to similar values when atigue ailure is approached. Thereore, due to the similar eective bond length, specimens with adhesive thicknesses o 1mm and 3mm have bond stress higher than static test, and they have a atigue lie shorter than expected. Comparing the three specimen represented in Fig. 7, all o which were under similar loading o around 20-21kN, FA-01 (adhesive thickness o 0.2mm) debonded at the load end ater ten o thousands o cycles, whereas FA-12 and FA-33 (adhesive thickness o 1mm and 3mm) reached debonding ater only several thousand cycles. That is, specimens with thicker adhesive rapidly debond over the ull length o the CFS. According to these indings, in static loading it is useul to increase the adhesive thickness, but where static and atigue loading takes place, the thinner adhesive is preerable. 245

6 FA-01 (ta=0.2mm, max load=20.2kn) FA-12 (ta=1mm, max load=20.1kn) FA-33 (ta=3mm, max load=21.3kn) Fig. 7 Strain distributions Fig. 8 Stress distributions at dierent adhesive thicknesses CONCLUDING REMARKS 1. In static loading, pull-out bond strength increases with adhesive thickness. Moreover, the incremental increase in pull-out strength becomes less signiicant with adhesive thickness. 2. In static loading, pull-out strength increases because o the increase in the eective bond length, which explains the eect o adhesive thickness. 3. Even or extremely thick adhesive, the experimental results correspond with Dai et al. s model 2). 4. In atigue loading, in contrast to the static test, atigue lie decreases with adhesive thickness at a particular loading level. 5. In atigue loading, in contrast to the static test, specimens with dierent adhesive thicknesses have the same eective bond length. REFERENCES 1) J. Dai, T. Ueda and Y. Sato: Uniied Analytical Approaches or Determining Shear Bond Characteristics o FRP-Concrete Interaces through Pullout Tests, Journal o ACT, Vol.4, No.1, ) J. Dai, T. Ueda and Y. Sato: Development o the Nonlinear Bond Stress-Slip Model o Fiber Reinorced Plastic Sheet-Concrete Interaces with a Simple Method, Journal o Composites or construction, Vol.9, No.1, ) Y. Sato: Fatigue Perormance o FRP Sheet Externally Bonded to Frost Damaged Concrete,graduation thesis at Hokkaido Univ.,