A PROPOSAL OF DESIGN PROCEDURE FOR FLEXURAL STRENGTHENING RC BEAMS WITH FRP SHEET

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1 N. Kishi, E-89, 1/8 A PROPOSAL OF DESIGN PROCEDURE FOR FLEXURAL STRENGTHENING RC BEAMS WITH FRP SHEET Yusuke Kurihashi Norimitsu Kishi Hiroshi Mikami Sumiyuki Sawada Civil Engrg. Research Muroran Inst. of Tech. Mitsui Construction Muroran Inst. of Tech Inst. of Hokkaido, JAPAN JAPAN T.R.I, JAPAN JAPAN Keywords: RC beams, FRP sheet, flexural strengthening, sheet debonding 1. INTRODUCTION In order to upgrade ultimate strength and/or ductility of exist Reinforced Concrete (RC) structures, many strengthening and retrofitting works have been performed in Japan. In these strengthening works, steel jacketing and/or concrete covering methods have usually been applied. On the other hand, in recent year, Fiber Reinforced Plastic Sheet (FRPS) jacketing methods have been sometimes applied since not only FRPS has high strength but also is easy to handle due to its lightweight and limp material. In general, FRPS strengthening method has been adopted to improve shear capacity and/or ductility by wrapping FRPS around RC/PC members and to upgrade flexural capacity by bonding FRPS onto the tension-side surface. Toward establishing rational upgrading methods for increasing these capacities for RC beam/plate, the plane-shear bonding properties of FRPS have been studied [1]~[4]. However, bonding properties of FRPS for flexural strengthening have not been adequately understood yet, because sheet debonding is affected not only by plane-shear force but also by flexural-shear force [5]~[7]. Until now, the authors have been conducting the static loading test on various rectangular RC beams strengthened with FRPS and investigating the load-carrying behavior of strengthened RC beams including sheet-debonding mechanism. Consequently, it has been clear that 1) two types of failure mode of the RC beams are experimentally confirmed: sheet-debonding failure (DF) type and flexural-compression failure (FCF) type, in which DF type beams are failed due to FRPS being debonded before surcharged load reaches analytical ultimate point, FCF type beams are failed due to FRPS debonding after getting to the analytical ultimate point with the upper cover concrete crashing; 2) FRPS is debonded due to a peeling action of concrete blocks formed at the lower cover concrete near loading point in the shear-span area irrespective of failure mode; 3) these failure modes can be predicted by using: the ratio of the main-rebar yield area in the shear-span to the shear-span length; and the ratio of the analytical yield moment to the ultimate moment [8],[9]. On the other hand, it is important study to investigate load-carrying capacity for T-shape RC beams strengthened with FRPS, because T-shape beams have been put to practical use very often. From this point of view, in this study, to establish a rational flexural strengthening design procedure for RC beams using FRPS, load-carrying behavior for T-shape RC beams strengthened with FRPS considering sheet-debonding mechanism is experimentally discussed. Because strengthening effect of FRPS is influenced mainly by axial stiffness of FRPS, Aramid FRP (AFRPS) is only addressed in this paper. Furthermore, prediction method for failure mode of RC beam strengthened with AFRPS is also discussed referring to author s recent work [9]. 2. EXPERIMENTAL OVERVIEW Six RC beams used in this study are listed in Table1. Nominal name of these RC beams are designated in order of main-rebar ratio p t (T1 (0.80 %), T2 (1.26 %), T3 (1.82 %), T4 (2.46 %)), shear span ratio r s (R5 or R7, rounded to the nearest integer), and sheet volume ratio p f (1 or 2 ). Here, p t and p f are estimated as an index of reinforcement volume and are calculated as: p t = A s /(b w d), p f = A f /(b w d), respectively, in which, A s and A f are sectional area of main-rebar and AFRPS, b w and d are web width and effective depth in cross section, respectively. The dimension and rebar arrangement for

2 N. Kishi, E-89, 2/8 Specimen Main rebar Rebar ratio p t (%) T1-R5-2 D T2-R5-2 D T3-R5-2 D T4-R5-2 D Table 1 List of RC beams Clear span Length (m) Shear span ratio r s Number of sheet layers Sheet volume ratio p f ( ) T2-R D T2-R r s = 6.9 r s = 5.0 Fig. 1 Dimensions of RC beams Fig. 2 An example of strengthened area with AFRPS and location of glued strain gauge (r s = 5.0) each type of T-shape RC beam are shown in Fig.1. RC beams used in this experiment are of sectional height: 250 mm, web width: 150 mm, flange thickness: 100 mm, flange width: 300 mm. The clear span is 2.6 m or 3.4 m long corresponding to shear span ratio r s, because of loading interval being fixed as 500 mm long. Figure 2 shows an example of strengthened area with AFRPS and the location of each glued strain gauge. The 130 mm wide AFRPS is bonded onto the central portion 100 mm inside the supporting points of each beam. The concrete surface for bonding AFRPS is chipped heavily to improve the bonding capacity of AFRPS [10]. Yield strength of rebar and material property of AFRPS used in this experiment are listed in Table 2 and 3, respectively. AFRPS with kg/m 2 was used for all RC beams considered here. All RC beams used here have been analytically confirmed that the bending capacity was less than the shear capacity even after strengthening. Here, the bending and shear capacities of each strengthened RC beam have been estimated by using multi-section method and modified truss

3 N. Kishi, E-89, 3/8 Table 2 Yield strength of rebar SD295A (Stirrup) SD345 (Main-rebar) Nominal Diameter D10 D13 D16 D19 D22 Yield strength (MPa) Table 3 Material properties of AFRPS Mass per unit area (kg/m 2 ) Thickness (mm) E-modulus (GPa) Tensile strength (GPa) Strain limit (%) Photo 1 Experimental setup theory [11], respectively. At commencement of experiment, the average compressive strength of concrete was 23.9 MPa. The ultimate compressive strain of concrete for numerical analysis was assumed as 3,500 µ based on the specifications of Japan Concrete Standard [11]. A surcharged load (hereinafter, load), the mid-span displacement (hereinafter, displacement), and axial strain distribution of AFRPS were measured and continuously recorded using digital data-recorder to precisely investigate the debonding process of AFRPS. Photo 1 shows the experimental setup. 3. EXPERIMENTAL RESULTS 3.1 Relationship between load and displacement Figure 3 compares the load-displacement curves of experimental results with analytical results. Analytical results are obtained by means of multi-section method, in which aforementioned material properties are considered and AFRPS is assumed to be perfectly bonded to concrete surface up to the analytical ultimate point. Each load P and displacement δ is normalized with reference to the values of P y and δ y at the main-rebar yield point, respectively. Figure 3(a) shows the results on four RC beams taking rebar ratio p t as variable. From these figures, it is seen that 1) from the analytical results, the smaller the rebar ratio p t, the larger the ultimate load and displacement (hereinafter, ultimate displacement) are; 2) from the experimental results, the smaller p t value, the slightly larger the ultimate load is; 3) all RC beams fail due to AFRPS debonding at almost similar displacement level (δ/δ y = 3.0). Here, it is seen that experimental ultimate load and displacement of T4-R5-2 beam are as large as

4 N. Kishi, E-89, 4/8 (a) The case of varying rebar ratio p t (p f = 1.98, r s = 5.0) (b) The case of varying sheet volume ratio p f (p t = 1.26 %, r s = 5.0) (c) The case of varying shear span ratio r s (p t = 1.26 %, p f = 1.98 ) Fig. 3 Comparison between experimental and analytical results on normalized load and displacement relation ( D and FC mean DF type and FCF type, respectively) analytical ones. This suggests that T4-R5-2 beam is belonged to FCF type. On the other hand, in case of T1~T3-R5-2 beams with smaller p t than that of T4-R5-2 beam, these load-displacement curves are lower than the analytical ones. This suggests that these beams are belonged to DF type. Moreover it can be seen that the smaller the p t value, the more remarkably the RC beams are failed with DF type. Figure 3(b) shows the results on two RC beams taking sheet volume ratio p f as variable. These figures reveal that 1) both RC beams are failed due to sheet debonding before getting to analytical ultimate state; 2) the difference between experimental and analytical results for ultimate load and displacement of T2-R5-2 beam is larger than that of T2-R5-1 beam. This suggests that RC beam tends to be DF type with increasing in p f value. Figure 3(c) shows the results on two RC beams taking shear span ratio r s as variable. From these figures, it is seen that 1) analytical results behave similarly to each other irrespective of r s value; 2) although the experimental ultimate load and displacement of T2-R7-2 beam is a little larger than those of T2-R5-2 beam, load-carrying behavior is almost similar to each other irrespective of r s value. From these results, it can be seen that 1) the smaller rebar ratio p t under keeping sheet volume ratio p f constant and/or the larger p f value under keeping p t value constant, the more remarkably the RC beams are failed with DF type; 2) shear span ratio r s has little effect on failure mode of the RC beam; 3) the influence of some parameters of T-shape RC beams on load-carrying behavior is almost similar to the case of rectangular RC beams [9]. 3.2 Strain distribution of AFRPS Figure 4 shows the strain distributions of AFRPS at the beginning of sheet debonding (hereinafter, sheet debonding point) comparing with the analytical results. In case of T4-R5-2 with FCF type, the distributions at analytical ultimate point are drawn, since AFRPS is debonded after reaching analytical ultimate state. Analytical results are obtained by means of the multi-section method. In these figures, L yu and L yd are analytically estimated as rebar yield area at analytical ultimate point and at sheet debonding point, respectively.

5 N. Kishi, E-89, 5/8 Fig. 4 Comparison between experimental and analytical results of AFRPS strain distribution at the sheet debonding point ( D and FC mean DF type and FCF type, respectively) L yu L yd Photo 2 Situation of sheet debonding due to peeling action (T3-R5-2 beam) These figures show that all DF type beams are failed due to AFRPS being debonded before L yd reaches L yu. In contrast, FCF type beam (T4-R5-2) is not failed at the analytical ultimate point, so that the length of L yd is similar to that of L yu. Observing these strain distributions of AFRPS in detail, it is recognized that the experimental strain distributions in equi-bending area give comparably good agreement with analytical ones. On the other hand, these in the region of rebar yield area L yd are larger than the analytical ones. This implies that AFRPS may be debonded due to peeling action of concrete blocks pushing out in the downward in the region of rebar yield area L yd. Photo 2 shows the beginning of sheet debonding in case of T3-R5-2 beam. From this photo, it is recognized that 1) in the equi-bending area, although bending cracks are developed, sheet debonding cannot be seen; 2) in the rebar yield area L yd of the equi-shear span, concrete brocks are formed due to interaction between bending and diagonal cracks developed around the lower cover concrete area; 3) sheet debonding is

6 N. Kishi, E-89, 6/8 Fig. 5 Experimental results on relationship between L yu /d and failure type spread toward the one-side supporting point due to peeling action of the concrete brocks pushing out AFRPS. This suggests that sheet debonding is closely related to the expansion of rebar yield area. Thus, it can be expected that the larger the rebar yield area L yu, the more easily the sheet is debonded. From these results, it is made clear that 1) the larger L yu, the more remarkably the RC beams are failed with DF type; 2) sheet debonding develops due to peeling action of concrete brocks pushing out the sheet in the rebar yield area of equi-shear span. Thus, it can be seen that sheet-debonding behavior in case of T-shape RC beam is similar to that of rectangular RC beam which was observed by authors [9]. 4. PREDICTION OF FAILURE MODE From former discussions, it is revealed that the larger L yu, the more remarkably the RC beams are failed with DF type. This suggests that T-shape RC beam is easy to fail due to sheet debonding before getting to analytical ultimate point when L yu is estimated as large value. The prediction method of failure mode has been proposed considering the relationship between L yu and sheet-debonding behavior [9]. Referring to this prediction method, the experimental results on failure type are plotted on L yu /d r s diagram (Fig. 5). In this figure, the results for DF type beams are indicated as the black marks. From this figure, it is revealed that 1) for an arbitrary r s, the larger the L yu /d ratio, the more remarkably the RC beams are failed with DF type; 2) the larger the r s value, the larger the L yu /d ratio for DF type is. Here, based on the prediction equation for failure type of RC beams proposed by Kurihashi et al. [9], equations for the lower and upper bounds are as follows. Equation for the lower bound of DF type: L yu /d = 0.35 r s (1) Equation for the upper bound of FCF type: L yu /d = 0.30 r s (2) From Fig. 5, it is made clear that both equations for the bounds can be applied to the case of T-shape RC beam strengthened with AFRPS. Here, replacing r s with a/d (a: shear span length), L yu /d r s relation can be converted into L yu a relation. Furthermore, assuming that ultimate moment M u is generated at equi-moment area of beam (Fig. 6), L yu a relation can be converted into M y M u relation (M y : main-rebar yield moment) and both equations are as follows.

7 N. Kishi, E-89, 7/8 Fig. 6 Reaction moment diagram at the analytical ultimate point Fig. 7 Experimental results on relationship between M y /M u and failure type Equation for the upper bound of DF type: M y /M u = 0.65 (3) Equation for the lower bound of FCF type: M y /M u = 0.70 (4) Figure 7 shows the relationship between M y /M u and failure type. From this figure, it is confirmed that aforementioned upper and lower bound equations (3) and (4) can be applied to the case of T-shape RC beam. If both failure types are estimated considering some safety margin, prediction equations of failure types are obtained as: M y /M u < 0.70 for DF type; M y /M u > 0.70 for FCF type. Thus, it is made clear that failure mode of RC beam strengthened with AFRP sheet can be expected using M u and M y estimated using multi-section method irrespective of cross sectional shape of RC beams. Furthermore, toward establishing a rational flexural strengthening method, it is important to investigate the strengthening method appropriately to each failure type. When failure mode is expected as DF type, the strengthening method considering appropriate anchoring treatment has to be investigated, since FRPS is debonded before getting to analytical ultimate point. On the other hand, when failure mode is expected as FCF type, the estimation method for appropriate sheet length has to be investigated, because ultimate load-carrying capacity of beam can be assured.

8 N. Kishi, E-89, 8/8 5. CONCLUDING REMARKS In this paper, in order to establish a rational flexural strengthening design procedure for RC beams in case using Fiber Reinforced Plastic Sheet (FRPS), static loading tests on T-shape RC beams strengthened with Aramid FRPS (AFRPS) are conducted to investigate sheet debonding behavior and load-carrying capacity of the RC beams. Moreover prediction method for failure mode of RC beam strengthened with AFRPS is also discussed. The results obtained from this study are summarized as follows: 1) Failure mode of T-shape RC beams strengthened with AFRPS is divided two types apart: sheet Debonding Failure (DF) type and Flexural Compressive Failure (FCF) type. 2) The smaller rebar ratio p t under keeping sheet volume ratio p f constant and/or the larger p f value under keeping the value of p t constant, the more remarkably the RC beams are failed with DF type. On the other hand, shear span ratio r s has little effect on failure mode of RC beams. 3) Sheet debonding is developed due to peeling action of concrete blocks pushing out in the downward direction. 4) T-shape RC beam is easy to fail due to debonding of AFRPS before getting to analytical ultimate point when the rebar yield area in equi-shear span L yu is analytically estimated as a large value. 5) Failure mode of T-shape RC beam strengthened with AFRPS can be expected using M u and M y estimated by means of multi-section method similarly to the case of rectangular RC beam. REFERENCES [1] Yoshizawa H. and Wu Z.: Experimental Study on Crack Behavior of RC Tensile Members Strengthened with Carbon Fiber Sheets, Journal of Materials, Concrete Structures and Pavements, No. 613 / V-42, pp , (In Japanese) [2] Kamiharako A., Simomura T., Maruyama K., and Nishida H.: Analysis of Bond and Debonding Behavior of Continuous Fiber Sheet Bonded on Concrete, Journal of Materials, Concrete Structures and Pavements, No. 634 / V-45, pp , (In Japanese) [3] Bizindavyi L. and Neale K. W.: Transfer Lengths and Bond Strengths for Composites Bonded to Concrete, Journal of Composites for Construction, pp , [4] Sato Y., Asano Y., and Ueda T.: Fundamental Study on Bond Mechanism on Carbon Fiber Sheet, Journal of Materials, Concrete Structures and Pavements, No. 648 / V-47, pp , (In Japanese) [5] Buyukozturk O. and Hearing B.: Failure Behavior of Precracked Concrete Beams Retrofitted with FRP, Journal of Composites for Construction, pp , [6] Takeo K., Matsushita H., Sagawa Y., and Ushigome T.: Experiment of RC Beam Reinforced with CFRP Adhesive Method Having Variety of Shear-span Ratio, Proceedings of the Japan Concrete Institute, Vol.21, No.2, pp , (In Japanese) [7] Kubota K., Harada T., Nagafuji M., and Takeo K.: A Study on Peeling Mechanism and Anchoring Method of CFRP Sheet Used as Flexural Reinforcement in RC Beam, Proceedings of the Japan Concrete Institute, Vol.23, No.1, pp , (In Japanese) [8] Mikami H., Kurihashi Y., and Kishi N.: Flexural Bonding Property of FRP Sheet Adhered to RC Beams, Congress of 16th IABSE (CD-ROM), paper 252 (8 pages), [9] Kishi N., Mikami H., Matsuoka K. G., and Kurihashi Y.: Failure Behavior of Flexural Strengthened RC Beams with AFRP Sheet, Proceedings of FRPRCS-5, pp.87-95, [10] Kurihashi Y., Kishi N., Mikami H., Sato M., and Matsuoka K. G.: Experimental Study on Flexural Bonding Property of AFRP Sheet Glued on RC Beams, Proceedings of the 7th EASEC, pp , [11] JSCE: Japan Concrete Standard, (In Japanese)