FLEXURAL BEHAVIOR OF RC BEAMS REINFORCED WITH NSM AFRP RODS

Transcription

1 Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds) 2005 International Institute for FRP in Construction FLEXURAL BEHAVIOR OF RC BEAMS REINFORCED WITH NSM AFRP RODS N. Kishi 1, H. Mikami 2, Y. Kurihashi 3, and S. Sawada 1 1 Dept. of Civil Engineering and Architecture, Muroran Institute of Technology, Japan kishi@news3.ce.muroran-it.ac.jp 2 Sumitomo Mitsui Construction Co., Ltd., Japan 3 Material Division, Civil Engineering Research Inst. of Hokkaido, Japan. ABSTRACT In this paper, in order to investigate an enhancement of flexural load-carrying capacity of the existing reinforced concrete (RC) members reinforced with near surface mounted (NSM) fibre reinforced polymer (FRP) rods and debonding behavior of those FRP rods, static four-point loading tests for RC beams reinforced with NSM Aramid FRP (AFRP) rods were conducted. For comparisons, RC beams reinforced in flexural with bonding AFRP sheet on the tension-side surface were also tested, in which axial stiffness EA between two reinforcing materials is similar to each other. In this study, the axial stiffness EA is varied in three levels of magnitude. The results obtained from this study are as follows: 1) It is experimentally observed that the similar reinforcing effects of AFRP rods/sheet can be expected if axial stiffness EA of reinforcing materials is similar to each other; and 2) The failure mode of the RC beams can be predicted using an empirical equation for the RC beams reinforced with bonding FRP sheet. KEYWORDS RC beams, flexural reinforcing, NSM FRP rods, FRP sheet. INTRODUCTION At present, FRP sheet bonding method and/or tension-side surface overlaying method with concrete have been applied for reinforcing the existing RC slabs and beams. However, applying those reinforcing methods, concrete surface of those existing RC members will be perfectly covered with sheet and/or overlaid concrete. As the results, following drawbacks are pointed out: 1) it is impossible to do a visual inspection of crack developed due to degradation of concrete; 2) anti-fatigue capacity of existing concrete tends to be decreased due to undrained water infiltrated in concrete and so on. As one of the methods for figuring out of those drawbacks, near-surface mounting method of FRP rod has been proposed. Lorenzis and Nanni (2001a, 2001b) have conducted static loading tests for RC beams reinforced in shear with NSM FRP rods and it is experimentally confirmed that the shear capacity of the RC beams can be effectively enhanced by applying this method. Nordin and Taljsten (2003) have conducted static loading tests for RC beams reinforced with pre-stressed NSM Carbon FRP (CFRP) rods and it is demonstrated experimentally that the pre-stressing of FRP rods in slots gives a very good bonding capacity for retaining the pre-stressing forces. Micelli et al. (2003) have studied the environmental effects to RC beams reinforced with NSM rods due to exposing environmental agents including freezing and thaw, high temperature and high relative humidity cycles under direct UV exposure. In this paper, in order to investigate the reinforcing effects and debonding behavior of NSM AFRP rods, static four-point loading tests for RC beams reinforced in flexure with NSM AFRP rods were conducted. Here, for comparisons, RC beams reinforced with bonding AFRP sheet on the tension-side surface were also tested, in which axial stiffness EA of those reinforcing materials is similar to each other. Here, three levels of magnitude for axial stiffness EA of the reinforcing materials were taken as variable by changing radius for AFRP rods, and width and mass for AFRP sheet. EXPERIMENTAL OVERVIEW Total six reinforced RC beams listed in Table 1 were used in this study, in which those three are of reinforced with NSM AFRP rods and the other three are of reinforced bonding AFRP sheet on the tension-side surface. Each beam was designated using two items: reinforcing material (R: AFRP rod, S: AFRP sheet) and index 337

2 number in ascending order of axial stiffness EA for each reinforcing material. The axial stiffness EA of reinforcing material was varied in three levels by changing rod size for AFRP rod, and width and mass for AFRP sheet. From Table 1, it is observed that the axial stiffness EA in same index number for both beams R and S are similar to each other. All beams were designed as double reinforced RC beam with a rectangular cross section as shown in Fig. 1. SD 345 D13/D19 and SD 295 D10 rebar were used as the axial rebar and stirrup, respectively. Here, SD 345 D19 and D13 rebar were used as top and bottom one, respectively, so as the RC beams to reach ultimate state with AFRP rods/sheet debonding failure mode. Schematic reinforcing diagrams for beams R/S and location of strain gauges glued on the reinforcing material are shown in Fig. 2, in which two AFRP rods were used as NSM one. NSM AFRP rods were mounted based on the following procedure: 1) two grooves with a little wider width and depth than the diameter of mounted AFRP rod at 40 mm inside from the side edges, where are the same lateral positions with the bottom axial rebar, were excavated using diamond disc saw; 2) those grooves were filled with putty made from epoxy resin; and 3) AFRP rods were mounted into the grooves. In case of beams S, unidirectional AFRP sheet was bonded on the tension-side surface of RC beams. Concrete surface was gritblasted to improve the bonding strength between AFRP sheet and concrete. Both beams R and S were reinforced in the region of RC beams leaving 100 mm between the supporting points and the rod/sheet end. Four-point loading test method with a 500-mm pure bending span was applied. Strain gauges were glued to the AFRP rod/sheet at intervals of 100 mm to measure the strain distribution within the AFRP rod/sheet during the whole loading procedure. In these experiments, surcharged load, mid-span deflection (hereinafter, deflection), and strain distribution of AFRP rod/sheet were measured and recorded continuously using digital data recorders. At the commencement of experiment, mechanical properties of concrete and rebar used in the experiments were as follows: compressive strength of concrete f c = 34.3 MPa and yielding stress of axial rebar f y = 362 MPa. Mechanical properties of AFRP rods and sheet are listed in Tables 2 and 3, in which the values for AFRP rods are nominal values of the pre-cured ones and those of AFRP sheet are tensile test results. Table 1 RC beams Specimen Reinforcing material Reinforcing volume Axial stiffness EA (MN) R-1 RA5 two rods (φ5.0 mm) 2.45 R-2 RA7 two rods (φ7.3 mm) 5.25 R-3 RA9 two rods (φ9.0 mm) 7.88 S-1 A200 width: 136 mm 2.46 S-2 A415 width: 140 mm 5.25 S-3 A415+A200 width: 142 mm 7.89 Figure 1 RC beam configurations Figure 2 Lower surface of reinforced RC beams and location of strain gauge 338

3 Table 2 Material properties of pre-cured AFRP rods (nominal value) AFRP rod Diameter (mm) Area of cross section A (mm 2 ) E-modulus (GPa) Tensile strength σ f (GPa) Strain limit (%) RA RA RA Table 3 Material properties of AFRP sheet (nominal value) AFRP sheet Mass (g/m 2 ) Thickness t (mm) E-modulus (GPa) Tensile strength σ f (GPa) Strain limit (%) A A EXPERIMENTAL RESULTS AND DISCUSSIONS Non-Dimensional Load- Deflection Curves Figures from 3(a) to 3(c) show experimental and analytical non-dimensional load-deflection curves for three different cases of axial stiffness EA of reinforcing material. In those figures, load and deflection are normalized with reference to each experimental and/or analytical load P ye, P yc and deflection δ ye, δ yc at the loading point of main rebar yielding, respectively, to compare the reinforcing effects of FRP material in the region over the rebar yield point based on the same scale among experimental and analytical results. Those analytical results were estimated by using multi-section method. Load and deflection at main rebar yielding, non-dimensional maximum load and deflection for each analytical and experimental results, non-dimensional load and deflection with reference to analytical results, and failure mode are listed in Table 4. From Fig. 3, it is observed that if the axial stiffness EA of reinforcing material is similar between both beams R and S, non-dimensional load-deflection curves for those beams are in good corresponding to each other from beginning to ultimate state. Then, if axial stiffness EA of NSM AFRP rods is similar to that of AFRP sheet, the similar flexural reinforcing effects of NSM AFRP rods with those of AFRP sheet bonded to tension-side surface can be expected. Even though non-dimensional load-deflection curves for beams S after main rebar yielding have a tendency to a little underestimate those of analytical results, the curves obtained from experimental results are generally better corresponding to analytical ones in spite of the type of reinforcing material. Then, it is supposed that a better bonding capacity can be obtained in cases of both AFRP rod and sheet from the beginning of loading to near ultimate state of the RC beams. From Table 4, comparing the non-dimensional maximum load and deflection of the experimental results with those of analytical ones, it is seen that the experimental results for all RC beams considered here are smaller than the analytical ones which were obtained by means of multi-section methods considering stress-strain relation for each material. Every RC beam reaches ultimate state with AFRP rod/sheet debonding failure mode. Based on the empirical equations for predicting the failure type of RC beams reinforced in flexural with bonding FRP sheet on tension-side surface developed by Kishi et al. (2002), all beams R and S are judged as reaching Figure 3 Comparison of non-dimensional load-deflection curves among experimental and analytical results 339

4 Table 4 Experimental and analytical results Beam At rebar yield point At maximum loading point (normalized) Load Deflection Load Deflection Exp. Ana. Exp. Ana. Exp. Ana. Exp. Ana. P ye P yc δ ye δ yc P ue /P ye P uc /P yc (i)/(ii) δ ue /δ ye δ uc /δ yc (iii)/(iv) (kn) (kn) (mm) (mm) (i) (ii) (iii) (iv) R R R S S S Experimental behavior at ultimate Rods Rods Rods Sheet broken Sheet Sheet ultimate state with rod/sheet debonding failure mode. It implies that the empirical equations may be applicable for RC beams reinforced with NSM AFRP rods. The equation for the lower bound of the sheet debonding failure mode is as follows: L yu /d = 0.35 r s (1) Where L yu is main rebar yield length in the equi-shear span at ultimate state estimated by using multi-section method, d is effective height of the cross section of RC beam and r s is shear span ratio which is obtained dividing shear span length a by the effective height d. Strain Distribution of AFRP Rod/Sheet Figure 4 compares analytical and experimental strain distributions of AFRP rod/sheet at three loading points in the region over the rebar yield point up to ultimate state: rebar yield point (i), midway point (ii), and measured maximum loading point (iii). In those figures, analytical results are drawn symmetrically with respect to the mid-span point and experimental results are illustrated using the strains measured over the entire length of the span. Since non-dimensional deflection δ max /δ y at maximum load point is different between two beams R and S, here the strain distributions at smaller non-dimensional deflection between them are illustrated. From those figures, it is observed that at rebar yield point (i), experimental and analytical results for three different levels of axial stiffness EA of reinforcing materials are in good corresponding among them and AFRP rod/sheet have still near perfectly bonded to concrete. From the experimental results at midway point (ii), even though bigger strains occurred locally in the equi-bending span due to flexural cracks, both experimental results are in better corresponding to the analytical ones. However, in the equi-shear span, experimental results in main rebar yield area have a tendency to be larger than the analytical ones, and this tendency is specially remarkable for the RC beams with larger axial stiffness EA of reinforcing materials. This implies that since critical diagonal crack (CDC) occurred in the lower concrete cover of the main rebar yield area, a peeling action of the CDC is remarkable due to pushing AFRP rod/sheet downward by increasing of shear force. From the strain distributions at the maximum loading point (iii), it is observed that bigger strain distribution comparing with the analytical one is spread toward the supporting point corresponding to the expansion of rebar yield area. The strain distribution in the equi-shear span obtained from beams R has a tendency to be larger than those from beams S. This implies that in cases reinforcing with NSM AFRP rods, all forces for peeling action of CDC have applied to only two AFRP rods locally. Figure 5 shows an appearance of development of CDC at the beginning of AFRP rod/sheet debonding for beams R/S-3. From those figures, it is observed that AFRP rods/sheet is pressed downward due to the peeling action of CDC developed in the lower concrete cover of the equi-shear span and reinforcing materials have a tendency to be. 340

5 Figure 4 Comparison of axial strain distribution of AFRP rod/sheet among experimental and analytical results Figure 5 Cracking of beam at the time of rod/sheet debonding for beams R/S-3 Therefore, it is seen that NSM AFRP rods will be peeled-off due to the tip of CDC pressing downward as well as in case of RC beams reinforced with bonding FRP sheet on the tension-side surface. This implies that the failure mode of the RC beams reinforced with NSM AFRP rods may be predicted by using an empirical equation as proposed for the RC beams reinforced with bonding FRP sheet on the tension-side surface. Figure 6 shows crack distributions of each RC beam after experiment. From this figure, in case of beams R, it is observed that the flexural cracks and splitting cracks are developed in the lower concrete cover of equi-bending span, but the concrete cover has not been still spalled yet. In the equi-shear span, since the formation of CDCs are observed, it is reconfirmed that the NSM AFRP rods have been due to the peeling action of CDC and the RC beams have reached ultimate state with AFRP rods debonding. On the other hand, in case of beams S, it is observed that in equi-bending span, flexural cracks and splitting cracks along the main rebar are developed. Since CDCs have been also developed in the lower concrete cover of the equi-shear span near loading point, it is supposed that AFRP sheet has been fully due to the peeling action of the CDC. However, since beam S-1 has reached ultimate state due to sheet rupture, the lower concrete cover has not been severely suffered from the damage. Figure 7 shows the damage conditions of the lower surface of beam R-2 after experiment, which has reached ultimate state with AFRP rods debonding. From this figure, it is observed that the concrete around the NSM AFRP rods has been spalled out corresponding to debonding of AFRP rods. Therefore, bonding strength between NSM AFRP rod and concrete may be similar or higher than the tensile strength of concrete. 341

6 Figure 6 Crack pattern for beams R/S after experiment Figure 7 Lower surface of beam R-2 after experiment CONCLUSIONS In this paper, in order to overcome the drawbacks of reinforcing methods for enhancing the flexural loadcarrying capacity of existing RC members such as FRP sheet bonding method and/or overlaying method with concrete, by proposing the use of near surface mounting method with FRP rods, the enhancing effects of flexural load-carrying capacity and debonding behavior of the FRP rods were experimentally investigated comparing with those for the RC beams reinforced with bonding FRP sheet on the tension-side surface. Here, AFRP rods and sheet were used for flexural reinforcing of the RC beams. Results obtained from this study are as follows: 1) If axial stiffness EA of NSM AFRP rods is similar with that of AFRP sheet, flexural reinforcing effects of the NSM AFRP rods may be similar with those of bonding FRP sheet; 2) RC beams reinforced in flexural with NSM AFRP rods reached ultimate state accompanying with the AFRP rods being due to a peeling action of critical diagonal crack (CDC) developed in the lower concrete cover of the equi-shear span near loading point as well as in the case reinforcing with bonding AFRP sheet; and 3) Failure mode of the RC beams reinforced with NSM AFRP rods can be predicted by using an empirical equation developed for that of the RC beams reinforced with bonding FRP sheet. REFERENCES JSCE: Standard Specifications for Concrete Structures-2002 [Structural Performance Verification], (2002) (in Japanese) Kishi, N., Mikami, H. and Kurihashi, Y. (2002). Experimental study for investigation of load-carrying behavior and prediction of failure mode of RC beams reinforced in flexure with bonding FRP sheet, Journal of materials, concrete structures and pavements, JSCE, 771(V-56), (in Japanese). Lorenzis, L. D., Nanni, A. (2001a). Characterization of FRP rods as near-surface mounted reinforcement, Journal of Composite for Construction, ASCE, 5(2), Lorenzis, L. D., Nanni, A. (2001b). Shear strengthening of reinforced concrete beams with near-surface mounted fiber-reinforced polymer rods, ACI Structural Journal, 98(1), Micelli, F. and Tegola, A.L, and Myers, J.J. (2003). Environmental effects on RC beams with near surface mounted FRP rods, Proceedings of FRPRCS-6, Singapore, 8-10 July, Nordin, H. and Taljsten, B. (2003). Concrete beams strengthened with pre-stressed near surface mounted reinforcement, Proceedings of FRPRCS-6, Singapore, 8-10 July,