BASIC CHARACTERISTICS OF FRP STRAND SHEETS AND FLEXURAL BEHAVIOR OF RC BEAMS STRENGTHENED WITH FRP STRAND SHEETS

Transcription

1 BASIC CHARACTERISTICS OF FRP STRAND SHEETS AND FLEXURAL BEHAVIOR OF RC BEAMS STRENGTHENED WITH FRP STRAND SHEETS A. Kobayashi 1, Y. Sato 2 and Y. Takahashi 3 1 Technical Development Department, Nippon Steel Composite Co., Ltd., JAPAN, a-kobayashi@nick.co.jp 2 Division of Built Environment, Hokkaido University, JAPAN 3 Department of Civil & Environmental Engineering, Hokkai-Gakuen University, JAPAN ABSTRACT When attaching continuous fiber sheets on concrete members on site, bonding defection of the continuous fiber sheets likely happens. FRP strips could solve the problem associated with the continuous fiber sheets, though they also have demerit due to less adhesion property caused by small adhesive area. To solve these problems, we have developed advanced FRP strand sheets which consist of bunch of individually hardened continuous fiber strands. The FRP strand sheets can be easily attached on the surface of concrete members with a single adhesive. Thus, the advanced strand sheets assure better construction quality and enable to cut down on application time and/or costs. Series of tensile, lap splice, and bonding tests were conducted in order to evaluate the material characteristics of CFRP strand sheets. Bending test of RC (Reinforced Concrete) beams was also conducted to evaluate strengthening effect of the CFRP strand sheets. In each test, specimens strengthened with CFRP strand sheets, with ordinal carbon fiber sheets or with CFRP strips were prepared. In the bending test, it was observed that both bending stiffness and capacity of RC beams were increased by strengthening with CFRP strand sheets. It was also observed that ultimate load, which was determined by delamination of strengthening material; of specimen strengthened with CFRP strand sheets was the greatest among all the specimens. Furthermore, the influence of concrete strength and depth of cover concrete on flexural strengthening effect of the FRP strand sheets were investigated. KEYWORDS FRP strand sheets, Continuous fiber sheets, CFRP strip, Overlap splice strength, Flexural strengthening INTRODUCTION As reinforcing methods for concrete structures, continuous fiber sheet bonding method and CFRP strips bonding method are widely put in practice. The continuous fiber sheet method is described as that the dry fiber sheets are impregnated with room temperature curing resin in order to form the fiber reinforced polymer (FRP), and bonded on concrete surface at the same time. However, continuous fiber sheet bonding method has following problems that are necessary to be improved. Since the process of forming fiber sheets into FRP is conducted at the work site, defects due to insufficient resin matrix impregnation might easily occur. Because of the restricted resin impregnation, it is difficult to produce a thick sheet; rather it is necessary to lay multi-sheets. Voids are easily generated in between the sheet and the concrete surface, or in between the sheet layers. Before applying the sheets, it is necessary to put adhesive paste such as epoxy putty on the concrete surface and make the surface smooth and hardened. In other words, the work needs multi tasks. Although continuous fiber sheet bonding method has a high reinforcing effect, it leaves some problems that are necessary to be solved when the quality, application period, cost, and ease of handling are considered. On the other hand, CFRP strip bonding method uses 1 to 2mm thick and to mm width Pultruted FRP strip, which is bonded on the concrete surface with adhesive in the form of putty. Since all the FRP strips are manufactured in the factory, its quality is very stable. The process of impregnating the fiber sheet in certain resin can be skipped, and in turn, it makes the bonding work easy and work period is shortened. However, it has a disadvantage as well. Compared with the continuous fiber sheet bonding method, since the thick FRP is partly 93

2 bonded on the concrete surface, bonding area is extremely smaller than when the continuous fiber sheet bonding method is applied. Because of this disadvantage, bonding shear stress is concentrated on the small bonding area, and FRP strips show debonding from its tips. Therefore, CFRP strip method has less reinforcing effect than that of the continuous fiber sheet bonding method. Furthermore, since the FRP strip is thick itself, it is unable to form layers by overlap joint, and it shows debonding before the FRP strip reaches its tensile strength. Due to the restriction that overlap splice cannot be applied with this method, it is necessary to produce long (continuous) FRP strips to cover the area that need to be reinforced. So this method is very difficult to be applied on long span structures. To solve these problems, we have developed advanced FRP strand sheets which consist of bunch of individually hardened continuous fiber strands. MATERIAL PROPERTIES OF FRP STRAND SHEETS Feature of FRP strand sheets In order to overcome with the problems of both the continuous fiber sheet bonding method and the CFRP strip bonding method, the strand sheet bonding method has been developed. As you can see in Figure 1, the strand sheet is made out of fine CFRP strands which are individually impregnated with resin and hardened. Hundreds of these strands are arranged horizontally in 1m width and woven with thread to make it into a sheet form. The strand sheet bonding method is the reinforcing method on concrete members by bonding the strand sheets on the concrete surface with epoxy resin in a form of paste. The strand sheet bonding method has following features; Since the quality of the FRP strands are maintained stably at the factory and are already hardened with resin, no impregnation process is needed at the work site. Thick sheet with high fiber mass per unit area can be produced. Since there is enough space in between strands, it easily lets the bubbles in between concrete surface and the strand sheets out, application failures will be decreased dramatically. Unlikely to the CFRP strip bonding method, because the thin layer of reinforcing sheet is bonded on the reinforcing area of the concrete element, the same reinforcing effect as the continuous fiber bonding method can be obtained. Overlap splice can be applied just as continuous fiber sheet bonding method, so that it is able to translate the stress between the separate strand sheets through the joint. Figure 1 FRP strand sheet Tensile Strength, Overlap Splice Strength and Bond Strength of FRP Strand Sheets Four types of strand sheets were made as shown in Table 1. HT and AK type were made from high tensile strength carbon fiber and aramid fiber respectively. MA and MB type were both made from high modulus carbon fiber, which had higher tensile stiffness compared with conventional continuous fiber sheet because of those large fiber weights per unit area. Tensile test and overlap splice test were conducted according to test methods for continuous fiber sheet of JSCE (Japan society of civil engineers) standards. mm length of overlap splice portion was employed for each overlap splice specimens. Five specimens for each type of strand sheets were tested. Results of tensile test and overlap splice test were shown in Table 1. Measured minimum tensile strength of each types of strand sheets were higher than the characteristic values of tensile strength of continuous fiber sheet made from same types of continuous fiber (namely 3400MPa for HT, 1900MPa for MA and MB, 2060MPa for AK). On overlap splice 94

3 test, all CFRP strand sheets specimens were broken with tensile rupture of CFRP strand sheet; however AFRP strand sheets (AK) specimens were broken at overlap splice portion with peeling off mode. Bonding test in which CFRP strand sheets and ordinary carbon fiber sheets were bonded to concrete prism specimens were carried out and the interfacial fracture energy of CFRP strand sheets bonded to concrete were investigated (see Figure 2). In these test, CFRP strand sheets and carbon fiber sheet had almost the same Young s modulus, design thickness, and width. Average interfacial fracture energy of CFRP strand sheets bonded to concrete was 0.94 MPa/mm and was 10% higher than 0.84 MPa/mm in those of ordinary carbon fiber sheets. Table 1 Material properties of FRP strand sheets. Type HT MA MB AK Continuous fiber High Tensile High Modulus High Modulus Aramid Fiber Carbon Fiber Carbon Fiber Carbon Fiber Density of fiber g/cm Fiber weight g/m Design thickness mm Tensile strength(av.) MPa 4,340 2,520 2,400 2,7 Tensile strength(min) MPa 4,009 2,269 2,035 2,683 Young s modulus GPa Overlap splice strength (failure mode) MPa 4,160 Tensile rupture 2,410 Tensile rupture 1,980 Tensile rupture 19 Overlap splice failure Figure 2. Bonding test of FRP strand sheet BENDING TESTS OF CONCRETE BEAMS STRENGTHENED WITH FRP STRAND SHEETS Outline of Bending Tests In order to evaluate strengthening effect of FRP strand sheets, bending test of RC (Reinforced Concrete) beams was also conducted. In each test, specimens strengthened with CFRP strand sheets, with ordinary carbon fiber sheets, and with CFRP strips were prepared. Beam sketches, dimensions and details of reinforcement are shown in Fig.2. Each test specimen was simply supported on a span of 1600 mm and subjected monotonically to two concentrated loads up to failure at increments of 5 kn. Four types of FRP strand sheets were used and those material properties were indicated in Table 1. Material properties of the carbon fiber sheet and CFRP strip are shown in table 2. Also note that the carbon fiber sheet and the CFRP strips had almost the same tensile stiffness and capacity as those of the HT type FRP strand sheet. 95

4 Parameters of bending tests were various type of FRP reinforcements, number of ply, with or without primer coating, absence/presence of overlap splice, strength of concrete and depth of cover concrete as shown in table 3. Table 2 Material properties of carbon fiver sheet and CFRP strip Type Carbon fiber sheet CFRP strip Continuous fiber High Tensile Carbon Fiber High Tensile Carbon Fiber Fiber weight g/m Design thickness mm Tensile strength(av.) MPa 4,190 2,707 Young s modulus GPa Width mm Type of FRP Table 3 List of specimen for beam bending test and results Primer Overlap S f F c P yex P yca P yex / coating splice Numbe r of ply No (kn) (kn) (kn) P yca (kn) N A01 HT 1 Yes No 17, A02 HT 1 No No 17, AL5 HT 1 No No 17, AL10 * HT 1 No No 17, A03 HT 1 Yes Yes 17, A04 MA 1 No No 38, A05 MB 1 No No 55, A06 HT 2 No No 34, A07 HT 2 No Yes 34, A08 MA 2 No Yes 76, A09 AK 1 No No 12, CF1 CF 1 Yes No 16, PL1 Strip 1 Yes No 17, PL2 Strip 2 Yes No 34, Sf: Tensile stifness of FRP sheet, Pyex:Experimental yilding load, Pyca:Calculated yielding load *: Low concrete strength and mm in depth of cover concrete P max 7@=700 7@=700 D10 D13 3 FRP sheets or strips 10 D FRP sheets w= FRP sheets w= CFRP strips w= w: depth of cover concrete unit: mm Figure 3 Specimen of bending test Result and Discussion The maximum loads and yielding loads of specimens strengthened with FRP strand sheets were significantly higher than specimen N0 without FRP sheet, as shown in Table 3. The relationships between load and deflection at center of the span for specimens strengthened with FRP strand sheets are indicated in Fig. 4. The relationship between load and deflection at center of the span for specimens strengthened with various kind of FRP reinforcements are indicated in Figure 5. Observed yielding loads were as higher as the tensile stiffness of FRP sheets. Where, experimental yielding loads P yex were determined by observed strain of main rebar at center of the span. The calculated yielding loads P yca were evaluated using the same method as for reinforced concrete 96

5 3 N0 A04 A06 A09 A02 A05 A08 3 N0 A06 PL1 A02 CF1 PL2 load (kn) 1 load (kn) deflection at center (mm) deflection at center (mm) Figure 4 Relationships between load and deflection at center for FRP strand sheets Figure 5 Relationships between load and deflection at center for various kind of FRP upper edge hight of section (mm) 1 calcurated neutral axis lower edge strain (μ) load kn Xcal rebar FRP sheet Figure 6 Strain distributions in vertical direction at center members, in other words, the fiber strain of FRP strand sheet is assumed to be proportional to the distance from the neutral axis of the section. For example, observed strain distributions in vertical direction at the center of the span for specimen A01 were almost liner before main rebar yielding as shown in Figure 6. The ratios of calculated and experimental yielding load (P yex /P yca ) for specimens strengthened with strand sheets (from A01 to A09) were in the range of 1.01 to 1.19, and the experimental values were nearly equal to calculated values. Therefore, it is considered that the yielding load of reinforced concrete beams strengthened with FRP strand sheet can be estimated using the same method as for reinforced concrete members, if peeling failure of FRP strand sheet does not occur before yielding. All specimens strengthened with FRP sheets except A4 specimen strengthened with MA type strand sheet and with CFRP strips were broken with peeling off mode. Because ultimate strain of MA type strand sheets made from ultra high modulus carbon fiber is small, A4 specimen strengthened with MA type strand sheet was broken with tensile rupture of strand sheet before peeling off. A01, A02 and A03 specimens had different details of strengthening, namely A01 was strengthened with primer coating, A02 was strengthened without primer coating and A03 was strengthened with overlap splice at the center of bending span. The maximum loads of specimens A01, A02 and A03 which had been strengthened with 1 layer of HT type CFRP strand sheet were almost same about 280 kn. These details were not so effective for maximum loads of strengthened beams with FRP strand sheets. The maximum loads of specimen AL5 and AL10 specimens which had lower concrete strength than A2 specimen with the same reinforcing details were also almost the same about 280 kn. Therefore, it is considered that the strand sheet bonding method is effective for concrete beams which had relatively low concrete strength. The maximum loads of these specimens strengthened with 1 layer of HT type strand sheet were higher than that of 255kN for specimen CF1 strengthened with ordinary carbon fiber sheet having same tensile stiffness as CFRP 97

6 strand sheets. As one of the reasons for this difference, it is considered that interfacial facture energy between FRP strand sheets and concrete surface is 10% higher than that of ordinary carbon fiber sheets. As shown in figure 5, the maximum load and deflection at where peeling off of FRP reinforcements occurs for specimen PL1 was smaller than those of specimens A02 and CF1 which had been strengthened with almost the same tensile stiffness and tensile capacity of FRP reinforcement. Those of specimen PL2 was also smaller than specimen A06 which had been strengthened with almost the same tensile stiffness and tensile capacity of FRP reinforcement. Compared with the strand sheet bonding method and/or the continuous fiber sheet bonding method, since the thick FRP is partly bonded on the concrete surface, bonding area is extremely smaller than when the strand sheet bonding method and/or the continuous fiber sheet bonding method is applied. Because of this disadvantage, bonding shear stress is concentrated on the small bonding area, and FRP strips show debonding from its tips. Therefore, it is considered that the CFRP strip method showed less reinforcing effect than the strand sheet bonding method and/or the continuous fiber sheet bonding method. CONCLUSIONS Series of tensile, lap splice, and bonding tests were conducted in order to evaluate the material characteristics of FRP strand sheets. Bending test of RC beams was also conducted to evaluate strengthening effect of the CFRP strand sheets. The following conclusions were deduced: Measured minimum tensile strength of each types of strand sheets were higher than characteristic values of tensile strength of continuous fiber sheet made from the same types of continuous fiber. On overlap splice test, all CFRP strand sheets specimens were broken with tensile rupture of the CFRP strand sheet. FRP strand sheets were effective for improving yielding and maximum load of reinforced concrete beams subjected to bending moment. It is considered that if peeling failure of FRP strand sheet does not occur before yielding of tension steel bars, the yielding load of reinforced concrete beams strengthened with FRP strand sheet can be estimated using the same method as for reinforced concrete members. REFERENCES Kobayashi, A., Tateishi, A., Sato, Y. and Takahashi, Y.(9). Study on basic characteristics of FRP strand sheets and its flexural strengthening effect for RC beams, Proceedings of the 9 th International Symposium on Fiber Reinforced Polymer Reinforcement for Concrete Structures, CD-ROM Takahashi, Y., Sato, Y. and Kobayashi, A. (9). Study on flexural capacity of RC beams reinforced with CFRP sheet, CFRP plate and CFRP strand sheet, Proceedings of the 9 th International Symposium on Fiber Reinforced Polymer Reinforcement for Concrete Structures, CD-ROM 98