Experimental study on flexural performance of ECC beams reinforced with CFRP bars
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摘要: 为研究碳纤维增强树脂复合材料(Carbon fiber reinforced polymer,CFRP)筋/超高韧性纤维增强水泥基复合材料(Engineered cementitious composite,ECC)梁的抗弯性能,对3根CFRP筋/ECC梁、1根玻璃纤维增强树脂复合材料(Glass fiber reinforced polymer,GFRP)筋/梁和1根CFRP筋混凝土梁进行了四点弯曲试验,分析了配筋率、纤维增强树脂复合材料(Fiber reinforced polymer,FRP)筋类型和基体类型对梁抗弯性能的影响。试验结果表明:CFRP筋/ECC梁与GFRP筋/ECC梁和CFRP筋混凝土梁类似,均经历了弹性阶段、带裂缝工作阶段和破坏阶段;配筋率对CFRP筋/ECC梁的受弯性能影响较大。随着配筋率的增加,CFRP筋/ECC梁的承载能力不断提高,延性性能逐渐减弱;ECC材料优异的应变硬化能力和受压延性,使得CFRP筋/ECC梁的极限承载能力和变形能力均优于CFRP筋混凝土梁;由于ECC材料多裂缝开裂能力,CFRP筋/ECC梁开裂后,纵筋表面应变分布比CFRP筋混凝土梁更均匀; 由于聚乙烯醇(Polyvinyl alcohol,PVA)纤维的桥联作用,CFRP筋/ECC梁破坏时,其表面出现了大量的细密裂缝,且能保持较好的完整性和自复位能力;正常使用阶段,CFRP筋/ECC梁的最大弯曲裂缝宽度均小于CFRP筋混凝土梁。最后,根据试验结果,建立了基于等效应力图的CFRP筋/ECC梁弯曲承载力简化计算模型,确定模型中的相关系数。由简化模型计算的极限承载力与试验结果具有较好的相关性。Abstract: To study the flexural performance of engineered cementitious composite (ECC) beams reinforced with carbon fiber reinforced polymer (CFRP) bars, four-point flexural experimental investigates were carried out on three ECC beams reinforced with CFRP bars, one ECC beam reinforced with glass fiber reinforced polymer (GFRP) bars and one concrete beam reinforced with CFRP bars. The main parameters were the reinforcement ratios, the reinforcement type and the matrix type. The experimental results show that the load-deflection curves of ECC beam reinforced with CFRP bars are similar with the ECC beam reinforced with GFRP bars and concrete beam reinforced with CFRP bars, which have an elastic stage, a working stage with cracks and a failure stage. The reinforcement ratio has a great influence on the flexural performance of ECC beams reinforced with CFRP bars. With the increase of reinforcement ratio, the ultimate bearing capacity of ECC beams is improved, and the ductility performance is gradually weakened. The excellent strain-hardening ability and ductility of ECC materials make the ultimate bearing capacity and deformation of ECC beams with CFRP bars superior to the concrete beam reinforced with CFRP bars. Based on the multi-cracking ability of ECC, the strain distribution on the surface of longitudinal bars is more uniform than that of concrete beams with CFRP bars after cracking. Due to the bridging effect of polyvinyl alcohol (PVA) fiber, a large number of fine cracks appear on the surface of ECC beams reinforced with CFRP bars. When ECC beams reinforced with CFRP bars fail, it could maintain good integrity and self-recovering ability. In service stage, the maximum crack width of reinforced ECC beams presents smaller than that of concrete beams. Finally, a simplified calculation model for ultimate bearing capacity of ECC beams reinforced with fiber reinforced polymer (FRP) rebars is proposed, predicting good agreement with the experimental results.
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Key words:
- CFRP bars /
- ECC beam /
- flexural performance /
- ultimate bearing capacity /
- crack width
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图 9 ECC材料单轴应力-应变曲线
fcr—Cracking strength; ftu—Tensile strength; εcr—Cracking strain; εtu—Ultimate tensile strain; σ0—Elastic compressive strength; ε0—Elastic compressive strain; εp—Strain corresponding to compressive strength; εc—Ultimate compressive strain; σc—Residual compressive strength; fcu—Uniaxial compressive strength
Figure 9. Uniaxial stress-strain curves of ECC
图 10 破坏阶段梁截面上应力和应变分布
Figure 10. Stress and strain distribution of the tested beams at failure stage
h, b—Height and width of the tesed ECC beams; εcs—Ultimate compressive strain of ECC; εcp— Compressive strain of ECC corresponding to the ultimate compressive strength; ε0.4—Elastic compressive strain of ECC; εf—Tensile strain of FRP rebar; εt—Tensile strain of ECC; εtc—Cracking strain of ECC; φ—Curvature of the beams; c—Height of the actual compression zone; σ0.4—Elastic ultimate strength of ECC under uniaxial compression; σcp—Uniaxial compressive strength of ECC; σcs—Residual compressive strength of ECC when reaches the ultimate compressive strain; σtc—Cracking strength ECC under uniaxial tension; σt—Tensile strength ECC; σf—Tensile stress of FRP rebars; Af—Cross-sectional area of FRP rebars; h0—Effective height of beam section; af—Distance from the resultant force point of FRP bars to the tensile edge of the beam; k1, k2, k3, k4, k5—Related material coefficients
表 1 梁试件主要参数
Table 1. Detailed parameters of tested beams
Sample Dimension of the
beam/mm3FRP rebar Matrix type Longitudinal
barReinforcement
ratio/%Stirrup Supplementary
reinforcement3CFRP(6)/ECC 120×160×2000 CFRP ECC 3Φ6 0.54 Φ8@80 2Φ8 2CFRP(10)/ECC 120×160×2000 CFRP ECC 2Φ10 1.01 Φ8@80 2Φ8 3CFRP(13)/ECC 120×160×2000 CFRP ECC 3Φ13 2.55 Φ8@80 2Φ8 3GFRP(10)/ECC 120×160×2000 GFRP ECC 3Φ10 1.51 Φ8@80 2Φ8 3CFRP(10)/PC 120×160×2000 CFRP Concrete 3Φ10 1.51 Φ8@80 2Φ8 Notes: yCFRP(x)—Number of FRP rebars designed in the beam is y, x means the diameter of used FRP rebars; PC—Plain concrete; CFRP—Carbon fiber reinforced polymer; GFRP—Glass fiber reinforced polymer. 表 2 筋材基本力学参数
Table 2. Mechanical parameters of FRP rebars
FRP D
/mmffu
/MPaEf
/GPaεfu
/%CFRP 6 2700.0 163.6 1.65 10 2436.0 143.3 1.70 13 2001.1 132.5 2.00 GFRP 10 1080.0 50.0 2.00 Notes: D—Diameter of FRP rebar; ffu—Tensile strength of FRP rebar; Ef—Elastic modulus of FRP rebar; εfu—Ultimate tensile strain of FRP rebar. 表 3 ECC配合比
Table 3. Mix proportion of ECC
Cement
+Fly ashSilica sand Water Fiber Super-
plasticizer1 0.2 0.28 0.009 0.006 表 4 聚乙烯醇(PVA)纤维的材料性能
Table 4. Material properties of polyvinyl alcohol (PVA) fiber
L
/mmdf
/µmffiber
/MPaδf
/%Efiber
/GPaρ
/(g·cm−3)12 39 1620 7 42.8 1.6 Notes: df—Diameter of PVA fiber; L—Length of PVA fiber; ρ—Density; ffiber—Tensile strength of fiber; Efiber—Elastic modulus of fiber; δf—Elongation of PVA fiber. 表 5 FRP筋增强ECC梁和FRP筋/混凝土梁试验结果
Table 5. Test results of ECC beam reinforced with FRP bars and concrete beam reinforced with FRP bars
Sample Mcr
/(kN·m)Mu
/(kN·m)Δcr
/mmΔu
/mmFailure mode 3CFRP(6)/ECC 1.09 19.67 0.29 38.40 ECC crushed 2CFRP(10)/ECC 1.13 22.83 0.36 33.28 ECC crushed 3CFRP(13)/ECC 1.14 31.41 0.37 28.73 ECC crushed 3GFRP(10)/ECC 1.11 20.76 0.44 45.31 ECC crushed 3CFRP(10)/PC 1.42 21.92 0.23 24.98 Concrete crushed Notes: Mcr—Crack moment; Mu—Ultimate moment; Δcr—Crack deflection at mid-span; Δu—Ultimate deflection at mid span. 表 6 挠度为l0/200时各梁跨中位置CFRP筋应变
Table 6. Strain of CFRP rebar at mid-span of the tested beam with deflection l0/200
Sample Strain of CFRP rebar at mid-span/10−6 3CFRP(6)/ECC 3065 2CFRP(10)/ECC 3066 3CFRP(13)/ECC 2349 3GFRP(10)/ECC 2660 3CFRP(10)/PC 3200 表 7 FRP筋/ECC梁受压区高度计算结果
Table 7. Calculation results of the actual compression zone height of ECC beams reinforced with FRP bars
Sample Reinforce-
ment ratio/%c
/
mmcb
/
mmFailure
mode3CFRP(6)/ECC 0.54 45.85 37.36 ECC crushed 2CFRP(10)/ECC 1.01 53.70 36.57 ECC crushed 3CFRP(13)/ECC 2.55 70.05 32.45 ECC crushed 3GFRP(10)/ECC 1.51 44.50 32.45 ECC crushed Note: cb—Actual compression zone height at boundary failure. 表 8 FRP筋/ECC梁等效矩形应力图系数
Table 8. Equivalent rectangular stress coefficients of FRP rebar reinforced ECC beams
Sample Reinforcement ratio/% α1 β1 α2 β2 3GFRP(10)/ECC 1.51 0.768 0.853 0.828 0.925 3CFRP(6)/ECC 0.54 0.768 0.853 0.820 0.927 2CFRP(10)/ECC 1.01 0.768 0.853 0.784 0.938 3CFRP(13)/ECC 2.55 0.768 0.853 0.735 0.954 Notes: β1—Ratio of compression zone height to neutral axis height of ECC; β2—Ratio of calculation height to actual height of tension zone of ECC. 表 9 FRP筋/ECC梁极限承载力试验值与计算值对比
Table 9. Comparison of experimental and calculated ultimate bearing capacity of FRP bars reinforced ECC beams
Sample Mu,exp/(kN·m) Mu,cal/(kN·m) Mu,cal/Mu,exp 3CFRP(6)/ECC 19.67 19.88 1.011 2CFRP(10)/ECC 22.83 21.47 0.940 3CFRP(13)/ECC 31.41 28.30 0.919 3GFRP(10)/ECC 20.76 20.06 0.966 Notes: Mu,exp—Test value of ultimate bending moment; Mu,cal—Calculated value of ultimate bending moment. -
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