Bearing capacity of reinforced concrete columns strengthened by engineered cementitious composite under small eccentric compression load
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摘要: 通过对工程水泥基复合材料(ECC)加固钢筋混凝土(RC)柱和未加固RC柱进行小偏心受压试验，研究ECC加固RC柱小偏心受压性能。试验结果表明，ECC加固层能有效约束核心混凝土；与未加固柱相比，加固柱的裂缝细而密，达到峰值荷载时受压区ECC尚未被压碎，破坏过程比较平缓，有较好的完整性，并表现出一定的延性特征；相对偏心距相同时，加固柱的开裂荷载、峰值荷载及延性相比未加固柱分别提高了107%~236%、45%~159%、37.4%~41.3%。依据试验结果，绘制出各加固柱跨中荷载-挠度曲线，可分为4个阶段：弹性阶段、裂缝稳定扩展阶段、最大荷载阶段及下降段。随着加固层厚度的增大，相同荷载下ECC竖向应变及钢筋应变越小；随着相对偏心距的增大，相同荷载下ECC竖向应变及钢筋应变越大。基于混凝土结构理论及力学原理，分析ECC加固层对核心混凝土柱的约束机制，提出ECC约束混凝土抗压强度和峰值应变的表达式，推导出加固柱受压承载力计算公式，承载力计算值与试验值相对误差在10%以内，二者吻合良好，为ECC加固混凝土柱在实际工程中的应用提供理论参考。
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关键词:
- 水泥基复合材料(ECC) /
- 加固钢筋混凝土(RC)柱 /
- 小偏心受压 /
- 试验研究 /
- 承载力分析
Abstract: Small eccentric compression tests on engineered cementitious composite (ECC)-strengthened reinforced concrete (RC) columns and the unstrengthened column as reference were carried out to investigate the influence of the thickness of the ECC reinforcement layer on the compressive performance of strengthened RC columns. The test results show that the ECC reinforcement layer effectively restrains the core concrete, and strengthened columns exhibit obvious ductile failure pattern. Compared with the unstrengthened RC column, the cracks of the reinforced column are thin and dense. When the peak load is reached, the compression zone is not crushed, and the failure process is relatively gentle, with better integrity, exhibiting certain ductility characteristics; The cracking load, peak load and ductility are increased by 107%-236%, 45%-159% and 37.4%-41.3%. The mid-span load-deflection curves are drawn based on the test results, which could be divided into four stages: The elastic stage, the stable crack propagation stage, the maximum load stage and the descending stage. With the increase of reinforcement layer thickness, the strains of ECC and reinforcing bars are smaller under the same load; With the increase of relative eccentricity, the strains of ECC and reinforcing bars are larger under the same load. Based on the theory of concrete structure and mechanical principle, the action mechanism of ECC reinforced layer on the core column was analyzed and the expressions of compressive strength and peak strain of the ECC-strengthened RC columns were proposed. The calculation formula of the bearing capacity of the strengthened column was established and the relative error between the theoretical results and the experimental values is less than 10%, which is in good agreement with the test results. Hence, the established calculation method could provide theoretical support for the application of ECC strengthened concrete column in practical engineering. -
图 1 水泥基复合材料(ECC)加固钢筋混凝土(RC)柱基本尺寸及截面配筋详图
Figure 1. Basic dimensions and section reinforcement details of reinforced concrete (RC) columns strengthened by engineered cementitious composite (ECC)
H—Specimen height; b—Section width of the RC column; h —Section height of RC column;
图 4 ECC加固RC柱加载装置示意图
Figure 4. Schematic diagram of ECC reinforced RC column loading device
LVDT—Linear variable differential transformer
图 6 典型ECC加固RC柱裂缝发展分布及破坏图
Figure 6. Photograph of crack development and distribution diagram of typical RC columns strengthened by ECC
图 7 ECC加固RC柱荷载-跨中挠度曲线
Figure 7. Load-mid-span deflection curves of RC columns strengthened by ECC
图 8 ECC加固RC柱荷载-纵筋应变曲线
Figure 8. Load-longitudinal reinforcement strain curves of RC columns strengthened by ECC
图 9 ECC加固RC柱荷载-ECC(混凝土)竖向应变曲线
Figure 9. Load-ECC vertical strain curves of RC columns strengthened by ECC
图 10 ECC加固RC柱的延性指标定义
Figure 10. Definition of ductility index of ECC strengthened RC columns
$\varDelta _y $—Yield displacement; $\varDelta _u $—Ultimate displacement; μ—Ratio of the ultimate displacement to the yield displacement; Nmax—Maximum load
图 11 ECC加固RC柱沿跨中截面的应变分布
Figure 11. Strain distribution along the mid-span section of RC columns strengthened by ECC
图 12 ECC加固RC柱的开裂及峰值荷载与加固层厚度的关系曲线
Figure 12. Relationship between cracking and peak load of RC columns reinforced with ECC and the thickness of reinforcement layer
图 13 ECC加固RC柱荷载-ECC横向应变曲线
Figure 13. Load-ECC transverse strain curves of RC columns strengthened by ECC
图 14 ECC加固RC柱约束混凝土截面应力分析
Figure 14. Stress analysis of confined concrete section of RC column strengthened by ECC
$ f_{{\rm{j}}} $, $ f_{{\rm{k}}} $—Maximum lateral restraint force in the width and height directions of the section, respectively; $ f_{\text {et }} $—ECC horizontal tensile force of the reinforcement layer
图 15 ECC加固RC柱小偏心受压计算模型
Figure 15. Calculation model of ECC reinforced RC column under small eccentric compression
h0, h1,0—Distance between the tension side reinforcement and the compression side edge of the concrete column and the compression side edge of the reinforced column; $A_{{\rm{s}}}$, $A_{{\rm{s}}}^{\prime}$—Cross-section area of reinforcement on the tension side and compression side; e—Distance between the point of bearing capacity and the resultant point of longitudinal tension reinforcement; $ \sigma_{\mathrm{s}} $, $\sigma_{{\rm{s}}}^{\prime}$—Stress value of reinforcement on the tension side and compression side; εs—Elastic modulus of reinforcement; fcc—ECC axial compressive strength; x—Equivalent rectangular compression zone height; xc—Section actual compression zone height; εec—ECC strain on compression side; εcc—ECC confined peak compressive strain of concrete; ε's—Reinforcement strain on the compression side; b1, h1—Width and height of reinforced column section; N—Bearing capacity of reinforced column; α1—Equivalent rectangular stress diagram coefficient of compression zone; as, a's—Distance from the center of tension or compression bar to the tension or compression edge of concrete section
图 16 ECC加固RC柱界限破坏截面应变分布
Figure 16. Strain distribution of the balance failure section of ECC reinforced RC column
$ \varepsilon_{\mathrm{y}} $—Yield tensile strain of the steel bar; $x_{{\rm{cb}}}$—Actual height of the compression zone of the limit failure
表 1 ECC加固RC柱试件参数
Table 1. Test parameters of RC columns strengthened by ECC
Group b1×h1/mm×mm H/mm e0/mm t/mm RC 200×250 1200 0.22h1 0 ECC-RCA1 250×300 1200 0.22h1 25 ECC-RCA2 270×320 1250 0.22h1 35 ECC-RCA3 290×340 1350 0.22h1 45 ECC-RCB1 250×300 1200 0.3h1 25 Notes: h1—Section height of the reinforced column; b1—Section width of the reinforced column; e0—Eccentricity; t—Thickness of ECC reinforcement layer; A and B—Relative eccentricity of 0.22 and 0.3, respectively; 1, 2 and 3—Thickness of the reinforcement layer of 25 mm, 35 mm and 45 mm, respectively. 
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表 2 ECC质量比
Table 2. ECC mass ratio
Cement Sand Fly ash Micro-silicon powder Water Water reducer PVA fiber Thickener 1 0.4 3 0.073 1.02 0.04073 0.072 0.00182
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表 3 聚乙烯醇(PVA)纤维性能参数
Table 3. Performance index of polyvinyl alcohol (PVA) fiber
Diameter/μm Length/mm Tensile strength/MPa Modulus of elasticity/GPa Fracture elongation/% Density/(${\rm{g}}\cdot $cm−3) 40 12 1560 41 6.5 1.3
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表 4 ECC抗压和抗拉试验结果
Table 4. ECC compressive and tensile test results
Group Compressive
ultimate load/kNfe/MPa fet/MPa Ultimate
tensile strain/%ECC-RCA1 194.5 38.7 3.65 3.12 ECC-RCA2 187.7 37.5 3.63 3.19 ECC-RCA3 192.9 38.6 3.72 3.26 ECC-RCB1 181.8 36.4 3.70 3.18 Notes: fe—Compressive strength of ECC; fet—Tensile strength of ECC.
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表 5 ECC加固RC柱主要试验结果
Table 5. Main test results of RC columns strengthened by ECC
Group fco/MPa fe/MPa Nk/kN ncr Np/kN n0 De/mm nde RC 29.6 — 121 1.00 600 1.00 2.16 1.00 ECC-RCA1 28.4 38.7 251 2.07 868 1.45 3.82 1.77 ECC-RCA2 29.8 37.5 352 2.91 1301 2.17 3.98 1.84 ECC-RCA3 29.9 38.6 406 3.36 1552 2.59 4.21 1.95 ECC-RCB1 29.7 35.5 181 1.46 661 1.10 6.03 2.79 Notes: fco—Axial compressive strength of concrete; Nk—Cracking load; ncr—Cracking load ratio of reinforced column and unreinforced column; Np—Peak load; n0—Peak load ratio between the reinforced column and the unreinforced column; De—Mid-span deflection corresponding to peak load; nde—Deflection ratio corresponding to the peak load of the reinforced column and the unreinforced column.
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表 6 ECC加固RC柱延性指标的对比
Table 6. Comparison of ductility indexes of RC columns strengthened by ECC
Group $ \varDelta _{\rm{y}} /{\rm{mm}}$ $ \varDelta _{\rm{u}} /{\rm{mm}}$ $ \mu =\varDelta _{\rm{u}}/\varDelta _{\rm{y}} $ $ n_\mu$ RC 1.39 2.16 1.55 1.00 ECC-RCA1 2.16 4.73 2.19 1.413 ECC-RCA2 2.22 4.80 2.16 1.394 ECC-RCA3 2.28 4.85 2.13 1.374 ECC-RCB1 3.31 8.01 2.42 1.561 Note: ${n_\mu } $—Ratio of the ductility coefficient between the reinforced column and the unreinforced column.
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表 7 ECC加固RC柱峰值应变及约束效应系数
Table 7. Peak strain and restraint effect coefficient of ECC-reinforced RC columns
Group $\varepsilon_{{\rm{s}}}'$/% $\varepsilon_{{\rm{ec}}}$/% $a_{\rm{s}}'$/mm $ t$/mm $\varepsilon_{{\rm{cc}}}$/% k2 ECC-RCA1 0.208 0.2172 40 25 0.2147 2.629 ECC-RCA2 0.184 0.2477 40 35 0.224 2.634 ECC-RCA3 0.1702 0.2836 40 45 0.2268 2.635 Note: k2—Strain constraint effect coefficient.
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表 8 ECC加固RC柱承载力计算值与试验值比较
Table 8. Comparison between the calculated and experimental values of the bearing capacity of ECC-reinforced RC columns
Group Np-theo/kN Np-test/kN Np-theo/Np-test ECC-RCA1 913 868 1.052 ECC-RCA2 1236 1301 0.950 ECC-RCA3 1471 1552 0.948 ECC-RCB1 698 661 1.056 B2 1505 1621 0.928 Notes: Np-theo—Calculated value; Np-test—Test value.
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