Axial tensile softening characteristics of PVA fiber concrete cured in super absorbent polymer under temperature effects
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摘要: 为研究不同温度下高吸水性树脂(Super Absorbent Polymer, SAP)内养护聚乙烯醇(Polyvinyl Alcohol, PVA)纤维混凝土试件的轴拉软化特性与内在断裂机制，采用MTS万能试验机进行单轴拉伸试验，分析了混凝土的轴拉力学性能、破坏形态、应力-位移曲线、断裂能、临界裂缝和特征长度的变化规律。根据试验曲线，采用Hordijk和Li等提出的软化模型拟合，分析得到相关参数规律及混凝土软化特性。试验结果表明，混凝土试件在薄弱处拉伸断裂，且断裂位置沿轴向随机分布；随着温度的升高，应力-位移曲线峰后软化段更加平缓，应力下降速度变慢；混凝土的断裂能随着温度的升高总体不断减小，特征长度介于970.6~2110.2 mm之间，随着PVA纤维掺量的增加总体先增后减，临界裂缝长度则随着温度的升高总体不断增大。研究结果表明，温度效应对混凝土强度、断裂性能及SAP与PVA纤维的协同作用影响显著，造成混凝土破坏模式、内在断裂机制的改变；加入适量的SAP和PVA纤维能够改善混凝土试件的抗裂性能和韧性，而过多掺量的PVA纤维会加快混凝土在高温下的损伤；通过对比发现，Hordijk模型能够更好的模拟不同温度下内养护纤维混凝土轴拉试验的软化段曲线。Abstract: To study the axial tensile softening characteristics and the intrinsic fracture mechanism of Polyvinyl Alcohol (PVA) fiber-reinforced concrete specimens cured in Super Absorbent Polymer (SAP) at different temperatures, the study was carried out by uniaxial tensile test using MTS universal testing machine, and the variation rules of the axial tensile mechanical properties, damage morphology, stress-displacement curves, fracture energies, critical cracks, and characteristic lengths of the concrete were analyzed. Based on the test curves, the softening model proposed by Hordijk and Li was used for fitting, and the relevant parameter laws and concrete softening characteristics were analyzed. The test results show that: the concrete specimen is tensile fracture at the weak point, and the fracture location is randomly distributed along the axial direction; with the increase of temperature, the softening section after the peak of the stress-displacement curve is more gentle, and the stress decreases slower; the synergistic effect of SAP and PVA fibers at room temperature can improve the toughness of the concrete better, and too much mixing amount of PVA fibers will accelerate the damage of the concrete at high temperatures; The fracture energy of the concrete generally decreases with the increase of temperature, and the characteristic length ranges from 970.6 mm to 2110.2 mm. With the increase of PVA fiber content, the characteristic length generally increases first and then decreases, while the critical crack length generally increases with the increase of temperature. The research results show that the temperature effect has a significant influence on the strength, fracture properties, and synergistic effect of SAP and PVA fibers in concrete, leading to changes in the failure mode and inherent fracture mechanism of concrete. Adding an appropriate amount of SAP and PVA fibers can improve the crack resistance and toughness of concrete specimens, while an excessive amount of PVA fibers can accelerate the damage of concrete at high temperatures. By comparison, it is found that the Hordijk model can better simulate the softening curve of internally cured fiber-reinforced concrete axial tensile tests at different temperatures.
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Key words:
- SAP /
- PVA fiber /
- high temperature /
- axial tensile mechanical properties /
- softening model
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图 2 混凝土试件在高温下的轴拉破坏模式
Figure 2. Axial tensile failure mode of concrete specimens at varying temperatures
图 3 SAP内养护PVA纤维混凝土在不同温度下宏观与微观断裂面形态
Figure 3. Macroscopic and microscopic fracture surface morphology of PVA fiber concrete cured in SAP at different temperatures
图 4 不同温度与纤维掺量下SAP内养护PVA纤维混凝土轴拉试验应力-位移曲线图
Figure 4. Stress-displacement curve of axis tensile test of PVA fiber concrete cured in SAP with different temperatures and fiber dosages
图 5 不同纤维掺量下SAP内养护PVA纤维混凝土力学性能与温度变化图
Figure 5. Plot of mechanical properties of PVA fiber concrete cured in SAP versus temperature change with different fiber contents
图 8 不同温度与纤维掺量下SAP内养护PVA纤维混凝土轴拉试验软化段曲线图
Figure 8. The softening curves of axis tensile test of PVA fiber concrete cured in SAP with different temperatures and fiber dosages
图 9 不同试验条件下SAP内养护PVA纤维混凝土断裂能、特征长度和临界裂缝长度变化趋势
Figure 9. Trends of fracture energy, characteristic length and critical fracture length of PVA fiber concrete cured in SAP under different test conditions
图 11 不同温度下0.20%PVA/C混凝土试件拟合结果与试验结果对比
Figure 11. Comparison between fitting and experimental results of 0.20%PVA/C concrete specimens at different temperatures
表 1 混凝土试件组分配合比(kg/m3)
Table 1. Mix proportions of concrete specimens(kg/m3)
Item Water Cement Sand Coarse aggregate Water reducer SAP PVA Water-cement ratio Internal conservation
water qualityC 155 480 640 1160 1.2 0.000 0.000 0.323 0.0 0.05%PVA/C 155 480 640 1160 1.2 1.116 0.645 0.323 27.9 0.10%PVA/C 155 480 640 1160 1.2 1.116 1.290 0.323 27.9 0.15%PVA/C 155 480 640 1160 1.2 1.116 1.935 0.323 27.9 0.20%PVA/C 155 480 640 1160 1.2 1.116 2.580 0.323 27.9 Notes: In the specimens of polyvinyl alcohol (PVA) fiber concrete cured in super absorbent polymer (SAP), a%PVA/C—PVA fiber volume fraction admixture of specimen is a%. The dosage of SAP of item C is 0, other items are 1.116 kg/m3. 
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表 2 不同温度与纤维掺量下SAP内养护PVA纤维混凝土断裂能、特征长度和临界裂缝长度平均值
Table 2. Average values of fracture energy, characteristic length and critical crack length of PVA fiber concrete cured in SAP with different temperatures and fiber dosages
Item Fracture energy GF/(N·mm−1) Characteristic length Lch/mm Critical Crack length wc/mm C-25°C 0.287 1341.945 0.535 C-200°C 0.251 1328.076 0.650 C-300°C 0.242 1245.240 0.720 C-400°C 0.226 1355.680 0.826 0.05%PVA/C-25°C 0.341 2110.220 0.731 0.05%PVA/C-200°C 0.290 1687.029 0.908 0.05%PVA/C-300°C 0.249 1698.376 0.960 0.05%PVA/C-400°C 0.225 1990.315 1.077 0.10%PVA/C-25°C 0.310 1762.373 0.639 0.10%PVA/C-200°C 0.251 1552.835 0.682 0.10%PVA/C-300°C 0.212 1407.621 0.869 0.10%PVA/C-400°C 0.167 1953.715 0.989 0.15%PVA/C-25°C 0.297 1579.185 0.583 0.15%PVA/C-200°C 0.248 1229.602 0.633 0.15%PVA/C-300°C 0.206 1316.608 0.733 0.15%PVA/C-400°C 0.163 1001.284 0.851 0.20%PVA/C-25°C 0.295 1167.763 0.502 0.20%PVA/C-200°C 0.243 1070.596 0.574 0.20%PVA/C-300°C 0.198 970.560 0.639 0.20%PVA/C-400°C 0.167 1845.660 0.785
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表 3 不同温度与纤维掺量下SAP内养护PVA纤维混凝土软化段模型参数平均值汇总表
Table 3. Summary of the average values of softening segment model parameters of PVA fiber concrete cured in SAP at different temperatures and fiber dosages
Item Hordijk[44]model Li[42]model c1 c2 R2 k n R2 C-25°C 3.048 6.967 0.999 0.0288 0.906 0.990 C-200°C 4.125 8.429 0.998 0.0184 0.764 0.996 C-300°C 3.797 6.590 0.983 0.0227 0.647 0.982 C-400°C 4.209 7.020 0.987 0.0279 0.613 0.994 0.05%PVA/C-25°C 3.700 7.447 0.983 0.0234 0.745 0.976 0.05%PVA/C-200°C 4.326 7.086 0.990 0.0301 0.591 0.995 0.05%PVA/C-300°C 4.758 7.955 0.992 0.0341 0.649 0.993 0.05%PVA/C-400°C 3.655 5.698 0.990 0.0381 0.814 0.992 0.10%PVA/C-25°C 4.970 9.000 0.990 0.0179 0.724 0.990 0.10%PVA/C-200°C 4.291 8.533 0.990 0.0160 0.738 0.992 0.10%PVA/C-300°C 2.678 2.608 0.996 0.0156 0.458 0.983 0.10%PVA/C-400°C 4.360 5.950 0.998 0.0471 0.621 0.993 0.15%PVA/C-25°C 4.988 9.639 0.997 0.0138 0.705 0.993 0.15%PVA/C-200°C 3.530 7.170 0.996 0.0186 0.783 0.983 0.15%PVA/C-300°C 2.812 7.064 0.997 0.0295 0.906 0.987 0.15%PVA/C-400°C 1.728 5.391 0.978 0.0227 1.070 0.956 0.20%PVA/C-25°C 6.203 10.890 0.959 0.0097 0.647 0.960 0.20%PVA/C-200°C 3.060 6.766 0.994 0.0174 0.854 0.982 0.20%PVA/C-300°C 4.330 8.557 0.981 0.0174 0.690 0.978 0.20%PVA/C-400°C 3.367 5.520 0.998 0.0371 0.689 0.994 Notes: c1、c2—the fitted parameters of Hordijk[44]model; k、n—the fitted parameters of Li[42]model; R2—The degree of fitting.
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