Effect of steel fiber-nano carbon black/concrete smart layer on crack self-monitoring performance
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摘要: 将结构型钢纤维及纳米炭黑作为导电材料加入混凝土形成智能混凝土，并将智能混凝土和素混凝土一起浇筑形成双层混凝土梁。在四点弯曲加载下，采用四电极法对混凝土试件的电阻进行测量，对比研究了复掺钢纤维-纳米炭黑/混凝土层中钢纤维掺量及智能层厚度对混凝土梁的弯曲性能及裂缝自监测性能的影响。结果表明：混凝土智能层内结构型钢纤维的掺量和智能层厚度的增加能够显著提高混凝土梁的弯曲性能；依据电阻变化率-时间(ρFCR-t)曲线的变化特征可对混凝土的裂缝出现时刻进行监测，裂缝出现前，ρFCR-t曲线基本维持在零点附近，智能混凝土梁出现裂缝时，ρFCR-t曲线开始急剧上升，出现新裂缝后，ρFCR-t曲线出现新的转折点，斜率发生明显变化；裂缝出现时，试件的ρFCR增长较迅速，后期ρFCR增长变缓，ρFCR-裂缝扩展宽度(ωCOD)曲线呈负衰减函数特征；对于双层混凝土梁，增加混凝土智能层钢纤维掺量和智能层厚度，减小了相同裂缝宽度下裂缝自监测信号ρFCR的数值；本文提出的一阶指数衰减函数模型拟合ρFCR-ωCOD曲线效果较好。Abstract: The macro steel fiber and nano carbon black as conductive materials were added into the concrete to make smart concrete. A new type of concrete beam with double layers combined with the smart concrete and plain concrete was produced. In addition, the effect of the steel fiber content and the depth of the steel fiber-nano carbon black/concrete smart layer on the post-crack toughness and self-monitoring performance were studied in this paper. In order to monitor the concrete crack, four-probe method was used to measure the electrical resistance of the specimens. The results show that the toughness of the concrete beams is increased with the increasing of fiber content and the depth of the smart concrete layer. The crack self-monitoring of the concrete can be realized by analyzing the characteristics of the fraction change in resistance-time (ρFCR-t) curve. Furthermore, the ρFCR value is nearby zero before cracking, then a significant increase of ρFCR can be found after the crack appearing. If more than one crack happens on the smart concrete beam due to the deflection hardening behavior, new turning points where the slope of ρFCR-t curve changes can be observed. Additionally, the self sensing behavior decreases with the increasing of the steel fiber content and the depth of the smart concrete layer. The ρFCR value of the specimen increases rapidly after crack happening, after that it increases slowly gradually. However, the fractional change in resistance value at a certain crack width (ωCOD) decreases with the increasing of the fiber content and the height of the smart concrete layer. The proposed first order exponential decay function fits well with the ρFCR-ωCOD curves.
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Key words:
- steel fiber /
- carbon black /
- smart concrete /
- fraction change in resistance /
- crack self-monitoring
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图 1 双层混凝土梁纵截面示意图
Figure 1. Diagram of longitudinal sections of concrete beams with double layers
图 2 加载装置(a)和电阻测量线路(b)示意图
Figure 2. Circuit diagrams of electrical resistance measurement (b) and testing set-up (a)
图 3 双层混凝土梁荷载-挠度曲线
Figure 3. Load-deflection curves of concrete beams with double layers
图 4 双层混凝土试件荷载-时间t-电阻变化率ρFCR的关系曲线
Figure 4. Load-time t-fractional change in resistance ρFCR curves of concrete specimens with double layers
图 5 不同智能层厚度的双层混凝土试件电阻变化率ρFCR-裂缝扩展宽度ωCOD的曲线对比
Figure 5. Comparison of fractional change in resistance ρFCR-crack opening displacement ωCOD curves of concrete specimens with double layers and different smart layer thickness
图 6 钢纤维掺量对不同智能层厚度混凝土ρFCR -ωCOD曲线的影响
Figure 6. Effect of steel fiber content on ρFCR-ωCOD curves of concrete with different smart concrete layer thickness
图 8 钢纤维桥接混凝土裂缝处的导电路径示意
Figure 8. Schematic diagram of bridging effect of steel fibers on conductive path at concrete crack
表 1 混凝土基准配合比
Table 1. Mix proportions of concrete
kg·m−3 Cement Fly ash Water Fine aggregate (0–5 mm) Coarse aggregate (5–10 mm) SP 390 155 272.5 848 822 5.5 Note: SP—Superplasticizer. 
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表 2 双层混凝土梁试件的导电相纤维掺量和智能层厚度
Table 2. Fiber content and depth of different layers of concrete specimens with double layers
Specimen Content of SF/(kg·m−3) Smart layer depth/mm PC layer depth/mm SF40-CB/concrete-20 40 20 80 SF40-CB/concrete-40 40 40 60 SF40-CB/concrete-60 40 60 40 SF40-CB/concrete-80 40 80 20 SF40-CB/concrete-100 40 100 0 SF80-CB/concrete-20 80 20 80 SF80-CB/concrete-40 80 40 60 SF80-CB/concrete-60 80 60 40 SF80-CB/concrete-80 80 80 20 SF80-CB/concrete-100 80 100 0 Notes: SF—Steel fiber; PC—Plain concrete; CB—Carbon black.
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表 3 双层混凝土试件受弯性能
Table 3. Flexure performance of concrete specimens with double layers
Specimen ${P_1}$/kN ${P_{\rm{P}}}$/kN $P_{{\rm{600}}}^{\rm{D}}$/kN $f_{{\rm{600}}}^{\rm{D}}$/MPa $P_{{\rm{150}}}^{\rm{D}}$/kN $f_{{\rm{150}}}^{\rm{D}}$/MPa $T_{{\rm{150}}}^{\rm{D}}$/(N·m) SF40-CB/concrete-20 16.1 16.1 11.3 3.4 8.7 2.6 21.3 SF40-CB/concrete-40 16.5 16.5 10.1 3.0 11.2 3.4 22.1 SF40-CB/concrete-60 14.0 14.0 11.0 3.3 12.9 3.9 24.2 SF40-CB/concrete-80 14.7 14.7 13.1 3.9 13.0 3.9 27.0 SF40-CB/concrete-100 17.7 17.7 17.3 5.2 15.5 4.7 32.6 SF80-CB/concrete-20 19.1 19.1 15.9 4.8 14.4 4.3 32.2 SF80-CB/concrete-40 18.4 18.4 17.8 5.3 14.5 4.4 33.3 SF80-CB/concrete-60 20.4 20.6 20.4 6.1 16.2 4.9 37.3 SF80-CB/concrete-80 16.8 19.4 17.3 5.2 18.7 5.6 35.9 SF80-CB/concrete-100 21.0 27.5 23.6 7.1 25.6 7.7 50.5 Notes: P1—First peak load; PP—Peak load; $P_{{\rm{600}}}^{\rm{D}}$—Residual load at deflection of 0.5 mm; $P_{{\rm{150}}}^{\rm{D}}$—Residual load at deflection of 2 mm; $f_{{\rm{600}}}^{\rm{D}}$—Residual strength at deflection of 0.5 mm; $f_{{\rm{150}}}^{\rm{D}}$—Residual strength at deflection of 2 mm; $T_{{\rm{150}}}^{\rm{D}}$—Area under load vs. deflection curve 0–2 mm.
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表 4 双层混凝土在不同裂缝扩展宽度ωCOD下的电阻变化率ρFCR
Table 4. Fractional change in resistance ρFCR of concrete specimens with double layers at different crack opening displacement ωCOD
Specimen ρFCR/% 0.2 mm 0.5 mm 1.0 mm 2.0 mm 3.0 mm 4.0 mm 5.0 mm 6.0 mm SF40-CB/concrete-20 1.61 10.35 25.16 44.29 52.87 63.87 71.77 82.63 SF40-CB/concrete-40 1.61 19.40 36.73 54.07 64.10 69.88 73.20 75.87 SF40-CB/concrete-60 2.68 10.60 23.36 42.08 50.93 57.33 63.58 65.42 SF40-CB/concrete-80 1.91 10.27 21.61 39.54 49.47 55.71 60.37 64.88 SF40-CB/concrete-100 1.94 9.58 18.18 26.99 33.20 37.14 40.47 43.01 SF80-CB/concrete-20 0.85 8.79 22.28 42.04 53.20 60.57 67.66 73.38 SF80-CB/concrete-40 1.14 7.49 17.56 32.26 44.70 51.48 57.98 62.69 SF80-CB/concrete-60 3.32 14.26 23.49 30.32 33.92 35.38 37.31 38.64 SF80-CB/concrete-80 1.44 8.44 17.74 29.42 38.73 43.86 47.51 52.58 SF80-CB/concrete-100 1.66 4.09 9.55 13.10 15.23 16.07 16.34 18.31
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表 5 ρFCR-ωCOD曲线一阶衰减函数拟合结果
Table 5. Fitting results of ρFCR-ωCOD curves by first order exponential decay function
Specimen y0 A B R2 SF40-CB/concrete-20 91.5 −92.1 3.2 0.998 SF40-CB/concrete-40 76.4 −76.6 1.6 0.999 SF40-CB/concrete-60 71.2 −72.0 2.4 0.999 SF40-CB/concrete-80 70.6 −71.4 2.5 0.999 SF40-CB/concrete-100 44.1 −44.1 2.0 0.999 SF80-CB/concrete-20 83.7 −84.7 3.0 0.998 SF80-CB/concrete-40 78.5 −79.2 3.7 0.998 SF80-CB/concrete-60 37.1 −36.7 1.1 0.997 SF80-CB/concrete-80 58.1 −58.5 2.8 0.999 SF80-CB/concrete-100 16.9 −17.0 1.3 0.994
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