摘要
采用恒电位驱动氯离子渗透并加速钢筋锈蚀来模拟海洋钢筋混凝土锈蚀,研究了水胶比、保护层厚度和钢筋直径对试件锈胀开裂的影响; 利用COMSOL Multiphysics软件建立了真实骨料混凝土模型并进行数值模拟. 结果表明:本试验制备的混凝土试件在恒电位加速锈蚀下钢筋发生非均匀锈蚀并导致混凝土产生3条主裂缝,钢筋临界锈胀应力为2.85~3.51 MPa,开裂时间为190~311 h,降低水胶比及增大保护层厚度均可延缓锈胀开裂时间; 数值模拟可以很好地再现钢筋混凝土非均匀锈蚀过程,其获得的锈胀应力演变及混凝土开裂模式与试验结果吻合.
钢筋混凝土结构暴露在海洋环境中,海洋中氯离子通过混凝土渗透至钢筋表面并破坏其钝化膜,导致钢筋出现点蚀和非均匀锈
对于钢筋锈蚀诱导混凝土开裂,国内外主要通过试验、理论模型和数值模拟等进行研究.试验研究中,一些学
因此,本文通过改进通电加速锈蚀试验来实现氯离子快速渗透至钢筋表面并诱导其非均匀锈蚀,精确测量钢筋锈胀应力,并考虑水胶比、保护层厚度和钢筋直径对混凝土锈胀开裂的影响. 并且,构建真实骨料混凝土模型,依据钢筋锈蚀电流密度计算锈胀应力,数值再现钢筋混凝土锈胀开裂全过程. 本文的研究为实现钢筋非均匀锈蚀及锈胀应力监测提供了方法指导,也对评估钢筋混凝土结构的耐久性提供了试验数据及模拟依据.
水泥采用P·Ι 42.5级基准水泥;粗骨料为花岗岩碎石,细骨料为天然河砂,细度模数2.7;选择Ⅰ级粉煤灰和S95级矿粉作为矿物掺合料;减水剂为JM‑PCA(Ⅰ) 聚羧酸高效减水剂,减水
Concrete No. | mW/mB | Mix proportion/(kg∙ | Compressive strength/MPa | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Cement | Mineral powder | Fly ash | Sand | Stone | Water | JM‑PCA(Ⅰ) | 3 d | 7 d | 28 d | ||
C1 | 0.36 | 337.0 | 45.0 | 68.0 | 650.0 | 1 130.0 | 162.0 | 3.4 | 24.5 | 38.8 | 46.8 |
C2 | 0.32 | 337.0 | 50.0 | 70.0 | 650.0 | 1 130.0 | 145.0 | 5.4 | 29.9 | 46.0 | 56.2 |
C3 | 0.28 | 380.0 | 50.0 | 70.0 | 650.0 | 1 130.0 | 130.0 | 6.8 | 34.8 | 52.6 | 65.9 |
钢筋选用HRB400型螺纹钢筋(钢筋直径D分别为16、20、25 mm),长度为295 mm,制成空心钢筋后在其内壁环向粘贴应变片,通过DH3816应变采集仪来采集钢筋锈胀产生的应变. 应变片的粘贴位置如

图1 应变片的粘贴位置
Fig.1 Position of strain gauge
钢筋混凝土通电加速锈蚀试验装置如

图2 钢筋混凝土通电加速锈蚀试验示意图
Fig.2 Schematic diagram of electrochemical accelerated corrosion device for reinforced concrete
采用电化学阻抗谱法测试混凝土中钢筋锈蚀电流密度,在通电加速锈蚀试验前期每隔12 h对混凝土试件进行1次电化学测试. 测试时采用三电极体系,电化学测试方法与试验装置如

图3 电化学测试示意图
Fig.3 Schematic diagram of electrochemical test
试验初期,由于氯离子未渗透至钢筋表面,钢筋锈蚀速率较慢. 但随着恒电位加速锈蚀时间(t)的延长,混凝土试件中部和表面逐渐因为钢筋锈蚀产物增加及膨胀而出现裂缝,且裂缝逐渐向钢筋两侧扩展. 同时锈蚀产物逐渐填充裂缝并从裂缝处溢出,试件表面出现黄锈,如

图4 试件裂缝观测图
Fig.4 Crack observation diagram of specimen
将锈蚀后的钢筋混凝土试件切开,观察试件裂缝及锈蚀形貌,如

图5 钢筋混凝土试件裂缝及锈蚀形貌图
Fig.5 Crack and corrosion development of reinforcement concrete specimen
采用超景深显微镜观测锈蚀钢筋混凝土界面区,得到其局部观测图,如

图6 钢筋混凝土试件局部观测图
Fig.6 Local observation view of reinforcement concrete specimen

图7 钢筋混凝土中钢筋的电化学阻抗谱
Fig.7 Electrochemical impedance spectra of steel bar in reinforced concrete
利用Zsimpwin软件中的R(QR)(QR)等效电路进行拟合,得到不同类型钢筋混凝土的极化电阻Rp,结果见
No. | mW/mB | C/mm | D/mm | Rp/(kΩ·c | |||||
---|---|---|---|---|---|---|---|---|---|
0 | 12 h | 24 h | 36 h | 48 h | |||||
1 | 0.36 | 15 | 16 | 543 | 235 | 129 | 89 | 46 | |
2 | 0.36 | 25 | 16 | 1 206 | 613 | 323 | 93 | 54 | |
3 | 0.36 | 35 | 16 | 1 772 | 498 | 379 | 110 | 63 | |
4 | 0.36 | 15 | 20 | 524 | 221 | 164 | 121 | 72 | |
5 | 0.36 | 15 | 25 | 522 | 324 | 206 | 102 | 54 | |
6 | 0.32 | 15 | 16 | 862 | 436 | 209 | 84 | 59 | |
7 | 0.28 | 15 | 16 | 1 024 | 372 | 193 | 72 | 53 |
(1) |

图8 不同类型钢筋混凝土拟合的锈蚀电流密度演化方程
Fig.8 Fitting regression equation of corrosion current density of different types of reinforced concretes
连续采集混凝土试件6(mW/mB=0.32,D=16 mm,C=15 mm)中钢筋内表面应变,结果如
(2) |
式中:q为钢筋外壁锈胀应力;为钢筋内壁环向应变,;r1为钢筋锈蚀前内半径;r2为钢筋锈蚀后内半径;Es为钢筋弹性模量;μ为钢筋泊松比.

图9 混凝土试件6(mW/mB=0.32,D=16 mm,C=15 mm)中钢筋锈胀应变及应力变化
Fig.9 Evolution of rust expansion strain and stress of concrete specimen 6(mW/mB =0.32, D=16 mm, C=15 mm)
从
钢筋直径为16 mm,保护层厚度为15 mm,混凝土水胶比为0.36、0.32和0.28的混凝土试件中钢筋锈胀应力计算结果如

图10 不同水胶比混凝土试件中钢筋锈胀应力变化
Fig.10 Evolution of rust expansion stress of concrete specimens with different water to cement ratios(D=16 mm, C=15 mm)
选取钢筋直径为16 mm、水胶比为0.36的混凝土试件,分析保护层厚度对钢筋锈胀应力的影响,结果如

图11 不同保护层厚度混凝土试件中钢筋锈胀应力变化
Fig.11 Evolution of rust expansion stress of concrete specimens with different cover thicknesses(mW/mB =0.36, D=16 mm)
基于混凝土数字图像处理技术建立真实骨料混凝土模

图13 真实骨料混凝土建模步骤
Fig.13 Modeling process of real aggregate concrete
通过考虑钢筋与混凝土界面“空隙区”的钢筋半椭圆锈蚀模

图14 钢筋半椭圆锈蚀模型
Fig.14 Model of semi‑elliptical corrosion of rebar
考虑钢筋混凝土二维平面模型,由此可以得到钢筋锈蚀产物总量的表达式为:
(3) |
式中:为铁与锈蚀产物相对分子质量的比值,一般为0.523~0.62
为了得到值,采用文献[
(4) |
钢筋锈蚀产物总量与锈蚀电流密度和钢筋直径D的关系为:
(5) |
据此,可以得到钢筋内边界的锈胀位移:
(6) |
式中:.
将电化学阻抗谱获得的钢筋锈蚀电流密度演化方程(见
选取混凝土水灰比为0.32,保护层厚度为25 mm,钢筋直径为25 mm的钢筋混凝土试件进行有限元模拟,模拟结果如

图15 钢筋混凝土试件开裂过程模拟图
Fig.15 Simulated diagram of cracking process for reinforced concrete specimen
把试验与模拟结果得出的钢筋临界锈胀应力和开裂时间进行对比,结果如
No. | Critical rust expansion stress of rebar/MPa | Critical cracking time/h | ||||||
---|---|---|---|---|---|---|---|---|
Point 1 | Point 2 | Mean value | Model | Point 1 | Point 2 | Mean value | Model | |
1 | 2.93 | 2.94 | 2.94 | 3.24 | 220 | 210 | 215 | 234 |
2 | 3.07 | 3.04 | 3.06 | 3.34 | 247 | 252 | 250 | 265 |
3 | 3.28 | 3.28 | 3.28 | 3.48 | 311 | 294 | 303 | 282 |
4 | 3.08 | 2.89 | 2.99 | 3.18 | 209 | 222 | 216 | 217 |
5 | 2.93 | 2.85 | 2.89 | 3.15 | 190 | 200 | 195 | 206 |
6 | 3.23 | 3.30 | 3.27 | 3.55 | 264 | 252 | 258 | 286 |
7 | 3.50 | 3.51 | 3.51 | 3.76 | 295 | 294 | 295 | 310 |

图16 试验与模拟的锈胀应力对比
Fig.16 Rust expansion stress of test and simulation
(1)采用恒电位驱动混凝土表面氯离子向内部渗透可以实现钢筋非均匀锈蚀. 试件靠近保护层侧的混凝土表面中间部分先开裂形成垂直裂缝,之后向两侧扩展形成3条主裂缝.
(2)混凝土试件的钢筋锈胀应力均呈现先增大后减小的趋势,临界锈胀应力在2.85~3.51 MPa,开裂时间在190~311 h;降低水胶比和增大保护层厚度均会增大钢筋临界锈胀应力,明显延长钢筋混凝土试件锈胀开裂的时间;钢筋直径对混凝土试件锈胀开裂时间和临界锈胀应力的影响不明显.
(3)真实骨料混凝土模型有助于更加真实准确地模拟钢筋混凝土裂缝模式和开裂过程,因此在模拟时应考虑混凝土真实骨料的影响. 此外,采用半椭圆锈蚀模型模拟的钢筋非均匀锈蚀结果与试验结果基本吻合,验证了数值模拟的可靠性.
(4)后续会进一步研究钢筋数量、钢筋位置以及骨料级配等参数对钢筋锈胀开裂的影响,并引入温湿度和氧气含量等环境条件参数进行分析,为钢筋混凝土构件的耐久性设计和评估提供指导.
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