Corrosion damage and bearing characteristics of bridge pile foundations under dry-wet-freeze-thaw cycles in alpine salt marsh areas
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摘要: 为探明青海地区桥梁桩基在干湿-冻融循环条件下的腐蚀损伤特性, 依托德香高速公路工程, 在现场埋设钢筋和混凝土试件进行干湿-冻融循环1年, 采用室内试验将混凝土试件进行干湿-冻融循环225次, 对比分析了不同位置和不同循环时间条件下混凝土质量、抗侵蚀系数、相对动弹性模量、抗压强度、微观机理以及钢筋锈蚀率的变化规律; 采用数值仿真分析了未防护桩基20年内承载力变化规律, 并提出了高寒盐沼泽区桥梁桩基防护措施。研究结果表明: 随着试件埋设深度的增加, 现场桩基混凝土试件的抗侵蚀系数相关度增大, 最大值为0.93;随着时间的增加, 桩基混凝土试件的抗压强度最大损失率为38.20%, 埋深0.25 m处钢筋的面积锈蚀率最大, 为91%;表面涂抹环氧树脂可以有效减少钢筋锈蚀率, 桩基混凝土试件与钢筋的质量变化不明显; 干湿-冻融循环225次时, 桩基混凝土试件的边角处出现脱落, 四周出现裂纹, 但质量变化较小, 相对动弹性模量降低了39.10%, 抗侵蚀系数降低到0.51, 混凝土的抗压强度损失率为65.88%, 其内部因出现Friedel盐等膨胀性物质而趋于破坏; 随着剥落厚度和腐蚀深度的增加, 前8年桩基的承载力基本不变, 8年后其承载力逐步降低, 若不进行维护, 第20年桩基承载力降低34.45%;建议在桩基服役8年后, 要进行重点防护。Abstract: To explore the corrosion damage characteristics of bridge pile foundation in Qinghai area under dry-wet-freeze-thaw cycles, relying on the Dexiang Expressway Project, the reinforcement and concrete specimens were embedded in the field to subjected to freeze-thaw cycles for one year. The laboratory test was used to conduct the dry-wet-freeze-thaw cycles on concrete specimens for 225 times. The variation rules of concrete mass, anti-erosion coefficient, relative dynamic elastic modulus, compressive strength, micro-mechanism and reinforcement corrosion rate at different positions and different cycle times were compared and analyzed. The numerical simulation was conducted to analyze the bearing capacity change rule of unprotected pile foundation over 20 years, and protection measurements for bridge pile foundations in alpine salt marsh areas were proposed. Research result shows that as the embedment depth increases, the correlation degree of anti-erosion coefficient of pile foundation concrete specimens in the field increases, and the maximum value is 0.93. As time increases, the maximum compressive strength loss rate of pile foundation concrete specimens is 38.20%. The areal corrosion rate of reinforcement at the depth of 0.25 m is the largest, and the value is 91%. Coating epoxy resin on the surface can effectively reduce the corrosion rate of reinforcement, The mass changes of pile foundation concrete specimen and reinforcement are not obvious. In the 225 th dry-wet-freeze-thaw cycles, the corner of pile foundation concrete specimen falls off and cracks appear around, but the mass change is small. The relative dynamic elastic modulus reduces by 39.10%, the anti-erosion coefficient reduces to 0.51, the compressive strength loss rate of concrete is 65.88%. Failare nearly occurs in the interior because of the presence of Friedel salt and other expansive substances. As the spalling thickness and corrosion depth increase, the bearing capacity of pile foundation in the first eight years remains essentially unchanged. After eight years, its bearing capacity gradually decreases. Without proper maintenance, the bearing capacity of pile foundation will reduce by 34.45% by the 20 th year. It is suggested that the key protection measures should be taken for pile foundations after 8 years of service.
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表 1 C30混凝土配合比
Table 1. C30 concrete mixing ratio
kg·m-3 种类 水泥 砂子 碎石 水 减水剂 配比 436 767 1 103 170 5.23 表 2 水中各离子含量
Table 2. Contents of various ions in water
水体 易溶盐含量/(mg·L-1) pH值 SO42- HCO3- Cl- 地下水 2 400.0 392.2 18 818.8 7.0 地表水 720.6 454.7 8 498.7 7.0 表 3 复合盐中各盐份含量
Table 3. Contents of salt in complex salts
侵蚀溶液 盐类型及其用量/(g·L-1) 溶液浓度/% Na2SO4 NaCl NaHCO3 含量 3.55 31.01 0.54 3.4 表 4 不同时间对应的桩基剥落厚度和腐蚀深度
Table 4. Spalling thicknesses and corrosion depths of pile foundations corresponding to different times
年份 0 4 8 12 16 20 剥落厚度/cm 0 3 6 9 12 15 腐蚀深度/m 0 1.6 3.2 4.8 6.4 8.0 表 5 有限元模型参数
Table 5. Finite element model parameters
参数 重度/(kN·m3) 黏聚力/kPa 弹性模量/MPa 泊松比 内摩擦角/(°) 桩 25.0 3.00×104 0.20 粉细砂 18.4 10.8 12.50 0.33 30 粉质黏土 18.3 20.0 5.60 0.30 20 剥落3 cm 腐蚀1.6 m 25.0 2.78×104 0.20 剥落6 cm 腐蚀3.2 m 25.0 2.65×104 0.20 剥落9 cm 腐蚀4.8 m 25.0 2.32×104 0.20 剥落12 cm 腐蚀6.4 m 25.0 2.06×104 0.20 剥落15 cm 腐蚀8.0 m 25.0 1.84×104 0.20 表 6 不同埋深桩基混凝土试件的抗侵蚀系数回归方程
Table 6. Regression equations of anti-erosion coefficients of pile foundation concrete specimens at different depths
位置 回归方程 水中 K=-8.0×10-6x2+0.003 4x+0.590 0R2=0.714 5 地表 K=-4.0×10-6x2+0.001x+0.792R2=0.820 8 地下0.25 m K=-5.0×10-6x2+0.002 1x+0.656 0R2=0.882 8 地下1.25 m K=4.0×10-6x2+0.002 5x+0.426 9R2=0.930 7 表 7 不同埋深不同时间钢筋的面积锈蚀率
Table 7. Areal corrosion rates of reinforcement at different depths and times
% 钢筋直径/mm 埋深1.25 m 埋深0.25 m 地表 水中 90 d 270 d 360 d 90 d 270 d 360 d 90 d 270 d 360 d 90 d 270 d 360 d Φ12 4.7 5.6 59.0 3.9 5.5 75.0 6.1 6.8 89.0 7.1 7.8 81.0 Φ25 4.6 5.3 65.0 3.9 5.2 66.0 5.8 6.5 91.0 7.5 7.4 76.0 Φ25′ 0.0 1.2 2.6 0.0 0.9 4.2 0.0 1.7 4.5 0.0 2.1 5.3 表 8 钢筋质量锈蚀率
Table 8. Tab 8 Mass corrosion rates of reinforcements
钢筋埋设位置 钢筋直径/mm 面积锈蚀率/% 质量损失率/% 地表 Φ12 89.0 0.5 Φ25 91.0 0.5 Φ25′ 4.5 0.2 地下1.25 m Φ12 59.0 0.3 Φ25 65.0 0.3 Φ25′ 2.6 0.1 表 9 各元素含量占比
Table 9. Proportions of various element contents
% 元素 C O Al Si Cl Ca 合计 原子百分比 11.93 71.32 0.83 5.18 1.57 9.17 100 -
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