摘要
基于新疆若羌盐渍土环境,对水胶比为0.32、0.35和0.38的混凝土试件,进行硫酸盐溶液浸泡腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护3 d)这3种腐蚀制度下的耐久性试验,分析腐蚀制度与水胶比交互作用下混凝土动弹性模量经时演变规律与作用机制.并基于混凝土动弹性模量动态变化过程,通过Wiener过程理论,建立了盐渍土环境下混凝土服役寿命预测模型,以定量表征混凝土损伤劣化规律.结果表明:在各腐蚀制度下,随腐蚀龄期增加,混凝土损伤度均出现先减小后增大的规律;在腐蚀终期,混凝土水胶比与其损伤度呈负相关;在相同水胶比下,硫酸盐溶液浸泡腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护3 d)对混凝土损伤程度的影响依次增加;由Wiener过程理论建立的混凝土服役寿命预测模型可知,混凝土预测寿命退化呈3阶段变化趋势,其趋势与试验中混凝土劣化过程保持一致.
关键词
中国西北地区分布着大面积的盐渍土与盐
在混凝土材料劣化机理方面,现有研究在硫酸盐、干湿循环等单一或耦合作用下,对各类混凝土材料的抗压强度变化、微裂缝产生与离子扩散现象进行了分析,明确硫酸盐对混凝土材料的影响机制.Haufe
现有研究主要通过拟合、不确定性理论及神经网络模型等方法,量化表征混凝土结构腐蚀损伤状态,并预测其服役寿命.基于硫酸盐全浸泡、半浸泡等环境下混凝土耐久性数据,通过线性/非线性方法,预测分析硫酸盐环境中混凝土结构剩余服役寿
西北盐渍土条件下混凝土耐久性研究较少,且该类环境下混凝土服役寿命预测模型缺乏相关研究.本文基于新疆若羌地区盐渍土环境,在硫酸盐溶液浸泡腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护3 d)这3种腐蚀制度下,对水胶
水泥采用P·O 42.5硅酸盐水泥,性能指标见
Specific surface area/( | Flexural strength/MPa | Compressive strength/MPa | ||
---|---|---|---|---|
3 d | 28 d | 3 d | 28 d | |
330 | 5.5 | 8.7 | 28.1 | 42.4 |
Strength grate | Water | Cement | Sand | Stone | Water reducer |
---|---|---|---|---|---|
C50 | 150 | 465 | 764 | 1 056 | 5.58 |
C40 | 147 | 420 | 850 | 1 083 | 5.04 |
C35 | 136 | 360 | 832 | 882 | 3.60 |
为研究西北盐渍土地区强腐蚀条件下混凝土材料的劣化损伤规律,以新疆若羌地区工程结构所处典型的格库铁路环境为背景,设置A、B、C这3种腐蚀制度,分别为硫酸盐溶液浸泡腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护3 d),来模拟硫酸盐浸泡环境中混凝土、盐渍土环境中混凝土、盐渍土环境中早龄期混凝土的劣化状态,以明确盐渍土环境下不同水胶比混凝土的耐久性劣化规律,试验工况设计见
Group | mW/mB | Strength grade | Curing method | Erosion method |
---|---|---|---|---|
A1 | 0.32 | C35 | Standard curing 28 d | 3%Na2SO4 |
A2 | 0.35 | C40 | Standard curing 28 d | 3%Na2SO4 |
A3 | 0.38 | C50 | Standard curing 28 d | 3%Na2SO4 |
B1 | 0.32 | C35 | Standard curing 28 d | Saline soil |
B2 | 0.35 | C40 | Standard curing 28 d | Saline soil |
B3 | 0.38 | C50 | Standard curing 28 d | Saline soil |
C1 | 0.32 | C35 | Standard curing 3 d | Saline soil |
C2 | 0.35 | C40 | Standard curing 3 d | Saline soil |
C3 | 0.38 | C50 | Standard curing 3 d | Saline soil |
Major ion | Ion content in water/(mg· | Ion content in soil/(mg·k |
---|---|---|
SO | 11 000.5 | 23 000.5 |
C | 760.3 | 193.1 |
M | 242.5 | 55.6 |
(1)腐蚀制度A:西北地区盐渍土环境中混凝土材料主要受硫酸盐腐蚀作
(2)腐蚀制度B:模拟盐渍土环境中混凝土劣化状态,混凝土放入标准养护室中养护24 h脱模,将养护至28 d的混凝土试件半埋于腐蚀介质(从格库铁路沿线典型盐渍土区域——台特玛湖取回的含硫酸盐土壤)中,并添加3%Na2SO4溶液使之高出土面约1 cm,待溶液蒸发至土壤表面时继续添加3%Na2SO4溶液,溶液更换周期为2月1次.
(3)腐蚀制度C:模拟盐渍土试验环境中早龄期混凝土劣化状态,混凝土放入标准养护室中养护24 h脱模,继续养护3 d后,半埋到从台特玛湖取回的含硫酸盐土壤中,并添加3%Na2SO4溶液使之高出土面约1 cm,待溶液蒸发至土壤表面时继续添加3%Na2SO4溶液,溶液更换周期为2月1次.

图1 各腐蚀制度下不同水胶比的混凝土损伤度演变规律
Fig.1 Evolution of concrete damage degree for different water‑cement ratios under each corrosion regime
在腐蚀前期,硫酸根离子扩散至混凝土内部,与水泥浆体中的氢氧化钙反应生成石膏,如
(2) |
混凝土体系中钙矾石活化能较低,石膏继续与水泥浆体中的水化铝相、含铝胶体以及未水化的铝酸三钙反应生成二次钙矾石固体,如式(
(3) |
(4) |
随着反应进行,铝相类物质逐渐被消耗,同时伴随着石膏晶体的不断析出,固体腐蚀产物体积增大了124
由
综上可知,在相同水胶比下,腐蚀制度对混凝土的劣化影响程度为:C>B>A;相较于腐蚀制度A,腐蚀制度B、C下的混凝土开始出现损伤的腐蚀龄期均有不同程度的“提前”,且腐蚀制度C下混凝土开始出现损伤的腐蚀龄期最短.即在相同水胶比下,腐蚀制度C对于混凝土的劣化程度影响较大.
基于Wiener理论对混凝土服役寿命(T)建模,可较好描述混凝土劣化过程中的不确定因
(5) |
式中:为混凝土退化指标;为漂移参数;为扩散参数;为标准Wiener过程函数;t>0.
同时,依据文献[
(6) |
式中:xk为退化指标;Df为耐久性阈值.
基于上式,得到可靠度函数Rk(t):
(7) |
式中:(x)为x的标准正态分布函数.
对
(8) |
(9) |
基于Wiener过程理论,对于不同工况下的混凝土损伤增量进行正态分布假设检验.绘制不同工况下混凝土损伤度增量P‑P图及其频率分布直方图,如

图2 不同工况下混凝土损伤度增量P‑P图及其频率分布直方图
Fig.2 P‑P plots of incremental concrete damage degree under different working conditions and their frequency distribution histograms
A1 | A2 | A3 | B1 | B2 | B3 | C1 | C2 | C3 |
---|---|---|---|---|---|---|---|---|
0.707 | 0.529 | 0.814 | 0.096 | 0.208 | 0.721 | 0.426 | 0.947 | 0.779 |
Note: Hypothesis testing within 95% confidence intervals.
由于概率密度函数为连续型函数,采用极大似然估计法对Wiener过程理论中的关键未知参数进行估计.通过概率密度函数可以推导出极大似然函数L():
(10) |
式中:为试件在区间的混凝土损伤度退化量;.
对
(11) |
(12) |
将各试件在不同工况下的混凝土损伤度退化量带入式(
Parameter | A1 | A2 | A3 | B1 | B2 | B3 | C1 | C2 | C3 |
---|---|---|---|---|---|---|---|---|---|
×1 | 0.772 | 1.471 | 2.222 | 1.463 | 3.387 | 3.877 | 2.049 | 4.415 | 6.451 |
| 8.852 | 2.386 | 3.932 | 8.521 | 6.576 | 8.122 | 6.847 | 8.320 | 5.013 |
基于Wiener过程理论建模时,失效阈值是判断混凝土是否能够正常工作的重要边界条件.根据GB/T 50082—2009标准可知:混凝土的失效阈值为混凝土动弹性模量损失率达40%,即混凝土损伤度失效阈值取0.4.通过建立Wiener过程理论混凝土服役寿命预测模型,对不同工况下的混凝土试件进行服役寿命预测.

图3 混凝土预测寿命可靠度函数及概率密度函数
Fig.3 Predicted life reliability function and probability density function of concrete
由
(1)A1、A2、A3的预测寿命分别为14 600、4 230、2 850 d,B1、B2、B3的预测寿命分别为5 990、1 910、1 690 d,C1、C2、C3的预测寿命分别为3 610、1 450、840 d.
(2)A1、A2、A3条件下混凝土预测寿命分别为B1、B2、B3条件下的2.43、2.21、1.69倍.这是由于腐蚀制度B下盐渍土环境中的混凝土不仅受硫酸盐腐蚀作用,还受干湿循环作用,因此其服役寿命显著降低.
(3)B1、B2、B3条件下混凝土预测寿命分别为C1、C2、C3条件下的1.66、1.31、2.02倍.这是由于腐蚀制度C下混凝土为早龄期混凝土,水化程度较低,内部孔隙较多,因而易受硫酸盐腐蚀.
(4)A1、A2、A3条件下混凝土预测寿命分别为C1、C2、C3条件下的4.04、2.90、3.43倍.这是由于腐蚀制度C为盐渍土环境,且混凝土预养护时间不足,因而C条件下混凝土劣化程度更高.
(1)在各腐蚀制度下,随腐蚀龄期增加,混凝土损伤度均出现先减小后增大的变化规律.在腐蚀终期,混凝土损伤度与水胶比呈负相关;另外,混凝土开始损伤对应龄期也与水胶比呈负相关.
(2)在相同水胶比条件下,硫酸盐溶液浸泡腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护28 d)、盐渍土腐蚀(标准养护3 d)这3种腐蚀制度对混凝土劣化程度的影响依次增加.
(3)基于Wiener过程理论可以较好地描述不同工况下混凝土动弹性模量退化过程:前期混凝土内部腐蚀物质积累;中期可靠度快速下降,混凝土内部产生损伤,开始劣化;后期混凝土耐久性失效.其变化趋势与试验中混凝土劣化过程保持较好的一致性.
(4)各工况下混凝土预测寿命存在显著差异,相较于硫酸盐溶液浸泡腐蚀(标准养护28 d),盐渍土腐蚀(标准养护3 d)条件下混凝土劣化程度更高.
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