摘要
在寒冷地区，冻融循环和除冰盐的耦合作用是一种非常典型的破坏模式，它能够导致严重的表面剥落和内部损伤。现有的耐久性评价手段多是基于质量损失和动弹性模量保留值的，评价方法的发展相对落后于实际的耐久性问题。而且多因素耦合作用下混凝土耐久性研究相对集中在混凝土基体的宏观性能变化上，忽略了混凝土的内部损伤和介观层次孔隙结构的劣化行为。不仅如此，在荷载、冻融循环和氯盐侵蚀的耦合作用下，冻融和荷载所导致的裂纹增大了氯离子渗透和钢筋锈蚀的风险，对多因素耦合作用下钢筋的锈蚀行为也值得进行系统研究。
为解决上述问题，本文利用应变计和七位半数字多用表监测了在冻融循环和氯盐侵蚀作用下水泥基材料的变形和电阻率演变，研究了残余应变与传统评价参数的关系，探索了用电阻率表征水泥基材料冻融损伤的方法和评价依据；通过改进的平板扫描法，着重考察了在冻融循环和氯盐侵蚀作用下混凝土在介观层次的孔隙结构劣化行为；系统研究了钢筋的锈蚀行为和机理；结合电阻率和孔隙结构试验结果，提出了混凝土冻融破坏的损伤模型。本文取得了以下研究成果：
1.建立了应变表征混凝土冻融损伤的方法和依据。在冻融循环和氯盐侵蚀作用下，饱盐混凝土的变形呈非线性，水胶比为0.65的饱盐混凝土在10次冻融循环内能产生约150με的残余应变，微裂纹萌生并不断扩展是导致混凝土残余应变逐渐增大的原因，应变监测表明混凝土的破坏是一个累积损伤的过程；应变—温度曲线具有明显的滞回，通过对干燥混凝土的应变—温度曲线在冻结段的拟合发现，混凝土的热膨胀系数α随加热—冷却循环次数的增大而下降，对于非引气混凝土，水胶比为0.51和0.61的混凝土经45次加热—冷却循环后α分别下降了0.87(10~(-6)/℃)和0.23(10~(-6)/℃)，而水胶比为0.51的引气混凝土其α下降了0.79(10~(-6)/℃)。
2.发现了冻融损伤过程中的临界残余应变。对于水胶比为0.61的试件，在未达到临界残余应变之前，混凝土存在轻微的损伤，超过临界残余应变之后，质量损失明显增大、相对动弹性模量迅速下降。对于引气混凝土以及水胶比为0.39的混凝土，则存在初始临界残余应变SICRS和临界残余应变SCRS两个临界点。在初始临界残余应变SICRS之前，混凝土基本不受影响，外部环境作用对混凝土基本不会造成损伤，超过SICRS后混凝土开始遭受轻微损伤直至达到临界残余应变SCRS。
3，建立了电阻率表征水泥基材料冻融损伤的方法和评价依据。电阻率曲线的增长幅度与冻融损伤成正比，电阻率曲线反映了损伤过程中孔溶液、毛细孔连通度和毛细孔曲折度的变化以及水泥基材料内部产生的裂纹和缺陷。拟合电阻率—温度曲线得到的斜率P和在Y轴的截距A与最大电阻率ρmax的皮尔逊相关系数分别达到0.926和0.958。电阻率—温度曲线同样具有滞回现象，利用电阻率—温度曲线得到了结冰形核温度Tint、临界孔隙封闭点、临界孔隙导通点和融点。此外，还发现低含水率的水泥基材料具有“斩波效应”。
4.证实了混凝土遭受冻融破坏所产生的内部损伤。经平板扫描法和图像统计分析证明，混凝土内平均裂纹数量和长度随冻融循环的进行而不断增加。水泥混凝土经过25次和100次冻融循环后，平均裂纹数量分别能达到0.25条和0.5条，平均裂纹长度分别能达到3.5μm和7μm；单位孔隙面积随着冻融循环次数的增加而逐渐增大，孔隙剖面的分形维数介于1.09~1.14之间，圆形度平均从1.6增加到2左右，混凝土孔隙结构全面劣化，损伤程度显著增大。
5.阐明了荷载—冻融—氯盐耦合作用对钢筋锈蚀的影响。在不施加荷载的情况下，经过200次冻融循环和氯盐侵蚀的耦合作用，钢筋并不会进入活化态。在施加了0.3和0.4倍的初始弯拉应力后，经过100次快速冻融循环后，阳极反应过程阻力变小，反应朝易于腐蚀的方向进行，钢筋混凝土将在10~15年内发生钢筋锈钢破坏，荷载的存在明显增大了钢筋发生锈蚀的风险。
6.建立了混凝土冻融破坏的损伤模型。根据残余应变变化速率的不同，混凝土基体的破坏过程可以分为诱导期、加速期和稳定增长期三个阶段，诱导期内饱水度不断增加，但基体的质量损失并不明显，加速期残余应变迅速增大，基体内部裂纹密度增加，损伤程度由饱水度控制，而稳定增长期则是长裂纹逐渐连通，浆体和骨料持续剥落的过程。
The coupling action of freeze-thaw cycles and de-icing salt is a typicaldeterioration mode in cold regions. It will result in severe surface scaling and internaldamage of the concrete matrix. However, measurement of existing durabilityevaluation mainly focuses on weight loss and retention value of dynamic elasticmodulus. The development of assessment method fall behind the actual durabilityissues obviously. Research on durability of concrete under combined actions mainlyfocuses on the degradation of concrete macro properties, while internal damage anddeterioration behavior of pore structure in mesoscopic scale as well as the relationshipbetween pore structure deterioration and permeability were neglected so far. Morethan that, cracks induced by freeze-thaw cycles and external load increase the hazardof steel corrosion and chloride penetration when concrete subject to freeze-thawcycles, chloride salt attack and external load. Therefore, corrosion behavior ofreinforced concrete subjects to combined action deserve to research systematically.
To solve the above problems, the deformation and resistivity behavior of cementbased materials were monitored by using strain gauges and7-1/2digital multimeter,respectively. Relationships between traditional evaluation parameters and residualstrain as well as test method and evaluation basis of resistivity to characterize thedurability of cement based materials were also studied. The deterioration behavior ofconcrete pore structure under the action of freeze-thaw cycles in mesoscopic scalewas investigated. Further, given that the possible effects of freeze thaw cycles andchloride salt attack combined with load, corrosion behavior of steel bar were studied.A deterioration process model of concrete exposed to freeze thaw cycles wasproposed according to the results of strain, resistivity and pore structure. Thefollowing research findings were obtained:
Test method of strain to characterize the damage of concrete expose to freezethaw cycles and chloride salt attack were established. Results show that deformationof concrete show nonlinear. The150με residual strain of saturated concrete with thewater to cement ratio of0.65were generated after10freeze thaw cycles.Strain-temperature hysteresis phenomenon was found. With regard to the driedconcrete, coefficient of thermal expansion decrease with the increase of freeze thawcycle numbers. Thermal expansion coefficient of concrete with the water to cementratio of0.51and0.61decrease by0.87(10~(-6)/℃) and0.23(10~(-6)/℃) respectively after 45heating and cooling cycles for non-air entrained concrete, and air-entrainedconcrete with the water to cement ratio of0.51decrease by0.79(10~(-6)/℃).
Critical residual strain during the process of freeze thaw damage was found.Concrete only suffer slight damage before the critical residual strain for specimenswith the w/c of0.61. Weight loss increases obviously and relative dynamic elasticmodulus decreases rapidly after exceeding the critical residual strain. Bothair-entrained concrete and concrete with the w/c of0.39have two critical residualstrain points, namely, initial critical residual strain SICRSand critical residual strainSCRS.Concrete can be unacted on the environmental load before SICRS. Then concretesuffer slight damage until it reaches the SCRS.
Test method and evaluation basis of resistivity to characterize the freeze thawdamage of cement based materials were established. Results show that resistivity isproportional to freeze thaw damage. Resistivity reflects the variation of pore solution,pore connectivity and pore sinuosity as well as cracks and defects of cement basedmaterials. Resistivity-temperature hysteresis phenomenon was also found. Pearsoncorrelation coefficient of ρmaxto slope of fitting curve P and Y-intercept A are0.926and0.958respectively. Further, ice nucleation temperature, depecolation point andrepecolation point were obtained from the resistivity-temperature curve. Furthermore,chopped wave of resistivity curve of unsaturated cement based materials wasobserved.
Internal damage of concrete caused by freeze thaw attack was confirmed by flatscanning method. Results show that average crack number and average crack lengthincrease with the progress of freeze thaw damage. Average crack number can reach0.25and0.5(average crack length can reach3.5μm and7μm) for concrete subjectedto50and100freeze thaw cycles, respectively. Moreover, per area increases with theongoing of freeze thaw damage. Fractal dimensions of pore sectional are between1.09and1.14and average roundness is between1.6and2.
Effects of external load, freeze thaw cycles combined with chloride slat attack oncorrosion behavior of steel bar were clarified. Results show that steel bar will keeppassivation state after200freeze thaw cycles combined with chloride salt attack withthe absence of external loads. depassivation time of steel bar moved up with the stressratio of0.4, the cathode reaction was restrained and resistance of anode reactiondecreases after the depassivation of steel bar. Steel corrosion damage may occur inreinforced concrete during10~15years.
Deterioration model of concrete exposed to freeze thaw damage was established. The deterioration process of concrete can divide into three phases, namely, inductionphase, accelerated phase and stabilization growth phase, according to the difference ofchanging rates of residual strain. Saturation degree of concrete is increasing ininduction period, but weight loss is not obvious. Internal crack numbers and crackdensity increase with the growing of residual strain, and the deterioration degree wascontrolled by the saturation degree in accelerated phase. With regard to thestabilization growth period, long cracks connect to each other and the connectivityincrease. Paste and mortar scaling from the surface of concrete steadily.
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