【作者】 延永东；

【导师】 金伟良；

【作者基本信息】 浙江大学 ， 结构工程， 2011， 博士

【摘要】 氯离子是影响海洋环境下混凝土结构耐久性问题的最主要因素,各种环境及荷载造成的损伤及开裂对氯离子的侵蚀有加剧作用,研究氯离子在损伤及开裂混凝土内的输运机理,对准确预测其使用寿命并进而采取相应措施提高其服役寿命具有十分重要的理论和应用价值。本文的主要研究工作如下：(1)分别采用双重孔隙介质模型和二维扩散模型,研究了溶液静止和流动状态下氯离子在开裂混凝土内的输运,模型同时考虑了表面氯离子浓度和氯离子扩散系数的时变性、氯离子的线性结合理论和裂缝的自愈合效应,得出了与实际试验数据相符合的结果。(2)采用双重孔隙介质模型,考虑氯离子在混凝土孔隙内的线性结合,将水分和氯离子在混凝土和裂缝内的迁移分别表示为扩散形式和对流-扩散形式,得出了不同饱和度下的水分扩散方程和氯离子对流-扩散方程,及Rayleigh-Ritz分布下的相应水分扩散系数随孔隙饱和度的变化规律,并采用有限单元法和有限差分法计算了干湿交替作用下的水分和氯离子在一规则开裂混凝土内的迁移过程,得出的干湿循环状态下的氯离子侵蚀结果与试验结果相吻合。(3)进行了普通混凝土、掺加0.1%聚丙烯纤维和掺加1%钢纤维的混凝土在不同湿度环境中的养护,并对养护好的部分试件进行75℃和120℃的烘烤,然后对所有试件进行了氯离子浸泡试验,60d和180d后的测试结果表明,养护时混凝土湿度越小,后期施加的温度越高,浸泡后相同深度的氯离子质量分数越大,且拟合后得到的表观氯离子扩散系数越大。掺加纤维后氯离子质量分数及表观扩散系数的增加程度均有所减缓。(4)研究了普通混凝土、掺加0.1%聚丙烯纤维和掺加1%钢纤维的混凝土在10次、25次、50次冻融循环后的动弹性模量、质量损失、抗折强度、抗压强度及120d的氯化钠溶液浸泡试验,结果表明,除冻融循环10次后的抗压强度及氯离子扩散系数外,三种混凝土的动弹性模量、抗折、抗压强度均随冻融循环次数增加而减小,掺加纤维的混凝土的动弹性模量、抗折强度、抗压强度降低速度及氯离子扩散系数的增大速度均小于普通混凝土。在普通体积掺量下,钢纤维抵抗微裂缝的能力要稍优于聚丙烯纤维,可以减缓混凝土在冻融过程中的损伤速度。(5)研究了加载中和加载后不同应力水平下的氯离子在普通混凝土内的侵蚀。通过对试件180d的取样测试,结果表明,当施加的压应力小于抗压强度的50%时,无论施加的是瞬时荷载还是持续荷载,混凝土内同一深度的自由氯离子质量分数随荷载水平的变化没有一定的规律性,当荷载施加到95%的抗压强度后卸载时,混凝土内相同深度的氯离子质量分数比其他应力水平时明显增大。且顺着荷载施加方向的氯离子质量分数要小于垂直荷载施加方向的值。施加不大于50%极限压应力的荷载后混凝土的表观氯离子扩散系数有所减小,其中瞬时荷载下混凝土内表观氯离子扩散系数大于相同应力水平下持续荷载作用下混凝土内的值。当应力水平达到70%时,瞬时荷载下混凝土内的表观氯离子扩散系数随着应力水平的增加逐渐增大。据此建立了相应的多项式计算模型。(6)采用两端对锚、三点加载的方式使钢筋混凝土梁式试件跨中开裂,研究了不同跨中裂缝宽度和加载方式对氯离子在混凝土内侵蚀和受拉区钢筋腐蚀的影响。通过对试件不同位置的取样测试,结果表明,弯曲裂缝处的氯离子质量分数远大于没有开裂区域,拉应力对开裂区和受拉区的氯离子输运也起到了一定的加速作用,受拉区的相对氯离子扩散系数在1.1-2.2之间。水灰比越大,应力水平越高,相同截面内的氯离子质量分数越大,变化荷载比持续静荷载更加速了氯离子的侵蚀速度。相同时间下,受拉区的钢筋锈蚀程度也要大于非受力区。水灰比越大,保护层厚度越小,受拉区的钢筋锈蚀程度越大,在本文所采用的应力水平及荷载作用方式范围内,钢筋锈蚀几乎没有变化。(7)将混凝土内钢筋初锈作为结构的耐久性极限状态,结合已经得到的参数关系,采用概率的方法分析了不同损伤情形下的耐久性失效概率和寿命,得出了随损伤参数变化的初锈概率分布曲线。结果表明,随着养护时混凝土饱和度的降低,其内部钢筋的初锈慨率逐渐增加,耐久性寿命相应缩短。冻融循环可使混凝土的寿命降低55%。较小的压应力延长了混凝土结构的使用寿命,但当压应力水平超出一定限值后,其使用寿命会逐渐减小。随着混凝土拉应力水平或钢筋混凝土构件中钢筋拉应力水平的增加,混凝土结构的使用寿命逐渐减小,当素混凝土达到80%的应力水平时,其使用寿命缩短了16.9%。当钢筋应力达到100MPa时,钢筋混凝土构件的使用寿命缩短了47.2%.更多还原

【Abstract】 Chloride ions have a tremendous influence on the durability of marine concrete structures damaged by environments and loads. It is important to study the chloride ions transportation in damaged and cracked concrete in order to have a better understanding on the ion transmission mechanism and to predict the reasonable service life of concrete structures. In this thesis, the following studies have been carried out.Modified dual porous media model and two-dimensional model are introduced separately to discuss the chloride ions transport in saturated and cracked concretes on basis of the stationary or flowing solutions. The change of time-dependent surface chloride concentration, the decrease of apparent diffusion coefficient, the linear bonding of chloride ions in concrete porous, and the self healing are considered in these two models. The calculated results are agreed very well with the experimental results.The dual porous media model is modified to analyze the chloride ions transport in unsaturated and cracked concrete. Water and chloride ions are expressed as diffusion and advection-diffusion equations respectively. The diffusion coefficients are expressed as saturation’s equation with the Rayleigh-Ritz distribution of concrete porous diameter and crack width. A cracked concrete specimen periodicity exposed to chloride solutions is calculated by finite element and finite difference methods, which results are agreed very well with the experimental results.Three different types of concrete materials, including OPC (ordinary performance concrete), PFC (polypropylene fiber concrete) and SFC (steel fiber concrete), were cured in different humilities for 28 days. To reflect the influence of high temperatures on the durability of concrete, parts of them were baked in oven. Some of these specimens were immersed into chloride solutions for 60 and the others for 180 days. Then the chloride in concrete was tested by RCT method. The results show that both the reduction of curing humidity and the increase of baking temperature can increase the diffusion coefficient. But this process can be slowed by polypropylene and steel fibers.After freeze-thaw cycles, the dynamic elastic modulus, flexural strength and compressive strength of former three types of concretes were tested firstly. Then some of the tested specimens were exposed to chloride solutions for 120 days. Results show that the dynamic elastic modulus, flexural and compression strengths decrease with the increase of freeze-thaw cycles. Chloride concentration at the same depth and diffusion coefficient increase with the freeze-thaw cycle times.The specimens with or after the compression stresses were exposed to chloride solution for 180 days. The experimental results show that when the stress level is less than 70%, chloride ions present little variations, the diffusion coefficient decrease a little. But when the stress level increase to 95%. both the chloride concentration and diffusion coefficient increase with the increase of stress levels. The chloride diffusion coefficient with stress is a little smaller than that after stress. A polynomial equation is proposed to predict this regulation according to the test results.Concrete beams were subjected to three-point bending by coupling in pairs, and cracks appeared in the mid-span of these beams. The influences of cracks and tensile tress on chloride concentration and diffusion coefficient were studied. Results show that the chloride concentration in cracked zone is much larger than that in uncracked section. The chloride diffusion coefficient in tensile zone increases 1.1～2.2 times. Chloride concentration increases with the increase of water to cement ratio and stress level. Varied load accelerates the chloride transportation. Tensile stress also speeds up the reinforcement corrosion process. The steels in tensile zone corrode much more when the water to cement ratio increases and the cover depth decreases, but vary a little in beams with different stress levels and crack widths in this thesis.The service life of damaged concrete is predicted on basis of the probabilistic framework of Monte Carlo Simulation when regards steel corrosion starting as limit state. Probabilistic information about uncertainties related to the surface chloride content and the threshold chloride concentration has been estimated from previous experimental or statistical studies, and the damage ratios are getting from test results. The predicted life reduces with the decreases of curing saturation, and with the increase of temperature, freeze-thaw cycle times and tensile stress. Compressive stress below 80% of the ultimate compressive strength increases the life, but decreases the life above that level.更多还原