混凝土及预应力混凝土冻融机理及耐久性评估研究

发布时间：2018-04-20 08:47

本文选题：耐久性 + 冻融循环　；参考：《东南大学》2017年博士论文

【摘要】：冻融循环破坏是寒冷地区建筑物的主要病害之一,严重影响了建筑物的正常使用性能及长期使用的安全性,带来的经济损失十分巨大。冻融循环对混凝土的微观、细观结构造成损伤,微细观结构的改变决定了混凝土材料宏观性能的退化、失效、破坏,材料宏观性能的退化对混凝土结构耐久性评估具有重要意义。本文从试验、理论和有限元模拟分析三个方面研究了冻融循环作用对混凝土材料细观损伤、宏观力学性能退化以及构件承载能力衰减的影响,主要成果如下:(1)通过材料性能试验,采用RapidAir、金相显微镜及IPP等微观测试手段,对混凝土细微观冻融损伤进行了定性和定量分析,结合宏观性能测试,认识到混凝土抗压强度在评价冻融损伤的作用。随着细观冻融损伤的发展,混凝土抗压强度呈三段式下降。含气量试验研究表明:当混凝土强度较高时(本试验中混凝土28天强度为66.55MPa),较大的孔隙间距系数以及较小的含气量并不一定意味着较差的抗冻性。细观研究表明:孔隙群更容易受到冻融循环的影响而出现微裂缝,出现时间较早,利于消散孔隙间的压力,该区域的损伤累积与冻融循环前期混凝土抗压强度的线性下降能较好地对应;相对孤立的孔隙和砂浆与粗骨料界面过渡区(ITZ)对冻融循环较不敏感,当冻融循环累计到一定程度后,这些区域才会出现明显损伤,预示着混凝土材料将出现宏观性能的加速退化甚至失效破坏。宏观研究表明:对混凝土进行耐久性全周期的评价时,尤其是混凝土损伤较严重,相对动弹性模量(RDME)已不再适用时,混凝土抗压强度是一个更加稳定、可信的冻融损伤评价指标。结合细观和宏观研究发现:随着细观冻融损伤的增长,混凝土的宏观抗压强度呈三段式下降。初始阶段,冻融引起的孔间裂缝改变了混凝土内部原有的缺陷分布,混凝土抗压强度下降较快。中间阶段,孔间裂缝稳定扩张,一些较大的孔开始因为冻融而出现孔壁剥落,混凝土抗压强度下降速度减缓。失效阶段,孔间裂缝形成一个更大的孔,与其他大孔之间形成更为宏观的裂缝,混凝土抗压强度下降速度再一次加快,最终导致混凝土冻碎。(2)编制了有限元分析子程序,基于对孔隙的模拟和改进的混凝土损伤塑性模型,模拟了混凝土细观冻融损伤,冻融损伤的混凝土抗压强度的变化与材料试验结果吻合。通过Python语言和Fortran语言的模块化混合编程,改进了混凝土损伤塑性模型,实现了受损混凝土单元的失效删除,开发了一套自动的混凝土细、宏观冻融损伤的模拟程序(源代码参见附件),程序涵盖了几何模型的生成、材料属性的定义、单元网格的划分以及重启动分析计算。该程序具有友好的用户交互界面,良好的通用性,允许用户自定义模型的几何参数以及材性参数,后续的数值模拟均由计算机自动执行,大幅提高了有限元模拟的效率以及可重复性。模拟出的细观冻融损伤与试验现象较一致,即首先在距离较近的孔间形成微裂缝,孔隙膨胀到一定程度后开始出现孔边剥落。随着细观冻融损伤的增长,模拟的抗压强度平均值呈三段式下降,与实测数据吻合较好:同时,模拟的抗压强度包络图能较好地覆盖实测数据散点。(3)基于预应力混凝土构件冻融试验,评估了冻融引起的预应力损失,结合有限元分析给出了考虑冻融的混凝土结构寿命预测的简化方法。完成了耐久性全周期的预应力混凝土构件冻融循环试验。同条件下的预应力混凝土构件与素混凝土材料表现出不同的破坏形态,这是因为预压应力改变了构件混凝土材料内的应力分布,这些应力会改变冻胀应力,导致构件内微裂缝的发生和发展改变。孔道内灌浆在冻融早期的不均匀破坏导致构件端部的预应力损失较大,在冻融后期灌浆完全破坏后对冻融预应力损失影响很小。即使混凝土材料完全冻碎,预压力和箍筋也还能将冻碎的混凝土保持为一个整体。混凝土冻碎时,测得的极限冻融预应力损失约为5%σcon(张拉控制应力),考虑到冻融引起的预应力损失是一个长期预应力损失,随着暴露在冻融环境时间的增长,混凝土材料损伤越严重,冻融预应力损失越大,因此,在评估寒冷地区预应力混凝土结构时尤其要引起重视,建议轴心预应力构件的冻融预应力损失的取值不小于5%σcon。以混凝土材料冻融循环损伤的研究成果为基础,构建了利用有限元模拟冻融预应力损失和冻融损伤后承载能力的方法,模拟结果与试验结果吻合较好。同时,改进了冻融预应力损失、冻融后承载力的简化公式,与试验结果、有限元模型结果对比发现,在混凝土的抗压强度高于初值的50%时,简化计算结果与实测结果及有限元模拟结果吻合较好,具有较高的可靠性;若混凝土抗压强度继续下降,因冻融损伤程度和分布的离散性而出现不可预测的误差。最后,结合对混凝土材料、构件等在冻融下的试验、理论和模拟研究以及基于性能退化的长寿命产品寿命预测技术,给出了基于混凝土抗压强度退化的混凝土结构冻融寿命预测的简化方法。
[Abstract]:The damage of freeze-thaw cycle is one of the main diseases of buildings in cold area, which seriously affects the normal use performance and the long-term safety of the building. The economic loss is very great. The freezing and thawing cycle causes damage to the micro and meso structure of concrete. The change of micro structure determines the degradation of the macroscopic properties of the concrete. The degradation of failure, failure and macroscopic properties of materials is of great significance to the durability evaluation of concrete structures. In this paper, the effects of freezing and thawing cyclic action on the meso damage of concrete materials, the degradation of macroscopic mechanical properties and the attenuation of the bearing capacity of the structures are studied in three aspects of test, theory and finite element simulation analysis. The main achievements are as follows: (1) By means of RapidAir, metallographic microscope and IPP, the concrete and microcosmic freezing thawing damage was analyzed qualitatively and quantitatively, and the effect of concrete compressive strength on the evaluation of freezing thawing damage was realized. With the development of meso freezing thawing damage, the compressive strength of concrete was three segments. The test of gas content shows that when the concrete strength is high (the strength of concrete 28 days is 66.55MPa), the larger pore space coefficient and the smaller gas content do not necessarily mean the poor frost resistance. Early, to dissipate the pressure between the pores, the damage accumulation in this area can correspond to the linear decrease of the compressive strength of concrete at the early stage of the freeze-thaw cycle, and the relative isolated pores and the transition zone between the mortar and the coarse aggregate interface (ITZ) are not sensitive to the freezing and thawing cycle, and when the freezing and thawing cycle is tired to a certain extent, the areas will appear to be obviously damaged. It indicates that the concrete material will have accelerated degradation of the concrete material and even the failure and failure of the macroscopic performance. The macro study shows that the concrete compressive strength is a more stable and credible evaluation of the freezing and thawing damage when the concrete damage is more serious, especially when the relative dynamic elastic modulus (RDME) is no longer applicable. With the combination of meso and macro studies, it is found that the macro compressive strength of concrete decreases with the increase of freezing thawing damage. In the initial stage, the gap between the holes caused by the freezing thawing changes the original defect distribution in the concrete, and the compressive strength of concrete decreases rapidly. In the middle stage, the cracks between the holes expand steadily and some of the cracks are larger. The hole wall exfoliates because of the freezing and thawing, and the compressive strength of concrete decreases. In the failure stage, a larger hole is formed between the holes, and a more macro crack is formed between the other large holes. The compression strength of concrete is accelerated again, and the concrete is finally frozen. (2) a finite element analysis subprogram has been compiled. Based on the simulated and improved plastic damage plastic model of concrete, the concrete meso freezing thawing damage is simulated. The compressive strength of concrete is in agreement with the test results of the concrete. The damage plastic model of the concrete is improved by the modular hybrid programming of the Python language and the Fortran language, and the damaged concrete single is realized. This program covers the generation of the geometric model, the definition of the material properties, the division of the cell grid and the reboot analysis and calculation. The program has a good user interface, good generality, allowing users to do so. The geometric and material parameters of the model are defined, and the subsequent numerical simulation is performed automatically by the computer, which greatly improves the efficiency and repeatability of the finite element simulation. The simulated micro freezing and thawing damage is the same as the test phenomenon, that is, the micro cracks are formed between the near holes, and the pore expansion will begin to appear to a certain extent. The average value of the simulated compressive strength decreases with three segments, which is in good agreement with the measured data: at the same time, the simulated compressive strength envelope can cover the measured data well. (3) based on the freezing thawing test of prestressed concrete members, the loss of prestress caused by freezing and thawing is evaluated, and the finite element method is combined with the finite element method. A simplified method for predicting the life of concrete structures with freeze-thaw is given. The freeze-thaw cycle test of prestressed concrete members with full durability is completed. The failure forms of prestressed concrete members and plain concrete under the same conditions are different, because the prestress stress changes the concrete material in the concrete. The stress distribution, these stresses will change the frost heave stress and lead to the occurrence and development of the micro cracks in the component. The uneven failure of the grouting in the early freeze-thaw process leads to the greater loss of the prestress at the end of the component, and the effect on the loss of the freezing and thawing prestress is very small after the complete failure of the grouting at the late freezing and thawing period. Pressure and stirrup can also keep the frozen concrete as a whole. When the concrete is frozen, the ultimate freezing thawing prestress loss is about 5% con (tension control stress). Considering that the prestress loss caused by the freezing and thawing is a long term prestress loss, the damage of concrete material is more serious as the exposure to the freezing and thawing environment increases. The greater the loss of the freezing thawing prestress, the more attention should be paid to the evaluation of the prestressed concrete structure in cold regions. It is suggested that the loss of the freezing thawing prestress loss of the axle center prestressed component is not less than 5% Sigma con., based on the research results of the freeze-thaw cycle damage of concrete materials, and a finite element simulation of the loss of frozen thawing prestress and the loss of prestressed concrete is constructed. The simulation results are in good agreement with the test results. At the same time, the simplified formula of the loss of freeze-thaw prestress and the bearing capacity after freezing and thawing is improved. Compared with the experimental results and the finite element model, it is found that the simplified calculation results, the measured results and the finite element simulation are obtained when the compressive strength of the concrete is higher than the initial value of 50%. The results are in good agreement and have high reliability. If the compressive strength of concrete continues to decline, the uncertainty of the degree of freezing thawing damage and distribution is unpredictable. Finally, combined with the experiments of concrete materials, components and other frozen thawing, theoretical and simulation research and life prediction technology based on the performance degradation of long life products, A simplified method for predicting freeze-thaw life of concrete structures based on deterioration of compressive strength of concrete is presented.

【学位授予单位】：东南大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TU528

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