Multi-Scale Nolinear Time-Varying Performance Analysis Of Concrete Under Freeze-Thaw Conditions

Multi-scale study of concrete is beneficial to deepen the understanding of macroscopic properties caused by the size,proportion and random distribution of mesocomponents,and the summary of relevant laws is beneficail to the actual production.The changes in the micro and meso scales of concrete ultimately lead to failures on the macro scale.In actual engineering,the failure of components is often determined by various factors such as material characteristics and loading conditions.The durability of concrete components in a freeze-thaw environment can also be studied.The random characteristics of materials and the influence of load and freeze-thaw coupling are considered in this article and the freeze-thaw failure of concrete is studied at the meso and macro scales respectively.The probability density evolution theory can be used to evaluate the variation of the reliability of macroscopic beam members with the number of freeze-thaw cycles.The main research contents and conclusions are divided into the following four parts:（1）According to the features and shortcomings of concrete 2D mesoscopic modeling methods have been developed,through the RGB recognition and intrusion determination of picture pixels,the random distribution of multiple different shape components is realized,and a large number of samples are obtained with high efficiency.The homogenization method is used to solve the macroscopic equivalent mechanical properties,so that the difference of the meso-aggregate gradation is reflected in the damage and destruction of the macroscopic beam members.（2）During the actual production process,there may be initial damage to the concrete member.Through fewer freeze-thaw cycles on the concrete specimen with prefabricated cracks,it is found that cracks caused by freeze-thaw are easy to occur at the location of the prefabricated cracks and extend downward along the plane they are in,and the damage plane is easy to pass through the interface between mortar and aggregate because of its weak performance.Prefabricating a certain depth of cracks on the specimen may lead to larger plastic strain under the condition of less freeze-thaw cycles,and then lead to failure.（3）The distribution characteristics of pore water in concrete are the main factors affecting its frost resistance.Exploring the proportion of saturated pore water area and random distribution in the meso-model can have a deeper understanding of its damage law.Simulations by multiple sets of models The comparative analysis of the results shows that during the freeze-thaw cycle,the damage is first generated around the saturated pore water,and the expansion speed of the damage in different areas is different.Therefore,the random distribution of the saturated pore water in the model will lead to different degrees of damage;load coupling As a result of the effect,a certain amount of tensile stress will significantly promote the extension of the damage,while the compressive stress will inhibit the expansion of the damage to a certain extent.（4）Considering the constitutive characteristics of concrete in the freezing and thawing environment and the random characteristics of concrete,steel bars,and external loads,based on the probability density evolution theory,a reliability calculation method for simply supported beams in the freezing and thawing environment is proposed.According to the constitutive relationship of concrete subjected to different freeze-thaw cycles,several groups of finite element models of reinforced concrete simply supported beams was established to analyze its nonlinear damage behavior.By extracting the deflection value,the probability density evolution curve and the reliability change curve of the concrete simply supported beam are obtained.From the analysis we can kown that the reliability of the beam members caused by freezing and thawing will decrease rapidly as the number of freez-thaw cycles increases.When the concrete strength level is determined,appropriately increasing the diameter of the bottom longitudinal ribs can effectively improve its reliability in the freezing and thawing environment.