| Almost all the concrete material in engineering practice is under damage andunsaturated situations. In this thesis, how elastic load influence the mass transportproperty of concrete is first focused. As an important part of the thesis, it is prettynecessary to study the effect of freeze-thawing action on the mass transport property ofconcrete from the macro-level and the microscopic distribution characteristics of thefreeze-thawing damage from the micro-level. Since the water saturation has alsoimportant influence on the mass transport properties, the permeability of concrete atdifferent degree of saturation is also experimentally studied.Before the test of mass transport properties, some concrete specimens with lowdamage are prepared by applying low-level load. Then, the difference between loadedand unloaded specimens is compared by measuring the mass transport property of thedamaged and un-damaged concrete. The concerned mass transport properties includechloride permeability, gas permeability and water sorptivity.The influence of freeze-thawing action on the mass transport properties ofdamaged concrete material is also experimentally studied. The first step is to createdifferent degrees of freezing-thawing damage in concrete specimens by rapid freezing-thawing method. Mass loss rate is used to characterize the concrete surface damage.Moreover, the decrease of relative dynamic modulus and increase of ultrasonic damageare also employed to characterize the internal damage of concrete material. From theviewpoint of mass transport properties, the damage index that is suitable to characterizefreezing-thawing damage of concrete material is discussed. Furthermore, three masstransport properties including gas permeability, water sorptivity and unsaturateddiffusivity calculated from water sorptivity are tested in the lab. The indicators lisedabove are used to characterize the mass transfer properties. Meanwhile, the relationshipsbetween them and macroscopic damage indexes of concrete are also investigated.To microscopically quantify the characteristics of freeze-thawing damage, onetypical damaged specimens were cut into sheets, whose surface were scanned by high-precision scanner to obtain the cracking maps at different locations. Furthermore, CADpackage is used to draw the cracking map. Auto-Lisp language is secondary developedto quantitatively analyze the detailed information of cracking map including cracklength distribution, crack density and crack orientation degree. Finally, these quantitiesof micro-crack network along the height direction are investigated.Besides, the self-healing of the damaged concrete is also studied. Moreover, thegas permeability and water sorptivity are measured at different saturation degree toinvestigate the effect of water content on mass transport property of concrete material. The results show that low-level load indeed influence the mass transport propertyof concrete, especially when the density of concrete is lower. Internal damage index ofconcrete is more appropriate to evaluate the freezing-thawing damage of concrete fromthe viewpoint of mass transport properties. Mass transport properties such as gaspermeability will gradually deteriorate with the increase of internal damage. From themicro-level, crack length of the freezing-thawing damaged concrete follows lognormaldistribution. The crack density on the two-dimensional sections at different heightschanges in a relatively small random range. According to the orientation index,freezing-thawing damage is close to isotropic. Moreover, concrete material afterfreezing-thawing action is approximately isotropic in the height direction. It is indicatedthat the mass transport indexes reduced about50%after self-healing for more than amonth immersed in the water. The gas permeability and water absorption will non-linearly decrease along with the increase of saturation degree. |
