Research On Mechanisms Of Thermal Deformation Damage Of Concrete Materials And Structures And Migration Methods

The cement concrete is a heterogeneous material that is composed of multiple ingredients with different thermal expansion properties. A drastic temperature variation causes different thermal volumetric changes of the components and hence thermal stress in the interface is generated. If the stress is greater than the bond strength between paste matrix and the aggregate, thermal cracking takes place and thus degradation of the concrete is observed. The thermal behavior and migration of thermal cracking of concrete in an extremely cold and dry climate with large temperature variations is investigated in this thesis via thermal compatibility study of components in concrete. The thermal expansion/shrinkage of cement paste and concrete in the cyclic thermal treatment is tested, based on which the evolution of thermal interface cracking is revealed. The effects of fly ash, silica fume and styrene-acrylic latex on the thermal expansion of hardened concrete are tested and analyzed, based on which practical solution for preventing thermal microstructural cracking of concrete in cold climate with large temperature variations is proposed. Thermal methods for detecting defects in concrete structure are established, which is also tested with a highway bridge in the North China.The research results have shown that, distinct thermal mismatch between the hardened cement paste and a coarse aggregate in concrete exists in the range of room temperature to 85℃. After number of thermal cyclic shocks, cracks in the interfacial transition zone of concrete are observed, propagating along the edge of the aggregates. The thermal cyclic shocks lower the compressive strength of concrete. The water to cement ratio of cement paste is an important factor on the thermal expansion coefficient of the hardened paste.The addition of fly ash, silica fume and styrene-acrylic latex lowers the thermal expansion rate and coefficient of hardened paste. The increase of addition level is accompanied by the decrease of the thermal expansion coefficient. The styrene-acrylic latex is the most effective in lowering the thermal expansion coefficient.The effects of fly ash, silica fume and styrene-acrylic latex on the thermal properties of hardened concrete are similar to those on cement paste. By adding fly ash, silica fume or styrene-acrylic into concrete, the thermal sensitivity is greatly reduced and hence the thermal stability is optimized by changing the microstructure of concrete.The introduction of fly ash and silica fume could reduce the amount of Ca(OH)2 in concrete and improve its pore structure, while the thermal expansion rate of the materials was thus limited. Besides, the addition of styrene-acrylic latex in concrete formed a impermeable membrane on the surface between C-S-H gel and unhydrated cement particles. As a deformation adsorption layer, the membrane thus enhanced the ductility of the material and lowered its thermal expansion coefficient.Based on the studies below, the thermal expension properties of cement paste were midified by the use of mineral admixture, arganic polymers and so on. The thermal deformation capability among cement, mortar and coarse aggregate was improved, which leaded to the development of the design and preparation of concrete with higher thermal expansion resistance and thermal deformation coordination.By using infrared imaging and temperature sensors, the temperature distribution of concrete structure under large temperature difference was observed and analyzed, focusing on the relationship between its surface temperature distribution and internal structure characteristic. Based on this, the non-destructive damage assessment was developed on trends of temperature distribution of the concrete structure. With this method, not only macro-structural defects could be found directly near the surface (hollowing, cellular, etc.), you could also determine the temperature distribution, structure, thermal stress and thermal injury trends under large temperature difference, in order to provide technical support for preventing thermal injury of structures. This paper introduced a hollow thin-walled structure of the bridge in Aershan region of Inner Mongolia in China, and carried out the study of design and preparation for low thermal expansion concrete. What's more, time-varying temperature field and thermal stress of thin-walled pier were investigated by monitoring the real-time temperature using infrared imaging.