| Summary: | 碩士 === 國立中央大學 === 土木工程研究所 === 97 === The barrier for low-level radioactive wastes disposal structures calls for the use of concrete. Taiwan is surrounded by marine environment where low-level radioactive waste disposal sites may be subject to intrusion of various elements from the external environment. When the disposal sites are exposed to such condition, it may cause concrete deterioration and reduces its durability.
This study focuses on the effect of leaching of concrete as simulated in the laboratory to assess the durability in long term use of concrete materials. The basic mixture for cements, mortar and concrete were used to produce test specimens that were exposed to different conditions such as pure water and artificial salt water to simulate calcium loss under different circumstances. The possible mechanism and potential influence of calcium loss and concrete durability were then carefully examined.
The results of the extensive laboratory experiments had shown that : (1) concrete materials has better resistance against calcium leaching in ammonium nitrate compared to cement mortar and cement when SEM-EDS measurement on the Ca/Si ratio were used. (2) The increase in duration of exposure of cement mortars resulted in the increase in porosity of the specimen and resulted in the decrease of bulk density. (3) Microstructural observation showed that mortar samples in ammonium nitrate solution experience accelerated leaching of Ca(OH)2, resulting in the severe deterioration of the surface of the specimen. The use of thermogravimetric analysis and X-ray diffraction analyses shows that the calcium content of the cement specimen decreases with depth.
Using the 4SIGHT program developed by NIST, the effects of the concrete deterioration were also evaluated. Parameter sensitivity analysis result had shown that formation factor, the water cement ratio and pozolanic material usage were essential in the resistance to chloride ions intrusion; it is also considered as an important formation factor in estimating the concrete material parameter. In addition, the concrete barriers would buckle under joint degradation mechanism, which would speed up the chloride intrusion rate, thus affecting the service life of the concrete barrier.
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