| Summary: | 碩士 === 國立高雄應用科技大學 === 土木工程與防災科技研究所 === 104 === The research objective was to evaluate Ordinary Portland Cement concrete subject to various elevated temperatures. Single OPC concrete mixture with water to cementitious (w/c) equal to 0.51 was proportioned. Concrete specimens were cast and placed in the curing tank in which water was saturated with calcium hydroxide. After ninety days of moist-cure, three elevated temperatures, namely 300, 600, and 900-°C, were carried out upon hardened concrete specimens. Furthermore, two post-damaged conditions were executed to recover damaged concrete specimens: one was to re-cure under 23°C with 50% humidity in a controlled environmental chamber and the other was to re-cure in the same curing tank. Mechanical properties including compressive strength as well as nondestructive approaches including ultrasonic pulse velocity and acoustic emission tests were performed on both damaged and recovered concrete specimens.
The experimental results showed that concrete specimens expanded in volume, lost in weight, and observed more surface cracks, as temperature elevated to 300, 600, and 900-°C. The compressive strength decreases 107.4, 51.1, and 16.0-% as well as ultrasonic pulse velocity declined 87.2, 45.6, and 22.8-%, as temperature increased. It has to be noted that the compressive strength was found to increase when the concrete specimens were subject to 300°C. Further study may be needed to investigate. However, compressive strengths were found more and less proportional to the UPV data, as higher temperatures elevated to 600 and 900-°C.
Acoustic emission apparatus coupled with the splitting tensile test was found able to assess damaged concrete. Before concrete subject to elevated temperatures, the development of indirect tensile strength versus displacement diagram fit well with the tendency of AE energy release. It was found there was a large amount of AE energy released when stress and displacement diagram developed about 40-50%. As such could be identified as the onset of fracture and the plain concrete generally exhibited a quasi-brittle fracture; however when concrete specimens were subject to elevated temperatures, neither the damaged nor recovered concrete specimens displayed the similar fracture pattern of plain concrete.
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