Chen, Lou; (2022) Effect of densification and post-densification regime on the microstructure and strength of microwave cured concrete. Doctoral thesis (Ph.D), UCL (University College London).

Text Final version-PhD Thesis-Lou Chen 16088188.pdf - Accepted Version Access restricted to UCL open access staff until 1 December 2027. Download (20MB)

Abstract

Microwave curing technique, as a promising alternative to steam curing, can provide volumetric heating as it can penetrate into cementitious materials instantly and convert the energy at the molecule level. Pilot research demonstrated the feasibility of densifying the microstructure of fresh concrete by utilizing the volumetric heating of microwave, thus increasing the strength of hardened concrete. However, in most of the previous studies, the curing process was conducted in a domestic microwave oven without applying proper control of temperature and humidity. Consequently, the densification regime employed could cause potential issues such as overheating and excessive water evaporation which may detrimentally affect the long-term performance of concrete. On the other hand, as rapid and volumetric heating can be achieved, it is questionable if the parameters such as slow temperature rising rate as specified in the current standard of steam curing would still be appropriate in the post-densification stage of microwave curing. Therefore, in this study, the effect of both the densification and post-densification regime on the strength and microstructure of concrete was systematically investigated for the first time. Concrete samples with w/c ratios of 0.3 and 0.5 were prepared and subjected to microwave curing, steam curing, and standard curing, respectively. Strength of bulk and near-surface concrete were characterized by compressive strength and rebound hammer tests at the ages of 1d, 7d, 14d, and 28d. Qualitative and quantitative approaches, including electrical resistivity, UPV, water absorption, BSE-IA, and 1H-NMR, were employed to examine the pore structure and ITZ of bulk matrix and near-surface region of concrete. The parameters affecting the densification and the post-densification, including densification temperature, end point of densification period, onset of temperature rising period, temperature rising rate,maximum curing temperature and total curing duration were carefully controlled through a tailored-designed, fully-instrumented, pilot industry-scale microwave system. Results indicate that optimal densification can be achieved when the volume of evaporated water equals to that of plastic shrinkage. When the excessive evaporation of water occurred, both the strength and microstructure could be detrimentally affected. Plastic shrinkage cracking can be avoided by ending the densification before the initial setting. The onset of temperature rising period at final setting could mitigate the heat-induced damage during the subsequent temperature rising period due to the conversion of free water into chemically bound water, thus contributing to better strength and microstructure. A temperature rising rate of 1°C/min is acceptable for microwave curing where the temperature gradient is still within the maximum allowable temperature gradient as specified in the current standards. This temperature rising rate (i.e., 1°C/min) is much higher than the 0.23°C/min temperature rising rate currently adopted for steam curing. The proper duration of the total microwave curing process is identified to be 9 hours, after which the gain of compressive strength for microwave-cured concrete is not obvious. This is consistent with the evolution of the content of both portlandite and chemically bound water. Compared to steam curing, under similar maturity, microwave curing results in enhanced macro-performance in terms of compressive strength, rebound number, UPV, electrical resistivity, and sorptivity. This improvement primarily derives from the refinement of microstructure, including the reduced porosity and ITZ width, particularly within the near-surface concrete. In addition, the total energy consumption of microwave curing is only about one third of that of steam curing, showing a great potential as a low-energy curing technique for concrete production. It is anticipated that the outcome from this research can provide a fundamental understanding of microwave curing regime and lay a foundation for the application of microwave curing technique in the precast concrete industry in future.

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