Abstract
In order to accelerate the application of magnesium oxide (MgO) expanding admixture in roller compacted concrete (RCC), aiming at common three-grading RCC used in water conservancy and hydropower engineering, the autoclave expansion of mortar specimens and the compressive strength of the autoclaved mortar specimens were studied by means of autoclave test and compressive strength test. Results showed that the balanced MgO content in RCC, which cannot be determined by the MgO content corresponding to 0.5% autoclave expansion rate of mortar specimens or the inflection point of the curve of autoclave expansion changing with MgO content like normal concrete, should be determined by the MgO content corresponding to the inflection point of the curve that compressive strength of autoclaved mortar specimens changes with MgO content to ensure long-term safe operation of RCC dams.
Highlights
IntroductionA large number of experimental studies and engineering practices show that the magnesium oxide (MgO) concrete, which is formed by mixing with MgO expansion agent in concrete, has excellent delayed microexpansion characteristics. is delayed microexpansion can compensate for the volume shrinkage of dam concrete during the long temperature drop. us, the desire of simplifying or even canceling the temperature control measures of dam concrete, speeding up the construction progress, and saving project investment can be realized [1,2,3,4,5,6,7,8,9,10]. erefore, MgO concrete has been increasingly used in small and mediumsized water conservancy and hydropower engineering in China, up to now more than 50, and its application has been extended from normal concrete dam to roller compacted concrete (RCC) dam [11,12,13,14,15,16,17,18]
Curve of autoclave expansion rate varying with added magnesium oxide (MgO) content Curve of compressive strength varying with added MgO content added MgO content has not reached 12%. is is an undesirable situation for concrete structure because long-term safe operation of buildings is the first priority for all engineers and technicians. erefore, for the mortar specimens prepared with reference to the mix proportion of roller compacted concrete (RCC), the balanced MgO content in concrete should be determined by the MgO content corresponding to the inflection point of the curve of compressive strength varying with added MgO content but not the autoclave expansion rate of 0.5% or the inflection point of the curve of autoclave expansion rate varying with added MgO content
(1) e autoclave expansion rate of mortar specimens increases with the increase of MgO content and increases rapidly when added MgO content increases to a certain value, which can be seen from the obvious inflection point on the curve of compressive strength varying with added MgO content
Summary
A large number of experimental studies and engineering practices show that the magnesium oxide (MgO) concrete, which is formed by mixing with MgO expansion agent in concrete, has excellent delayed microexpansion characteristics. is delayed microexpansion can compensate for the volume shrinkage of dam concrete during the long temperature drop. us, the desire of simplifying or even canceling the temperature control measures of dam concrete, speeding up the construction progress, and saving project investment can be realized [1,2,3,4,5,6,7,8,9,10]. erefore, MgO concrete has been increasingly used in small and mediumsized water conservancy and hydropower engineering in China, up to now more than 50, and its application has been extended from normal concrete dam to roller compacted concrete (RCC) dam [11,12,13,14,15,16,17,18]. When MgO concrete damming technology is applied to actual engineering in China, the added MgO content in normal concrete is generally determined by autoclave test referring to the Chinese National Standard Autoclave Method for Soundness of Portland Cement (GB/T750-1992) (hereinafter referred to as GB/T750-1992) to take mortar whose material composition is closer to the concrete than neat cement paste as the autoclave specimens. This method is named mortar autoclave method [26, 27]. RCC which is commonly used in dams of modern water conservancy and hydropower engineering contains fly ash up to 55–70%, which is significantly higher than 25–45% in normal concrete. en, compared with normal concrete, how to determine the balanced MgO content in RCC? At present, there are few reports about this problem, and it will be discussed in this paper so that it is helpful to apply MgO expansion agent to RCC
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