Expansive Property Of Concrete Incorporated With Lightly Calcined MgO Under Different Autoclave Conditions

The technique of dam construction by weakly expansive concrete added with magnesia (WECAM) is a new technique developed in China, in which appropriate light-burnt magnesia is added to dam concrete when produced to generate a special delayed expansive stress to compensate shrinking stress due to decrease of temperature so as to prevent concrete cracks. It not only is in favor of simplifing the method of temperature control and an effective solution to the crack problem of concrete dam, but also simplify the technology of dam construction and bring down the project cost, further more, speed greatly up the construction of concrete dam. With the advent of 21st century, a greater development is taking place in the dam construction in china, where this technique must play more important role. Therefore it has great advantage of technology and economy and will bring a bright prospect to the development in the dam construction in China.Soundness, as a vital quality index of weakly expansive concrete added with magnesia, attracts much attention of dam constructers. If magnitude of expansive stress caused by adscititious magnesia is too large, or its distribution is not uniform, it trends to damage the structure of cement concrete, and then affect seriously dam durability. The evaluating standard for soundness of added with magnesia, which is based on the standard for cement paste at present and criticized by many experts of dam construction, have to be improved.In this paper evaluating standards for cement and concrete in china and abroad are reviewed, and expansive behaviors of various micro-expansive concrete, added with various content of magnesia (MgO) and fly-ash (PFA) and autoclaved under different autoclaving condition, are studied. On this base proper autoclaving condition of weakly expansive concrete withmagnesia and its evaluating standard are put forward. The main conclusions are as follows:1.The autoclave expansion of WECAM goes up and its rate trends to mitigation when autoclave temperature rise under other fixed condition. The autoclave expansion of WECAM is distinctly different from one another with temperature from 150℃ to 200℃, but it comes near from 200℃ to 216℃ under other uniform condition.2. The autoclave expansion of WECAM rises under other fixed condition when autoclave time is prolonged. Its rate is obvious at 150℃ and 180℃,but unobvious at 200℃, 210℃ and 216℃. It shows that prolonging of autoclave time increases autoclave expansion of WECAM effectively.3. It is in favor of evaluating soundness of WECAM by means of combining concrete autoclave expansion with its strength. The experimental results reveal that autoclaved concrete strength ascends its autoclave expansion ascends, but autoclaved concrete strength descends at the same time its autoclave expansion ascends when autoclave expansion is more than 0.1%.4. The autoclave expansion of WECAM and its rate rise under other fixed condition when the magnesia content increases. If magnesia is added excessively, it will cause crack, even crash.5. The autoclave expansion of WECAM and its rate reduce under other fixed condition when fly ash (PFA) content increases. It shows that fly ash added may restrain expansion of WECAM and improved its soundness.6. As for WECAM, its soundness should be tested appropriately with concrete specimens autoclaved at 200℃ for 6h, its limit expansion should be determined from 0.1% to 0.2%. All these are the foundation of determining optimal content of magnesia added in WECAM.