STUDIES ON FROST DAMAGE OF PRECAST CONCRETE USED TO AGRICULTUREL ENGINEERING

1.在蒸压混凝土中，夹带的空气具有很强的抗冻融性能。即使在一天两次固化和较短的预固化时间的模塑样品中也表现出如此高的耐热性。混凝土的抗压强度与耐久性之间没有相关性。预制混凝土的耐久性完全取决于夹带空气。预制混凝土表面的气泡区域通过反复冻融而扩大。然而，充气预制混凝土即使在300次冻结和解冻后也没有坍塌。相比之下，素混凝土仅在90次循环后就坍塌了。试验结果表明，在预制混凝土未硬化状态下，含气量为6.5%是极限。即使在未硬化状态含气量为4.5%的引气混凝土中，如果含气量为r…，也不一定能获得高抗冻性和抗融性。更多的是受到暴力的启发。在预制混凝土工厂中实际测量的振动台上的含气量下降了约50%。对于含气量为2.1%的素混凝土，振动1分钟后；对于含气量为4.5%的引气混凝土，振动2分钟；对于含气量为6%的引气混凝土，振动4分钟。在较窄的振动幅度下，空气含量的下降速度约为1/7。本试验测得的硬化混凝土中夹带空气含量最小为2.5%，最大为3.8%。测量表明，气泡的平均含气率为220亩。即使具有最大空隙分数为254亩的样品也表现出足够的抗冻性。气泡的粒级积累以100-200亩以下的气泡为主。这可能表明，这些微泡具有防止混凝土因冻害而劣化的作用。较少

1. Enen in steamecured concrete, the entrained air provides a large resistance against deteriortion by freezing and thawing. Such a high resistance was also demonstrated even in the sample molded with 2-stage curing a day and a short pre-curing time.2. No conrelation was noted between the compressive strength and durability of concrete. The durability of precast concrete depends completely on the entrained air.3. The bubble area on the surface of precast concrete expands by repetition of freezing and thawing. However, airentrained precast concrete did not collapse even after 300 cycles of freezing and thawing. In contrast, plain concrete collapsed after only 90 cycles.4. The present experiment results indicate that 6.5% is the limit of the entrained air content in an unhardened state of precast concrete.5. A high resistance to freezing and thawing may not always be obtained even in air-entrained concrete that had the air content ot 4.5% in the unhardened state, if the air content was r … More educed by vidration.6. An approximately 50% decrease in the air content during vibration on the vibration table as actually measured in a precast concrete plant occurred after 1 minute of vibration for plain concrete with 2.1% air content, after 2 minutes of vibration for air-entrained concrete with 4.5% air content and after 4 minutes of vibration for air-entrained concrete with 6% air content. With an narrower amplitude of vibration, the rate of the decrease in the air content was reduced to approximately 1/.7. The entrained air content in hardened concrete measured in this experiment ranged from the minimum of 2.5% and maximum of 3.8%.8. The measurement revealed that the mean air void fraction of bubbles was 220mu. Even the sample having the maximum air void fraction of 254mu demonstrated an adequate freezi-resistance.9. The grain size accumulation of bubbles was predominated by air bubbles of the size below 100-200mu. This may suggest that these micro-bubbles have an effect of preventing deterioration of concrete due to frost damage. Less