STUDIES ON FROST DAMAGE OF PRECAST CONCRETE USED TO AGRICULTUREL ENGINEERING

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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