Which test is used to determine the air content in fresh concrete?

The test used to determine the air content in fresh concrete is the pressure method outlined in ASTM C231. This method involves placing a sample of concrete into a pressure chamber, where the air is displaced by applying pressure. The amount of air present in the concrete can then be calculated based on the difference in pressure readings before and after the test.

This is essential in concrete work because a certain amount of air voids is necessary for preventing damage due to freeze-thaw cycles; however, too much air can weaken the concrete. By accurately measuring the air content, this test helps ensure the performance and durability of the concrete once it sets.

The other methods do not directly measure air content. For instance, ASTM C31 pertains to field sampling of concrete, while ASTM C39 focuses on measuring the compressive strength, which evaluates the concrete's load-bearing capacity after it has cured. ASTM C143 is designed to measure the workability or consistency of fresh concrete through the slump test, not to assess air content.