Measurement of Air Content in Concrete
The air content of concrete is determined using the pressure air measurement method, which relies on Boyle’s law. Boyle’s law states that the volume of a gas is inversely proportional to its pressure. In this method, pressure is applied to a known volume of concrete that contains air voids. As pressure is exerted, the concrete undergoes compression, leading to a reduction in volume. The change in volume is measured and compared to the initial volume, providing the air content of the concrete. This test was first developed by Klein and Walker in 1946, and later refined by Menzel, who standardized the apparatus. One significant advantage of this method is that it does not require knowledge of weights and specific gravities.
ASTM Method of Air Content Measurement in Concrete
There are two types of meters specified in the ASTM test method for pressure air measurement. The Type A meter allows for direct measurement of volume change in concrete. It involves using a column of water over a known volume of concrete and applying pressure to determine the volume reduction. This method is straightforward but has a limitation. It requires recalibration when there are elevation or barometric pressure changes exceeding 183m, which is equivalent to a 2% change in barometric pressure at sea level.
On the other hand, the Type B air meter utilizes a known volume of air at a higher pressure. It equilibrates with the concrete in a sealed container, and the pressure drop in the high-pressure chamber reflects the amount of air in the concrete. Unlike the Type A meter, there is no need for recalibration with the Type B meter. However, this method has its own limitation due to potential leakage from the valves. To ensure accurate results, the operator must be prepared to address any leakage and variations in the dial readings. Failure to do so can lead to incorrect measurements.
The pressure measurement technique requires the concrete sample to undergo complete consolidation in a bowl. This is important because air voids formed due to insufficient consolidation would be measured as part of the air content. Proper rodding or internal vibration is necessary depending on the slump of the concrete. For slumps above 75mm, rodding is used, while vibration is used for slumps below 25mm. Vibration should continue until the surface is free from aggregate and exhibits a glistening appearance. Over-vibration can lead to the removal of intentionally entrained air voids.
When testing concrete with aggregate sizes larger than 50mm, it is necessary to screen them through a 3.75mm sieve prior to the test. This is because sampling becomes more challenging with larger aggregates. A strike plate or bar is used to level off the concrete surface before conducting the test. The pressure method is less commonly used with concrete that has dense aggregates. In such cases, the air within the aggregates’ porosity becomes compressed, leading to higher air content values. To correct for this, an aggregate correction factor is applied. However, this correction factor is not suitable for concrete that uses lightweight aggregates, where the correction factor exceeds 0.5%.