The pressure air measurement method, which is based on the principles of The Boyle’s law, is used to determine the air content of concrete. According to Boyle’s law, the volume of a gas is inversely proportional to its pressure. Compression occurs when pressure is applied to a concrete of known volume (that contains air voids), resulting in a reduction in volume. The volume change is calculated and compared to the initial volume. The air content in concrete is determined by the difference between the original and reduced volume of concrete. Klein and Walker used the following test for the first time in 1946. Menzel was the one who perfected the device and turned it into a regular test. One of the most significant advantages of.
Measurement of Air Content in Concrete
The air content is measured using two metres according to the ASTM test procedure for pressure air measurement. The Type A metre aids in the direct measuring of concrete volume change. A column of water is poured over a known volume of concrete to accomplish this. A calibrated sight tube can be used to determine the volume reduction after applying pressure. The Type A technique follows a simple procedure. One of the method’s drawbacks is that it necessitates recalibrating the elevation whenever the barometric pressure rises above 183 metres. This difference is approximately identical to a 2% change in sea level barometric pressure.The use of a Type B air metre necessitates the use of a.
A known volume of air at a known higher pressure is used in the Type B air metre. In a sealed container, this can equilibrate with the known volume of concrete. The amount of air within the concrete is proportional to the pressure decrease seen in the high-pressure chamber. There is no requirement for recalibration as compared to a type A air metre. However, it has its own leaking limits due to the valves. As a result, the operator must be ready at all times with the instruments necessary to fix the leakage and variations in the dial readings. Ignorance can lead to erroneous conclusions. The pressure measurement technique necessitates complete consolidation of the concrete sample in a bowl. This is due to the formation of air voids.
Air Content in Concrete Mix Design
To achieve adequate consolidation, proper rodding above a droop of 75mm and internal vibration below 25mm is required. The vibration must be sustained until the surface is clear of aggregate and has a gleaming appearance. Over vibration can cause purposefully entrained air gaps to be removed. Concrete with aggregates larger than 50mm must be screened through a 3.75 mm sieve before being tested, as representative sampling becomes problematic with larger aggregates. The concrete is struck off with a strike of the plate or a bar. When concrete has dense aggregates, the pressure method is employed less. The air in the porosity of the aggregates will be compressed in the same way that the air in the cement paste is compressed.
In locations of the country where frost damage is a possibility, air-entrained concrete is commonly required. The pressure method is commonly used to determine the amount of air in fresh concrete of normal density (ASTM C 231). ASTM C 173 is another helpful test. The pressure approach, on the other hand, is typically favoured due to its speed. Within 15 minutes after receiving the composite sample, you should begin the test. Fill the 0.25 ft3 base of the air-content test equipment with three equal layers of material, then rod each layer 25 times. After rodding, use a mallet to beat the outside of the base 12 to 15 times to seal any air voids. Strike off the bowl flush at the top after completing the three equal layers.
Fill the air gap between the top of the struck-off concrete and the bottom of the top of the air metre with water and lock the top of the air-content test device over the base. The built-in hand pump is then used to pressurise the metre top until it is zeroed out (or as calibrated). After a period of stability, release the pressure in the top and read the air-void content on the meter’s dial. Calculate the final result by subtracting the aggregate correction factor from the dial reading. Testing tip: A typical air content for concrete with a 34-inch maximum-size aggregate is around 6%, and air content specified ranges are normally minus 112% and plus 112% of the goal value.
There are three main ways for determining the air content of fresh concrete. The following are the methods: Gravimetric Method Volumetric Approach Applying Pressure Gravimetric Method: Because this method is simple and requires no special equipment, it is the first method for calculating the air content in concrete: The procedure primarily entails using a conventional method to determine the density of freshly compacted concrete. The theoretical density of air-free concrete, determined from mix proportions and specific gravities of the constituent materials of the concrete, is then compared to this density.
Methods of Air Entraining:
This procedure has been found to be useful in the laboratory. It is not well suited for field use because it necessitates knowledge of concrete mix proportions. Volumetric Method: This method is used to directly measure the volume of air in a fresh concrete sample. After removing all air from a sample of known volume of concrete using any method, the amount of water required to restore the original volume is calculated. This procedure is time consuming and laborious. Pressure Method: This is the most common and well-suited method for use on the job site. It is based on Boyle’s law, which states that the volume of air and the applied pressure (at constant temperature) are proportional.
This metre is directly graduated for air percentages, thus no knowledge of material proportions or qualities is required. This metre should not be used with porous aggregates and should be calibrated if used at a high angle depicts a simple metre. When a sample of compacted concrete is subjected to a known pressure, the test procedure involves watching the volume of the sample shrink. A small pump, similar to a bicycle pump, is used to apply pressure, which is then measured using a pressure gauge. Because of the increase in atmospheric pressure, the volume of air in the concrete reduces, causing the level of water above the concrete to fall. By adjusting the water’s level.
Why is it Important
The major goal of entraining air in concrete is to make it more durable when it is subjected to freeze/thaw cycles in the presence of water. Air-entraining admixtures for concrete produce small, consistent, and stable air bubbles in the cement matrix ranging from 0.01mm to 1mm in diameter throughout a concrete mix. The bubbles in the hardened concrete become air spaces, producing a relief system that allows water to expand as it freezes without destroying the concrete. Even while concrete that is not subjected to freeze/thaw cycles may not require air-entraining admixtures, air content testing is nevertheless recommended to profile the properties of a specific batch. The majority of non-air-entrained concrete has between 1% and 2% entrapped air, and other admixtures may unintentionally entrain even more air. Specifications for Structural Concrete and Related Materials (ACI 301-16)
The measurement of air content in fresh concrete of normal density is typically performed using the pressure method (ASTM C 231).
The methods are: 1. Gravimetric Method 2. Volumetric Method 3. Pressure Method.
Concrete having a total air void content of about 6.5% seems optimal. A mix having 6.5% total air voids will have approximately 1.5%.
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