Concrete Air Content TesterDetailed explanation of working principle and operation process:
1、 Working principle: Barometric method based on Boyle's law
Concrete Air Content TesterThe core principle is Boyle's law (under constant temperature conditions, the pressure and volume of gas in a closed container are inversely proportional). The working logic is as follows:
Balance of pressure between chamber and measuring bowl
Inject air into the air chamber through a hand pump to the initial pressure (such as 0.1MPa), and release compressed air into a sealed measuring bowl filled with concrete. The air in the concrete is expelled, causing a decrease in pressure between the air chamber and the measuring bowl, ultimately reaching equilibrium.
Calculation of gas content
Given the chamber volume (V ₁), initial pressure (P ₁), and equilibrium pressure (P ₂), according to the formula P ₁ V ₁=P ₂ (V ₁+V gas), the gas volume (V gas) in concrete can be calculated. Gas content (%) is V gas/measuring bowl volume x 100.
Optimization accuracy of water injection method
The improved air pressure method (such as CA-2/CA-3 type) injects a water layer into the surface of concrete to form a flat interface, eliminating volume errors caused by uneven plastering, especially suitable for dry and hard concrete.
2、 Operation process: Standardized steps and key details
1. Preparation for the experiment
Equipment inspection: Confirm that the measuring bowl, bowl cover, and sealing ring are not damaged, and that the pressure gauge pointer is reset to zero and flexible.
Sample processing:
When the particle size of coarse aggregate is greater than 40mm, it needs to be removed by wet screening.
Concrete with a slump of ≤ 70mm shall be compacted using a vibration table (frequency 50Hz, amplitude 0.5 ± 0.1mm); When the slump is greater than 70mm, it should be compacted in layers (25 times per layer, with the upper layer inserted 10-20mm into the lower layer).
2. Loading and sealing
Layered loading:
Vibration table compaction: The concrete is loaded slightly above the bowl mouth and vibrated until the surface is smooth and glazed before stopping.
Manual compaction: It is packed in three layers, with each layer compacted to a height of 1/3 of the measuring bowl. The bottom layer is compacted to a depth, and the upper layer is compacted to a depth of 10-20mm into the lower layer.
Surface treatment: Use a scraper to scrape off excess concrete, and use a spatula to smooth it out to the level of the measuring bowl mouth, ensuring that there are no bubbles.
Sealing operation:
Clean the edges of the measuring bowl and bowl cover, place an O-ring (such as 238 × 8.6mm), cover the bowl cover and clamp it with a clamp.
Check the level with a spirit level to avoid tilting and affecting gas discharge.
3. Water injection and pressurization
Water injection and exhaust:
Open the inlet plug and outlet valve, use a suction ball to inject water until the outlet valve is open, and close both valves.
The core function of water injection method is to fill the small bumps and depressions on the surface of concrete with a water layer, ensuring sealing and improving testing accuracy.
Pressure operation:
Use an air pump or hand pump to increase the pressure to a gauge pressure slightly greater than 0.1MPa, gently tap the dial to stabilize the pointer, and fine tune to 0.1MPa.
Key details: The initial pressure should be precise to avoid equipment damage caused by overpressure.
4. Pressure balance and reading
Balance operation:
Press the valve stem 2-3 times and lightly tap around the measuring bowl to evenly distribute the pressure.
Read after the pointer stabilizes (direct reading instruments directly display gas content; non direct reading instruments need to be converted according to the calibration curve).
Repeated testing:
The same sample needs to be tested twice, and the average value should be taken. The difference between the two results should be ≤ 0.2%, otherwise retesting is required.
5. Cleaning and maintenance
Exhaust pressure relief: Open the exhaust valve and drain valve to release pressure; Release the clamp and remove the bowl cover.
Equipment cleaning: Pour out the concrete residue, rinse the measuring bowl, bowl cover, and sealing ring with clean water, dry them, and store them in a dry and ventilated place.
Long term maintenance: Regularly check the measuring bowl volume (weighing method) and sealing (whether the pointer returns to zero after being pressurized to 0.1MPa).
3、 Key precautions
Avoid water being sucked into the chamber: If the pressure in the chamber has been released but there is still pressure in the measuring bowl, do not press the valve stem, otherwise water will be sucked into the chamber, affecting subsequent testing.
Correction of aggregate air content: If the aggregate is a porous material (such as lightweight aggregate), its air content (C) needs to be measured separately, and the final result should be corrected according to A=A ₁ - C (A ₁ is the instrument measurement value).
Temperature effect: Changes in water temperature can affect gas volume. During testing, it is necessary to record the water temperature and refer to the correction factor at the standard temperature.