Air Content test and Significance
This test method is suitable for fresh concrete containing normal aggregate with a maximum size of 50 mm or less, but not suitable for fresh concrete containing porous aggregate for which the aggregate correction index cannot be accurately determined. This standard covers the method of determining the air content of fresh self-compacting concrete by a pressure reduction in the air chamber. The measurement of air content in fresh concrete of normal density is typically performed using the pressure method.
Air Chamber: – As shown in diagram
Calibration of apparatus
4Calibration of the container
Fill the container with water and weigh the mass of the water. To fill water in the container, thinly apply cup grease to the flange of the container and place a polished glass plate on the flange. Fill the container with water while moving the glass plate along the flange carefully so as to allow no air bubbles to remain. Weigh the mass with a scale having a reciprocal sensitivity higher than 0.1% of the total mass of the container and water.
Determination of initial pressure
1) Fill the container with water and attach the cover gently to the container with the vent hole open. After attaching the cover, pour water until the air enclosed between the cover and the water level is discharged.
2) Close all the valves, and operate the hand pump to make the pressure in the air chamber slightly exceed the initial pressure. Approximately 5 seconds later, gradually open the regulating valve until the pointer of the pressure gage points accurately to the initial pressure mark.
3) Fully open the actuating valve to equalize the air pressures of the air chamber and the top of the container, and take the pressure gage reading. Check if the reading exactly agrees with the graduation for 0% air. If they do not agree, check for any leakage of air or water or any other defect, and repeat the calibration procedure. When the pointer points to the same position deviating from the zero point in the calibration procedure repeated several times, slide the position of the initial pressure mark so that the pointer points to it. Repeat the calibration procedure to conf1rm that the new position for the initial pressure mark is correct.
Calibration of graduation for air quantity
(1) Follow the same procedure as above, and then proceed to the following:
a. Transfer an adequate quantity of water from the container to a graduated cylinder using the tool described in 2.6 and express the quantity in percentage by volume of the capacity of the container.
b. After equalizing the air pressure in the container with atmospheric pressure, close up the apparatus and increase the air pressure in the air chamber to the initial pressure.
c. Open the actuating valve to introduce the high-pressure air into the container.
d. Take an air-quantity reading after the pointer of the pressure gage has been stabilized.
(2) Transfer water from the container to the graduated cylinder again as instructed, and express the total quantity of the water collected in percentage by volume of the capacity of the container. Take a reading of the air quantity similarly to (1).
(3) Repeat the above operations several times and compare the percentages of the collected water with the graduation marks for air quantity. When these values agree, the graduation marks are correct. When they disagree, make a diagram showing their relationship. Use this diagram for calibration of the air quantity graduation.
Measurement of aggregate correction factor
1) Calculate the mass of fine and coarse aggregates in a concrete sample with a volume V, of which the air content is to be determined, by the following equations: (Eq.)
wf = mass of fine aggregate in sample concrete with a volume of V (kg)
V = volume of sample concrete (equal to the capacity of container) (liter)
B = volume of as-mixed concrete per batch (liter)
Wf = mass of fine aggregate used per batch (kg)
wc = mass of coarse aggregate in sample concrete with a volume of V (kg)
Wc = mass of coarse aggregate used per batch (kg)
2) Collect representative samples of fine and coarse aggregates, w f and wc in mass, respectively. Immerse fine and coarse aggregates separately in water to make the moisture conditions of the sample aggregate particles equivalent to that of the aggregate particles in the sample concrete. Place aggregate in the container containing water to approximately one third of its capacity. To place aggregate, place a scoopful of fine aggregate followed by two scoopfuls of coarse aggregate so that all aggregate particles can be immersed in water. Minimize air entrapped in water when placing aggregate. Emerging air bubbles must be removed promptly. Tap the side of the container
with a mallet to release air. Also, rod approximately 10 strokes with a tamping rod to a depth of approximately 25 mm each time fine aggregate is added.
3) After having placed all the aggregate in the container, remove all air bubbles on the surface of water, wipe the flanges of the cover and container and fasten the cover to the container with a rubber gasket in between. Follow the, procedure described in 6. (2) and take an air quantity reading by the pressure gage This is taken as the aggregate correction factor, G.
Measurement of air content of concrete
1) Place the sample in the container, the sample is filled in three layers of nearly equal depths, each layer being rodded, distributing approximately ten strokes uniformly distributed over the area, and the side of the container being tapped with a mallet approximately five times. In both methods, the sample is leveled with the top rim, with the excess being scraped off with a straightedge.
2) Thoroughly wipe the top surface of the container flange and bottom surface of the cover flange and attach the cover gently to the container, with the cover being vented. Fasten the cover to allow no air leakage and pour water in accordance with 4.2 (1). Adjust the air pressure in the air chamber to the initial pressure in accordance with 4.2 (2). Approximately 5 seconds later, fully open the actuating valve. Tap the side of the container with a mallet to distribute the pressure within the container. Fully open the actuating valve again and take an air quantity reading to the nearest 0.1% by the pressure gage after the pointer is stabilized. Take the reading as the
apparent air content of the concrete (A1).
Calculate the air content of concrete (A) by the following equation:
where A = air content of concrete (%)
A1 = apparent air content of concrete (%)
G = aggregate correction factor
Note: An aggregate correction factor of 0.1% or less may be omitted.