Mainly there are three methods of measuring air contents of fresh concrete. The methods are: 1. Gravimetric Method 2. Volumetric Method 3. Pressure Method.
1. Gravimetric Method:
This method is simple and does not require any special equipment, as such it is the first method used for calculating the air contents in concrete: The method principally comprises in the determination of density of fresh concrete compacted by a standard method. This density is then compared with the theoretical density of air free concrete calculated from mix proportions and specific gravities of the constituent materials of the concrete.
Let the density of air free concrete = 2380 kg/m3
Measured density of concrete = 2220 kg/m3
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Then 1 m3 concrete will contain solid & liquid matter = 2220/2380m3
= 0.924 m3
∴ Air content = 1 – 0.924 = .076 or 7.6%
This method is found useful for laboratory use. It is not well suited for field use as it requires knowledge of mix proportion of concrete.
2. Volumetric Method:
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This method aims at measuring the volume of air in the sample of fresh concrete directly. The sample of known volume of concrete is taken and all air is removed from it by any method and then amount of water required to restore the original volume is determined. This method is tedious and time consuming.
3. Pressure Method:
This is the most popular and best suited method for site use. It is based on the relation between the volume of air and the applied pressure (at a constant temperature) given by Boyle’s law. This meter is directly graduated for percentage of air and proportions or properties of materials need not be known. This meter is not suitable for use with porous aggregates and need calibration if used at high attitude.
A simple meter is shown in Fig.6.11. The procedure of the test consists of observing the decrease in the volume of a sample of compacted concrete when subjected to a known pressure. The pressure is applied with a small pump as that of bicycle pump and measured by a pressure gauge. Due to the increase in pressure over atmospheric the volume of air in the concrete decreases and this causes a fall in the level of the water above the concrete. By arranging the level of the water to vary within a calibrated tube, the air content can be read even by an unskilled operator.
Methods of Air Entraining:
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Air entrainment can be effected by adding finely divided aluminium powder to the cement. By the addition of this powder a chemical reaction takes place and hydrogen is liberated. The addition of 0.1% to 0.2% of aluminium powder of the weight of the cement produces sufficient gas to form a cellular structure in the cement paste producing a light weight concrete on hardening. This method however is not found suitable for air entraining as the bubbles produced are relatively large and the process is effective when cement with little sand or neat cement is used.
Therefore the best method of air entrainment is, to use surface active agents, which can be used from 0.025 to 0.1% by weight of the cement.