How does air content affect concrete?
They increase the freeze-thaw durability of concrete, increase resistance to scaling caused by deicing chemicals, and improve workability. Air entrainment will reduce concrete strength. As a general rule, a 1% increase in the concrete air content will decrease the 28-day compressive strength by about 3 to 5%.
How do you find the air content of concrete?
The gravimetric method (ASTM C138) in which the air content is obtained by subtracting the sum of the absolute volumes of the ingredients in a batch from the measured volume of the mixed concrete as calculated from its density.
How much air should be in exterior concrete?
concretes specify 7.5 ± 1.5 percent air for resistance to freezing and thawing; for workability, specify 5.5 ± 1.5 percent for lightweight ag- gregate concretes in the 3000- to 4500-psi (21- to 31-megapascal) strength range and 4.5 ± 1.5 percent in the strength range above 4500 psi (31 megapascals).
What does full air in concrete mean?
Air-entrained concrete is ordinary concrete that contains controlled amounts of air in the form of microscopic bubbles. These intentionally entrained air bubbles are extremely small. Most concrete contains some moisture which expands during freezing temperatures.
Is air in concrete good?
The incorporated air helps preserve the hardened structure of the hard concrete over time, thus, making it more durable. Air entrainment reduces the occurrence of surface scaling within the concrete which arises due to the use of de-icing products like antifreeze.
What is air content of fresh concrete?
Testing tip: A typical air content for concrete with a ¾-inch maximum-size aggregate is about 6%, and specified ranges in air content are typically minus 1 ½% and plus 1 ½% of the target value.
What is a concrete air test?
An air content test is required for checking air entrained concrete. A sample of concrete is placed in the cylinder of an air meter in three layers, each being fully compacted by hand or by vibration. The cylinder is filled level with the top.
Should exterior concrete have air in it?
In fact, it’s a good idea to measure the air content of the fresh concrete. If it’s above 3%, it has entrained air and should be rejected. But for an exterior slab in a freezing climate, you need to design the concrete for that exposure.
What is the percentage of air by volume in air entering cement?
The amount of entrained air is usually between four and seven percent of the volume of the concrete, but may be varied as required by special conditions.
Do you add air to exterior concrete?
The primary use of air-entraining concrete is for freeze-thaw resistance. Hard-troweled surfaces are not recommended for exterior concrete slabs, because the smooth finish becomes slippery when wet. Hard troweling is also not recommended for air-entrained concrete for several reasons.
How is the air content of hardened concrete determined?
How is the air content of hardened concrete determined? A. Hardened concrete samples from cylinders or cores can be lapped (ground smooth, not polished) and subjected to a microscopic examination to determine the total air content. The spacing factor determined indicates the average distance from a point in the cement paste to an air void, and the specific surface indicates the average size of the voids (ASTM C457).
Does Cement brand affect air content?
Portland Cement – Air content can be affected by cement brand, cement type, alkali content, cement fineness, cement content, and contaminants. o AEA dosage rates may vary between cement brands. o Generally AEA dosage rates utilizing straight cement mix designs are approximately 40 – 50% of AEA dosage rates of mix designs which
Concrete entrained air testing is a must for such concrete. Air content of the freshly mixed concrete can be measured by gravimetric, volumetric, or pressure methods. The pressure method is the most commonly used, and in this method a calibrated air meter is used to test a known volume of concrete that is subjected to a defined pressure.
What is air content test?
Air Content Test. An air content test is required for checking air entrained concrete. The method of testing is given in BS EN 12350-7, which replaces BS 1881: Part 106. Method A outlines the principle of the test.