Concrete is one of the most durable building materials. It is a composite mixture of sand, cement, aggregates, and water. Apart from the mentioned ingredients, it also has some amount of entrained air in it. This air finds its way into the concrete during the mixing of concrete ingredients and in some cases, it is intentionally incorporated by adding air-entraining admixtures. In this blog, Gharpedia shares the importance of air content in concrete and the measurement of air content in concrete.
What is the Air Content in Concrete?
Courtesy - Concrete construction
The amount of air that makes up a certain concrete element is known as its air content and is typically represented as a percentage. The main objective of entraining air in concrete is to increase its durability when subjected to freeze/thaw cycles. Hence, concrete air-entraining agents are added to the concrete mix to produce tiny, consistent, and stable air bubbles in the cement matrix that range in size from 0.01mm to 1mm throughout the concrete mixture.
Importance of Measurement of Air Content in Concrete
The main reason behind adding air to concrete is to withstand the wear and tear in concrete caused by repeated freeze-thaw cycles in wet environments. The purpose of using air-entraining admixtures in concrete is to evenly disperse tiny air bubbles (0.01 mm to 1 mm in diameter) throughout the cement matrix. Once the concrete has hardened, the bubbles create air gaps that work as a relief mechanism against cracking of the concrete due to the expansion of freezing water.
Air in concrete determines the quality of the concrete, hence, measurement of the air content in concrete is important. Concrete mix without air-entraining admixtures also contains some amount (1% – 2%) of entrapped air (large air voids due to poor compaction). Henceforth, air content testing is still advised for profiling the qualities of a given concrete batch.
Methods of Measuring Air Content in Concrete
01. Pressure Methods for Air Content
Courtesy - Indiamart
There are two different pressure meters used to measure the air content of concrete specimens, namely, type A and type B. In both methods, pressure is applied to the concrete specimen to compress the air voids, and then the air content is calculated by the difference in volume before and after compression. In the type A method, pressure is applied to the consolidated concrete by filling the water in a graduated cylinder in the cover, whereas the type B method has an air chamber in which air is pressurised to compress the consolidated concrete.
Pressure methods for air content cannot be used for concrete mixes containing lightweight aggregates, blast furnace slag, or other porous aggregates.
02. Volumetric Methods for Air Content
Courtesy - certifiedmtp.com
03. Gravimetric Method for Air Content
It is the first technique ever used to determine the amount of air in concrete without any specialised tools. This process largely comprises calculating the density of newly compacted concrete traditionally. This density is then compared to the theoretical density of air-free concrete, which may be calculated from the mix proportions and specific gravities of the constituent ingredients of the concrete.
Conclusion
The main purpose of this process is to directly measure the amount of air in a sample of fresh concrete. A concrete specimen of known volume is first compacted inside a container for air content testing by volume. Water, isopropyl alcohol, or both are poured into a container with an in-built graduated cover and then sealed. After that, the whole thing is shaken up well to eliminate the air bubbles in the specimen. The amount of air present is calculated as the difference between the fluid levels before and after agitation. This method is tough and takes a long time.
Measuring air content in concrete gives you the amount of entrained air in your concrete batch. Knowing this will help you decide whether the air content in your concrete batch is sufficient to improve the durability of the concrete or whether there is a need for a new batch of concrete for your construction work.
FAQ
01. Why do we measure the air content of concrete?
An air content test is usually needed to check air-entrained in concrete in areas where frost damage can happen. Air-entrained concrete mixes improve the durability of concrete in harsh weather.
02. What is a good air content for concrete?
An ideal air-void percentage for concrete is about 6.5%. The percentage of air in a mixture with a total void volume of 6.5 percent is roughly 1.5 percent.
03. What causes high air content in concrete?
The air content will increase with longer mixing times, reaching a maximum of two minutes in stationary or paving mixers and 15 minutes in most transit mixers. As a result of the concrete’s loss of slump, the amount of entrained air gradually starts to diminish.
04. How does air content affect slumps?
Air content increases as the slump increases. This is true up to 6 inches. For slumps above six inches, the water content of concrete decreases, and hence the air content also decreases. However, when the slump goes below two inches, the air content decreases, as more water makes it difficult to entrain air.
05. How do you calculate air voids in concrete?
The volume of this area was thought to be the volume of voids (Vv). The percentage of voids (V %) can be calculated using the formula Vv/(Vv+VM)100.
Vv – volume of voids
VM – Volume of filling material
06. What happens if air content in concrete increases?
Increased air content in the concrete increases the workability, however, decreases the compressive strength of the concrete. The ideal air-void percentage for concrete is 6.5%. For every 1% increase, there is 3 – 5% decreament of concrete strength.
Courtesy: Image 1 – Indiamart
Author Bio
Sikandar Choudhury – Sikandar Monwar Huda Choudhury is a freelance article writer who is passionate about sharing his knowledge and experience with others through writing. With several years of experience in the engineering field and having written 100+ articles related to construction, Sikandar is a skilled writer with a talent for breaking down complex concepts and making them accessible to a wider audience. Sikandar is always looking for new opportunities to share his knowledge and experience with others through writing and is available for hire as a freelance civil engineering article writer. He is easily reachable on LinkedIn https://www.linkedin.com/in/sikandar-monwar-huda-choudhury-2b3a1a20a/.