While concrete is among the most durable building materials in the construction industry, many factors impact its strength and durability over time. Perhaps the most difficult one of these qualities to control is the air content in the mix. Air content plays an important role in preventing a variety of problems for both exterior and interior concrete projects. Incorrect air content is one of the main reasons loads get rejected at the job site, even on loads with an air entrainer or detrainer added at the batch plant. Understanding the factors that can cause fluctuation in the air content during transit can help ready-mix operators better prepare their mixes for job specifications.
Temperature
Changes in temperature can significantly impact the air content of the mix during transit. In some cases, an increase of 30 degrees decreased the air content by up to 25%. Conversely, if the temperature drops by 30 degrees, it can increase the air volume by up to 40%.
Mixing
Air pockets in the concrete generate through a foaming action in the water during the mixing process. There is a limit to how much foam will be created by simply mixing the concrete in the truck. After the first fifteen minutes, the amount of air generated starts to level off. In some cases mixing the concrete longer than that can cause the air content to decrease, even if you added an air-entraining admixture at the plant.
Admixtures
Other admixtures, such as set delay admixtures or superplasticizers, can increase the amount of air entrained in the mix. If your concrete has these products in the design, you will need to adjust the amount of air-entraining or detraining admixtures accordingly.
Control Your Mix
Controlling the air content of your concrete mix can be challenging and adding an air-entraining admixture at the plant may not be enough to ensure the load has the right amount of air by the time you arrive at the job site. Fortunately, Fritz-Pak air admixtures can easily be added at the job site to ensure your concrete is ready for any project.