Why does certain concrete have to have air entrainment added to it?
Answer:
I am assuming that you are asking based on a civil engineering standpoint, or at least a materials standpoint. The above statement, however, isn't entirely accurate. Concrete trucks, discharge chutes, pump trucks and dump buckets dont' care how heavy the concrete is, because adding air is such a marginal difference in weight and they are capable of lifting the heaviest concrete. The proof is in the pudding. Visit the websites below and see for yourself!
Concrete is air entrained because of freeze thaw cycles. I live in Michigan, and we air entrain all of our concrete that is above the frost line and outside. Inside, or heated slabs do not get air.
The purpose of the air is to prevent cracking when the winter freeze happens. When the water in the concrete freezes and expands, the tiny air bubbles in the concrete give the water a void to fill, instead of cracking the slab/structure.
We typically look for between 5-8%, or in some calses 4-8% entrained air.
1. To make it lighter in weight (when strength is not needed)
2. To make it more of an insulator to heat.
3. To decrease the specific heat of the material.
Cool-deck around pools uses this to keep the concrete cooler on your feet, because the specific heat of the air-concrete mixture doesn't hold as much heat as 'solid' concrete.
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totally agree with Lindsey. we use air entraining agents in our concrete due to freeze-thaw. these agents create microscopic air bubbles in your concrete to accomodate for the expansion of your concrete therefore minimizing cracks. If you want lighweight concrete, you use lightweight materials instead like lightweight aggregates, air-entrainment have nothing to do with it.
Lindsey is exactly right and tlbs is exactly wrong.
good job lindsey
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