Air Content in concrete for trench footing
Air Content in concrete for trench footing
(OP)
They typical air content I have seen in mix designs for formed footings and foundation walls is 5.5 +/- 1.5 %. Now I have a project where we are using trench footings. Is there any reason why the aircontent should be lower? The supplier is indicating 0-2% air for a 4000 psi conc (W/C=0.5).






RE: Air Content in concrete for trench footing
RE: Air Content in concrete for trench footing
RE: Air Content in concrete for trench footing
We don't usually require air in the concrete for these because of the above reasons.
RE: Air Content in concrete for trench footing
RE: Air Content in concrete for trench footing
RE: Air Content in concrete for trench footing
Yes, I do have 6" of grade above the footing on the exterior side. But is there an ill-effect for having say 5" air anyway? I know it can reduce the strength a bit but since we are calling out 3500 psi for the trench, it should not a problem.
RE: Air Content in concrete for trench footing
Entrapped air does nothing to inhibit freeze-thaw resistance. The air voids are too large to be effective.
Do you need it? Not likely, but that's a local call, depending on your site-specific conditions.
If you specify entrained air, it should certainly be more than 2%.
RE: Air Content in concrete for trench footing
RE: Air Content in concrete for trench footing
RE: Air Content in concrete for trench footing
RE: Air Content in concrete for trench footing
You might not need it, but if you want to be conservative, OK to do so.
RE: Air Content in concrete for trench footing
RE: Air Content in concrete for trench footing
Air-entrained Concrete: Over and above resistance to freeze-thaw, there are other benefits of entrained air in both freshly mixed and hardened concrete. Workability is one of them.
If you decide to specify air-entrained concrete, I would suggest conforming to codes based on the type of exposure.
For example, ACI 318-02, Table 4.2.1 gives total air content of 5% for nominal aggregate size of 3/4" and moderate exposure.