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Air Content in concrete for trench footing

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

(OP)
Also, another question. Isn't it required to have some air content in the trench footing since it will be subject to freeze and thaw?

RE: Air Content in concrete for trench footing

Footings are traditionally placed so Top-of-Concrete is just below freeze line.  Thus no entrained air required.  Might want to check depth of this particular set of footings.  If above freeze line, I would want a fair ammount [>2%] of air.

RE: Air Content in concrete for trench footing

Trenched footings have their top level usually just below grade line - in this case the upper "corners" of the footing are not really participating in the structural support and also do have some earth over the top which inhibits freezing/thawing activity (temp swings are muted).

We don't usually require air in the concrete for these because of the above reasons.

 

RE: Air Content in concrete for trench footing

(OP)
Duwe, the footing is the foundation wall and vice versa. The whole trench will be poured from top to bottom (3'-6" min deep) in one pour because that's where the freeze line is. So surely a large portion of the trench will see free-thaw cycles.

RE: Air Content in concrete for trench footing

JAE, how far north does the "light dirt cover and no entrained air work", please?

RE: Air Content in concrete for trench footing

(OP)
JAE,

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

0-2% is not really "entrained air".  You generally will get 1 to 1-1/2% by agitation entrapment or "entrapped" air.

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

(OP)
Ron, the project is in southern WI, so yes freeze-thaw could be an issue. I want to be on the safe side and call out 4+/-1% entrained air. Are there any other issues I should be concerned with though when it comes to air-entraiment for a trench footing?
 

RE: Air Content in concrete for trench footing

For a trenched footing I wouldn't worry about specifying air.   We typically don't and I agree that the top portion of the trenched footing will experience freeze-thaw cycles.

RE: Air Content in concrete for trench footing

why not pick up the phone and have a conversation with the supplier? not the contractor, the ready mix plant...

RE: Air Content in concrete for trench footing

WWT...no other problems.  Air entrained concrete, provided it is designed for the needed strength, will work fine in your application.  Your specification of 4 +/- 1% is good.

You might not need it, but if you want to be conservative, OK to do so.

RE: Air Content in concrete for trench footing

If you do choose to be safe, why not expand the spec if its only in there "just in case". Remember someone will be testing on the job (more time/$) and it would be silly to reject concrete or slow down the contractor (reject/addition of AE, test, ect.) if the air is slightly higher or lower than on "just in case" spec.

RE: Air Content in concrete for trench footing

WWTEng - Although Building Codes have sections devoted to frost protection of foundations, they are typically overruled in part by local jurisdictions.
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.
 

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