Air Content in concrete for trench footing

[WWTEng]

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).

 

[WWTEng]

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?

 

[Duwe6]

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.

 

[JAE]

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.

 

[WWTEng]

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.

 

[Duwe6]

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

 

[WWTEng]

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.

 

[Ron]

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%.

 

[WWTEng]

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?

 

[JAE]

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.

 

[cvg]

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

 

[Ron]

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.

 

[scone38]

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.

 

[DST148]

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.