Lean Concrete

[Dad2one]

Hello.

We had to over-excavate for retaining wall footings in Atlanta while chasing competent bearing soil. The contractor wants to use a lean concrete to fill the over-excavation and submitted a mix design that has a w/c ratio of 1.06 for 1000 psi.

Info I've found about lean concrete just says that it is characterized by 'high' w/c ratios. 1.06 seems well beyond 'high' to me.

What do you think?

Thanks.

 

[woodman88]

I would pass this to the soil engineer for their recommendations. The soil engineers I have worked with have been most helpful with these types of problems.

Garth Dreger PE
AZ Phoenix area

 

[TXStructural]

Your fill needs to be unreinforced concrete of virtually any strength. The reason for such a high W/C is to keep workability (in this case, the ability to get it out of the truck) while using low cement content to reduce cost.

Because the mix is obviously very lean, you will likely have very little paste, so shrinkage will not be a problem (a problem with typical high W/C ratio mixes.) I'd be inclined to use slightly more cement than you mention to provide a margin of error, but there is no strong reason to demand higher strength.

 

[apsix]

A 0.9 max w/c ratio for lean mix is not uncommon.
As TXStructural said, add cement to bring it down to that.

 

[dcarr82775]

It is fill. At 1000psi it is much better than compacted soil. I've used flow fill/lean mix that gets 500-600psi for the same condition and not had any problems.

 

[Dad2one]

Thanks for the input. As it turns out, it's based on a standard GDOT mix....which allows a w/c up to 1.4.

 

[Ron]

Dad2one...this is essentially flowable fill (controlled low strength material). Works for you application in your area.

Before such materials became "accepted" we simply used a weak concrete mix for the same thing.