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MoreCowbell 05-02-2012 08:36 PM

Mixing concrete for column footing
1 Attachment(s)
I've posted a number of questions on this forum as I planned for my crawlspace-to-basement conversion project. Most of which were about inane details that I thought were major issues.

Anyway, I am actually doing work on this now. I knocked out the first column, pulled out it's footing and formed out the new footing in it's place (only a little deeper). :)
Attachment 3874
The county inspector was out yesterday and gave me his blessing and I will be mixing concrete tomorrow. So, it's time for another question about something that is probably not as significant as I am thinking it is.

I'm using Quickrete pre-mix concrete (the 4,000 PSI product in the yellow bag). My question is how stiff or loose to mix it. I know the ideal amount of water will yield a stiff mix that will take some work to get the air pockets worked out. On the other hand, more water will make the concrete weaker but easier to work out the air.

Should I go with a stiffer mix and just try to work out the air pockets with a 1x2 until I think I got it all? My concern is that I tried that recently with some concrete weights I formed up for a lawn aerator and they looked like swiss cheese when I removed the forms. :(

Thanks for the input (past and present).

mudmixer 05-02-2012 09:01 PM

If you have the right slump, any holes (honeycombing) or voids when you remove the forms it is usually due to the placement/vibration and not the low slump.

When dealing with pre-proportioned mixes, you lose the advantages of admixtures that can help, but cannot be used in a DIY job.

With a DIY reinforced spread footing, just use a higher strength (5000 psi) enough water to make placement easier. The major problem with water is the shrinkage and cracking that ios not a problem with a spread footing on grade. Judging by the footing size, depth and reinforcement, concrete strength is not a major factor unless you have huge columns and many stories.


nealtw 05-02-2012 09:25 PM

Throw some fill around your forms, they really are not heavy duty enough to hold the weight.

MoreCowbell 05-02-2012 11:43 PM


Originally Posted by mudmixer (Post 72237)
If you have the right slump, any holes (honeycombing) or voids when you remove the forms it is usually due to the placement/vibration and not the low slump.

If you don't have a concrete vibrator, is there a decent alternative? I was using a length of 1x2 making those aerator weights. Is something narrow like rebar a better choice?


Originally Posted by nealtw (Post 72238)
Throw some fill around your forms, they really are not heavy duty enough to hold the weight.

I'll definitely be packing in some fill as the concrete fills the form. I'm hesitant to do it beforehand since it may cave the form in. The other footings I have to do will be 100% cut into the clay so I will not need to worry about this on those ones.

Thanks for the replies.

BridgeMan 05-05-2012 10:44 PM

Your forms are too flimsy, and you also ended the rebar too short. Bars should be no more than 2" from exterior concrete faces, while yours look to have more than 3" clear. Hope the stress distribution wedge doesn't crack the unreinforced outsides of the footing.

MoreCowbell 05-06-2012 12:31 AM

My original plan was to mimic the dimensions of the original footing (30x30x10). That would have given me the 2" clearance with the rebar. However, I ended up digging the footing another 2" in each direction to end up with 32x32x12, but the rebar was already cut and I didn't know better at the time.

I'm pretty sure that the original footing was not reinforced anyway (the builder lives down the block from me and he said they usually don't use rebar in the column footings), so I hope that I'll be OK.

At any rate, I poured the concrete a few days ago. I'll wait the full 28 days to get maximum curing before putting weight on it.

BridgeMan, what angle does the stress wedge diverge at? The main reason for deciding to go wider and deeper at the last minute was because I will have a much narrower column than before concentrating the weight (4" pipe vs. 16" block).

nealtw 05-06-2012 10:01 PM

Weight travels out at 45 degrees, at 10" deep the weight will desperce to 24" for a 4" post. I would use a 6x6 wooden post instead of steal but that's me. You builder down the street is outdated by more that a few years. He didn't buid a footing just a platform for blocks, it had a big footprint so it worked fine but the weight wasn't traveling at an angle.

CallMeVilla 05-06-2012 10:31 PM

Nobody really answered your question about the slump of the mixture. I'd like to know if there is a field technique for determining adequate slump WITHOUT using a tester. If you eyeball the mixture, it is easy to get it wrong.

Anybody have thoughts? Experience?

nealtw 05-07-2012 12:09 AM

CallMeVilla 05-08-2012 09:03 PM

Interesting slump test . . . Now think of the guy sitting in his basement with one footing to fill and a wheelbarrow of cement. How does he do a DIY field test to get the right slump? Just askin.

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