Mixing concrete for column footing

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MoreCowbell

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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). :)
DSC_1492.jpg
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

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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.

Dick
 

nealtw

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

MoreCowbell

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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?

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

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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

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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

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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

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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?
 

CallMeVilla

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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.
 

MoreCowbell

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Well, it turned out OK after all (I think).

DSC_1509.jpg

I was mixing batches of about 2 cu.ft. at a time in my small mixer. I didn't use any slump tester, just eyeballed it. The first couple batches were probably stiffer than normal and the last few probably a little looser than they needed to be. In any case, it's been almost a week now and I'll probably wait the full 28 days before taking my temporary supports out and putting weight on it.

Nealtw, I think I'm safe with that 45 deg stress cone. There is 14 inches between the column and the edge of the footing. Including the 3.5" curb on top, I have 15.5" of concrete depth.

Now, just 4 more to go. :eek:

Thanks for all the input.
 
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mudmixer

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Cowbell -

You do not have to wait the fictitious 28 days. That number is only for standard laboratory testing procedures and they frequently test much more often and after a couple of days you will have 70% of ultimate and your loads are no where high enough to have to wait.

Your soil was warm and you are buried, so the conditions for curing are good.

Dick
 

MoreCowbell

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Your soil was warm and you are buried, so the conditions for curing are good.
Well, I was iching to remove the temporary supports and get the weight on the new column. You gave me that last push. I did that a few hours ago. That made 8 full days of curing. It looks like it's supporting the weight just fine.
DSC_1511 - Copy.jpg

Thanks for all the tips.
-Rob
 

BridgeMan

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Regarding slump (although too late now), if he's really anal about it, he will acquire a K-slump tester. It's a handy little device that you poke into a pile of concrete, and it immediately gives you a slump reading. Uses a calibrated, perforated hollow tube that lets the mortar in the mix flow into the tube. Paid around $25+ for mine 30 years ago (made in Canada, as I recall), so they're probably double that by now.

If he decides to "guesstimate" his slump, placing a shovel-full of mud (on a dry surface) that almost stands up by itself will be good-to-go. Somewhere between 2" and 3".
 
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