weather and concrete?

[Currmudgeon]

</font><font color="blue" class="small">( Dirt guys are guessing 98 - 99% compaction. Will have it tested before the pour.)</font>

If you have to trench in your plumbing, I would try to get as much compaction on those trenches as I could with had tools or a vibrator plate. That's pretty much what they do in the trades. After the rebar is in, you're pretty much locked off the surface. If it is well compacted, it should be OK until you pour. I;d try yo pour as soon as reasonable.

</font><font color="blue" class="small">( RE freezing concrete. I live in Texas. Granted, it's the highest and coldest part of Texas (near Amarillo), but it's pretty rare for the ground to freeze much here.)</font>

Then the concrete in the post holes won't freeze either. The warmth of the dirt will protect it. Concrete is exothermal, so it gives off some heat when it sets. I'd insulate the top surfaces with a couple inches of straw, if it was going to get down near freezing.

 

[drm]

Phantom,

Here is the opinion of a structural engineer who lately has gone from designing buildings to investigating failures and construction problems.

The temp issue will most likely be minimum if at all. If temps are going to go below 40 deg F (not 32) you need to cover the post holes with a few inches of straw. If you pour the slab when temps will dip below 40 deg F the best cover is an insulating blanket (provided by the concrete contractor up north) or with a 4-6" layer of straw. This is ACI (American Concrete Institute) recommendations. Most likely covered in the pdf cowbydoc sent.

The compaction over the plumbing is a big deal as is the condition of the soil prior to pouring concrete. My suggestion is to place the plumbing prior to setting forms. Make sure the area over the trenches is compacted to the same level as the rest of the pad or you will get cracking there.

Here are some other pointers:
1. Provide 4" of a gravel base #57 crushed limestone has been working well up here. Gravel could be river run without fines with max 1 1/2" stone size & graded.
2. Install 4 mil plastic under the gravel to reduce moisture from coming up to the surface of the slab.
3. Install rebar at 12" oc minimum. Place bars on rebar chairs or precast blocks not brick. Wire at each crossing of bars.
4. Install control joint every 10' for a 4" slab, 12.5' for 5" slab.

 

[phantom309]

Thanks mucho for all of the great info everybody.

I'm proceeding on with setting the poles, with an eye on the weather a few days down the road. This time of year, the cold snaps are few and short so I think I can avoid any cold weather problems.

Here's a related question. I set six poles yesterday and late last night we got a bit of rain. I guess the pad is well compacted because it sure doesn't soak up the rain. Unlike the rest of my property, the bldg pad is standing in water. And a couple of the holes for the posts I planted filled 100% up with water! (I didn't fill the holes completely with concrete yet - will do so when the slab is poured and the two are tied together)

The rain didn't come until six or seven hours after the concrete was poured, so I assume that the concrete had set up enough to prevent the water from causing a problem. Is this OK? I've always heard that concrete cures hardest under water, but I didn't know how long the concrete should be allowed to start curing before being exposed to water?

FWIW, I took a portable water pump out and pumped the water out of the holes this afternoon.

Thanks again for all the great tips. When I have something that looks a little more interesting, I'll post a few pictures!

Mark

 

[drm]

Pumping the water out was fine. A few hours to set is good as long as the rain is not hard or does not faw directly on the concrete. You do not want the concrete paste to be washed away. BTW, concrete actually needs additional water on exposed surfaces for 7 days to cure properly.

 

[patrickg]

3. Install rebar at 12" oc minimum.

Think maybe you meant 12" OC maximum rather than mimimum? Puting rebar closer together till you get ridiculously close should be better not worse.

/forums/images/graemlins/smile.gifPat /forums/images/graemlins/smile.gif

 

[drm]

More steel is for the most part better but not always necessary. If using #3 rebar 12" spacing is adequate for most applications with a 4" thick slab. If using #4 rebar 16" to 18" spacing is normally sufficient. The reason for not spacing rebar closer than 12" oc is so that you can step between the bars while placing the concrete. This is because if you use small bars and do not space the support chairs (or precast blocks) close enough the bars will bend when stepped on.

For this reason I like #4 rebar at 16" oc for most 4" slabs with a good subbase, 4000 psi low w/c ratio concrete, control joints at 10' oc max...

 

[phantom309]

DRM-

Thanks for the great info in this tread, as well as others.

A couple days after I planted my last batch of poles, we did get an unexpected dip in the temp overnight to about 27. I had not covered the concrete in the holes. The concrete was about 12" below grade.

Is there any way for me to determine if my concrete suffered any damage from the cold temps? If it freezes, will any damage be visible?

Thanks again for the great tips everybody!

Mark

 

[cowboydoc]

The damage is from cracking or the concrete not curing properly. Aside from seeing any cracks there's really now way to tell. Why did you put so much concrete in? Usually if people do put concrete in it's just down about a foot or so and the remaining 3' is tamped in with dirt.

 

[phantom309]

After all of the poles are in, I intend to dig a trench between the poles, put up forms and pour a slab. The rest of the post holes will be filled with concrete when the slab is poured.

RE tamping with dirt - I am bypassing the concern of rotting posts by using steel posts. 3.5 x 3.5 x 3/16" A500 steel tubing. I built a steel and rebar cage at the bottom of the poles which will then be tied in with the rebar in the slab.

Attached is a picture of the bottom of one of the poles.

Mark

 

[drm]

All I can say is wow; and, of course I am jealous. Looks great.