#11  
Old 06-22-2011, 06:55 AM
Peasant
 
Join Date: Jun 2011
Location: Ohio
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Default Re: Concrete Slab Depth and Reinforcement

Thanks, Ben.

I have one additional concrete slab question --

I mixed and poured the base slab on Saturday. I covered it with plastic and occasionally watered it through yesterday. Pending the weather, I want to lay cinder block tonight.

When I removed the plastic tarp today (to let it dry-out in preparation for mortar and blocks tonight) rain water on the surface poured on to the slab. And it poured through the slab somewhat easily...like a sieve.

I use a hoe to tamp the concrete as I poured. It seemed very dense at the time.

Is this slab porosity normal 3-days after the install? Will it diminish as the slab dries? Do I need to apply a sealant? Did I horrifically screw-up the mix?
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  #12  
Old 06-22-2011, 02:19 PM
benguilford's Avatar
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Default Re: Concrete Slab Depth and Reinforcement

Can you describe the consistency of the concrete mix that you used? You said you used bagged mix - so you can't have gone wrong in regards to cement, sand and aggregate content. That just leaves the possibility of a really dry mix, which if properly vibrated during the pour wouldn't be an issue, however if it has just been tamped down then there is a chance the grout or 'slurry' in the mix hasn't worked its way fully around the aggregate in that area, leaving small voids for water to penetrate.

How much water does it absorb? Are we talking buckets? I still wouldn't be concerned about the strength at all. It'll hold up your oven just fine. The problem is, that if there is a path that water can travel through the concrete to reach the reinforcement in the slab, it will eventually rust the reo and cause cracking in the slab. Additionally, if you live in an area that experiences extremely low temperatures, this water will go through cycles of freezing and thawing - on freezing the water in the concrete expands, creating cracks. On thawing, more water fills the new cracks, then it freezes again and you ned up with pretty severe cracking.

For what it costs, I'd seal it and never worry about it again.
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  #13  
Old 06-22-2011, 03:48 PM
Peasant
 
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Location: Ohio
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Default Re: Concrete Slab Depth and Reinforcement

If I pour a quart of water on it, the water disappears like magic. It shocked the heck out of me. In northern Ohio, freeze/thaw is a definite issue. I gotta fix it.

I called Quickcrete and they recommended I place a 3/8" layer of sand topping over the surface. I guess that is what I'll do. Hopefully I can simply raise my slab form 1/2 inch.

They agreed that the concrete strength would be fine.
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  #14  
Old 06-22-2011, 04:34 PM
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Default Re: Concrete Slab Depth and Reinforcement

Sounds like a plan my friend. When you say 'sand topping' do you mean some kind of cementitious grout? Like a concrete mix without the coarse aggregate? That would do the job well. You could just get 3/8" strips of timber and nail them to the top of your existing formwork to raise it up, PLUS this will mean that you can level your foundation slab and screed it off to a perfectly smooth finish - makes it a bit easier to lay that first course of blocks.

Block work is good fun, it's very satisfying to see the structure come together. Good luck, and it's about time you gave us some pictures!
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  #15  
Old 06-22-2011, 09:15 PM
Peasant
 
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Location: Ohio
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Default Re: Concrete Slab Depth and Reinforcement

The mix I mentioned is exactly what you thought. I raised the form tonight, and I believe it will make a nice level surface. I expect to pour it tomorrow.

I called the people at Quickcrete (concrete mix manufacturer), and they offered to have a local rep come out and review my base. I need to call them back tomorrow, but if they are sincere, I'm going to take them up on their offer. I know I must have mixed it too dry, but I followed the directions to a T. Over the course of 19 bags, any overage or underage should have averaged-out?

Pictures? I'll post a before/after shot of the base once it's done. I'll also post the rep's thoughts on what went wrong. Maybe it keeps someone else from making the same mistake. Although some extensive Google searches lead me to believe I'm the first person in the world to experience the sieve-base concrete phenomena.

Thanks to everyone for the guidance and tips. I really appreciate it.
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  #16  
Old 06-22-2011, 09:32 PM
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Default Re: Concrete Slab Depth and Reinforcement

Getting the rep out there sounds like a great idea, unless they're going to charge you for it!

I would suggest the problem is this; your mix may have been slightly dry when you poured it, but then the really critical part is that the concrete wasn't vibrated thoroughly. Have you ever seen a stiff mix of concrete being vibrated? It behaves like a liquid, and flows into all corners and around the reo. Without some kind of immersed vibration, dryish concrete even if tamped down, will potentially set with voids all through it. Check out this Youtube clip showing some relatively dry concrete being vibrated to see what I mean; Concrete Sample under Vibration

I'm the Precast Concrete engineer for a large freeway construction project here in Melbourne, so I play with concrete all day! The key to working with stiff concrete is to vibrate it into place (among other things we use pencil vibrators - looks like a brush cutter with a big... well.. 'thick metal rod' on the end that vibrates like mad). If you don't use one of these, at least get a trowel or shovel or something and just give the mix a really good 'poking', this will help the cement slurry work its way around the aggregate and allow air bubbles to work their way to the surface. Definitely do this when you pour your hearth slab, and you'll see the difference.
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  #17  
Old 06-22-2011, 11:12 PM
Neil2's Avatar
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Default Re: Concrete Slab Depth and Reinforcement

" If I pour a quart of water on it, the water disappears like magic"

I suggest you take it all out and do it over. A slab with that kind of voids is likely to fail.
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  #18  
Old 06-22-2011, 11:42 PM
benguilford's Avatar
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Default Re: Concrete Slab Depth and Reinforcement

Could you put up some photos of your slab so we can get a visual for what you're dealing with?
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  #19  
Old 06-23-2011, 10:53 AM
Peasant
 
Join Date: Jun 2011
Location: Ohio
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Default Re: Concrete Slab Depth and Reinforcement

Quickcrete rep is stopping by at 8am tomorrow. No fee. We'll see what he has to say.

I'll try to post some photos tonight.
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  #20  
Old 06-23-2011, 09:23 PM
Peasant
 
Join Date: Jun 2011
Location: Ohio
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Default Re: Concrete Slab Depth and Reinforcement

Here's a pic of the problematic base.
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