It begins!

[mfiore]

Re: It begins!

George, thanks for the input. I looked back at your photos to see how you tied in the cores with the slab. Thanks for posting. These little things are less glamorous than the dome, but very helpful. I have 1/2 inch re-bar. Is that much harder to bend? I guess I'll find out.

Daren, I am very glad to be out of that hole! I've had fortunate weather and time over the past few days, an unusual combination for me. I looked at your photo album. Any updates? I reread the thread you posted on plastic under the slab, and see that it didn't get a very robust response. My questions may have taken that thread elsewhere (sorry )

What is the purpose of rigid insulation under the slab? I didn't even consider that (hadn't come across that in the forum, but sure could have missed it.) I presume it prevents frost from entering the slab, but couldn't the ground directly underneath freeze and push up just as easily?

As always, thanks for your input.

[dmun]

Re: It begins!

Also, what is the best way to bend re-bar (without buying a "re-bar bender")

Two lengths of iron pipe, either black or galvanized, make a great rebar bender. Thread the rebar into the pipes, stand on one, pull up the other.


Remember that you have to overbend, pull it to a hundred degree angle for it to spring back to ninety degrees. This makes it impossible to use the pipe trick to bend a length of rebar sticking out of a masonry structure to be parallel to it. You will also break the masonry by trying to get it that last ten degrees by hammering on it.

</The voice of experience>

[dmun]

Re: It begins!

What is the purpose of rigid insulation under the slab? I didn't even consider that (hadn't come across that in the forum, but sure could have missed it.) I presume it prevents frost from entering the slab, but couldn't the ground directly underneath freeze and push up just as easily?

This is a fairly new (to us) technique to avoid frost heave.

ESB: Frost-Protected Shallow Foundations

I asked the same question the other day. It seems that the wide spread layer of insulation prevents the frost (which happens from the top down) from penetrating the soil. The warmer soil below keeps the footings from freezing.

[mfiore]

Re: It begins!

Now it's coming back to me.. If you are not at that point of your build yet, when you read something, it doesn't make as much sense. The questions all come out when you are in the middle of everything.

Now that I have a deep foundation, below frost-line, does this still serve a purpose? In the traditional method, with a traditional deep footing, how are we preventing frost heave from the center portion of soil (within the walls)?

[dbhansen]

Re: It begins!

I'm hoping that the "pillar" of earth under my (and your) slab will stay mostly dry, which should help prevent heaving under the slab. I'm not sure how much water could "wick up" into the center; I suppose it depends on the local geography (totally flat here). My "pillar" got completely dried out while I was digging the hole, unlike the outer "walls," thus the plastic wrap to hold the center together. My thought was that having the rigid insulation under my slab would further raise the frost line (i.e., the frost would not penetrate as far into the soil = less heave). And having a layer of gravel under the slab should provide a cushion against heaving too. So hopefully the combination of all those things will prevent any lifting or cracking. But I didn't base this on any scientific research, just various recommendations, so hopefully you'll get an expert to comment here. I ended up using the plastic vapor barrier, but I don't know if it or the insulation will serve a real purpose. The insulation itself is impermeable to water, so I probably didn't need the plastic. If you don't use insulation, you'll need the plastic to properly cure your slab.

[mfiore]

Re: It begins!

I spent some time filling the cores of my foundation with concrete. It went pretty quickly. I did add several rebars and bent them over to incorporate into the slab. Also backfilled most of the "trench". If I didn't say it before, man am I glad to be out of that hole!

I'm going to put a slab of concrete on top of the block foundation. For the base under the concrete slab, what type of gravel is used? The high school kid at Home Depot tried to sell me pea-gravel, which I could tell would not work well. The stuff just moves around, it doesn't compact down in any way.

[gjbingham]

Re: It begins!

My guide for building a paver patio recommends compacted class 5 crushed limestone or a gravel substitute. Call around to the local gravel establishments in your area. 4 -6 inches tamped by hand or a rented compactor should suffice. That's my guess. I've been wrong before.

[egalecki]

Re: It begins!

I am not sure if this is what you need, but my husband says he uses "57's" for under concrete pads. That's apparently a size of gravel (at least around here). We just order it from the quarry, though, so I am not sure how it comes from Lowe's or HD.

[dmun]

Re: It begins!

You've already got your footings, you may not need a slab. If you just set bolts in your next layer of block you can bolt a pressure treated floor for your wood storage. Less weight to lug. Of course, if you want a concrete floor in your wood storage area, then go right ahead.

[dmun]

Re: It begins!

Not a home depot item. You want crushed stone from your masonry supply or brickyard. Bulk sand was much cheaper from the same source. They will deliver pretty reasonably if you don't have a pickup truck.

[dbhansen]

Re: It begins!

This doesn't advance the conversation, but I'm very curious what the purpose of the gravel is in a situation like this. Not drainage, since water won't be entering the area below your slab anyway (like it would under a floating slab). Does it provide cushion against frost heave? If that's the case, then it seems like it should be UNcompacted. Compacting it in frost-free areas makes sense, to prevent settling, but there is no chance of your slab settling after it dries. It almost seems like a better idea to just dig out 6" of soil and use plywood (and maybe some steel supports?) to form the bottom of the concrete, leaving 6" of air under the plywood. Does the center of the slab need support from below?

This seems like a unique type of construction that we're doing -- a relatively small slab resting on a block wall, with lots of weight around the perimeter but little in the middle. Maybe the "normal rules" don't apply.

Sorry to hijack your thread, but it got me to thinkin'....

[mfiore]

Re: It begins!

Sorry to hijack your thread, but it got me to thinkin'....

I'm happy for the discussion! Always better to discuss these issues before I get to that point in the build than after. Seems like a solid base below the concrete would prevent it from settling/cracking (like our sidewalks)?

[mfiore]

Re: It begins!

I think a concrete slab will work best for me. Even though I have solid block footings, the block stand will be slightly smaller (4 inches in each dimension). This will give me a little support outside the stand for future finish material (whatever that will be) in the future.

I also will explore alternative shapes for the stand (possibly an 'H' shape, see other threads). The concrete will be needed in the middle to support this.

Questions: Since I have a 48 inch block foundation, do I need 6 inches of concrete (per plans) or will 4 inches suffice?

Is there a significant difference in the support provided by 1/2" re-bar vs 3/8? When I had the block delivered, my great home improvement store sent the wrong size, and I didn't realize it until using half of it.

[mfiore]

Re: It begins!

Further questions. The guys at the local concrete yard recommended compacted sand as a base below the concrete, rather than gravel. Any thoughts or suggestions?

[dmun]

Re: It begins!

I'm suspicious of sand under footings. Sand can shift and wash out, where crushed rock stays put, and has bigger voids and less likelihood of getting silted up and not draining. But we're talking about the inside of your footings, right? It doesn't matter what you put there. Nothing is going to shift or wash out inside the block enclosure.