What is used under rebar in concrete?

[mancave]

I recently saw a concrete parking lot being made. This was after hours, so no one was around to ask. They had the forms made, and had rebar placed about every 2 feet in both directions. Under the rebar they had small plastic "stands" to keep it off the ground.

What are these things called, and where does someone buy them? Also, what did they use before they made these plastic things? I plan to pour a pad of concrete in front of my garage soon, and think rebar should be used, but I was just going to use rocks under the rebar. Actually the thought was to buy some of those cheap patio blocks, which are about 1.5 inches thick and bust them up into pieces for this. That way the rebar would all be about the same space off the ground, and those patio blocks are pretty low cost. Possibly cheaper than those plastic things.

On the other hand, I was wondering if using that metal mesh material which is sold in rolls would work just as well as rebar and which would be cheaper to buy.

Any concrete experts out here?

[Bob_Villa]

Google is your friend

[gfretwell]

When commercially made chairs are not available, installers use pieces of brick, concrete block or whatever they can find. Worst case is when they use wood, foam or a coffee cup which weaken the pour.,

A responsible contractor will provide real chairs so their workers can build a better product at a faster pace. These things are dirt cheap in contractor qualities anyway. That is peanuts compared to the time a crew wastes looking for something to use on the site and try to make it work.

[nestork]

What you should do is excavate and then:

1. put down landscaping fabric. This is a tight weave material that won't rot due to exposure to moisture in the ground. It's purpose is to prevent the limestone from dissipating into the soil to prevent soft spots in either the asphalt or concrete you put over top.

1. Spread crushed limestone over the area you intend to pave or place concrete over. Typically, the limestone will be anywhere from 3 to 4 inches thick and will be compacted down to 2 1/2 to 3 inches thick with a plate compactor you can rent.

2. Put your rebar in. Ideally, it's best to put in TWO layers of rebar; one layer near the top of the slab and one near the bottom. That way, for the slab to bend far enough to break, the rebar has to stretch. Since steel is very strong in tension, it's the rebar's resistance to stretching that makes the slab more rigid and resistant to bending. Where you only see one layer of rebar supported in the middle of the slab, it's not there to strengthen the concrete, it's only there to prevent the pieces of concrete slab from spreading apart when (not if) the slab breaks. You can buy supports for rebar at any place that sells rebar. Look under "Steel" or "Concrete" in your yellow pages phone directory.

1. HIRE someone to finish your concrete for you. You can save 90 percent of the cost of the job by doing all the bullwork yourself, but placing concrete and finishing it is a skill that takes a lot of experience to master. And, with concrete, any screw ups are permanent screw ups. Do all the bull work yourself, and then hire a concrete contractor to actually place the concrete for you, and you'll end up paying less for a professional looking job.

[gfretwell]

How do you get 2" of cover on the concrete in a typical 4" slab?

[nestork]

Can you make your slab thicker, say 6 inches?

And, can you squeeze that cover down to 1 1/2 inches so that you have your two layers 1 1/2 inches from the top and bottom of the slab instead?

If it's only going to be a patio slab, and you're not going to be putting anything heavy on it, then stick with the 4 inch thickness and put the rebar in the middle just to hold the slab together if it breaks.

[mancave]

I would actually think that the brick or concrete block pieces, would make the best slab because it's the same material, and becomes part of the concrete. I do agree that wood or foam would be a bad choice. But I do wonder if plastic "chairs" would also weaken the slab to a small degree. Plastic is not a very solid material, but I suppose it wont decay under a slab. Wood would definately rot, and I would not evne consider using it.

[mancave]

While this does sound like the top of the line method, it seems rather excessive, not to mention costly, for a home driveway. This sounds more suited to a highway. My plan is to just pour the concrete over some gravel, and add either the rebar or that rolled steel material that looks like fencing. My only modification is on one side of the driveway, the soil tapers down a little bit, so I intend to make it a little thicker on that side to prevent washing out under it. So, while the slab is 4 inches, that edge will be about 6 inches. I plan to finish it myself. I've done it several times in the past and it's always been smooth enough for my needs. The surface will just be a broomed finish. A little roughness is desired to prevent slipping when it's wet.

[gfretwell]

A good way to go with rebar is to use the 6 over wire, stiffened with #3 rebar every few feet to make a flat mat. (takes out the set from being rolled)

[Derek Lindstrom]

replying to mancave, Derek Lindstrom wrote: Yeah they're called Dobies! We use them for keeping the steel or rebar off the ground and in a central location so the it's ready for the concrete and wont need any extra work lifting or placing the rebar in the center of the slab

[Hunter]

Before the spacers people used to use brick take a break it in two that provides 2 or 3 spacers

[gfretwell]

Back in the olden days concrete guys knew enough to just "pull up the wire" (rebar or whatever) but these days in the dumbed down building trades they require "chairs". You can use a piece of brick, paver "twinkies" are common or there are a number of commercial products. Most big jobs use commercial chairs because they don't want to screw around with finding appropriate chunks of concrete. The chairs are too cheap and always pass inspection.