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Unread 06-11-2014, 08:34 AM   #1
Topspin
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Concrete pad quandry - help a neighbor quick

My young neighbors back yard just got cleared of everything and some cement forms were set up. I asked the contractor whats up. He said he is putting in a sport court for my next door neighbor. Concrete pad only.

I asked if he was going to put his crushed rock/gravel on top of the scraped off dirt and he said that "concrete already has gravel in it so theres no need to go to the expense of putting gravel down first."

I know nothing about new construction concrete pads (as I deal with what I get in tile remodels) but isn't it customary to compact the soil, put a layer of crushed rock, some type of vapor barrier prior to the concrete?

Trying to offer advice to a young newbie neighbor so I can save him before the concrete arrives in a couple days - assuming its an issue. CX?

Here's the situation:

Outdoor - all exposed to elements (and in WA state we get a fair amount of rain and some snow)
Size: 40'x30'
Looks to be sloped at 1% grade away from the house
Framing looks like it will be a 4-5" thick pour

I asked the contractor if he'd give me a bid on a walkway (before hearing about his method above) and I asked him if he uses rebar. He said he would throw some rebar into the concrete as they go. that was enough for me. However, I really like my young neighbors and don't want to see them get something that will fail. Any of you builders care to chime in over the next few days?
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Unread 06-11-2014, 08:53 AM   #2
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I think it depends a lot on what kind of dirt you have under there. It it's fill, you have to take it out and compact it in 6" lifts. If it's native soil, you can pour it. you also need some drainage under the slab, or chances are it will frost heave. Again, depends on what's there. Some soils drain very well, some not at all.
Vapor barrier is for houses, not necessary for outdoor slabs. Around here we put in rebar on 2 foot centers, all tied and chaired up before the pour. I'm sure Jmezzanote will chime in, he's the real expert.
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Unread 06-11-2014, 09:10 AM   #3
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Not too much of a chance of frost heave here, but sometimes we get months of continuous rain which can run rivers in a yard. My thought is without some crushed rock underneath to allow safe passage of excess runoff, the dirt will just erode under the concrete.

The home has been in place for 24 years, and ground undisturbed. Most of the soil is a harder packed dirt with higher clay content and a little gravel mix. This hardpan is the kind of dirt (down 10" or so) when you dig post holes for a fence you can't pound it into submission. When you hit it with any chisel type tools it acts like concrete. You could use a little three-pronged gardening tool to wear a hole faster than hitting it with a big spike or pick. Those two man, handheld post hole diggers often just bounce on the surface and can't get in. But bobcat mounted diggers have no problem. If that gives you any better idea of the type of surface were talking about.
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Unread 06-11-2014, 11:13 AM   #4
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They always pour right on that around here, Kevin. It's a pretty much a standard method, including no reinforcement, even though it's risky. If you start looking around you'll see a lot of cracked slabs here too.
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Unread 06-11-2014, 03:08 PM   #5
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My comments/suggestions:
It sounds like the subgrade is a decent undisturbed material. I don't see the benefit to further compaction and/or a layer of gravel/sand assuming the subgrade is reasonably flat.

I would suggest 6 mil polyethylene be placed on the subgrade. As an alternate to save money, someone should keep the subgrade damp as the concrete is being placed to prevent the water in the concrete from being sucked out by the dry subgrade.

As far as reinforcing goes, I would use, in order of preference, fibermesh, flat sheets of welded wire fabric, rolls of welded wire fabric or rebar properly placed and supported.. Either type of wwf should be properly supported (not placed directly on the poly or subgrade). If the contractor is really going to throw some rebar in as he goes, I'd get a credit for the rebar because it will more than likely be a waste of money.

With the amount of rain you get, erosion of the subgrade could be a problem around the perimeter of the slab. If the top of the slab is not fairly close to the surrounding finish grade I would do one of two things. First I would thicken the edge of the slab to get it a few inches below finish grade. Another less effective option would be to berm soil against the edge of the slab to try to prevent erosion. This option will require ongoing maintenance.

Two items you didn't mention were expansion joints and curing. A slab this big will more than likely crack like a potato chip if expansions joints are not provided at proper intervals and properly constructed. Some method of curing the slab should be undertaken. A combination curing/sealer compound can be applied after the slab is finished. You probably don't want to do this if another finish material is going to be applied on top of the slab. Two other options are to cover the slab with poly or place some lawn sprinklers on the slab to keep it damp.

I know, too long didn't read LOL.
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Unread 06-11-2014, 07:41 PM   #6
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Thanks, very thorough. I appreciate your input. I will pass this information along to my neighbor to make sure that he's at least getting the minimum on his project and not substandard installation.

Like I said, I am not a concrete guy so I am not aware of the proper/acceptable methods but thought concrete and dirt just didn't sound kosher to me. With all our education separation layers I thought putting down some crushed rock would not only improve water migration, but would also act as a type of cleavage membrane.
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Unread 06-12-2014, 05:33 AM   #7
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not sure why you need vapor barrier on an outside slab, but you definitely need reinforcing. 6x6 wire mesh throughout, minimum. score control joints on a 10'x10' grid to help with cracking

EDIT: oh yeah, curing is important too...see Ed's post above. your best bet for a homeowner, outside patio, is probably wet-curing (sprinkler). keep the top moist for a few days.
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Unread 06-12-2014, 06:22 AM   #8
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a little add to the moisture curing--lots of people make the mistake of hosing down the pad and then letting it dry out and watering it again repeatidly. The water evaporating off the top and drying out will actually pull more moisture out of the slab. That's why you see sidewalks covered in burlap and wet. Set up sprinkler and let it run so that the slab never dries out for the first day or two.
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Unread 06-12-2014, 09:24 AM   #9
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My parts we don't usually use a membrane outside, we use welded wire mesh supported about half way into the slab, never seen anyone not use gravel base ..if for no other reason but to control the thickness of the slab and allow for independant movement. Different thicknesses will cause cracks to form also.

My climate is not that much different then Seattle, I don't see how the type of dirt has any relevance. There is alot of types of dirt..even expanding dirt.

I think the slab would dry more evenly and have less curling without plastic under it. sure keeping the slab constantly wet(forever?) may make it harder but does it really have to be that hard..time will make it harder. the surface does needs to be hard to prevent erosion that will eventually happen anyways..but thats about it. Watering or preventing water from leaving the surface to me is a good method.
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Unread 06-12-2014, 05:06 PM   #10
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Sorry Jim, I didn't mean forever, just the first few days. I witnessed far too often where people water the concrete, let it dry out and bake for several hours and then sprinkle a bit of water on it again and leave it again to bake.
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Unread 06-12-2014, 05:09 PM   #11
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oh and yes, if you are just throwing rebar in as you go. might as well save the money and not throw any in at all. Rebar is suppose to be tied all together with a min. of two feet overlap. Least in the commercial world.
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Unread 06-13-2014, 10:05 AM   #12
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Well, things look swimmingly well next-door. Turned out he contractor was going to put down plastic as shown below, and the rebar is more than thrown down. Set off the surface and tied and grids. Not a lot of rebar but still something.

Also looks like they're going to put in some preformed crack isolation strips now. So I'm feeling a lot better. Will talk to them about the way they finish it off with the water too. Thanks for comments.
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