Help pouring a concret pad.

[MFL]

Gentlemen,

I've spent the last two days using my FEL and box blade to mix in 40 tons of fill dirt and level and area so I can pour a 20x20 concrete pad to serve as the floor for a small metal toolshed/workshop. I am trying to do as much of the work myself to save some money, but keep getting conflicting advice. Here's the lowdown so far.

The building will be a standard metal building with a garage door, a walk-in door and two windows.The heaviest individual thing that will be stored in the building will be my lawn tractor, along with some lawn implements and some of my tools. I will not be storing the Kubota here as it has it's own covered storage until I can finish the barn. I am not in an area where I have to worry about the ground freezing or heaving during the winter. If we get 6-8 days below 30 degrees, it's alot. Here are the points in question:

I've heard 6" depth and 4" depth.
Some say it is absolutely essential to have a vapor barrier (4mil plastic), and a layer of gravel to help keep out moisture and prevent cracking. Some say that plastic is all I need in my area.
Some say wire and rebar are needed. Some say no rebar, and some say that if I'm using fibered concrete, that neither is needed.
Cure for at least a month before putting anything on the pad, others say all that is needed is to wait a week.

Could anyone with experience give me an idea of what you would do? Are there any other considerations I'm missing?

All help, as always, will be GREATLY appreciated! /forums/images/graemlins/smile.gif

 

[Cliff_Johns]

Will the slab have a thickened edge, or were you planning on 4-6" edge to edge?

 

[chim]

The floors on grade I see poured around here all have crushed stone (around 4") under them, and most are reinforced with wire. Plastic film is cheap, and I'd use it to keep moisture from coming up through the concrete.

Some projects require concrete testing. This involves pouring test cylinders and having a testing lab crush them at 7 and 28 days. Many crush at or near the design strength on the 7th day test. Concrete floors on a lot of jobsites see some fairly heavy loads long before they have cured for a month.

For pours like yours we would use a minimum of 3000# air-entrained concrete. Since it doesn't sound like your pour would be subjected to cold temps, mebbe it wouldn't need air. The concrete plant would be able to help with some info more specific to your locale............chim

 

[daedong]

If it were me the first thing i would do is form a well compacted base then build the shed and then pour the concrete. I would use a light gauge concrete mesh and 4" of conrete is plenty thick for this shed.

 

[v8dave]

4" of concrete and 4" of gravel under the concrete. If you get a weekly rain, then I'd put in a vapor barrier to keep out the moisture. Although, if you don't have the whole pad site elevated for water to drain away from the site, then the vapor barrier will just hold the water in.

You didn't ask, but I'd make a foundation around the edges of the slab. The edges of a slab are the most fragile part of the structure. You may drive a truck to edge to drop off something and that will crack off the edge without some reinforcement.

Rebar or steel mesh is to keep cracks from spreading when they happen and it will crack. Lay a rebar around the perimiter in the foundation. Use mesh in the slab--its cheap insurance. 20' x 20' is a large surface, if you can, score some "break here" lines in the pad--say quarters (i.e., 10'). You can float some "cream" back into the cracks after scoring them in to keep the surface smooth.

I just had a 30' x 50' pad poured here and the county says no further construction for a week.

Dave

 

[GerardC]

I have a 28 x 40 pole barn. I put a fiber reinforced 6" concrete pad (NO rebar, steel wire etc.) on top of 6mil plastic and it has yet to crack anywhere (going on 2 years now) with only perimeter separation. I have NO expansion joints. I was told to wait a week before parking my vehicles.

I would go for the thicker pad as you never know what the future holds. G

 

[GRUMPA]

This is pretty much what I did when I built mine. And all the advice you got so far is pretty accurate.

http://www.hometime.com/Howto/projects/garage/garage_4.htm

Water barrier is very important in wet climates. Where I am its very dry so I omitted that on my pads. Hope this helps.

 

[GRUMPA]

I just noticed this.

http://www.tractorbynet.com/forums/showflat.php/Cat/0/Board/photos/Number/450398/page/0/view//sb/5/o/all/fpart/1/vc/1

 

[Egon]

The plastic barrier keeps the water in the concrete while it is curing. Dry soil will take moisture from the concrete. This will have negative effects on the ultimate concrete strenght. Normally seven days is an acceptable time period before working on the concrete. 28 days is an accepted time for the concrete to be considered fully cured. Actually the concrete spends the rest of it's life curing but at ever decreasing rates.

As per others I'd suggest a monolithic slab with 6 in. exterior footer incorporated with the 4 in. floor.

Egon

 

[MFL]

Thanks Everyone!! As always, great info from the TBNers!! All the info has helped me to finalize the plans. I think a 4" slab with dugout 6" footers around the perimeter. Vapor barrier, 2" of gravel, and wire mesh. I'll pour a fiber mix to help with the overall strength.

The ready-mix companies I've talked to say that their guys will screed the pour, but that it's on me to float and finish the edges. Sound right?