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?

 

[JJT]

I did a similar pad a few years ago for a 16x16 shed. I went with a 6" pour, with rebar and no vapor barrier. I'm up north, there are no footers on this slab and there hasn't been a singe crack. I ordered 4000 psi, large aggregate concrete.

Up here the redi-mix companies dump and run, you will not find one that will screed the concrete, that's the contractor/home owner's job.

Make sure you have someone reliable help you out, my help was a no show the day I poured, so I ended up screeding and steel troweling the pad and a 30x8 garage apron single handed.

 

[GareyD]

I live in Bremen, just about 60 miles from you and just completed a 30x50 pour with a steel building...

Here is what I did..

First you need to make sure that any fill you have placed is WELL COMPACTED...this red clay we have around here has a tendency to crust on the top few inches and stay soft deeper down...any movement in the base will show as cracks in the slab...I put the fill in, in 2 inch lifts, drove over it with the tractor many times and water settled it as I went...

I backhoed a 12 inch wide by 6 inch trench 2 inches inside the perimeter and formed with 2x6 lumber...hand dug the last 2 inches out to the form where the overhead doors would be to the width of the door opening plus a foot on either side of the door...tied 1/2 inch rebar in the trench at the door openings, and placed wire on the floors in front of the doors where anything heavy could be parked in the future. Dug 16 inch piers where the uprights of the building would be and placed the bolts for the columns. Used 6 mil plastic vapor barrier (ya know we get a good amount of rain, and clay doesn't drain well around here) didn't want the slab to dry too fast or have moisture wicking in the future.

Poured the slab with fiber reinforced crete, and had the luxury of curing for 45 days before placing any weight on the slab (my building was delayed 3 weeks due to the steel shortage????)

I ended up with a 4+/- inch slab with 6+/- edges...I've parked the L3430 w/BH - the Harley - the boom truck that was used to install the building - my neighbors big ole IH tractor on this slab, that about 90 days later doesn't have as much as a hairline crack anywhere on it...and no I didn't make a single expansion cut on it anywhere...I hate sweeping out cracks!!!

Now I've been told I went to overkill on this project...all the extras...rebar, mesh, extra crete...added less than $500 to the total, but I am fairly certain that I have a building that I can use anyway I want, park anything I'd care to in/on, and not have a problem with for the rest of my life /forums/images/graemlins/smile.gif

Daddy always told me, it's cheaper and easier to do it right the first time, than to have to redo it!!!

GareyD

 

[GareyD]

I got an 8 year old grandson I'd think about rentin' out to you...attached is a pix of him helping float my slab...the big feller behind him is the guy I hired to do the crete work...

GareyD

 

[Junkman]

A contractor friend pointed out to me that a undisturbed pad location is always best. He told me that when doing a shed, or out building that they just remove the top soil and then frame the sides of the pad with 2"X6" or 2"X8" depending on what the owner wants. Put down plastic, wire mesh and drill the outer frame for rebar in the apron area. Then they pour the floor and let it "rest" for a week. Remover forms and backfill to the concrete and never have any problems with settling. He said that undisturbed earth is the best way to do this and having the slab a little higher than the surrounding ground they don't have water problems either. If the "topsoil" is thicker than 3", they remove it and put in stone/stone dust mixture and compact that first. His suggestion to me was to just get the top soil off and not worry about the rest, barring any roots, boulders, etc. This was good advice for me when I poured my concrete garage 6' x 40' apron in the rear of the garage. Not one crack or settling in 3 years...

 

[MFL]

GareyD,

When can I pick him up?? So far the building crew consists of me, Max, my 110lb mutt, whose in charge of pulling out roots, and Hannah, my 35lb mutt, whose in charge of chasing off any dangerous looking rabbits. /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

The wife does come out with a cold drink every once in a while and tells me not to come in the house dirty!

 

[MFL]

JJT,

No gravel layer either? Your pour sounds like the perfect answer for me! I'm just afraid that if I don't include some of these other steps, I'll end up with a two piece slab and a crumbling building. This is my first pour of this size, and I'd rather over-build now than live with the consequences.

 

[GerardC]

</font><font color="blue" class="small">( The wife does come out with a cold drink every once in a while and tells me not to come in the house dirty!)</font>

LOL. I wonder if our wives are related /forums/images/graemlins/grin.gif! She will however say that the project looks good though /forums/images/graemlins/wink.gif. G

 

[MFL]

Junkman,

Unfortunately, I didn't have the option to put it in an existing clear area. I cleared a 36x36 area of trees, ground the stumps and am leveling and grading right now. Tomorrow, I'm renting a plate compactor, and shaking my insides lose for eight hrs. getting it as compacted as I can. I'm also driving over it as much as possible with the tractor and letting the un-ending rain we've been having work on compacting.

 

[MFL]

GareyD,

Great Post!! Thanks for the tremendous detail on your project. I hear ya on the properties of Georgia clay!! /forums/images/graemlins/shocked.gif Tomorrow will be spent entirely on compacting!!

 

[mopower440]

..lol..well, maybe finally i can help someone out on this board, about 3 months ago, i was on here asking the same question as i wanted to do as much of the build myself, i am building a 24x30 pole building, what i did was build the frame of the building first, i used 2x6 skirt boards, then i did the floor before putting the metal siding on, i leveled the floor best i could and packed it down by driving the heaviest vehicle i had accross it several times on several different days, then i put down about 3" of gravel, got it leveled out and packed it down by driving over it also, then, i put a divider right down the middle of the barn staked it in with rebar spikes and got it level with the side skirt boards, this way i could do 1/2 of the barn one day and the other half the following week, i ordered #3500 fiber mesh reinfoced concrete, i got 3500 as i would be putting cars on it, if you use the fiber mesh you do not need rebar or the wire mesh..i also got about an inch less gravel around the edges so it would be a little thicker on the edges, the slab is 4" and about 5 1/2 around the edge, the concrete truck came, we had him add a little water to make it move easier, we spread it out with a rented concrete rake and 2 regular garden rakes, we then screeded it from the side skirt board to the center divider i put in, we screeded it 3 times as it would rise a little by the time we would get to the end, after 3 times it looked really nice, we then used a rented bull float and got it nice and glassy looking, then my neighbor talked me into going and getting a broom to put a broom finish on it wich i would have rather left the smooth finish on it, but oh well, it turned out as good as any professional job ive seen and wasnt hard at all, the only hard part is screeding, its not mentally hard, but you will work your butt of working the screed board back and forth, after that was done, we sat back and couldnt believe how nice it looked, we did the following side 3 weeks later, turned out good also, ill tell you this, i was afraid at first, but there is nothing to it, just prepare the ground really good first, get it level and tamp it down the best you can, also, you can use a barrier shhet on the ground before you pour, this keeps a lot of moisture from coming up through the concrete , doesnt make the concrete as cold and damp feeling in the winter..