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Old 11-20-2014, 08:58 PM   #1
Sharps-Nut OP
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Cement floor help.

Having a new shop built. Last time went all out and went 5.5 thick and 1/2" rebar tied on if memory serves me 24" grid? Its been built 13 years no cracks unless they are in the cuts they made on 10' kerf grid. This was a 40'x60' slab with thicken edges for a structural steel building.
We are moving and the same company is building the shop but his son is now the cement man where last time it was subbed to the man of my choice. The son is suggesting 4" thick, 4,000 psi slab , 8" thicken edge, with wire instead of rebar. All other specs same as above, and if memory serves they put 1 yard below the structural steel arches. My worries are the same, I don't want a crappy floor as the years pass with big open cracks and crap like that. I thought my last shop was my one and only so I tried to go all out. Now there are kids and money is tighter. The bid is 10,400 for the floor mentioned, up from 6500. 14 years ago. He felt there would be 1500 worth of rebar not counting labor. I build stuff, but cement work is not my calling. Money is going fast from driveway to clearing a spot, but cement is next and heck that what the hole thing rides on. Help I am lost. Thanks, SN
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Old 11-20-2014, 09:30 PM   #2
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Shouldn't it be at least 6" thick?
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Old 11-21-2014, 07:57 AM   #3
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Can you get ahold of the contractor who did the floor 14 years ago and get some input from him? Or have him talk to your general contractor?

My experience with concrete is that neither wire or rebar will keep it from cracking, it's intended to hold the pieces together after they crack.

Key ingredients to a quality floor are good compaction of the base, enough depth, a good mix and a proper curing. I'll also include properly spaced saw cuts,within 24 hours of the pour.

My slab sounds very similar to your first one, 6 inches and eight years later there are no cracks. I had to stay out there with the contractor to keep them from taking shortcuts and ignoring the contract, it was a real headache and they were not used to or receptive to an owner who paid attention to what they were doing. Maybe you can hire your original contractor as an owner's representative superintendent. I know you said money's tight and as you know, that's a problem with concrete, it's pretty much permanent and no going back and fixing a cheap job.

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Old 11-21-2014, 11:23 AM   #4
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If heating is any concern at all where you're located consider foam under the slab. http://www.finehomebuilding.com/how-...-on-grade.aspx It will go a long way to making the garage more comfortable for working (in addition to wall/ceiling sealing and insulation).
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Old 11-21-2014, 11:40 AM   #5
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Just FYI cement is one of the things that make up concrete, not the finished product.

The saw cut joints are supposed to control where the cracks occur. They are supposed to be controlled cracks for the concretes expansion and contraction.

As someone above correctly stated, the bar or mesh (WWF) will just kind of hold it all together and level.

In larger commercial construction the slab would be specified by an engineer, they would use geo-tech reports and soil conditions, etc.

You might also consider fiber-mesh, as it's some pretty tough stuff. It can be annoying in some ways (the little fibers sticking out until they wear off). But having done a bit of destructive testing on it, it's hard to destroy purposely, so I would imagine it would work well.

One of the keys in a slab on grade is getting the reinforcing into the center of the slab. Your original slab was bar, so they had to use supports to keep the bar up (generally just a concrete brick). One shortcut that is well known, is when reinforcing with mesh rolls, the contractor throws down the mesh and pours the concrete. Then the contractor just pulls the mesh up into place through the fresh concrete. This rarely works very well and the mesh will be wavy, sometimes close to the surface and sometimes close to the ground.
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Old 11-21-2014, 12:03 PM   #6
groundrules
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just my opinion, for what it's worth- mesh is fine, tied rebar is overkill/not necessary.
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Old 11-21-2014, 05:22 PM   #7
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At $5.00 for a twenty foot stick rebar is cheap insurance.
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Old 11-21-2014, 07:31 PM   #8
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Any concrete slab or asphalt paving job depends upon properly compacted subgrade.
Consult a local geo tech soils engineer for recommendations on your specific soil type.
Have him conduct a few compaction tests to see if your "native undisturbed" soil is suitable to pour directly over?

If you need to elevate the building site (pad), use compacted sand or aggregate base (AB) and roll with a roller.

Decide if you need a moisture barrier (geo tech question).
Moisture barrier usually installed to keep ground water from pushing up thru slab due to hydrostatic pressure of new building or building in area where surface ground water passes around building slab.
Moisture barrier can also be used to promote slow curing by preventing the water in the mix from just soaking away into subgrade.
Some say moisture barrier in direct contact with concrete promotes cracking and so place 1 to 2 inches of sand over barrier prior to pour.

The mix is the next important factor.
What use the slab is going to have dictates the compressive strength required and the thickness.
2500 psi concrete is standard but adding sacks to increase psi is not a bad thing unless budget is tight.
Additives can also help, talk to your ready mix supplier for advice.
Also pay close attention to the "slump" of the mix.
Too much water makes it easy to place but creates a weak slab, again talk to your supplier and watch to make sure the crew onsite does not add water to the truck without your approval.

Additional thickness is not a bad thing unless not need by your usage or if your budget is tight.
You'll need and additional 14 CY of concrete to go from 4 to 6 inch thick.

Built many 40x60 metal buildings in Calif and most were 5 sack mix, 6 inch thick with either wire mesh or fibermesh.
Wire and rebar are for maintaining surface alignment WHEN the concrete cracks, which I guaranteed every customer that it would/must crack due to shrinkage during the cure process (again too much water means more shrinkage).

Crack control is the secret.
If you look at the edge of your existing shop I guarantee you will find cracks that align with your sawcuts.

I prefer to pour the spread footings under each column a day prior to the slab.
Pour up to finished grade with a triangular shaped form and when you pour the slab place sand on top of the footing and felt expansion board to the face of the column footing to isolate the slab from the footings.
Saw cutting "control joints" on 10ft grid is perfect but most concrete contractors will try to just cut down the center the 40 ft direction and at 15 ft the 60 ft way.
This leaves rectangular shapes and concrete shrinks evenly all direction so wants to be in squares.
This is why sidewalks have so many scored lines. there needs to be one line at the distance of the width of the walk creating squares.

The ultimate way to control cracks and an alternate to saw cutting is to form the slab in 10x10 squares egg crate fashion and pour every other square creating "construction joints".
These joints should have dowels to pin the slabs together and/or the metal joint form to create a tongue and groove edge to the slab.
OR
make 2 30x40 pours with a "construction joint" at the center column line and saw cut the 10x10 "control joints".

All this is costly and creates a minimum 3 day pour cycle.

How the concrete is cured matters.
Hwy construction requires covering with wet carpet.
Minimum should be a curing compound spray ASAP after finishing.
I've done a few slabs where we build a dirt berm around the slab and flooded the entire slab under water.
Review what your contractor plans to do.
Periodically wetting with water spray hose does not work. Once the surface dries even for a short period, rewetting is too late and does no good.

Another thing that can be costly is getting the structural column anchor bolts in the wrong place.
I write into all concrete and metal building subcontracts that NO concrete was to be poured until the bolts were checked by the metal building erector.

Knowledge is power or maybe just confuses the situation.
I know your subcontractor will guarantee no cracks but the truth is it's a crap shoot even if everything is done perfectly.

FYI
Most jobs were 6 inch 2500 psi with fibermesh, poured monolithic in one shot and sprayed with cure and saw cut as soon as you could walk on surface without leaving marks.
Sometimes you could see the cracks "take off" ahead of the cut as the shrinkage has already set up tension in the slab within hours.
OH well.

Good Luck
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Old 11-21-2014, 08:16 PM   #9
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Concrete for around $250 a yd in place isn't a bad price
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Old 11-22-2014, 10:22 AM   #10
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Underlayment

Not a lot of talk about underlayment here. IMO it's crucial. Study-up.....
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Old 11-22-2014, 08:14 PM   #11
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Dirt work

Been a busy week. $930.00 got me 3 hours of dozer, 3 hours of scraper, and 3 hours of track hoe work done. He scraped off all topsoil down to clay, then dug out clay with a scraper from a proposed pond enlargement project, nice dry clay. Back filled to grade with the clay form the pond. A couple large oaks had to go, but are now saw logs. It allowed us to tuck the building in behind the house and existing shop to avoid spoiling the view. Hulckster thanks for the education. I think I will spec out having the rebar upgrade from the wire. I feel good about my site prep and was delighted to get this much work done for the price. Upping the cement to 5,000 and 5" thick and the proper site work I hope is enough to help guarantee me as nice of floor as I am leaving. Some of you asked that the shop was for, just basic dry storage, tractors, pick up, old boat and camper misc ag equipment. I will try to photo the progress as it occurs if there is interest. My current shop has been such a joy, tight, dry, and what few rodents make it in around the overhead doors are easily kept in check with a little decon, leaving no mouse damage to vehicles, camper and such. Hoping for the same success here, the floor spec just scared me a bit. 4" 4,0000 lbs with only wire seemed light. Of course the edges are thickend, and piers 3' but still. Thanks again for everyone's help. The depth of knowledge here always astounds me. Thanks, SN

Sharps-Nut screwed with this post 11-24-2014 at 08:52 PM
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Old 11-23-2014, 06:13 AM   #12
Dan V.
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Been years since I worked/sales at a concrete contractor. You have some excellent advice here. I also told all the customers that concrete is going to crack - expect it.

Sorry to hear about your clay. Where we worked it was pretty much all clay, and there were problems. Heck, my own drive will heave as much as two inches in the spring with the freeze/thaw cycles. Most people don't want to pay for all the extra prep, so there were a lot of unhappy customers. Not all ours, but a lot who didn't want to hear that their site work was crap and that's why the need for premature rework.
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Old 11-24-2014, 08:57 PM   #13
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Clay.

Quote:
Originally Posted by Dan V. View Post
Been years since I worked/sales at a concrete contractor. You have some excellent advice here. I also told all the customers that concrete is going to crack - expect it.

Sorry to hear about your clay. Where we worked it was pretty much all clay, and there were problems. Heck, my own drive will heave as much as two inches in the spring with the freeze/thaw cycles. Most people don't want to pay for all the extra prep, so there were a lot of unhappy customers. Not all ours, but a lot who didn't want to hear that their site work was crap and that's why the need for premature rework.
Is clay a problem. Here most dig down to clay then gravel and build on that as its supposed to be more stable than the top soil. Or at least I thought that was the reason. Not questioning you just trying to learn. My first shop I had 12 truck loads of hillrock brought in, its a red clay and fint composition. The dnr shut down the guy that was selling it as he was not properly permitted to do so. It was good stuff.
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Old 11-25-2014, 10:49 AM   #14
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All grate info ,like the others stated lose the wire mesh. Compaction ,rebar and a 5" to 6" of 6 sack you well be happy for years, control joints do just that control the crack they let it fallow the path of least resistance 10x10 is a good number and a line off any inside corners and or thickness changes make equal squares I do disagree that ALL concrete cracks If you look around you well see menay large pieces of concrete not cracked the floor of any big box store ,Tennis courts, Walls. I to am starting a new shop/garage looking at 75yrds 30x35 shop the rest in flatwork estimates from 10,000 to 27,000 and I well SUPPERVISE
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Old 11-25-2014, 06:38 PM   #15
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sounds to me like holkster summed it up pretty well
a monolithic pour will work also-the key to all slabs is compaction of the ground under the concrete-insist on compaction
I recommend a vapor barrier so the slab doesn't sweat in high humidity conditions
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