Garage Concrete Slab - Rebar Schedule

[Tractor Seabee]

The axiom "all concrete cracks" is true; the key is to get it to crack where you want it to. That is what control, joints, and expansion joints is all about construction joints. Putting all this in the right places is the key to no cracks in the field of the slab; along with all the other things that have been discussed. I can show slabs (including driveways and parking areas) jobs I have done; 4" thick, 4K PSI, <4" slump, fiber reinforcement, control joints that have no visible cracks for 15-20 years. 2500PSI concrete is OK if the subgrade exceeds that, does not frost heave, does not get overloaded, and stays that way. Not worth taking a chance on. Foundations are static loads not dynamic; big difference. Concrete is an engineered solution not an architectural solution. A lot of architects get it wrong. ACI certs are your best QC.

You can drive a loaded dump truck over a properly placed 4" slab with no problem. Note: concrete is "placed" not "poured", different concept. If you can pour it, it has too much water. Pumped concrete has water reducing additives to keep it thinner with less water. Tell the plant you only want water on the truck equal to the mix design slump requirements, you will provide the was-out water. If your finisher balks at this you have the wrong guy.

Last caveat; saw cutting the next day is wasted money, cracks have already started where you do not want them. Cut as soon as you can get the machine on the slab even if it is midnight.

Ron

 

[comancheflight]

I worked in material analysis lab most of my life. And dito what seabee said. ALL concrete cracks. You want controlled cracks. Metal and glass is used to keep cracks to minimim.
I stand on my first post.- your money - your garage - nuf said.

 

[Pettrix]

Found these rebar chairs. I will give them a try on my slab...



HERCULES REBAR CHAIRS

 

[Oldpath05]

When my was built my garage 3 years ago, 6x26x36, I knew it was to big for my skills, called 3 or 4 people about doing it, 2 said they would be right over such a day, no show, called another, he came right over end of the day after doing a foundation, looked it over got his transet thing out, tweeked the for m a little, said he could do it fallowing week, I wanted the slab this week ASP!!. The guy was quite accommodating, came over on Fri.

Me not knowing much about cement slabs, ask what size rebar, how many, he said 3/8, 3ft. on center squares, double where I might put a car lift someday, double layer around the outer edge, he said get those plastic trees and wire + the wire tool thingy. The rebar came 20', I welded them together so they made 36' and 26', he came back on Wednesday for final check, he said the rebar look great, Friday its a go, took the day off, 5:30 AM, still in bed, cement trucks come rumbling down the road in front of my house. 11 AM new cement slab smooth as glass, warm day, he said keep spraying water on it till evening. So far have 1 hair line crack where the slab was on new gravel, I expected that, I'd say the guy did a good job.

 

[vlcakc]

4" concrete slab - I called out for #4 bar at 12" oc for the house and garage. Of course the contractor says that's "overkill" and he recommended #3 bar @ 24" oc. I am a little leery with #3 bar since it's so thin and the spacing is bigger than I like.

Rebar is actually inexpensive. #4 rebar in 20' lengths is $5 per stick. So I don't see much savings in a 24x28 deep garage

Had the same argument with a contractor when I was constructing a 48' X 40' steel building. I spec'd #4 @ 12" oc. When I pointed out that I planned to park a 34,000# gvw motor home in the building he backed off. He then suggested "Interstate highway mats" that gave the same reinforcing area as the #4 @ 12" oc. I researched them and gave the approval to substitute. Saved a lot of labor tying off the individual rebar. I've used the mats several times since then with good results.

Using Bar Mats to Increase Productivity| Concrete Construction Magazine | Reinforcement, Concrete Slabs and Floors, Mike Artman, Joe Neuber, John Ortiz, Maher Tadros, American Bar Mat , Neuber Concrete, Ortiz Engineered Products, University of Nebraska, Associated General Contractors of America

Vic

 

[Redlands Okie]

Your 12 inch #4 rebar center specs are fine it just is not fun to tie of that much rebar Be sure base is compacted with a packer. Rebar needs to end up in center of slab height wise. Nothing but a post tensions cable slab replaces rebar. Wire mesh or its modern day replacement fiber mesh does not replace rebar. Fiber mesh does help control expansion and contraction cracking, alligator cracking and helps the concrete to not absorb water after it cures.
Every truck load of concrete comes with a free set of cracks. The rebar holds it all together and saw cuts controls the cracking. Saw cuts at 25% of concrete thickness. Slab will have a tendency to crack where ever there is a inside corner, plumbing block outs, post (especially near edges as in pole barn style), etc. 10 feet squares are usually as big as you should go.
A bit thicker edge at drive in entrance areas if you have no footing. Maybe a extra bit of rebar or two near the edge also at those areas.
Less water in the pour is better, the concrete plant can add chemicals to make the concrete flow better using less water. Be sure to check it out. The chemicals only last a short time and then it thickens up, so it usually does not affect finish times by a lot.
The slower it cures and the less water at pouting time the less cracking.
Concrete sealers that are applied at the time of finishing usually are silicone based. This prevents most concrete coatings from sticking to the concrete properly later on. If you plan on coating the concrete with a clear coat later be sure to check the issue out or just not use a concrete sealer at finish time.
I make a living fixing what many have done wrong the first time so have some ideas of what not to do.

 

[Egon]

the edge also at those areas.
Less water in the pour is better, the concrete plant can add chemicals to make the concrete flow better using less water. Be sure to check it out. The chemicals only last a short time and then it thickens up, so it usually does not affect finish times by a lot.

Just change the slump required.

 

[Streetcar]

Rebar in the center of the slab adds very little strength, it helps out the cracked concrete together
Fiber will stop the cracks in the concrete before they grow larger
Higher strength concrete will also help with cracking
Applying curing agent immediately or keeping slab wet during curing will help the concrete from drying and cracking.
Control joints try to make the concrete crack where you want it to crack
Concrete likes to crack

 

[dfkrug]

Out here a #4 bar at 20' length is $5.00 delivered per stick

So for my pad that's around $280 in rebar.

9 yards of concrete or $975 in concrete ($108 per yard for 4,000 psi mix)

That's $1,252 in rebar and concrete. Not a lot of money for the materials. Labor is another story.

I have not priced rebar in a long time, but the last time I bought a bundle (150 sticks), it was only
a bit more than HALF the one-at-a-time big box store price. Delivered. And steel prices are down
right now, tho most rebar is prob still imported from Mexico, Korea, and China.

That is a great price on readi-mix concrete. Sounds like a 5-sack mix. A 6-sack mix will be
stronger and pump easier, but cost another $10-15 or so per yard.

I am sure this varies by region, but in my big metro area (SF Bay), most concrete is pumped,
and pumping contractors prefer 3/8" pea gravel concrete WAY over more conventional 3/4".
I never let them pour a slab with 3/8". The bigger the aggregate, the better the strength, and
less the cracking, all other variables being the same.

 

[dodge man]

One thing I'm suprised no one has commented on is the 4" thickness. I consider 4" to thin for anything except sidewalk. I'd go with 6" thick. It could be in your area 4" is the norm, you might not have to deal with the freeze thaw we do.