Expansn joints NOT needed in 3-car slab?
Streamline
| Posted in Construction Techniques on
My concrete contractor tells me that if I order stealth fiber in my 6-bag concrete mix, I do not need expansion joints in my 3-car garage concrete slab. Is this true? The slab is sitting on 10″ of 5/8″ structural fill.
Replies
First off, those are crack control joints, not expansion joints.
If you go more than about 12' without putting in control joints, almost any concrete will crack randomly and then you will have a mess to look at.
You need to edumacate your concrete contractor.
and you need to cut them deep 1/4 the thickness of slab and cut within 24 of being poured. we usually cut as soon as you can walk on it. I would cut at leats 1 1/2 inches deep
3-car slab is pretty big. While the fiber will reduce smaller cracks, which is what the stuff is used for in decrative concrete, and adds some strength to flatwork, but to not get a crack in a big slab over time is doutful.
An admix to reduce water would help to reduce shrinkage and there's a good chance shrinkage cracks would be minor if at all, but that's only half the story. With a large slab you'll have some settling of the gravel base, it's just not possible to have zero settling regardless of how well it's compacted, and then a crack will popup.
Today there's no doubt that I'd go with cutting joints since they always look better than any random crack.
Your concrete guy is partially right and partially wrong. Most of the long term cracks are never seen by the concrete guy.
Good cracking
Beer was created so carpenters wouldn't rule the world.
If your "concrete guy" is the one who called them "expansion joints" get a new, real concrete guy.
Even if he called them control joints, that is an awfully optimistic view of what fiber will do.
BTW, cut joints work as well as struck (?) joints (possibly better) but look crummy, in my opinion. At least have them tool the cuts to round the corners off.
See, e.g.,
http://tinyurl.com/ywkxjw
QUOTE
Control Joints in Decorative Concrete
Control joints in concrete are used control cracking that would naturally occur. Read on to find tips on placing and cutting control joints in decorative concrete, including how to incorporate them into your design.
by Stacey Enesey Klemenc
If you pour it, it will crack.
This unwritten law of concrete may as well be written in stone because — try as you might — there's no getting around it. So what's a contractor to do? In short, learn to master the art of placing control joints to encourage concrete to crack where you want it to crack.
According to the Portland Cement Association, control joints — also called contraction joints — should be placed at two times the concrete slab thickness in feet for a maximum aggregate size of less than 3/4 inch.
For example, the PCA maintains, the maximum joint spacing for a 5-inch concrete slab with a 3/4-inch coarse aggregate would be 10 feet. If the maximum coarse aggregate size is greater than 3/4 inch, the spacing could be increased to two-and-a-half times the thickness. This would increase the spacing to 13 feet.
With my mouth I will give great thanks to the Lord; I will praise Him in the midst of the throng. For He stands at the right hand of the needy, to save them from those who would condemn them to death.
- Psalms 109:30-31
Not being a concrete guy, I see 3 different kinds of lines cut in concrete slabs. They are (1) saw cut lines in concrete (2) rounded concrete cuts that look nicer and concrete slabs are butt up against each other (3) some kind of soft dark boards sandwiched between 2 slabs in the cut often seen on sidewaks. Can you help me understand the names and purpose of each?
1) control joint
2) construction joint
3)expansion joint
I take it that each serve a special function. Can you clarify? Thanks.
Control joints, typically sawn in the slab, but sometimes done using inserts, are made to control cracking. Shrinkage, substrate movement, and even loading can cause cracks, and sawn or formed joints tend to "control" the cracking so that the breaks occur where intended. 12' x 12' modules are a good guideline for control joints.A three car garage, somewhere near 24 x 36 in size, would be sawn into a 2x3 grid, or six squares each 12 x 12.Construction joints are for when slabs are poured in sections. The joints between sections are control joints. Often a material is placed between sections.Expansion joints are formed joints with a material in the joint, and usually a flexible sealant at the top surface. The joint and its material is there to absorb slab horizontal expansion.
yes,yes, yes correct on all three point. but last add one more point. concrete will always crack on an inside corner. That meaning if you have a pole, block out, another slab, where the new slab wraps around a corner, it will crack at a 45 degree. slip in a piece of expansion board and it will take care of it.
find concrete products wholesaler in your area lotta product out there e z use plastic control joints are avail height, thickness tailored to depth slab being poured they can tell you how often to space have plastic tab to help embed product into the mud, then tab peels off use expansion joint material on perimeter of slab to provide movement space, reduces cracking control drying time to reduce cracks, mist w water if poured in sun or open air, cover w kraft paper or similarproduct for that purpose fiber helps control small cracks but not bigger stuff use heavy gauge highway wire embedded in slab to help control movement, and higher psi like 4,000 test concrete
Most of the people here will probably tell you you should have control joints cut or placed in your slab. If you do, you'll probably not have major cracks appear anywhere else. If you don't, you're taking the risk of having cracks show up somewhere you don't like.
In my case, I've got a 24x36 slab going on 2 years now. No control joints, no cracks (yet). I fully expect that somewhere along, I'll get some cracking, but I doubt it will be a problem. I prepped very well, and used a reasonable amount of rebar.
My dad's workshop a few miles away is the same size, and after about 20 years, it has 2 or three noticeable cracks - all of which are a lot smaller than a control joint would be.
A neighbor of mine has a smaller garage, but apparently didn't prep very well, and has a large crack running parallel to the doors a few feet back, and the front of the slab has tipped slightly.
Part of it is local weather & soil conditions, part is prep & design, part is concrete slump & placement, & the rest is just dumb luck.
Don
There's a article in the new mag. that explains how to reduce the chances of cracks in a big slab. Looks as if it might be the answer you are looking for. I'm getting ready to have a floor replaced in a basement so I'm looking to reduce (really eliminate if possible) the chance of cracking and if this is not possible then where to put the joint strips. The old slabs rarely had cracks, I'm always asking myself why they always crack now, is it the material or the process, or something else?
Why not make the joints a feature? Design a pattern, cut the slab and grout the cut lines. My house is like that and some people (ok some dim people..) think it's marble.