My concrete slab was poured yestday for the basement. Today, they made control cuts (my term) from corners to corners. They said this is so when the slab cracks, it will be limited to the cuts.
Huh? I’ve never been in a basement that had these cuts. What’s up with that? Won’t this compromise the integrity of the slab?
Join some of the most experienced and recognized building professionals for two days of presentations, panel discussions, networking, and more.
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.
Start Free Trial NowGet instant access to the latest developments in green building, research, and reports from the field.
Start Free Trial Now
Replies
I know two things about concrete...
It gets hard.
It cracks.
I've seen plenty of basement slabs with cracks everywhere, so that sounds right.
There are no electrons! It is all made up. Don't believe it.
Electricity is made by GREENIES.
I have poured a few basements and have never been asked to do control cuts. I would do so on exterior work but basement concrete floors with proper compacting of the 3/4 stone under and vapour barrier act as a protected area from the moisture below. I would guess that a cut in a basement floor would allow moisture to enter the basement.
Concrete comes with two guarantees. It will crack. It won't burn. Perhaps its regional, but I've never heard of not cutting control joints. Not just typical here, pretty much a given.
"If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man." - Mark Twain
Control joints are standard here. Sometimes they are tooled in, sometimes they are saw cut. Saw cut ones tend to work better as, generally they are deeper. Saw cut is normal for interior concrete since you end up with a more a more level surface. Tooled joints are more common on exterior concrete as they look a little better in standard applications.
I haven't seen a tooled joint in a coon's age, interior or exterior.--------------
No electrons were harmed in the making of this post.
Must be a regional thing. Here, all driveways, porch slabs, sidewalls, etc get tooled control joints, with a few expansion joints on each pour too.
Seems like it must be regional. I've never seen a basement slab with control cuts.
There is but one just use of power and it is to serve people. [George Bush]
Yeah, when I was a kid, of course, everything was tooled. But I noticed especially when I moved to MN in 74 that tooled joints were rare, even in slightly older (60s, probably) work.--------------
No electrons were harmed in the making of this post.
Control joints are usually cut into all engineered slab floors such as the malls, low rise industrial, etc. Page 14 of the Portland Cement Association's book Design and Control of Concrete Mixtures states "Drying shrinkage is an inherent, unavoidable property of concrete, therefore, properly positioned reinforcing steel is used to reduce crack widths, or control joints are used to predetermine and control the location of cracks."
They are not cut into basment slabs (1) to save money, (2) the contractor doesn't know the difference, (3) the homeowner doesn't know the difference, and (4) codes don't require them (since codes are minimum standards, not neccessarily good standards)
They do not compromise the strength of the slab which by the way is nothing more than a heavy concrete carpet. The slab should have no bearing on the integrity of the structurs unless it was designed with footing strips or post bearing pads under the support points. The slab does not prevent the exterior walls from being pushed inward by soil/moisture pressures. The properly designed/executed footings take care of that.
The cracks do not allow soil moisture upward as there should be a moisture barrier under the slab. They are usually cut only 1/3 or 1/4 the thickness into the slab to make a weak point so that the shrinkage crack will occur where you plan it to.
Guess your name says it all. Thanks.
Edited 10/26/2005 3:29 pm by tufenhundel
Excellent post Experienced. (Apt name there, buddy)
I'm going to have to agree that it must be a regional thing. I have never seen a slab that didn't have control cuts in it.
Those contractors that do the "radon sealing" would probably hate to walk into a basement that didn't have them. imagine having to go along and try to caulk all those crooked hairline cracks running all hither and yon. " If I were a carpenter"
Good post.
My guy does toole joints all the time. I hate the dust left over from saw cuts. My neighbor is still cleaning up concrete dust in her basement.
I'm not sure you could even find someone who knows how to tool joints here.--------------
No electrons were harmed in the making of this post.
I guess it's a lost art. Strange, from watching the two crews (my guy tooled the joints & the other guy cut them later), I don't see how it saved any time at all.
Edited 10/26/2005 1:37 pm ET by Soultrain
Our basement has control joints saw-cut resulting in squares - are you sure they cut yours corner to corner?
The joints should extend 1/3 down thru the slab to "direct" the crack direction. If they're not cut, shrinkage cracking can take several months to occur but they'll be random (and ugly). My in-laws had no control joints placed in their ~30 x 40 cellar slab and you stick your fingers into the (only) shrinkage crack. Otherwise the floor looks good.
The guy said cracks usually start at corners, and with all the corners in our basement, that's where they all seem to start and terminate.I saw the cuts today, they seem alright. From the responses here, it looks like a regional thing. I've never seen one in Chicago, here in Indiana--my first.
As for cracks in corners -- are we talking about outside corners or inside (reentrant) corners?You can pretty much count on cracks at the reentrant corners. The corner acts as a stress riser. Stress risers and crack propogation is really pretty interesting, but also sort of off-topic.To follow upon Experienced's observations, concrete shrinks at a definite rate depending upon the mix design and a few other factors. That shrinkage will create a stress that will exceed the tensile strength of the concrete, which is very low. That's how you get cracks. There are other kinds of cracks, but that's another subject.While it is entirely possible to pour a slab that does not crack (yes, it's done all the time, just not usually in residential work), it is also cost prohibitive and really unneccesary for most applications.So the control joints you see are something that is used to propogate a crack in a more desirable and aesthetically acceptable location. As already described, the cut that is 1/4 to 1/3 the thickness of the slab causes that inevitable crack to happen someplace that is not ugly.You will often see better-quality concrete subs put extra steel at reentrant corners for the reason Experienced describes.Why are the cracks irrelevant to the strength? As has already been discussed, the slab has no structural value, but even if it did, those cracks less than 1/8" really don't mean much.There are lots of different meanings for strength, mostly we talk about compresive strength and tensile strength. Concrete has great compressive strength, and as long as aggregate interlock is maintained, that strength is pretty much maintained. By aggregate interlock, we mean that the lateral displacement does not exceed about half the diameter of the coarse aggregate, so in other words, all the little rocks in the concrete overlap. That's part of what re-mesh (like what we used to call 6/6x10/10 welded wire fabric) does. It provides enough tensile strength to resist lateral displacement, and maintain aggregate interlock. But it is not a structiural component, and should not be mistaken for such.Rebar is a different material with a different purpose. In fact, reenforced concrete is a different material than concrete without steel. But, another subject. <G>
You obviously know a little bit about concrete! The more I learn about this stuff, the more fascinating it is. I heard on NPR some time ago about a school, or university program, solely dedicated to the science and application of concrete. I've never thought about it much, but what an amazing material.I plan on getting some hands-on experience when I try casting counter tops for the house later on.Thanks.
I've pushed a few wheelbarrows of the stuff. <G>Yes, it really is a fascinating material. I feel like I've only begun to learn about it.
I've never seen control joints in a basement slab. In my home the slab has random hairline cracks, which is normal for concrete. If the cracks open up or there is a change in elevation from one side of a joint to another then the concrete was not done properly.
Quote:"If the cracks open up or there is a change in elevation from one side of a joint to another then the concrete was not done properly."
If the cracks open up much more than 1/8-3/16" or there is a change in elevation from one side of crack to the other, there was a problem with the compaction/noncompaction of the underlying crushed stone or soil that it was placed on and not the concrete pour.
Yes. I consider preparation of the base as part of the job.