Just out of curiosity (I'm trying to learn about concrete), can anyone tell me how weak a cold joint is. Assuming there is rebar crossing the joint, that is. All the books and articles seem to pretend that the joint is completely not connected at all, and even a "key" between the two will only kind of help a little and the two pieces will still be free to rotate against each other, shift, crack, etc.
I see buildings going up all the time here, with what must be hundreds of pours (these are complicated university buildings, lots of foyers, atriums, suspended walkways, etc.). Are those all flimsy cold joints? Or do they do something special to make them stronger.
BTW -- I just learned about slip-forming... looking at the Millau bridge project
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-- Yikes, that is one rediculous huge concrete project.
All huge slabs are poured in sections. Where possible, the seams are placed where walls will cover them rather than in the middle of a room. Seams are good - they allow the concrete to expand & contract without cracking.
It would be good if you noted what sources you were refering to
joint is completely not connected at all, and even a "key" between the two will only kind of help a little and the two pieces will still be free to rotate against each other, shift, crack, etc.
You mean how strong? Tension, compression, or shear?
Concrete is much weaker in tension vs compression, and cold joints even weaker.
Tile and stone mortar is a species of deliberate "cold joint". Look at pull-off strength for some cementitious products on a manufacturer's Web site, like
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Should specify something like 50 psi. Compare that to 3500 or 5000 psi for concrete in compression.
If a 4-inch thick wall 8 feet high would pull apart at 50 psi, that amounts to about 10 tons total force, best case.
Of course. All a key or hardware will do is convert the failure from that of poorly bonded surfaces to homogeneous tension concentrated over a smaller area. Such a conversion cannot be much stronger, and may in fact be weaker.
Another factor is the "coldness" of the joint. Freshly set concrete is still active on the surface and will bond well to a fresh pour, if done within hours or a day or so. Aged concrete has an inert surface that doesn't bond well to new.
I th> Of course. All a key or hardware will do is convert the failure from that
Huh? Seems like adding steel, which is just fine in tension, will add a huge amount of strength, since it will take the tension, and the concrete will take the compression.
And if a key makes it weaker, why would anyone ever add a key? Again, it seems like a cold joint resisting a shear force might be flimsy because it is just poorly-bonded concrete in sheer, but a keyed cold joint puts part of the bond in compression, and puts the well-bonded concrete into sheer instead.
This is useful (if it is correct). So that means if you pour the second section just a day later, it is much better than if you wait a week or two?
The problem is not the steel, but the steel-concrete bond. Why is that any better per unit area, or any more area, than the concrete-concrete bond? The steel just pulls out of the concrete.
All a key does is trade an area of bond tensile strength for a smaller area of bulk tensile strength. Whether the trade is an improvement depends on the geometry and the relative values of the two types of strength per unit area. The problem is that neither is that good.
Imagine that you joined the edges of two slabs with a butt joint versus a dovetail key. In the former you have the bond strength per unit area over the full area of bond. In the latter you have the tensile strength per unit area, but over a much smaller area that is the base of the dovetail. If the tensile-vs-bond strength ratio has to be large enough to overcome the geometrical disadvantage.
Consider a dovetail joint in a wooden drawer. It is an improvement only if the tensile strength of the wood is much larger per unit area than the pull-off strength of adhesives, because the tensile area of the dovetails is much less than the bonded area of a butt joint.
Absolutely. Concrete takes weeks to cure to nearly its ultimate strength. The chemistry of the bulk material and surface activity changes during that time.
Why? If the steel pulls out, it wasn't designed or placed right.
Tensile strength is irrelevant in concrete design. A key minimizes the likelihood of shear displacements. It can also increase bond surface area, but that's a secondary consideration.
I think much of the confusion is coming from terminology.
A cold joint in concrete implies to me that the pour was held up (form failure/truck breakdown/pump failure, etc) that was not planned. These can be cosmetic and/or structural problems. A construction joint is a deliberate stopping point that is part of the building design. Dowels, keyways, etc are designed primarily to work in sheer. Reinforcing steel or mesh in slabs doesn't keep the concrete from cracking, it keeps the cracks from spreading. In structural concrete, the steel reinforcing is used to overcome concrete's poor tension properties. A contraction joint is a deliberate tooled or sawn joint to accommodate concrete shrinkage. Concrete will crack, the contraction joint tries to tell it where to do it. ______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net
All concrete will expand & contract with variations in temperature & humidity. If the slab is too large without controlled expansion points, the slab will crack where you don't want a crack. That's why a large slab is poured in sections, with the section lines located along future wall locations - if the joints expand or contract you won't see them. To make multiple sections stick together as one huge piece is asking for trouble down the road. Maybe you should define how large a pour you're talking about.
Look closely at the image and you will see the concrete is being poured and worked in sections. There are obvious horizontal lines between the columns and in the upper right next to the shadow you see a verticle line. Another verticle line is to the left of the workers feet extending up into the overpour of the next section. True, it's being poured at the same time in the photo, but work could stop for any reason and resume the next day with no problem. The verticle at the workers feet seems to be a board - the typical way to make a tongue & groove joint for the next day's pour.
Sorry, let me go back to the original post Forget slabs, forget expansion joints, forget control joints to control cracking. The question is about *cold joints*, where we want to permanently join two pieces of concrete. For example, the joint between a footer (say a 1' x
1' x 4' block, to be specific), and a short stem wall sitting on the footer (say 4" thick, 4' long, and 2' high). Imagine the two pieces could be poured separately, on different days (say, 1 month apart), or together at the same time on the same day with no pause between the two.
In the latter case, I think it is clear that there is no joint at the intersection of the wall and the footer, unless someone wants to
*deliberately* put one there by sawing, trowling, etc. In the former case, there is, what I think is called a cold joint. A new piece of concrete is joined to a cold, already cured piece.
Most of you seem to be missing the question, I think. The question is simply, how much worse is it to do it 1 month apart, rather than at the same time? Will the rebar crossing between the footer and the wall completely more or less make up for the fact that it was poured separately? Or if a key is installed, or a combination of key and rebar, would that make up for it? Or is the tensile and shear strength of concrete so low anyway that the further weakening due to the "cold joint" negligible, because even the continuous pour would be so weak as to be negligible.
Well, maybe my terminology is not up to speed -- please let me know. But when I say "poured together", I mean that the concrete is poured, you know, together. On the same day, from the same truck, at the same time. It really has nothing to do with whether someone cuts a control joint, or puts in a board or key or whatever as an expansion joint. I think they only way to get a cold joint (which is what I am asking about) is to do two separate pours, on (say) different days of the week, where one side has already cured substantially.
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