Concrete question

[5wndwcpe]

My contractor and I have been going back and forth about this so I thought I'd put it out to you guys. The question is should I put expansion joints (not control joints) in the new garage floor I'll be pouring this fall. It's a slab on grade, 6" thick, 4000lb, with radiant heat. There will be 2'' foam around the perimeter which would in itself act as an expansion joint. The slab will be 28' x 56'. If I should go with the joints, how many and where ? I did pose this question a few months ago and never really got an answer. Help me out fellas.

 

[z28toz06]

i just did radiant in my 24 by 32 pour and we put an expansion joint in at the 16 foot mark. I would ask whomever is going to pour it what they suggest. You will probably need to cut a kerf in there somewhere while it is still green, just in case it ever decides to crack on you. At least it will follow the kerfs.

I also put a 12 inch sleeve of 1/2 inch foam on the pex pipe where the expansion joint will be. Just in case it does crack the pex pipe will have a little extra cushion.

 

[PAToyota]

With proper reinforcing I'd split the 56' in half and put an expansion joint there. Squares tend to do better than rectangles as far as cracking and such. We've done up to 50'x50' for spacing on expansion joints with no control joints.

 

[Falcon05Dad]

I did not have any expansion joints placed into my recently poured 42x32 5" thick 4000psi floor. It does have control joints cut halfway in each direction. I water cured it for three days and no cracks have shown up anywhere as of yet.

I would have considered expansion joints if I had put in a radiant heat system, however. Best of luck with your project.

Greg

 

[bluesman2a]

I have a couple of other concrete questions:

1) I have seen references to "water curing" here. What is it? What are the benefits of doing it, and for how long?

2) I saw another reference that said you should be there for the pour to make sure no water is added. Could somebody please explain this one as well? I thought water was used during the pour process to keep the correct consistency, or am I just being stupid?

Any education would be appreciated.

 

[PAToyota]

Concrete "cures" - it doesn't "dry." Well, actually it can dry out, but that is a bad thing.

If you look at the strength of concrete as it cures, there are points at seven days and twenty-eight days where the graph of the strength tapers off. At seven days it has reached 2/3s to 3/4s of its design strength and at 28 days it has reached the design strength - but it actually continues to get stronger after that. But if it dries out early, the reaction of the cure stops and with it the increase in strength. So if you let it dry out in two or three days, it could only be half the strength you ordered - 2000psi instead of 4000 psi. The other issue is letting the surface dry out while the rest of the slab is still moist - this will result in spalling.

So the idea with water curing is to keep the slab moist long enough for it to reach a high enough strength. A lot of places use a sealing compound that is sprayed on to trap the moisture in the concrete - like putting plastic wrap over food in the refrigerator to keep it from drying out. But many people feel that actually keeping the concrete moist is better. There are different ways to do this. Burlap laid over the concrete and kept moist is one. Plastic sheeting is another. There are others. However, you do want to let the concrete set up so first so that you aren't washing any of the cement away from the sand and aggregate - one problem with people just leaving a sprinkler running or something like that.

Adding water during the pour is another matter. Typically you ask for a certain "slump" to the concrete - that is that the concrete has stiffness when it is poured. This is all part of the calculation of its design strength. If you add too much water while pouring, you dilute the mixture and it again never reaches its design strength. Again, this is part of the equation with the concrete curing instead of drying out. A certain amount of water initially is part of the equation - if there is too much water it "overloads" the equation.

Hope that helps somewhat.

 

[bmwpower]

I agree with all of what PA said, but I'll add that water is often added during the pour so the concrete doesn't setup too quickly. That's part of the problem with high strength slabs - they setup faster. Masons don't like to work with high strength concrete since they have little time to finish the slab.

The trick is to get a mason who (1) doesn't have the truck driver add water to the mix and (2) who doesn't mind working with high strength concrete and (3) who hustles to finish the slab nice and smooth.

Good luck.

 

[64auto]

I had my concrete poured yesterday. Lastnight I flooded it good. Is this enough or should I put water on it for a few days? Thanks in advance

 

[Namocsid]

I'd water it several times a day for the next 5 days to a week

 

[PAToyota]

You not only want to keep it wet for about a week, but you want to keep it continuously wet. Just flooding water on it and then letting the surface dry out before flooding it again won't help. Once it is dry, the reaction stops.

 

[Junkman]

One of the best ways is to flood the slab, and then put a few layers of burlap over the concrete. Make sure that the burlap never dries out, keep watering the slab. If it is in the sun, then it will take more diligence to keep it wet. Some people cover it with plastic, but that can lead to high temperatures under the plastic, so I don't recommend it. Water is your friend, and heat / drying is your enemy......

 

[bmwpower]

One of the best ways is to flood the slab, and then put a few layers of burlap over the concrete. Make sure that the burlap never dries out, keep watering the slab. If it is in the sun, then it will take more diligence to keep it wet. Some people cover it with plastic, but that can lead to high temperatures under the plastic, so I don't recommend it. Water is your friend, and heat / drying is your enemy......

Yea, but if it's covered with plastic and it's wet underneath, the water won't go anywhere. evap/condense/evap/etc

 

[Junkman]

Yea, but if it's covered with plastic and it's wet underneath, the water won't go anywhere. evap/condense/evap/etc

This is correct, but if the temperatures under the plastic heat the concrete excessively, then you can create thermal cracking of the concrete as a result. You need to balance moisture along with temperature.

 

[dyson74]

Wet Wet Wet keep it wet.

Under plastic i've found it gets extremely humid (and hence wet) but the temp can be over come by covering with a second layer of shade material.