Water curing my 23x24 slab?

[Chucktown]

My slab is scheduled to be poured on Friday afternoon. 23x24, monolithic slab, 6" think with 18"x18" footers. #3 rebar along the footers with wire mesh on the slab. My plan is to polish the concrete at some point, but until then, I'd like the strongest slab possible.

I have a "grand" tree within 12 inches of the slab that the city won't let me remove, so we've had to engineer around it. Those of you living in and around Charleston know that the city loves their trees almost as much as they love their houses...it ends up being a bureauratic nightmare between the historical and environmental conservationists.... At any rate, I have a 100 year old American Elm that I have to work around, so I asked the engineer to give me a slab that wasn't going anywhere.

My main concern at this point is about shrinkage cracks, so I'd like to give the most optimal conditions for curing.

I'd like to water cure the slab, but ponding is not practical. Will lawn sprinklers suffice in the winter? It's not scheduled to get below freezing here in the short term, heat will not be a problem. Temps will be in the mid 30's at night and low 60's during the day.

I'd love any recommendations anyone can give. Thanks.

 

[imnutz]

I would just cover it with visqueen for a few weeks

 

[Gary S]

You are overthinking it. Unless you have miserable hot weather, concrete cures itself perfectly. This time of year, the hot weather shouldn't be a problem anymore. Even as far south as you are, the weather should be decent by now. If not, pick a cool stretch.

 

[uppster]

Why not do what the man you are paying tells you to do, instead of asking a bunch of strangers?

 

[Chucktown]

Why not do what the man you are paying tells you to do, instead of asking a bunch of strangers?

Because I have never found anyone as meticulous as I am, and I know there are quite a few people with my "disease" on this forum!

My contractor will do whatever I want, for a price. I'd prefer to get the maximum reward for the least amount of money possible. And that means I have to do more research myself instead of paying the experts.

 

[theoldwizard1]

My slab is scheduled to be poured on Friday afternoon. 23x24, monolithic slab, 6" think with 18"x18" footers. #3 rebar along the footers with wire mesh on the slab. ... I have a "grand" tree within 12 inches of the slab that the city won't let me remove, so we've had to engineer around it.

That is one serious footer, but over 50 years, my money is on the tree !

 

[ConCretin]

If you really want to do it right, pick up a roll of this stuff;

http://www.pna-inc.com/products/hydra_cure/

If you don't want to spend that much, simply wet down the slab and cover it with poly.

Your goal is to maintain the slab in a moist condition for at least seven days to keep the mix water from evaporating and halting hydration before the concrete gains enough strength to resist early loads such such as shrinkage.

Ignore those who advise against curing - that is just dumb.

 

[Chucktown]

That is one serious footer, but over 50 years, my money is on the tree !

That's my concern! I brought out two arborists, one from the city and another one. Both said that the tree will probably not grow much more than it is now. I'd have to measure it but it's ~4ft in diameter. They had to notch one large root to make way for the footer, and both agreed that it shouldn't kill the tree. To cut the root would compromise the stability of the tree, so notching won out.

I'll attach some pictures when I get a chance.

 

[Chucktown]

If you really want to do it right, pick up a roll of this stuff;

http://www.pna-inc.com/products/hydra_cure/

If you don't want to spend that much, simply wet down the slab and cover it with poly.

Your goal is to maintain the slab in a moist condition for at least seven days to keep the mix water from evaporating and halting hydration before the concrete gains enough strength to resist early loads such such as shrinkage.

Ignore those who advise against curing - that is just dumb.

Thanks, LLWillysfan, I've been following your build, and I think it goes without saying you are an authority on concrete! I've just called the local distributors to see if they have any Hydracure in stock. The only one I was able to get a hold of would have to special order it, so that is out. I may have to just cover it with poly, tape the seems, and cover it.

I really appreciate your help!

 

[Herb]

I would, and always do, install a vapor barrier under all concrete work, inside and out. Plastic under your slab will prevent the water in the concrete from getting sucked up by the dirt which in theory should make the slab dry slower and stronger. This has always worked for me; I have two 30' x 30' slabs, a 20' x 20' apron, a 17' x 15'apron, and two 5' x 15' sidewalks that all have fibermesh additive, a plastic barrier, no saw cuts and no cracks yet- but it's only been thirteen years so I guess they could still crack. Every time I request the vapor barrier I was ALWAYS told that it wasn't neccessary, but I think that's because it slows the finishing process dramatically if you are going for a really nice whirlybird finish.

 

[Jackfre]

A surviving American Elm. I'm with the city. Don't hurt the tree!

 

[Chucktown]

I would, and always do, install a vapor barrier under all concrete work, inside and out. Plastic under your slab will prevent the water in the concrete from getting sucked up by the dirt which in theory should make the slab dry slower and stronger. This has always worked for me; I have two 30' x 30' slabs, a 20' x 20' apron, a 17' x 15'apron, and two 5' x 15' sidewalks that all have fibermesh additive, a plastic barrier, no saw cuts and no cracks yet- but it's only been thirteen years so I guess they could still crack. Every time I request the vapor barrier I was ALWAYS told that it wasn't neccessary, but I think that's because it slows the finishing process dramatically if you are going for a really nice whirlybird finish.

Hi Herb. Thanks for the reply. I forgot to mention that they have already installed the vapor barrier.
Here are a few pictures from this morning, tying rebar.

 

[Chucktown]

A surviving American Elm. I'm with the city. Don't hurt the tree!

It really is a beautiful tree. There was really never any discussion about removing it. "Grand" trees over 24" (I think) are pretty much untouchable. It would have to be compromising an historic structure for it to be removed. The previous garage structure, a "suburban looking" (<--insult! as described by a member of the board of architecture review) garage, was already so termite infested that nothing could be salvaged.

The only real negotiations took place on how to handle the root. And I think we found a happy medium and little cost to me.

 

[Will67]

my 0.02
add chairs or dobies under your remesh to place it in middle of the slab. Dont let the concrete people tell you that they will lift it up as they pour.

 

[Chucktown]

my 0.02
add chairs or dobies under your remesh to place it in middle of the slab. Dont let the concrete people tell you that they will lift it up as they pour.

Interesting, that's exactly what they told me.

I work from home so I'll be around when they pour. How effective is the "lift"method? There are chairs under the rebar, but that's it.

I can pick up some at HD today...will them walking on the mesh while pouring negatively affect the mesh, puncture the plastic too much, or bend the mesh, etc?

 

[pfarber]

Well, you are in for a surprise.

Your concrete will crack. Not if, just when. Rebar doesn't prevent cracks, just the spreading of the two halves once the cracks are made.

That tree, of some other root system, will heave the ground and cause a crack.

At least your contractor made his house payment off that extra thick slab and footings.

Should have went with some sort to stonework.. that way you can repair/dig up heaved sections.

 

[Gary S]

Well, you are in for a surprise.

Your concrete will crack. Not if, just when. Rebar doesn't prevent cracks, just the spreading of the two halves once the cracks are made.

.

The best thing you can do is cut relief cuts in the concrete within 24 hours of pouring it. That way, any cracks that form should follow the cuts nice and straight and prevent your concrete from becoming a mess.

 

[Chucktown]

The best thing you can do is cut relief cuts in the concrete within 24 hours of pouring it. That way, any cracks that form should follow the cuts nice and straight and prevent your concrete from becoming a mess.

That's the plan right now, to cut them Saturday morning, then water cure. Thanks.

This board is awesome for feedback.

 

[Rob73]

I have poured a few driveways and quite a few slabs. In my experience with concrete pours, there is no optimal conditions. Every weather condition is different and every load is different. Seems like you just have to adapt to those conditions.

I poured a new garage floor of 35 yards last week, and they delivered it in 4 loads. Every load had a different slump(consistency) regardless of what I ordered. If you work it right & finish it right, it will work out just fine. There is so much different information on concrete. How best to work it. How best to finish it. How best to cure it.

Sounds like you are getting good advice from the guys on here. Plastic on bottom to keep the dirt from sucking the moisture from it during curing. I had always heard that it would prevent the concrete from wicking moisture from the ground later too. Rebar & wire to keep cracks from opening up. A good finish to it. Then what ever your preference is to keep the moisture from leaving it too early as it cures.

Seems to me concrete is predictably unpredictable. There is a lot of things that you can predict for sure, but how, where, when its gonna crack is kinda unpredictable. You can try to control it with grooves, but that is no guarantee thats where it is gonna crack. And, it may never crack. Go figure.

I am sure your contractor will do a good job and you will be satisfied. If he has done much concrete work at all, he will probably do a quality job.

I am not a professional, so I may be way off base.

Rob

 

[c39er]

Why not install hidden zip strips for control of cracking? Looks a lot better than saw cuts that sometimes are cut crooked and hold debris after sweeping.
This slab is 20 years old-no cracks except smooth cracking at zip strips.

 

[pfarber]

My 24 x 40 slab on grade is on #2 gravel... no plastic or insulation. Water will drain away via the gravel. My slab is 4 years old and the joints are cracked.

If they could stop cracks, they would put it in writing. I have never seen a contractor provide a 10 or 20 year no crack warranty.

The cut joints don't stop cracks either. They simply allow for a predetermined point for the crack to develop (aka stress relief point)... but you can still get diagonal cracks that make life ****.

 

[uhcrandy]

When your done finishing the concrete, just spay a Sealer and be done with it. Any concrete supply will carry it. This time of year anything else is overkill.