Lower Temperature for working concrete


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Old 02-12-06, 08:35 AM
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Lower Temperature for working concrete

I am putting down a granite walk way with large granite scaps I got from a local quarry. I want to throw mud in the joints between the granite slabs to finish the walkway and make it cohesive; however, the temperature is only 39 today and may freeze tonight....

what is the lowest working temperature for concrete... and because it is on the ground and not structural can I throw mud today and then cover it with a tarp and pine straw (I know the curing process is exothermic, so some heat will be release as the mud sets up).... what are the consequences if the concrete is exposed to freezing temperatures?

Thanks
Jaymes
 
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Old 02-12-06, 09:17 AM
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Are you tallikg about actually pouring a concrete slab, and then laying the granite on top, or just tuckpointing a mortar mix between the slabs of granite? Unless there is a concrete slab underneath the granite, mortaring would be a waste of time and money. The ground shift and consequent movement of the granite pieces would cause the mortar to break out in no time.
Regardless of the method you plan on using, consider this: The water in a concrete or mortar mix will freeze just like any other water. The small amount of heat produced by hydration doesn't go far in protecting the concrete in any temperature below about 27 degrees or so, and that's only over the first night. Ambient ground temperature will play a part. If the ground is not frozen, items placed in the ground will not be frozen. If the ground freezes, chances are excellent that the concrete will freeze too. Once the chemical reaction of hydration is stopped by freezing water, it does not start back up again when the concrete thaws. Therefore, the concrete will never reach it's intended strength.

Pecos
 
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Old 02-12-06, 09:33 AM
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tuck pointing

actually, I am "tuckpointing", but this is not a problem. I have graded the ground underneath the granite and packed the granite in with sand. These pieces are also about 2 - 4 inches thick and about 2 X 3 to 3 X 4 feet in size... ie they weigh a lot (my back can attest to that) and are not going to move. I am filling in the voids with concrete to give a "solid" walkway to the front. I did this on a granite patio in the back (laid out in the summer) and have had very little shifting or movement over the past 5 years, so I think it is a fairly solid method. My only concern is the concrete setting up properly, which you have addressed.... thanks.

Since I live in NE GA, the ground here rarely freezes (although recently we have had some frost in the morning). I am thinking that I can prevent freezing by covering with a tarp and then straw. Temperature here is suppose to get into the upper 20's tonight (28 according to the forecast), but I am counting on the ground to stay warm enough to get me through....

I am really pressed for time on this project and need to get it done. Keep your fingers crossed?
 
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Old 02-12-06, 09:54 AM
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Yes, you should be fine with your method in your location. If you lived up North, you'd be amazed at the amount of ground shift due to freezing. It is enough to raise an entire 4' thick concrete driveway an inch or more! The weight of your granite slabs and the mortar would be no match for it.
To correct a point I made earlier, hydration will not stop completely until the temperature reaches as low as about 14 degrees Farenheit, or -10 degrees Celsius. The concrete will continue to hydrate and gain strength, but VERY slowly.

Pecos
 
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Old 02-12-06, 10:19 AM
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can I still work it?

Originally Posted by Pecos
To correct a point I made earlier, hydration will not stop completely until the temperature reaches as low as about 14 degrees Farenheit, or -10 degrees Celsius. The concrete will continue to hydrate and gain strength, but VERY slowly.

Pecos
That's good to hear, because I just went outside and took the tarp off the mud I threw yesterday. It got down into the very upper 20's last night and the concrete is still very green and actually somewhat pliable (gives a little or is "soft" when poking), but at the same time somewhat friable (crumbles when rubbed)..... decided to wait the week and hope this cold snap breaks soon for the remainder..... assuming that what is all ready down will cure slowly over the next week or so, until then I will keep traffic to a minimum.

one question... can I use a damp sponge or cloth and smooth rub some of the seams if the concrete is workable, although it has been almost 24 hours since I put it down?
 
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Old 02-12-06, 11:25 AM
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I'm not much of a mortar guy beacuse I do concrete flatwork and not masonry. However, if you introduce more water to the surface of concrete to aid in finishing, it can have a negative effect on the finished product. It creates a high water to cement ratio which significantly weakens the surface. I'm not sure if it's the same with mortar, but I would assume so. That's why you see the instructions on the bag warning against re-tempering the mix with additional water if it starts to get hard. I would think it would be better to use a steel joint tool of some sort to scrape away excess mortar instead of slicking it down with water, especially in low temps. I'm sure someone else can answer definitively.

Pecos
 
 

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