Pouring concrete in high temperatures

[bones68ss]

We are going to do a pour and the weather forecast shows 110 degrees (F). What would be the best way to water cure the concrete. Flooding the slab, using a spray on water cure, or using a mat? We are worried about losing moisture due to the high temps.

 

[SlideRuleEra]

IMHO, flood the slab and maintain a slow, but steady flow of water for the duration of the cure. This inflow should help to keep otherwise stagnant water from getting too hot.

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[BigH]

Don't forget the preparation before curing (make sure you concrete temperature is low, make sure you have misters/foggers to prevent plastic shrinkage cracking).

 

[Zambo]

BigH is correct, there is a time between initial placement of the concrete and finishing of the complete slab where flooding will not be possible.

I advise shading at the front face of the placement where the concerete is just reaching intitial set (I haven't tried misting before so I don't know if this can be used as a direct substitution). Then there will be a section which has reached initial set and hessian sacking soaked with water can be used. Then on completion move to flooding the slab.

I'm not sure that the water getting hot is a problem. If anything the high temperature of the water will reduce the temperature gradient through the thickness of the concrete.

 

[SlideRuleEra]

bones68ss - Since you did not ask about PLACING concrete in high temperatures, I did not address that issue. However since this is now wisely being discussed, I'll add a few more comments:

1. Consider use of Type IV cement in the concrete mix to minimize the amount of heat produced.

2. When the concrete is batched, use ice (flakes) instead of water. Replacing the water completely (100%) with ice is practical. This will "help" keep the temperature at time of placement below required limit - this may not be enough, the concrete temperature could still be too high.

3. Make detailed arrangements to have concrete delivered on a strict, well planned time schedule. This will allow the concrete placement to proceed without interruption and minimize the chance of cold joints.

4. If these steps are not enough, the concrete placement may have to be made at night (carefully consider the entire process of concrete placement when selecting a start time). Make sure that all support activities, such as necessary lighting, employees' work schedules, etc. are coordinated in advance.

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[concretemasonry]

bones -

Is the problem just high temperatures (arid climate) or a combination of heat and humidity?

How big is the slab?

You perceive your enemy to be the high air temperature, but he real situation is the temperature of the concrete prior to and after placing, and controling the release of the moisture necessary for the proper hydradration of the cement.

If the slab is not too big, protection of the concrete surface from the sun does wonders and a temporary tent cover is really quite economical and effective. Everyone knows what direct sun can do to cause high surface temperatures (race tracks can easily hit 140 degrees with and air temperature of 80 degrees and no heat of hydration generated by the curing cencrete).

If it is a large slab, misting is a good way of offsetting the effects of the sun and reducing the moisture loss, but it can be difficult to control depending on size, wind and weather conditions. Covering with poly is also effective, but can effect the visual appearance.

The first step is using the right mix at the coolest temperature reasonable and place quickly. Unfortunately, cool concrete still has to cure for proper slab finishing. After that, it just is a combination of protective measures that are apprpriate for that size project.

Dick

 

[bones68ss]

Thank you guys all. The info has helped me think. We have an admixture in the mix(Eclipse)which will slow down the hydration process. And the mix design (2500 PSI) is going to be good. Our saw joints are spaced every 15 feet so this should help with cracking. I think we are going to go with what Dick said as far as the tent cover. Putting water on the slab, covering it, and then putting water on top of the mat. We found a cover that they say won't leave effervescence. The slab is about 45,000 SF and we are going to start our pour at 12:30 am and should be poured out by about 6:00 am. What do you guys think? Does that sound like a good plan?

 

[BigH]

Sounds reasonable - especially the time of pour. The saw joints don't really help, though, for plastic shrinage cracking which is caused by evaporation of water faster than hydration. Ensuring the concrete arrives at a proper temperature is required. The readymix supplier should also be misting his aggregate stockpiles to keep the aggregate as cool as possible. SlideRuleEra talked about using Type IV cement - but slower hydration can also be obtained by using a pozzolan (flyash) cement. Your compressive strength requirements are rather low (16MPa or so) - is this a problem with durability in the use of the slab?

 

[Zambo]

Just out of interest what is this? With saw cuts at around 4.5m and a compressive strength of 16MPa sounds like a road-base slab.