I've been poring (pun intended) over a ton of threads about the million different ways to aid the curing process. Sprinklers, flooding, plastic mat, burlap mats, liquid curing aids, etc, etc.
My pad just finished getting prepped today and concrete is scheduled for next thursday. I was hoping it would cool off, but this is Texas, no such luck. 100+ with full sun and no rain for the forseeable future. My concrete guy says they can spray a curing sealer on it, no charge. But I'm afraid that it's just going to get fried with the super high temps and direct sunlight.
I was thinking of putting up some posts and having a tarp suspended above the slab with sprinklers going on it to keep the area moist and shielded from the sun, but I'm not sure if that's a great idea either.
Either way, Im spending a ton of money on this project and I realize you get one shot at the concrete, so I want to make sure I do it right.
I've been a commercial concrete contractor for 10 years, we do a ton of flatwork year round in all conditions. Hot weather is in some ways easier to deal with than cold weather.
Basic guidelines:
1. Most specs limit as-delivered fresh concrete temps to 90 or 95 degrees. This almost always requires using chilled water in the mix, which is an added cost of $3-5 per CY. Any batch plant without chillers doesn't get our business, and not having chillers in TX would be suicide. You can replace part of the mix water with ice as well, that's usually $10-15 per CY depending on the ratio. One trick we sometimes use is to withold 2 gallons of mix water per CY at the plant, and have the trucks fill their tanks with cold water, and then add it back and re-slump the mix on site just before the pour. This is different from retempering which is adding water to an old/tight mix to loosen it up, which is NOT recommended.
2. Heat of hydration is reduced by using a cement substitute such as Flyash or slag, which slows the initial set and therefore the heat generated. Most specs limit cement substitutes to 25% of the cementitious content by weight. This is highly recommended for summer pours without shade, as an all Portland mix will take off quickly and might get away from the finishers. Set-retarding admixtures can help buy some time up front to get it down and start working it, but when they wear off you better be ready for it.
3. Curing by spray applied curing compound is cheap and easy, which is why all contractors prefer this. Don't do it. Wet cure all slabs whenever possible, especially in high heat and direct sun, for the best chance at reducing random cracking to a minimum. For a trowel finished slab with a good vapor barrier (!!!) underneath it the usual 5-7 days can be reduced to 3 so long as the slab stays wet. Flood/sprinkle, or use a moisture retaining cover like hydrasorb or burlene. You can also use cheap 6 mil poly, but you'll have to keep pulling it up to rewet the slab and put it back down with something to ballast it.
4. Plastic shrinkage cracking is a concern during and at the tail end of the finishing operation. This can be partly due to high evaporation rates, so the use of an evaporation reducer like Confilm is recommended during concrete finishing. This is different from a curing compound and is only meant to reduce evaporation at the surface between finishing operations. The use of synthetic microfibers in the mix can also help reduce plastic shrinkage cracking, without adding much cost or difficulty in finishing.
5. Pour EARLY. Ideally all of the concrete should be down before the sun comes up. This sometimes means paying an early opening fee to the batch plant, but that's usually negotiable. We start most of our summer slab pours at 3AM or earlier. Once the sun hits the slab, it'll take off and playing catch up is a losing battle. Finish crews don't mind the early pours, keeps them out of the worst heat of the day.
Concrete is complicated, I still learn something new with every job. ACI writes the concrete Bible that every spec references, linked below is my copy of ACI 305 which has all the hot weather concreting info you could want. Go ahead and download and save this, I'm not going to leave it up for long. Let me know if you have any questions.
https://www.dropbox.com/s/wvnemriuxu7yj8z/ACI 305R-10 Guide to Hot Weather Concreting.pdf?dl=0
