Why does nobody talk about the new quick-set mixes in hot weather?

I poured a small patio slab in my own backyard here in Phoenix about six weeks ago, using a bag of that fast-track stuff because I wanted to finish before noon. The before and after difference is wild. It looked perfect when I troweled it off, super smooth and tight. But by the next morning, I had a spiderweb of hairline cracks all through it. I'm talking hundreds of them. The timeframe was maybe 8 hours from finish to full cure in 95 degree heat. I think the cause was the mix setting up way too fast with the sun beating down, and I didn't keep it wet long enough. It just sucked all the moisture out before it could cure right. Has anyone else run into this with the summer heat hitting now? What's your trick to slow it down?

4 comments

4 Comments

ross.kim1mo ago

Holy crap, hundreds of hairline cracks overnight? That's brutal. I used a fast set for a mailbox post base last summer and it went off in like 20 minutes flat. You gotta soak the area first with ice water and then cover it with a wet tarp immediately, or it just turns to dust.

john5061mo ago

Fast set concrete is a nightmare for anything bigger than a post base. That rapid heat buildup is what causes all those cracks. A wet tarp helps a little, but it can't stop the internal stress. You really need a proper mix with additives to control the cure, or just use regular concrete and wait. Rushing it always leads to problems down the road.

cora8131mo ago

Disagree, used fast set for a big slab last month and it's holding up perfectly fine.

jenny4227d ago

John's right about the internal stress, that's the real killer. The heat builds up way too fast in a big pour and the concrete just can't handle it, it wants to shrink and pull apart. Even a wet tarp only cools the surface, the middle is still cooking itself. You'd need special mix designs with slow acting stuff to manage the heat, which most DIY bags just don't have. It's a recipe for trouble if you're not super careful.