We did a slab last fall that has an extreme amount of surface cracking on about 3-foot centers.
The slab is six inches thick, over a vapor barrier, then over 4-inches of bule foam, on a 6-inch gravel base. The concrete is 4000-psi, design with water reducer added, and 25% of the cement replaced with fly ash. The water to cement ratio was about 0.35. The 28-day breaks came in between 5500, and 6000-psi.
It was around 35 to 40-degrees, with the wind blowing about 10 to fifteen with occaisional gusts to 35.
The sub got on it with a power trowel as soon as he could and worked it well, and power cut the shrinkage joints, before covering it with a heated glycol loop from a ground heater, and two layers of blankets for three days. The pour was on Thursday, the heat came off on Monday, and the blankets a week later.
The entire slab exhibits a lot of shrinkage cracks.
My thought is that the foam held the heat in the bottom of the slab, and working it with the power trowel burnished and cooled the top of the slab. Then when the bottom of the slab cooled off, and shrank, it cracked the surface.
The concrete sub thinks that is was because the foam and vapor barrier trapped water and that they are drying cracks.
Anybody want to weigh in with an opinion?
Replies
Concrete cracks.
How big is this slab overall, and how far apart were the expansion cuts (and how deep)?
No expert here
Most of that sounds good stuff
But My thought is that the surfaced cooled and evaporated too much faster than the bttom. Instinct - no reasoning to back it up