Cracked concrete

Gordy

In this particular case if there was not any perimeter expansion, and zero control joints.

You fired up the slab at 40 degrees. The slab is warming. The perimeter walls are not. Slabs expanding as it warms, and the perimeter walls are not. Something has to give.....

nibs

The case for warming a slab slowly.
Hot water in the pipe begins to warm the concrete adjacent to the pipe, the local concrete begins to expand. relative to the cooler more distant concrete. Concrete is a very good conductor of heat so the warmed concrete warms adjacent concrete. If the co-efficient of expansion local to the pipe. exceeds the ability of the concrete to sufficiently warm and expand adjacent concrete. sufficiently quickly, local areas that are expanded will create stress with the areas not yet warmed.
Air entrained concrete has been observed to be less prone to these stresses perhaps because the micro bubbles allow the concrete to expand against the air bubbles.

In hundreds of pours have never observed fibers on the surface of a properly finished slab, Not saying it never happens, it has not been my experience.

hot_rod

nibs said:

The case for warming a slab slowly.
Hot water in the pipe begins to warm the concrete adjacent to the pipe, the local concrete begins to expand. relative to the cooler more distant concrete. Concrete is a very good conductor of heat so the warmed concrete warms adjacent concrete. If the co-efficient of expansion local to the pipe. exceeds the ability of the concrete to sufficiently warm and expand adjacent concrete. sufficiently quickly, local areas that are expanded will create stress with the areas not yet warmed.
Air entrained concrete has been observed to be less prone to these stresses perhaps because the micro bubbles allow the concrete to expand against the air bubbles.

In hundreds of pours have never observed fibers on the surface of a properly finished slab, Not saying it never happens, it has not been my experience.

My memory of slabs with fiber is they look like someone dumped dog hair on the finish My finishers like to use power trowels as it burns (melts) the fibers for a perfect mirror finish. Hard to do that with hand trowels.

My office is a red colored heavy fiber pour that was power troweled, and it is a mirror finish. And it did "road map" crack even poured within ICF walls.

Gordy

crazing is usually due to concrete skin drying out to fast. Maybe poured on a hot, dry,windy day. Also over machining the finish.

keepingyawn

That shouldn't happen if the system was designed and installed properly. However, I've heard of many cases in which polished concrete floors cracked because of the installed floor heating. It happened a few months ago to a friend of mine, but he was lucky to find the guys from concrete Odessa TX in time before the situation got worse. The floor heating piping must be at a specific depth within the concrete in order for everything to work well. The ones who installed your heating system probably put it too close to the floor or didn't use expansion joints.

Jamie Hall

It's worth remembering that there are concrete cracks -- and then there are bigger concrete cracks. All concrete will at least microcrack if it is subject to much tension. Fibers or no fibers. What fibers will do is distribute the stress in the concrete, so that no bigger (easily visible) cracks occur. Mesh reinforcement will do much the same, except the cracks are likely to be big enough to be visible.

The purpose of rebar (not mesh) is to take any tension forces which develop in the slab or beam. In concrete design, the assumption always is that the concrete contributes nothing to resisting any tension forces and converestly, the rebar contributes nothing to resisting compression (that latter is not actually quite correct, but for the sake of reducing potentially messy arithmetic; the contribution varies with the modulus of elasticity, and steel is much more elastic than concrete).

HomerJSmith

It is hard to say how strong the concrete is. One can test the compression strength...but who does.

Curing needs water to reach maximum strength over 30 days. The strongest concrete are the piers under the Golden Gate bridge. The concrete that I have done and I've done some, I have flooded the concrete for at least a week or covered it with plastic sheeting to retain the moisture. Who does that?

mattmia2

Commercially they usually spray on a coating that retains the water for it to cure, but I have covered and sprayed concrete I have poured.

ratio

The last water-cured slab I saw had $14k in plastic over it. Not the thick stuff either.