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Old 07-22-2007, 07:36 AM
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Originally Posted by Bryan59EC
It will break off in chunks or sheets, if cured too rapidly.

How can you tell this will happen? what would be a visual, or this is what is and a always will thing
There are a few things that will cause concrete to crack, break off in chunks, or sheets.
Compaction is critical, nothing I know of compacts better than sand and water. Walk over a beach after the tide goes out, on the wet sand, it's pretty tight. Walk over where someone has driven a vehicle, it's very tight. Same principle for compaction in concrete.
The concrete itself is important. A load that has too much water added will lose strength, this can result in the 'sheets' or finish breaking down. A dry load can crumble after time. Not enough concrete in the mix will also result in crumbling, a mix your own load can do this easily if they think they are saving a bit by cutting back on the concrete as they mix.
The finish? It's easier to finish if it has more water added, too much and you're screwed. Tamping pushes the rock down, making it stronger where it is needed, throughout the concrete. The bowlfloat or bullfloat, as it is called, brings the 'butter' to the surface, the butter being the concrete-sand, this is where too much water or not enough water comes into play. The right combination makes a stronger finish. I like to use 3/8 rock in the mix for light applications, sidewalks, mowing curb, stuff like that. If it has to take quite a bit of weight, use 1 inch rock, it's stronger. 3/8 is a bit easier to tamp and finish.
Also, some finishers will add calcium chloride (CC) to speed curing time, especially if rain is a concern or in damp conditions like fog. I don't like it, have seen most of these crack, some will crack in just a few days. A 'hot' or 'green' load can take off on you also, turn your head for a few minutes in the right conditions and it has almost completely cured before you can finishwork it. A real bummer.
Expansion joints, around here every 10 feet, there's a mess of ways to do those, strips, cuts, etc.
The final finish? Do you want glossy smooth, may be slippery but will be easier to clean. Burn it in with the trowel like pigjam did. Troweling in itself is an art, the wetter the concrete, the flatter you want to keep your trowel to the surface to keep it flat. As it cures, you can use more of the trowel edge to "burn' the surface, you won't be rippling the surface as you trowel. A sidewalk or driveway finish, trowel till it takes quite a bit of pressure to leave a fingerprint, broom finish with a fine bristled push broom, keeping it fairly wet. Pull the broom, don't push. If it's a warm day, mist with water for a day or 2, every 3-4 hours, depending on how hot it may be. Cold? cover with plastic and straw. Hope this helps. Dan
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