yesterday i asked about hairline cracks in a small slab pour. you and samt mentioned the bar being to close to the surface and possibly temp causing the cracks. after refinishing the slab it looked good. but if the bar is too close will these crack reappear? i have not poured a lot but some. always used 1 1/2 dobies and #4 bar and never saw this befor. what to expect in the long run? thank you. paul.
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rebar itself does nothing untill a load is applied to the slab. with the bending of the concrete because of loads factor, the rebar will counteract this force. rabar itself needs 1 1/2 time the largest aggregate for clearance. so unless a load is apply the cracking will not appear, and most likely if you overload the slab it still might not appear. A regular house is not enough weight for the calculation of rebar but code required it.. Now there might be shrikage cracks appear which happen due to top surface drying quicker than bottom. This is why you need to keep wet for seven days.
The myth that all concrete crack is 100 % wrong.
thanks so much for your quick response. we kept it wet the rest of friday and then covered it with plastic for the weekend. paul.
"The myth that all concrete crack is 100 % wrong."All concrete contracts as it cures --- an unreinforced concrete slab, cast on a slip-plane, will crack at roughly 10' centers because of contraction.
The rebar controls the cracking but you will still have micro-cracks [less than 0.3mm wide is the general acceptable standard for pre-cast panels] visible on the slab face that is in tension.
For a large slab, pouring in a checker-board pattern with crack-control joints between sections is the best way to go.
IanDG
Edited 6/28/2004 2:29 am ET by IanDG
Paul,
From what you said,
even if you had a true 4" and not 3 1/2" (2x4 forms), you still had only 1 1/2" mud on top of the rebar. For best practice it should have been in the bottom 1/3 of the thickness.
(WAG) Your mix was a little wet to make it easier to work. You were in a hurry and finished before enough water had evaporated out.
Personally I hate to use rebar on a 4" slab because I want 3" of protective mud on all sides of the 'bar. I don't even like WWM on such a thin piece.
I would rather depend on good compaction of the substrate, a dry mix and proper curing to prevent cracks and provide strength.
I think ALL concrete cracks are caused by improper practices, poor design, or catastrophic acts of god. Cracks are not an inherant attribute of the concrete itself.
Water is the enemy of concrete.
SamT
"Personally I hate to use rebar on a 4" slab because I want 3" of protective mud on all sides of the 'bar. I don't even like WWM on such a thin piece."The in-situ concrete topping to the pre-cast floor slabs on this project was 2" thick, reinforced with welded mesh.
Suspended slabs 3" or 4" thick wouldn't be possible if rebar wasn't used.
IanDG
Edited 6/28/2004 12:56 pm ET by IanDG
And your point, re, slab on fill?
SamT
My point re: rebar in slabs thinner than 4" -- not advisable according to your post.
IanDG