huangyhg

no cracks in sog
fellers,
i am designing a containment control system. it is all concrete inside an existing building measuring 8" deep, 70'-8" long x 23'8" wide. the sog is 8" thick and we will place rebar 3" from the bottom. we need the concrete sog not to crack. we will add wwf mesh in the top and possibly also use fibermesh concrete. what spec should i use for shrinkage compensating concrete?
thanks all
t

try system k (
i would suggest rebar within 2" of the top. i would suggest that you calculate the area of steel you need by the subgrade drag equation (found in "designing for floor slabs on grade" and other sources).
you might also consider one or two layers of poly under the slab to reduce the subgrade friction coefficient. do not use any thickened footings as they will add to the drag. make sure the edges are free to shrink and not restrained by adjacent construction.
you might also check the area of steel provided against that recommended in aci 350.
cracking can be minimized, but truly eliminating cracking is elusive. one way to minimize cracking to the point that cracks are usually not visible is to use at least 0.6 percent reinforcing steel, located at the proper depth. this us federal highway administration document outlines the details:
go through aci 223-98 for design. also suggest system k as mentioned above and burlap curing.
a post-tensioned slab on ground would be my choice. sand bed, two layers poly, no thickenings, 3 mpa prestress, tendons at mid-depth, fully bonded by grouting, thorough curing.
all concrete cracks! the best you can do is minimize the size of the cracks with generous rebar, patterned placements and shrinkage compensating cement.
i disagree completely with civilperson's comment. it is true that all concrete shrinks, but restraint is what causes it to crack. a slab cast on a frictionless surface would shrink from all the edges toward the centre, but would not crack.
hokie- as soon as you load that slab, to a point where the tension stress is greater than the modulus of rupture, the concrete will crack...
if you need to absolutely eliminate cracking, i think post tensioning is the way to go
if you just need to eliminate cracking due to shrinkage... why not add pour strips around the perimeter?

and as soon as you load that slab, to a point where the tension stress remains less than the modulus of rupture, the concrete will not crack ...
a. assuming the slab won't crack b/c of temp and shrinkage good luck trying to get that to happen the tension stress less than the mor
b. ... concrete will still be exposed to temp changes
c. ... shrinkage
i think you guys are playing to much of the "well if this happens... and then if this happen... and if that happens... the slab won't crack" game
you can't eliminate cracks in the rc structures only control them... you need to pt if you want to be sure the slab won't crack

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