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cracks in concrete SOG

cracks in concrete SOG

cracks in concrete SOG

(OP)
Getting lots of cracks in a SOG. Size of slab was 85' x 65' control joint were sawed in after set same day the joints were 1/4" by 1" deep. and done at the column block outs in 20x20 panels. We are told that that we conrol joint were to far apart and the saw cuts were to small. That why it crack so much. slab was poured on was poured on rock and 8" of stone. Is ther eany other reason why so many cracks. We can not belive it was the control joint placement and size

RE: cracks in concrete SOG

(OP)
the slab did have #4 12" each way about 3"from top this was a 6" slab.
Any other reasons

RE: cracks in concrete SOG

Did your slab have a slip membrane inserted between the underside of the concrete and the stone layer.  If not then you may find that the concrete has bonded to the stone subgrade.  This will restrain the shrinkage in your slab and will therefore render your contraction joints relatively ineffective.

Another thought may be to check your rebar quantities (if you have reinforced the slab)

Regards

Andy Machon

 
 

RE: cracks in concrete SOG

Control saw joint should be 25% depth of the concrete slab.  I would have recommended an expansion joint to have been part of the slab design.  Any opinions to suggest yea or nay for this addition?

RE: cracks in concrete SOG

I agree re the 25% depth of joint.
The need for expansion joint depends more on temperature ranges to which slab will be exposed.  For an indoor or covered slab, an expansion joint is probably not reqd.
Reinforcement quantity should be checked against recommendations from several authorities.
Note also that the slip membrane, which is necessary, will become ineffective if the compacted stone was not levelled with a blinding layer of sand before laying the membrane.  An uneven stone surface will give mechanical keyed interlock through the membrane, and defeat the usefulness of the slip layer.

RE: cracks in concrete SOG

How do you get 20' joint spacings on a 65 x 85 slab?
If reinforcing steel was continuous through joints, they wouldn't work. Joints should have been 1 1/2" deep. 20' was a standard spacing for a long time, but now 30x the thickness for plain slabs has been found to work better. With the heavy reinforcing in this slab, the spacing of the joints could be very wide, but some of the steel should have been cut at the joints. Some people don't like t he drag formula for calculating joint spacings any more, but it has worked. Expansion joints are unnecessary and a maintenance problem. Slip sheets are unneccessary.

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