reduction of contraction stress due to friction of slab on g

huangyhg

reduction of contraction stress due to friction of slab on g
hi
i suppose to overlay a new concrete slab with 4" thick (each 12' x 20' panel) on the exisitng cracked one with 6"-8" thick. since the existing concrete slab provides very rare contraction joint,my question is how can i reduce the risk of differential contraction and restraint force due to friction against the exsiting slab when i provide more joint in the new slab. will plastic membrane work?
thanks
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there are three ways you can handle this....
1-you can cut joints in the existing slab where they should have been to begin with, then epoxy the existing random cracks, and overly the slab with corresponding joints in the overlay.  this will reduce the reflective joint cracking in the new slab.
2- you can use an isolation membrane over the old slab, assuming you have no faulting at the existing cracks.  the isolation membrane should be plastic sheeting, bonded to the old slab (glue it down so that you don't have wrinkles), then place the new overlay slab on top with proper jointing.
3- you can "crack and seat" the existing slab by breaking it up in place, compacting the pieces with a heavy vibratory compactor, then place the new slab above with proper jointing, but make sure the sawcut joint depth is deeper than typical to cause early cracking and reduce the potential of random cracking.
as noted in all three, proper control jointing is essential.
hi ron
thank for your suggestions. for me, i prefer the second one; however, it makes me some wonder if the non- lubricated sheeting will work since there is some aggregate locking between those two rough surface. unfortunately, it is diffcult to core the existing slab to know its actual thickness. have you face any problem in this case when the non-lubricated plastic sheeting doesn't work?
dollarbulldog...the plastic sheeting will work, even non-lubricated.  the coefficient of friction between the two is low.  if you want to increase your potential for slippage, don't glue the one-side down....use mineral oil on the old slab, squeegee the wrinkles out of the plastic, then place the new concrete.  this will give you an almost frictionless interface assuming the old slab is relatively smooth.