New Concrete On Top of Old

Canarywood1

Any construction adhesive will work,and it's an easy way to anchor them.

kwikfishron

If you're going to pour a slab you should bust out and remove about a third to a half of what you have now on the high side. As mentioned 7 1/2 to zero is a bad plan.

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St. A

I have decided to go with the sleeper method, at least on the interior project. My intention was to do concrete, vapor barrier barrier, sleeper, ply sub-floor. A construction adhesive would just adhere my sleeper to the vapor barrier and not to the concrete. Would you put the vapor barrier elsewhere?

Canarywood1

In that case a ramset would work,or maybe tapcons.

Guywithskills

I have done lots of pours over old concrete for a variety of reasons. Tried lots of "quick solutions." My experience says, if you pour over old, the cracks and joints will mirror up no matter what you do. That is guaranteed. You will not bond to it. It is all we could do to bond to Cool Decking around a pool that was already rough, TSP washed and acid washed.
If the old slab is without cracks and decent shape, pour the new one, match the expansion joints up exactly by marking so they can be seen to follow after the new poor place 15lb roofing felt over old slab. If old slab has cracks, we bust it up with an 8lb sledge hammer into 8" pieces approximately, actually not that hard. Takes an hour for a 10' x 20' slab. If it is especially hard, use a pry bar and just leverage the slab up a couple of inches and start at the far end and break your way to the lever. This works very well and Ive seen lots of guys in the trade use this. We were not the only ones. Dont pull it up, you are just breaking it up and leaving in place. Then lay 15lb felt over and pour your new slab. You do not want it binding to the old and you can put your expansion/cut joints where you please. This will give a good result. In the early days of my concrete life in the 70s, we tried probably all the short cuts. The real labor is in breaking up the old. Like I said, maybe an hour of sledge hammer. If out of shape and fortunate enough to not have to do such hard labor, bring a buddy and trade off. Charge him $20 for the work out...

itsreallyconc

interesting - yes, crks/jnts do reflect - that's why there are specific respective methods, tools, & mtls to deal w/'em,,, we routinely overlayed conc & never had any debonding issues - i think the success is due to 3 parts: 1, choosing the right mtls; 2, proper repairs & surface prep; & 3, having artisans w/necessary skill.

kool-dek type mtls require more prep than tsp ( mild acid ) & ' acid washed ' it also helps knowing different types of overlays ( bonded, unbonded, thin, decorative ), & proper mtls avail to do the work - usually NOT found at apron/vest store

' lots of guys in the trade ' isn't a ringing endorsement to me unless the trade is ' hacking ' there are specific guidelines for when / where to place contraction jnts,,, expansion joints aren't rqd in a slab - perhaps an isolation jnt

another point - we still own 8, 12, & 16# sledge hammers but they rarely see any use - for $60, rent a bosch brute electric jackhammer if you want a workout, go to a gym

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if you hear it from a guy in the apron store, be VERY suspicious the mtl/method will work,,, when it time to build something together, they won't answer phones NOR help
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