I am planning to build a 24 X 24 garage. I am placing it at the same place as my old, smaller single car garage. I removed the old garage, broke up the old slab into 10 inch chunks. Can I pour the new slab over the old? do you recommend using 2" styrofoam?What about vapour barrier? located in Edmonton, Alberta.

ballengerb1

Do not pour on top of broken concrete. A newly leveled gravel base must be used for a garage slab. Are you heating this garage and thats why you are concerned about a foam /barrier?

smearcase

I am an old roadbuilder and the only method I know of similar to what you are talking about is called crack and seat. A drop hammer is used to crack the existing concrete and then it is rolled with a 35 ton roller to compress the cracked sections. It is not broken into small pieces like you have. Those 10 inch pieces create a fulcrum like breaking a stick over your knee (dont do that-I have knee trouble).
Maybe if you can get the pieces close to level, then set rebar (#6 bars) about two inches above the bottom of the new slab (1 foot apart both ways) the slab might bridge over the many voids in the broken concrete. So all that we have achieved so far is that we need expensive steel so we can pour over broken concrete?
Just pouring over the broken concrete might work for a while but I think you will eventually get serious cracks, many of them structural. Shocks from blasting in the area or even earthquake-type tremors do occur, which can shift the concrete pieces creating a void under the concrete, then the load of a vehicle damaging the concrete slab.

westnlas

I think the only way that you might be able to pour over those old chunks would be to bury them so deep that it wouldn't be worth the effort. Even larger rocks a few inches below grade will cause the slab to crack. Since concrete is so very expensive and you expect to use it for many years, Get rid of the debris, compact a proper base and do it right. That way, it's once in a life time job.

ablanch

Remove all broken concrete to bare earth. Remove any foreign material from slab area, (roots, large chunks of concrete, etc,) , and provide a minimum 4" thick base course consisting of clean graded sand, gravel, crushed stone or crushed blast-furnace slag passing a 2" sieve over the compacted earth sub grade. A base course is not required when the concrete slab is installed on well-drained or sand-gravel mixture soils classified as Group I according to the United Soil Classified System.

Unless the garage is to be used as a habitable space, no plastic is required. If garage is to be heated, then perimeter insulation should be used. Slabs-on-ground with turned down footings shall have a minimum of one #4 bar at the bottom of the footing. For slabs-on-ground cast monolithically with a footing, one #5 bar or 2 #4 bars shall be located in the middle third of the footing.

A 4" min. 3,500fc, air entrained not less than 5% or more than 7%, concrete slab should be used, reinforcing of slab is optional, assuming cars or light trucks are to be stored inside. Slope garage slab to entrance.

ablanch

Also, I recommend that you provide control joints in the slab at 100 sf intervals (10'x10")to a depth of 1/4 of the slab thickness to control any slab shrinkage cracks.

westnlas

Shouldn't the control joints be about 3/4" instead of 1/4" to insure the cracks follow the control joints ? I have used a nylon line and buried it in the fresh concrete about an inch with the tamp and bull float. During finish, Pull the rope out and run an edger over the line left in the slab. I've not had a crack that wasn't in the control joint by using this method.
Good luck with it,

ballengerb1

He did not say 1/4", he said one fourth the thickness of the slab. In most cases that would be about one inch. You just though you had a " after the 1/4, its not there.

westnlas

I stand corrected ! Thank you for pointing me in the right direction.

ablanch

Click on my Moniker "ablanch" I will help anyone that has questions about Building, Electrical, Plumbing, Mechanical or Fire (including fire sprinklers} and some Erosion Control methods.