Expert: Bill-C - 10/27/2007

Question
I am looking to lay a slab foundation for a timber framed garage, it is to be located on the drive which has different levels of concrete slab all a bit tatty.
Question is can i lay the new slab directly on top of the existing slab if so how do i key it and should i use mesh reinforcement.
Many Thanks

Answer
Hi Ian,

I presume with you saying different levels you mean uneven?
I f there is no visible cracking or movement in the existing slab you can lay directly onto with the following preperation,You can scabble the existing slab with a hammer drill to rough it up so to speak or use a propriety bonding agent like uni bond or equvilant.
Remove all laitance from it and erect your formwork /shutters around perimeter. Lay a minimum of 100mm concrete c35 or c30 , and install A242 reinforcement mesh fabric. The fabric can be placed prior to the pour on deckchair stools or engineering brick .you want to achieve 40mm 50mm cover over the fabric or you can lay it onto the concrete during the pour and press into it and ensuring it gets laps of at least 200mm.
I would advise to cut in induced joints with a stihl saw approx 30mm DEEP every 6m across its breadth to prevent cracking. This depends on the slab dimensions.
Hope this helps.

Regards
Bill