Expert: Bruce E. Johnson - 9/9/2008

Question
Hi,
 Hoping that you are keeping good.I ;m planning to extend  
concrete slab(floor) of first story. The new slab poured
will be a two way slab having dimensions of 18'-3" x 17'-3"
. It will be tied with the old slab reinforcement from 3
sides. what measure shall I take to ensure good bondage of
new slab reinforcement with the old slab reinforcement.
Also what steps to be taken to ensure good bond between the
old and new concrete ??

Answer
Hi Maktzumi, drilling into the existing slab with a 3/4" concrete drill bit at least 6" and inserting a piece of 5/8" rebar one foot long and using epoxy to bond it in place will work fine.  Do this every two feet and then lay a bar of #4 steel across these dowels to help tie it all together when you pour the new slab.  You really don't want to bond the new slab to the old slab, in fact a layer of 1/2" expansion joint material should be used to separate the two slabs so that when the concrete expands and contracts it won't crack.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com