Expert: Bill-C - 4/1/2008

Question
I HAVE A NUMBER OF CONCRETE SECTIONS AT OUR HOA THAT TEND TO RAISE AS MUCH AS 3 INCHES WHEN THE TEMPERATURE IS BELOW FREEZING.  I AM LOOKING TO HAVE THESE SECTIONS REMOVED AND TO POUR A NEW 4 INCH SLAB.  I AM LOOKING TO REMOVE 6 TO 8 INCHES OF THE CURRENT SUB BASE MATERIAL (MOSTLY CLAY I THINK) AND PLACE THIS DEPTH OF STONE FOR A NEW SUB BASE.  IS THIS SUFFICIENT TO SOLVE THE LIFTING PROBLEM AND WHAT SIZE STONE IS BEST FOR THIS APPLICATION.

THANKS FOR YOUR ADVICE     RON

Answer
Ron,


Carefully remove the clay subsoil and try not to disturb areas where your sections of slab are to remain , best to excavate the perimeter at a 45 degree splay just beyond the area of repair.
You dont indicate any damproof membrane or reinforcement or expansion movement joints in your question.

Normal preparation as follows working down from concrete finish slab.
100mm concrete slab, A252 mesh fabric with 40mm top cover acts as anti crack reinforcement, laid on visqueen 1200 guage plastic dpm on 20mm sand blinder on 250-300mm type one inch hardcore.
Ensure the hardcore is compacted in layers no greater than 200mm in thickness.
Normal concrete sets /slabs etc should have movement joints every 6m approx to allow expansion and contraction with temprature.
An induced stihl saw cut to a depth of 30mm will also help and induce any natural cracking drying out to be directed to that particular point.

Movement joints shall have 12-15mm expansion foam full depth of slab or no less than 150 if slab is greater and ensure reinforcement is set back at a minimum of 75mm from the joint.

Hope this helps , if you require further information please feel free to write.

Regards
Bill