ACI 360

[Lion06]

ACI 360 and 302 talk about placing bonded topping slabs over existing slabs with control joints that have already activated. Both documents say that you should match the joint locations in the topping slab to the base slab, which makes sense. They also say that additional control joints in the topping slab serve no purpose, because relief between the existing joints can't take place.

I don't follow that last statement, and it actually seems counterituitive to me. I would think that placing a topping slab over a slab that has seen 90% of the shrinkage it will ever see is similar to placing a new slab (in the fact that they won't shrink similarly), and it further makes sense that an epoxy bonding agent (required for the bonded topping slab) makes the shrinkage restraint worse.

Does anyone know of an explanation for this or have any literature on the topic? ACI doesn't say much in 360 and 302 other than the statement above.

 

[JAE]

To get a crack you need two things - restraint...and restraint that gives way to allow the crack.

With dirt/granular subbase you have both. With a bonded overlay the restraint will never give way. Thus the shrinkage cracking, if any, will be expected to be dispursed in more microcracking.