Bonded structural topping
Bonded structural topping
(OP)
Although I have on a number of occasions used structurally bonded concrete toppings to old concrete slabs, and got good bond strengths when done by shotblasting the old concrete surface and using a proprietary bonding agent (most recent project: 1.4 MPa to >2 MPa with a coefficient of variation of <10%) when tested two weeks after placing), I am wondering why the topping does not debond when the topping concrete shrinks. Wouldn't the shrinkage shortening of the topping create an interface shear stress between the topping and the base concrete that would shear the topping off along the interface plane?






RE: Bonded structural topping
Absolutely. It seems to me that we must implicitly be relying on the topping to crack due to shrinkage stress prior to the horizontal shear bond failing. That said, I can't recall any code guidance or research on the subject.
One of the things that we have going for us is that the area of cracking topping cross section is relatively small relative to the area of the horizontal shear interface. If toppings were 6' thick instead of 2" thick, we might need to rethink our assumptions. I wonder about it when thick raft slabs are cast in multiple lifts which is, in part, why I often use vertical shear friction bars in that situation.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Bonded structural topping
All our raft slabs are cast in one go, to the best of my knowledge. I thought that was standard practice, but maybe not.