Bonded Overlay

[MacGruber22]

Have existing masonry plank with existing 2" composite concrete topping. There is a crappy asphalt and waterproofing topping above that is going to be removed. We are replacing with a bonded concrete overlay that will end up with an average thickness of about 3.5 inches and a minimum thickness of 1 1/2" (sloped to drains). Some of the existing composite topping will likely need to be replaced due to extensive water intrusion. At the end of the day, the overlay will be protected with a traffic membrane.

At a minimum, I am going to place crack control joints over beams that support the plank (since there is only a minimum amount of continuity top bars in the existing topping), in addition to typical perimeter isolation joints, etc. We are going to install epoxy mesh in the thicker areas of the overlay. The question I have been pondering over is if contraction joints are warranted for early plastic shrinkage. We typically require an aggregate/cement ratio of at least 5 for repairs which in combination with a low w/c ratio is the best way to limit shrinkage without relying on more expensive compensating additives.

Part of me wants to minimize the number of joints cut into the overlay. More joints, means more sealant, means more maintenance, means more chance for places for water ingress. The other part of me thinks that the average thickness may be too much to not provide contraction joints. Thoughts about the substrate bonding/aggregate interlock restraining early plastic shrinkage? As an extreme example, if my overlay was 6 inches thick, I would be confident in saying the substrate is too far away from the overlay surface is help restraint the early shrinkage - thus requiring a grid of contraction joints within 8-hours of pouring.


One in the hand is worth two in the bush.

 

[oldestguy]

What is meant by term "plank"? If these are separate structural units, such as pre-cast concrete "planks" side by side, isn't there some temperature movements of these blanks which would tend to have the overlay crack over every plank joint, following the side-to-side joining of planks? What did the old overlay have for joints? I'd use that as a guide. How are you bonding the overlay to the planks? Is there significant temperature change, day-night, summer - winter that this facility has? I'd look more at long term temperature changes than shrinkage of a low moisture overlay..

 

[MacGruber22]

Masonry plank was a very conventional precast material back in the day. Trade names such as Dox Plank were earlier versions. Structural guys here should be very familiar with them. I hear you on the separation between the planks, but my thought was the existing integral structural topping in combination with the side walls of the structure would provide enough restraint. Also, the masonry plank systems have keyways between each member to provide distribution to adjacent planks under point loads. But, now that I am typing, I believe your concerns regarding differential temperature above and below are good ones considering the lower level is partially below grade, though I was counting on the fact that we will end up with 12+2+3.5 inches of total composite structural depth with new heavy mesh in the top 3.5 inches (0.1 in^2 per foot of mesh). I do agree the differential could be large.

Construction hasn't started so we have no idea the condition of the existing structural composite topping, or if it had joints. Existing structural drawings are comprehensive but do not indicate joints in the structural topping. The asphalt, drainage board, and cold/hot applied waterprooing needs to removed first to observe. I believe there was no reason to put joints in the existing structural topping, because it was going to have the bituminous waterproofing and asphalt on top - what would be the point of joints in the structural topping in that case.

One in the hand is worth two in the bush.