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Diamond Grinding Concrete Pavement

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bridgebuster

bridgebuster

Active member
Jun 27, 1999
3,969
I’m reviewing a procedure for repairing a bridge deck overly, 2” thick concrete. Actually, the repair involves replacing the entire overlay, which was recently installed. Here’s what happened, among other things, the contractor didn’t turf drag the bridge, and none of the inspectors seemed to mind. Subsequently, the overlay cracked and debonded.

I recommended that the contractor roughen the bridge surface approximately ¼” . He’s p---ing & moaning that there’s no machine that can do this – I know there are walk behind machines for floors, but it’s slow work, and small milling machines, but these are sometimes hard to find. He wants to bring in a huge machine, something on the order of a Roadtec 900. I think this would do more harm than good.

I think diamond grinding would be the best way to go; just enough to roughen the surface to get some bond. Any thoughts?
Thanks
 
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We commonly apply 2 inches of grout to clarifiers for leveling purposes. When we have problems with delaminating, it's usually a result of trying too hard (doing too much to the surface). A old timer here said that grinding, sand blasting and almost any operation that tries to roughen the surface does just the opposite, especially on a microscopic level. I don't think that diamond grinding is the way to go. It might look more rough, but as far as the concrete molecules know (and they know), it's slick as snot.
I'd hold them to using a walk behind or larger scabbler. Possibly they can do a test area if the bridge is big enough.
 
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Thanks Jed. Interesting observation about the roughness caused by diamond grinding.
 
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2 inch may be a bit thin for concrete overlay, but 1/4 inch removal is recommended. The method of removal should not create micro-fractures, not sure if diamond grinding will do that.

I have used a layer of bedding mortar, nominal 1/4" thick (cement, sand, water, and a water reducing, set-retarding admixture, 7 - 9 inch slump) for bonding roller compacted concrete layers to prevent de-bonding. not on a bridge deck, but on a spillway apron.

 
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A few thoughts:

2" structural topping is common in building work. Not sure about bridges, but it does seem a bit thin for existing bridges, considering construction timing and dynamic loading.

My experience is that the most important thing in achieving bond is not roughness, but cleanliness. Normally, this can be achieved with a light shotblast to remove the laitance. Rigorous efforts to achieve roughness do lead to microcracking in the substrate. But if there are contaminants present like you would find in bridge decks, achieving cleanliness might be problematic.

Any chance they used a bonding agent? They are notorious for acting as debonders if the timing is not perfect.
 
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BB...I think the diamond grinding will not produce the profile you want. Grit blasting will. As hokie66 noted, cleanliness is important. Have them pressure wash after grit blasting. I would lean toward removing the 2" overlay...too thin to top again.
 
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Thanks for the responses and information everyone.

Perhaps my OP wasn't clear: This is a new bridge; it hasn't been opened to traffic. At first, the overlay cracked; then it was noted that the overlay was "peeling off". It turns out the contractor 1)didn't turf drag or roughen the deck surface; 2) didn't pre-wet the deck before placing the overlay; 3)didn't properly cure the concrete.

cvg - good information about overlay curling.
 
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An overlay like that will always crack, as the restraint will overcome the tensile strength of concrete. But delamination need not follow, and that is where good construction practice comes in.
 
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hokie66,

The 2" thickness is a DOT standard for a reconstructed bridge deck; it's meant to be a bonded overlay. Normal DOT practice is to build in a monolithic overlay. This isn't a DOT project but it is my understanding the owner wanted concrete.

 
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I was informed today that the contractor milled the deck today with a big @$$ machine. He said there is no machine anywhere capable of milling/grinding 1/4 inch. It doesn't matter to me; however, in the process they ripped out/ cut up about 15% of the top mat.

My boss is in a panic "what should we do?" I said tell them to hire a engineer to figure out a repair. Hit rebar once, Understandable; hit rebar all day long by doing the same thing, can't fix stupid.
 
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did you forget to hold that partnering meeting at the project kickoff?
 
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Beats me; I wasn't involved in the project, until now. I typically get involved in these things when a problem comes up. I think the owner should have hired a CM who pays attention to things.
 
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