Concrete Demolition and Surface Preparation

[Everynameistaken]

Hello,

We are looking at two items, concrete demolition in which we need to remove several inches and expose the existing reinforcing and secondly to prepare an existing concrete surface to receive new concrete.

We have a tech spec that allows the concrete to be removed via chipping or light jack hammer but we want to make sure we are able to reduce the incidents of microcracking/ bruising. This can be done by coming back and blasting with high pressure water jet. What I am trying to figure out is at what pressure of water will we be removing the surface with any micro cracks? Is it a commercially available pressure washer or does it need to be a hydro demo tool?

Secondly, we need to prepare some old concrete to receive a new concrete structural layer above. We have asked for "intentionally roughened surface to 1/4"" which is pretty standard and easy to achieve with new concrete by greencutting. On old concrete the contractor is looking into different means, one is hydrodemo which is perfect, but maybe cost prohibitive. The other two are mechanical methods, either bush hammering or mechanical scarifiers or scabbers. My concern here again is bruised concrete or micro cracks, and the need to come back over with high pressure water or very high pressure like a hydro deom tool.

Any thoughts would be appreciated,

 

[BridgeSmith]

You can't use a pressure washer for hydro-demolition, the pressures required are much too high.

The only time we use mechanical scarification before hydrodemolition is when we have a high strength concrete overlay. The pressure required to take off an inch of 7ksi silica fume concrete will blow through the other 7 inches of regular concrete in no time.

As far as cracking from chipping or scarifying, it's a function of the thickness of the remaining concrete. For guidance I'll tell you what we do for bridge decks. On our 7"-8" bridge decks, we allow them to chip away concrete up to 1/2 the thickness. If they have to go deeper, they have to go the full depth. We typically have #5 bars at 6" to 8" in the bottom of the slab. I'd be very concerned about doing anything that causes impact beyond reinforcement.

 

[Canuck65]

If you remove the concrete with hydrodemolition, then you are correct that the surface will be prepared and ready to go after a wash off of any remaining slurry and drying to SSD.

If you jackhammer, and limit the size of jackhammer to 15lbs, the contractor should be able to prepare the surface in a suitable manner. The last inch or so should be removed with the small hammers. The contractor won't like that, but that's what they are getting paid to do. If an inspector goes around and identifies any loose material on the surface (there is always some), it can be marked for the contractor to go back through with a 15lb'er. The surface should have a good profile. Sandblasting is a great way to clean the surface, however, water blasting could also be used. Hydrodemolition is usually in the order of 18,000 psi, I would imagine something lower, in the 10 - 12,000 psi range would be suitable, however, I have never done it this way. We always get sandblasting in these types of cases. Bush hammering and scarifying is not required if several inches have been removed by jackhammer or hydro. Normally jackhammering and hydrodemolition are not mixed, except on a hydro job where you can't get access up tight to walls or columns and the last bits are jackhammered out.

The IRCI used to have some great background info on this topic.

 

[Everynameistaken]

Hi Guys,

Thanks for the comments.

We understand that typically you would not mix hydro demo and mechanical means. If hydro demo was used then it would work much easier to do that for everything.

I do still have a question about what to do about the potential for remaining microcracks/bruising. I note that it has been suggested to use small chipping hammers to try and reduce the effect, however if this is a large area then this maybe a problem, and currently we have not specified that they have to use small hammers so that maybe a problem. We can tell them that they much remove any non-sound concrete so that could mean coming back with a wire brush, commercial pressure washer or smaller hammer after a larger one used to get the profile, but also means lots more work.

In an ideal situation (fingers crossed) they will elect to use hydrodemo for both cases noted above, but we do not know this. It maybe that a bush hammer is used and then it would again be great (fingers still crossed) if we could have them use a commercial pressure washer say (5000-6000 psi) to go over the mechanically prepared surface to remove any non-sound concrete but leave the sound stuff. Any idea of what pressure would be required to do this?

 

[BridgeSmith]

"I do still have a question about what to do about the potential for remaining microcracks/bruising."

The potential for that to happen is a function of the thickness of the remaining concrete and the impact loading applied to it. As I said, we generally want at least 3.5" of remaining (reinforced) concrete, and if it will be less than that, we make them remove the full depth. Ours are, of course, suspended structural slabs spanning up to 10' between girders. We limit the size of the chipping hammers to the 15lb class for removal below the top mat of reinforcing.

"...if we could have them use a commercial pressure washer say (5000-6000 psi) to go over the mechanically prepared surface to remove any non-sound concrete but leave the sound stuff."

That is unlikely to remove unsound concrete, possibly loose concrete, but hydrodemolition equipment operates at pressures twice that and more.