Hydro demolition of concrete - control

[Edu09]

Hello Everyone,

I have a project where there is a requirement to hydrodem part of a concrete structure to a depth of 4in - section is approx 4ft thick. What I am wondering is if there is a good way to visually see how much has been demolished. I am thinking of something like a marker that could be pushed into the concrete and the contractor could see when the required depth has been reached.

Any thoughts would be appreciated. THanks in advance

 

[Canuck65]

On hydrodemolition projects I have worked on, as soon as the protective cowling surrounding the water jets passes, you can visually see how deep the removals are. Perhaps some slurry and water in the way, but easy enough to put a measuring tape in to check. There is some adjustment by the operators when they first start, but then it should continue reasonably uniformly unless there are significant variations in the concrete quality, and then the operator usually adjusts. As engineers we normally set the requirements (i.e. 4" removal depth), then leave it to the hydrodemo operators.

 

[Edu09]

Thanks @Canuck65. It sounds like they will have good control of the process. I've heard of a project where this was overdone and they had to end up closing a bridge because such a large quantity was removed. This must have been down to a misunderstanding of what should be removed.

 

[BridgeSmith]

I've heard of a project where this was overdone and they had to end up closing a bridge because such a large quantity was removed.

That usually happens when there's a higher strength overlay that hasn't been completely removed. The pressure needed to remove a 1/2" of 7000psi concrete is enough to blow through the entire 8" deck of 3000psi concrete in the same amount of time.

For a 4' thick section, you shouldn't have to worry about it blowing through. If it's a single concrete pour, you can likely expect variations in depth from the 4" minimum to about 8".

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Edu09]

That's interesting. I'm going to keep this in mind about the pressure required for concrete strength. Thanks Bridgesmith!

 

[STrctPono]

Yes, this is really a test and tune type of operation when they start the hydrodemo. They can adjust the pressure and speed based on the strength of the concrete and the depth of removal that they are getting. Of course we have all heard stories of hydrodemo machines cutting through 8" of concrete bridge deck and I attribute this to errors in judgement by the operator. We had a contractor profile the surface of a bridge deck a few years back. Most of it was existing 2500psi concrete and the deck overhangs had recently been poured with 6000 psi concrete. The difference in removal/profiling was considerable. Provided CSP7 profile on the old concrete but didn't even phase the newer concrete.

 

[Edu09]

Thanks STrctPono. This is how they are suggesting controlling this, expertise of the nozzleman.