concrete delamination detection

[ajk1]

Sounding the top of an existing waffle slab (2½" slab with 5"x7½" ribs at 24" centres) gives various sounds that have been interpreted by one person as delamination and by another as just the type of sound that you get when sounding such a thin slab. Would ground penetrating radar survey provide a more reliable answer as to whether there is delamination and its location and extent?

 

[Ron]

ajk1...if the delamination is pronounced enough, such that an air gap exists, GPR might pick it up, but "iffy".

We use a device called a Delam 2000....a simple pair of sprockets on an axle that gives a distinct sound for delamination. If you hear the same sound over the entire surface, there is less of a likelihood of delamination. If the sound changes with location, investigate destructively by drilling small holes.

Here's a link to the device: Delam 2000

I have the optional wireless microphone, but find it more annoying than useful.

 

[Canuck67]

ajk1,

Not sure about GPR, but I will add this comment. I have sounded many square feet of waffle slab. An experienced person will have no doubt as to whether it is a thin slab or delamination. Yes, the sound is different between the rib and the pan, however, a delamination is yet again different. This is similar to situations I have been brought into where someone else has identified unusually large swaths of delamination, when in fact it was clearly nothing but a debonded topping. In most cases, a debonded topping sounds different than a concrete delamination.

What would be the cause of the concrete delaminations in the thin slab if there is no mesh? (I may be mistaken and perhaps the waffle slab you are looking at has mesh.

 

[ajk1]

To Ron: ok, that is interesting.I will look into it.
Drilling small holes to check sounds like an interesting idea, but how would the delamination be detected by that?

To Canuck67: the topping has all been removed so debonded topping is not what they (the contractor) claimed that he heard when sounding after it was removed. There is no steel in the 2.5" slab, so I asked them the same question as you just asked: what would be the cause of the delamination?

 

[Ron]

ajk1....assuming the delamination is somewhat above the bottom of the slab, if you drill into but not through the slab, you'll likely intersect the delamination. If you pour water in the hole and get a rapid loss of water, you've found a delamination. Make sure no one is standing around the hole when you do this....you don't want the delamination compressed.

You can also seal a clear tube above the hole and pour the water into the hole and up the tube. It allows you to better see the water column fall, if it does.

Not foolproof, but can work.

As for the GPR, we have tried to build numerous calibration sections to check such things. We have built a wall behind our office with a variety of different conditions of lack of continuity of sections, various rebar locations, voids, etc. Our GPR picks up lots of things, but unless there is a clear air gap, it won't show.

 

[Canuck67]

ajk1, Without steel in the thin slab, there can't be a delamination. However, there can be a delamination related to corrosion of the top steel in the ribs that affects the adjacent thin slab. The delaminations over the ribs should be quite pronounced.

Perhaps for some training, identify location of all ribs in one bay and chalk line them. Try and choose a bay that also has obvious delamination. This should allow a few people to get together to determine what all the different sounds relate to, with a senior, experienced person overseeing. It can be difficult. Hammer may be better than a chain, however, hammer will be tedious and lengthy which in itself will reduce the accuracy of the results given the amount of square footage you likely have to sound.

 

[Ingenuity]

I have GPR and the Delam 2000 (and ultrasonics too) and the Delam 2000 is my go-to for fast and accurate delam surveys. My Delam 2000 was purchased many many years ago when it first came out. Let's just say it is the non-digital, non-wireless version. Two Harley-Davidson sprockets on a axle with a handle.

 

[ajk1]

To Ron - the water ideas sounds interesting. What diameter hole do you drill, and how do you feed water into it? With a syringe?

To Canuck67 - yes our people told me earlier in the week that hammer tapping the thin slab was better than chain dragging it.

To Ingenuity - yes I knew that attempts were made about 20 years ago to automate the means of chain dragging, but I had thought I had read that in the end they were no better than a chain drag. But you seem to have found differently so that is interesting. I suppose that the system that you described saves the fatigue of swinging a chain around, but is it more reliable at finding hollow sounding areas than a chain drag?

 

[Ron]

ajk1...anthing above 1/2 inch for the hole. Clean the drill cuttings out as you would for setting an epoxy anchor. A piece of 1/2" to 1" clear, rigid tubing can be sealed at the top of the hole with plumbers putty to allow a taller water column...easier to see.

Also, the Delam 2000 closely mimmicks a hammer tap moreso than it does a chain drag. We use ours mostly for detecting delaminations in direct applied stucco.

 

[ajk1]

Thanks Ron. I will try to convince our people to give it a try. Sounds like a bit of fun to do. Much appreciated.