GPR vs Ultrasonic

[swearingen]

We have a case where some new, large columns were constructed of CIP concrete and there are some surface voids after the form work was removed. The client is concerned that there may be voids in the center of the column as well. We don't believe this to be the case, despite having ties passing through the column, however the client still wants to check.

The client would like to use ground penetrating radar equipment, while we advocate using ultrasonic scanners. Do any of you have any comments on the advantages and disadvantages of these two technologies to be used in this case?

Thanks in advance...


-5^2 = -25 ;-)

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[bridgebuster]

Maybe Six of one, half a dozen of the other.

We've used GPR on vertical surfaces of some concrete rigid frame bridges to search for voids. It's a simple process. The technician uses a probe that looks like a paint roller. If the rebar is closely spaced it sometimes creates problems picking up voids behind the bars. I's relatively quick but has depth limitations, about 18"

I also had another project - retaining wall replacement - that used ultrasound (impact-echo) to determine the thickness of the stem (no as-built drawings available). It's not as fast as GPR. It seemed to work; we got a good image of the wall. I say seemed because this part of the project was done by a subconsultant and he hired the ultrasound service. I never witnessed anything; it was about 8 -9 years ago and I don't have the project files in the office anymore.

 

[ImminentCollapse]

If you are worried about areas behind the areas with surface problems, GPR may not work on rougher surfcaes. If you have good access to opposite faces, I would go with pitch and catch UPV. You can use the rubber mold (we've even used plumber's putty) on rough surfaces to get good transducer contact. If you want to go high-tech (and high $$), then go shear-wave tomography on smooth surfaces.

Also, recommend a few small cores or small bit drilling to validate NDT.

IC

 

[dik]

...and if nothing is found, testing is at the client's nickel... a criteria for costs should be determined first. Often with improper consolidation, voids occur between the rebar cage and the formwork.

(Added)Do you have any photos?

Dik