Determining an Existing Foundation Thickness 1

[PEFLWI]

I need to determine the thickness of an existing foundation. The original drawings are missing. Would a rebar locator be able to determine the thickness? What other ways could I determine the thickness of he foundation?

 

[jerseyshore]

Dig a hole?

 

[80PercentTruth]

I've done this before to check the thickness of an existing warehouse slab. Drill a hole with a hammer drill until you encounter dirt, stick a long paper clip or rod down the hole with the end bent at a 90 degree angle. Stick it all the way to the bottom of the hole and pull upward until the hook catches on the bottom of the slab. Make a tick at the top of the rod, pull out of the hole and measure.

"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."

 

[phamENG]

I'm with jersey here. I did it last week. Dig down to the bottom of the footing and use a measuring tape to figure out the distance from the bottom to the top. Don't try to get fancy with it - GPR will show you were voids are and, if you have a good operator, where rebar is, but rarely will it even tell you the size of the rebar much less the thickness of the concrete element.

 

[hawkaz]

Depending on the thickness, GPR might work

 

[JStephen]

For measuring the thickness of steel plate (as in, the tank industry), they have little ultrasonic thickness testers. But I have no idea if they are usable where the solid is not reasonably homogenous, or where the far surface is very rough.

 

[jerseyshore]

80% has the right strategy for slabs, little wire hanger or something with a hook is great for checking a slab depth quickly and easily.

I do a ton of checking foundations for additions/ elevations and the first thing I tell people if they don't have original drawings is to dig a hole and call me when it's done.

I've used GPR on a number of jobs and about 75% of the time the results are inconclusive and we have to assume worst case/ not touch the existing structure. Those GPR guys are damn good salesmen though.

 

[PEFLWI]

Thanks everyone.

 

[a2mfk]

We have had good experiences with a local GPR company (central FL) for scanning CMU walls, slabs, foundations, concrete beams and columns, etc. The tolerances for rebar are within one size, rebar spacing within a couple of inches, and thickness also within an inch or so. I suppose like with everything, quality can vary greatly from company to company, and also depends a LOT on their equipment and experience. It also is relatively economical, enough so where I have used it on residential projects. If in doubt as to their ability, ask for an example report they recently did of something similar that you need.