Analysis of Existing Concrete Slab

[sklev]

My firm was hired to analyze the structural capacity of a building in our city. The owners want to use the 3rd floor as storage for paper products weighing up to 250 psf. The building is very old; we believe it to have been built in the 1920’s. There are no plans for the building save floor plans. It is constructed of masonry walls and concrete slab floors and columns.

Our first step was to contact the local testing company to help us determine the make-up of the slab so we could analyze it. The tester performed Ground Penetrating Radar (GPR) testing. We did determine the thickness of the slab to be 8” with a 5” finish on top. The GPR picked up rebar, but the tester could not determine accurately the size of the rebar because the magnetometer could not read deep enough in the slab. The report we received was vague and inconclusive and does not instill much confidence in our understanding of the slab.

I’ve also done research on construction in the 1920’s. We believe the slab to be of flat slab design similar to the C.A.P. Turner Mushroom Slab design (image attached). It was a very common type of construction in that time period. The rebar report from the testing company reflects this type of construction to some extent.

Basically, we are at a point where we do not feel confident with what we know about the slab, even after testing.

Are there any other means of non-destructive testing out there that could help us determine the make-up of the slab? I have read past forums on this site about non-destructive testing. We are familiar with x-ray, but it is too expensive and was ruled out. How does impact echo testing work?

Is anyone familiar with these types of slabs?

 

[Ron]

A good GPR survey and interpretation should give you the rebar spacing and pattern...a core down to the top of the rebar would confirm its size.

If the slab is only 8" thick, you could have a radiograph done over an area about 4 film widths (4, 4x17 film sheets placed side by side). From this you can determine an accurate spacing and with a little geometry, get the rebar size.

 

[MiketheEngineer]

Even if you "see" the rebar - only God knows from what it was made. Things were petty loose back then.

However - how about a simple deflection test. Get some good dial micrometers - put a fairly heavy load on the floor in a few place and measure the deflection. Work backwards.

That should give you some comfort level.

 

[JAE]

You should do both what Ron and Mike suggest for something that significant (large floor, large building, large live load).

Do an analytical approach by researching the make-up of the structure and attempt....attempt...to estimate a live load capacity under the current code.

Then do a load test per ACI 318 criteria.

Compare both and use your engineering judgement to establish a recommended live load capacity.

Then present that capacity to your client in a way that you know they will understand...some clients don't understand what a psf is.

 

[sklev]

Thanks all. Any insight is good insight.

I was disappointed with the GPR report that we got back. It was very vague and didn't give any firm information, besides the slab thickness. We have an idea of what we think the slab design is, and the report only partially backs this up. My boss and I have talked quite a bit about it, where we would want cored that would give us more information about the rebary layout. The one particular portion of the GPR report that is unsettling was the fact that he didn't report any rebar in the middle of the slab. I've done quite a bit of reasearch, and even engineers in the 20's knew that the moment was maximum in the center. I was present when the tester was scanning and he picked up some diagonal rebar in the center that supports the design that we think the slab is, but he left it out of his report. I suppose he didn't feel confident about it? But, I feel it's important to go off of the facts, which is the limited information we have in the report.

Ron - Does radiograph testing work on 13" slabs? The total thickness is 13", but there was an 8" slab and an obvious 5" added layer when the tester was reading the slab. I've assumed that the slab is 8" with a 5" finish because there is no way that I can know if the two were poured close enough together to work as one.

Mike - I agree. I've found values of rebar from that time period, along with strengths of steel and concrete and have been using these in my calculations. I don't feel comfortable with using today's values because they might not have been achievable with the technology of the time period.

I am just not comfortable with what I have. I'm new to the work force; less than a year from graduation. I don't feel comfortable giving a client a firm value when my calculations are based off of vague and spotty information.

Thanks for all the posts. I am just looking for some more insight to lead me in the right direction.

 

[rapt]

sklev,

One thing to watch out for with flat slabs designed back then was that they often designed for only half of the load in each direction, grossly underestimating the amount of rinforcement required. The moments in your extract would tend to suggest that htis was done in this design method with M-ve support strip being wL3/27 (if I am reading it correctly)! These days it would be about .11 * wL3 * .75. So much less!

 

[Brad805]

Sklev, welcome to reno work. It never gets any better.

The GPR results seemed fairly useless. The Hilti PX-10 works quite well for measuring the overall slab thickness too. I have borrowed it before from our Hilti rep.

If you have two mats of rebar at any locations, the X-rays can be difficult to interpret. It is rare that the top and bottom mats will ever perfectly align. I will leave Ron to answer the question about X-raying a 13" slab. I know we did one that was close to 10" thick, and the exposure time was around 45min - 1hr. We only had access to the lower grade x-ray source.

Have you considered removing some regions of the topping at the key areas? Keep in mind who benefits the most from this investigation.

Have you tried any of the handheld rebar locators? They are not perfect by anymeans, but might help. You can use these easily from the top and the bottom of the slab. The model below are commonly rented from NDT retailers. I think last we rented was something like $150 per week. I wish Hilti would rent.

http://www.proceq.com/products/conc...tection.html?gclid=CP2Ih5LtyaYCFQpvbAod6xuTFw

Brad

 

[hokie66]

Sounds to me like analysis of this slab is not going to prove much. Load testing may be your only option. That would involve propping the floors below, so not very palatable to the owner.

 

[csd72]

Time for a radical suggestion:

5" of topping weighs in the realm of 600psf.

Have you considered removing this topping and replacing it with a much thinner self levelling screed?

Just a thought.

 

[Lion06]

I haven't seen any responses that indicate you can get the rebar location within the depth of the slab with non-destructive testing. Can this be determined without coring?

 

[ron9876]

I would want some destructive demo to see the what and where of the reinf. Don't forget the load on columns and footings.

 

[Brad805]

sklev, I was thinking about this a little more. You know if you post the typical bay arrangement dimensions and the original building usage I bet many will offer a best guess estimate of the original design load. Not that this would mean anything since this is a internet forum, but having followed this forum for a while now, I know there are a few guys that clearly know their stuff and are not just tossing out guesses.

What is the general condition of the structure? Any large cracks or noticable floor deformation? You can always find clues about the original designer by looking at the building from a more general perspective.

If this was designed for something like 100psf, I think this study may prove to be a waste of your time and energy. If the client is prepared to upgrade this floor and everything below regardless of the cost, then it is a different story. Typically, basic discussions about money and the reality of renovating (disruption and costs) a third floor will sway a client in the right direction.

I find the 5" topping intriguing. I can't quite evision a fundamental reason for it to be 5" unless they were trying to increase its strength previously. Especially if that topping has no reinforcement or is not anchored properly to columns or walls. Without reinforcement or connections it seems to be a useless dead weight. A couple of inches I could understand, but 5"?

Brad

 

[todh]

sklev,

I think you need a better GPR company. I am a structural engineer that also happens to own my own GPR equipment (although I don't do a lot of GPR). You should easily be able to determine the spacing, depth of the reinforcing steel and thickness of the slab as long as you have a known thickness of concrete somewhere within the building (near a core hole or other penetration) so the GPR can be calibrated. Typically most people I have run into completing GPR are not structural engineers and therefore can not adequately interpret what they are seeing in the GPR images.

A 1600 MHz antenna on the GPR will get you through your 13" slab. Using GSSI's GPR and software, you can complete a grid of the entire surface to map out your reinforcement pattern. It is typically done for small (2'x2') areas, but I have also done it on larger 10'x10' areas with good results as long as you know what you are expecting to see and can set up your grid spacing and size in the right location.

I have done renovation work on a 1911 building in Texas and got my first exposure to GPR there. Use the GPR data to locate the reinforcing steel and then expose it to determine size of steel. We had twisted square bars in our 1911 second floor slab. If you want, you can cut out of a section of the steel for testing too.

Good luck!

 

[grantstructure]

Would a delamination between the 5" topping throw off the GPR reading? Just a guess.

I wouldn't be surprised to find no reinforcing in the center areas of the slab, and as rapt noted, understanding of two way action was decidedly incomplete in that era.

There are those who claim that rebar distribution between middle and column strips is actually inconsequential--that is, that so long as you have the steel present somewhere in the bay, you can count on it for the total moment across the bay. I find that difficult to swallow, but the person making the assertion is PhD.

I like the suggestion of removing the topping slab and replacing it with a new slab of known quantity. You could come back with a lightweight topping as well, if that helped. Another possibility is FRP reinforcing applied to the soffit for positive moment regions. I think you could FRP the top if you included a protective topping. Anyone want to second that (or shoot it down?)

 

[rscassar]

I agree, both yourself and the PE shouldn't stick you head out to far on this one.

How much was paid for the GPR report. If the report only indicates the slab thickness and does not give bar layout than I would not be happy with the report or testing company.

There was an article in the 2004 Concrete International concerning the development of building codes and briefly described a history of floor slab construction.

It says that floor slabs were patent designs back in the 1920's and that some of the early building codes only accounted for 85% of the total static moment.

I am always on the FRP bandwagon to increase the strength of an existing slab. FRP won't do much to increase stiffness for deflection and vibration calculations. Be careful when specifying FRP. You will need to write a project specific specification for the installation of the FRP as well as a an inspection programme.