Typical density for lightweight concrete

[OHEcoEng]

Hello, everyone.

I have been trying to figure out a concrete density for a month or so now. I went so far as to have a core sample of the roof taken. Unfortunately, I didn't call for the sample personally; it went through the building owner, and calculating density wasn't part of the job.

In performing my roof structural analysis, I have been assuming that the minimum concrete density is 8 psf/in (per ASCE 7-05).

Plans for this building are from 1998, and I have not been successful in tracking down all relevant sheets, or finding who actually did the concrete work. The plans do call for "insulating concrete fill" and the core sample revealed EPDM covering 2.25" concrete, 2 or so inches of EPS, and metal decking.

Do any of you guys/gals have any experience with specifying this type of concrete, and what is a typical density used for a "flat" roof ? Is there ever any situation when non-structural 96 pcf lightweight concrete would be used for a roof deck?

I have run an analysis on some of the OWSJ in the building, and "existing conditions" reveal significant overloading of the joists in some situations when I use my 96 pcf assumption. It is leading me to believe that this concrete is more in the range of 36 -50 pcf.

Any advice would be greatly appreciated!!!

Mike

 

[MiketheEngineer]

That is pretty LOW (36-50pcf)for any kind of concrete that I know of -

 

[OHEcoEng]

Not knowing much about these systems, I've been looking on the 'net for info.

This is one that I had come across, with a description of "typical densities and uses...

http://www.litebuilt.com/lightweight-concrete-handbook.html#D-1

 

[paddingtongreen]

Does this help?

http://www.perlite.org/product_guides/25_perlite_insulatin_rfdeck.pdf

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dicksewerrat]

There is a "concrete" mix that floats. I think it is called Elastoseal. Not sure. It is filled with air bubbles. This may be what you have.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[OHEcoEng]

Thanks for the feedback. Unfortunately, I am to the point of guessing, and I need to make a reasonable determination. What would you all do?

I know that standard "lightweight concrete" is too heavy for this roof. It's just a matter of making an educated guess at what is actually there. It's just that I am uneducated when it comes to what is most common to use for concrete roof decking, or "insulated concrete fill". I thought this term might ring a bell with some of you.

And Michael: I don't know how he knows, but the roofer said it was not perlite (this was suggested some time ago).

Richard: good lead, and I'm going to look into this. This has similar dry densities in the range of that other link (it's actually elastizell).

 

[paddingtongreen]

If not perlite, it also could been vermiculite or polystyrene bead concrete. Any of those could get you down to 30pcf.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[OHEcoEng]

Here is a photo of the core that was taken. Lots of air and no aggregate. This is obviously a very friable concrete.

 

[hawkaz]

I tried uploading a file, but it was too big. See ICC ER-3627.

Per the manufacturer product sheet, this has a wet density of 30-45 pcf and a dry density of 25-38 psf depending on what you need. f'c varies from 140 psi to 450 psi.

 

[SteelPE]

Couldn't you just weight the sample and calculate the density? I would think that would be much better than guessing. Somebody must still have the sample somewhere. If not, don't guess just have another core taken. There is no sense in guessing.

If something happens, and someone asks you how you calculated the weight of the concrete do you really want to say "I guessed".

 

[IFRs]

Weigh the sample, put it in a loose plastic bag and see what it displaces, calculate the density

 

[concretemasonry]

It certainly looks like "Gypcrete" or a similar expanded lightweight material that is generally used as a reasonably hard cover over a floor/roofing system and really has little structural benefit since it is frequently used to a provide smooth surface to place roofing or other materials on.

Determining the density for the purposes of loading can be acceptable with an accurate determination of the average thickness.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[paddingtongreen]

Search for "Cellular lightweight concrete". That has voids.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Ron]

What you have is lightweight insulating concrete. If it is the perlite or vermiculite variety, then the density will be from about 25 to 40 pcf. If it is cellular concrete (Elastizell or similar), then the density will be around 35 to 45 pcf.

Both are insulating fill...non-structural. Since you found expanded polystyrene in the mix as well, it is likely a perlite or vermiculite type of mix. The most common is a material called "Zonolite" originally manufactured by the W.R. Grace Company, now provided by Siplast.

 

[OHEcoEng]

I was out yesterday taking care of kids on a snow day, so sorry for the late response.

As far as weighing the sample and calculating the density based on core volume, that was my intention. I was expecting a full report. Needing the info ASAP, I called the roofer, and he said that's not part of their report...I guess they pull it, take pictures, and then put it back and reseal the roof.

The EPS is a separate layer, not part of the concrete mix.

Anyhow, thank you all for the feedback. Very helpful!! This just has to be a very low density insulating fill (like 40 PCF), otherwise, the 20K3 joists under it would have already failed!!

 

[Ron]

OHEcoEng...The EPS is perforated sheets that allow the LWIC to flow and bond the sheets together. The density values I gave are for the actual lightweight insulating concrete itself...not the system including the embedded EPS.

 

[mbullism]

We've been doing a lot of existing roof analysis for photovoltaic panel installations, and this comes up quite a bit... here are a couple articles I found related to what I think you have. I don't think it's a guess, so much as an engineering judgement. Hope these help-

 

[mbullism]

and another...