Lightweight Concrete - criteria for acceptance? Proper testing regimen?

[SAS209]

Hello,

We have a project where we will be placing 115 pcf lightweight concrete for a hospital, and came across an interesting issue during our preinstall meeting. For background, I am a licensed CE working for a construction management company that also has a self-perform concrete arm.

The engineer specs out 115pcf as the required equilibrium density per ACI 301 (Section 7). ACI 301 references ASTM C567 for lightweight concrete density testing. The testing agency says how they do density testing for lightweight concrete is to take a set of 3 6"x12" cylinders for every ~50yds, takes them back to the lab, and does an oven dry density test. They add 3pcf and that's the equilibrium density they report back to the engineer to determine whether or not the density of the placed (and now hard) concrete is acceptable. They do not take wet density values in the field. They don't do any test densities before the project starts.

However, if you read ACI 301 (at least how I read it), this seems incorrect. ACI seems to say: do a test batch before the project starts, correlate wet/fresh density back to its lab-determined dry density, add 3pcf to determine equilibrium density. Then, when the project starts and the decks are being placed, only take the field wet/fresh density, use it's pre-determined correlation back to equilibrium density, and if it's within +/- 4pcf of the specified equilibrium density, accept it. There is no discussion of taking lab cylinders every pour, baking the cylinders down to oven dry density, and reporting back to the project +3 days after the pour to tell everyone if the density was correct. And this makes sense to me - why would a project want to wait 3+ days to know if they should have rejected the concrete based on density? It's too late.

Can anyone comment on this that has experience with LW concrete density testing? Which way is correct per industry standard?

More supporting info, if needed:
ACI 301: this is a "specification for structural concrete" document, so this is basically used by engineers etc. to tell a contractor how they want concrete work done on a job. Section 7 is specific additional info/requirements for lightweight concrete. Section 7 incorporates all of Sections 1-5 through reference, and only tells you what's additional/different than the basics outlines in Sections 1-5.
Section 7 says:

7.1.3.1 - For lw, need to also submit ASTM C567 density results (before starting the project) such that equilibrium density can be correlated with a fresh density (which will be needed/taken later on during the project in the field).
7.1.3.3 - Architect/engineer needs to review these submittals prior to the project starting
7.2.2.1 - ACI 301 wants equilibrium density to be calculated (not measured) based on either calculated (based on dry material weights per a formula in ASTM 567) or measured (based on baking a cylinder per ASTM 567) oven dry density.
7.3.1.1 - This goes into field quality control using fresh density (+/-4 pcf) correlated back to the acceptable equilibrium.
FYI: This is the only acceptance criteria ACI gives specific for lightweight concrete. Note that it's a field acceptance criteria (not a lab acceptance, like e.g. a cylinder break strength test would be). So, to me, this means the only density to accept/reject lightweight concrete on is based solely on fresh density (during the pour) and it's pre-determined correlation back to an acceptable equilibrium density value. I.e. there's no need to take cylinders for each field pour, send them back to the lab, cure them, bake them, and get an oven dry density.
Side note: this is where we had our challenges at another project. Testing agency reported on oven dry density instead of fresh density with a correlation; caused lots of confusion and cost to us as the contractor.

 

[darthsoilsguy2]

the biggest trouble I've had with lightweight concrete density is whenever somebody gets the crazy idea of questioning a UL design because something that doesn't matter is slightly different. I've never met a structural engineer who had issues with an extra couple of pcf. It is always the nitpicking UL# chicken littles who cause nightmares because there really isn't anything built to the full letter of the UL test assembly.... so there is plenty of nonsense out there in fireratingland if you go fishing for it.

It's been 10 years since I was more involved in testing and inspection work so i will need to recuse myself from getting into a deep procedural discussion..... However, the acceptance and rejection of FRESH lightweight concrete is not something the industry on a whole would support, certainly not the NRMCA members who pack the ASTM committees... unless the fresh concrete was clearly the WRONG mix (i.e. 145 pcf).

The problem with fresh acceptance is that lightweight concrete gets to be the way it is by using porous coarse aggregate. There is a lot of variability in the fresh concrete condition by the placement method. Ever do a lightweight pour by pump and experience slump loss? Where does that water go? We can break a lot of laws on the jobsite, but we just can't break the Law of Conservation Of Mass. The pumping of the fresh concrete several stories puts a lot of pressure on the concrete mix and cementious water invades the porous aggregate in ways that wetting the agg overnight doesn't accomplish. Even if you pump to a wheelbarrow on the ground next to the truck.... look up and see how high that concrete went up before it came down. The fresh density goes up because the porous aggregate is saturated deeper into the pores than it did during the trial batchs of the the mix design. I would assume that all mix designs are done by straight mixing WITHOUT any pressurization. When you have slump loss, a lot of that water that finds its way into the porous aggregate is water that doesn't end up consumed by hydration and it doesn't drag paste with it since the sand is too big....so it is water that evaporates as extended release of free moisture vapor in the slab AND it is water that bakes out of the cylinder for greater drying loss..... so when this happens, there is a real potential for LWT mixes to test heavy fresh and comes back into mix design spec during the labwork.

Since LWT mixes are almost always elevated slabs and pumped..... you can see why the ready mix suppliers who have strong expertise and representation on the testing committees like ASTM and ACI aren't going to volunteer to do things that would increase the amount of false rejections of concrete that could very well be batched completely in-spec..... AND we're talking about increased rejections of in-spec trucks that are out of spec resulting from the contractor's placement means & methods after discharge from the chute.

my explanation above about pumping is just to give a practical example why the industry does seemingly inconvenient things sometimes. There is big money in the acceptance and rejection of concrete. 50 years from now we're still going to be arguing whether concrete should be tested at the truck or at the point of placement.