Floor Slab concrete tests coming in low......

[Brewdude]

I have a question about construction of floor slabs and whether or not a much lower strength than specified would be an issue to other structural engineers. I have a project that has a small elevated slab as well as some of the slab on grade that came back with f'c test results of around 2200 psi.

My drawings spec'd 4000 psi concrete and all of the rest of the concrete on the project has been fine. Apparently, the concrete that did come back negative was a different mix design than the rest of the concrete for the project to enable them to pump it through a 2" line. It is a 4000 psi mix that the ready mix plant has used in the past with success.

After the 28 day results came back so poorly (2200 psi average), we had them take some cores and saw as bad as 1450 psi on the worst core and as good as 2300psi in the best case. The contractor is obviously trying to avoid tear out and replacement since it will back up their schedule but I don't know if I have much of a choice other than to have them tear it out.

Now they went ahead and did an ASTM C803 Winston probe test and are seeing results of 3600psi or greater. While this test yields better results, I'm hesitant to accept it to be the same as the cylinder tests.

Any feedback would be great. Is the biggest concern durability for a slab? What would you do as the engineer?

 

[Brewdude]

Additional Information:

ACI 22.1.1.1 states that 2500psi is he minimum compressive strength of "structural concrete".

Of course ACE 22.1.1.2 states "It is not within the scope of this code to provide design and construction requirements for nonstructural member of plain concrete such as soil-supported slabs (Slabs on grade).

 

[hokie66]

A 2" pump line? Amazing that any contractor would allow this, or any concrete supplier would supply concrete to be placed that way.

I would reject. Windsor probe results are useless in addressing concrete understrength without calibration with the actual strengths, and that has obviously not been done. The cylinders and cores prove that you have substandard concrete.

 

[Brewdude]

That is my thought, but you know how these things go. The customer starts seeing if they can keep their contractor on schedule and starts asking if they can do this or that. I was considering taking the route of telling them that the elevated slab (over composite deck, 8' spans or so) needs to to be torn out and replaced as I would consider it to be structural. The slab on grade could be up to them, but we are talking mortar strengths here. I didn't even know concrete could be produced so weak. I think it makes sense to tear all bad areas up and replace.

 

[Brewdude]

Mix design information for the 2" line pump.


Strength: 4000 Psi 2” line pump With Mid Range


MATERIAL SOURCE DESCRIPTION SPEC. GRAV. WEIGHT YIELD C.F.
Cement Lafarge Type I 3.15 500 Lb. 2.55
Fly Ash Sefa Type F 2.53 94 Lb. 0..6
C. Agg. Winn #7 Limestone 2.65 1280 Lb. 8.12
F. Agg. Ingram River Sand 2.6 1800 Lb. 11.1
Water 1 258 Lb. 4.14
A.E.A W.R. Grace Darex AEA 5% 2.00 Oz. 1.35
Admixture W.R. Grace Addva 140 25.00 Oz. 0

TOTAL 27.39

Note: This mix design is designed with air so it can be pumped through a Two inch line pump.

 

[hokie66]

They must not have much concrete experience, as trying to pump this mix through a 2" grout line is futile.

 

[Ron]

Agree completely with hokie66...reject it. This contractor is an idiot. As hokie noted, a 2" pump is not for appropriate for structural concrete.

Don't feel bad for the contractor. Stick to your guns and reject it. The results are awful and the long term result could be even worse (yes...it will gain a little strength, but the compromise in the concrete integrity and durability has already been made. If you accept it, you put your integrity and your license on the line...don't do it.

 

[JAE]

Agree with the others (just chiming in here!).

I'd not depend on the Windsor probe at all. The cores are much more representative.

 

[Brewdude]

Thanks for all of the feedback. I just wanted to see if anyone who think like an engineer had thoughts on this whole ordeal since getting pressure from the field can make you think twice sometimes.


Another thing that I don't like is that the contractor used concrete with air entrainment. Our spec states that interior concrete shall no have air entrainment. Funny thing is, they just happened to not even test for air entrainment on the day of placement of the concrete in question.

 

[Duwe6]

Concur with taking cores. Use a diamond-tipped hole-saw. If your sample 'cookies' are too thin for valid crush results, have them machined into grout cubes.

And contract a *major* concrete testing lab, that has at least one experienced Civil Engineer that specialized in concrete. Thye can guide you to the best testing approach. With results from the actual slab, your Contractor is legally prooven to have placed below-spec concrete. Now he has no legal cohoice but to jackhammer it out and start over.

Been there, done that.

 

[msquared48]

I'm interested in why the contractor wanted to use the 2" line in the first place. What was different about the area to pour as compared to the other successful areas?

Mike McCann
MMC Engineering

 

[Duwe6]

MUCH cheaper to rent a 2" placement pump, vs. renting a Putzmeister pump truck. Contractor gambled and lost.

 

[MiketheEngineer]

Structural slab has to go. Slab on grade could remain - BUT it will WEAR OUT rather quickly if you have even a slight amount of traffic on it.

 

[Teguci]

Suggest that you'd have it replaced if it was your house but tell the client that the slab on grade will always look good from your house (helps if you are smiling)!

Also helpful to let him know how much it will cost to have it replaced and give him your restoration card.

Bottom line, if its not structural, the client gets to decide.