Floor Slab concrete tests coming in low......
Floor Slab concrete tests coming in low......
(OP)
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?
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?






RE: Floor Slab concrete tests coming in low......
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).
RE: Floor Slab concrete tests coming in low......
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.
RE: Floor Slab concrete tests coming in low......
RE: Floor Slab concrete tests coming in low......
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.
RE: Floor Slab concrete tests coming in low......
RE: Floor Slab concrete tests coming in low......
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.
RE: Floor Slab concrete tests coming in low......
I'd not depend on the Windsor probe at all. The cores are much more representative.
RE: Floor Slab concrete tests coming in low......
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.
RE: Floor Slab concrete tests coming in low......
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.
RE: Floor Slab concrete tests coming in low......
Mike McCann
MMC Engineering
RE: Floor Slab concrete tests coming in low......
RE: Floor Slab concrete tests coming in low......
RE: Floor Slab concrete tests coming in low......
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.