low density / low strength concrete

[structSU10]

I have a situation where the design concrete mix was 4000 PSI, with 145 pcf density - pretty standard mix, with components as follows:

cement 520 lbs
fly ash 91 lbs
water 32 gals
fine agg 1220 lbs
coarse agg 1760 lbs
WRA - 12.2 oz sika plastocrete

There was an issue with the concrete in the field, with strengths coming back on cylinders of 2000 PSI, and the laboratory test came back at 125-128 pcf density. The contractor took cores to test the concrete in place, and a couple cores had the 128 pcf density, and strength about 3000 psi, and one have density of 136 pcf and strength of 4500 psi.

These elements, for strength, require a minimum of 3000 psi. Do I rely on the concrete cores that I have adequate strength and can accept the concrete as placed? Should I be concerned about quality of concrete away from the cores and the cylinder results?

 

[jayrod12]

Yes I would be concerned. Inadequate density could mean inadequate vibration/consolidation. Generally speaking 2500 psi concrete and 5000 psi concrete should have the same density within a reasonable range. You're 25-30% short on the specified strength and about 7-15% short on density. I would not be accepting that level of deviation from the specs typically.

 

[hokie66]

Something drastically wrong there. I don't even consider 2000 PSI to be structural grade concrete. What elements contain this "concrete"?

 

[Ron]

The mix constituents and quantities are consistent with a 4000 psi mix with a w-c ratio of 0.43 and a unit weight of 145 to 146 pcf. The results are obviously not. Check the trip tickets to see what was delivered. Check the testing lab to make sure they didn't mix up samples. Check the placement...was it pumped? If so, was there water added at the pump?

Pumped concrete should always be observed by someone knowledgeable of concrete. It is common for the pumping crew to try to add water at the pump, particularly with such a low w-c ratio.

Where were the lab samples taken? At the transit mixer or at the end of the pump line if pumped?

For core tests, if the strength of the cores reaches 85 percent of the design strength, they are considered acceptable under US criteria. The variation seen in the core tests shows variability in the field placement, likely from differences in added water.

 

[structSU10]

Thanks for the insight. They did pump this, and said they only added more admixture, no extra water, at the pump. The low density makes me think otherwise though.

 

[jayrod12]

In our office we generally give the contractor the option to attempt to remove the unsound concrete, but in a case such as this one we'd always recommend a complete removal and replacement as there is so much wrong we wouldn't be confident of any concrete being ok, even if both sides of the wall looked ok, who knows what's going on between the faces.