Frozen concrete

[bouk715]

I'm hoping that someone here will be able to shed some light on this topic...

We have a project currently in construction here in the northeast. Like most of the country, we've had some very cold temperatures the last few days. The contractor placed a concrete foundation wall early last week (roughly 10"-12" thick). The wall was covered with insulated blankets after placement. The ambient temperature dropped well below freezing shortly after placement (within 24-36 hours). Test cylinders were cast and a couple were left at the site for field curing to determine strength gain during cold temperatures to determine acceptable form removal and backfilling time-frames (the project specs had timeframes, but the contractor wanted to see if he could hasten it by reaching strengths earlier). According the testing agency, the cylinders were kept under the blankets.

A cylinder was picked up from the site to check 4-day compressive strength, and it was determined that the sample was frozen. So what I'm not sure about is if the wall itself froze or if it was just the sample. I assume since the sample is so small compared to the larger wall, they wouldn't freeze at nearly the same rate. The testing agency came back the next day (after 4 day break) and did 3 windsor probe tests. Results came back between 2,300 psi and 5,000 psi. Mix design is 4,000 psi at 28 days. The testing agency will be back again to take 3 concrete core tests after the concrete has been in place at least 14 days to test the compressive strength and confirm the windsor test results.

My question is, assuming the core compression tests come back okay (say at least 7-day strength), should I request any other tests on the wall? Could there be voids in the concrete if it partially froze, even if it looks okay from the exterior and compression tests come back acceptable? Is petrographic the only reliable test to check for the voids?

Any comments would be greatly appreciated. Thanks.

 

[dcarr82775]

If they got blankets it quick I doubt it froze. The temperature is most important in the first 24 hours or so. I think the windsor probe and core tests are a good way to go. You could also walk the wall and look for any soft areas (car key, screw driver, etc.) that would indicate potential freeze. You could beat on the wall with a hammer and listen for sound changes to indicate something wrong, but I don't think your internal void fear is likely. I think the windsor probe and core tests are a good way to go.

 

[FEM4Structures]

Cylinders were kept under blankets and were frozen. Testing Lab will require additional testing of concrete as the cylinders specified by ACI were compromised.

Generally with frigid temps as seen during last week, General Contractor should have taken appropriate measures to cure the concrete. I have seen them using heaters under the tarps. Admixture in Mix Design geared towards cold weather.

Windsor Probe is not a conclusive test since the probe has to be calibrated with concrete meeting 4,000 psi comp strength onsite.

Your choice should the onus of concrete acceptability fall on your shoulders should be destructive testing. You may want to go through ACI specifications to choose the number of cores required to accept the concrete. You may want to select locations where the cores should be taken and representative of concrete strength.

Thanks

Thanks,

FEM4Structures

 

[concretemasonry]

There is always the conflict between the ASTM standards for making, storage and curing of cylinders and the desire to use cylinders to predict the strength of the actual concrete placed and cured. The individual cylinders are fragile and not subjected to the same conditions as concrete placed in a larger amount that has the advantage of the real heat generated during the actual curing process.

After all, cylinders are meant to determine the quality and compliance of the concrete mix delivered. After that, it is a question of the placement and the curing conditions, both of which are or should be done according to the construction documents. The use of probes or cores will only provide a sample of the properties of the accessible surface of the concrete for testing/sampling, but for a 10" or 12" placed on acceptable base might be representative.

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

 

[bouk715]

Thanks to both of you for the quick and informative responses.

I reviewed the wall today and sounded it with a weld hammer. The concrete sounded firm and I didn't find any dead spots (voids/delamination). There were some areas where the exterior surface was "chalky" and could be scraped, but it generally appeared superficial. My guess is there was some frost damage in areas (but perhaps not frozen).

There will be 4 test cores taken from the hardened concrete. One core will be taken in one of the "chalky" areas, so we should have a decent idea of how the concrete is doing.

Thanks again!

 

[madmantrapper]

Post back on this thread to let us know the outcome please.

 

[bouk715]

Will do. Core tests will be taken late next week, with the breaks/reports scheduled for early the following week.

 

[bouk715]

Just got back the test results today. Four cores were tested and results ranged from 3,750 psi to 6,000 psi. Age of the concrete at time of testing was 19 days.

Thanks to everyone for all your input and help on this!