Frozen concrete cylinder 1

[gosai]

I had a test report from testing company, which show lower strength of cylinder at seven days and more than specified strength at 28 days. The reason specified by testing company for lower strength at 7 days was frozen cylinder during initial curing. Is it possible for concrete to gain strength at 28 days if it frozen during initial curing?

I also checked actual concrete at poured location, there was no signs of frost on the surface.

 

[HeavyCivil]

Samples are usually placed in a cooler on site for the first day - the heat of hydration should be enough to keep them from freezing during that time. I would say the testing company did something wrong if they were actually allowed to freeze.

Are you sure this wasn't a field-cure? Either way, more important that the actual placement was not allowed to freeze. You should ensure proper cold-weather concreting practices are utilized by the contractor.

If the concrete does freeze prior to reaching 1000psi, it probably would not have gained specified strength. If you got your strength it's probably okay.

Low temperatures obviously result in slower curing. Cold weather concreting with moderate to high fly-ash contents can result in some really low 7-day breaks, but normal performance at 28 days.

 

[Ron]

Check your mix design. Your testing lab might be guessing as to the cause of the lower 7 day results. If the mix has fly ash or ground granulated blast furnace slab cement in it, the 7 day strengths will be retarded; however, the 28 days strengths are usually unaffected.

If there was no evidence of freezing at the site, then the cylinders probably didn't freeze either.

 

[dik]

As long as the 'gel' hasn't started to form, your concrete cylinder can likely be frozen and when thawed, it should regain the strength of an unfrozen cylinder. It's only if the concrete cement has started to hydrate that the freezing breaks up the cement matrix.

Dik

 

[concretemasonry]

Are the reported strengths from the same cylinder tested at both 7 and 28 day strengths? This is what the basis of the "healing" of concrete was really discovered. - Old marked cylinders retested later for training purposes for employees.

How many cylinders in the batch? - Individual or averages?

In many modern labs the cut-off on the testing machine for routine testing cuts off as soon as there is a yield, but the cylinder can increase in strength.

Were the samples taken by the testing lab and curing controlled by the lab?

Who transported the cylinders from the site to the lab? - Important because 7 day cylinders are especially fragile and if they are not strapped to a pallet and/or cushioned a bump could show and early failure that is not representative of the in-place concrete strength.

Dick

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

 

[Ron]

After over 25 years of being in a testing lab environment, and having had to defend our practices and procedures many times over the years, I agree with concretemasonry.

Many testing labs are very good at what they do. They are careful not to compromise data in any way. Over the years, profit has become more difficult to achieve and training has suffered. This is certainly true of engineering technicians who do sampling and testing.

A universal testing machine, particularly with a high capacity range, is very expensive. As CM noted, many technicians are taught to drop the load before breaking the cylinder all the way down....this can lead to erroneously low results. It does prolong the life of the machine, but it does not follow the testing protocol they say they are following. Not good!

To evaluate this discrepance properly, you must go back over each step of the batching, delivery, placement, sampling, transport, testing, review and reporting of the data.

 

[gosai]

The testing company also sent a photograph of surface condition of the cylinder which was tested for 7 days. The photograph of cylinder was consisted of signs of frost on the surface (flakes on the surface). As per testing company the cylinders kept in the lab under controlled curing conditions helps concrete to develop specified strength at later age.

 

[Ron]

If that is the case, then why did the testing lab allow the cylinder to freeze in the field? That's improper protection of the specimens and typically violates most sampling and testing protocols.

Specimens in the field are cured and tested in the lab under STANDARDIZED conditions to check the mix design...not the concrete in the structure. One reason standardized procedures are used is to have repeatable results from person to person and lab to lab (within the criteria of within-test and within-lab variations).

 

[gosai]

Thanks to all for giving me such nice information

 

[cvg]

given the fact that you have doubtful results from the lab, you could spend a lot of time reviwing the procedures they followed and perhaps not ever get to the bottom of this or you could obtain cores from the actual structure, give them to another lab and check the actual compressive strength