Initial 24 hr curing temeratures

[JoeManNorth]

I recently had a few test cylinders cure in the initial 24 hr below the CSA standard of 20 +/- 5 deg. C. My cylinders cured between 15 deg C to in one extream case 2 deg C. All my samples however passed the 7 day test above the required 30 mPa averaging 31 mPa. Would the initial curing time have a tremendous affect on the strenght? If it did would my cylinders not have a low mPa value? And finaly should I be really worried about this, Keep in mind that up here in the north temperatures can drop down quickly at night.

On smaller projects I have been through this and never had any problems. This projects engineer is laying a egg.

I am now curing my cylinders in a heated room. Even though I told them the trailer vibrated too much.

 

[eric1037]

If your cylinders freeze, it will significantly affect the strength. We have insulated boxes with a 100-watt lightbulb in them that we use to keep the concrete warm.

 

[mitchelon]

How does you curing temperature compares with the temperature of the in-place conrete? Cold curing decreases the 7 and 28 days strenght and increases the long term strength.

 

[civilperson]

Cylinders cured to the ASTM standard do not necessarily reflect the strength of the concrete in the structure. They are a test of the ultimate strength of the mix as delivered. Field cured cylinders which have the same conditions as the structure placement for cure are a better indication of the inherent strength of the concrete in the forms. The low temperatures cited for the test cylinders would delay the cure process, (assuming no freezing), for the the time it takes to get back to the 20 degree temperature range. So your 7 day breaks were more like a six day cure strength.

 

[dik]

Eric... if the concrete freezes before it starts to hydrate, then there is little effect on the concrete. There was a multistorey building in Winnipeg about 30 years ago where an upper floor slab froze! The floor was protected with insulating tarps and heated and unthawed... it works just as well as any other. I don't recommend it... As long as the freezing occurs and doesn't affect the cement gel, it could be OK... notice the 'could' not should!

Dik

 

[JoeManNorth]

Good stuff guys, thanks alot for your input. My cylinders did not freez however, they just got very cold, 2C (I think thats like about 35F) so that is not the issue. It's having to try to justify to the engineer that the concrete results are still valid that is being a tough sell. He does have the right to say that my cylinders where not made up to code and reject all the results, even though the only thing that's not up to code is the initial temp.

I had a engineer in our office tell me that there are some cases that the concrete stregth will be higher then what it is if you cure it below the corect temp. Has anyone heard of this before? All studies and reports I've read show that the 7-28 day stregth is 20-30% lower then a properly cured cylinder but the concrete will achieve a higher mPa pass the 90 days then if properly cured.

Freezing a floor slab or a mass amount of concrete is a little different then freezing a cylinder. I have froze a cylinder once apon a time and it broke at 12.5 mPa, but the barier that the concrete was used for was fine due to the heat the concrete released.

 

[bb29510]

The cold retards the concrete. I myself think the first 24 hours is not critical as long as they are protected as good as possible. If cold I would reccomend not moving for 48 hours, but then and big then. the book says this is wrong, no matter what you say he going throw the book in your face, it happen daily to me. I have seen studies where one set was left out for seven days while its twin went into curing room on time.Then at seven days the other went into the curing room for a 21 day cure. At 28 days the differnet was only a couple hundred psi. Which is not much being so many variable are there. It still comes back to the book and some believe the book is law. There really not much you can do but test the slab itself.

 

[boffintech]

brownbagg, To ensure an accurate comparison between field cast concrete strength test specimens and mix design values, ACI and ASTM specifications require that during Initial Curing, the first 24 to 48 hours after field casting concrete strength test specimens, that the cylinders be maintained in an environment in which the ambient temperature is maintained and controlled within a given range and in which moisture loss is prevented. Additionally, the verification of required temperature with a maximum-minimum thermometer or other suitable electronic temperature recording device is required.

Concrete strength test specimens not provided the specified Initial Curing environment may not produce meaningful reliable results. High strength concrete mixes (5K psi to 10K psi), high early concrete, and slag concrete mixes, are particularly sensitive to daily high and low temperature extremes during the Initial Curing period. Concrete strength test specimens subjected to overnight temperatures lower than the specified standard may produce slower strength gain than the mix design values. If early cylinder strength tests are required for post-tension tendon stressing and/or formwork stripping the schedule may be impacted by low breaks caused by inadequate Initial Curing.