Minimum Concrete Temperature

Agreed, do not let the concrete to become frozen, otherwise it'll become a lump of mush.

I like to limit concrete temps on my project to above 40 degrees F; anything less than that is rejected.

Caution: do not allow de-icing salts or any admixtures containing chloride ions with concentrations above 0.5 mg/l to be added to the mix because those ions will attack the steel rebar. Also makes for weakened concrete.

Please be more specific. Are you talking about curing? Today, Around here, My minimum concrete Temp was running at about 44°F at about 7:00 this morning... Good questions get great answers.

Pour temps are not as critical as actual curing temps. It can be below freezing when you pour, although it is highly not recommended! The temperature you cure at is what is important. That must be at a higher than freezing temp. The cooler and wetter you can keep the concrete when curing, the stronger it will be.

Highway paver's like to use chloride to increase the cure time in cold weather, but it isn't something I have ever been in favor of, due to being corrosive to the rebar as previously mentioned

First of all thank you for all your informations about this matter. It really gives me additional knowledge.

I'am talking about during concreting of structural elements...as per standards when the ready mix concrete arriveat the site we have to check the slump and the temperature before pouring it to the structures..sometimes during cold weather the temp. of the concrete will run at about 18 degree C going down to 16 degree C at about 8:00 in the morning...so are these temperature rates are safe for the concrete or not? Becaused the contractors will insist that it is still safe..is he cheating or not?

thank you,

Is 18 degrees C above freezing?

C=(9/5)x+32'F

Where x=18'C

Therefore:

C=(9/5)18+32'F = 64.4'F>40'F above freezing

The temperature must be kept above freezing so the water can chemically react with the portland cement. Also because it is a exothermic chemical reaction the reaction will generate its own heat. When the water freeze the reaction stops so keep it above freezing for at least 21-28 days (or use additives for low temperature).

All those answers are really funny - I am from a place where the best concrete practices keep on being perfected : Pour concrete below 0C, let it cure half the time at -20C and in the warmer months, be sure to leave fresh concrete exposed to the sun (will "dry" much faster this way :-). The method has been perfected to the point where sometimes, the concrete will crumble to dust leaving previously painted rebars (!!!) ready for another pour of fresh concrete. That much saved in terms of preparation for the next recurring intervention. Of course, such extreme case won't allow the economic system to provide its fullest added value (shame).

Per ACI 40'F for the first 120 hours is recomended 72 hours required.

i have worked on smoke stacks where that was a problem. i believe calcium and hot water was the answer. another answer is covering the concrete. it makes its own heat. of course, the answer to your question is, never let it freeze.