Editing Concrete (section)

====Cold weather placement====
[[File:Pohjolatalo Kouvola 001.jpg|thumb|''Pohjolatalo'', an office building made of concrete in the city center of [[Kouvola]] in [[Kymenlaakso]], Finland]]

[[Extreme weather]] conditions (extreme heat or cold; windy conditions, and humidity variations) can significantly alter the quality of concrete. Many precautions are observed in cold weather placement.<ref name="FPrimeC">{{cite news|url=http://www.fprimec.com/cold-weather-concreting|title=Winter is Coming! Precautions for Cold Weather Concreting |date=14 November 2016|newspaper=FPrimeC Solutions|language=en-US|access-date=11 January 2017|url-status=live|archive-url=https://web.archive.org/web/20170113155033/http://www.fprimec.com/cold-weather-concreting|archive-date=13 January 2017}}</ref> Low temperatures significantly slow the chemical reactions involved in hydration of cement, thus affecting the strength development. Preventing freezing is the most important precaution, as formation of ice crystals can cause damage to the crystalline structure of the hydrated cement paste. If the surface of the concrete pour is insulated from the outside temperatures, the heat of hydration will prevent freezing.

The [[American Concrete Institute]] (ACI) definition of cold weather placement, ACI 306,<ref>{{cite web|title=306R-16 Guide to Cold Weather Concreting|url=https://www.concrete.org/store/productdetail.aspx?ItemID=30616|url-status=live|archive-url=https://web.archive.org/web/20170915204757/https://www.concrete.org/store/productdetail.aspx?ItemID=30616|archive-date=15 September 2017}}</ref> is:

* A period when for more than three successive days the average daily air temperature drops below 40&nbsp;°F (~ 4.5&nbsp;°C), and
* Temperature stays below {{convert|50|°F|°C|abbr=on}} for more than one-half of any 24-hour period.

In [[Canada]], where temperatures tend to be much lower during the cold season, the following criteria are used by [[Canadian Standards Agency|CSA]] A23.1:

* When the air temperature is ≤ 5&nbsp;°C, and
* When there is a probability that the temperature may fall below 5&nbsp;°C within 24 hours of placing the concrete.

The minimum strength before exposing concrete to extreme cold is {{convert|500|psi|MPa|abbr=on}}. CSA A 23.1 specified a compressive strength of 7.0&nbsp;MPa to be considered safe for exposure to freezing.