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Whatever its size and purpose, every concrete structure requires close temperature monitoring to verify proper strength development. For mass concrete structures, temperature differentials and actual temperatures need to be closely tracked, as a large gradient between a warmer interior and cooler surface can lower a structure’s durability, shorten its service life, or even lead to cracks, as well too high a temperature as well as too low a temperature can impact proper strength gain and durability.
In the simplest terms, mass concrete is any concrete structure large enough to require monitoring of internal temperature differentials. There are no specific dimensions or measurements. Foundations, mat slabs, large bridge piers, radiation containment structures and dams are all examples.
After concrete is poured, it hardens or “cures” which is a chemical reaction that generates heat. It can take weeks for a concrete pour to finish curing an during that time heat is being generated. The internal temperature differentials of mass concrete can make it more susceptible to improper curing.
For a mass concrete pour to fully cure and reach its maximum strength, the ambient temperature must be between 50-60 °F (10-16 °C) when the pour is performed. If is too cold, the curing chemical reactions will slow down. Under extreme conditions, they may even stop. If it is too warm during the pour, the concrete will develop less strength in the later stages of curing. This will make it less durable.
Concrete generates heat as it cures. In mass concrete, this leads to temperature differentials as the core of the structure dissipates heat more slowly than the surface.
This, in turn, creates tensile stresses and strains within the concrete structure as the warmer concrete expands and the cooler concrete contracts. These stresses can lead to thermal cracking. They are worsened by cold weather, which quickly cools the surface of the concrete structure but not the core.
ACI structural concrete specifications mandate that a concrete structure’s temperature differential does not exceed 35 °F (19 °C) during curing. In Canada, these differential temperatures are related to thickness of concrete and height to length ratio. The thicker the concrete the lower the temperature differential
In most situations, this approach is conservative, but can be an overestimation of the allowable temperature differential.
Temperature differentials in mass concrete curing, if not monitored and managed, can lead to serious problems like cracking, lower service life of the structure, noncompliance and project delays.
Thermocouples or data loggers are the industry standard for monitoring temperature differentiates on-site. However, newer technologies offer superior efficiency, accuracy, and reliability for contractors and engineers.
Specifically, in-situ sensors can continuously measure a concrete structure’s temperature, strength and maturity and then relay that information to a construction management platform like Job Site Insights. These sensors can also issue automatic alerts when conditions arise that can impair concrete curing, such as freezing weather or a heat wave. The sensors are easy to install and offer precise real-time temperature readings for both the surfaces and centers of mass concrete pours.
When water freezes, it expands. Within curing concrete, this freezing can crack and weaken a concrete structure and even render concrete useless. Pouring concrete in conditions where this may occur is called cold weather concreting.
The ACI defines cold weather concreting as “a period when for more than three successive days the average daily air temperature drops below 40°F (5°C) and stays below 50°F (10°C) for more than one-half of any 24 hour period.”
Cold weather concreting presents unique challenges to building teams, especially when pouring mass concrete structures, which are already more vulnerable to thermal cracking while curing due to their internal temperature differentials. In fact, placing mass concrete be it either in the summer or winter may mean taking actions that are counter intuitive. You may in some circumstances have to insulate the concrete even in the summer of temperature loss is too high at the surface, or cool the inner core of the mass concrete in the winter if the heat generation is too high in the middle.
However, with the right precautions, placing mass concrete—winter or summer—can be performed successfully. These include: