Field problems: Usually problems occur during the fall of the year, when unexpected cold nights occurs. Air temperatures, subgrade temperatures, concrete stockpile temperatures, and of course fresh concrete temperatures all drop. The cold subgrade temperatures coupled with possible variations in slab thickness create a non-uniform/spotty set. Concrete in contact with this cold subgrade sets slower because the subgrade is drawing the warmth away from the concrete. This lowers the concrete temperature, slowing the cement hydration. At the surface, which is exposed to the sun and wind we can see both warming effects and drying actions…both leading to lower relative humidity, which is not necessarily good and can create problems.
In cold weather, and because of the slower set, concrete bleeds for a longer period of time. Bleed water continues to travel up from the slower setting, plastic, lower portion of the slab as the top surface may be setting or drying. If the surface is crusting over, due to the drying action, it may appear to be ready for finishing, but in reality the concrete is still bleeding. Water gets trapped just under the sealed surface possibly leading to surface defects such as blisters and delamination
Operating a power float over a crusted surface can cause the “body” of the slab to “roll”. This has sometimes been referred to as a “jello-like” slab. This can result in a wavy, sometimes cracked surface. Further, if the finisher waits until the underlying concrete sets…it may be too late to produce an acceptable smooth, hard, durable surface.
Regarding strength and durability: Concrete needs to be protected from freezing temperatures until it has reached 500 psi before being exposed to its’ first freeze thaw cycle, which is a minimum time of the first night…but more likely about 2 days after placement at about 50 degrees. This is said to be enough strength for the concrete to withstand one freeze cycle.
A note about freeze-thaw: Here in the northern portion of Alabama as well as in central and north Georgia, it is not unusual to have several cycles of freezing and thawing throughout the winter months. We see the overnight temperatures dip into the teens or low 20’s followed by warming trends in the upper 30’s and 40’s during the day. This constitutes one freeze-thaw cycle. When the water in concrete freezes, it expands by about 9%. As the water/ice expands it creates internal pressures within the concrete that can, and usually will, exceed the developed tensile strength of the concrete. Multiple freeze-thaw cycles can result in internal cracking and disruption or failure of the bond between the cement paste and aggregates. Eventually, the result is concrete cracking, scaling, and crumbling.