Q: Is there any way to determine from a visual examination if my concrete foundation, which is curing now at temperatures ranging from 15º to 30ºF, has been damaged by these freezing temperatures?
Formed concrete surfaces sometimes show ice-crystal imprints, indicating that fresh concrete was frozen. The absence of ice-crystal imprints is one indication that concrete hasn't been damaged by freezing.
Since corners are most vulnerable to freezing damage (because two surfaces are exposed), you could also chip off a small piece of the corner and examine the fractured surface. Imprints are especially likely to occur in sockets where coarse-aggregate particles have pulled out. A two-power hand lens is useful in checking for ice-crystal imprints in either formed surfaces or aggregate sockets.
Wetting the concrete can also make the imprints more readily visible. If you need positive proof of strength loss caused by freezing, you would need to have a testing laboratory take cores and determine compressive strength. Examination of the core surfaces with a hand lens also might yield further evidence of ice-crystal imprints.
Do you know how long the concrete cured before temperatures fell into the freezing range?
A.: If concrete reaches a compressive strength of 500 psi before it freezes, usually not enough water is left in the capillary pores to cause destructive pressures. Water expands about 9% when it freezes, so if capillary pores are only 90% full due to hydration of some of the cement, the 9% expansion can be accommodated within the pores.
Bruce A. Suprenant, "Protecting Fresh Concrete from Freezing Weather," Concrete Construction
In commenting on a wall exposed to below-freezing temperatures, you say, "The absence of ice-crystal imprints is one indication that concrete hasn't been damaged by freezing." However, the presence of ice-crystal imprints is no proof that the concrete has been damaged; all it proves is that the concrete has been frozen while not very hard.
- Bryant Mather Department of the Army Waterways Experiment Station Corps of Engineers Vicksburg, Miss.