Question: How do I resolve the following contradictory published recommendations on protecting fresh concrete against early freezing?

Your reprint collection, Curing Concrete: Methods and Materials, says on page 11, "Protection should not be discontinued before a minimum compressive strength of 1500 psi is reached; it is assumed that by this time capillary pores in the concrete will have emptied sufficiently to avoid damage if the concrete freezes."

In three other places I find similar explanations, but the value given is 500 psi. These three references are:

  • ACI 306R-78, "Cold Weather Concreting," Section 1.10.1
  • Concrete Construction, "What To Do With Concrete in Cold Weather," September 1984, page 789
  • Concrete Construction, Problem Clinic, "How Accelerators Protect Against Freezing," April 1985, page 372

Which recommendation is right?

Answer: The recommendation accepted today is 500 psi. The article in the reprint collection that gave 1500 psi was originally published in the September 1984 issue of Concrete Construction, page 270.

This article showed data originally published in the article by H. J. Gilkey, "Curing Structural Concrete," Journal of the American Concrete Institute, May 1952, page 711. In that article the author said "It is assumed that protection against freezing may be discontinued when the concrete has attained a compressive strength of 1500 psi.

By this time the capillaries are believed to be sufficiently emptied to obviate freezing damage."By 1956, however, ACI 60456, "Recommended Practice on Winter Concreting," was setting this value at 500 psi, the same value still given in the three later references you cite. ACI 306R-78, Section 1.10.1 now reads "Prevent damage to concrete from freezing at early ages. The degree of saturation of newly placed concrete, which has no access to external sources of water, will be reduced as the concrete hardens and water is combined in the hydration process. Under such conditions, the time at which the degree of saturation becomes reduced below that level which would cause damage by freezing corresponds roughly with the time at which the concrete attains a compressive strength of 500 psi. At temperatures of 50º F most well proportioned concrete will reach this strength during the second day."