Is it true that concrete will no longer freeze after it reaches 500 psi and, if so, what is the best method for field-testing the concrete to determine when it reaches 500 psi?
It is not true that concrete will not freeze after it reaches a strength of 500 psi. However, it is a rule of thumb that concrete will not be destroyed by a few cycles of freezing and thawing after it has attained about 500-psi strength. In other words, 500 psi is the minimum strength required to resist the first exposure to freezing. In most cases it is not really practical to determine that concrete has attained this minimum "safe" strength. Most engineers would be more inclined to estimate the rate of strength gain on the basis of the known low-temperature behavior of the concrete mix being used. Then if there is any likelihood whatever that the concrete might not attain 500 psi before it freezes, they would make provision for heating or insulating it enough to prevent freezing. Actually, the recommended method is to not rely at all on either tests or estimates but to follow the practices for protecting concrete against freezing in accordance with ACI 306, "Recommended Practice for Cold Weather Concreting." That standard provides tables which show the amount of protection required for various concrete sections, types of cement and degree of load as well as the length of time the protection should be supplied.