When placing concrete in cold weather, fresh concrete must generally be protected from freezing until it reaches a compressive strength of at least about:

Explanation

Concrete is vulnerable to permanent damage if it freezes before reaching roughly 500 psi, so it must be kept above freezing until that early strength is achieved. 100 psi is too low to resist damage, while 1,500 and 3,000 psi are well beyond the point where freezing is critical.