When Concrete Freezes. The concrete isn't strong enough to take the stress of the moisture expanding inside of it. If the moisture freezes inside the concrete, it will expand causing damage.
Freezing of freshly mixed concrete can be very harmful. Staying off the concrete for the recommended amount of time ensures that it has enough time to cure, preventing future damage and making your investment worth the money. Adding water will keep the concrete saturated so that freezing will damage it even after it reaches 500 psi compressive strength.