On purely theoretical grounds shouldn't the compressive strength of a concrete increase as it gets colder?
True. But over the normal range of ambient temperatures the effect has no practical value. For certain industrial applications (for example for tanks for the storage of liquefied natural gas) tests have shown that the compressive strength of a normal moist sand-and-gravel concrete will increase from 5000 psi at 75 degrees to 17,000 psi at minus 150 degrees F.