Cyclic freezing and thawing tests on structural clay materials are reported. These results and those of recent studies on the freezing of water in consolidated porous materials and the theories of frost damage are used to discuss the mechanisms of pressure generation and relief mechanism. It is concluded that the major mechanisms of stress generation and relief during the freezing of water in porous materials may be different for small or large cooling rates, and that it is probable that freezing and thawing tests can be accelerated, although it should be done with great caution.