Durability testing has been for many decades an important element in concrete research and specifications. Concrete structures are expected to have a service life of at least several decades and they are very difficult and costly to repair if degradation occurs. The penetration tests can be classified according to the type of fluid involved as well as the nature of the penetrating medium, whether chemically reactive with the cement matrix or whether largely inert. When nonreactive fluids are used, the results of the penetration test can be interpreted on the basis of the pore structure by considering models of physical interactions between the penetrating substances and the pores. The most common gases used for testing for penetration into concrete are air, oxygen, and carbon dioxide. The penetration of chlorides into the concrete can take place by a variety of mechanisms, the most important being convection and diffusion.