This study investigated the effects on concrete transport properties provided by a commercial hydrophobic admixture called Hycrete, and used the results in a proprietary reactive transport model called STADIUM. This model, incorporated into Unified Facilities Guide Specification 03 31 29 (Marine Concrete), supports a performance-based approach for estimating the service life of a concrete-based system in a marine environment. The researchers developed specific laboratory modules to characterize the moisture and chloride transport properties of mixtures with and without Hycrete to serve as input parameters for estimating the service life of structures. Physical tests were performed on three different concrete mixtures, each prepared with 0 (reference case), 1, and 2 gal/yd3 of admixture. One test series indicated that Hycrete has no effect on compressive strength and total pore volume or, in most cases, diffusion coefficients. Moisture-transport tests showed that the admixture mildly reduced drying rates and clearly reduced water-absorption rates. With the data implemented in STADIUM it was possible to reproduce the measured chloride profiles in samples exposed to wetting/drying cycles in sodium chloride solutions. The modeling results showed that a mixture prepared with Hycrete and exposed to exposure cycles exhibits a lower chloride-ingress rate, reasonably replicating the experimental observations.