This paper reports on silicone-based water repellent systems which were tested for chloride and carbon dioxide diffusion, accelerated carbonation, penetration depth, effect on substrate pore structure, and moisture loss from the concrete. Four different silanes were tested with a pure monomeric silane showing the best resistance to diffusion of chloride. Penetration depths were low: 1-2 millimeters, carbonation rate, carbon dioxide diffusion, and moisture evaporation were not significantly modified by any of the silane treatments. For a second paper on this topic, see <i>AATA </i><b>25-1125</b>. -- AATA