Corrosion of re-bar within reinforced concrete is a major problem in countries where salt is applied to roads for de-icing. Concrete structures are periodically inspected in order to monitor possible damage caused by chloride induced corrosion of the reinforcement. However, bridge decks covered with asphalt pavements are not accessible for visual inspection and probing is limited. As a result, the planning of rehabilitation of bridge decks is usually based on a small number of probes. Consequently, the condition of bridge decks can only be assessed with low certainty. Therefore, a method enabling to study the conditions of concrete bridge decks covered by asphalt pavements is desirable. This paper describes a laboratory experiment aiming at the investigation of the effects of moisture and chloride content on the amplitudes of radar signals. It can be shown that both, moisture and chloride content have a measurable influence on signal amplitudes. This may enable the future use of ground penetrating radar for the planning of probing campaigns or for the extrapolation of results obtained at single probing points.