Can we detect internal moisture content in hardened concrete with an infrared camera?

Abstract

The main goal of this paper was to assess the ability of infrared thermography to detect non-superficial moisture content in concrete elements. For this purpose, a commercial camera (Flir i5) was used and firstly its capacities and technical limitations were analyzed by comparing it with other commonly commercialized. Secondly, the experiments were undertaken using a concrete specimen (water/cement=0.5) and water under a pressure of 500 kPa was injected over 72 hours into one side of the specimen. The specimen was then left at room temperature (20ºC) for 24 hours, so the surface moisture disappeared. Prior to the images acquisition, the specimen surface was heated by a lamp located at a distance of 0.68 m from the central point of the specimen. In this way, two areas (dry and injected) to be registered would receive the same energy, since they were equidistant from the heating source. The first record was acquired before heating the specimen and then, the heating process was interrupted every 10 minutes to perform the infrared images acquisitions. Finally, by means of the destructive test, the average water penetration depth was assessed to be 3 cm. The infrared images acquired before heating the specimen showed a small temperature variation as a result of the presence of water, 16,3 ºC in the dry area and 15,8 ºC in the injected area. However, after the heating, due to the greater specific heat of water, the injected part achieved a temperature 4º C lower than the symmetrical dry area. This result was very interesting because, in the initial moment (before heating) the surface specimen was dry, but there was a wet area inside of the specimen that was not visually identifiable. Nevertheless, by means of infrared imaging an indicator was obtained that allowed to detect the presence of this internal moisture content.