4:00 PM - 4:15 PM
[HTT22-03] Thermal response of building stones contaminated with salts
Keywords:Infrared Thermography, Salt weathering, NDT, Limestone
Infrared Thermography (IRT) is a well-known and non-destructive technique (NDT) used in a passive way to detect salt presence linked to moisture and their resulting decay forms. To understand the stone behavior under salt crystallization and to predict its evolution, the active IRT was tested. The main objective was to trial this technique in order to obtain more information such as the type of salt, its precise location and its concentration.
Three building stones were studied: two natural limestones (Bajocian (BJ) and Sinemurian (SN)) and a reconstituted stone (RS) made with debris of these limestones. These three materials were used simultaneously in buildings of South of Wallony (Belgium) as their color and origin are close to each other.
Forty-five disk-shaped samples of each stone were submitted to immersion/drying tests in water (used as reference) and in salt solutions of Na2SO4, NaCl and MgSO4 at 7%, 14% and 26% in weight during 20 cycles. The weight of the samples and the surface color were recorded after each cycle. After 1, 5, 10 and 15 cycles, 3 samples were taken out for active IRT measurements.
Active IRT was conducted by means of a FLIR infrared thermography camera SC655 with two flash lights placed at 45° of the sample, symmetrically along the axis of the camera, producing a light of 4800 J for 5 ms. The flux of photons emitted by the flash excited the sample leading to an increase of its temperature, and the corresponding thermosignal was recorded by the camera. The thermosignal (TS) depends on i) the mineralogical composition and the physical parameters of the material: density, porosity and pore structure, specific heat, emissivity, conductivity and diffusivity, ii) the surface properties (roughness, color). Thermosignal (TS) images were recorded and thermal curves over time were extracted from them.
Macroscopic observations showed that all the stones developed efflorescences on their surface with different morphology depending on the type of salts. Only BJ and RS, the most porous stones, showed a notable sanding starting from cycle 10.
Colorimetry showed that Na2SO4 efflorescences tended to enlighten the different stones whereas NaCl and MgSO4 darkened them. Weight increase of the samples was different depending on the stones and the salts properties. But the overall tendency revealed that the most concentrated the solution was, the most the weight increased.
To avoid the influence of color in the IRT response, the samples were black painted before the IRT analysis. For the most porous stones (BJ, RS), the TS depended on the salt concentration and the number of cycles. The Na2SO4 contaminated samples had the highest TS compared to MgSO4 and NaCl that were similar and close to the initial TS. Especially for the Na2SO4, the TS increased with the concentration. It also increased with cycles until the beginning of sanding and cracking (cycle10). After that, TS decreased and stabilized to the initial value. On the least porous stone (SN), no change of the TS was observed in relation to the salt type, the concentration or the cycle number.
The active IRT seemed to be a non-destructive technique adapted to identify and quantify the salts. Though, it is necessary to use it during the first’s states of salt accumulation. Further tests will used longer heating of the samples thanks to halogen or ceramic lamps in order to have a deeper thermal response of the sample.