## 5 Discussion
### IRT as a Tool to Detect Microstructural Changes  
The capillary water uptake can be followed by IRT since the variation of emittance in wet porous materials allows the movement of water to be visualised on stone sur  
Figure 10: Relationship between the capillary coefficient (C), the cooling rate index (CRI), and the MIP porosity for the Thermal Fatigue test (TF).  
faces [36; 81]. Regarding the fresh stones, the thermographs showed a correlation between porosity and water absorption by capillarity [37; 38], with a faster rise in the chalk, whilst slower in the sandstone and the granites, with a stabilisation of the wet fringe at the middle of the samples.  
The capillary water uptake test, and more precisely the penetration index, determined a bottom wet part, in which the water circulates, and an upper dry zone, separated by its observable moist surface (wet fringe). The IRT monitoring made it possible to define these two visible zones, with different migration interpretations for porous and fissural rocks. During the first minutes, the water-rock interaction reached a very fast temperature equilibrium at the bottom part of the sample. From that, the void network controlled the wet fringe movement (Figure 11).  
For sedimentary rocks (Figure 6e,j,o,t), the water contained in the pore system rose and occupied the pore space uniformly so that the wet fringe migrated according to the water filling, delimited by the upper part of the capillary zone (Figure 11a). A better arrangement and connection of the grains favour the kinetics of internal migration of the fringe and the cooling by water. When the wet fringe reached the top, the sample compensated for the loss of water through evaporation.  
Fissural rocks presented a heterogeneous pattern. The profiles in Figure 7e,j,o,t and the visual observation revealed similar evolution for the wet zone and wet fringe. The interconnected cracks of the porous network became saturated from the first minutes. That included the wet zone and the capillary zone. The water saturation continued with the occupation of the transverse cracks, showing a heterogeneous migration of the wet fringe through the first 1.5 cm (Figure 11b). Monitoring the rise in water by capillary using the IRT  
Figure 11: Fluid circulation in porous media. (**a**) Granular rock. (**b**) Polycrystalline rock.  
detects variations in the microstructure (cooling/variation of the wet fringe) but without quantification.  
Infrared thermography also allowed us to quantify the cooling rate index for the upper part of the sample, by simple exchange with the environment. This parameter is well correlated to porosity for stones with a void volume over 10%, although with approximate values that allow us only to divide into groups (10%, 15%, 20%) [(41; 42; 82)]. The studied rocks did not follow these statements. For fissural rocks and special microporosity rocks, such as chalk, the surface roughness and the microcrack connectivity will play a more important role than the porosity itself. The CRI of granite with 1% porosity and sandstone with 5% were similar, while the CRI of high microporous chalk was very low and the CRI of a weathered granite with the same porosity as the sandstone was three times.  
The CRI varied during the heating cycles, although not in the same sense that the bulk microporosity. However, the changes on the network observed by SEM explained completely the cooling rate behaviour. That indicated that the CRI was not a probe of the small development of microcracking within the rock but a good indicator of the damage to the outer layers, from which the temperature changes started.