Wet front penetration with unsteady state wicking in mortar studied by Magnetic Resonance Imaging (MRI)

The movement of water or water carrying aggressive ions is one of the leading causes of deterioration of concrete structures worldwide. Moisture profiles during unsteady state wicking were determined by one dimensional magnetic resonance imaging of 10 cm mortar specimens. Bulk free induction decay and T₂ ^* lifetime mapping results show a bi-exponential behavior of the MR signal lifetime, T₂ ^*, in all samples, indicating at least two different water populations. From T₂ ^* mapping, the short T₂ ^* lifetime, assigned to interlayer water (water between C–S–H layers), and its associated amplitude are constant along the sample. The long T₂ ^* lifetime and its associated amplitude, related to water in the pore space (micro and macropores), change with local moisture content. To the best of our knowledge, this is the first time that interlayer water has been spatially resolved in MRI of cement-based materials. Gravimetric sorptivity measurements show two regimes of water absorption in the four samples studied. In the first regime, a capillary transport mechanism is dominant in filling the macropores. In the second regime, the interaction between water and solid matrix is postulated to cause swelling that results in a reduction of the water absorption rate. After swelling, diffusion controls pore filling. Water front penetration behavior observed with magnetic resonance imaging, and bulk free induction decay measurements showed changes in behavior that can be linked to sorptivity changes from an initial to a secondary regime. Inverse modelling was conducted to extract the transport properties, using the Hydrus program with the one-dimensional moisture content profiles. Modelling results showed a decrease in saturated hydraulic conductivity with water exposure time.