Quantifying sorptivity using the contact sponge method: an improved calculation method validated by classical capillary rise experiments and neutron radiography

Abstract Non-destructive techniques such as the contact sponge method (CSM) and the Karsten tube have been developed to assess the water absorption properties of building materials during field measurements. In contrast, laboratory investigations typically rely on capillary rise tests (CR) on centimetric specimens imposing a one-dimensional water flow. Although the results obtained from these methods are qualitatively comparable, a robust quantitative correlation is still lacking. An analytical expression has previously been proposed to estimate the sorptivity from measurements with non-destructive techniques, based on the assumption that the water penetration depth equals the lateral spreading of water on the material surface. In this study, we investigate the water absorption processes induced by the contact sponge method, by combining neutron radiography, which allows direct visualization of water penetration, with conventional capillary rise tests based on gravimetric measurements. Sandstones and limestones with porosities ranging from 10% to 30% were examined. Neutron imaging reveals that the standard assumption of equal penetration depth and lateral spreading is not valid. These observations were used to propose an improved analytical expression that accounts for the three-dimensional water flow induced by the non-destructive method. This significantly improves the quantitative agreement between sorptivity values obtained from CSM and CR tests. This revised analytical framework results in an adaptation of the CSM field-testing protocol to ensure reliable sorptivity assessment.