Discussion: Neutron radiography: an excellent method of measuring water penetration and moisture distribution in cementitious materials

Neutron radiography is an excellent method for determining water penetration or moisture gradients in cementitious materials, since water attenuates thermal neutrons more strongly than the other components (cement, aggregate, etc.), while the opposite is the case for X-rays or gamma radiation. The image of the cross-section of the irradiated area of the sample is transferred indirectly to a photographic film through the β- decay of a dysprosium foil which becomes the primary target of transmitted neutrons. Moisture gradients can then be read from the film by a densiometer or studied in detail by an image processing system. Examples given are the detection of the water front during the capillary suction of water by a cement mortar, a dried concrete core drilled out of an old structure and a sealed cured concrete cast in the laboratory, and a subsequent image analysis of the mortar samples.