Monitoring moisture movements in building materials using X-ray attenuation: Influence of beam-hardening of polychromatic X-ray photon beams

X-ray attenuation measurements are commonly used as a non-destructive method to monitor internal concentration changes of moisture (i.e., moisture content) and other chemical compounds in porous building materials. The technique provides direct measurements of moisture content changes through analysis with a composite model consisting of a dry porous material and a thickness of water equivalent to the moisture content of the material. The current formulation of this composite model relies on certain assumptions, including a monochromatic X-ray photon beam source (i.e., X-ray photons of a single, consistent energy) and that interactions between the X-ray photons and the materials (water and porous material) are independent. However, X-ray sources typically used by researchers in this field of study produce X-ray photon beams over a spectrum of energy levels, or polychromatic X-ray photons. Implications of this inconsistency are introduced and discussed. This paper presents both an overview of fundamental descriptions of the X-ray attenuation measurement technique and results from a parametric experimental study of various porous construction materials, including calcium silicate board, aerated autoclaved concrete, clay brick, cementitious materials, and wood. Results from the parametric investigation indicate the attenuation coefficient of water is dependent on the type and thickness of the porous material.

► The controlling physics of the X-ray attenuation technique are reviewed.
► A composite model is used to assess moisture content changes in porous materials.
► Typical model assumptions are reviewed and impacts are investigated.
► Results indicate porous material thickness affects water’s attenuation coefficient.
► Results may be off by up to 25% if wrong attenuation coefficient of water is used.