Detailed Modelling of the Masonry Unit-Mortar Interface Using Hygrothermal Simulation

Abstract

Hygrothermal models are important tool for assessing the risk of moisture-related decay mechanisms in historic masonry structures. However, there are significant uncertainties in the process related to material properties, boundary conditions and quality of construction that effect confidence in the model's predictions compared to measured values. This paper examines one potential source of uncertainty; the imperfect nature of mortar joints in masonry walls, exemplified by such things as open joints, hairline cracks and imperfect bonds at the interface between mortar and unit. These are rarely considered in hygrothermal modelling in detail, where perfect interfaces are typically inferred. The premise is that at this interface, liquid transport behaviour is more similar to that of a fracture than that of a bundle of capillaries. These fractures of varying heights (or aperture) can affect transport into and out of the plane of the wall (perpendicular plane) and impede the liquid transport between mortar and the masonry unit (in-plane). This could lead to the "effective" moisture transport being different than what would be predicted using measured bulk material properties. A more detailed method for modelling this interface, borrowing techniques from the field of geohydrology is presented which demonstrates the effect that detailed modelling of the mortar joint has on moisture transport in masonry. A brick wall with cement mortar is studied. A twodimensional hygrothermal model was created to demonstrate the effect of increased liquid conductivity into the wall cause by fractures.