J.P. FORTH and J.J. BROOKS
University of Leeds
The long-term irreversible moisture expansion in clay brickwork is thought to be the result of chemical reactions between adsorbed water and the amorphous silica, y-aiumina and the glass phase of the clay unit. A further mechanism has now been identified, which in some cases may be far more dominant, causing an “enlarged” vertical moisture expansion of free-standing (not subjected to external load) clay brick masonry. The enlarged expansion, which is greater than the expansion of the unbonded clay brick, would appear to be caused by cryptoflorescence.
The results of strain readings and chemical and microstructural analysis from a preliminary experimental investigation indicate that the enlarged expansion occurs just below the surface of the brick at the interface between the brick and mortar bed joint of the masonry. The mechanism leading to cryptoflorescence is complicated, but it stems from the concentration of soluble salts crystallising in the pores below the surface of the brick and creating internal pore pressure. There appear to be several influencing factors, such as type of brick (material constitution, water absorption, pore size), storage or curing conditions of the masonry and its volume/surface ratio.