An assessment of binder leaching in traditional mortars for mass masonry structures

BACKGROUNDIn Scotland alone there are an estimated 446,000 pre 1919 dwellings (totalling 20% of the building stock) and of these the vast majority of these are stone masonry (McKinney, 2007, Urquhart, 2006). Innumerable low rise domestic buildings, historic bridges, viaducts, infrastructural masonry (i.e. retaining walls etc), castles and palaces exist in the UK and across the world. These are built with traditional lime mortars which are widely believed (Ashurst, 2007, Beckman, 1995, Fawcett, 2001, Kent, 2005, McKibbins, 2006) to be susceptible to soluble binder leaching and consequent deterioration. Understanding the rate and severity of deterioration in these materials is extremely important, and is a matter of informed concern about structural safety (Chiesa, 2006) and risks of falling masonry (Halstead, 2005). 1275 incidents of falling materials and debris from buildings were reported by 25 local authorities, over a 2 year period, and of these 80% involved buildings of 100 or more years old (Soanne, 2008). Masonry accounted for 40% of the reports, with the greatest number of issues relating to external walls.The ability of soluble binder (free lime) to leach from the body of the mortars is related to the dampness of the fabric, with saturated, cold conditions being ideal for its propagation. The future likelihood of these conditions in building fabrics is increased by the consequences of global warming (Cassar & Hawkings, 2007). McKibbins (2006) suggested that saturation of porous masonry materials is a major problem in traditional arch bridges, with increased, lime leaching and creation of secondary porosity leading to washout of fines, increased susceptibility to freeze thaw cycling, frost attack and concentration of stress in localised regions. It is evident that this has implications for the structural integrity of bridges but traditional mass masonry walling is equally vulnerable. Both Kent (2005) and Fawcett (2001) support this view and have shown that binder migration in traditional mass masonry structures may potentially lead to wall core voiding, believing that this may have implications for the stability of the structure with the loss of bond potentially occurring between the inner and outer leaves of masonry, and consequent reduced stability.PROGRAMME & METHODOLOGYThe research will investigate the rate and quantity of soluble binder leaching from traditional lime mortars, assessing the material's physical performance. In addition, the laboratory work will be correlated with samples taken from various projects to cross check the effect of soluble binder leaching upon the macro and micro pore structure and surface characteristics of the mortars. All proposed tests will be undertaken initially on sound samples and then on those that have been subject to accelerated deterioration using electrochemical migration and chemical extraction. An assessment of the basic moisture transfer mechanisms will be undertaken using sorptivity tests based on 'sharp front theory' modelling (Hall & Hoff 2002) which is suitable for fragile (deteriorated) samples. Physical tests will be undertaken to determine the tensile and compressive strength of the mortar samples, and thin section petrographic analysis and Environmental Scanning Electron Microscopy will be used to assess the surface characteristics and pore structure of selective, representative materials.The data will be cross correlated with on site samples provided by the industrial partners. The work will lead to the development of predictive models for the assessment of soluble binder leaching in traditional mortars and it affect upon masonry structures.INDUSTRIAL COLLABORATORSThe following will contribute technical expertise and access to construction sites; Historic Scotland; City of Edinburgh Council; National Trust for Scotland; Construction Materials Consultants; Laing Traditional Masonry; Scottish Stone Liaison Group and Jacobs Engineering.