VAPOR PERMEABILITY AS A CRITERION FOR CHOOSING A PLASTER MIXTURE FOR LIMESTONE-SHELL WALLS

The reasons for the degradation of load-bearing structures and the collapse of buildings made of limestone-shell rock are considered, the main of which is the moisture content of the material due to damage to the plaster layer. Repair and restoration work on the facades of historic buildings should be carried out using special plaster compositions whose properties are consistent with the main material of the structures. Imported general-purpose materials do not take into account the characteristics of limestone materials. An important aspect of creating domestic plaster compositions for repairing the walls of historic limestone-shell rock buildings is to ensure that the vapor permeability of the plaster and the material of the main wall are consistent; it is the mismatch of materials in terms of vapor permeability that causes the cement-based plaster to delaminate from the limestone surface. For a reasonable design of the composition of a special-purpose plaster mixture, objective information about the vapor permeability of the interacting structural layers is required. Based on the analysis of research and publications, the factors that affect the transfer of water vapor in materials are identified. The method of determining the vapor permeability of shell rock and ashlar-cement plaster mixture is the "dry" cup method with the placement of a desiccant in it. Based on the requirement to establish a steady-state diffusion flow of steam in the test sample and taking into account the saturation of the desiccant, the optimal time for conducting vapor permeability measurements was determined in terms of the duration of the experiment. The vapor permeability coefficients of shell rock and the developed repair compositions were experimentally determined. The average vapor permeability coefficient of the proposed plaster compositions exceeds the vapor permeability coefficient of shell rock. The calculation and graphical method shows that when using ash and gypsum cement compositions, there is no probability of condensation in the thickness of the limestone wall.