The properties of plastering mixes in the laboratory and in practice

In a composite masonry-mortar system the compressive strength is correctly regarded as a critical parameter for assessing the compatibility of the building materials. As a rule the characteristic values of tensile strength and deformation behaviour, which are needed for estimating cracking, correlate well with the compressive strength. However until now the compressive strength of plasters has, for various reasons, been measured under conditions which deviate from those found in practice. It was established in a few quite early trials that these practical conditions led to compressive strengths which were different from those obtained in the laboratory. As part of a wide ranging investigation the compressive strengths and other characteristic mortar values were measured in the laboratory and under practical conditions. The plasters were mixed at the building site with 2 mixer pumps, processed under varying weather conditions, and applied to masonry made of the 4 types of masonry unit normally used in Germany. Investigations when producing the mortar showed that, contrary to the assumptions which have often been expressed, fresh mortar mixed in the laboratory mixer as specified in, the standard does not always have more air voids than mortar mixed on site with mixer pumps. The type of masonry involved was found to have no clear influence on the compressive strength of the layers of plaster which hardened on the different types of masonry. However, it was apparent that the weather conditions during the production and hardening of the plaster had a significant influence on the strength development. Of the methods used for testing compressive strength the partial area load method proved more suitable than testing the surface hardness with the Schmitt hammer The trials confirmed that even with large free contraction movements there were no harmful cracks in plastering mixes where the strengths had been adapted specifically to suit the surface to be plastered.