Performance assessment of custom-made replications of an original historic render - A study of application influences

This performance study focuses on a custom mortar designed and produced as a material and technological copy of an original. The study was carried out in order to determine mortar properties and evaluate influences on practical application. Two mixing water amounts and four different floating compaction levels were compared. The influence of a limewash coating was also studied. The assessed performance properties were related to water transport mechanisms and mechanical strength. The properties were determined on standard specimens cast in steel moulds according to the EN 1015 standard. In order to also be able to take into account the application effects, render panels 60 cm x 60 cm were prepared. The mortar was made of natural hydraulic lime slaked to powder mixed with locally dug sand in 1:1 vol ratio. The specimens were cured in laboratory conditions with regular weekly wetting, and were tested at the age of 120 days. The assessed physical properties were open porosity, 24-hour absorption, capillary absorption, unidirectional drying, water vapour permeability, compressive and flexural strength. In addition, carbonation degree was quantified by thermal analysis. XRD QPA was used to confirm the carbonation. Unhydrated hydraulic compounds were not identified by the XRD; only amorphous content was quantified. SEM observation confirmed the presence of hydraulic compounds. All tests were carried out on the standardised prisms as well as on the non-standard specimens cut from the render panels. This allowed for determination of not only the average properties but also their approximate ranges due to the various application factors. The determined properties were proportionally comparable with other NHL mortar mixes. This confirmed that the mortar designed as a material and technological copy is generally applicable for restoration works, and the results provided the basic data for the assessment of its suitability. The experiments showed that the most crucial influential factor was the initial moisture content which affected all the studied properties. The higher compaction increased the compressive strength and decreased open porosity. The compaction was applied as repeated floating of the surface, and this also had a significant effect on the reduction of plastic shrinkage cracking. The floating alone closed the surface pores and changed its properties in comparison with the rest of the mortar. It strongly affected the drying and permeability properties that were up to 4 to 5 times higher for the panels than that of the standard specimens. Curious behaviour was observed on the limewash coated surfaces. The limewash diminished the closing effect of the floating and relatively increased the drying and permeability. The results obtained on the panels differed from that of those determined on the standard specimens. The differences between the properties of standard and non-standard specimens are discussed. (C) 2019 Elsevier Ltd. All rights reserved.