Durability of renderings of historical buildings

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Insinööritieteiden korkeakoulu | Master's thesis
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Structural Engineering
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Rakenne- ja rakennustuotantotekniikan koulutusohjelma
Historical, cultural and architectural values of historical buildings and structures signify the importance of their conservation. Several deterioration mechanisms associated with different environmental or weather conditions are responsible for damaging rendering mortars of historical buildings. These deterioration mechanisms may act individually or jointly, making it important to understand each deterioration mechanism and their coupled effects. This research investigated the deterioration mechanisms of renderings in a test wall and conducted a condition assessment based on laboratory and in-situ tests and observations. A test wall was built to explore how different rendering mortars behave in the Nordic climate and to suggest design-suitable repair mortars for historical buildings. Several studies have determined mortar deterioration mechanisms of the specimens prepared in the laboratory but in this research, the test specimens were drilled out from a structure that has been exposed to outdoor environment and exposure conditions for a period of over seven years. Several in-situ tests and laboratory experiments were conducted to analyse the durability properties of the rendering mortars. The durability properties of the test mortars were investigated based on visual inspection, microscopic camera analysis, hammering test, ultrasonic testing, relative humidity and temperature assessment, microscopy analysis of thin sections, carbonation test, water absorption and desorption test, tensile strength test, protective pore test, vapour diffusion test and chloride resistance test. WUFI Pro one-dimensional heat and moisture calculation software was adopted to model the test wall and make comparisons of the simulation results with field measurements. The choice of binding materials, correct water-binder ratios and working methods were among the factors influencing the durability properties of rendering mortars. Moisture played the most significant role in the deterioration of the structure. Moisture acted as a transportation medium carrying unwanted materials into the structure responsible for other deterioration mechanisms. The results obtained from the WUFI Pro model simulation were consistent with the field measurements but still further studies are needed to assure its use for long-term prediction of the moisture and heat distribution in rendering mortars. The results suggested that modern cement mortars were in better condition than the traditional lime mortars in the test wall renderings. With correct mix proportions and working methods historical building renderings made of traditional lime mortars can be repaired with modern lime cement mortars to increase their durability and service life. The results obtained can be applied to manage the deterioration mechanisms of rendering mortars exposed to the Nordic climate.
Puttonen, Jari
Thesis advisor
Sistonen, Esko
lime rendering, mortar durability, historic mortars, Suomenlinna test wall
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