In the past the earthen materials were accounted as the most widely used materials both structurally as well as finishing of traditional and historical buildings in urban and rural areas across the country. However, due to the wear and tear and lack of resistance to humidity and subjected loads, the roof and the walls covered with thatch mortars need to be annually reconstructed and repaired, which today is a serious maintained predicament. Low resistance against the lateral loads and cracking in walls, rapid erosion due to humidity, low-resistance against decay and impacts are major disadvantages of such mortars. Thus, durability of thatch as one of the stable and eco-friendly materials is very important. 6 The present research aims to find the optimal mix design of thatch mortar based on the examining some of the physical and mechanical parameters of thatch mortar built with various additives. The present study at the beginning investigates the optimal percentages of water-soil ratio. In this regard, samples of different combinations of thatch with water at ratios of 10 to 45 percent weight of the soil are prepared and their surface features including colors and cracking of the mixture are assessed. The second phase is to provide the appropriate percent of straw and soil relative to each other that are formulated after interviews with traditional craftsmen, and literature review. To find the appropriate mix percentage of straw and soil (calculation criteria: crack length), several mixtures of thatch are built and eventually the most optimum mix design of normal thatch is chosen to investigate the role of stabilizing additives. The compressive strength tests are conducted based on ASTM C109 standard for each mix design in the laboratory and flexural strength tests are conducted based on the ASTM C78 for each mix design. To investigate the resistance to humidity (weathering resistance), the water spray test is also conducted for samples of 1*10*10 dimensions based on Craterre institute in France standards and then subjected to the centralized pressure of 100 ml water. The time at which a complete cavity is created in the specimen and the water can flow through the pit, is considered as completion time of the experiment. All phases including preparation, construction and testing are conducted based on standards in the soil mechanic laboratory at University of Yazd, Yazd, Iran. The results show that the composition of two acrylic and sodium silicate additives in thatch mortar during the drying period will further reduce the contraction and cracking rather than the thatch separate composition. Also, the composition of sodium silicate additive in normal thatch results in an increase in mortar density to 2.03 g/cm3. Investigation of thatch resistance to the compression and bending as well as various humidity-induced erosive behaviors suggested that the optimum composition of thatch mortar is obtained using sodium silicate additive that is resistant to various loadings and humidity. It has constituent elements of soil, water, and sodium silicate. The most resistant composition for thatch mortar construction includes the following mix percentages: Soil, straw (80 % straw by volume of soil) + Water (42 % water by volume of soil) + sodium silicate (100 % sodium silicate by volume of water). The results indicate that the thatch ratios of the soil, 80 % straw, 42 % Water and 100 % sodium silicate by volume of water, considerably increase mortar structural as well as humidity resistance . Similarly, the durability of thatch additive effect, consist of 100 % sodium silicate, against the water pressure was measured considerably above previous samples. Thatch implementation as a covering and finishing material in facades is currently very prevalent. However, if it is considered as a structural or finishing construction mortar it seriously requires water proofing and strengthening, proper solutions should be provided for this kind of covering. with increasing acrylic content, more surface adhesion is created between grains.
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