engleski

A contribution to durability of masonry structures

The paper deals with durability of materials in wall structures and masonry with respect to freeze and thaw cycles. It also examines the effect of the selected brick firing regime on the resistance of bricks to freeze and thaw cycles. Results of the examination lead to the conclusion that the longer a brick is fired in the kiln at a maximum achieved temperature, the better is its resistance to freeze and thaw cycles. Another conclusion is that the brickmaking method and its firing regime also affect the pore system developed and that the pore system will be more similar in bricks made using the same method. The following parameters were determined based on 16 batches of bricks: compressive strength of bricks before and after exposure to freeze and thaw cycles and ratios of measured values; saturation coefficient; specific surface area determined by applying the BET method; portion of pores of particular sizes and total portion of pores; total pore volume and average pore radius measured by a mercury porosimeter; total portion of pores measured based on hydrostatic weighing; Maage’s index; initial absorption coefficient and water absorption. In addition, water absorption and desorption of bricks was monitored through a 24-hour period. Based on said parameters, the possibility of their use for the purposes of classifying bricks as resistant or non-resistant to freeze and thaw cycles was mathematically calculated. In addition, a mathematical model describing the ratio of compressive strength values of bricks, measured before and after exposure to freeze and thaw cycles, was also sought. It was concluded that water absorption and the water desorption coefficient of bricks, measured for a period of 180-360 minutes, are excellent classifiers for classification of bricks as resistant or non-resistant. Nanotomography was performed on selected bricks to gain an insight into the portion of open and closed pores in bricks. It was determined that bricks with a higher portion of open pores relative to the total portion of pores have a higher ratio of compressive strength values measured after and before the freezing cycle and are, therefore, more resistant to freeze and thaw cycles. What that indicates is that, in addition to pore distribution, the type of pore also plays a significant role in the resistance of bricks to freeze and thaw cycles. Furthermore, the paper examined the effect of freeze and thaw cycles on different types of mortars: lime; lime-based; cementitious; and cementitious mortar with added air-entraining agent. Ratio of the volume of components in said types of mortars was varied for each individual group. The highest resistance, in terms of compressive strength ratio of values measured before and after the freeze and thaw cycle, was measured in cementitious mortars with added air-entraining agent. They were followed by cementitious mortars, while the lime and lime-based mortar samples decomposed during exposure to freeze and thaw cycles, thus making their resistance impossible to determine. Thermal characteristics of the selected cementitious mortar and of two batches of bricks were determined before and after exposure to freeze and thaw cycles. It was observed that freeze and thaw cycles have a negative effect on thermal conductivity and that it increases after the samples are exposed to such processes. This means that both the mechanical and thermal insulation characteristics decrease after the materials are exposed to freeze and thaw cycles. Masonry was constructed by using selected bricks and mortar and, through tests, its initial shear strength was measured before and after exposure to the freeze cycle. In addition, the heat transfer coefficient was calculated before and after the freeze cycle, after which a comparison of these two values was made and their mutual ratio determined. The conclusion was that freeze and thaw cycles diminish both the mechanical and thermal characteristics of masonry, which, in turn, leads to greater energy consumption and higher heating costs in buildings, as well as higher annual CO2 emissions. This points to the need for a faster and more economical method of evaluating the resistance of bricks to freeze and thaw cycles, because masonry structures are, for the most part, constructed from bricks. Based on the research conducted for the purpose of this paper, the recommended method is water absorption testing. Water absorption testing is a simple method which could be used by manufacturers of clay masonry units within their production quality control in the factory to verify the resistance of bricks to freeze and thaw cycles.

masonry structures; brick; mortar; durability; resistance to freeze and thaw cycles; water absorption parameters; porosimetry; specific surface area; thermal properties

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