Masonry Magazine October 2002 Page. 22
Cover
The absorption is measured in two ways: 1) submerging the test specimen in room temperature water for a period of 24 hours, and 2) submerging the test specimen in boiling water for five hours. These are known as the 24-hour cold-water absorption and the five-hour boiling water absorption, respectively. These two are used to calculate the saturation coefficient by dividing the 24-hour cold-water absorption by the five-hour boiling. The saturation coefficient is used to help predict durability.
The initial rate of absorption or suction is the rate of how much water a brick draws in during the first minute after contact with water. The suction has a direct bearing on the bond between brick and mortar. It has been shown by test results that when a brick has high suction (over 30 grams/min/30-square inches), a strong, watertight joint may not be achieved. Therefore, high suction brick should be wetted for three to 24 hours prior to laying to reduce the suction and allow the brick's surface to dry.
Very low suction brick should be covered and kept dry on the jobsite. Brick manufacturers can furnish values of IRA and saturation coefficient of the selected units. The material specifier or supplier should inform the mason contractor about the suction of the brick prior to construction.
Suction is perhaps the most important physical property to the mason contractor. Bricklayer productivity is influenced by the suction and the layout of the portion of brickwork being laid. Weather conditions when working also influence the reaction of brick and mortar.
Building codes require that brick with an initial rate of absorption greater than 30-grams/min/30-square inches at the time of laying be wetted. This places added requirements on the contractor. Knowing the brick's initial rate of absorption when preparing the bid will permit the mason contractor to provide a more accurate price.
Compressive strength. The strength of a unit is used to determine durability and also compressive strength of a wall assembly. While most brick is specified on the basis of strength, it is important not to sacrifice properties of durability and bond for higher compressive strengths. Most brick currently produced have strengths ranging from 3,000 psi to over 20,000 psi, averaging around 10,000 psi. Achieving sufficient compressive strength with brick is seldom a problem.
Application. A building must perform the functions for which it is designed. The materials selected for a project must also perform as intended. "The designer must consider all factors that a wall or material must withstand," Bryja notes. "Some of the more important factors include moisture penetration, accommodating anticipated movement of materials, and structural loads. No one standard assembly is suitable for all localities, occupancies or designs; therefore, the designer must evaluate each factor and its relative effect on the selection of a material or assembly."
Moisture penetration. The use of quality materials and workmanship is essential in obtaining a satisfactory degree of water resistance. When water passes through brick masonry walls, it invariably does so through separations or cracks between the brick units and the mortar. It is virtually impossible for significant amounts of water to pass directly through a brick unit. Therefore, brick units that develop a complete bond with mortar offer the best moisture resistance. Brick and mortar properties should be compared in order to provide compatible materials that result in more watertight walls.
Currently, there are no requirements for the degree of water resistance of a wall. The recommended practice is to use a drainage type wall, regardless of the brick selected.
Accommodating material movements. Brick undergoes movement from both temperature variation and moisture expansion. These properties are not critical to the selection or laying of brick, but they are important to designers and these movements should be provided for in design and construction.
Structural loads. Ability to withstand either gravity or lateral loads relies heavily on brick strength, mortar strength, and strength of the wall assembly. Compressive strength requirements found in the ASTM specifications for brick are based on durability performance. Structural analysis may require a higher compressive strength in order to resist the applied loads. It is common to use high-strength units in load-bearing or reinforced brick masonry projects.
Cost. Material selection is often based on initial cost only. Although initial cost is important, lifecycle cost is a better tool for making critical decisions. "When deciding between different materials, all costs involved including labor and maintenance costs, future value and life expectancy should be considered," explains Bryja.
The selling price of brick is governed by many factors, including manufacturing methods, size and appearance of the