Brick Selection – Tech Note 9B
Brick selection is based on a number of factors. Not only are aesthetics and durability important, but strength, absorption, availability and cost are important to the owner, designers and contractors. The selection process can be difficult since each group is trying to satisfy different requirements. Typically, the final selection is based on a compromise from all parties involved.
The use of brick as a building material dates back centuries. Because of brick’s enduring qualities and limitless appearances, designers can satisfy their creative styles with brick. Brick is readily available in many sizes, colors, textures and shapes. These can be adapted to achieve virtually any desired style or expression. Brick’s small module can be related to the scale of the wall. These sizes can be combined in such a way as to create different appearances and patterns. Not only does brick size influence scale and appearance, but the size of brick influences wall cost because larger units require fewer brick, normally resulting in less labor. When specifying the size of units, dimensions should be listed in the following order: thickness (width) by height by length.
Brick manufacturers also offer a wide variety of colors to choose from. Units whose colors range from reds and burgundies to whites and buffs are manufactured today. Many manufacturers produce over 100 colors. Many of these color variations are created during the firing process. Temperature variations and the order in which the units are stacked in the kiln determine shades of light and dark. Ceramic glazes, slurries or sand coatings can be applied to the surface to achieve colors not possible with some clays. The possibilities of using units of contrasting colors in bands or other patterns are endless. Sample panels, or mockups, can aid in selecting the desired color by showing the finished appearance. Another aesthetic feature to consider when selecting brick is the texture. Textures on brick can be smooth, wire cut (velour), stippled, tumbled, brushed, rolled, and more. The texture interacts with light and creates differing and interesting shadows. Unique design features can easily be achieved by using special brick shapes. Brick can be molded and formed into any shape, from simple sloped sill shapes to fancy water table brick. For most manufacturers, molded shapes are easier to produce than extruded shapes, because the molded, or soft-mud process is more adaptable to making brick shapes than the extruded process. Making very large shapes can be difficult in either process because of problems with proper drying and firing.
There are many physical properties which may influence the selection of brick. Some of these include durability, absorption, compressive strength and abrasion resistance. Physical properties required for proper performance are given in the appropriate American Society for Testing and Materials (ASTM) specification for brick.
Durability. Currently, there are two accepted methods for demonstrating durability under ASTM standards: 1) durability as predicted by compressive strength, absorption, and saturation coefficient, or 2) durability as determined by compressive strength and passing 50 cycles of the freeze and thaw test. Criteria in each ASTM specification determine grade or class designations. Because of the varying climates and applications of brick, specific physical properties are required. Brick are classified into these grades or classes according to their resistance to freezing when wet.
Absorption. Absorption can be broken into two distinct categories absorption and initial rate of absorption (IRA). Both are important in selecting the appropriate brick. Absorption of a brick is expressed as a percentage, and defined as the ratio of the weight of water that is taken up into its body divided by the dry weight of the unit. Water 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 5 hour boiling water absorption, respectively. These two are used to calculate the saturation coefficient by dividing the 24 hour cold water absorption by the 5 hour boiling. The saturation coefficient is used to help predict durability. The initial rate of absorption (IRA) or suction is the rate of how much water a brick draws (sucks) in during the first minute after contact of the bed surface 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 in2 [30 grams/min./194 cm2]), a strong, watertight joint may not be achieved. Therefore, high suction brick should be wetted prior (3 hrs to 24 hrs) 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.
Compressive Strength. The strength of a unit is used to determine durability and also compressive strength of the resulting brick masonry. Typically, most materials are judged on the basis of strength. However, 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 (20.7MPa) to over 20,000 psi (138 MPa), averaging around 10,000 psi (68.9 MPa). Achieving sufficient compressive strength with brick is seldom a problem.
Abrasion Resistance. This property is important when brick is used as paving. The resistance to abrasion is affected by the degree of firing and by the nature of the raw material. Abrasion resistance is predicted in two manners. It is evaluated in terms of cold water absorption and compressive strength. These two properties produce an abrasion index which is used to determine the type of traffic which is suitable for a particular brick. Alternately, volume loss is determined by sand blasting the paver surface.
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 which a wall or material must withstand. Some of the more important factors include moisture penetration, temperature variations 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 which develop a complete bond with mortar offer the best moisture resistance. Brick and mortar properties should be compared to provide compatible materials which result in more watertight walls. Currently, there are no requirements for the degree of water resistance of a wall.
Temperature Variations. Brick must withstand daily temperature cycles and seasonal extremes (-30°F to 120°F [-34°C to 49°C]) depending on location, throughout its life. Thermal expansion and contraction of brick is not critical to the selection of brick, but it is important to designers and this movement should be provided for in design and construction. Brick also withstands temperature extremes in fires. Since brick is a fired material, it will not burn and acts as an excellent barrier to fire because it is non-combustible.
Structural Loads Ability to withstand either gravity or lateral loads relies heavily on brick strength, mortar strength and dimensions 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. Compressive strength of masonry may be a governing criterion in load bearing or reinforced brick masonry projects.
Cost Material selection is often based on cost, usually 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. The selling price of brick is governed by many factors, including manufacturing methods and appearance of the unit. When considering different brick, one must take into account shipping costs. Since most prices quoted are plant prices, distance between the manufacturing plant and the jobsite is a major determinant of these shipping costs. Brick manufacturers and distributors can supply brick prices and shipping prices. Brick price is only one part of the in-place costs. Labor and overhead costs are approximately twice the brick and mortar costs. Many of the Masonry Institutes throughout the country provide cost comparisons between different materials.
Availability The availability of brick fluctuates with the time of the year and current construction trends and demands. On the average, brick production time runs about 5 days, from pugging of the clay to the finished, fired product. This can change depending on many factors such as variations in raw materials, forming process, and kiln types. Many brick manufacturers have stockpiles of brick, but usually only a small quantity of each brick type. This may satisfy smaller jobs, but for large projects requiring large quantities of brick, a special production run must be made for the job. Most manufacturers have a set schedule as to when they produce a certain brick shade. It is at this time that the size of the run will be increased to accommodate the large order. It is wise to determine the brick’s availability from the manufacturer. It is best to purchase all brick from the same production run because there are typically slight color variations between runs. All manufacturers have quality controls to keep this at a minimum.
Resource: Brick Industry Association-www.gobrick.com