A High Demand For Brick
Surveys indicate that an increasing number of construction projects involve additions, rehabilitations, or maintenance of existing or historic brick masonry buildings. As a result, the demand for brick that matches the original materials has surged. This article addresses the key properties to consider when selecting matching and replacement brick.
Brick used in additions and rehabilitations can be broadly classified as either matching brick or replacement brick, depending on their properties and use.
Matching Brick are units that have similar aesthetic properties to those found in the existing brickwork. Typically, these aesthetic properties include color, texture, and size. Matching brick may not necessarily have the same physical properties as the existing brick.
Replacement Brick are units that will be bonded into the existing brickwork. These bricks should have similar aesthetic properties and, to a certain extent, similar physical properties to the brick in the existing building. Replacement brick is typically used to repair existing brickwork or in new brickwork that is “toothed” or bonded into the existing brickwork. When adding to an existing brick structure, it might be possible to detail the new brickwork so that it is not bonded into the existing brickwork, often by using an expansion or separation joint between the new and the existing brickwork.
Mortar plays a crucial role in the performance of matching and replacement brick. Using the wrong mortar can cause spalling and cracking in the existing brick masonry. The mortar used with matching brick does not necessarily need to have similar properties to the mortar in the existing brickwork. However, for extensive replacement of existing brick bonded into the brickwork, a mortar with similar physical properties to the existing mortar is recommended. Generally, a soft mortar with high lime content and low or no cement content is ideal for repairs to existing or historic brickwork. For more information on traditional lime mortar, refer to ASTM C270, ASTM C1713, and NPS Preservation Brief 2.
Aesthetic Properties are paramount when restoring a building or constructing an addition. Maintaining the integrity and character of the original structure involves finding brick that matches in color, texture, and size.
Color and Texture: Brick has always been valued for its wide range of colors and textures. Modern manufacturing techniques use advanced equipment, mineral additives, surface texturing, and coatings to produce a variety of brick.
Size: It is most economical to obtain matching and replacement brick by specifying a contemporary brick currently made by the manufacturer. The critical dimensions to match are those on the surface exposed to view. Dimensions not exposed to view can differ if they do not impact the design and constructability of the brickwork. If the size of the existing brick varies greatly from currently available sizes, it may be necessary to use a larger brick that matches in color and texture and cut it to the proper size. If only a small number of bricks are required, this is likely the most economical solution. For larger quantities, a manufacturer may produce the brick to the correct size, but this will require setup charges.
A Sample Panel can be used to evaluate the accuracy of a match. The color, texture, and size of matching and replacement brick can be compared to the original brick using a sample panel. The brick in the sample panel should represent the full range of colors and textures of the selected brick and be distributed within the panel to match the existing brickwork. Both the original brickwork and the sample panel should be cleaned before a visual evaluation. Slight changes in mortar color can significantly affect the perceived appearance of the brickwork.
Regardless of which brick is used, both matching and replacement brick should comply with the appropriate ASTM standard, usually ASTM C216. For most restoration and repair projects, matching the aesthetic properties of the original brick is the primary concern, while closely matching the physical properties is usually not necessary. However, when replacement brick will be bonded into the existing brickwork and extensive replacement is needed, the physical properties of the brick should be considered to ensure compatibility. Extensive replacement typically involves spot repairs larger than 2-3 square feet, replacing more than 20% of the existing brickwork, or replacing 8-10 feet of an existing brick course.
For extensive replacement, achieving compatible movement when loads are applied and accommodating irreversible moisture expansion of the replacement brick are critical. Improper combinations of physical properties can result in cracking or spalling of softer brick.
Deformation: Brickwork deforms as loads are applied. When constructed with the same mortar and brick, the deformation experienced by each brick is similar. However, if the brickwork contains both hard and soft bricks, the harder brick will not deform at the same rate as the softer brick, potentially causing cracking in the softer brick. When small areas of brickwork are removed and replaced, the existing brickwork can usually redistribute the load around the repair area if a softer mortar is used. For large areas, the replacement brick should be similar in hardness to the existing brick to ensure similar elasticity and avoid cracking.
Irreversible Moisture Expansion: A brick is smallest when it exits the kiln and undergoes irreversible moisture expansion afterward. Most of this expansion occurs shortly after the brick leaves the kiln. Existing brick in brickwork will have already undergone most of its expansion, but new replacement brick may not have. If extensive repairs are completed with new brick, it may induce stress within the existing brickwork. For small repairs, a softer mortar can handle the expansion. For large areas, isolating new brickwork from existing brickwork with expansion joints is recommended. If expansion joints cannot be used, allowing replacement brick to undergo moisture expansion before being laid in existing brickwork is common. This typically involves stocking the brick for 2-3 months before use.
Brick Briefs are short discussions on a particular topic, intended to provide guidance. The information must be used with good technical judgment, and final decisions rest with the project designer and owner.
