by Kenneth Itle, AIA, Mike Ford, AIA, and Timothy Penich, AIA
Contemporary masonry veneer construction consists of a single wythe of masonry such as brick or stone mechanically anchored to a backup structure, typically either wood or steel framing or concrete masonry units (CMUs). The veneer is supported laterally by the backup structure and bears on the foundation or a shelf angle. Veneer construction incorporates an air space to deter water penetration into the building and reduce heat transmission. Water-resistant barriers (WRBs), flashings, weeps, and drainage systems are also installed to facilitate drainage. Masonry veneer wall systems are composed of many different materials. Thus, masonry veneer construction must be designed with the goal to accommodate differential movement.
Code requirements for masonry veneer walls are outlined in the Masonry Society (TMS) 402/602-16, Building Code Requirements and Specification for Masonry Structures. Additional guidance for specifiers based on the code is provided in the Masonry Designers’ Guide, published by TMS. The Brick Industry Association (BIA) has authored various technical notes on brick construction to provide guidance on individual materials and general construction of masonry veneer systems. They serve as useful references. Finally, ASTM standards define the various characteristics of masonry materials.
Brick are manufactured from naturally occurring substances that are fired. Typically, test data including absorption, saturation coefficient, compressive strength, efflorescence, and initial rate of absorption can be obtained from brick manufacturers and suppliers. Brick testing is conducted in accordance with ASTM C67, Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile. ASTM C216, Standard Specification for Facing Brick (Solid Masonry Units Made from Clay or Shale), can be used to evaluate the testing data. This standard also defines two grades of facing brick. Grades refer to a brick’s resistance to damage caused by freeze-thaw cycles. Grade SW (severe weathering) brick is meant for areas where high resistance to potential damage caused by freeze-thaw cycles is necessary. Grade MW (moderate weathering) brick is intended for use where only moderate resistance to damage caused by freeze-thaw cycles is needed. Almost all of United States requires Grade SW brick for veneer construction.
The physical requirements for each grade include minimum compressive strength and maximum water absorption and saturation coefficient. The saturation coefficient is a ratio of the rate of absorption by 24-hour submersion in cold water to five-hour submersion in boiling water. ASTM C216 provides an absorption alternate that states the saturation coefficient requirement does not apply if the 24-hour cold water absorption of the five tested brick units does not exceed eight percent.
Initial rate of absorption and efflorescence are also included in the test data. Initial rate of absorption data should be requested from the brick manufacturer, as this property is used to assist in mortar selection as well as the installation process. For example, brick with a high initial rate of absorption may potentially experience increased levels of efflorescence. This can be avoided by wetting bricks during installation to prevent absorption of moisture from the mortar.
Results from efflorescence testing are also included in manufacturer’s data. The specified rating for efflorescence should be “not effloresced.”
Three brick types are outlined in ASTM C216: FBS, FBX, and FBA. FBS is brick for general use and is the most commonly specified type. FBX is for projects requiring a high degree of precision and low variation in size, and FBA is meant for producing architectural effects resulting from non-uniformity in size and texture of individual brick units. ASTM C216 outlines physical requirements for each brick type.
While selecting brick with the appropriate grade is important, it is also crucial to minimize the volume of moisture to which the brick is exposed by proper detailing to ensure a successful veneer wall.
Mortar is typically composed of cement, hydrated lime, and sand, mixed with water. Admixtures and pigments are used as well. ASTM C270, Standard Specification for Mortar for Unit Masonry, outlines standards to reference when specifying mortar.
Cement-lime mortar, masonry cement, and mortar cement can be used in mortar. Portland cement is blended with hydrated lime to produce cement-lime mortar. Masonry cement contains Portland or blended hydraulic cement and limestone or lime. Mortar cements are similar to masonry cements in content but there are controls on air content and minimum strength requirements in the former. These are outlined in ASTM C1329, Standard Specification for Mortar Cement. Both masonry and mortar cement may contain additives specific to each manufacturer. Therefore, careful review of product data and safety data sheets is necessary.