The Emergence of Thin Brick: Understanding adhered veneer

ASTM C1088, Standard Specification for Thin Veneer Brick Units Made from Clay or Shale, provides requirements for thin brick. Much of what is found in ASTM C1088 is also found in ASTM C216, Standard Specification for Facing Brick (Solid Masonry Units Made from Clay or Shale), for facing brick. In this case, the brick is thinner, not to exceed 44 mm (1 ¾ in.). There are no minimum thicknesses, but generally the thinnest is 6 to 10 mm (¼ to 3/8 in.) since they are more prone to breakage at this thickness.

The durability of thin brick is determined by its grade. Instead of the SW and MW grade found in C216, Grades ‘Exterior’ and ‘Interior’ are employed. The weathering index is used to differentiate between the two; in most of the United States, Grade Exterior is the requirement. There are no compressive strength requirements since the thinness gives a higher apparent reading than is true. The absorption limitations for durability are the same here as in ASTM C216.

As in C216, there are different types of thin brick as they relate to dimensional tolerances:

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TBS is for general use;
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TBX is for use where dimensional tolerances are important; and
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TBA is where aesthetics are more important than tolerances.

When using thin brick for precast concrete applications, Type TBX is generally recommended since they are placed in forms before the concrete is poured and require a tight fit.

Mortars can be a whole topic in and of themselves, but briefly there are three ways to bond thin brick to its backing:

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Type S mortar (ASTM C270, Standard Specification for Mortar for Unit Masonry) has always been used for field-installed applications—the higher bond strength is required since it is often the only thing holding the brick on to the wall;
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conforming to tile standards set by American National Standards Institute (ANSI), a latex or polymer-modified mortar is often used instead of Type S to provide for better bonding and durability; and
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adhesives may be used for modular panels and should conform to ANSI A118.3, Specifications for Chemical-resistant, Water-cleanable Tile-setting and Grouting Epoxy Adhesive, or ANSI A136.1, Organic Adhesives for Installation of Ceramic Tile.

Installation
There are just a few basic thin-brick installation methods. One option is directly to the substrate which typically occurs in the field. Both thick- and thin-set methods apply to this category. Another method involves modular panels where the thin brick are set into preformed panels, typically adhered with an adhesive. Lastly, there are brick-faced precast panels, which are brick-set into a precast concrete panel. When using proprietary systems, one must always follow the manufacturer’s installation instructions.

In any system, if the backing varies more than 6 mm in 3 m (¼ in. in 10 ft), then an additional leveling coat should be first applied. This is critical since any imperfection in the backing will be seen in the finished wall. Additionally, if any applied coating or form-release agents are used on the backing wall, adhesion should be verified before installation. Substrate preparation is always worth the effort. Another installation recommendation for all wall systems is installing a mortar joint between the units after they are set. Using a concave or V-joint profile helps resist water penetration.

The thin-set method of installation is the most popular since there is only one coat necessary, which reduces installation costs. Since such systems produce a thinner layer, the backing must be fairly level or plumb—otherwise, it reflects through the thin-brick surface. In most cases, a WRB or air barrier is applied to the backing, followed by the thin-bed mortar. Latex or polymer-modified mortar is then used to adhere the thin brick to the wall.

The thin-set method follows typical tile-setting procedures. The mortar material is applied to the unit and/or backing as the units are set into place. Full bedding of each brick unit is necessary for long-term durability. This installation method is more prone to mistakes since there is not much room for error. This lack of redundancy may result in more failures in the future unless the manufacturer’s installation recommendations are closely followed.

Common in the past, the thick-set method installation may still be used in some areas of the country. This method is very similar to plaster installation. TMS 402/American Concrete Institute (ACI) 530-11/American Society of Civil Engineers (ASCE) 5-11, Building Code Requirements and Specification for Masonry Structures and Related Commentaries, actually has requirements for this particular installation method.

First, the WRB and/or air barrier is installed onto the backing along with wire lath. Appropriate fasteners for the lath and methods for overlapping the lath should be followed. A scratch coat is then applied atop the lath. The thickness of the scratch coat is between 10 and 32 mm (3/8 and 1 ¼ in.) thick. If an expansion or control joint is shown, then the joint is also formed in the scratch coat. Finally, a thin bond coat is applied. A portland cement paste should be applied to the back of the thin-brick unit before it is placed, ensuring it spreads over the entire surface. The brick are then manipulated to align them with the other units. Corner units are typically applied first and used as a lead.