Monday, November 12, 2012

Burnt Clay Facades

Terra cotta, or “burnt clay,” is a hard baked, high grade of weathered or aged clay. It is similar to brick but the clay is of higher quality and fired at higher temperatures. This article focuses on exterior architectural terra cotta as distinguished from statuary, pottery , and terra cotta blocks used as inner wythes of wall or fill material.

The 1893 Columbian Exposition in Chicago demonstrated the versatility and ornamental qualities of terra cotta.* It highlighted the great variance in color and shapes possible with terra cotta and began the demand in the United States for terra cotta that lasted through the late 1930s. Terra cotta is prized for its light weight, longevity, aesthetic qualities, and unit construction. At the peak of production, almost every urban area in America was producing architectural terra cotta in some variation. Today, most replacement units are produced by either Gladding McBean or Boston Valley Terra Cotta.

Specific forming techniques including hand press, machine press, slip casting, and extrusion are used depending on the shape and style of unit required. In the analysis of terra cotta failure the forming techniques are less critical than the strength characteristics of the fired clay, the integrity of the exterior surfaces, and structural support systems.

Exterior ornamental terra cotta was marketed as a light weight water proof cladding. If proper construction techniques were employed, and the system was maintained, and the local climates were mild, terra cotta performed as sold. However, terra cotta adorns buildings in severe weather climates, and is installed with structural materials affected by environmental conditions, and located on fa├žade elements inaccessible for routine maintenance.

The mortar joints are the material most susceptible to failure. Joints often exist on all three axis with some units of terra cotta designed for flat horizontal surfaces. Over time and exposure, the mortar fails providing a means for water intrusion. Sever cycling of weather in simultaneous freeze/thaw conditions can cause the terra cotta clay to expand and contract, accelerating the crazing or cracking of the protective glaze. Extensive crazing can lead to glaze spalling and allow for further water intrusion.

Once water enters the system there is no weep path allowing for water egress. Construction means and methods, as well as the cellular unit design, trap water and contribute to the potential corrosion of steel lintels, wire ties, steel structural support members, and other miscellaneous metals. Rapid freezing and thawing cycles, in addition to steel corrosion, can crack terra cotta units. If the units remain unrepaired, further water intrusion and/or absorption will occur.

 The repair of terra cotta will depend both on the cause and manifestation of the defect. Typical defects include crazing of the glazed finish, shallow surface spalls, deep spalls affecting the bisque, cracked units, inadequate support and / or anchorage, corrosion induced stress fractures, impact damage, mortar degradation, lack of maintenance, and inadequate repairs.

Proper terra cotta repair methods are linked to the cause of defect. Repair techniques are often performed on-site by skilled tradesmen. When damage to the terra cotta unit is severe, full replacement may be required. Defects due to inadequate support or a result of corrosion to supporting steel members is likely to require more invasive repair strategies including removal and replacement of several courses of interlocked terra cotta units.

When replacement units are not required and the scope is limited to on-site repair, labor costs exceed material costs. Since many historic terra cotta units were specialty designed and installed for the structure, a premium price is paid for replacement. New exterior decorative terra cotta is available only from the sources referenced and with small quantity orders, the first unit is approximately $5,000 with much of the costs attributed to making the form and determining the finish color and texture. Subsequent costs per unit will decrease with the range of decrease dependent upon quantities required.

The most important component of terra cotta repair is an understanding the cause of deterioration and the proper repair specifications. Both are derived after a full condition assessment and evaluation of the existing conditions.

·         First two images from Wikipedia; others property of PMA
·         Last of the Handmade Buildings, Virginia Guest Ferriday, Mark Publishing Co., Portland, OR 1984
·         National Park Service, Preservation Brief No7, Preservation of Glazed Terra Cotta
·         APT Pacific NW Chapter 2005 workshop
·         Terra Cotta, Standard Construction, Revised Ed., National Terra Cotta Society, 1927

Thursday, November 1, 2012

The Challenges of Assessing Structural Brick Veneer Panels

The origin of Structural Brick Veneer Panel dates back to the early sixties when new "tensile strength intensive" exotic mortar combined with steel reinforcing to create a 4-inch thick, single wythe brick panel. Developments continued to occur throughout the 1960s and 1970s, peaking in use during the 1980s. The system was relatively expensive due to the use of the high tensile strength mortar.

Developments in both the high tensile strength mortar and the clay units continued to reduce cost and allow the use of regular reinforcing and standard mortar and grout. Newer systems and manufacturing processes accommodated both horizontal and vertical reinforcing and permitted high-lift grouting. Later advancements in the connections of the brick veneer panel system to the building frame resulted in the use of brick veneer panels system on multi-story high-rise office buildings, schools, apartment buildings, residences and many other applications throughout the United States and the Pacific Northwest.

There are two major failure mechanisms of Structural Brick Veneer Panels: water intrusion and mortar/grout additives.  Water intrusion can occur from a lack of adequate flashing at the window head and sill interface, carbonization of the mortar, and structural cracking. Brick veneer panels are commonly designed to allow for limited cracking at the horizontal bed joints at the brick to mortar interface. Masonry veneer panels leak more through the mortar and brick interface than through the masonry unit itself. If the mortar and brick interface is cracked, as is permitted under structural design calculations, water infiltration will increase. A cement based material, panel mortar will carbonize over time decreasing the protective alkalinity environment surrounding the reinforcing bar and thus increasing the potential for corrosion. The largest volume of water intrusion is typically associated with inadequate window systems that fail to keep water out of both the structural brick veneer panel and the cavity interface.

The durability of the wall is highly influenced by the quality of the mortar joints and interior cell grout. The specification should require re-consolidation of the grout or the incorporation of additives that balance expanding, retarding, and water reducing agents to provide a slow, controlled expansion prior to the grout hardening. Mortar/grout additives, particularly those developed in the 1970s, containing vinylidene chloride can initiate and accelerate reinforcing steel corrosion under the right conditions. The composition of the mortar/grout is determined through laboratory analysis of chloride content, vinylidene chloride, and pH level.

Repairs to structural brick veneer panels is labor intensive and may involve panel replacement, panel encapsulation, window system replacement, and/or extensive individual masonry unit repair.

  Image Acknowledgement: Tawresey, John G. & John M. Hochwalt, KPFF Structural Engineers, Design Guide for Structural Brick Veneer, 3rd Ed, Western State Clay Products Association