Tag Archives: terra cotta

Critical Bonds

slaton patterson sutterlinFAILURES
Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE

In the southeastern United States, a 12-story office building, constructed in the early 1930s, experienced uncontrolled water leakage through its mass masonry exterior wall. As this was attributed to weathering of the existing mortar joints in the brick and terra cotta façade, the joints were specified to be repointed with a 1:1:6 mortar mix (Type N per ASTM C270, Standard Specification for Mortar for Unit Masonry). The work was performed during the summer.

Example of water-damaged plaster finish on interior of recently pointed masonry wall. Photos courtesy David S. Patterson

Example of water-damaged plaster finish on interior of recently pointed masonry wall. Photos courtesy David S. Patterson

To match the original joint profile and aesthetic, the relatively wide (25-mm [1-in.]) mortar joints were struck flush with the brick face. Soon after repointing, water damage to the interior plaster wall finish was observed in localized areas adjacent the existing steel-framed punched windows. Leakage (and subsequent plaster damage) occurred primarily on the building’s north elevation, which is vulnerable to wind-driven rain. While diagnostic water testing of the window system did not result in leakage to the building interior, testing of the repointed masonry wall did.

Close-up examination of the exterior masonry revealed widespread bond line separations at the interface of the mortar and adjacent masonry in the vicinity of the interior water damage. Although many of these separations were very narrow (hairline), some were wider than 0.51 mm (0.02 in.). Even minor bond separations can represent potential leak sources—water can pass through gaps as narrow as 0.3 mm (0.012 in.).

Mortar-bond-line separations in exterior masonry are of particular concern in mass walls due to the network of inherent voids in the back-up masonry, which is typically in contact with the exterior assembly. During testing, water entered the exterior wall construction through mortar-bond-line separations and traveled inboard via voids in the terra cotta tile and brick backup wall, wetting its interior surface to which the plaster finish was applied.

Example of mortar-bond-line separation—the area of missing mortar was a unique condition. The dark-gray mortar on the face of the brick remains from an earlier repair campaign using face grouting (shell pointing). The irregular surface of the face of the brick also made tooling the repointing mortar more challenging.

Example of mortar-bond-line separation—the area of missing mortar was a unique condition. The dark-gray mortar on the face of the brick remains from an earlier repair campaign using face grouting (shell pointing). The irregular surface of the face of the brick also made tooling the repointing mortar more challenging.

While many factors can affect mortar bond, in this example installation and curing during hot weather without following proper procedures likely contributed to lower mortar bond strength and increased drying shrinkage, and thus to bond separations. American Concrete Institute (ACI) 530/530.1, Building Code Requirements and Specification for Masonry Structures, a joint publication with the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE), and The Masonry Society (TMS), provides code requirements and specifications for hot weather practices for masonry construction, and is referenced in the International Building Code (IBC). A concise summary of the mandatory code practices required by IBC and ACI 530 with additional commentary is also provided in the Brick Industry Association (BIA) Technical Note 1, Cold and Hot Weather Construction, which serves as a good refresher for specifiers.

Water leakage at the interior face of the backup wall during testing of areas exhibiting mortar-bond-line separations.

Water leakage at the interior face of the backup wall during testing of areas exhibiting mortar-bond-line separations.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of the CSI or The Construction Specifier.

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates, Inc. (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com.
David S. Patterson, AIA, is an architect and senior principal with WJE’s Princeton, New Jersey, office, specializing in investigation and repair of the building envelope. He can be e-mailed at dpatterson@wje.com.
Jeffrey N. Sutterlin is an architectural engineer and senior associate with WJE’s Princeton office, specializing in investigation and repair of the building envelope. He can be contacted via e-mail at jsutterlin@wje.com.

Missouri Campus Features New Terra Cotta Application

By Dirk McClure

Terra cotta tile was clad onto this project's insulated composite precast concrete sandwich panels.  Photos © Jacia Phillips

Terra cotta tile was clad onto this project’s insulated composite precast concrete sandwich panels. Photos © Jacia Phillips

The University of Missouri, located in Kansas City, boasts the country’s first terra cotta-clad insulated composite precast concrete panels assembly.

Before this installation, terra cotta had been clad into non-insulated panels in a few projects. At the Henry W. Bloch Executive Hall for Entrepreneurship and Innovation, however, terra cotta tile was clad onto 1783 m2 (19,200 sf) of 3.6-m (12-ft) wide insulated composite precast concrete sandwich panels.

Designed by BNIM Architects and Moore Ruble Yudell, an important aspect of the project’s scope was to ensure it maintained the aesthetic of the university’s campus as a whole. The modular appearance of the five-color terra cotta pattern provided the desired masonry appearance.

Two sides of the building feature darker red colors on the terra cotta; the other sides feature a lighter palette.

Two sides of the building feature darker red colors on the terra cotta; the other sides feature a lighter palette.

Rather than specify a conventional rainscreen cladding system, the project team relied on the terra cotta to provide a rain barrier. A cost analysis by the general contractor also demonstrated terra cotta clad into an insulated precast sandwich panel would yield a 25 percent cost saving over a steel frame and air barrier application. The joints were concealed to provide the aesthetic of a more traditional rainscreen.

Designers outlined the seemingly random pattern of the terra cotta tiles. Every 152 mm x 1.2-m (6 in. x 4-ft) section of this five-color tile pattern was sorted by the precaster. The terra cotta was then cast into the panels in their offsite production facility. The use of sealants kept the gray-colored precast set back and invisible on the finished product. Two sides of the building feature darker red colors on the terra cotta, which complement the adjacent brick masonry campus buildings. The panel’s interior faces were left exposed in areas, with a sandblast interior finish. This helped reduce volatile organic compounds (VOCs) by limiting the use of paint.

The continuous insulation (ci) used in the assembly met American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, requirements. The precast high-performance insulated wall panels used to connect concrete wythes provided a low thermal transfer system and contributed to the project’s overall thermal mass goals. The extruded polystyrene (XPS) panels achieved an R-value range of 16.37 to 17.65. Finally, the radiant heating mechanical system was installed under the floor.

Thermal performance, high-performance integrated design and overall sustainability were also goals outlined by the project team. The project incorporates energy efficiency and daylighting strategies, and is targeting Gold certification under the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental (LEED) program.

The high-performance insulated wall panels used carbon fiber grid to connect concrete wythes, which provided low thermal transfer system and contributed to the project’s overall thermal mass goals. The extruded polystyrene (XPS) panels achieved an R-value range of 16.37 to 17.65.

The high-performance insulated wall panels used carbon fiber grid to connect concrete wythes, which provided low thermal transfer system and contributed to the project’s overall thermal mass goals. The extruded polystyrene (XPS) panels achieved an R-value range of 16.37 to 17.65.

Tests were performed to determine the correct level of using terra cotta in an insulated panel. These tests included determining:
● coefficient of thermal expansion;
● allowable bowing factor;
● optimal tile thickness; and
● freeze-thaw cycles.
Finally, full precast mockup panels were also designed and poured before the project was in full production.

Dirk McClure is regional director of sales and business development for Enterprise Precast Concrete. He has a bachelor’s degree in interior architecture from Kansas State University. McClure currently sits on Precast/Prestressed Concrete Institute’s (PCI’s) International Sustainability Committee and is a NCIDQ Certificate Holder, LEED Accredited Professional, and a member of the Construction Specifications Institute (CSI) Kansas City Chapter. He can be contacted by e-mail at dmcclure@enterpriseprecast.com.

Behind the Scenes: Another Look at a Terra Cotta Failure

Whenever we do readership surveys, the Failures column consistently ranks as one of the most popular parts of The Construction Specifier. Over the years, Deborah Slaton and David S. Patterson (along with many guest authors) have used the magazine’s final page to delve into what went wrong with a building, and explore how it could have been prevented. Continue reading