Standard Core Material

When ACM was first introduced to North America, the common core material was an extruded polyethylene. Many of the standard products available today continue to use this type of core. The common practice is to extrude a flat layer of core material that was bonded to the metal skins in a single continuous process. This bonding method allows the use of the heat, pressure, and tension to aid in the creation of the composite panel.

The standard core material meets all of the code requirements for panel use up to 40 feet above grade. The primary criteria governing standard core material is ASTM E84, which measures the surface flame spread of a material. The code requires this value to be less than 25. As a point of reference, the flame spread of a red oak flooring panel is used as a baseline and equated to a value of 100. The performance of the panel is considered in whole, so the metal skin material protects the core material from contributing to the fire during initial exposure.

There are other plastic materials that have been used successfully in place of polyethylene; however, the industry definition of an MCM is a panel that contains a solid plastic core bonded in a continuous process. The batch process does not meet the intent of this MCM panel criteria.

Companies have promoted core color or core density as a performance attribute; however, the most important points regarding the core remain:

• Solid core material: The skins of ACM are relatively thin (0.019 inch) and can easily telegraph any surface imperfection, including a discontinuity in the core, such as a honeycomb or corrugated core would produce. Higher-gloss or highly reflective finishes exaggerate these discontinuities and lead to visual problems with the panel and finish.
• Bond strength between the core and skins: The standard is set at 22.5 inch-pounds/inch for the bond strength between the core and skin material. This strength was not a simple shear or tension test, but rather a peeling test that demonstrates that the material will not delaminate over time. This test, ASTM D1781, has been used by this industry since the 1980s and is included in the acceptance criteria (AC25) used to develop evaluation reports for MCM products and systems.

Fire-Resistive Core Material

As MCMs gained in popularity for their aesthetic opportunities and performance attributes, the application of MCM cladding expanded into high-rise construction.

Concern over fire performance is different once the cladding is used above 40 feet.

In the United States, NFPA 285 has been developed to exhibit relative real-world fire performance. A similar test has been developed for Canadian use, NRC/ULC S134. The Class A flame-spread certification remains an additional requirement for those MCMs to be used above 40 feet.

The 2012 IBC established criteria to determine when a standard or fire-retardant core must be used. The major elements that dictate the type of core material to use include: panel height above grade or grade plane; wall construction type (rated or non-rated fire assemblies); and proximity to the property line or other structures within the property boundaries. The alternative performance criteria to NFPA 285 for MCM is referenced in the 2018 IBC in Sections 1406.10 and 1406.11 and include testing requirements, including ASTM E84, ASTM D635, ASTM D1929, and NFPA 285. Use of these sections is complex and should be considered only after discussion with the MCM manufacturer.

Typically, a manufacturer’s standard panel material meets the performance requirements for the first three tests only, while the fire-retardant core material meets the performance requirements of all four test standards. When the construction conditions are within the limitations outlined below, a combination of some or all of the first three fire tests are required in the IBC, and a standard core material can be used. When these installation conditions are not within the defined limitations, either the fire-retardant core material must be used, or the authority having jurisdiction (AHJ) must accept the material in accordance with Section 104.11. Should the building require fire-rated construction, another important consideration is whether the manufacturer of the MCM has performed third-party-verified testing to show compliance with the requirements of the applicable fire tests.

Generally, MCM is required to meet the performance criteria of NFPA 285 when installed higher than 40 feet above the grade plane. However, there are certain installation conditions that may allow use up to a height of 75 feet above the grade plane without this requirement. The applications are defined in Section 1406 and are based on the allowable use of other combustible materials throughout the code.

In the 2018 IBC, the use of combustible materials on all construction types to a height of 40 feet above grade plane is allowed. The only limitation is a fire-separation distance of less than 5 feet. If that limitation cannot be met, a fire-retardant material or AHJ acceptance must be obtained. Installations of standard core MCM up to 50 feet above grade plane are defined in Section 1406.11.2 and based on the allowable use of plastic veneer defined in Chapter 26. If the ASTM D1929 and section size and vertical separation of section limitations cannot be met, fire-retardant material must be used.

There is no single formulation of fire-retardant core material required to meet code criteria. Each MCM manufacturer develops its own formulation and production parameters. The most common solution is to replace a portion of the combustible material found within the core material with either fire-retardant chemistry or an inert filler that would not promote flame spread.

MCMs IN ACTION: CASE STUDY #2

Photo courtesy of 3A Composites USA

MASSACHUSETTS COLLEGE OF ART AND DESIGN STUDENT RESIDENCE HALL, BOSTON

Project: Massachusetts College of Art and Design Student Residence Hall
Location: Boston
Architect: ADD
MCA/MCM Alliance Member Manufacturer: 3A Composites USA
Composite Fabricator/Installer: Lymo Construction
Contractor: Suffolk Construction
Completion: May 2012
Size: 150,000 square feet, 20 stories tall, 493 bed capacity

For the team at ADD that created the new residence hall at the Massachusetts College of Art and Design in Boston, the collaborative process gave birth to a piece of architectural artwork that garnered it the 2013 MCA Chairman’s Award for Education – Colleges and Universities.

Using 5,900 MCM panels in seven different colors and five different widths, depths, and gloss levels, lead architect B.K. Boley and his team sought to infuse the building’s exterior with the look and feel of the project’s aesthetic inspiration, Gustav Klimt’s 1909 painting “Tree of Life.” Like the painting, the building uses an eclectic arrangement of colors to mimic an organic form—a concept that quickly gained approval from the group ADD invited into the charrette discussions used to plan and design the project.

“They were looking for a new building that was expressive of their mission, which is to train artists to engage with the public and to work in industry as fine artists,” Boley says. “We came up with the idea of a tree of life as a symbol for the project because it represents rebirth, change, and optimism.”

The building—which the students, in particular, wanted to be identifiable as “a painting in the sky,” Boley recalls—houses 493 students in a 20-story structure that creates the image of organic order from chaos. There is a progression of darker and more muted browns at the bottom to lighter and glossier tones at the top, mimicking the way light plays off of a real tree. The mottled look of the exterior’s profile is actually designed around a rational pattern in the panel width and depth and window arrangements: every two stories, the pattern repeats until it reaches the top of the building.

“We knew we wanted it to be organic, but we also knew it would have to be buildable,” Boley says. “And composite metal panel ended up to be the best material to work with because we could pick our own colors—they are all custom colors—[and] we could have depth in the facade as opposed to a flat facade.”

Boley’s team also came up with panel designations and drawings that essentially acted as a map for the contractors to follow during installation.

Using the composite aluminum panels allowed the project to achieve its artistic goals and stay within its $48-million budget. Boley’s team also chose a pressurized rainscreen system that has insulation behind it and spray foam insulation in the cavities, delivering a high R-value and overall energy efficiency, pointing the building toward LEED Gold status.

The team was able to use a rout-and-return method to rout and fold the panels so the contractor did not need to weld them together. That tight arrangement turned out to be emblematic of a project that almost everyone involved could appreciate.

Photo courtesy of 3A Composites USA

EXO APARTMENTS, RESTON, VIRGINIA
MCMs offer exceptional design opportunities while providing reliability, economy, and environmental benefits.