Vibing Masonry #6: Concrete Masonry Units and Fire Resistance

Words: Steve Blye
Photos: Wikimedia, Department of Defense, Getty Images

Concrete masonry units (CMUs) are valued in construction for their exceptional fire resistance, stemming from their non-combustible composition and robust performance under high temperatures. This quality makes CMUs a preferred choice for fire-rated walls in buildings, offering significant advantages over alternatives like gypsum board on steel studs, metal, wood, and phenolic panels. This article outlines the technical basis for CMU fire resistance, including composition, standards, and practical applications. 

On September 11, 2001, American Airlines Flight 77 crashed into the only one of five sides of the Pentagon that had recently completed major internal renovation for security upgrades. In this First Floor West, new CMU block walls and other safety features had just been installed for blast and fire protection. Miraculously, because the renovation was completed days before, only 800 workers were present in this section, far fewer than the typical 4,500 staff. The newly installed CMU walls helped limit damage, protecting many lives. Sadly, 184 people were still killed, including 125 inside the Pentagon and 59 on the plane (excluding hijackers).

Composition and Inherent Properties 
CMUs are made from Portland cement, aggregates (e.g., sand, gravel, or lightweight materials like expanded shale or pumice), and water. This combination yields a non-combustible material that does not burn, emit toxic fumes, or contribute fuel to a fire, unlike wood, which ignites easily, or gypsum board, which fails under prolonged fire and water exposure. 

Non-Combustible Nature: As per the International Building Code (IBC) and National Fire Protection Association (NFPA) standards, CMUs are classified as non-combustible, as they do not ignite or sustain combustion. In contrast, metal panels, while non-combustible, lose structural integrity due to heat-induced deformation, and wood burns readily. 
High Melting Point: CMU components—cement and aggregates—have melting points exceeding 2,000°F (1,093°C), well above typical building fire temperatures (1,000°F to 1,800°F). For comparison, aluminum melts at 1,200°F, and steel weakens above 1,000°F, melting to liquid around 2,500°F.

Fire Resistance Ratings 
CMU fire resistance is evaluated through standardized testing, such as ASTM E119, which assesses a wall’s ability to withstand fire while maintaining structural integrity and limiting heat transfer. CMUs excel due to their density, thickness, and thermal properties. 

Fire Ratings by Thickness and Type: The Concrete Masonry & Hardscape Association provides fire resistance data:
- A 4-inch solid CMU wall achieves a 1-hour fire rating.
- An 8-inch hollow CMU wall, commonly used, achieves a 2-hour rating without additional finishes.
- A 12-inch hollow CMU wall, when filled with concrete or grout, can reach a 4-hour rating.
- Lightweight CMUs (e.g., using pumice) may have slightly lower ratings (e.g., 1.5 hours for an 8-inch wall) but still outperform non-masonry alternatives. 
Comparison to Alternatives: Metal panels require additional fireproofing (e.g., gypsum board or mineral wool) to achieve a 1-hour rating, as they conduct heat rapidly. Wood panels offer minimal resistance (15-30 minutes for untreated assemblies). Precast concrete panels match CMU performance but are often heavier, less cost-effective, and far less flexible for renovations.

Thermal Insulation and Heat Transfer 
CMUs resist fire by slowing heat spread through walls, leveraging their thermal mass and low conductivity to protect occupants and adjacent spaces. 

Hollow Core Advantage: Hollow CMUs, with internal air pockets, reduce heat transfer compared to solid materials. The NCMA notes that these air spaces act as insulators, delaying temperature on the unexposed side, a key factor in achieving higher fire ratings. 
Test Data: In ASTM E119 tests, an 8-inch hollow CMU wall maintains an unexposed side temperature below 250°F (121°C) for 2 hours, even when the fire side reaches 1,700°F (927°C). Metal panels, without insulation, transfer heat almost immediately.

Practical Applications and Standards 
CMUs are critical for fire-rated construction, such as firewalls, stairwells, and occupancy separations, as mandated by building codes. 

Building Codes: The IBC (Section 721) and NFPA 101 (Life Safety Code) specify CMUs for fire barriers. For example, a 2-hour fire-rated wall is often required between residential units, a role CMUs fulfill efficiently. 
Historical Context: The Great Chicago Fire (1871) highlighted masonry’s durability, as wooden structures were destroyed while masonry remained intact. Early CMU buildings, like the Unity Building (1892), met emerging fire safety demands.
Modern Use: CMUs are prevalent in schools, hospitals, and industrial facilities. A 2019 NCMA, now CMHA, study found CMU walls in schools consistently exceed code requirements, providing up to 3-hour ratings in critical areas.

Supporting Evidence 

CMHA TEK Notes: TEK 7-1C details how CMU wall thickness, aggregate type, and reinforcement affect fire ratings, with tables showing up to 4-hour ratings. 
UL Listings: Underwriters Laboratories certifies CMU assemblies (e.g., UL Design U902) for fire resistance, confirming their reliability.
 
Case Study: A 2015 Texas warehouse fire demonstrated CMU walls containing a blaze to one section, while metal-clad areas suffered extensive damage.

Conclusion
Concrete masonry units’ non-combustible composition, high thermal mass, and proven performance in tests and real-world fires make them a superior choice for fire-resistant construction. Compared to gypsum board, metal, wood, or other wall finishes, CMUs offer a robust, code-compliant solution, ensuring safety in critical applications. When adding up their many other attributes—resistance to storms, floods, pests, ballistics, impacts, and more—concrete masonry units should be considered first.

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