QUESTION: How does one determine the fire resistance rating of an existing, single wythe, hollow concrete masonry wall?

ANSWER: In its Resource A, the 2012 IEBC1 provides detailed guidance for evaluating fire resistance of archaic materials, defined as those utilized prior to the 1950s, including concrete masonry units (CMU). The CMU size, percent thickness, and aggregate type must be known to use the tables in that document.

In Section 722, the 2012 IBC2 provides information on calculating the fire resistance of materials for new construction, including tables that list concrete masonry wall fire-resistance ratings based on the equivalent thickness and the type of aggregate used in the concrete units. For similar guidance, ACI 216.1/TMS 02163 can be consulted.

The equivalent thickness of the CMU wall is calculated as the net volume of the masonry unit divided by the product of the unit length and unit height, where these parameters are determined in accordance with ASTM C1404. An estimated equivalent thickness can be obtained by consulting NCMA TEK 7-1C5, which lists values for typical two-core units. However, significant changes were made to the ASTM standard for hollow CMU subsequent to publication of the TEK note, rendering its information invalid for newer units.

Both IBC and ACI 216.1/TMS 0216 categorize concrete masonry unit aggregate into four types, listed in order from lowest fire resistance to highest fire resistance:

  • calcareous or siliceous gravel (other than limestone)
  • limestone, cinders, or air-cooled (unexpanded) slag
  • expanded clay, expanded shale, or expanded slate
  • expanded slag or pumice

 

Table 3.1 of ACI 216.1/TMS 0216 lists the fire resistance rating of the concrete masonry unit wall based on the minimum equivalent thickness and type of aggregate in the unit. IBC Table 722.3.2 expands this table to provide fire-resistance ratings in quarter-hour increments.

As a first step, the tables should be consulted based on the estimated average equivalent thickness per NCMA TEK 7-1C, provided that the construction predates 2009. If the required fire rating can be achieved regardless of the type of aggregate used, then no further effort is required, unless the required thickness value is very close to the estimate. If the value is close, or if the required fire rating cannot be achieved unless certain types of aggregate are present, then the equivalent thickness and/or aggregate type should be verified. Equivalent thickness can be determined by removing several units and assessing them in accordance with ASTM C140. Aggregate type can be determined by subjecting a CMU to petrographic (microscopical) analysis.

If the required fire-resistance rating cannot be achieved by the hollow CMU, after equivalent thickness and aggregate type are known, then wall modifications such as filling the cells or adding wall finishes, are required. Refer to other FAQs.

EXAMPLE: Consider an existing 10-inch CMU wall circa 1970. According to NCMA TEK Note 7-1C, the typical two-core 10-inch CMU has an equivalent thickness of 4.5 inches. If a 2-hour fire rating is required, the wall is sufficient no matter what type of aggregate is in the CMU because the required equivalent thickness ranges from 3.2 to 4.2 inches. If a 3-hour rating is required, however, the wall is only sufficient if it has all expanded slag, pumice, expanded clay, expanded shale, and/or expanded slate as its aggregate. The fire rating is not sufficient if the CMU includes other types of aggregate.

Resources:

  1. 2012 International Existing Building Code (IEBC), International Code Council
  2. 2012 International Building Code (IBC), International Code Council
  3. Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, ACI 216.1-07/TMS 0216-07
  4. ASTM C140 Standard Test Methods for Sampling and Testing Masonry Units and Related Units
  5. NCMA TEK 7-1C, Fire Resistance Ratings of Concrete Masonry Assemblies, National Concrete Masonry Association, 2009

 

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