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Can the fire resistance rating of an existing, single wythe, hollow concrete masonry wall be improved by filling the cells?


If an existing masonry wall is determined to have a fire rating less than that required (Refer to FAQ “How does one determine the fire resistance rating of an existing, single wythe, hollow concrete masonry wall?”), one option is to fill the hollow cells of the concrete masonry units (CMU). Generally, this approach is only practical when the top of the wall is accessible so that the fill material can be poured in from the top and complete filling can be achieved.

Per ACI 216.1/TMS-02161, the following cell fill materials are approved for improving fire resistance:

  • sand, pea gravel, crushed stone, or slag that complies with ASTM C33
  • pumice, scoria, expanded shale, expanded clay, expanded slate, expanded slag, expanded fly ash, or cinders that comply with ASTM C331
  • perlite that complies with ASTM C549
  • vermiculite that complies with ASTM C516


If the cells can be completely filled with one of these materials, then the equivalent thickness of the filled CMU wall is equal to the actual unit thickness. This results in a substantial increase in fire resistance.

Example: Consider an existing 10-inch CMU wall, whose equivalent thickness is 4.5 inches. Suppose that a 4-hour rating is required. The required minimum equivalent thickness, per ACI 216.1/TMS-0216, ranges from 4.7 to 6.2 inches (depending on aggregate type), so the existing wall is not adequate. If the cells are completely filled with one of the approved cell fill materials, then the equivalent thickness is the actual unit thickness, or 9.625 inches.  In that case, the wall easily meets the requirement.


 The decision to utilize this information is not within the purview of the MIM, and persons making use of this information do so at their own risk. MIM makes no representation or warranties, expressed or implied, with respect to the accuracy or suitability of this information. MIM and its members disclaim liability for damages of any kind, including any special, indirect, incidental, or consequential damages, which may result from the use of this information. This information is not to be interpreted as indicating compliance with, or waiver of, any provision of any applicable building code, ordinance, standard or law.