Average Thickness Passes but Individual Readings Fail
On a 12-story office project, the approved fire-resistance design requires 3/4 in. of SFRM on W18x50 floor beams for a 2-hour rating. The project specification also requires a field density of at least 15 pcf when tested by ASTM E605. After floor 5 was sprayed, the inspector took 12 thickness readings on each of 20 beams and one density sample. The 240 readings average 0.68 in.; 38 readings are between 0.55 and 0.70 in.; the rest are at or above 0.75 in. The density sample is 14.2 pcf. The applicator argues that the average is close enough and that the low readings are on beam webs that are hard to reach. The inspector must determine acceptance, what repair is required, and whether the density result changes the decision.
1. How should the inspector evaluate the thickness readings against the approved design? Is the 0.68 in. average acceptable?
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No. The approved fire-resistance design establishes a required minimum thickness, not an average that can be traded against low spots. The 3/4 in. thickness is what supports the 2-hour rating on the W18x50 beam, so each reading below 0.75 in. is a deficiency even if the arithmetic average is 0.68 in. The 38 readings between 0.55 and 0.70 in. cannot be excused because they are on webs; inaccessible or difficult geometry is a reason to improve access, not a reason to accept less fire protection. ASTM E605 provides the measurement method, but it does not create a tolerance that permits random low points. The inspector should identify the deficient beams, require repair to the full required thickness, and expand sampling if the pattern suggests the application method is not controlled.
2. What repair and reinspection should be required for the low-thickness areas?
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The contractor should prepare the existing surface as required by the manufacturer, remove any loose or unbonded material, and apply additional SFRM in lifts that achieve the required 3/4 in. minimum without trapping air or causing delamination. Simply troweling material into a thin spot or relying on a later cosmetic coat is not acceptable if the installed thickness remains below the design. After cure, the inspector should reinspect the repaired beams using the same sampling approach and compare each reading to the required minimum. The repair records should identify the beams, locations, material batch, date, and readings. If low thickness is widespread, the inspector should require a larger sampling area and a review of the applicator's nozzle, air pressure, and pass technique, because the problem is likely procedural rather than isolated.
3. Does the 14.2 pcf density result change the acceptance decision? Explain.
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Yes, it is a separate nonconformance. The specification requires at least 15 pcf, and density affects both fire resistance and cohesion. Adding thickness over low-density material does not correct the density deficiency. The inspector should verify the density result with the approved sampling method and compare it to the product's approved design and the project specification. If 15 pcf is required, the contractor cannot simply add more material; the affected material may need removal and reapplication, or the contractor may seek a written approval from the registered design professional and code official based on the product's evaluated design. A new density test should be taken after corrective work. The inspector should report the density failure separately from the thickness failure so that each corrective action is tracked and closed.