When qualifying a primer to be used beneath SFRM, what is the minimum allowable bond strength of SFRM on the primer?

• A. Half the bond strength on steel
• B. Equal to the bond strength on steel
• C. Double the bond strength on steel
• D. One-quarter of the bond strength on steel

Answer: (A)

This checks that the layer between the sprayed fireproofing material and the primer isn’t the weak link in the system. The SFRM must bond to the primer with at least 50% of the strength it has to steel. Why this is right: if the SFRM/primer bond were weaker than half the SFRM/steel bond, delamination or peeling could occur at the primer interface during fire exposure or thermal cycling, compromising the protective system. Requiring at least half keeps the primer from being the limiting factor and helps ensure predictable performance. The other options would either demand too much adhesion (equal to steel), be unnecessarily strict (double), or allow too weak a bond (one-quarter), none of which align with how the system is designed to behave under service conditions.