Fire Performance of Fiber Reinforced Polymer
When designing a structural upgrade to an existing structure, it is important to take into consideration a few calculations to determine the fire rating of a structure. Fiber Reinforced Polymer (FRP) includes a system of both carbon fibers and the bonding epoxy. The carbon fibers themselves are great in fire as they will not lose strength, even while glowing red hot. However, the epoxy is more like a plastic and will soften or even melt when exposed to temperatures as low as 150 degrees.
We know from ASTM 119 that “standard fire” temperatures can rise to 1,100 degrees in 7-8 minutes and up to 2,000 degrees in 4 hours. As I mentioned, epoxy tends to melt in as low as 150 degrees, so the FRP system will no longer have a valuable bond to the existing structure within a few minutes. So before we can design the upgrade, we need to evaluate the existing structure’s material and associated fire rating based on ACI 216.
To calculate the existing strength, based on ACI 216, we can use the example of a loss of 75% of 60ksi in a steel beam would leave you 45ksi after 2 hours exposed in the extreme load of a fire. From these data points you can understand the reduced capacity of the cross sections. While calculating the reduced capacity, you will need to assume the FRP is gone and will not add additional strength to the beam.
Fire proofing the FRP is an option such as V-Wrap™ FRS; however, the costs for specialized fireproofing material can be expensive. So from the calculations, we need to determine if the reduced strength is sufficient if the building fails. If it is, there is no need in fireproofing the FRP. If it is not, then we need to evaluate is common troweled / sprayed on material will increase the time enough to meet satisfactory fire ratings, as these materials tend to be a better price point. The down side of troweled / sprayed on material is that it is difficult to have an aesthetic finish. If the finish needs to be aesthetic or the fire rating cannot be met using troweled / sprayed on materials, then we consider the fireproofing coatings for FRP systems to meet the capacity needs.