As you’ve seen in ‘composites 101’, FRP is a Fiber Reinforced Polymer (or Plastic). The fibers carry the load and the resin bonds the fiber to the structure and transfers the shear load amongst the fibers. Typically the choice of fibers play no part whatsoever in the performance of FRP in a fire. The common strengthening fibers maintain their properties at very high temperatures; Carbon Fiber for example is manufactured at approximately 1,900 degrees F.
The temperature sensitive part of FRP strengthening is the polymer resin used. There are essentially two types of polymers, thermoplastics and thermosets.
Thermoplastics are what most people typically call plastics and include ABS, acrylic and polyethylene.
Thermosets include all two-part resins such as epoxies, polyesters, vinyl esters and phenolics.
Just as with steel, both exist as a solid at room temperature but once the temperature rises, all polymers/plastics will soften and then melt. The temperature at which the polymer softens is called ‘Glass Transition Temperature’ or Tg. At ‘glass transition temperature’, or Tg, the polymer begins to lose its ability to transfer shear load amongst the fibers, and subsequently between the fibers and the surface. The big difference comes when the temperature is lowered. Thermoplastics (like steel) will re-harden and regain most of their original properties although possibly in a different shape! Thermosets on the other hand do not re-harden and are structurally unsound.
The Tg of a thermoset resin is usually determined by its cure temperature. An epoxy resin designed for, and cured at 400 degrees F will have a Tg of about 450 degrees F. A phenolic resin designed for, and cured at 800 degrees F will have a Tg of about 850 to 900 degrees F. Epoxy resins used for field strengthening are typically cured at ambient temperature and have a Tg of approximately 140 degrees F. The application of heat (between 120 to 140 degrees F) will not only accelerate the cure, but will also raise the Tg to above 140F, but not much above. The Fiberbond Veloxx system has seen a Tg of over 190 degrees F.
In conclusion, a self extinguishing FRP system with, a class 1 fire rating can be safely used for seismic strengthening without a TPS. The same system can also safely be used to strengthen a structure that has a safety factor of 1. Strengthening a structure that would fail if the FRP burns off (+ 200 degrees F) should not be done unless the FRP is protected with a Thermal Protection System.
