Fibre-Reinforced Polymers

In North America, the restoration and maintenance of deteriorating infrastructure has become a major component of the construction market.

One of the most promising materials that has emerged for prolonging the life of structures is fibre-reinforced polymers, or FRPs. These composite materials also reduce maintenance costs.

FRPs have been used in the aerospace industry for decades, where their high strength and light weight can be used to great advantage. An FRP is essentially a two-component material, consisting of high-strength fibres embedded in a polymer matrix. In civil engineering applications, the most commonly used fibre types are carbon and glass.

While FRP products have been shown to enhance the capacity of bridges and other outdoor structures such as parking garages, the application of FRPs in interior spaces has been questioned because of concerns about fire. There has been a lack of information regarding the behaviour of FRP reinforcing materials at elevated temperatures.

To breach this information gap, researchers at the NRC’s Institute for Research in Construction in Ottawa have been collaborating with other researchers to study the fire performance of typical building structures that have been wrapped with FRPs to strengthen them, and then insulated to provide fire protection (see figures 1 & 2). The other research team members are ISIS Canada at Queen’s University, and industry partners Fyfe Co. and BASF Building Systems.

The research involved both experimental and numerical studies of the fire performance of reinforced concrete columns, beams and slabs. These members had steel rebar reinforcing and had been strengthened by wrapping them with FRPs. The thickness of FRPs varies from one proprietary product to another. The FRPs tested are Tyfo and Mbrace. They were installed on the different concrete systems using sprayed tacky primers and saturants.

The structures were also protected with two different types of fire insulation consisting of spray-applied cementitious plasters with thicknesses varying between 25 and 60 mm. The insulation was applied to the exterior of the FRP wrap.

A total of nine full-scale tests (five columns and four beam/slab assemblies), and four intermediate scale tests (slabs) were conducted using standard fire exposure. The researchers developed numerical models and then validated these against the experimental data obtained from the tests (Figure 1).

The study demonstrated that:

* FRP-strengthened systems that were insulated with spray-applied fire resistive material achieved a minimum four-hour fire resistance rating under service load. The insulating fire protection systems maintained low temperatures in the concrete and its reinforcing steel, thus enabling the concrete and steel to retain most of their ambient strength during the fire tests.

* Even though the glass transition temperature of the FRPs was exceeded, the systems demonstrated satisfactory fire resistance because the insulation stayed in place. (The glass transition temperature is the temperature at which the FRP experiences degradation in strength, stiffness and bond, typically less than 200C but for the material in this study, it was less than 100C.) The insulation and its proper installation were key to protecting the structure. The FRP reached its glass transition temperature and degraded within two hours

Phase 1 of the research has concentrated on beams, slabs, and circular columns strengthened with FRP sheets. The researchers are now developing design guidelines for using FRPs as exterior reinforcing for slabs, beams and columns.

Phase 2 will investigate rectangular concrete columns repaired with FRP sheets, and beams and slabs repaired with near surface mounted FRP reinforcement.

With respect to behaviour in fire, near surface mounted, or “NSM” strengthening techniques may be preferable to externally bonded repair systems for at least two reasons. First, the FRP is placed within the concrete cover, thus providing some measure of thermal insulation to the FRP and the adhesive. Second, adding supplementary fire insulation on the exterior of a near surface mounted FRP-strengthened concrete member would be more easily accomplished than for an externally bonded FRP system, since delamination of the FRP resulting in complete loss of the insulation would be less of a concern. The potentially superior behaviour of near surface mounted FRP reinforcement in fire thus requires investigation.

Noureddine Benichou, Ph.D. is a senior research officer with the Institute for Research in Construction, National Research Council of Canada, Ottawa. Dr. Benichou is a speaker at the NRC/IRC Building Science Insight seminar on “Fire Safety Research for Better Building Design,” being held in cities across Canada October 2007-February 2008. See irc.nrc-cnrc.gc.ca/pubs/bsi/2007