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New Research on the Behaviour of FRP Strengthened Concrete Elements in Fire

18 May 2004

The University of Glamorgan, in conjunction with the BRE, has carried out a pilot study on the behaviour of Fibre Reinforced Polymer (FRP) strengthened concrete beams in fire.

Preparatory findings were presented at the recent Composites Processing Association, in Birmingham in April this year.

The need for repair and strengthening of civil infrastructure is well documented. In recent years, various FRP composites (Carbon, Glass and Aramid) have been increasingly used to strengthen existing building and highways structures in the UK and worldwide. The pultruted FRP plates or woven fabrics are adhesively bonded to the tension surface of concrete or steel structures, which leads to substantial increase in flexural or share capacities. However, there is little knowledge of the behaviour of FRP strengthened concrete structures in fire.

The lack of research into FRP composite strengthened structures led Larry Craigie of the Composites Fabricators Association (now ACMA) to state last year that “restrictive codes and regulations along with a lack of understanding of material fire performance have been key barrier to widespread FRP use”.

In the preliminary experimental programme, four carbon and aramid FRP strengthened concrete beams (of 2.6 m in length and 200 mm x 100 mm section) were placed in a gas-fired furnace. The FRP composites were bonded to the beams using typical epoxy based resin adhesives. The pure bending zone of 1.25 m was exposed to fire, and subject to service and ultimate load, as shown in the image.

Test results indicate that FRP strengthened beams could sustain over 25 minutes of controlled fire condition before ultimate failure occurred. At the end of the test, the FRP composites were still well bonded to the concrete tension face and it appeared that in the temperature range of 300-480 oC where the concrete fabric showed no apparent signs of overstressing, the load carrying capacity of such beams was mostly unaffected from that of the control beam.

The tests showed further that the beam curvature increased significantly in comparison with the corresponding values under ambient conditions. There were some signs of flash burning of parts of the FRP, and there were noxious gases from burning of chemicals present prior to beam failure, although it was unclear whether the source was the FRP, adhesive or a combination of both.

The test findings also revealed that the ultimate tensile strength of both Aramid and Glass fibres decreases with the increase in temperature. (AFRP: -7.5% for 20 hours; -17% for 30 hours at 180 C; GFRP: 2% for 20 hours; - 11% for 30 hours at 180C)

Adhesives with higher glass transition temperature and good fire retardant properties could be developed and used in FRP bonding (vs. the commonly epoxy based resins). The commonly used FRP matrixes in construction (Epoxies, Polyester, and Vinylesters) could reformulated/supplemented by other types of matrices for better fire resistance properties where necessary.

The result of the pilot study is encouraging, albeit a number of the testing parameters need to be modified and further studied. Due to their lower heat conductibility, FRP composites are expected to have superior fire resistance performance than the traditional steel plate counterpart. The use of phenolic based matrix and resin adhesive shall further improve the fire resistance period.

Ultimately, the project aims to determine the minimum fire resistance period of various FRP strengthened elements in fire, and the effectiveness of any suitable fire protection systems.

A larger, more formal study is currently being planned at the University of Glamorgan in conjunction with the BRE. The team members consist of Professor R Delpak, Dr D B Tann (Glamorgan Researchers) and Professor DB Moore (from BRE).

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