Project: Carbon Fibre Composites for Structural Upgrade and Life Extension
Reference: CSU 28/4/53
Last update: 04/08/2003 09:29:13
The project relates to a novel use of carbon fibre composites for upgrading and extending the life of bridge structures by creating an independent roof structure and reinforcing the existing cast iron beams.
The research will investigate the combined effect of fatigue loading, creep loading and the environment on the properties of both the independent structure and the reinforced cast iron beam. A monitoring system capable of recording and reporting on the continuing materials and structural performance will also be developed.
University of Southampton
Transportation Research Group, Dept of Civil and Environmental Engineering, Southampton, Hampshire, SO17 1BJ
London Underground Ltd
30 The South Colonnade, Canary Wharf, London, E14 5EU
0207 308 2697
MSL Engineering Ltd
MSL House, 5-7 High Street, Ascot, SL5 9NQ
Burntwood, King's Worthy, Winchester, SO21 1AD
Defence Science and Technology Laboratory
Structural Materials Centre, R178 Building, RAF Farnborough, GU14 6TD
Cost to the Department: £221,194.00
Actual start date: 01 March 1998
Actual completion date: 04 January 2001
Carbon Fibre Composites for structural upgared and life extension - validation and design guidance
Author: Dr S Moy
Publication date: 01/01/2000
Source: DML Devonport Ltd
Summary of results
- Strengthening of Cast Iron Cruciform Struts.
The work confirmed the benefit of the carbon fibre strengthening, demonstrated that the performance could be predicted when applied to structures already under load and so validated the design methods used to make the prediction. Materials tests, large scale laboratory tests and full scale tests were completed.
The materials data were measured for the carbon fibre reinforced composites and some specially cast iron (to replicate the materials and geometry of struts typical of those in use in structures built in the mid-1800's). Three pairs of struts 2.5m long were cast in three different slenderness ratios and strengthened using different thicknesses of carbon fibre strengthening and with different levels of pre-load. One strut from each of two of the pairs was pre-loaded to 50% of their predicted buckling load prior to the application of the carbon fibre to replicate the dead load condition of those in service. The load was applied eccentrically at both ends of the strut.
Three cast iron struts 12.2m long that had been removed from Rotherhithe Station were provided by LUL for the project. The struts were cut in half so they were of a reasonable size to test and then one half was strengthened using carbon fibre and the other tested unstrengthened to provide a benchmark. The first round of testing did not lead to failure of the composite, and so further specimens were cut from the reinforced sections and these were then re-tested.
The tests conducted on the bespoke cast struts were less influenced by material variability and geometric imperfections because of the improved casting process. The results demonstrated that the carbon fibre applied, increased the load carrying capacity, bending stiffness and axial stiffness of the struts. The increase can be predicted using either hand-calculation or FEA techniques, even when the struts are carrying a dead load prior to the application of the carbon.
Strengthening of Tensile Flange of Metal I-Beams
This section of the work sought to investigate the level of surface preparation required on metal beams to achieve the desired bond strength, write and validate an installation procedure to enable carbon fibre strengthening to be bonded in place reliably and demonstrate that the degree of strengthening predicted in design was achieved in service.
Bond strengths of various adhesive systems were studied, and the variation in strength with bond line thickness, surface preparation and how quickly the strengths develop once the plates are installed. Further practical tests and literature surveys were completed to recommend installation procedures, to investigate how to apply the adhesive and prepare bonding surfaces to ensure a good bond is achieved. Operationally it is attractive to be able to strengthen beams without taking them out of service, and a series of tests was completed to determine the influence on bond strength of applying load to the beams whilst the adhesive is still curing.
Grit blasting has proven to be the most effective and reliable method, but it is not always the ideal solution from a practical point of view. International standards relating to preparation only refer to steels, whereas strengthening is more often required on cast iron structures. A series of photographic benchmarks have been produced to guide installation in the same way as is done for steel. It was concluded that grit blasting is still the preferred route for surface preparation.
One of the original aims of the project noted that the novelty of carbon fibre strengthening is such that end-users may require some additional demonstration that it is working beyond the data developed to demonstrate its benefit. A structural monitoring system to do this was developed and applied to a live railway bridge, D65A Acton Town. The system was able to automatically collect and process data and these data could then be monitored remotely. The results demonstrated that the design methods were still valid at full scale. The system offers a method for improved management of structures.
Strengthening of web of metal I-beams
Corrosion of the web of main girders of railway under-bridges is quite common. It is not possible to access the web directly and a geometrical '?' shape was devised that could be installed simply to provide a shear connection between the two flanges and that aimed to restore the original shear capacity of the damaged web. Only theoretical analysis was completed. Whilst the geometrical section would have been easy to install the detailed FE analysis completed demonstrated that the concept would not work in practice. Composite materials can provide a solution to this, but not in the manner investigated here where the transfer of shear forces between the two flanges was sought to be transferred by a folded laminate plate.
120 Year Design Life
Carbon fibre materials were developed at the end of the 1960's and so have only been in use for about 30 years. The civil engineering industry desires a 120 year design life. Given actual experience it is not possible to guarantee this, but a test programme was devised to look at known effects and attempt to predict their influence over the 120 year time span. The effects of moisture absorption have been documented by previous defence related programmes. The effect of acid and alkalis that could be encountered in the civil industry have not been so well researched. Work was therefore completed to investigate whether degradation due to acids and alkalis reached a lower bound or whether it was likely to continue indefinitely. This was achieved by exposing the samples in these environments at elevated temperatures and monitoring the change in performance with time. The combined effects of dynamic loads and thermal cycling were also studied to try and identify if any synergistic effects would be expected. All current indications are that the materials will be suitable for 120 year service.
The project has achieved its aims. A Design Guidance Note covering the design and installation of carbon fibre strengthening has been produced. This is backed up by test results at materials scale, component scale and full scale.
A structural monitoring system has been developed that can record and report on the performance of the strengthening and this has been demonstrated in the field. The results show that the design methods validated at laboratory scale can be extrapolated up to full scale. The system will be a valuable tool for future management of structures.
Materials tests designed to replicate a 120 year service life have been completed, and the results indicate that carbon fibre reinforced composites will be suitable for this service.
A seminar was held at the Institute of Civil Engineers on May 11th 2000 which presented the results from the programme to a capacity audience of nearly 100 people. In addition many research papers have been published and Thomas Telford published the Design Guidance Note formally in 2001.
Departmental Assessment Status: Project completed prior to implementation of Departmental Publication Scheme.