Distributed sensing of physical and chemical parameters for structural monitoring

The need to monitor and undertake remedial works on bridges has greatly increased due to the appearance of widespread faults in bridges of 20 or more years of age. These faults have arisen due to a combination of poor initial designs, materials and construction together with changes in design rules and loadings. The magnitude of the problem was highlighted in an article by Dunker and Rabbat (1993) reporting on the structural condition of bridges in America. They suggest that of all the State steel bridges built before 1960 some 40% have significant problems, with 20% of Federal steel bridges in a similar condition: for concrete bridges the figures are 10% and 5% respectively. Clearly this is a serious situation which cannot be remedied in the short or even medium term. These observations, coupled with changes in the contractual conditions taking place throughout the civil engineering industry towards Design, Build and Operate contracts, which can include extended warranty periods, are forcing the contractor, the client and the operator to monitor structures to determine their condition. It is becoming apparent that some system to continuously assess the state of the structural health of bridges is essential. The work reported here addresses two areas of interest: the determination of strain distribution along an optical fibre length and the detection of chemical parameters as a function of linear position along a fibre length. Strain measurements, or measurements of movements, are one of many tools traditionally used to evaluate structural integrity. Fibre optic sensors are emerging as an important technology in this area due to their ability to provide distributed information. Chemical changes in structural materials are viewed with increasing importance since it is chemical activity, such as corrosion, which often is the root cause of the structural movements detected by strain monitoring apparatus