Cement and Concrete Composites 19 (1997) 35-44 0 1997 Elsevier Science Limited Printed in Great Britain. All rights reserved 09%9465/97/$17.00 + 0.00 ELSEVIER PlI:SO958-9465(96)00039-X Distributed Water Ingress and Water Potential Measurements using Fibre Optics W. C. Michie,” B. Culshaw,” A. McLean,a M. Konstantaki” & S. Hadjilouca@ “Optoelectronics Division, University of Strathclyde, 204 George Street, Glasgow Gl lXW, UK bCybernetics Department, University of Reading, Reading RG6 2AY, UK Abstract We report on a distributed moisture detection scheme which uses a cable design based on water- swella ble hydrogel polymers. The cable modulates the loss characteristic of light guided within a multi-mode optical fibre in response to relative water potentials in the surrounding environment. Interrogation of the cable using conventional opti- cal time-domain rejlectometry (OTDR) instruments allows water ingress points to be iden- tified and located with a spatial resolution of 50 cm. The system has been tested in a simulated tendon duct grouting experiment as a means of mapping the extent of fill along the duct during the grouting process. Voided regions were detected and identified to within 50 cm. A series of salt solutions has been used to determine the sensor behaviour over a range of water potentials. These experiments predict that measurements of soil moisture content can be made over the range 0 to - 1500 kPa. Preliminary data on soil measure- ments have shown that the sensor can detect water pressure changes with a resolution of 45 kPa. Applications for the sensor include quality assurance of grouting procedures, ven’fication of waterproofing barriers and soil moisture content determination (&or load-beating calculations). 0 1997 Elsevier Science Limited Keywords: Optical fibre sensors, distributed sensing, hydrogels. INTRODUCTION The appearance of widespread failures in bridges which are 20 years of age or older’ has highlighted the importance of effective monitor- ing systems which are able to identify structural problems at an early stage. In Western Europe, the annual civil infrastructure repair/mainten- ance costs were estimated in 1990 to be around 190 billion ECU, representing 32% of the total investment in building and construction.2 In the United States the problems are on a similar scale.2 Approximately 40% of all US steel bridges built before 1960 and 10% of concrete bridges are having significant problems. The potential for monitoring systems to reduce operational maintenance costs by identifying problems at an early stage and by verifying the effectiveness of repair procedures, is clearly sig- nificant. The means of installing effective monitoring equipment and processing the col- lected data represent a considerable challenge which is being actively pursued world-wide. The bulk of commercialised sensing instru- mentation measures physical parameters such as loads, distances or movements. In many instances these measurements are secondary indicators of other problems such as corrosion. By the time that the physical movements are such that the problem can be identified the corrosion may well be at an advanced state and expensive repairs unavoidable. In this present work we have focused attention on a means for performing distributed chemical measurements as a means of complementing the physical parameter instrumentation base. The measurement system presented here uses optical time-domain reflectometry (OTDR) and a novel cable design which enables chemical parameters to modulate the loss of an optical fibre. To date this approach has been demon- strated as an effective method for detecting locations of water ingress,3 with potential appli- cations in monitoring waterproofing layers. It 1E