Temperature characterisation of LPG sensors for monitoring deterioration in reinforced concrete

Optical fibre technology is increasingly being used to monitor stresses and strains in structural members, thus leading to the development of intelligent structures. A research project is currently underway to further expand on the development of optical strain sensors by developing sensors for deterioration monitoring in reinforced concrete. A prototype sensor based on long period fibre gratings (LPGs) is being produced that will detect chemical changes within the concrete and on the steel reinforcement surface, thus indicating initiation of corrosion. Novel coatings will be adhered to the surface of the LPG which react with chemical changes and corrosion inducing substances at the steel/concrete interface thus changing the optical properties of the LPG. The paper presents the results of preliminary tests to characterise the LPG for temperature variations. The purpose of the tests is twofold: firstly, to determine if the LPG is sensitive to a varying external medium and secondly, to compensate for the effects of temperature in the working sensor. It is shown that the temperature coefficients for the LPGs considered do not vary too much if water is used as the external medium instead of air. It is also shown that the temperature response of LPGs can be both positive and negative. Low order resonance bands are shown to be as sensitive to temperature changes as high order bands. Finally, the importance of keeping LPGs taut when testing is emphasised as results are affected by bend- induced spectral changes.