Hybrid structures, notably bridges, consisting of steel girders and concrete decks, are designed to exploit in the best way the mechanical properties of both materials. In order to guarantee safe, durable and low-cost service of such structures, a long-term quality and a robust connection between steel and concrete must be guaranteed. Even if experience in construction, maintenance and exploitation of these structures is large, a good and long-lived connection remains a challenging point in their realisation. This is mainly due to the initial incompatibility in the dimensional behaviour of two materials: early and very early age deformations of concrete (thermal expansion and total shrinkage) generate stresses in both materials even before service and imperils their good interaction. With the intention of a better understanding and an accurate numerical modelling of the steel-concrete connection in hybrid structures, measurements are indispensable and, in particular the deformation evolution in both materials during the early and very early age of concrete. In this paper we present examples of this type of measurements, the monitoring of the total dimensional evolution of hybrid elements, the determination of hardening time of concrete and structural monolithisation and, indirectly, the determination of concrete's Young modulus evolution. An approach to concrete delamination detection is also assessed. All these parameters are evaluated using fibre optic sensors directly embedded in the fresh concrete or attached to steel elements. Their high accuracy, long-term stability and especially their ability to measure early and very early deformation of concrete are essential for monitoring of mentioned parameters.
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