STRUCTURAL MONITORING OF HYBRID SPECIMENS AT EARLY AGE USING FIBRE OPTIC SENSORS
Hybrid structures, notably bridges, consisting of steel girders and concrete decks, are designed to explo it 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 steelconcrete 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, longterm stability and especially their ability to measure early and very early deformation of concrete are essential for monitoring of mentioned parameters.
Characterization of Early Age Curing and Shrinkage of Metakaolin-Based Inorganic Binders with Different Rheological Behavior by Fiber Bragg Grating Sensors
This paper reports results related to early age temperature and shrinkage measurements by means fiber Bragg gratings (FBGs), which were embedded in geopolymer matrices. The sensors were properly packaged in order to discriminate between different shrinkage behavior and temperature development. Geopolymer systems based on metakaolin were investigated, which dealt with different commercial aluminosilicate precursors and siliceous filler contents. The proposed measuring system will allow us to control, in a very accurate way, the early age phases of the binding systems made by metakaolin geopolymer. A series of experiments were conducted on different compositions; moreover, rheological issues related to the proposed experimental method were also assessed.
Submicron polymer structures with X-ray lithography and hot embossing
This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiOx membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λc ≈ 0.4 nm) at a dose of 4 kJ/cm3 using a Si3N4 membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.