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Vibration-based damage identification can constitute a successful approach for Structural Health Monitoring (SHM) of civil structures. It is a non-destructive condition assessment method, dependent on the identification of changes in the modal characteristics of a structure that are related to damage. However, the damage identification from the modal characteristics of existing structures currently suffers from a low sensitivity of eigenfrequencies and mode shapes to certain types of damage. Furthermore, the sensitivity of eigenfrequencies to environmental influences may be sufficiently high to completely mask the effect even of severe damage. Modal strains and curvatures are more sensitive to local damage, but the direct monitoring of these quantities is challenging when the strain level is very low. In the present work, the identification of the modal strains of a pre-stressed concrete beam, subjected to a progressive damage test, is performed. Dynamic measurements are conducted on the beam at the beginning of each cycle and its response is recorded with multiplexed Fiber-optic Bragg Grating (FBG) strain sensors. Bending, lateral and torsional modes are accurately identified from dynamic strains with a typical measured root mean square (RMS) strain value of about 0.35 micro-strains. The evolution of the modal characteristics of the beam after each loading cycle is investigated. Changes of the eigenfrequency values, the amplitude and the curvature of the strain mode shapes are observed. The changes in the strain mode shapes appear at the locations where the damage is induced, and are already identified from an early damaged state. [ABSTRACT FROM AUTHOR]

Abstract: The (DSM) direct strength method [1] is used in order to predict the axial compression capacity of innovative cross-section shapes for built-up columns from cold-formed steel (CFS) profiles. The proposed four composed shapes have been adapted in order to obtain members of increased strength-to-weight ratio. After excluding various buckling mechanisms from the columns response to compressive loads, highly stable members with reduced sensitivity to initial imperfections are obtained. DSM predictions are validated experimentally for the four built-up cross-sections types. The investigation shows that DSM could easily be extended towards composed members if provisions for members sensitive to global-distortional buckling interaction are included in the method. The good agreement with experiments also indicates that the proposed built-up assemblies are reliable and can be integrated into everyday construction practice. [Copyright &y& Elsevier]

Abstract: The influence of mineral additions and chemical admixtures on the shear thickening flow behaviour of powder type self-compacting concrete (SCC) is studied by means of a wide-gap concentric cylinder rheometer. The Couette inverse problem is treated by means of the integration method in order to derive the flow curve τ(γ̇) from the torque measurements. According to the experimental results, the shear thickening effect is found to be strongly influenced by the addition of the chemical admixture (a polycarboxylate ether based superplasticizer), whereas mineral additions were found to modify the intensity of shear thickening. The limestone, quartzite and fly ash addition used in this research project, respectively increase, unalter and decrease the shear thickening intensity. The powder volume and the available amount of free water proved to have a major impact on the viscosity of the powder type SCC mixes. Increasing the powder volume or decreasing the amount of free water results in an increased viscosity of the SCC mix. [Copyright &y& Elsevier]

Abstract: Based on an experimental programme, including autogenous shrinkage tests on concrete, ultrasonic monitoring of fresh concrete, and mercury intrusion porosimetry, the influence of the filler type on the autogenous shrinkage of self-compacting concrete has been investigated. The onset of percolating structure formation (time zero) is influenced by the filler type due to a possible accelerating effect of the filler on the cement hydration. Limestone filler accelerates the hydration process, and reduces time zero, while this is not the case when using quartzite filler. Using fly ash, showing only a slight acceleration, a slight reduction of time zero is obtained. A very significant swelling peak is noticed during the first day. This swelling peak is not a measuring artefact, but is caused mainly by water absorption on the filler surface and resulting disjoining pressure. Some part of the swelling peak might also be caused by Ca(OH)2 crystallisation. The fineness of the filler is an important factor for this swelling behaviour, although also the nature of the filler seems to have an influence. The superplasticiser also interacts with the fillers, influencing the swelling behaviour. The different filler types used in this study did not lead to significant differences in critical pore diameter of the microstructure. This might explain why no significant differences have been found in final autogenous shrinkage values using the different filler types. [Copyright &y& Elsevier]