Ultrasonic transmission measurements have been used for decades to monitor concrete elements, mostly on a laboratory scale. Recently, coda wave interferometry (CWI), a technique adapted from seismology, was introduced to civil engineering experiments. It can be used to reveal subtle changes in concrete laboratory samples and even large structural elements without having a transducer directly at the place where the change is taking place. Here, several load tests until failure on large posttensioned concrete beams have been monitored using networks of embedded transducers. To detect subtle effects at the beginning of the experiments and cope with severe changes due to cracking close to failure, the coda wave interferometry procedures had to be modified to an adapted step-wise approach. Using this methodology, we were able to monitor stress distribution and localize large cracks by a relatively simple technique. Implementation of this approach on selected real structures might help to make decisions in infrastructure asset management.
In steel-concrete composite girders, innovative composite dowels can be used to transfer the shear forces between the concrete slab and the steel section. Today, composite IntroductionComposite dowels are efficient, innovative shear connectors consisting of interlocking steel and concrete dowels for use in filigree composite beams (Fig. 1). The steel dowels are produced by oxygen cutting in which the cutting torch burns regular recesses into a steel plate or the web of a steel section in a continuous pass. In the composite connection, the steel dowels embedded vertically and the interstitial concrete dowels ensure an interlocking structural connection. The use of composite dowels yields very economical solutions, especially in composite sections with single-flange steel beams, since the relatively ineffective cross-sectional area of the structural steel near the plastic neutral axis is reduced [1]. Today, composite dowels are predominately used in engineering structures such as prefabricated composite bridges [2]. However, due to their ease of manufacture, excellent loadbearing and deformation properties and suitability for slender concrete slabs, these dowels are being applied more than ever in building construction as well, e.g. in integrated composite floor sys-The design of conventional shear connectors is addressed in EN 1994-1-1 [5], where approaches for determining the shear capacity of headed studs are given. EN 1994-1-1 allows for the use of alternative shear connectors other than headed studs if distinctive qualities in terms of shear and deformation behaviour are met. In conjunction with EN 1994-1-1, the design of composite dowels with puzzle-or clothoid-shaped geometry under predominantly static and cyclic loading can be carried out according to a general technical approval [6], which was certified by the DIBt in May 2013.If the concrete slab of a composite beam is exposed to tensile stresses in the longitudinal direction, transverse cracking occurs; for example, cracking can arise in the region of the interior supports of a continuous composite beam. In EN 1994-1-1, as well as in the general technical approval [6], the impact of transverse cracking on the static shear capacity of the shear connectors (headed studs as well as composite dowels) is neglected. Indeed, the effect of transverse cracks and longitudinal tension in the concrete slab on the structural behaviour of composite dowels has not yet been systematically studied. In the few known shear tests under longitudinal tension in concrete slabs [7], [8], no significant deterioration of the dowel's loadbearing capacity due to transverse cracking has been observed. In [7] it is assumed that a local pressure field in the concrete dowel resulting from the transferred dowel force leads to closure of the transverse cracks. According to [7], this crack closure yields a reduced stiffness in the shear connector, but without a significant decrease in shear capacity. However, the small database of usable experiments does not allow for a definitive asse...
According to current design codes, many existing road bridges exhibit shear capacity deficits. This is partly due to increased traffic loads and partly due to changes in the code provisions. In order to extend the service lives of these bridges, either refined design approaches may be used or strengthening measures performed. This paper describes the results of experimental investigations into how additional external prestressing influences the shear capacity of continuous prestressed concrete beams. Within the research project, six shear tests were performed on three test beams with parabolic internal post-tensioning and additional, variable external prestressing. The aim of the project was to determine the effect of external prestressing on the shear capacity of existing bridges, and whether current design approaches lead to conservative results when used for recalculating existing bridge structures.
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