Shear transfer across a crack in concrete subjected to repeated loading. Experimental results: Part I

This research relates to the behaviour of cracked concrete under repeated shear loading. Pre-cracked monolithic specimens were subjected to a repeated shear load. The normal restraint stiffness was applied by means of embedded reinforcing bars with a diameter of 8 mm. The variables of the tests were the concrete grade, the steel grade, the reinforcement ratio, the number of cycles, the applied shear stress level and the initial crack width. The maximum applied shear stress tm was in the range of 50%-90% of the static shear strength. The experimental program on reinforced specimens comprised 42 tests with repeated load and four tests with an increasing static shear load. The repeated tests were so-called 'high-cycle low-amplitude' experiments, i.e. the number of cycles exceeded 1,000. The displacements of the crack faces were measured by scanning the electronic signal nine times per load cycle. The load was sinusoidal. The experimental results are represented by the relations between the displacements of the crack faces and the number of cycles, the crack-opening path and the behaviour of the crack in a cycle. In the second part of the research program tests were performed on similar specimens without embedded reinforcing bars. The normal restraint stiffness was governed by four external steel bars having no bond with the concrete. The force normal to the crack face was measured by means of strain gauges glued to the steel bars.Design & ConstructionCivil Engineering and Geoscience

Shear transfer across a single crack in reinforced concrete: Additional detailed tests

This report gives information on additional detailed tests concerning shear transfer across a single reinforced crack in concrete. The bond characteristics of 8mm diameter deformed reinforcing bars have been investigated by means of pull-out tests. The bars were embedded in concrete over a length of 40mm and were tested at three different ages of the concrete. Static and sustained as well as repeated loading conditions were applied. Experiments were also carried out on push-off specimens. The axial steel stresses of the embedded reinforcing bars were measured using strain gauges stuck to the centre of each bar. After these tests the microstructure of the steel was investigated to verify whether or not yielding of the bar had occurred.Design & ConstructionCivil Engineering and Geoscience

Photonic calibration for fluorescence microscopy

Based upon a collection of compact LEDs (light-emitting diodes) and a compact photodiode, we have developed a calibration tool for fluorescence microscopes that are used as digital imaging devices. The entire device (excluding a USB connector) measures 25 mm x 80 mm x 12 mm. Virtually all commonly-used fluorophores can be simulated with one of the six LEDs. An LED is chosen from the host computer and its current range is selected (digitally) so as to provide a test of the complete dynamic range of the imaging system. Thus by varying the current through an LED in a controlled way, a controlled amount of “emission” light can be produced, transmitted through the chosen optical path of the microscope, and measured by the image sensor. The digitized intensity can then be determined as a function of the LED current. Any other (fluorescence) intensity measured through the same electro-optical path can then be characterized (and thus calibrated) by an equivalent electrical current. The excitation light is calibrated by a photodiode which has a dynamic range of 10^5:1 and thus is suitable for a variety of light sources: mercury lamps, lasers, LEDs, etc. The integration time of the photodiode as well as its gain can be digitally selected from the host computer. Further, using a Spectralon® reflector, the inherent non-linearity of the LED emission versus current can be measured by the photodiode and used to provide a look-up table compensation independent of the image sensor used in the fluorescence microscope system.IST/Quantitative ImagingApplied Science

LEDs for fluorescence microscopy

Traditional light sources for fluorescence microscopy have been mercury lamps, xenon lamps, and lasers. These sources have been essential in the development of fluorescence microscopy but each can have serious disadvantages: lack of near monochromaticity, heat generation, cost, lifetime of the light source, and possible distortions due to coherence effects. We are examining the possibility of using the new high-power light-emitting diode (LED) sources as alternatives to the above mentioned sources. LED sources are near monochromatic, are inexpensive, produce little heat, have no coherence problems, have extended lifetimes, are small, and can easily be modulated. We describe experiments comparing various LEDs to other light sources. We compare, for example, a 530 nm LED to the 546 nm line from a mercury lamp on a fluorophore whose absorption maximum is broad and in the middle between these two wavelengths.Quantitative Imaging GroupApplied Science

Adaptation of nanoarrays for the study of ?-synuclein aggregation - preliminary results

In previous publications we have shown that we can perform enzymatic reactions in nanoarrays by means of a microarray-reader based on a conventional microscope. In this publication we report on a modification of this system in order to monitor the aggregation kinetics of the natively unfolded protein ?-synuclein. We describe the motivation for this development, the problems associated with the miniaturization of the aggregation assay, and the validation of our modifications.Imaging Science and TechnologyApplied Science