Transient thermal tensile behaviour of novel pitch-based ultra-high modulus CFRP tendons

A novel ultra-high modulus carbon fibre reinforced polymer (CFRP) prestressing tendon made from coal tar pitch-based carbon fibres was characterized in terms of high temperature tensile strength (up to 570 °C) with a series of transient thermal and steady state temperature tensile tests. Digital image correlation was used to capture the high temperature strain development during thermal and mechanical loading. Complementary thermogravimetric (TGA) and dynamic mechanical thermal (DMTA) experiments were performed on the tendons to elucidate their high temperature thermal and mechanical behaviour. The novel CFRP tendons investigated in the present study showed an ambient temperature design tensile strength of 1400 MPa. Their failure temperature at a sustained prestress level of 50% of the design tensile strength was 409 °C, which is higher than the failure temperature of most fibre reinforced polymer rebars used in civil engineering applications at similar utilisation levels. This high-temperature tensile strength shows that there is potential to use the novel high modulus CFRP tendons in CFRP pretensioned concrete elements for building applications that fulfill the fire resistance criteria typically applied within the construction industry

Environmental assessment of radical innovation in concrete structures

In the building sector, the contribution of concrete structure to the overall emissions of greenhouse gases is significant. Switzerland is engaged in a 2050 energy strategy where the reduction of the embodied energy of buildings is a key aspect. In this study, we assess the environmental impact of different low energy concrete solutions. The study focuses on technologies that use cement with very high substitution rate (up to 65%) and other tensile resistant materials than steel in order to keep high durability targets. Hybrid wood-concrete structure, low carbon high performance concrete prestressed with carbon fiber reinforced polymer, and ultra-high performance fiber reinforced concrete with synthetic fiber reinforcement are among the studied options. The environmental assessment is done through life cycle analysis using Ecoinvent database for Switzerland and SimaPro software. Results of initial environmental assessment of production of the new technologies present huge energy and emission savings potential for the energy turnaround

Development of novel low-clinker high-performance concrete elements prestressed with high modulus carbon fibre reinforced polymers

In this thesis, new ideas for reducing the environmental impact and, at the same time, increasing the mechanical performance of carbon fibre reinforced polymer (CFRP) prestressed high performance concrete (HPC) elements were studied. This involved in particular, the initial characterization of sand coated ultra-high-modulus (UHM)-CFRP tendons and the assessment about their suitability for prestressing applications, the development of novel low clinker high performance concretes (LCHPCs) and the final proof of concept on structural level with the development of a 2nd generation of UHM-CFRP LCHPC beam elements. At first, the sand coated UHM-CFRP prestressing tendons were investigated on their bond to concrete. With the aid of a combined experimental and numerical approach, employing X-ray CT, scanning electron microscopy (SEM) and the finite element software Abaqus 6.14, a numerical model could be formulated to describe the tendon pull-out behaviour up to failure. The tendon draw-in behaviour was significantly affected by the longitudinal stiffness of the CFRP tendon. In contrast, the experimentally tested ultimate bond strength between sand-coated tendon and concrete was only dependent on the chosen sand-coating and found independent from the tendon`s stiffness. Secondly, starting from an industry reference HPC, novel LCHPCs were developed by substituting significant amounts of cement with limestone filler, metakaolin and silica fume. Three LCHPC recipes were developed with clinker replacement levels of 54, 58 and 70 %. All three recipes reached a compressive strength between 77 MPa and 88 MPa. Due to their low cement content, they showed less shrinkage and creep in comparison to a reference HPC. Based on these results a finite element model was developed in Abaqus 6.14, considering concrete shrinkage and creep, to estimate the performance of the novel LCHPCs and the UHM-CFRP prestressing tendons in a fictitious prestressing application. This model showed that high longitudinal stiffness of the UHM-CFRP tendons will lead to increased prestress losses. Low shrinkage and creep of LCHCPs, in contrast, were predicted to contribute to a high remaining prestress level the fictitious prestressed elements. Thirdly, the gained knowledge on LCHPCs and UHM-CFRP prestressing tendons was combined and three meter long UHM-CFRP prestressed LCHPC beam elements were designed. In these elements, the prestress loss over time was experimentally studied by the aid of fibre optic sensors placed inside the CFRP-prestressing tendons. Further, the beams were tested in 4-point bending and their structural behaviour was analysed by a digital image correlation system (DIC). The experimental results confirmed the previously developed numerical model. UHM-CFRP tendons showed much higher prestress loss over time. In the four point bending tests, UHM-CFRP tendons contributed to a significantly reduced beam deflection in particular when the beam was loaded in the cracked state. The LCHPCs showed no significant effect during the 4-point bending tests performed 28 days after casting. This confirmed the expectations and showed that these recipes are ready for application in CFRP-prestressed concrete elements. This work was concluded by performing a life cycle assessment (LCA) on the new beam elements using the measures of global warming potential (GWP), cumulative energy demand (CED) and ecological scarcity method (UBP). In comparison to a reinforced concrete structure savings of 80% for the CED measure and even up to 90% for the GWP and UBP measure could be reached by using CFRP-prestressed LCHPC beam elements. A direct application of LCHPCs as replacement for HPC would lead to savings between 25% and 50% for recipes containing metakaolin and up to 55-70% for a recipe which used only limestone and silica fume as cement replacement. The results of this research could be transferred without large adaptions into praxis and would significantly help to reduce the CO2 footprint of future infrastructure. In addition, this thesis sets the basis for the use of UHM-CFRP prestressing tendons in prestressed concrete and developed the first LCHPC recipes for applications in CFRP prestressed structural concrete elements

Von Bigband bis Urban Gardening : Motivationale Gründe Studierender für ein kulturelles Engagement an Hochschulen

Abstract English Artistic offerings at universities have a long tradition. Almost every university in Germany offers its students a variety of opportunities to engage in artistic and creative activities. This study investigated why students participate in these offerings. The results show that the need for community experience plays a very important role. At the same time, artistic engagement eases students' transition into college. The opportunity to try out new things in a protected setting, the experience of self-efficacy and a compensation to their studies are also important factors. It becomes clear that artistic activities at universities are not just a leisure activity, but also make an important contribution to students' personal development. For this reason, the authors suggest establishing an accompanying format at universities for reflective participation in these offerings. Abstract Deutsch Kulturelle Angebote an Hochschulen haben eine lange Tradition. Fast jede Hochschule in Deutschland bietet ihren Studierenden vielfältige Möglichkeiten, sich kulturell und kreativ zu engagieren. In der vorliegenden Studie wurde untersucht, warum Studierende an diesen Angeboten teilnehmen. Die Ergebnisse zeigen, dass das Bedürfnis nach Gemeinschaftserfahrung eine sehr große Rolle spielt. Gleichzeitig erleichtert das kulturelle Engagement den Studierenden den Übergang in die Hochschule. Die Möglichkeit, in einem geschützten Rahmen Neues ausprobieren zu können, die Erfahrung von Selbstwirksamkeit sowie der Ausgleich zum Studium sind ebenfalls wichtige Faktoren. Es wird deutlich, dass kulturelle Angebote an Hochschulen kein reines Freizeitvergnügen sind, sondern einen wichtigen Beitrag zur Persönlichkeitsentwicklung der Studierenden leisten. Aus diesem Grund schlagen die Autorinnen vor, an Hochschulen ein begleitendes Format für die reflektierte Teilnahme an diesen Angeboten zu etablieren

Model-based quantification of the effect of wood modifications on the dynamics of the violin

International audienceIn this study, a finite element model is used to simulate the impact of wood modifications on the dynamical behaviour of a violin. Chemical and fungal modifications of the wood selected for violins (tonewood) are considered. In addition, the impact of spruce indented rings anatomic singularities is also studied. Models are used to compute modal bases up to 4000 Hz, and eigenfrequencies, eigenvectors and bridge admittance are used to compare different dynamical behaviours. The impact of wood modifications over the geometric tolerances inherent to instrument makers methods is discussed. This study shows that some modifications or anatomic singularities are not sufficiently efficient to be observed reliably over the geometric uncertainties. In contrary the fungal treatment method also known as mycowood sufficiently modifies the dynamical behaviour to to overcome the luthiers tolerances. Moreover, results give a wood modifications threshold, which can be used as a support for further wood modification methods