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
