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

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

Prestressing low clinker structural concrete elements by ultra-high modulus carbon fibre reinforced polymer tendons

The combination of low clinker high-performance concrete (LCHPC) and ultra-high modulus (UHM) carbon fibre reinforced polymer (CFRP) tendons was recently proposed for prestressed structural elements. The 70% reduction in cement content resulting in limited creep and shrinkage of the LCHPC in comparison to a conventional high-performance concrete (HPC) and the very high UHM-CFRP tendon stiffness (> 509 GPa) were expected to impact the mechanical behaviour of such structures. This study focuses on the behaviour of 3 m-long beam specimens during prestressing, concrete hardening and in 4 point-bending experiments. Fibre optic sensors were implemented inside the CFRP tendons to measure strain during those stages and a digital image correlation system was employed to monitor the 4-point-bending tests. After 28 days, the LCHPC recipe, despite a 70% cement reduction and much smaller environmental footprint, did not show measurable differences in the prestress loss behaviour in comparison to a conventional HPC. The UHM-CFRP prestressing tendons, because of their stiffness, showed both higher prestress losses of around 40% and on average a nearly doubled prestress transfer length. However, they increased the beam`s maximum load-bearing capacity by 21% and showed 47% less deflection at failure in comparison to beams prestressed with the standard modulus (UTS)-CFRP tendons.ISSN:1359-5997ISSN:0025-5432ISSN:1871-687

Bond Performance of Sand Coated UHM CFRP Tendons in High Performance Concrete

The bond behaviour of novel, sand-coated ultra-high modulus (UHM) carbon fibre reinforced polymers (CFRP) tendons to high performance concrete (HPC) was studied by a combined numerical and experimental approach. A series of pull-out tests revealed that the failure type can vary between sudden and continuous pull-out depending on the chosen sand coating grain size. Measuring the same shear stress vs. tendon draw-in (τ-δ) curves in the same test set-up, for sand coated CFRP tendons with a longitudinal stiffness of 137 and 509 GPa, respectively, indicated that the absolute bond strength in both cases was not influenced by the tendon’s stiffness. However, the τ-δ curves significantly differed in terms of the draw-in rate, showing higher draw-in rate for the UHM CFRP tendon. With the aid of X-ray computed tomography (CT), scanning electron microscopy (SEM) and visual analysis methods, the bond failure interface was located between the CFRP tendon and the surrounding sand-epoxy layer. For further investigation, a simplified finite element analysis (FEA) of the tendon pull-out was performed using a cohesive surface interaction model and the software Abaqus 6.14. A parametric study, varying the tendon-related material properties, revealed the tendon’s longitudinal stiffness to be the only contributor to the difference in the τ-δ curves found in the experiments, thus to the shear stress transfer behaviour between the CFRP tendon and the concrete. In conclusion, the excellent bond of the sand-coated UHM CFRP tendons to HPC as well as the deeper insight in the bond failure mechanism encourages the application of UHM CFRP tendons for prestressing applications.ISSN:2073-436