Three novel low-clinker-high-performance concretes (LCHPC) with clinker replacement levels of 54, 58 and 70 % were developed to substitute high-performance concrete (HPC) in carbon-fiber-reinforcedpolymer (CFRP) prestressed structural elements. The clinker substitution was achieved by a high amount of limestone filler and smaller amounts of metakaolin and silica fume. After 28 days, the LCHPCs reached high compressive strength values, between 77 MPa and 88 MPa, and Young's moduli between 35 GPa and 44 GPa. The LCHPCs showed self-compacting properties and low creep and shrinkage in comparison to a reference HPC. A finite element analysis (FEA) of the internal stress development over time in CFRP-prestressed structural elements was performed, including creep and shrinkage. A higher prestress level was maintained in the LCHPCs compared to the reference concrete, thanks to their low shrinkage and creep. In contrast, the use of ultra-high-modulus CFRP prestressing tendons leads to increased pre-stress losses.

Abstract: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.