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

Tobias Lämmlein,Michele Griffa,Pietro Lura,Francesco Messina,Giovanni Terrasi

doi:10.3390/polym9020078

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.

Highlights

Slender carbon fibre reinforced polymers (CFRP) prestressed concrete elements have beneficial properties compared to steel prestressed structures
The maximum bond strength obtained with the different sand coatings (Scheme 1) was evaluated by pull tests. of
Carbon fibre fragments were found rarely and only very isolated (Figure 6, upper row). These results indicate a bond failure between sand-epoxy coating and the CFRP tendon

Summary

Introduction

Slender CFRP prestressed concrete elements have beneficial properties compared to steel prestressed structures. These elements are much lighter, mostly due to the fact that CFRP does not corrode and the concrete cover can be reduced at a minimum dictated only by statics and ease of fabrication [1]. The use of HPC minimizes loss of prestress due to creep, which would lead to considerable loss of prestress in the UHM tendons with time and temperature. In these applications, the elements need to be anchored to a primary structure. The prestress transfer length, starting at the end of each element, should be kept as short as possible to reduce the dead weight and the non-usable area of the elements

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