Bond Durability of Carbon Fibre Reinforced Polymer (CFRP) Tendons Embedded in High Strength Concrete

Abstract

The structural performance of Carbon Fibre Reinforced Polymer (CFRP) pretensioned structures is controlled by the bond between the CFRP tendons and concrete. The bond strength of CFRP sand-coated tendons can be affected by humid environments due to the porous epoxy matrix structure or by defects in the external sand coating layer of CFRP tendons e.g. due to storage conditions. Pull-out tests were carried out to assess the bond strength performance of sand-coated tendons embedded in high strength concrete and immersed in water at either 23°C or 40°C. Sand-coated CFRP tendons with two different core diameters of either 4.2 mm or 5.4 mm were studied. To assess the effect of the sand coating coverage on the bond, half sand-coated and uncoated tendons were also tested. An image processing technique was developed to help correlate bond strength variations with variations in the sand coating. An average difference of 24% between the bond strengths of the half sand-coated and full sand-coated tendons was recorded. A large scatter in the pull-out results for the sand-coated tendons of diameter 5.4 mm was observed and this was attributed to the manufacturing process. There was no clear trend of bond strength degradation in the sand-coated tendons even after roughly 1.5 years of full immersion in water irrespective of the exposure temperature. However, an increase in the bond strength of the uncoated tendons and in the bond stiffness of all CFRP tendons was observed. This was felt to be the result of concrete autogenous shrinkage in high strength concrete and the potential swelling effects of the tendons in a humid concrete environment. Analytical models are used to describe the bond stress-slip behaviour and their suitability for sand coated CFRP tendons is studied.