Mechanical anchorage of prestressed CFRP tendons : theory and tests

Abstract: Fibre Reinforced Polymers (FRPs) are slowly becoming important materials to consider also for a structural engineer. They are light-weight, insensitive to corrosion and have highly modifiable mechanical properties. Strengths five times higher than that of ordinary reinforcing steel are common and that combined with the possibility to vary the modulus of elasticity makes them suitable to use in combination with concrete. Carbon fibre based polymers (CFRPs) especially serve as an excellent substitute for steel in the rehabilitation of structures. A case study on that subject is presented in this thesis while the focus lies on the use of CFRP as a material for use in prestressing tendons, and to be more precise, on the anchorage of prestressed CFRP tendons. FRPs orthotropic properties highly influence their behaviour in different directions. The best properties are reached through tension in the fibre direction, and as such CFRP is as good for prestressing tendons as any prestressing steel. It is also not sensitive to corrosion and easy to work with due to its light weight. Mechanical properties in the transverse direction are however not that advantageous and early attempts to anchor CFRP bars by traditional mechanical prestressing anchorages have consistently failed. A thorough program for the development of a successful anchorage has therefore been undertaken. In a first step a literature review was conducted to investigate CFRPs possibilities to replace steel in prestressing applications, internally and externally, as well as traditional anchorage techniques for steel tendons. From the literature study it was concluded that CFRP may very well serve as tendons but some doubts also arose concerning the environmental effect on the CFRPs long term behaviour and the materials ability to work under bent conditions in multispan applications. The traditional anchorages will however not work properly, all of them use mechanical grip to keep the steel stressed. This is possible through the steels capacity to yield but not suitable to anchor the brittle CFRP. A state-of-the-art survey on attempts made globally during the last 15 years to come up with a suitable frictional anchorage has also been performed. It can be seen that several ideas are discussed, often in one or two publications. One Canadian research team, Al-Mayah et al. (2001-2008), has taken the development further and focused on variations of the traditional wedge anchorage. Based on the knowledge gained from the literature it was decided to further concentrate on a conical anchorage with a barrel of steel and three smooth wedges in aluminium. Simple analytical approaches to the conical wedge anchorage with smooth interior surfaces prove the importance of the angle in the wedge-barrel interface. Also frictional behaviour in the rod-wedge and wedge-barrel interfaces proves to be important factors. Numerical studies of these and other geometrical and mechanical properties give further input into the development of a pilot anchorage to be tested in the laboratory. The optimum angle of the wedge towards the barrel seems to be between 2-3°. The thickness of the wedge should be kept as small as possible and it is favourable with high strength steel in the barrel. A small displacement of the wedges towards the unloaded end of the tendon in the design of the anchorage does also reduce the overall slip of the rod during tension. After overcoming initial problems not discovered in the analytical or numerical models the developed anchorage performed well during laboratory tests. In short term tests performed on an 8 mm thick circular rod 100 % of the rods ultimate capacity was reached. During the tests measurements of displacements and strains were performed. Fibre Optical Sensors (FOS - Bragg gratings) were for the first time included in the interior of the anchorage to give a complete picture of the load phase. These measurements were compared to a refined finite element model and show reasonable agreement. The largest source of error is assumed to be the complicated frictional behaviour in the material interfaces and the transverse material properties of the CFRP. Lastly a case study on the strengthening of a 50 year old trough bridge in Frövi is included. The bridge was successfully strengthened for bending in the transverse direction with 23 Near Surface Mounted Reinforcement (NSMR) bars in the lower part of the slab while 11 holes are drilled underneath the upper steel reinforcement to facilitate CFRP tubes with an outer diameter of 32 mm and a thickness of 4 mm. The lack of bending capacity was discovered by a consultant in 2005 and calculations with a new approach in this thesis show that the strengthening was necessary although on a minor scale. New calculations of the capacity show that the bridge's capacity after strengthening is well above the design load and measurements on site secure that the CFRP is utilized correctly as a load carrier.