ANCHORING METHOD FOR PRESTRESSING OF FRP REINFORCEMENT

D Ďurech, Brno University of Technology, Czech Republic F Girgle, Brno University of Technology, Czech Republic D Horák, Brno University of Technology, Czech Republic

I Laníkovcá, Brno University of Technology, Czech Republic P Štěpánek*, Brno University of Technology, Czech Republic

  

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35th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 – 27 August 2010, Singapore

ANCHORING METHOD FOR PRESTRESSING OF FRP REINFORCEMENT

D Ďurech, Brno University of Technology, Czech Republic F Girgle, Brno University of Technology, Czech Republic D Horák, Brno University of Technology, Czech Republic

I Laníkovcá, Brno University of Technology, Czech Republic P Štěpánek*, Brno University of Technology, Czech Republic

Abstract

Non-metallic reinforcement has many advantages, but there are some areas of application that need to be resolved to improve the usage of FRP reinforcement in real conditions. One of the disadvantages of FRP reinforcement is its lower modulus of elasticity, which leads to greater deflections of structures and can also cause early propagation of cracks. The paper deals with the possibility of eliminating this problem by prestressing the reinforcement. It covers several problems related to anchoring and current anchoring methods used world-wide. The paper also mentions some drawbacks of these methods. In an effort to bypass these drawbacks a new anchoring method has been developed. It differs in the way longitudinal forces are transferred from the bar to the surrounding concrete. Basically, it is based on the addition of an additional anchoring member (cylinder) on the surface. This member is made of polymers so the whole system remains completely steel-free and very simple to produce. Before using the system in real structures a series of tests were performed and the results are presented. Also, some prestressed test panels were prepared and compared with standard panels. Keywords: FRP Reinforcement, Prestressing, Anchoring System, Fully Non-Metallic Anchoring System

1 Introduction

Application of prestressed reinforcement has been tested since 1990. First anchoring systems based on the same idea as anchoring of steel tendons (mechanical anchoring) are published in PCI Journal, [10]. Mostly were combined FRP and steel (or stainless steel) members – see fig. 1a, 1b, 1c, 1d.

A new system for the reinforcement of structures has been developed within the framework of research projects carried out at the Faculty of Civil Engineering. This system uses non-metallic materials based on carbon (CFRP) and glass fibers reinforced polymers (GFRP). Such composite reinforcement has many advantages1 but there are some problematic areas to solve and the usability of the developed reinforcement needs to be increased.

A distinctive disadvantage is the relatively low modulus of elasticity (45-60 GPa, the value varying depending on material composition) in comparison with classical steel reinforcement. This causes larger deformations of loaded structures and early crack propagation. Neither aspect affects the bearing capacity of carrying elements reinforced with FRP bars, but they certainly cause problems

when SLS are taken into account. Even though it is debatable whether or not crack formation needs to be checked (there is no risk of corrosion), a high amount, a high amount of relatively wide cracks on a deformed structure negatively affects its aesthetic appearance. The visual aspect of hidden elements does not matter (one proposed utilization, among others, is as prefabricated sewer piping and in collectors – i.e. non-accessible, thus non-visible structures in aggressive environments), but it can be the decisive negative factor in a decision regarding whether or not to use FRP reinforcement in a visible structure.

Fig. 1a – d: FRP tendons and anchoring components, [10]. a) Arapree; b) FiBRA; c) Carbon Stress Flat tendon; d) Leadline

There are basically two ways to solve this problem. One is to increase the reinforcement area – this is economically unfavourable due to the high prices of FRP materials. The second possibility is to prestress the reinforcement. This solution is smooth, and in addition to this it increases the bearing capacity of reinforced elements. However, such prestressing is complicated by the anisotropic material properties of FRP reinforcement, namely its low compressive strength. The aim of the research was to develop a reliable method of anchoring and prestressing non-metallic reinforcement. Furthermore, the requirements specified ease of use and quick installation. At the same time, the system had to maintain all the advantages of non-metallic reinforcement.

2 Current technical state

Standard techniques of anchoring prestressed reinforcement are based on the use of anchoring wedges – while using them the tensile strain originates in the reinforcement bar as well as the longitudinal shear tension and transverse pressure caused by the clinch of the wedge. Because it is not possible to transfer such a high compression force into FRP reinforcement it is necessary to modify these techniques. Currently, there are two main anchoring systems2 used around the world. They differ in the anchoring method:

2.1 Wedge system

This system is very similar to the classical anchoring system. It uses shear force between the prestressed tendon (bar) and the anchoring device to transfer the prestressing force. Sufficient shear bearing capacity is achieved mostly by:

coarsening the wedge contact surface (non-standard shape), varying the inner shape of the wedge (parabolic or graduated inner shape of the anchor) or by applying an additional metallic surface layer to the reinforcement (die-cast wedge system, Fig.

2a, 2b).

a b

c

d

The latter system is a partial hybrid between the wedge and grout system because of the possible modes of failure. This can occur as shearing failure between the prestressed reinforcement and the sleeve, or by the metallic coating being pulled out.

The problem is that the sleeve is mostly made of steel.

Fig. 2a: Die-cast wedge system Fig. 2b: Mechanical wedge system

Other types of mechanical anchoring designed and tested on some research institutions are drown on fig. 3; see [9].

Fig. 3: Mechanical anchor: Example of a clamping anchor with an inside sleeve

2.2 Grout system

The prerequisite of this system is the use of the cohesion between reinforcement and grouting (epoxy resin, special cement mortar) for the transfer of tension force into a structure. A crucial point for its function is a sufficient anchoring length that prevents the tendon from being pulled out of the sleeve. The sleeve is usually made from composite based material or a thin-walled non-corrosive metal tube (Fig. 4). It can be a straight cylindrical tube or it can be cone-shaped to enhance the transfer of the prestressing forces.¨

Looking at the descriptions of these systems it is obvious that most of them rely on steel parts to anchor the reinforcement. However, the use of such systems is in contradiction with the initial intention to use non-metallic reinforcement and to take advantage of its lack of corrosion risks or to use its other

positive aspects. That was the main reason why we started the development of our own anchoring method.

Fig. 4: Grout system (Bonded anchors: example of contoured sleeve and straight sleeve bonded anchor)

3 The developed anchoring system In the case of the newly developed anchoring system, the method used to safely transfer prestressing force into the surrounding concrete lies in the creation of a sufficient additional anchoring surface. This anchoring area is created by adding one or more anchoring cylinders (made of a special epoxy mixture) of a large diameter to the reinforcement bar. The cylinder material has higher cohesion with FRP reinforcement than concrete or other standard building materials have. Thus, it is possible to transfer a higher amount of tension force (shear force between the reinforcement and the anchoring cylinder respectively) via a shorter anchoring length. Due to the larger diameter of each cylinder, an additional surface is created around the bar that serves in transferring the force into the surroundings. The total carrying capacity is given by a combination of front side compressive strength and the shear carrying capacity of the anchoring cylinder’s surface (Fig. 5).

Fig. 5: Working mechanism of the anchoring cylinder with acting forces

Of course, it is possible to combine the cylinders and chain them into a series – either to increase carrying capacity or to improve the safety and reliability of the anchoring system.

The creation of the anchoring device itself is very simple and its implementation is undemanding. It allows the preparation of reinforcement right where members are produced, or even later directly at the construction site.

4 Performance tests

4.1 Load carrying capacity of the anchor The first sets of the experiments were prepared with different types of materials and different shapes of anchoring devices. The aim was to create such an element as would meet the requirements for sufficient compression strength (crushing of the front side) and excellent cohesion with FRP reinforcement, and also be easy to produce. After a number of tests, a simple cylindrical shape was

selected as optimal. Resins with mineral filler proved to be a suitable base material. Furthermore, this material can be complemented with non-metallic fibers (dispersed or oriented) to increase the toughness preventing the cylinder from undergoing transverse rupture.¨

From the very beginning the anchoring system was tested with developed FRP reinforcement. The purpose of these tests was to determine the cohesion between the cylinder and the reinforcement bar. During the experiment the anchoring cylinder was placed with only its front side leaning against the plate (i.e. the transverse deformation was not restrained). The force needed to pull the reinforcement out was measured. During this setup a large problem emerged – the cylinders themselves are not capable of transferring the evolving transverse forces and they are inclined to rupture before full pull-out force is reached (Fig. 6). The maximal reached load level was around 60% of the total tensile strength of anchored GFRP reinforcement.

Such a form of stress does not occur very often (supposedly it can arise during additional prestressing if the final anchoring device is not wrapped in concrete) but it can happen. This problem can be solved quite easily by adding a surface layer with higher tensile strength on top of the cylinder. A significant improvement was gained by wrapping the anchor with carbon fabric or casting the anchor into a composite tube of high tensile strength. Another possibility is to provide the wrapping by surrounding concrete.

Fig. 6: Transverse ruptured anchoring cylinder (loaded without the surrounding grip of concrete)

A further set of experiments was prepared to examine the behavior of pre-cast prestressed concrete elements, i.e. in the case that the anchoring cylinder is wrapped in concrete that prevents transverse rupture. Using this working scheme, it is possible to anchor the prestressing force equal to the maximal tensile strength of the used reinforcement by using only one single cylinder of appropriate length.

The experiments focused on the search for a sufficient cylinder length were based on pull-out tests described in ACI standards3. The test scheme consisted of the developed reinforcement with glued anchors (of different lengths and a diameter of 45 mm) and a concrete block of the dimensions 300x300x300 mm. The reinforcement was separated from the concrete along the zone in front of the anchor. Based on the test results it appears that 70 mm is a sufficient length for the anchoring cylinder (for the given type of reinforcement and used materials and with a diameter of 14 mm). Such an arrangement is capable of transferring the forces needed to rupture the reinforcement, which is one way the failure occurred – the other failure mode was the failure of the joint between the reinforcement and the anchoring cylinder (Fig. 7). An important outcome is that the front side of the cylinder was not damaged by pressure, so the material has sufficient compression strength.

Fig. 7: Cast-in anchoring cylinder after a pull-out test (with a marked crack open between reinforcement and anchor)

The next question was regarding the influence of chained cylinders. These tests were mostly carried out by numerical analysis. In this way it was possible to prepare a parametrical study that covered the influence of the total amount of the cylinders and also the distances between them. As the results indicate (Fig. 8), the optimum number of anchoring cylinders is three. Adding additional cylinders does not increase the maximum anchoring capability of the system.

Fig. 8: Influence of the total number of anchoring cylinders on the maximum anchored force (constant distance between cylinders: 50 mm)

The system mechanism can be divided into three phases (see the graphical description in Fig. 9):

1) Only the first cylinder is fully activated while the subsequent cylinders are not. The tension force in the reinforcement bar is almost completely transferred into this first cylinder and only a little part

of it is transferred further along the bar. The cohesion between the reinforcement bar and the first anchoring cylinder is at its maximum.

2) The cohesion between the reinforcement and the first cylinder is not capable of transferring the acting tension force and the contact between them is partially damaged (the post-peak part of the stress-displacement cohesion diagram is active). A portion of the tensile force is still being transferred into the first cylinder and the rest activates the second cylinder. Meanwhile, the residual force passing further beyond this cylinder increases slightly. This process repeats through all the cylinders. The maximum anchoring force is reached when the cohesion force is maximal (i.e. the area covered by the diagrams in Fig. 9 is maximal). After this moment the failure mode quickly begins.

3) Just before the failure of the whole anchoring system the front cylinders are damaged (i.e. the cohesion between them and the bar is not capable of transferring any force) and the last cylinders are fully activated. However, the total cohesion is no longer able to balance the acting tension force in the bar. Therefore, the reinforcement is pulled out of the rest of the cylinders and the system collapses.

However, it has to be mentioned that these results are highly dependent on the space between the cylinders. If this space is too short (i.e. around 2-3 cm), the failure mechanism utterly changes. In such cases, after the activation of the second cylinder high-pressure zones are caused between the cylinders above the front surface. Because the concrete is not capable of withstanding such pressures it is crushed and the whole anchoring zone is severely damaged. So far the optimum distance between the cylinders seems to be around 5-7 cm.

The other limit condition is the bearing capacity of the reinforcement. Should the maximal cohesion between reinforcement and cylinders be higher than the bearing capacity of the bar itself, the reinforcements would rupture. In such case the capacity of the anchor is not fully used and the economy of such a design is questionable.

Fig. 9: Scheme of the cohesion stresses during the individual phases of loading

(the small diagrams under the stress schemes show the stress-displacement diagram for cohesion and the currently active part of it)

4.2 Behaviour of prestressed specimens

After evaluation of the initial experiments, a set of prestressed concrete elements was prepared (concrete panels with the dimensions 2700 x 450 x 150 mm). They were reinforced with three GFRP bars prestressed up to 30% of their tensile strength (i.e. the approximate tension in each bar was 250 kN). The relatively low prestress proportion was chosen according to the limitations mentioned in ACI4,5 standards and other publications6,7 covering the long-term behaviour of FRP materials. To gain the most exact data possible the reinforcement bars were fitted with built-in strain gauges8; the exact locations are shown in Fig. 10 and Fig. 11. These panels were prestressed before the actual casting of the concrete.

The precast concrete beams were chosen for these initial experiments, because of an early state of prestressing mechanism. In such condition, it was easier to apply and to control the prestressing forces in the GFRP bars and to avoid the undesirable anchorage seating.

Fig. 10: Locations of the built-in strain gauges in the reinforcement

Fig. 11: Real picture of the built-in strain gauges between the anchoring cylinders in the reinforcement bars placed in the formwork

The prestressing force was applied symmetrically to the middle bar and then to both the lateral bars. The prestressing force was then checked and the additional prestressing was applied twice to balance out the losses of strain in reinforcement caused by deformation of the prestressing frame (see the peaks and depressions in Fig. 12). After the prestressing the reinforcement and the frame were left for about two hours and after that time the concrete was cast in place. The specimens were cured for

three days. After that the anchored ends of the reinforcements were cut and thus the prestressing force was applied.

Fig. 12: Strain in the reinforcement bars during the prestressing

At the age of 28 days the panels were exposed to a four-point bending test. The objective of the test was to study the behaviour of the prestressed panels especially compared with their non-prestressed variants. Also, one specimen was prepared using standard steel prestressing tendons. In this case the total prestressing force was the same as with the GFRP reinforced panels.

During the tests it became clear that the prestressing caused significantly distinct behavior compared (Fig. 13) to the non-prestressed specimen. While the latter specimen suffered from early crack propagation the prestressed specimens were capable of withstanding a load force more than twice as high (38 to 15 kN total load). Also, the number and maximal width of cracks were reduced in comparison to the non-prestressed specimens. The deflection of the prestressed panels was approx. three times lower (80 to 26 mm).

All specimens reinforced with GFRP bars broke due to the rupture of the reinforcement, as intended. The specimen reinforced with steel tendons was not destroyed. Because the tendons were anchored only by cohesion they were not able to anchor the necessary forces. Both ends of the tendons started to slip and thus the panel bent without the load increment while the cracks opened and widened.

The behaviour of the steel tendon prestressed specimen can be used as a illustrative example of the benefits the developed anchoring system can provide concerning also pre-cast structures. Since the observed failure mode is not normal for the typical steel prestressed beam with an adequate development length, it is evident that its anchoring length was not sufficient (although it was the same as in the case of FRP prestressed beams with the anchoring cylinders). This confirms the theory that the anchor shortens the development length and the anchoring length of the prestressing bar. So, the anchoring cylinders can be used

- either to shorten the necessary development length, or - to apply the prestressing force in the exact spot, if we use the separation of the bars in front of

or around the anchoring cylinder.

Fig. 13: Differences in the behaviour of the prestressed and non-prestressed specimens

5 Conclusion

Results and experience gained during the experiments show that the developed system of non-metallic reinforcement is fully functional and can be used for the construction of new structures. Furthermore, it succeeded in simplifying the method of anchoring prestressed FRP reinforcement. This allows the usage of FRP reinforcement in a much wider range of possible applications and it also improves such products significantly. The improvement consists in the reduction of the deflection of bending structures and subsequent cracking. While both phenomena are not limit factors for the ultimate limit state of structures they can cause severe problems in serviceability limit state design. If such cases occur then prestressing is an elegant way to solve such problems.

The ongoing research doesn’t only aim to improve and introduce new materials; the gained data gathered during experiments serves as input data for related theoretical research. This work is focused on creating and verifying numerical equations describing the anchorage zones. After that, the deduced relations could be used for the design of generally any structure reinforced with FRP reinforcement. This is very important because of the lack of any local design code intended for FRP reinforcement.

At the same time, several long-term experiments are currently running. These will provide data needed for the further development of design recommendations and for their extension to cover time dependent influences. Also, the influence of changing temperature (standard weather changes or even fire impact) is being closely observed and will be taken into account.

6 Acknowledgement

This research has been carried out with the support of the Czech Ministry of Industry and Trade within the framework of research task FT TA 5/036 Management of risk, reliability and durability of concrete structures, Czech Grant Agency GAČR 103/09/H085 Modern Composite Structures and Research task of Czech ministry of Education, Youth and Sports MSM0021630519 “Progressive reliable and durable load bearing structures”

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structures under various load schemes”, Proceedings of the Fifth International Symposium Computational Civil Engineering 2007”, Editura Societatii, Iasi, Romania, 2007

[2] Vistaps M. Karbhari, “Use of composite Materials in civil infrastructure in Japan”, WTEC Study on Use of Composite Materials in Civil Infrastructure in Japan. University of California, San Diego 1998.

[3] ACI 440.3R-04 Guide Test Methods for Fiber-Reinforced Polymers (FRPs) for Reinforcing or Strengthening Concrete Structures, 2004.

[4] ACI 440.1R-06 Guide for the Design and Construction of Structural Concrete Reinforced with FRP Bars, 2006.

[5] ACI 440.4R-04 Prestressing Concrete Structures with FRP Tendons, 2004. [6] Dolan W. C., Hamilton H. R., Bakis E. C., Nanni A., Design recommendations for concrete

structures prestressed with FRP tendons – Volume 1, University of Wyoming, 2001. [7] Robert M., Benmokrane B., "Characterization and long-term durability of GFRP reinforcing bars

embedded in moist concrete", Proceedings of the 9th International Symposium on Fiber Reinforced Polymer Reinforcement for Concrete Structures, Sydney, 2009.

[8] Horak D., Krupa P., Prokes J., Stepanek P.: “Development of new methods of measuring non-metallic reinforcement in concrete”, Workshop NDT 2006 - Non-destructive testing in engineering practice, 2006, pp. 38-42.

[9] Schmidt J. W., Täljsten B., Bennitz A, Cowi A. S. FRP tendon anchorage in post-tensioned concrete structures. Concrete Repair, Rehabilitation and Retrofitting II – Alexander et al (eds), © 2009 Taylor & Francis Group, London, ISBN 978-0-415-46850-3

[10] Nanni, A. – Bakis Ch. E – Dickson, T. O.: Performance of Tendon Anchor Systems for Prestressed Concrete structures. PCI Journal, 01-02/1996