BOND PROPERTIES OF A NEWLY DEVELOPED COMPOSITE REBAR
A. Weber Schöck Bauteile GmbH, Baden-Baden, Germany
Email: [email protected] ABSTRACT Load transfer along the surface of unidirectional composites such as GFRP rebar is problematic. The properties of the rebar itself and the surrounding concrete are the critical factors. The newly developed composite rebar ComBAR has a volume content of 75% ECR-glassfibres and is based on a vinyl-ester hybrid resin. The surface geometry of the trapezoidal ribs has been optimised for a secure bond behaviour. A testing program on the relevant properties is presented. The ductility of the bond and the dependence of the bond strength on the concrete strength are shown in RILEM pullout tests. In eccentric pullout tests the low splitting forces are shown. Reinforced beams are tested to show the behaviour of standard development lengths. A detailed comparison of the bond behaviour with that of reinforcing steel is presented. KEYWORDS GFRP reinforcement, rebar, RC member, bond tests, failure mode. INTRODUCTION When talking about reinforced concrete we normally think of steel rebar in concrete. If another material is used as rebar the properties of that material can be compared with the conventional reinforcing material steel. Whereas the tensile properties of the rebar depend on the material and its composition, the bond behaviour depends on its surface geometry and the resin properties[7, 9]. Once the initial cracks have occurred in a loaded concrete member, several effects are to be observed.
Figure 1: Crack principle
1. Slip characteristics of the rebar: Due to the movement of the rebar relative to the surrounding concrete, a shear stress is activated. The steeper the slip vs. shear stress relationship, the harder is the bond characteristic. Of interest in practical applications is the range below 0,5 mm slip, which is smaller than a typical crack width.
2. Maximum bond stress: The maximum allowable shear force which can be transferred without damaging the
rebar can be derived from this parameter[1]. The maximum bond stress depends up to a certain level on the concrete strength. This level is different for each type of rebar. For steel rebar there is no limitation.
3. Splitting forces: The surface of the rebar has to transfer the forces into the surrounding concrete. This leads
to splitting forces. Depending on rib geometry and surface treatment these forces lead to cracks parallel to the reinforcement, which reduce the shear force transfer.
4. Failure Modes: In steel-reinforced concrete the concrete corbels are sheared off. This failure mode is well
known and safe. The reinforcing bars remain unharmed. Most commercially available GFRP rebar show other failure modes in concrete of normal strengths: shearing off of the bar ribs, shearing off of the sand coating, splitting of the concrete cover and failure of the indentations in the bar are the most common failure modes[7]. Important for safety and durability reasons is that damage to the bar does not take place at slip values which occur in the serviceability state.
rebar
crack width
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5. Security of bond: Of interest for an overall security concept is the total energy of the stress-slip curve. Is it possible to anchor the whole force of a rebar in the concrete? Is there a zipper-like failure, where the first ribs are overloaded and sheared off, the loads is then transferred to the next ribs, which are in turn overloaded, and so on?
6. At elevated temperatures the tensile properties are guaranteed by the glass fibres. What is the dependence of
the bond strength on the temperature? What is the maximum allowable temperature?
Answers to these questions are reviewed in the following paper. TEST METHODS To get answers to the above mentioned questions, several different tests has been performed. To determine the slip characteristic and the maximum shear force the well known RILEM test setup with a short embedded length of 5 times the bar diameter has been used[1]. What we get from this test is a bond-stress vs. slip law without disturbances due to other failure modes.
Figure 2: Standard RILEM pullout test setup
To determine the resulting splitting forces an eccentric test setup has been used. A thin metal sheet at the end of the bond length has been installed to induce a crack. For comparison, all tests have been performed also using steel rebar of the same size with the same concrete made from the same mix[2].
Figure 3: Eccentric pullout test setup
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For the tests at elevated temperatures a “push-through” test setup has been used. A concrete cylinder containing the rebar has been cut to the desired length. After curing and drying completely in an oven at 100°C, the specimen have been heated to the desired temperature. The temperature is held for half an hour. Then the specimen were brought to the testing machine and tested immediately. Using this simple test setup it was possible to perform the tests at elevated temperatures without excessive handling and clamping of the hot specimen. The results and the failure mode of the “push-through” test were similar to the pullout tests at normal conditions [3].
Figure 4: “push-through” test setup RESULTS AND DISCUSSIONS First the results of the centric RILEM tests are shown. The maximum bond stress is the same as that of reinforcing steel. The maximum bond stress occurs at lower slip values. If we analyse the bond stress vs. slip relationship we see that for a slip lower than 0,25mm (which corresponds to a crack width of 0,5mm) the bond stress is as much as 50% higher than the value for reinforcing steel. In a safety discussion the run of the curve after reaching the maximum is important. From this point of view the new rebar shows a comparable bond energy.
centric RILEM test d=16mm, 5 ds bond length, : Combar, BST 500 S
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Figure 5: RILEM pull out test
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eccentric RILEM-test, 16mm diameter, 5ds bond length, cover 20mm
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Figure 6: eccentric pull out bond test with low concrete cover
The result of the eccentric pullout test shows that cracking as the cause of splitting forces occurs later with the presented rebar than with conventional reinforcing steel. Shear force transmission is much lower than it is in the ideal conditions of the centric test. However, the conditions of the eccentric test are closer to reality. The low splitting effect is very important because GFRP rebar are often placed close to the surface of a concrete section. There is no concrete cover necessary to ensure the durability of the GFRP rebar. The concrete cover is used only to anchor the forces in concrete. Another important function of the concrete cover is the protection against fire. With reinforcing steel a concrete cover of 35mm guarantees a fire protection for approximately 90 minutes. While the thermal tensile behaviour of GFRP rebar is dominated by the properties of the fibres, which are in the same order of magnitude as those of reinforcing steel [6], the bond behaviour at elevated temperatures is dominated by the resin properties, which should be limited to much lower temperatures [3]. The following figure shows the bond behaviour for the critical temperature range.
push through test, concre te C25/30, Combar16mm
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Figure 7: bond behaviour at elevated temperature
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The results show that the concrete dominated failure extends to temperatures of approximately 200°C. Up to this temperature the bond depends on the concrete strength. At higher temperatures the resin becomes weaker than the concrete and the ribs of the rebar are sheared off. From 350°C on the decomposition of the material takes place and there is only a low residual bond strength. If the design value of the bond stress of 2,7 N/mm² is considered for the concrete grade used in the test, and the live loads are reduced for the fire load, a temperature limit for the anchorage zone in the range of 350°C is determined. If all material properties are described sufficiently, the behaviour of the system rebar concrete can be tested under typical loads like 3-point bending of a beam[5] or 4-point bending of a slab what is shown in the next figure.
4-point bending test / concrete C25/30, 18cm thickness, 1m bending without shearforce, 0,5m lever
As, Agfk =3,5cm² cv =20mmleverarms with shear reinforcement
release ComBARrelease steel
test procedure: stepwise increase of load 10 kNhold for 5 minfurther steps for ComBAR 150 kN, 170 kN, 200 kN, 240 kN and 300 kN
Figure 8: 4 point bending test as result of bond behaviour and mechanical properties
In a 4-point bending test load deflection behaviour was analysed. In addition crack widths and patterns were recorded. In the concrete slab reinforced with steel rebar seven cracks formed in mid-span. In the slab reinforced with GFRP rebar twelve cracks formed. Because of the limited tensile strain of the concrete, these lead to crack widths in the same order of magnitude as those in the steel-reinforced slab. Deflections in the load range between 70 and 130 kN were up to three times greater than those of the steel-reinforced slab. The good bond behaviour of the presented rebar could be shown in laboratory test as well as in tests of realistic concrete members. CONCLUSIONS In the following figure a clear relationship between the bond strength and the concrete strength is shown. In realistic concrete sections the bond is limited by splitting, which occurs earlier than that in the test due to the lower concrete cover and by the occurrence of bending cracks. That means we have the security of an undamaged bar for all practical concrete strengths. The design values for reinforcing steel are also shown in the figure. They are substantially lower than the values determined in the tests. The suggestion for the newly developed rebar is to use the same design values for bond strength as those used for steel rebar. Bond is a basic property of every rebar system. But the maximum value of the bond strength is not the only thing that counts. It is shown that ductility and failure behaviour as well as a high bond stress at a realistic slip are the key issues for such a system. With these properties a zipper like failure mode can be prevented. Low splitting forces allow the application of the new rebar near the surface without the risk of longitudinal cracks or preliminary bond failure. With higher splitting forces the advantages like corrosion resistance can´t be used in slim structures. From a safety point of view, an undamaged rebar surface at realistic slip values must be shown
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to guarantee the durability of the whole system. The clearly improved thermal bond properties against conventional GFRP rebars[8] allow the application not only in non critical sections of a building.
evaluation of bond strength in centric RILEM tests in ideal conditions
Figure 9: Evaluation of bond strength - concrete strength relationship for the newly developed rebar
ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the DBU German environmental foundation. REFERENCES ACI 440.3R-04 Guide test Methods for FRPs for Reinforcing or Strengthening Concrete Structures, ACI
Committee 440, American Concrete Institute, Farmington Hill, Michigan; 2004 Weber, A. Prüfbericht zum Verbundverhalten, Internal report of Schöck Bauteile GmbH, Baden-Baden 2004 Weber, A. Prüfbericht zum Temperaturverhalten,.Internal report of Schöck Bauteile GmbH, Baden-Baden,
2005 Weber, A. Juette, B. GFRP – Reinforcement ComBAR® in Diaphragm Walls for the Construction of Subway
and Sewer Tunnels, Proceedings CCC 2005 Lyon 2005 p.881 Keller, T. Biegeversuche an mit GFK-Stäben bewehrten Betonbalken Versuchsbericht CCLab2005.6-1
Lausanne 2005 Nause, P. Stellungnahme zum Brandverhalten... Internal Report IBMB Braunschweig, 2005 Füllsack-Köditz, R. Verbundverhalten von GFK-Bewehrungsstäben und Rissentwicklung in GFK-
stabbewehrten Betonbauteilen, Dissertation, Weimar 2004 Katz, A. Berman, N. Bank, L.;Effect of High Temperature on Bond Strength of FRP-Bars, Journal of
Composites for Construction, May 1999/ 73 Tepfers, R. Bond Clause Proposal for FRP-bars/rods in concrete based on CEB/FIP Model Code 90 with
discussion of needed tests, Chalmers University of Technology, Göteborg, Sweden 2004
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