Fourth International Conference on FRP Composites in Civil Engineering (CICE2008) 22-24July 2008, Zurich, Switzerland

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1 INTRODUCTION

The tensile rupture load of tendons stressed with removable anchors is usually significantly lower than the theoretical value based on the tendon strength. The tensile strength of tendons is typically established by gluing the tendons into conical or cylindrical sockets. In order to avoid high stress concentrations at the transition tendon to grouted cone, a gradually increasing shear modulus for the grouting resin is used, or the resin is gradually reinforced, e.g. with steel balls (Sumida et al. 2007). The on-site pretensioning with removable wedges generally creates higher stress concentrations, which explains the reduced fracture load. A low prestressing level of the CFRP-tendons means low material utilisation, leading to a higher amount of reinforcement and thus to higher costs of prestressed concrete elements.

The goal of this project was the improvement of the anchorage performance through a shape optimization of the clamping devices. The relevant stress peaks in the tendons had to be re-duced, and a failure tolerant applicability of the system had to be achieved (mounting, prestress-ing). The taper angle difference and thus the pressure distribution on the tendon as well as local notch stresses at edges had to be optimized in order to reach a smooth load transfer between wedges and tendon.

The improvement of the newly designed anchorage system (collet and wedges) had to be validated by means of laboratory tests (static, creep), and by means of on-site tests to proof the new system’s applicability (static and creep for bundles of tendons).

The paper finally describes a selected architectural project realized by using thin-walled structural elements made of high performance self compacting concrete (HPSCC) prestressed with CFRP, and it gives information about design and field implementation.

Increasing the acceptance of high performance concrete prestressed with CFRP

G.P. Terrasi1, Ch. Affolter1, M. Barbezat1 & G. Bättig2 1 Empa, Swiss Federal Laboratories for Materials Testing and Research,

Dübendorf, Switzerland 2 SACAC Schleuderbetonwerk AG, Lenzburg, Switzerland

ABSTRACT: The efficient use of pultruded carbon fibre reinforced plastic profiles (CFRP-tendons) to prestress high performance concrete (HPC) highly depends on the performance of the anchorage system and on the material choice. The current applications only utilized approx. 40 % of the CFRP material strength in prestressed concrete elements. A higher prestress impli-cates lower costs of fully prestressed concrete elements. The aim of the present project was a design optimization of the pretensioning anchorage system, which has to be removable and re-usable. The optimized design was achieved by means of finite element (FE) calculations, where parametric studies were complemented with extensive experimental work for validation. The analytical results showed a reduction of up to 25% for the relevant stress peaks in the ten-dons. The static rupture load under laboratory conditions increased by 24%, and the pretension-ing level on-site could even be increased by 50%. This improvement in production efficiency can be explained with the easier applicability of the new system (i.e. failure tolerant assembly and prestressing process).

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2 CONSTITUENT MATERIALS: CFRP AND HPC

The prestressing reinforcements chosen for this work are round pultruded and sand-coated CFRP-profiles (Ø 5.4 mm). The carbon fibres are from type Tenax UTS with a volume fraction of approx. 62%, and the used resin is an epoxy type Bakelite 4434. The same resin was also used for the sand coating. The tendon’s design tensile strength is 2’000 MPa (5% fractile value), with the elastic modulus being 150 GPa. The low density and the excellent stress corrosion resistance of CFRP are well known. Other fa-vourable properties of CFRP tendons are the superior fatigue behaviour and the absence of creep and relaxation (Uomoto 2001). The above properties make the still expensive unidirec-tional CFRP-profiles particularly suitable as prestressing reinforcements for concrete elements (Burgoyne 1997). The high quality and the cost of CFRP tendons require a correspondingly high quality of the concrete matrix: HPC of strength class C90 to C100 is a suitable partner for this advanced composite reinforcement class. The compaction of HPC can be achieved by cen-trifugal casting, vibration or by the self compacting method (HP Self Compacting Concrete HPSCC (Persson & Terrasi 2002)). The advantageous characteristics of CFRP and HPC and an appropriate bond between them (over sand-coating of the CFRP) allow to minimise the weight of planned pretensioned bending elements by reducing their wall thickness while guaranteeing an excellent serviceability (no susceptibility to corrosion, high bending stiffness and high fa-tigue strength). The key advantage in manufacturing slender prestressed concrete elements is the durability of the CFRP prestressing material. Steel corrodes and a significant concrete cover (45-65 mm) is required to protect the prestressing steel from aggressive internal and/or external environments. On the contrary, only a relatively small concrete cover is required with durable CFRP tendons (15-20 mm for CFRP tendons of diameter 4-5 mm when embedded in HPC). Prefabrication using HPSCC, precise formwork and accurate positioning of the prestressing ten-dons makes it possible to pretension filigree bending elements with very small dimensional im-perfections, the deviations from nominal values being lower than +/- 5 mm.

3 OPTIMIZATION OF THE ANCHORAGE SYSTEM

3.1 Understanding the current system Figure 1 shows a typical longitudinal cut of the existing anchorage system (conical collet and wedges, left) and a cross section of the sand coated tendon in the clamping device. It is obvious, that the sand coating reduces the contact stiffness compared to a smooth tendon. The sand with a layer thickness of approx. 0.6 mm is impressed locally into the wedge material.

Figure 1. Longitudinal cut of the anchor with typical stress distribution (path 1-2; not optimized, left), and cross section of the sand coated CFRP-tendon (view A-A. right)

The currently used anchorage system had to be characterised and modelled as accurate as

possible, in order to gain the necessary parameters for the optimization of a new system. Trans-

CFRP-tendon

layer of silica sand

collet

wedge 1 2

A-A:

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verse compressive tests were performed to identify the contact stiffness between the CFRP-tendon (including the additional coating with silica sand) and the wedge material. This contact stiffness together with an assumption for the friction coefficients allowed the recalculation of the prestressing procedure. The calculated data were validated against measured load-displacement curves of the wedges recorded during tensile tests in the laboratory.

3.2 Contact stiffness measurement and recalculation The contact stiffness was measured with a test set-up close to the real conditions, but with all parts having a prismatic shape, which resulted in a plain strain distribution. The set-up is shown in Figure 2 (left).

Figure 2. Test set-up for measurement of the contact stiffness (left) and validation after the im-plementation into the finite element model

The established dependency between contact pressure and model overclosure was imple-mented into a finite element solver (Abaqus V6.6-ef1, 2006). With the derived parameters, a tensile test was recalculated. Figure 2 (right) shows the comparison between two measured and a calculated load-displacement curve.

3.3 Optimization of the clamping system The following parameters were considered to optimize the design of collet and wedges:

• Wedge (and collet) length considering a max. outer diameter of the collet of 38 mm • Taper angle difference between conical bore in the collet and the wedges • Local modifications (chamfers, fillets), i.e. shape optimization of the contact area

Several designs were elaborated and calculated. The stress distributions were compared

among each other and correlated to the possible failure criterion. Figure 3 shows a previous de-sign (right) and one alternative of the redesigned clamping device (left).

Figure 3. Comparison of new (left) and old design (right)

The following figures show the comparison of the relevant stress components for the differ-

ent designs on the path 1-2 (cf. Figure 1): Radial stress σR in Figure 4 (compressive) and the shear stress τRZ on the tendon surface in the contact area to the wedge (Figure 5).

press

CFRP-tendonprism (PPS)

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Comparison of σR vs. axial position (inner path, wedge symmetry)

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-80

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0

20

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Axial Position Z [mm]

Stre

ss [M

Pa]

current-20_sigRlength-54_sigR

length-60_sigR

Comparison of τRZ vs. axial position (inner path, wedge symmetry)

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0

10

-10 0 10 20 30 40 50 60

Axial Position Z [mm]

Stre

ss [M

Pa]

current-20_tauRZ

length-54_tauRZlength-60_tauRZ

-25%

Figure 4. Radial stress σR on the tendon (1-2) Figure 5. Shear stress τRZ

The first curve (‘current-20’) refers to the original design with no taper angle difference. The

2nd and the 3rd curve refer to new design options with wedge length of 50 mm and 55 mm, re-spectively (collet length: 54 and 60 mm). The old design shows a constant pressure distribution over the full length with a peak for the shear stress near the pulled end, whereas the new options have a gradually increasing pressure over the tendon surface, with a flatter shear stress distribu-tion. The max. reduction in shear stress equals to 25% for a collet length of 60 mm.

4 EXPERIMENTAL VALIDATION

The increase in rupture strength was verified in laboratory tests. The influence onto the creep rupture strength and the failure tolerance of the new system (repeatability, reliability) was exam-ined in laboratory as well as in on-site tests (pretensioning in a bed in the production plant). The static fracture strength was determined with a minimum of 6 specimen. The creep resis-tance strength was established with a load controlled creep test over 3 days (3 samples per load level). After the specimen had passed a load level, the prestress was increased by 100 MPa. The creep rates were recorded. No failures occurred for the indicated creep strengths.

Figure 6. Installation for creep resistance

testing in a pretensioning frame (12 CFRP-tendons, 3 days) in the pro-duction plant

Figure 7. Cross section of a three-storey rec-tangular HPSCC façade element (win-dow connection over stainless steel ‘cup’ inlays at the inner short edge)

The applicable prestress in production was verified with on-site tests on bundles with twelve tendons of 11 m each (Figure 6). An initial overload of 50 to 100 MPa at stressing is aimed for,

CFRP-tendons(Ø 5.4 mm)

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before the stress is relieved to the prestress σP necessary for the HPC element. The results are summarized in Table 1. The current prestressing level of CFRP tendons in daily production is 800 MPa. This is considerably lower than the original anchors creep strength, which is due to high geometric tolerances of the original wedge design.

Table 1. Rupture strength (s.d.), creep strength and allowable prestress σP for new and old anchor design Static strength [MPa] Creep strength, lab. [MPa] σP on-site [MPa] Old design 1’185 (83) 1’100 800 New design 1’468 (76) 1’300 1’200 Improvement % 24 18 50

5 FIELD APPLICATION

The extension of the college “Falletsche” in the city of Zurich with a new 3-storey classroom-building was designed by architect Rolf Mühlethaler of Berne. A glass-concrete façade with very slender concrete border elements was planned. This project required the production of 905 m slender rectangular concrete beams with very low geometrical tolerances, that were produced by SACAC in CFRP prestressed HPSCC. The main reason for the choice of CFRP prestressed concrete elements was the high slenderness ratio of the beams imposed by the customer under fulfilment of high durability requirements: Aesthetic criteria were decisive for the materials’ choice. 157 vertical façade beams with a length varying between 0.5 m and 11.1 m and a cross section of 100 mm x 300 mm (Figure 7) were designed and manufactured in the winter 2005-2006. The elements are supporting glass-windows.

Figure 8 shows two three-storey (11.1 m) and one two-storey (7.6 m) façade beams during quality assurance operations in the SACAC production plant.

Figure 8. CFRP prestressed HPSCC façade beams with total length of 11.1 m (first two from the

left) and 7.6 m (right)

The beams were prestressed with 8, 10 or 12 pultruded and sand coated CFRP tendons of di-ameter 5.4 mm at a total prestress of 146 kN, 183 kN, 220 kN respectively: The CFRP prestressing ratio achieved with the old anchorage design of Figure 3 was 40% of the design tensile strength of 2’000 MPa. The minimum concrete cover of the CFRP tendons was 25 mm (cf. Figure 7). Design of the elements was performed considering a simply supported beam un-der self weight (determining load case: transport and installation). The deflection criteria to be fulfilled by the beams under maximum service load was field (l) deflection < l / 300, with l = 0.5 to 11.1 m. In order to avoid cracks and limit deflections the beams were fully prestressed for maximum service loads. Relevant cross-sectional data of the rectangular façade beams prestressed with 12 CFRP tendons are the cracking moments Mcrack

y +/- = ± 18.8 kN·m, Mcrackz +/-

= ± 6.2 kN·m and the moments of resistance MRy +/- = ± 48.5 kN·m and MR

z +/- = ± 14.6 kN·m.

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The vertical façade beams were fixed to the building’s structure/windows by standard inlays in the HPSCC (stainless steel Halfen-railsTM with nail anchors for connecting the windows, see Figure 7, and hanging tensile anchors for the anchorage of the beams). Besides this, 326 thin-walled (thickness 50 mm) CFRP HPSCC screen-panels with L-profile and length 1-1.5 m were manufactured and serve as horizontal joining elements in the façade, that were fixed to the sides of the vertical façade beams at the height of the slabs of the building. The building façade was installed in spring 2006 (Figure 9).

Figure 9. College Zurich Falletsche: a slender prestressed HPSCC façade beam is highlighted.

6 DISCUSSION

The calculational improvement of the clamping device could be proved with static tests per-formed in the laboratory. The rise in rupture strength of 24% is satisfactory, and the value can be further increased with improved pultrusion matrixes of the tendons. The currently proved short-term creep strength is only 18% higher for the new clamping sleeve, as the time consum-ing creep tests were performed in rather large steps of 100 MPa.

The highest improvement was demonstrated in the on-site application, where the maximum applicable prestress in the tendon could be increased from 800 MPa to 1’200 MPa. The better overall performance originates in an additional factor besides the reduction of local stress maxima: the system provides a higher failure tolerance and it is easier to apply in production. Where the old wedges required an additional silica sand layer glued onto the tendon in order to prevent slipping of individual wedges at the start of the prestressing process, the new system does not need any additional treatment or preparation. Nonetheless, no slipping of individual wedges during pretensioning could be observed.

Although the allowable prestress of the CFRP tendons could be increased by 50%, the reached tensile capacity of the system is still 27% lower than the value based on the design ten-don strength. A further objective in the project is the optimization of relevant material properties of the pultruded CFRP tendons (transverse compressive strength and interlaminar shear strength). Eventually, prestressed HPC elements will be produced with the new components to examine their mechanical performance and fire resistance.

7 REFERENCES

ABAQUS Analysis User’s Manual (Vers. 6.6-EF1) 2006, Abaqus, Inc., Providence, RI (USA) Burgoyne, C.J. 1997, Rational Use of Advanced Composites in Concrete. In Third International Symposium on Non-Metallic (FRP)

Reinforcement for Concrete Structures (FRPRCS-3). Sapporo: Japan Concrete Institute. p. 75-88. Persson B, Terrasi G.P. 2002, High Performance Self Compacting Concrete, HPSCC. In: König G, Dehn F, Faust E, editors. 6th Int.

Symposium on Utilization of High Strength/High Performance Concrete. Leipzig, p. 1273-1290. Sumida, A., Mutsuyoshi, H., and Pandey, G.R. 2007, Development of new heat-resisting FRP bars, at FRPRCS-8 in Patras, Greece Uomoto, T. 2001, Durability Considerations for FRP Reinforcements. In Fifth International Symposium on Non-Metallic (FRP) Re-

inforcement for Concrete Structures (FRPRCS-5). Cambridge, p. 17-32.