trc pedestrian bridge – design, load-bearing behavior … · the use of a fine grained concrete,...

International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. I, ICTRC 353

TRC PEDESTRIAN BRIDGE – DESIGN, LOAD-BEARING BEHAVIOR AND PRODUCTION PROCESSES OF A SLENDER AND LIGHT-WEIGHT CONSTRUCTION

J. Hegger, C. Kulas, Institute of Structural Concrete, RWTH Aachen University, Germany H. N. Schneider, Chair of Building Construction and Design, RWTH Aachen University, Germany W. Brameshuber, M. Hinzen, M. Raupach, T. Büttner, Institute of Building Materials Research, RWTH Aachen University, Germany

ABSTRACT: The pedestrian bridge over a state road in Albstadt, Germany, had to be torn down due to immense corrosion damages of the steel reinforcement. The design of the new bridge allows a slender construction, thus, the new composite material textile reinforced concrete (TRC) is used. By using textiles made of non-corrosive materials like alkali-resistant glass rovings, concrete covers can be reduced to a minimum of only some millimeters to minimize the cross-sections.

The paper describes the design, load-bearing behavior and production processes of a 100 m long pedestrian bridge subdivided into six prefabricated TRC parts, each offering a maximum length of 17.20 m and a maximum span of 15.05 m. The 3.21 m wide cross-section, which is a T-beam, has a height of only 43.5 cm resulting in a slender bridge construction with a slenderness ratio of only H:L = 1:35.

1 INTRODUCTION

Existing pedestrian bridges made of steel reinforced concrete often show damages caused by the corrosion of the reinforcement. The concrete covers of those constructions were designed in accordance with former standards, but are too small with regard to the required corrosion protection of steel reinforcement against carbonation and chloride ingress. The corrosion of the steel reinforcement leads to cracks and spalling of the concrete. These damages cause optical detractions on the one hand, and a reduced load-bearing capacity of the construction on the other. In dependency of the state of corrosion these structures have to be rehabilitated by cost-intensive actions or replaced by new structures. One example of an older pedestrian bridge, which is damaged by corrosion of the reinforcement, is the pedestrian bridge over a state road in Albstadt, Germany. Due to the immense corrosion damages, this bridge has to be torn down and has actually been replaced by a new bridge.

The aim of the design of the new bridge was a slender fair-faced concrete superstructure fulfilling demands on a frost resistant construction. Thus, the innovative composite material textile reinforced concrete (TRC) was chosen as construction material instead of the commonly used steel reinforcement. The main advantage of using textiles like alkali-resistant glass (AR-glass) is the possibility of reducing the concrete cover to a minimum because the corrosion protection needed for steel reinforcement is not required. The concrete covers can be reduced to a minimum of only some millimeters resulting in slender, light-weight and sharp-edged structures with high-quality surfaces.

The paper deals with the used materials, the design, production processes and gives detailed information on the experimental investigation of a large-scale TRC construction.

354 HEGGER ET AL.: TRC Pedestrian Bridge – Design, Load-bearing Behavior and Production Process

2 MATERIALS

2.1 Textile reinforcement

TRC uses mesh-like non-corrosive reinforcement structures, which are usually made of AR-glass or carbon. The advantage of carbon is its high tensile strength of over 2000 MPa, but there is a lack of availability and it is more expensive in comparison to AR-glass. Thus, for the pedestrian bridge AR-glass rovings are used as basic material, which are finished to a laid-scrim – a mesh-like structure – which is shown in Fig. 2.1.

Fig. 2.1. Textile reinforcement

In order to activate nearly the full tensile strength of the AR-glass reinforcement, the textiles were impregnated using a warm hardening epoxy-resin – so-called PrePreg System (PREimPREGnated), [Rau06]. After the impregnation process, the resin was transferred from the sticky B-stage to a rigid system by hardening at 120 °C for two hours.

Table 2.1. Tensile strength of a roving in 0° direction

Parameter Unit 23 °C 80 °C Tensile strength 1950 1880

Standard deviation MPa

113 77

Due to the slenderness and the exposure of the construction it can be expected that the reinforcement will be exposed to elevated temperatures during spring and summer. So it is required that the tensile strength of the impregnated reinforcement is not reduced due to an elevation of the ambient temperature. The specimens used for the tests were single rovings with a length of 150 mm, which were glued into threaded sleeves with a warm hardening epoxy-resin and then connected to the test frame. The values in Table 2.1 represent the mean out of five specimens.

Furthermore, inherently stable reinforcement structures with a good workability and handling during the concreting process are achieved by impregnation. In [Heg09] the production processes in an industrial scale of planar and spatial epoxy-impregnated textile structures are described.


Table 2.2 shows the main properties of the textile reinforcement.

Table 2.2. Textile properties

Property Unit Roving Producer - OCVTM Reinforcements

Denotation - LTR 5325 Titer tex 3600 (= 1200+2400) Elastic modulus MPa 64800

Impregnation - Epoxy resin (Hexion Specialty Chemicals, Inc.)

Roving distances 0° / 90° mm 5 ; 15 / 7.5 ; 15 Section 0° / 90° mm²/m 134 / 119

Tensile strength1 0° / 90° MPa 1035 / 1194 1 Tensile strength of the reinforcement in concrete (mean values)

2.2 Concrete

The concretes used for TRC members have to meet special requirements with regard to the production process and the mechanical properties. As a rule, a small maximum grain size of 4 mm is used and the mix design adjusted to a highly flowable consistency with the aid of superplasticizers to ensure a good penetration of the textiles. To reduce the alkalinity of the concrete and to improve the contact zone between matrix and textile, pozzolanic additives, e.g. fly ash and silica fume, are used. Since the above-mentioned requirements frequently contradict each other, the composition of the fine grained concrete is usually a compromise between the requirements made on the fresh concrete, the mechanical properties, the durability and also the economical aspects for an industrial production of textile reinforced concrete members.

These fine grained concretes, owing to their composition, are currently not covered by German standards. The maximum grain size is below the minimum size of 8 mm specified in German standard DIN 1045-2:2008-08 [DIN08]. An excessively high powder content and the highly flowable consistency can also deviate from the specifications in the standards. This is the reason why construction projects processed with textile reinforced concrete are currently still implemented based on individual approvals in Germany. In the present case, a concrete mix had to be developed that fulfils the requirements. The mix design and the concrete tests for the individual approval are described in chapter 4.2.

3 DESIGN OF THE TRC SUPERSTRUCTURE

Fig. 3.1 shows the visualization of the pedestrian bridge, which has a total length of nearly 100 m. The superstructure is subdivided into six prefabricated parts, which are erected on slender steel columns. The maximum length of each part is 17.20 m while the maximum span is 15.05 m. Regarding the circular footprint of the bridge, the parts have to be produced with a radius of 112 m.


Fig. 3.1. Visualization of the bridge (Hartwig N. Schneider Architekten)

The cross-section of the superstructure is a 3.21 m wide concrete T-beam with seven webs, each pre-stressed by four single-strand cables and reinforced with three GFRP-bars in addition to the textile reinforcement, Fig. 3.2. Shear forces are borne solely by the spatial textile web reinforcement.

By using TRC as construction material, minimized concrete covers of 1.5 cm are achieved. This value was determined considering the maximum grain-size, geometrical tolerances during the concreting process and ensuring a sufficient bond between textile and concrete. The small concrete cover and small bending diameter of the textile of approximately 0.8 cm affords a minimum member thickness of only 9 cm at the end of the two cantilevers and 12 cm as minimum web thickness.

The slab thickness is 12 cm, where the upper 1 cm is used as an abrasion layer to resist the mechanical action of pedestrians, bicycles and snow removal vehicles during winter times. Thus, no further covering of the walkway is necessary. With a height of the superstructure of only 43.5 cm the whole construction has an extremely small slenderness ratio of H:L = 1:35.

Fig. 3.2. Cross-section of the superstructure


4 INVESTIGATIONS

4.1 General

Today textile reinforced concrete is not regulated by any standards in Germany. Thus, an individual approval of the construction by the building authorities is required. In the scope of this approval process a large scale testing program was conducted. The use of a fine grained concrete, the durability of the AR-glass reinforcement as well as the load-bearing behavior was investigated and is presented in the following chapters.

4.2 Concrete

The specifications for the concrete used for the bridge deck were specified as following: the compressive strength class had to be at least C 55/67. In addition, a high freeze-thaw and de-icing salt resistance had to be achieved (XF4, according to German standard). Furthermore, a high abrasion resistance had to be achieved, as the bridge is regularly cleared by a snow removal vehicle during the winter times. To ensure a good penetration of the reinforcing textiles, the maximum grain size of the aggregates was limited to 4 mm. At the same time, the fresh concrete was adjusted in such a way that a sufficiently high processing time and a flowability as high as possible would be achieved, together with a high stability to vibration. Entrapped air on the surface of the concrete members should be avoided. Also a surface of light colour was to be aimed for. In view of the increased binder content, the development of the heat of hydration had to be taken into consideration.

To obtain the above-stated properties, both the content of various cement types and fly ashes were varied. A combination of 450 kg/m3 Portland limestone cement and 100 kg/m3 fly ash with an equivalent w/c ratio of 0.41 led to overall best results. To promote the strength development of the concrete and to get a light colored surface, metakaolin was used instead of the customary dark silica fume. Metakaolin is a white powder and has a somewhat higher pozzolanic reactivity than silica fume. To achieve the specified freeze-thaw and de-icing salt resistance for exposure class XF4, DIN 1045-2:2008-08 [DIN08] prescribes the use of an air-entraining agent. Due to the inadequate air-pore stability of highly flowable mixes and the loss of strength that results from the use of conventional air-entraining agents, alternative use was made of micro hollow spheres (MHS). MHS are preformed synthetic hollow spheres and are added to the fresh concrete mix. As a result, a very good frost resistance was achieved in laboratory tests. With the aid of a superplasticizer based on polycarboxylate ether, a highly flowable and still vibration-stable mix with a spread of up to 750 mm in the flow table test was designed. The conventionally measured air content was established at 2.8 Vol.-% at a density of 2,256 kg/m3. The composition of the concrete M9-15 used for the bridge as well as the essential properties of the hardened concrete are summarized in Tables 4.1. and 4.2. Also the properties of two concrete mixtures needed for investigating the durability are given in these tables.


Table 4.1. Mix proportions of the bridge concrete and two reference concretes

Mix proportions Unit M9-15 PZ-0899-01 M1 Cement CEM II/A-LL 42.5 R 450 490 450

Fly ash 100 175 - Metakaolin 31.5 - - Silica fume - 35 -

Water 213.8 280 202 Quartz powder - 500 -

Sand 0 - 4 1457.4 - 1754 Sand 0.2 – 0.6 - 714 - Binder content 581.5 700 450

Micro hollow spheres

kg/m3

3 - - Eq. w/c ratio - 0.41 0.47 0.45

Table 4.2. Fresh and hardened concrete properties of the bridge concrete and two reference concretes

Properties Unit M9-15 PZ-0899-01 M1

Spread (flow table test) mm 750 mm n.d. n.d. Air content % by vol. 2.8 0.4 n.d.

Concrete density kg/m3 2256 2239 2490 Compressive cube strength 28d 87.1 n.d. 68.7

Flexural prism strength 28d 10.7 7.6 8.9 Young’s Modulus 28d

N/mm² 33,600 33,000 37,200

Capillary water adsorption kg/m2h0,5 0.018 0.100 0.132 Total pore volume by MIP Vol-% 13.0 12.6 10.6

pH Value of the pore solution (28 d after concreting) - 13.2 13.5 14.0

4.3 Durability

Even the reinforcement is called alkali resistant glass, there is a noticeable strength loss due to the alkalinity of the concrete [Büt06]. This strength loss has to be known during the design process of a load bearing structure, because only the remaining tensile strength of the reinforcement at the end of the service life can be taken into account as maximum tensile strength. The basis for the prediction of the long term behavior is the durability model, which was developed within the SFB 532 (collaborative research centre) [Orl05].

Due to the tight schedule of the project, the durability had to be investigated parallel to the concrete development. Consequentially the final concrete matrix was not known at the beginning of the investigations concerning the durability. So two well investigated concrete matrices were selected as reference concretes – these concretes are denominated as PZ-0899-01 and M1. Selected parameters of these mixtures are given in table 4.2. The concrete PZ-0899-01 is the basic concrete matrix within the SFB 532 and so a large database is available. In order to represent the influence of a CEM II 42.5 R and a larger aggregate size than used in the PZ-0899-01 mixture, the concrete M1 was selected as second mixture.


The following graph shows an excerpt of the gained laboratory data. The triangles and the line represent the strength loss measured with the PZ-0899-01 concrete in combination with a non-impregnated AR-glass due to accelerated ageing (storage under water at 50 °C). It can be noticed that by changing the concrete mixture (M1 - squares) in the present case the run of the curve is not altered significantly. By using an impregnated reinforcement, the measured strength loss (in the accelerated ageing) is reduced significantly. This corresponds to other investigations carried out within the SFB 532 [Büt09].

Fig. 4.1. Loss of strength within accelerated ageing (storage under water at 50 °C) for two different types of concrete and reinforcement

In order to achieve a safe approximation of the strength loss, two major factors were neglected within the prediction of the long-time strength loss. The impregnation of the reinforcement as well as the carbonation of the concrete was not taken into account. So the long term loss of strength was predicted by using the values derived with the PZ-0899-01 concrete and non-impregnated AR-glass.

a) b)

Fig. 4.2. a) Climatic data recorded for Aachen and Balingen; b) loss of strength for the non-impregnated AR-glass reinforcement


Besides the calibration data derived out of laboratory testing, the model uses the climatic data of the location to calculate the long term loss of strength [Orl05]. In Fig.4.1 the run of the temperature and rain curves of Balingen, which is approx. 15 km away from the location of the bridge are shown. The climate of Aachen is given as comparison. Regarding the climate, the AR-glass corrosion is mainly affected by the temperature and humidity. It can be seen that in Balingen the maximum amount of rain coincides with the maximum temperatures during the summer months, which might accelerate the AR-glass corrosion.

The prediction of the long term loss of strength for Balingen leads to an estimated loss of strength of 33 %, being on the safe side, since the carbonation of the concrete as well as the impregnation of the concrete have been neglected. Due to these assumptions the total loss of strength of the reinforcement was set to 30 % for the designed life time of 50 years.

4.4 Load-bearing behavior

In the scope of this paper, the load-bearing behavior of the slab and the longitudinal beam under shear force are presented exemplarily, describing the main tests in transverse and longitudinal direction of the bridge.

a) b)

Fig. 4.3. Slab: a) failure mode; b) detail of crack pattern before failure

The load-bearing behavior of the slab is determined with standard four-point bending tests, while the specimens (L x W x H = 130 cm x 30 cm x 11 cm) match with the real dimensions of the bridge. The slab thickness of the specimens was set to 11 cm in order to include the effect of abrasion, which was estimated to be 1 cm at the end of the lifetime. The bridge deck will have a slab thickness of 12 cm at the beginning.

Due to the high reinforcement area of the slab, the whole specimen failed to reach the maximum shear force. Fig. 4.3 a) depicts the ultimate limit state (ULS), where a diagonal tension failure as first failure is shown. Secondly, a horizontal crack on the level of the reinforcement is recognized. Both, the primary and the secondary failure are typical for slabs without shear reinforcement and have been investigated also on slabs with a steel reinforcement, [Leo84]. The tests show a distinctive crack pattern with small crack widths and distances, which is typical for TRC members, Fig. 4.3 b).

Fig. 4.4 shows the textile stress over the deflection. Compared to the values of the textile strength in Table 2.2, which are determined in a tensile test, it is typical that in bending tests higher textile stresses are activated. In this case a mean value of about 1500 MPa. Higher bond stresses between textile and concrete can be activated due to the bending of the member,


[Vos08]. The characteristic value of the resistance Rk and action Ek indicate a global safety factor of 5.0.

The customer requires a maximum crack width of 0.3 mm under service loads. This was fulfilled, because in serviceability limit state (SLS) all specimens remained uncracked.

0

400

800

1200

1600

2000

0 10 20 30 40 50

resistance Rk (characteristic value)

resistance Rd (design value)

action Ed (design value, ULS)

Deflection in mm

Textile stress in MPa

action Ek (characteristic value, SLS)

ηglobal = 5.0

Fig. 4.4. Load-bearing behavior of the slab: textile stress – deflection diagram

The next test represents the investigation of the shear resistance of the longitudinal beam, Fig. 4.5. Due to a limited width of the testing machine, the specimens were tested with three instead of seven webs. By applying the point load at a distance of 1.20 m apart from the support, the ratio of the distance from the support over the effective depth is a/d = 3.15. Thus, it is ensured to determine the minimum shear resistance of the beam, [Leo84]. Furthermore, the point load is applied eccentrically to consider torsion effects.

Fig. 4.5. Test-setup of the longitudinal beam


Fig. 4.6. Longitudinal beam: failure of the compression zone

The ULS is characterized by collapsing of the compression zone as depicted in Fig. 4.6. On the horizontal axis of the diagram in Fig. 4.7 the deflection related to the span of the specimens is shown. Here, the members achieve high deformations in ULS in the range of 1/200 to 1/150 of the span. High deformations and a stabilized crack pattern signalize the collapsing of the member, thus, no sudden failure was recognized. Overall, a global safety factor of 5.0 is achieved here as well.

0

100

200

300

400

500

600

700

800

0 0 0 0 0Deflection related to span

ηglobal = 5.0

1/500 1/250 1/167 1/125

Shear force in kN

resistance Rk (characteristic value)

resistance Rd (design value)

action Ed (design value, ULS)action Ek (characteristic value, SLS)

Fig. 4.7. Load-bearing behavior of the longitudinal beam: shear force – related deflection diagram

5 PRODUCTION PROCESSES

Due to the cross-section of the bridge and the inclination in longitudinal and transverse direction a special formwork made of coated wood was constructed. During the mixing process the required concrete spread was adjusted with superplasticizer. The concrete was cast


by means of a special concrete bucket, fitted with a funnel-shaped outlet, Fig. 5.1. As the concrete could not be cast through the small meshes of the upper textile layer it was poured into openings specifically arranged for this purpose. Additionally, internal and external vibrators were used. Finally, the surface was finished with a vibratory beam.

a) b)

Fig. 5.1. Concreting process: a) openings in upper reinforcement layer, b) concrete bucket

To reduce heat and moisture loss the bridge elements were covered with a plastic foil and an insulating blanket. For demoulding a partial prestressing was applied to the elements. The full prestress was applied after 28 days. The required slip resistance was achieved by sand-blasting.

6 CONCLUSIONS

In recent years, TRC has been often applied for small scale structural elements with simple load-bearing behavior and basic configurations of textile reinforcements. This project demonstrates, that TRC can also be applied for large-scale and slender members like the 100 m long pedestrian bridge. The cross section is a T-beam, i.e. even members with a more or less complicated configuration of the reinforcement (intersection of the web and lower slab reinforcement) can be realized.

By using TRC as construction material it is possible to design slender and light-weight concrete members in comparison to steel reinforced members. The micro-grained concrete allows for sharp-edged members, which meet the needs of modern architecture.

PROJECT TEAM

Building principal: City of Albstadt, Groz-Beckert KG, Albstadt

Construction supervision: Regional Commission Tübingen – Branch of Structural Engineering

Architect: Hartwig N. Schneider Architekten, Stuttgart

Structural Engineer: H+P Engineering GmbH & Co. KG, Aachen

Inspection Engineer: Bornscheuer Drexler Eisele GmbH, Stuttgart


Expertise: Prof. Hegger, Institute of Structural Concrete, RWTH Aachen University

Prof. Brameshuber, Prof. Raupach, Institute of Building Mate-rials Research, RWTH Aachen University

Construction Company: Sebastian Wochner GmbH & Co. KG, Dormettingen

ACKNOWLEDGMENT

The authors gratefully acknowledge the Groz-Beckert Group and the town of Albstadt, Germany, for the financial support and their willingness to undertake this pilot project. Special thanks go to the German Research Foundation (DFG) which financed the Collaborative Research Center 532 at RWTH Aachen University.

REFERENCES

[Büt06] Büttner, T., Orlowsky, J., Raupach, M., Investigation of the Durability of Textile Reinforced Concrete - Test Equipment and Modelling the Long-Term Behaviour: [ISBN 978-2-35158-046-2] Bagneux: RILEM, 2007. - RILEM Proceedings PRO 53. - In: High Performance Fiber Reinforced Cement Composites (HPFRCC5), Proceedings of the Fifth International RILEM Workshop, Mainz, July 10-13, 2007, (Reinhardt, H.W. ; Naaman, A.E. (Eds.)), pp. 333-341

[Büt09] Büttner, T. ; Orlowsky, J. ; Raupach, M., Textile Reinforced Concrete - Durability Issues: Changes of the Bond and Tensile Strength Due to Ageing. Cambridge : Woodhead Publishing Limited, 2009. - Proceedings of the 9th International Symposium on Brittle Matrix Composites, Warsaw, 25-28 October 2009, (Brandt, A.M. ; Olek, J. ; Marshall, I.H. (Eds.)), S. 101-110 ISBN 978-83-89687-48-7

[DIN08] DIN 1045-2:2008-08 Tragwerke aus Beton, Stahlbeton und Spannbeton; Beton – Festlegung, Eigenschaften, Herstellung und Konformität

[Heg09] Hegger, J., Kulas, C., Horstmann, M., Tailor-made 3D-reinforcements for TRC structures. In Oehlers, D. J.; Griffith, M. C.; Seracino, R.: Proceedings of the 9th International Symposium on Fiber-Reinforced Polymer Reinforcement for Concrete Structures. ISBN 978 0 9806755 0 4. Sydney, Australien. 13.-15.07.2009. S. 57. CD-ROM:\papers\frprcs9final00222.pdf.

[Leo84] Leonhardt, F., Vorlesungen über Massivbau – Teil 1 Grundlagen zur Bemessung im Stahlbetonbau, 3rd edition, Springer-Verlag, 1984, ISBN 0-387-12786-0

[Orl05] Orlowsky, J., Zur Dauerhaftigkeit von AR-Glasbewehrung in Textilbeton (Durability of TRC; PhD Thesis). Berlin: Beuth. - In: Schriftenreihe des Deutschen Ausschusses für Stahlbeton (2005), No. 558

[Rau06] Raupach, M. et al., Epoxy-impregnated textiles in concrete - load bearing capacity and durability. In: Hegger, J.; Brameshuber, W.; Will, N. (Edt.): Proceedings of the 1st International RILEM Conference. September 6-7, 2006, Aachen, Germany. RILEM Publications S.A.R.L., 2006, pp. 77-88.

[Vos08] Voss, S., Ingenieurmodelle zum Tragverhalten von textilbewehrtem Beton. PhD-Thesis, series of the Institute of Structural Concrete, RWTH Aachen University, issue 24, 2008, ISBN 3-939051-03-9RWTH

trc pedestrian bridge – design, load-bearing behavior … · the use of a fine grained concrete,...

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