10th International Conference on Short and Medium Span Bridges

Quebec City, Quebec, Canada, July 31 – August 3, 2018

PRECAST UHPFRC DECK OF THE ISABEY-DARNLEY PEDESTRIAN BRIDGE

Charron, Jean-Philippe1, Desmettre, Clélia1 and Cantin Bellemare, Étienne2,3 1 Department of Civil, Geological and Mining Engineering, Ecole Polytechnique Montreal, Quebec, Canada2 Bridges and Tunnels Division, City of Montreal, Quebec, Canada3 etienne.cantinbellemare@ville.montreal.qc.ca

Abstract: A cast-in-place reinforced concrete footbridge built in 1967 suffering from extensive concrete spalling and rebar corrosion was replaced by a steel Pony-Warren truss combined with precast UHPFRC slabs linked by UHPFRC field-cast joints. The mechanical and durability performance provided by UHPFRC led to concrete and steel rebar reductions in the UHPFRC slab of 64 and 91 %, respectively, when compared to a conventional reinforced concrete slab. Construction of the precast slab was carried out on site and took 5 periods of 8 hours, while equivalent work for cast-in-place slab takes about 19 days. The fabrication and installation of the slabs was successful. The long-term performance provided by the UHPFRC slab will be followed up in the next decades.

1 PROJECT OVERVIEW

The Isabey-Darnley pedestrian bridge is located in Montreal (Quebec, Canada) over Highway 520 close to the International Airport. This new structure replaces the former reinforced concrete footbridge that was built in 1967, which suffered of early concrete spalling and rebar corrosion after only 40 years of service. The footbridge allows pedestrians to cross the 9 lanes of the highway safely. In this area, the average annual daily traffic of the highway is 30 000. Construction of this 61 m long footbridge of 3 span supported by steel Pony-Warren truss as shown in Figure 1 was completed in December 2016. The deck slab is made of ultra-high-performance fiber reinforced concrete (UHPFRC) precast elements. The development of UHPFRC precast deck slabs for bridges is part of a 10 year ongoing research project involving Ecole Polytechnique Montreal, the City of Montreal and other partners.

Figure 1: Isabey-Darnley Pedestrian Bridge

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2 INNOVATION IN THE ISABEY-DARNLEY PEDESTRIAN BRIDGE

Among the requirements for the replacement of the Isabey-Darnley pedestrian bridge, traffic delays and redirection below the structures had to be minimized and quick access for pedestrians to this industrial area also had to be provided. Moreover, the new structures had to ensure an excellent durability under severe exposure conditions. In this context, a slab system composed of 11 precast UHPFRC thin elements linked by short transversal UHPFRC field-cast joints was selected (Figure 2).

Figure 2: UHPFRC Precast Slab And Field-Cast Joint Geometry

2.1 Design and materials rationalization

UHPFRC distinguishes itself from other concretes available in the market by a very dense microstructure, obtained with a very low water-to-binder ratio (about 0.15 to 0.25) and optimised granular distribution, and a high content of micro-fibers (generally between 2 and 6 % in volume). This basic composition provides a self-compacting concrete with outstanding mechanical properties (Habel 2004) and extended durability perspectives (Charron and Desmettre 2015). The compressive strength varies between 120 and 180 MPa, the tensile strength between 8 to 15 MPa. The tensile behaviour is especially improved by a significant hardening phase in the pre-peak behaviour during which only microcracks develop in the material and the presence of a softening phase once a macrocrack develops in the post-peak phase. This tensile behaviour leads to the development of more cracks, but thinner, in service conditions than conventional concrete, thus lowering penetration of water and aggressive agents in UHPFRC.

Technical specifications of the designers for the UHPFRC slabs and joints aimed to maximise flexural and shear strength of thin slab, minimize rebar splicing in the joint, and guarantee exceptional durability (crack widths inferior to 0.1 mm in the slab and at the joints in service condition). Composition requirements were water-to-binder ratio inferior to 0.25, 8 % minimum content in silica fume for the binder and 4 %-Vol. of steel micro-fibers. Mechanical properties specified were a minimum tensile strength of 10.5 MPa, a minimum compressive strength of 100 MPa and some specifications to ensure a hardening behavior in the tensile pre-peak phase. The UHPFRC tensile behavior was verified by four-points bending tests on 400 x 100 x 50 mm3 (length x width x height) prismatic specimens and application of an inverse analysis as specified in the Swiss Standard SIA 2052 (SIA 2015). Durability performance criteria for freeze-thaw resistance, scaling and chloride permeability were also specified and a 7-day water curing was required for the slabs and field-cast joints

The design of the precast slabs and field-cast joints were made considering the Canadian Highway Bridge Design Code (CSA 2006) for loads (dead, live, snow, ice, handling and exceptional loads). Load combinations were based on the recommendations of the French Association of Civil Engineering (AFGC) for UHPFRC structures (AFGC 2013). The later provided detailed information on mechanical constitutive laws, structural design equations, durability requirements, quality control recommendations, etc. As specified in AFGC (2013), the design values for UHPFRC tensile properties were obtained considering the variability of the UHPFRC mixture (determination of a characteristic law) and then applying a security factor (-25 %) as for conventional concrete and an global orientation factor (- 20 %) to take into account

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disparity in fiber orientation due to placement. Finite-element analysis with Atena 3D software were also performed to confirm the satisfactory structural behavior of the precast slabs and joints (deflection and crack opening in service, rebar yielding and failure mode at ultimate).

The final design comprises precast slabs of 75 mm thick from 2 to 6 m long linked by 125 mm wide UHPFRC field-cast joints (100 mm rebar splices for 10M) (Figure 2). The precast slab includes curbs which serve as a security cladding for pedestrians, favour water drainage and strengthen the 75 mm-thick slab. The critical load combination for the precast slabs occurred during the construction process (manutention and installation) whereas the field-cast joints were the critical part at the serviceability and ultimate limit states. The precast slabs adopt a bidirectional mechanical behavior. After connection on site, the continuous slabs required very low positive and negative bending reinforcements and crack control reinforcement (Figure 3). The mechanical and durability properties of UHPFRC led to significant reductions in concrete and rebar volumes in the UHPFRC slab in comparison to a conventional reinforced concrete slab (Table 1).

Figure 3: Comparison, at an identical scale, of the cross sections of the UHPFRC slabs (top) and of the conventional reinforced concrete slab initially planned (bottom)

Table 1: Comparison of the concrete and steel quantities in the UHPFRC and conventional slabs

Materials Conventional slab UHPFRC slab Reduction with the UHPFRC slab

Concrete 35.1 m3 12.6 m3 64 %Steel 4672 kg 435 kg 91 %

2.2 Durability perspective

Due to its denser matrix (lower water-to-binder ratio and optimized granular distribution), uncracked UHPFRC presents a durability index (water and air permeability, diffusion, sorptivity) over 60 times inferior to an ordinary portland concrete (OPC) and over 20 times inferior to an high-performance concrete (HPC) (Charron and Desmettre 2015). In the cracked stage in service condition, the thinner cracks (microcracks) which characterize UHPFRC provide a permeability more than 1000 times inferior to OPC or HPC and more than 300 times inferior to conventional fiber reinforced concrete (FRC) (Charron and Desmettre 2015).

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The extremely low transport properties and high deterioration resistance of UHPFRC provide an exceptional durability at the material scale. As most UHPFRC structures were built less than 10 years ago, little data is available for UHPFRC long term durability at the structural scale. Thanks to the Barr pedestrian bridge also built in 2016 with conventional cast-in-place reinforced concrete slab and located about 1 km from the Isabey-Darnley Bridge over the same highway, the comparison of the long-term performance provided by the UHPFRC slab will be possible with a realistic reference. Except for the slab, the two pedestrian bridges are identical. Concrete deterioration, crack opening, rebar corrosion, vibration mode, esthetic appearance, maintenance works and expenses will be followed and analyzed in the next decades.

2.3 Construction time

The objectives of this project were to design an aesthetically pleasing footbridge that was durable and cost effective, while minimizing impact on highway traffic during construction.

The installation of the steel structure took 12 hours. The installation of the precast slabs of the Isabey-Darnley footbridge was achieved in two phases, each taking 8 hours, while 3 periods of 8 hours were required for the casting of the UHPFRC field-cast joints under winter conditions. As a comparison, the construction period required for the cast-in-place reinforced concrete slabs of the Barr footbridge was 19 days. The use of precast UHPFRC has increased the durability of the bridge, reduced the loads on the foundation and reduced the construction time, without additional cost incurred compared to that of a cast in place reinforced concrete deck slab. The use of UHPFRC is an economical choice when the life cycle cost in considered.

3 CONCLUSIONS AND PERSPECTIVES

The fabrication and installation of the slabs were carried out without any problems and the result was very satisfactory. Following this successful project, the City of Montreal is interested in using UHPFRC in the future.

UHPFRC properties give to civil engineers new perspectives for thin and light structure designs, new structural applications and repair configurations, wider shape liberty (hole, geometry, aesthetic) and long lasting applications. However, the use of UHPFRC needs specific and detailed specifications in terms of design, batching, placement, finishing, curing and quality control (for example, mechanical properties have to be determined with adapted test setups and analysis procedures). These specifications are being prepared for the next edition of the Canadian Highway Bridge Design Code and will require training for all participants involved in the construction process (from owners to designers, contractors, supervisors and workers). It will be important to make project stakeholders aware of the importance of following adapted specifications, to plan special start-up meetings with participants explaining the materials requirements, and to provide strict supervision during construction involving UHPFRC.

AcknowledgementsThe authors would like to acknowledge Ville de Montreal and Ville Mont-Royal for accepting this first pilot application of UHPFRC on one of their bridge infrastructures, and underline the contribution of the general contractor, Entreprises Pera, the precast slab producer, Betons Genial, and the UHPFRC provider, King Material, to this project.

ReferencesAFGC. 2013. Bétons Fibrés À Ultra-Hautes Performances - Recommandations Association Française de

Génie Civil. France.Charron, J.-P. and Desmettre, C. 2015. Intérêt De l'Utilisation Des Bétons Renforcés De Fibres Pour La

Construction d'Ouvrages d'Art Durables - Rapport de recherche SR13-01. École Polytechnique de Montréal. Montréal, Québec, Canada.

CSA. 2006. Canadian Highway Bridge Design Code - CAN/CSA S6-06. Canadian Standards Association. Mississauga, Ontario, Canada.

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Habel, K. 2004. Structural Behaviour Of Elements Combining Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) And Reinforced Concrete. Ph.D. Thesis, École Polytechnique fédérale de Lausanne, Lausanne, Suisse.

SIA. 2015. SIA 2052:2015, Béton fibré ultra-performant (BFUP) – Matériaux, dimensionnement et exécution. Societé suisse des ingénieurs et des architectes. Zurich, Switzerland.

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