UHPFRC - Mechanisms and applications
TOUTLEMONDE ; BERNARDI
Type de document
CONFERENCE INVITEE (INV)
Langue
anglais
Auteur
TOUTLEMONDE ; BERNARDI
Résumé / Abstract
Ultra-high Performance Fibre-Reinforced Concrete (UHPFRC) has emerged around 25 years ago as a fruitful result of dedicated research efforts based on the combination of three main ideas : Strength and compactness improvement of cement materials by intense reduction of the water to binder ratio, which was made more and more possible and efficient with R&D advances in superplasticizers and mineral additions ; Use of fibers to provide post-cracking tensile capacity and pseudo-ductility, which was made possible thanks to more than two decades of conceptual and exploratory development of conventional fiber-reinforced concrete, and really
efficient as compared to it due to the high quality of the matrix and the possiblity to incorporate high amounts of fibers ; Reduction of natural imperfections due to aggregate in limiting their size and selecting very high quality materials in an
optimized grading, which took benefit of aggregate packing models having being developed in the late 80s. This combination enabled to produce several industrially controlled patented materials markedly overpassing the highest high-performance concrete available at this time, with 150 MPa-characteristic compressive strength at least, and possibly dispensing with the traditional use of secondary reinforcement due to material non-brittleness and with thick cover in aggressive environment. This allowed dispensing with keeping systematic continuity with traditional reinforced concrete
design provisions. Moreover, mixing, placement, thermal treatment and formwork technology could be kept understood in the continuity with emerging self-compacting concrete. Twenty years ago, the first major applications, Sherbrooke footbridge (Canada) and prestressed beams for the renovation of the heat exchange zone in the cooling towers of Cattenom power plant (France), demonstrated outstanding strength and resistance to transfer, so that durability and lightness of prestressed members have appeared as main advantages to be searched in structural applications. The first recommendations for production, characterization of UHPFRC and structural
design using these materials, published in France in 2002 [1], made it possible to explore the implementation possibilities of such materials especially for pre-stressed structures and non-reinforced thin elements. Application to bridges and footbridges (Bourg-lès-Valence in 2001, Sakata Mirai and Seoul - Seon Yu Footbridges in 2002, Pinel bridge in 2006, passerelle des Anges in 2008) motivated consistent research efforts to ensure controlled quality of the material produced and placed, and safe design provisions of lighter and thinner structures. Optimized shapes, like ITE® beams, ribbed slabs or shells, were demonstrated as favourable for valuable implementation, while cost-efficiency also relied on indirect material savings or lightness benefits for the method of execution. Besides valuable application in structural components, UHPFRC has been adopted for a decade by architects to develop attractive façade and roofing components, thanks to new aesthetic possibilities associated to durable mineral surface quality, lightness and possible complex shapes and semi-transparency, with typical examples of Villa Navarra roof and 'Les enfants du Paradis' net panels in 2007. For such elements, the use of organic or stainless steel fibres was developed, the range of
mixes was extended addressing the fire-resistance demand and the possibly lower strength requirements, and ribbed plates or shells with passive reinforcement in the stiffeners tended to appear as structurally efficient. Growing interest and
economic significance of this development in building applications turned out evident from the first international symposium strictly dedicated to UHPFRC applications, organized in Marseille (France) in 2009 [2]. From the same time however, UHPFRC produced volume took off also for less visible, however technically optimized projects of the extension of Haneda Airport in Tokyo, light prestressed bridges in Malaysia, or joint fills between precast beams in North American projects of 'accelerated bridge (re)construction'. The year 2013, associated to the second UHPFRC international symposium organized in Marseille [3] and revised edition of AFGC Recommendations, has constituted a significant milestone for UHPFRC in France. Two major projects associated to urban renovation, the MuCEM in Marseille (Fig. 1) and the Jean Bouin Stadium in Paris, had been completed, having led to widened awareness of technical and architectural capabilities of these 'new concretes' both among professional (architects and engineers) and for the public, including clients of constructions. Technical acceptance of the design, industrial processes and details associated to these projects, as well as previous satisfactory French 15 years-experience of building components
and bridges made of UHPFRC, has made it possible to launch the standardization process in France. This has resulted in the elaboration of three complementary standards, related to UHPFRC structural design, material production and control, and execution of structures. The first two documents have been published in French and English in 2016 [4-5], the latter one is expected for 2017, as well as updating of the standard 'common rules' for precast concrete products. Although based on
the technical consensus expressed in AFGC recommendations revised in 2013, the standards elaboration has promoted clarification and simplification of the ordering / qualification processes for easier UHPFRC contract implementation. Since 2013, international recognition especially within ACI Excellence in Concrete Construction Awards program has been gained not only for the MuCEM and Jean Bouin Stadium in 2015, but also for the 'Ring of Memory' at International Memorial of Notre-Dame de Lorette in 2016. Noticeably enough, due to optimization in conceptual design, UHPFRC has made possible competitive solutions not only for tailor-made projects, but also for typical bridge situations exemplified by the Buthaumont Bridge and the footbridge at Le Cannet des Maures. In Switzerland with the iconic example of Chillon viaducts, in the US with the Pulaski viaduct restoration, and progressively also in France, bridge deck repair or protection using UHPFRC has deserved increasing interest. Repair solutions, for buildings also, are increasingly considering UHPFRC due to specific versatility, structural and durability performance and weight savings, which results in cost-efficiency despite
a possibly still high unitary material cost. However, the driving field of UHPFRC application in France has concerned
cladding and roofing panels, for buildings (e.g. 'Vente Privée' Headquarters, 'La Mantilla' buildings...) and for large
infrastructure projects (e.g. Montpellier high speed railway station). UHPFRC solutions based on ultra-thin, highly
transparent and architecturally appealing elements have thus been made possible in a cost-efficient way due to the lightness
of these secondary elements.
Consolidation of engineering and industrial know-how is a key condition for further development of such applications. This
has been the case among several architects, designers, checkers, engineering offices, and precasting plants, although quite
few. Education associated to the standards dissemination should strengthen these capabilities. Further research and
development efforts should address advanced UHPFRC modelling, seismic design with UHPFRC, development of typical
'UHPFRC solutions', and non-conventional process optimization (sprayed UHPFRC, 3D printing etc.) which could widen
the scope of cost-efficient UHPFRC implementation.
