Rendement parasismique de colonnes préfabriquées avec chemisage en béton

La construction accélérée d’infrastructures routières est une solution à préconiser pour favoriser le remplacement des ponts sur des axes routiers existants et ainsi maintenir de façon efficiente l’offre en service aux usagers. Cette méthode de construction peut également s’appliquer en zones propices aux séismes, notamment par l’utilisation de bétons performants. Ce travail doctoral se penche sur la faisabilité d’une telle approche pour la construction accélérée des éléments de fondation pour les ponts. L'étude porte sur l'utilisation d’éléments préfabriqués pour la fabrication des colonnes des piles de ponts. L’objectif est de concevoir des colonnes répondant favorablement aux efforts sismiques ainsi qu'aux exigences de durabilité pour des conditions environnementales sévères. Le système étudié consiste en une coque préfabriquée d’une colonne évidée faite en matériaux performants, tels les bétons fibrés à ultra hautes performances (BFUP) ou en béton hautes performances (BHP), que l’on dépose sur la semelle de fondation, centrée sur les armatures d’ancrage en attente. Par la suite, un béton normal est mis en place dans l'élément évidé afin de constituer le noyau de la colonne. La coque préfabriquée, en plus de servir de coffrage participatif, permet de profiter de la forte résistance en compression et traction de ces bétons performants afin de confiner le noyau de la colonne en plus de conférer à l'ensemble une la tenue améliorée aux agents agressifs en comparaison avec les solutions coulées en place avec des bétons normaux. Le confinement amené par la coque permet la formation de la rotule plastique à la base selon les mêmes principes que les solutions en béton conventionnel. Cette approche est en accord avec la majorité des codes internationaux quant aux exigences de chevauchement d’armature dans la zone de rotule plastique. Différents critères et hypothèses de calculs sont mis de l’avant afin d’apporter des recommandations à l’assemblage de structures d’éléments préfabriqués. Le concept proposé requiert des équipements de levage conventionnels que l’on retrouve sur la majorité des chantiers.»

Abstract

Accelerate Bridge Construction is one solution for maintaining an efficient service to users. This method of construction can also be applied in seismic-prone regions by using High-Performance Concrete. The present study looks at the feasibility of implementing accelerated construction of the substructure for bridges. The aim is to design a stay-in-place structural formwork that responds favorably to seismic forces and environmental conditions. The precast column consists of a hollow-core section made of high-performance materials, such as Ultra-High-Performance Fiber-Reinforced Concrete (UHPC), which is placed in the center of a radius of anchoring reinforcement from the footing. In addition to serving as a permanent formwork, this construction technique not only benefits from the high compressive and tensile strength of the Ultra-High-Performance Concrete but it also improves durability. Afterward, normal concrete is filled inside the UHPC tubular section as the core of the column. It therefore allows the continuity of the longitudinal bars from the footing as in cast-in place construction. This approach is in line with most international codes regarding reinforcement overlap requirements in the plastic hinge region. Various scenarios and hypotheses have been proposed to provide appropriate guiding principle for the assembly of prefabricated structures. The concept requires conventional lifting equipment found on most construction sites. Three aspects will be considered: the use of prefabrication for bridge pier construction, the seismic resistance of prefabricated piers and the durability of the substructure. Two separate experimental aspects will be analyzed, the first will assess the contribution of confinement to strength and ductility gains. The second assesses the seismic behavior and structural damage of a scaled-down bridge pier. A total of 15 circular specimens representing short columns were subjected to monotonic axial loading until failure. In these tests, the axial load is applied directly on the core of the column. The prefabricated outer shell is loaded indirectly by the radial expansion of the compressed core. Three scaled-down circular pier specimens were subjected to low-cyclic fatigue protocol in a multi-axial subassemblage test system. These specimens were built using conventional and commercially available materials. For the experimental program on short columns, we noted an average gain of 140% in maximum load for reinforced concrete specimens x encased in a 70 mm UHPC reinforced shell. The ductility performance, even for strain hardening UHPC, was unsatisfactory without the presence of conventional transverse reinforcement in the shell. Varying the amount of transverse reinforcement in the central core proved to have an influence on the maximum sustained load. Whereas lowering the percentage of transverse reinforcement in the shell appears to reduce ductility. The contribution of a thick UHPC shell with 1% transverse reinforcement showed similar performance to a thick high-performance concrete (HPC) shell with 2% transverse reinforcement. The enhanced tensile strength and strain-hardening behavior of this high-performance material enabled a reduction in the amount of confinement reinforcement required. For design purposes, yielding of the transverse reinforcement at maximum load can be considered. Based on literature confinement models and force equilibrium, it has been demonstrated that the strength of UHPC exceeds postcracking stress. Its behavior is in the softening branch following the opening of the main cracks. It is therefore not advisable to take the contribution of the UHPC and transverse reinforcement at their maximum simultaneously when calculating the effective confinement stress on the core.»