Polymer concrete-GRP plate hybrid joints for transverse moment-shear continuity in GRP bridge decks

Thomas Keller, Wendel Michael Sebastian
Elsevier, 2010
Journal paper

Abstract

This paper describes an experimental study of two joint configurations, one with only reinforced polymer concrete (RPC) infill, the other with RPC infill and bonded GRP plates, to overcome shear and moment discontinuities in GRP bridge decks transverse to the bridge's main girders. Fabrication, instrumentation and combined moment/shear force loading of these joints are reported. The joints occupied the full 0.9 m widths and the central 1 m lengths of simply supported deck specimens of span 2.7 m. Cutting out of GRP from the deck to create holes for pouring the concrete into the voids of the deck did not appear to compromise joint integrity under short-term load. Rosettes on the webs of the deck lead to the deduction that over 90% of shear force in the joint was carried by the RPC infill at only 30% of the length of the joint inwards from one end of the infill. Beam theory reliably predicts the neutral axis heights in both joint sections after cracking of the polymer concrete, and also predicts the cracking load for the joint with only RPC infill. For the joint with RPC infill and GRP plates, the test data appear to be elusive in showing a clear cracking load. The GRP plates significantly improved cracked section stiffness beyond that for the RPC alone. A peak load of 500 kN over a 300 mm×300 mm patch damaged but did not fail the deck specimen with either joint. For the RPC-only joint, an unexplained 45% drop in strain occurred in the compression concrete during sustained load over 80 minutes. Practical fabrication issues and further potential research into these joints are discussed. © 2010.

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Thomas Keller, Wendel Michael Sebastian
Elsevier, 2010
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/150059?ln=en

About this result

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The design of reinforced and prestressed concrete girders is usually performed so as to avoid any non ductile failure mode. Amongst these failure modes, one of the most undesirable occurs by crushing of concrete in the web. To avoid this brittle type of failure, it is of paramount importance to know precisely the effective compressive strength of concrete in the web. It is reduced by cracking (transverse strains), which is due to the shear deformations. In the presence of post-tensioning cables in the web, cracks may form along the cables inside the web, which also weakens the concrete of the web. Numerous bridges have been built in Switzerland by using profiled girders with thin webs containing only a minimal amount of stirrups but large post-tensioning ducts, which occupy a considerable part of the web width. Both phenomena mentioned above play a role for these bridges, especially for more brittle high performance concrete. The two phenomena are only partly understood. A sound physical model for the shear behaviour as well as a failure criterion are necessary to achieve a uniform safety level for new structures and to evaluate the strength of existing structures. Large-scale laboratory tests on prestressed concrete girders have been conducted in order to improve understanding and to investigate both phenomena in detail. Web crushing along the post-tensioning cables was the mode of failure of all girders. In addition, laboratory tests have been conducted on prismatic specimens to investigate the effect of the presence of a post-tensioning cable in an isolated manner. A physical model has been developed for the shear behaviour of reinforced and prestressed concrete girders. The variation of loading, stresses and strains along the girder axis is taken into account. The shear capacity of the compression flange is considered on the basis of equilibrium and compatibility. The load increase in the post-tensioning cables is calculated with a bond condition. A physical failure criterion has been developed for the effect of the presence of a posttensioning cable. The failure load of the conducted prism tests and a large number available in the literature has been estimated with a good precision for various post-tensioning duct types and concrete cylinder strengths. Based on physical considerations a failure criterion has been developped that takes into account the effect of transverse strains. Both effects are more pronounced for a higher cylinder strength. With the developed physical model and the failure criteria, the failure load of a large number of reinforced and prestressed concrete girders (laboratory and from the literature) has been estimated with good precision. The interaction of the effects of the presence of a post-tensioning cable and of transverse strains has been taken into account for the development of a combined failure criterion. Its format is compatible with the current design codes.

EPFL2006

Ultra high performance concretes (UHPFRC) are characterized by a dense matrix and a high fibre content. These materials exhibit exceptional mechanical and durability properties making them an ideal material for rehabilitating existing structures. The primary interest in UHPFRC focuses on their uniaxial tensile performance. When they applied in a cast in place overlay configuration, they provide increased rigidity to the global element and localized protection to the steel reinforcement embedded in the concrete core during the service by minimizing surface cracking and inhibiting the diffusion of aggressive agents. At the ultimate state, and under certain configurations UHPFRC significantly improves the element's carrying capacity. The overall goal of this research is to study the UHPFRC tensile behaviour (hardening and softening behaviours) of characteristic test specimen and to apply this knowledge in analyzing the structural response of various composite elements. The objectives related to this study are: To determine the factors influencing the UHPFRC uniaxial tensile test specimen behaviour To illustrate the importance of UHPFRC tensile hardening and softening responses in UHPFRC structural plates and UHPFRC-reinforced concrete composite beams. To determine the rupture mechanism and overall contribution a UHPFRC overlay offers to a composite UHPFRC-reinforced concrete slab loaded to failure in flexural/punching shear. To study the parameters influencing the UHPFRC uniaxial behaviour in tensile composite structural elements. The behaviour of specimen with different configurations and compositions were studied in uniaxial tensile tests. The variables significantly influencing the fibre orientation were isolated by studying the fibre orientation at localized cuts in the various specimen. These controlling variables include: mixture viscosity, casting method, casting direction, fibre aspect-ratio and element geometry. Furthermore, a meso-level model was developed to predict the uniaxial tensile behaviour and was validated against the experimental test results. With the help of this model, the influence of the main fibre characteristics (Vf, Lf, df) on the resultant fibre orientation and matrix quality were examined. This model has determined that the extent of hardening does not increase linearly with the quantity of fibres but rather follows an asymptotic curve converging to a maximum possible hardening. A second model was developed to randomly orient and place fibres. This model has helped to explain the uniaxial tensile response variability of different mixtures ranging from conventional Fibre Reinforced Concrete (FRC) to UHPFRC. A finite element program has been employed to study the influence the UHPFRC softening behaviour has on the composite structural beam and UHPFRC plate responses. For a flexural plate (length 500mm, width 200mm, height 30mm), the bearing capacity was significantly increased by the very pronounced UHPFRC softening behaviour. Additionally, the UHPFRC layer noticeably increased the rigidity of the composite beam and slab. For example the UHPFRC layer increased the slab bearing capacity by 40% in comparison to a purely reinforced concrete slab by bridging the punching shear mechanism with a UHPFRC membrane effect. The structural responses of the two element types were then further analyzed using a finite element analysis program. An inverse analysis methodology was employed to obtain the UHPFRC tensile material laws in respective composite elements. These findings documented that a similar materials exhibited significantly different responses when applied in different applications. This analysis documents that the UHPFRC layer in a slab closely follows the uniaxial tensile test specimen response, while the UHPFRC layer in a composite beam exhibits a significantly reduced performance. The primary reasons for these variances are: the relative fibre distribution, the interface rugosity, the internal residual stresses and the extent of the maximum stress zone. Recommendations are made concerning the implementation of UHPFRC and the influence the fibre aspect ratio and volume (Vf, Lf, df) imposes on the resulting fibre orientation in different applications. Finally, a method for determining the UHPFRC characteristic curve which is required to design a composite UHPFRC-reinforced concrete element is presented.

EPFL2007

Poinçonnement non symétrique des dalles en béton armé

Luca Tassinari

Punching shear of reinforced concrete flat slabs supported on columns is a failure mode in which the slab fractures around the column. This type of failure is sudden and can be catastrophic in nature. Punching shear generally governs the ultimate limit state design of flat slabs and slab bridges. The Critical Shear Crack Theory (CSCT) developed by Prof. Muttoni can be used to calculate the punching shear strength in axis-symmetrical cases. The main objective of this work was to investigate non-symmetrical cases of punching shear and to extend the CSCT to such cases. The failure criterion proposed by Prof. Muttoni allows estimating the punching shear strength in terms of the slab rotation in the radial direction. A more general model is needed to predict the punching shear strength of slabs with non-symmetrical conditions. A numerical model was developed, which is based on the finite difference method, to predict the flexural behaviour of flat slabs. It takes into account the non-linear behaviour of concrete and reinforcement steel and can be applied to non-symmetrical cases and complex structures such as flat slabs with different span length in each direction and slab bridges. Moreover, this model satisfactorily predicts the behaviour of slabs with orthogonal reinforcement and general load cases. A series of punching shear tests on non-symmetrical slabs was carried out to validate an analytical approach which considers the redistribution of shear stresses along the perimeter. This refined approach provides accurate predictions of strength and slab rotations in both orthogonal directions at failure. Moreover, this approach can be a useful tool for the structural assessment of existing structures. An analytical model is also presented which extends the CSCT to punching shear cases of internal columns with moment transfer, edge columns, and corner columns, which are commonly found in practice. The comparison between the analytical and experimental results shows that the approach developed combined with the failure criterion used in the CSCT provides accurate predictions of strength and deformation capacity for internal columns with moment transfer, edge columns, and corner columns. This work was also used to validate the formulation of the first complete draft of the Model Code 2010. In this work, the punching shear behaviour of flat slabs with bent-up bars was also investigated. Bent-up bars were commonly used in the past and are still used nowadays for particular design situations. A reliable model is needed to design structures and to assess existing structures with bent-up bars. To address this, five punching shear tests were performed on symmetrical slabs with bent-up bars. The strain measurements along the reinforcement bars provided a better understanding of the anchorage mechanisms. A model was developed, which is consistent with the CSCT and provides satisfactory predictions of the distribution of strains along the bent-up bar. The proposed model can be used to calculate the contribution of the bent-up bars, which needs to be added to the concrete contribution from the CSCT to obtain the punching shear strength.

EPFL2011

EPFL2006

EPFL2007

Poinçonnement non symétrique des dalles en béton armé

Luca Tassinari

EPFL2011