Reinforced concrete bridges under increased railway traffic loads

The aim of this thesis is to provide rational engineering methods for the fatigue safety examination of existing railway bridges. Increasing railway traffic loading and enhanced structural analysis allow for higher exploitation of the capacity of bridge elements. This implies that increasing action effects due to higher service loads may become fatigue relevant in the future. The application of the results allows for more precise fatigue examinations. As beneficial consequence, most railway bridges may be operated safely in the future without costly strengthening measures. Calculated stress ranges in the reinforcement that governs the fatigue resistance of structural elements are too high when using conventional resistance models. The load carrying contribution of non-structural elements and a not entirely developed crack pattern lead to fatigue relevant reduction of the stress range in the rebars. A theoretical study shows that reductions of 5 – 15 % may be obtained by considering the composite effect of the continuous rail grid. Measurements of the dynamic traffic effects were conducted on a single-track bridge. The effects of passenger and freight trains at different velocities on the structural response were measured. The results are interpreted based on the observed vertical shape of the railway track. The observed dynamic behaviour could be correctly represented by simulations using simple dynamic models. The effect of other velocities and in-creased carriage loading degrees are investigated in a parametric study. The findings of previous investigations for road bridges and the investigations of this research work for railway bridges show that the Dynamic Amplification Factor (DAF) decreases for increasing loadings. Thus, smaller DAFs are applied for fatigue relevant causing heavy vehicles. Fatigue stress ranges may be reduced by supplementary 3 – 6 %. A method based on Linear Elastic Fracture Mechanics theory is presented for fatigue life determination. Fatigue life typically is very sensitive to different calculation parameters, rendering the calculation of a reliable fatigue life difficult. The refined analysis of fatigue tests reported in literature allows better understanding of the fatigue behaviour of bridge elements. Thus, a fatigue safety concept is established that compensates the inconvenience of the high sensitivity of fatigue life calculations. This fatigue safety concept is based on the observed load carrying behaviour of inspected bridge elements. The specific fatigue behaviour with visible signs allows for the examination of fatigue safety in a pragmatic way. Specific inspection of bridge elements becomes pertinent as soon as general examinations exhibit an insufficient fatigue resistance. Fatigue behaviour of Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) is characterised by increasing deformations during cyclic tensile loading. Under a maximum tensile stress level of 4 MPa the deformation of the UHPFRC layer becomes stable during cyclic loading. For slightly higher stresses, the deformation increases and the UHPFRC layer fails due to steel fibre pull out. Application of the UHPFRC layer in the flexural tension zone considerably reduces the fatigue stresses in the steel reinforcement. As the UHPFRC layer deforms, the stress range in the reinforcement is effectively and permanently reduced and may remain below the nominal fatigue limit. For design of members with a layer of UHPFRC for waterproofing and fatigue strengthening, the maximum allowed strain for stable behaviour under tensile fatigue loading should be verified.

Connections by Adhesion, Interlocking and Friction for Steel-Concrete Composite Bridges under Static and Cyclic Loading

Dimitrios Papastergiou

Steel-concrete composite bridges with twin-I steel girders and prefabricated slabs constitute an economical and competitive solution for small and medium bridge spans. In recent years they have started to gain their share in the construction market. However, the current slab-to-girder connection with groups of headed shear stud connectors is not well adapted for prefabrication and fast erection, and does not respond successfully to the durability of the construction. New types of connections by adhesion, interlocking and friction constitute a conception that aims to answer these demands. The resistance of these connections is based on the development of longitudinal shear stresses in the interfaces which form the connection. The goals of this study are to complete the existing research in this field by proposing scientific tools in order to predict the connection's structural performance, i.e. prediction of the connection's ultimate resistance and deformation capacity, including the post-failure behaviour. In addition, the behaviour of the connection under cyclic loading is also investigated to assess the connection's resistance to fatigue and to propose a design method for composite bridges under fatigue loading. To fulfill theses goals a combinational methodology was applied which included: study of the state of the art: a) in composite bridges so as to define the requirements and b) in interface behaviour in order to emphasize the parameters which are responsible for the resistance and obtain information for the nature of cyclic loading damage mechanisms, experimental investigations: a) to reveal the laws describing the interfaces and the connection's behaviour for static and cyclic loading, and provide data for development of analytical expressions and model validation and b) demonstrate the capacity of a composite beam under cyclic loading and the extent of its remaining resistance at ultimate limit state, analytical studies: a) to produce the laws describing the interface behaviour under static and cyclic loading and b) to evaluate the impact of the cyclic loading to the slip in the interfaces and in the connection and thus propose a damage factor for cyclic loading, finite element analysis in order to enable an analytical description of the confinement effect, caused by the reinforced concrete slab, which contributes to the resistance, the development of a numerical model to predict the connection's structural performance and to propose a design method for composite bridges fabricated with the new connection. Application of the mentioned methodology resulted in several useful conclusions: The interface behaviour can be described by analytical laws. The post failure behaviour of the interfaces was explained by accounting for the kinematics of the failed surface and basic notions of fracture mechanics. The damage due to cyclic loading on the interfaces and on the connection is expressed by the accumulation of a residual slip. Provided that the shear stresses remain lower than an elastic limit and the accumulated slip does not exceed the failure slip for static loading, fatigue failure is avoided. The confinement effect was incorporated, analytically, together with the laws of the interface behaviour in a numerical model to predict the connection's structural performance and allow a parametrical study. A design method for the verification of composite beams under static and cyclic loading was developed.

EPFL2012

Connexions par adhérence pour les ponts mixtes acier-béton

Michel Thomann

When building new bridge structures, or widening or replacing existing bridges, the duration of on site work has a significant influence not only on the costs, but also on the potentially harmful effects (noise, pollution, traffic jam, deviation) of the construction work. Thus, it is of interest to design structures in such a way as to minimize the construction time. Steel-concrete composite bridges are ideal for this purpose: the steel beams may be welded and the concrete slab precast in the shop, leaving only the erection and assembly work to be performed on site. Currently, however, the steel-concrete connections used in composite bridges are not well adapted for the use with precast slabs, as they tend to slow down the assembly work and decrease the durability of the slab. Consequently, there is a need to develop new types of connections. Connections by adherence, whose resistance is due to friction between the various interfaces, constitute a very promising solution to this problem. The current state of knowledge is incomplete, however, and should be improved in order to make the use of connections by adherence possible in practice. The goals of the research presented herein are to evaluate the practical value of connections by adherence, to develop analysis method and tools making it possible to predict their behaviour and to propose a design method for composite beams employing connections by adherence. To achieve these goals, the requirements that must be met by connections in steel-concrete composite bridges are first defined. Then, the behaviour of connections by adherence is studied in the following steps: - model the behaviour of confined interfaces loaded in shear with the help of experimental and analytical studies, - study the mechanical behaviour of connections by adherence, and especially their deformation behaviour, with the help of push-out tests, - create and validate an analysis tool that makes it possible to predict the behaviour of connections by adherence, - use this model in a parametric study to obtain results to be used in the development of a simplified design method capable of predicting the ultimate shear resistance, - develop and validate a design method for steel-concrete composite beams with connections by adherence. Finally, a comparison between the requirements defined at the beginning of the study and the calculated or measured characteristics of connections by adherence is performed. This makes it possible to evaluate the practical value of these connections and to define the needs for further research. Several important conclusions are drawn from this work. First, it is shown that connections by adherence meet the specified requirements of robustness, economy and reliability. Their high longitudinal shear resistance and stiffness ensure excellent static behaviour, both under service and ultimate loads. Secondly, it is demonstrated that the behaviour of these connections may be explained and modelled with the help of laws governing the behaviour of confined interfaces loaded in shear. The confinement is due partially to the strong interaction existing between the different interfaces and partially to the effect of the concrete slab around the connector. Finally, it is shown that, although a plastic calculation of the shear forces along the bridge axis can not be performed with these connections, it is possible to perform a plastic design of the cross section provided that the consequences of this calculation on the elastic longitudinal shear force distribution are taken into account. The work carried out in this thesis has made it possible to propose design tools for composite beams with a connection by adherence. Those tools, scientifically founded, should help to increase the use of such connections in practice.

EPFL2005

Structure et mécanismes de microdéformation de polymethylmethacrylates renforcés au choc

Laure Dupuits Lalande

Polymethylmethacrylate (PMMA) is used in many applications requiring good optical properties, resistance to ageing and resistance to UV irradiation, in spite of its relatively poor mechanical response to impact loading. In order to extend the range of application of this transparent but fragile material, and improve its competitiveness with engineering polymers such as polycarbonate, it is necessary to improve its fracture resistance. Several generations of toughened PMMA reinforced by addition of rubber particles have therefore been developed by Arkema over the last decade ("rubber toughened PMMA" (RTPMMA)). However, there remains a need for substantial improvement even in these relatively fracture resistant materials, providing the overall motivation for this project. The first type of material considered is based on RTPMMA in which the matrix ductility has been improved by copolymerization with up to 25 % ethyl acrylate (EA). The reinforcing particles used in these new modified RTPMMA grades comprise a PMMA core, either one (conventional 3L particles), or two rubbery layers (experimental 4L particles), and finally an outer grafted PMMA shell. A second, entirely new type of rubber modified PMMA, based on Poly(MMABA-MMA) copolymers (MAM) prepared by controlled radical polymerization (PRC) is then considered. PRC is carried out using nitroxide initiators, making it possible to polymerize the blocks of PMMA from the blocks of PBA in a continuous mass process. Although little was known about their mechanical response prior to the present work, the MAM copolymers represent an attractive alternative to RTPMMA from an economic point of view since their production involves essentially only one step. The scientific aim of this study is to understand the relationship between the mechanical properties, the crack propagation mechanisms and the microstructure of these various impact reinforced PMMA materials, and, if possible, to use this knowledge to identify strategies for improving their performance. It is first shown that there is an increase in the strain to break from 10 % to up to 30 % on addition of 25 % EA to the PMMA matrix, and a corresponding reduction in the yield stress from 95 to 60 MPa. The TEM observations of thin films deformed in-situ show these changes to be associated with a change in microdeformation mechanism from crazing in the pure PMMA to homogeneous plastic deformation. However, they are not accompanied by any substantial improvement in the high speed fracture resistance, the value of the maximum stress intensity factor, KImax, at 1 m/s remaining at about 2 MPa.m1/2 in all the matrices investigated. The ductility improves significantly on addition of the 3L and 4L particles, and this improvement is particularly marked with the 4L particles, the strain to break reaching 80 % and the yield stress dropping to about 40 MPa, regardless of the matrix EA content. The fracture resistance also increases over the full range of speeds investigated, and optimum performance is obtained in the matrix containing 25 % EA combined with 4L particles, KImax ranging from 3.2 MPa.m1/2 at 10-4 m/s to 3.5 MPa.m1/2 at 1 m/s. At low speeds, both the 3L and 4L particles deform by cavitation of the rubbery layer and, as the EA content is raised, the matrix microdeformation mechanism at the crack tip changes from crazing to simple shear, consistent with the observations for the matrix. Nevertheless, at very high speeds, at which crazing dominates in all the materials, the rubbery phase of the particles is argued to become relatively rigid and hence increasingly ineffective in decreasing the crack propagation energy. Thus, the overall improvements in mechanical behaviour obtained on EA addition are not reflected by an improvement in the notched Charpy impact toughness, which does not exceed 5.5 kJ/m2 in any of the RTPMMA. There may be scope for further optimizing the particle size at high EA contents owing to an observed decrease in entanglement molar mass, as demonstrated in previous work by Béguelin et al., but this is not thought to be viable from an economic point of view. MAM copolymers provide an alternative, low cost means of introducing a rubbery phase with controlled dimensions and morphology into a PMMA matrix. These MAM copolymers show similar ductility in tension to the RTPMMA (strain to break around 70 %) and an even lower yield stress (around 25 MPa depending on the grade). MAM is found to show a lamellar morphology for PMMA contents, ∫PMMA < 75%, and deformation of this morphology results in extension of the PBA phase and formation of chevron-like regions of shear deformation. Even so, the fracture resistance remains relatively low in the first generation of these materials, KImax not exceeding 2 MPa.m1/2 at 1 m/s, and Charpy impact tests on notched samples showing relatively little improvement between MAM and the PMMA homopolymer (notched Charpy impact toughness of up to 2.25 kJ/m2 compared with 1 kJ/m2 for PMMA and 5.5 kJ/m2 for RTPMMA). The morphological observations and mechanical test results for MAM nevertheless point to the important role of the molar mass and ∫PMMA, and also to the interest of stripping, i.e. removal of residual BA monomer prior to polymerization of the MMA block. It has hence been possible to design and produce a second generation of stripped MAM copolymers with higher molar masses (Mn > 100 kg/mol) and an optimum ∫PMMA (< 65 %). These are found not only to show ductile behaviour in tension and excellent optical properties, but also notched Charpy impact toughnesses approaching 6 kJ/m2. Although this is still significantly less than for polycarbonate (12 kJ/m2), it demonstrates the promise of MAM for expanding the range of applications of MMA-based systems beyond that currently accessible to conventional RTPMMA.

EPFL2007