Design principles of surfacings on orthotropic steel bridge decks

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Design Principles of Surfacings on Orthotropic Steel

Bridge Decks

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft

op gezag van de Rector Magnificus prof. dr.ir. J.T. Fokkema voorzitter van het College van Promoties,

in het openbaar te verdedigen op maandag 13 maart 2006 om 15.30 uur door

Tarig Obeid MEDANI

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Dit proefschrift is goedgekeurd door de promotor: Prof.dr.ir. A.A.A.Molenaar

en de toegevoegd promoter: Dr.ir. M.Huurman

Samenstelling promotiecommissie: Rector Magnificus, voorzitter

Prof.dr.ir. A.A.A.Molenaar, Technische Universiteit Delft, promotor Dr.ir. M.Huurman, Technische Universiteit Delft, toegevoegd promotor Prof.ir. F.S.K.Bijlaard, Technische Universiteit Delft

Prof. A.Collop, BSc., MSc., PhD., University of Nottingham

Prof.dr.dipl-ing. B.Steinauer, Rheinisch-Westfälische Technische Hochschule Aachen Prof. K.Jenkins, BSc., MSc., PhD., University of Stellenbosch

Dr. A.Scarpas, BSc., MSc., Technische Universiteit Delft

Published and distributed by: T.O.Medani

E-mail: t.o.medani@tudelft.nl

Road and Railway Engineering Section Faculty of Civil Engineering and Geosciences Delft University of Technology

P.O. Box 5048 2600 GA Delft The Netherlands ISBN 90-9020478-4

Key words: orthotropic steel bridge, mastic asphalt, material characterisation, material modelling

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written context from the publisher.

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ACKNOWLEDGEMENTS

The research presented in this dissertation was carried out at the Road and Railway Engineering Section of the Faculty of Civil Engineering and Geosciences at the Delft University of Technology. Three sections of the department of Design and Construction were involved in the project “Prolonging the lifespans of orthotropic steel bridges”. The section of Road and Railway Engineering was responsible for research into the behaviour of surfacing materials. The section of Steel Structures is doing research into the development of fatigue damage in the steel components of the bridge. The later is a subject of a PhD study conducted by my colleague Peter de Jong. The Group of Mechanics of Pavement Structures and Materials of the Section of Structural Mechanics was responsible for the computational plasticity and the numerical techniques used to describe the response of the different surfacing materials.

I would like to thank all those people who helped, encouraged and supported me in the different phases of this project, without their efforts it would certainly never have been completed.

First, I would like to thank Henk Kolstein who initiated this project and worked very hard to bring it to a successful end. Special thanks are due to the following companies and institutions that supported this project:

SBO B.V. Specialistische Bedekkingen en Onderhouds Technieken, Smits Neuchatel Infrastructuur B.V., Shell Nederland Verkoopmaatschappij B.V., Bolidt Kunststoftoepassingen B.V., Aachen University of Technology, Vereniging Centraal Bureau voor Constructiewerkplaatsen, Bouwdienst Rijkswaterstaat, the Ministry of Economic Affairs via the Research School Structural Engineering and the Technology Foundation STW.

I acknowledge with profound gratitude and sincerity the contribution of Xueyan Liu who executed most the numerical aspects in this project. Without his contribution, the numerical work would not have been possible. Apart from his devotion to his work, I appreciate a lot his friendship and personal support. I extend my gratitude to Tom Scarpas who was always there for us whenever needed. His ideas, suggestions and constructive criticism have contributed a lot to this project. My gratitude is extended to Cor Kasbergen, Frank Clusters and Yani Sutjiadi for their help and support.

The experimental work would have been impossible to realise without the tremendous amount of hard work by Marco Poot and Robin van Dijk to whom I am grateful. Thanks are extended to Abdol Miradi, the manager of the laboratory, for his endless support.

I gratefully acknowledge the contribution of Allert Bosch, the MSc student who graduated in my PhD project.

I wish to express my appreciation to the members of the users panel (“gebruikerscommissie”) for their comments and discussions during our meetings.

I also acknowledge the continuous support and encouragement of my colleagues at the Road and Railway Engineering Section and I thank them for creating a pleasant working atmosphere.

My gratitude is extended to colleagues and friends from the Building and Road Research Institute of the University of Khartoum and the Ministry of Interior Affairs in Sudan for their endless support.

Tarig Medani,

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SUMMARY

This dissertation describes the research into the behaviour of surfacings of orthotropic steel bridge decks. The motive for this research is the frequently reported problems of this type of structures including cracking and rutting of surfacing materials and fatigue related cracks in the steel plate. These problems are believed to be related to the changes in traffic in terms of increased number of trucks and heavier wheel loads. Because these changes are far beyond our experience, prevailing design methods of wearing courses for orthotropic steel bridges seem to have very limited success.

An intensive experimental program was carried out on three candidate wearing course materials, namely mastic asphalt, guss asphalt and an open synthetic material and on a bituminous-based membrane material used to connect the mastic asphalt to the steel plate of the Moerdijk Bridge in the Netherlands in 2000.

With regard to the wearing course materials, the test program was divided into two categories based on the purpose of the tests:

1. Basic tests. The purpose is to characterise the materials using “traditional tests”. This program included:

• fatigue testing to determine the fatigue characteristics of the candidate materials;

• repeated load indirect tensile tests to determine the mix stiffness and the compliance characteristics of the mixes at several temperatures and load frequencies;

• four-point bending tests to determine the mix stiffness and the compliance characteristics of the mixes at several temperatures, load frequencies and strain levels;

• monotonic indirect tensile tests to determine the tensile strength and the fracture energy of the mixes tested.

2. Monotonic uniaxial compression and tension tests. These tests were performed to serve two purposes:

• to characterise the response of the materials to temperature and strain rate;

• to determine the parameters of the material model, which was used to describe the non-linear response of the wearing course materials.

With regard to the bitumen-based membrane material, the four-point shear test was used to serve two purposes:

• to characterise the shear strength and stiffness of the material at different deformation rates, temperatures and confining stresses;

• to determine the parameters of the material model, which was used to describe the non-linear response of the membrane material.

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Furthermore, constitutive relations for elastic as well as inelastic response of surfacing materials were developed. The Asphalt Concrete Response (ACRe) material model, developed earlier at Delft University of Technology, was used to describe the response of the wearing course materials. A Desai-type material model was used for the characterisation of the response of the bitumen-based membrane material.

To determine the mechanical behaviour of the wearing course materials and the membrane material, a general-purpose concept called the “global model concept” was introduced. This concept is a general purpose concept and can be used for e.g. the determination of the parameters of material models. In this concept, a global model is fitted to data obtained from all relevant experimental results and not to results of individual tests.

All the developed models were implemented in the finite element code CAPA-3D, developed earlier at Delft University of Technology. For verification of the different models results, of laboratory tests and full-scale experiments in the LINTRACK APT (Accelerated Pavement Testing) facility were used. Furthermore, results of non-linear FE simulations were used for identification of the different distress phenomena that occur in surfacings of orthotropic steel deck bridge.

A scientific approach in which the non-linear response of the materials, the geometry and the load are well presented was used to give an insight into the interaction between the different components of the structure and the vehicle at different temperatures. This scientific approach has shown that the assumptions upon which available design methodologies are based are not true.

An apparent disadvantage of the scientific approach is that it is too sophisticated and expensive for routine analyses and design of the structure. For this reason a practical design concept is proposed. In this concept, the geometry and the load are well presented and the material behaviour is simplified. This simplification consists of assumption of linear behaviour of wearing course materials but an important phenomenon which is normally ignored, namely the degradation of the stiffness with increasing strain, is catered for. This concept can be used for the design of surfacings for new orthotropic steel bridges as well as for the estimation of the remaining lifespan of surfacings on orthotropic steel bridges in use.

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SAMENVATTING

Dit proefschrift behandelt het onderzoek naar het gedrag van deklagen op orthotrope stalen brugdekken. De reden voor dit onderzoek ligt in de problemen die bij dit type constructie frequent ontstaan, zoals scheurvorming en spoorvorming in de deklaag en aan vermoeiing gerelateerde scheurvorming in de stalen dekplaat. Naar verwachting houden deze problemen verband met de toename van de verkeersbelasting, zowel wat betreft het aantal vrachtauto’s als de grootte van de wiellasten. Omdat deze veranderingen in verkeersbelasting ons ervaringsgebied te buiten gaan, zijn gangbare methoden voor het ontwerp van deklagen op orthotrope stalen brugdekken weinig succesvol. Een uitgebeid meetprogramma is uitgevoerd op drie mogelijke slijtlaagmaterialen, namelijk gietasfalt, guss-asfalt en een open kunststof-gebonden materiaal. Tevens zijn proeven uitgevoerd op een bitumineus membraan dat in 2000 op de Moerdijkbrug is gebruikt om de asfalt deklaag aan de stalen dekplaat te hechten.

Het proefprogramma op de slijtlaagmaterialen bestond uit twee delen met elk een verschillend doel: 1. Standaard proeven. De reden van deze proeven was om de materialen te karakteriseren met “traditionele proeven”. Dit testprogramma omvatte:

• Vermoeiingsonderzoek om de vermoeiingseigenschappen van de deklaagmaterialen te bepalen;

• Dynamische indirecte trekproeven om de mengselstijfheid en de dynamische viscositeit van de mengsels bij verschillende temperaturen en frequenties vast te leggen;

• Vier-punts-buigproeven om de mengselstijfheid en de dynamische viscositeit van de mengsels bij verschillende temperaturen, frequenties en rekniveaus vast te leggen;

• Monotone indirecte trekproeven om de treksterkte en de breukenergie van de mengsels te bepalen.

2. Monotone uniaxiale trek- en drukproeven. Deze proeven zijn uitgevoerd met als doel:

• het karakteriseren van de materiaal respons als functie van de temperatuur en reksnelheid; • de bepaling van de parameters voor het materiaalmodel dat is gebruikt om de niet-lineaire

respons van de deklaagmaterialen te beschrijven.

Op het bitumineuze membraan zijn vier-punts-schuifproeven uitgevoerd. Deze proeven hadden als doel:

• het karakteriseren van de schuifsterkte en stijfheid bij verschillende vervormingssnelheden, temperaturen en normaalspanningsniveaus;

• de bepaling van de parameters voor het materiaalmodel dat is gebruikt om de niet-lineaire respons van het bitumineuze membraan te beschrijven.

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Verder zijn constitutieve vergelijkingen opgesteld ter beschrijving van de elastische en de niet-elastische respons van deklaagmaterialen. Het Asphalt Concrete Response (ACRe) materiaalmodel, dat eerder aan de Technische Universiteit Delft is ontwikkeld, is gebruikt om de respons van de slijtlagen te beschrijven. Een Desai-achtig materiaalmodel is gebruikt bij de beschrijving van de respons van het bitumineuze membraan.

Om het mechanisch gedrag van de deklaagmaterialen en het membraan materiaal goed te bepalen is het “global model concept” geïntroduceerd. Dit concept kan onder andere worden toegepast bij de bepaling van de parameters voor materiaalmodellen. Binnen het concept wordt een globaal beschrijvend model gefit aan alle beschikbare data verkregen uit verschillende soorten proeven. Het model wordt dus niet gefit aan de resultaten van een enkele proef.

Alle ontwikkelde modellen zijn opgenomen in het eindige elementen pakket CAPA-3D dat eerder is ontwikkeld aan de Technische Universiteit Delft. Voor de verificatie van de verschillende modellen is gebruik gemaakt van de resultaten van laboratoriumproeven en full-scale metingen in de LINTRACK installatie voor het versneld belasten van verhardingen. Bovendien zijn de resultaten van niet-lineaire eindige elementen berekeningen gebruikt om de diverse schademechanismen te indentificeren die in deklagen van orthotrope stalen brugdekken optreden brugdekken optreden.

Een wetenschappelijke benadering waarbij het niet-lineaire gedrag van de materialen, de geometrie en de belasting goed zijn gemodelleerd, is gebruikt voor het verkrijgen van inzicht in de interactie tussen de verschillende componenten van de constructie en het voertuig bij verschillende temperaturen. Deze wetenschappelijke benadering heeft aangetoond dat de aannamen waarop beschikbare ontwerpmethoden zijn gebaseerd niet correct zijn.

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1. INTRODUCTION AND SCOPE OF THE STUDY

1.1 INTRODUCTION

Orthotropic steel bridges consist of a 10-14 mm steel deck plate supported in two perpendicular directions, by U-shaped stiffeners and crossbeams in the longitudinal and transverse directions, respectively. Crossbeams are welded to the steel plate every 2-4 m [Kolstein and Wardenier, 1997]. Usually, the deck plate is surfaced with a 30-70 mm thick surfacing material e.g. mastic asphalt. A typical transverse cross-section of an orthotropic steel bridge is shown in Figure 1.1.

Figure 1.1: A typical cross section of an orthotropic steel bridge deck [Huurman et al., 2003]

Following World War II, German engineers developed the modern orthotropic bridge design as a creative response to material shortages during the post-war period [Troitsky, 1987]. Since that time, the experience with asphalt mixes on orthotropic steel decks spread gradually over the rest of Europe, like the Netherlands and France. As far is known, the first orthotropic steel bridge was the Kurpfalz Bridge over the river Neckar in Mannheim (Germany), opened in 1950.

The lightweight and flexibility of orthotropic steel bridge decks make them an option for cases where a high degree of pre-fabrication or rapid erection is required [Gurney, 1992], in seismic zones, for movable bridges, for long-span bridges and for rehabilitation to reduce bridge mass [Mangus and Sun, 1999]. Furthermore, they could be built in cold climates at any time of the year [Mangus, 2002].

535 mm 325 mm 300 mm Asphaltic surfacing Girder

Binder layer Steel deck plate

Steel stiffener Spacing in girder

Transversal cross section 300 mm 200 mm cross section x-x x x 535 mm 325 mm 300 mm Asphaltic surfacing Girder

Binder layer Steel deck plate

Steel stiffener Spacing in girder

Transversal cross section 300 mm

200 mm cross section x-x x

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SBS gemodificeerde bitumen

verdwijnt onder nieuwe laag asfalt

op de Moerdijkbrug

…... Soprema Nederland levert deze zomer ook een bijzonder gemodificeerd bitumen dat moet dienen als waterkerende laag op de stalen Moerdijkbrug. Hij is één kilometer lang en heeft zes rijstroken. En wanneer deze zomer de Moerdijkbrug voor een slordige 27 miljoen wordt opgeknapt zal dat ongetwijfeld enorme files veroorzaken, misschien wel van Rotterdam naar Breda voorspellen pessimisten. De Moerdijkbrug is de intensiefst gebruikte brug van het land met een totaal van 120.000 passages per etmaal. Een viervoud van het aantal dat ieder etmaal de filetopper

Brienenoordbrug te verwerken krijgt. Vanaf 12 april wordt het wegdek van de Moerdijkbrug over de volle breedte opnieuw geasfalteerd met een mengsel waarvan verwacht wordt dat het in elk geval acht jaar in topconditie zal blijven.

On the other hand, high strain levels in the order of 1200 µm/m were measured at the top of a surfacing of an orthotropic steel bridge [RHED, 1985], compared with strain levels generally encountered in ordinary pavements (50-200 µm/m). Furthermore, the life spans of surfacings on such steel bridges are quite short when compared with those of ordinary road pavements. Moreover, steel deck plates prove to be very vulnerable to fatigue related cracks. With regard to the surfacing, several countries e.g. the Netherlands, Great Britain and Germany in Europe; Thailand, China and Japan in Asia, have reported several problems including rutting and cracking

The following two articles from Dutch newspapers serve as an excellent example to illustrate the problem [Figure 1.2]:

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Introduction 3 Figure 1.2: Newspaper articles about the resurfacing of the Moerdijk Bridge in the

Netherlands

The first article dated April 1999, gives general information about the Moerdijk Bridge and a new SBS-modified asphaltic mix, which will be used for resurfacing of the bridge. Furthermore, it was expected that the new mix would show an excellent performance for at least 8 years. In the second article dated April 2000, it was announced that two of the eight lanes of the bridge renovated in 1999, must be resurfaced again! And guess what: they were the outer two lanes (where trucks normally drive).

1.2 OBJECTIVES AND LIMITATIONS OF THE RESEARCH

PROGRAM

To the best of the author’s knowledge, there is no universally accepted methodology for the design of surfacings on orthotropic steel deck bridges. The traditional design techniques are merely based on linear elastic theory, experience and some norms obtained from structural tests.

It is known that, especially in the last three decades, there have been changes in traffic in terms of increased number of trucks, heavier wheel loads, introduction of super-single tyres etc. Because these changes are far beyond our experience, prevailing design methods of wearing courses for orthotropic steel bridges seem to have very limited success.

On the other hand, there have also been positive changes e.g. improved qualities of binders and road materials. In the opinion of the author, these positive changes are overshadowed by the absence of reliable design techniques for orthotropic steel bridge decks, which can deal with the current challenges. Because of these factors, many problems, including rutting and cracking, have been reported nationally and internationally. Furthermore, these problems have been more severe than ever.

Moerdijkbrug opnieuw op de

schop

Een deel van de vorig jaar voor 35 miljoen gulden gerenoveerde Moerdijkbrug moet weer van nieuw asfalt worden voorzien. Twee van de acht rij- en vluchtstroken voldoen na een jaar al niet meer aan

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In the Netherlands, a number of its 86 orthotropic steel bridges constitute part of the main national and European road networks e.g. Moerdijk, Ewijk, Caland and Van Brienenoord Bridges [Bosch, 2001]. Resurfacing and/or maintenance of these bridges either results in strong reduction of road capacity or long detours, which were not accepted by the public and private enterprises. This has led to an increased pressure on the road authorities to significantly reduce resulting hindering of traffic.

All these factors have led to the initiation of this research program at Delft University of Technology in December 1999. The principal objective of the research program is to get a better insight into the response of the different components of the structure and their, poorly understood, interaction with the vehicle and the environmental conditions. Furthermore, it is essential to get a better understanding of the structural distress phenomena and the parameters that influence them. This should lead to a design procedure that enables the design of the structure with a longer lifespan. Furthermore, the procedure should be able to determine the structural value of the surfacing, so that steel plate fatigue damage may be prevented by proper surfacing design.

Three sections of the Faculty of Civil Engineering and Geosciences were involved in this research program. The Structural Mechanics Section was responsible for providing support with regard to computational plasticity and the numerical techniques that were involved. Furthermore, the 3D finite element program CAPA (Computer Aided Pavement Design) [Scarpas, 1992] was used for the numerical computations presented in this dissertation. The Steel Structures Section is doing research into the development of fatigue damage in the steel components of the bridge. The section of Road and Railway Engineering is responsible for research into the behaviour of candidate surfacing materials and a membrane material.

To this end, special attention has been focused on fundamental research into the characterisation of three candidate surfacing materials namely: mastic asphalt, guss asphalt and open synthetic wearing course. In addition, the behaviour of the membrane connecting the steel plate to the mastic asphalt has been investigated. The information obtained from the materials testing was used for the development of different models that describe the response of these materials. Furthermore, constitutive relations were developed for the candidate surfacing materials and the membrane material. These constitutive relations were implemented in the finite element program CAPA-3D. The different models, except the material model of the open synthetic mix, were verified using laboratory tests and measurements obtained from a bridge prototype tested at the Accelerated Pavement Testing (APT) facility of Delft University of Technology (LINTRACK).

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Introduction 5 parameters that determine the response of the structure, namely geometry, loading and materials behaviour, are well respected.

1.3 ORGANISATION OF THE DISSERTATION

In this dissertation, four main features can be identified, namely experimental work, description of the time-temperature dependency characteristics of several materials, material response modelling and numerical simulations. Followed by this introductory chapter, a literature review on orthotropic steel bridges, surfacing materials, design issues and distress phenomena in surfacings of orthotropic steel deck bridges will be presented in Chapter 2. The development of the framework of the research approach is described in Chapter 3. In Chapter 4, the results of the “basic” testing program carried out on the three candidate surfacing materials will be presented. The basic testing program includes four-point bending fatigue tests, indirect tension fatigue tests and indirect monotonic and repeated load tensile tests. Chapter 5 describes the results of uniaxial monotonic compression and tension tests carried out on the three candidate surfacing materials. In Chapter 6, the results of the four-point shear tests and tensile tests carried out on the membrane material will be discussed. A new unified model, which describes the time-temperature dependency characteristics of several road materials, will thoroughly be presented and discussed in Chapter 7. In Chapter 8, the response of the three surfacing materials to any state of stress will be presented. In addition, a general-purpose concept called the “global model concept” which can be used for e.g. the determination of the parameters of material models will be described. Moreover, examples of application and verification of material models will be discussed. In Chapter 9, the response of the membrane material, connecting the mastic asphalt and the steel plate, to any state of stress will be presented. In addition, the determination of the parameters of the material model and results of numerical simulations of a number of four-point shear tests will be presented. Chapter 10 provides verification of the deck surfacing system (surfacing wearing course material and the membrane connecting it to the steel plate) using measurements of the APT Lintrack facility, carried out on a prototype of a bridge section surfaced with mastic asphalt. In addition, results of several numerical simulations of the response of the bridge structure to different loading and environmental conditions are presented and discussed. Furthermore, the outlines of a design concept for surfacings on orthotropic steel bridge decks are presented. Finally, conclusions and recommendations are presented in Chapter 11.

1.4 REFERENCES

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Kolstein, M.H. and Wardenier, J., “Stress Reduction Due to Surfacing on Orthotropic Steel Decks,” Proceedings of the ISAB Workshop: Evaluation of Existing Steel and Composite Bridges, Lausanne, Switzerland, 1997.

Mangus, A.R. and Sun, S., “Orthotropic Bridge Decks,” edited by Chen, W. and Duan, L., Bridge Engineering Handbook, C. R. C. Press, Boca Raton, USA, 1999.

Mangus, A.R., “Orthotropic Design Meets Cold Weather Challenges,” Welding Innovation Volume XIX, No 1, 2002, Ohio, USA, 2002.

Huurman, M., Medani, T.O., Scarpas, A. and Kasbergen, C., “3D-FEM for Estimation of the Behaviour of Asphalt Surfacings on Orthotropic Steel Deck Bridges,” International Conference on Computational & Experimental Engineering and Sciences, ICCES'03 Corfu, Greece, 2003.

Road and Hydraulic Engineering Division of the Ministry of Transport, Public Works and Water Management (RHED), “Research on the Fatigue Behaviour of Steel Bridges,” (in Dutch), Report BR 82-01, The Netherlands, 1985.

Scarpas, A., “CAPA-3D Finite Elements System User’s Manual, Parts I, II and III,” Dept. of Structural Mechanics, Faculty of Civil Engineering, Delft University of Technology, Delft, The Netherlands, 1992.

Troitsky, M.S., “Orthotropic Bridges - Theory and Design,” 2nd ed., James F. Lincoln Arc Welding Foundation, Cleveland, USA, 1987.

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Orthotropic steel bridges 7

2. SURFACINGS ON ORTHOTROPIC STEEL DECK

BRIDGES: MATERIALS, DESIGN AND

DISTRESS

2.1 INTRODUCTION

In this chapter, several topics related to surfacings on orthotropic steel bridge decks will briefly be reviewed and discussed, including deck surfacing materials, stress reduction in the steel deck due to surfacings and some design issues related to orthotropic steel bridge decks. Furthermore, background information on the three candidate surfacing mixes which were investigated in this program, namely mastic asphalt, guss asphalt and open synthetic wearing courses, will be presented. In addition, issues related to distress phenomena in surfacings of orthotropic steel bridges and their mechanisms will be reviewed.

2.2 ORTHOTROPIC STEEL DECK BRIDGES

2.2.1 Introduction

Modern steel deck bridges consist of a 10-14 mm steel deck plate supported in two perpendicular directions; by ‘crossbeams’ in the transverse direction and by stiffeners in the longitudinal direction. To support the steel deck crossbeams are welded to the structure every few meters (typically 2-4 m) [Kolstein and Wardenier, 1997]. The load in the crossbeams is transferred into the longitudinal main girders, which form the main support of the bridge. Therefore, the elastic properties of the steel deck structure in the two orthogonal directions are different. In other words, the steel deck structure is an orthogonal and anisotropic plate, for short it is called an orthotropic plate [Gurney, 1992].

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Figure 2.1: Two basic layouts of an orthotropic bridge deck [Gurney, 1992]

2.3 HISTORICAL BACKGROUND

In the 1920’s, American engineers began using steel plate riveted to steel beams for large movable bridges [Paul, 1931]. The purpose was to minimize the dead load of the lift span. In 1938, the American Institute of Steel Construction (AISC) began publishing reports on the steel-deck system. AISC called this the “battledeck floor” because it felt the steel deck had the strength of a battleship. The orthotropic deck was a result of the ‘battledeck’ floor. This floor consisted of a steel deck plate, supported by longitudinal (normally I-beam) stringers. In their turn, these stringers were supported by cross beams. The principle of a bridge with a ‘battledeck’ floor is shown in Figure 2.2.

Figure 2.2: Principle of a bridge with a ‘battledeck’ floor [Gurney 1992]

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Orthotropic steel bridges 9 Following World War II, German engineers developed the modern orthotropic bridge design as a creative response to material shortages during the post-war period [Troitsky, 1987]. The orthotropic deck reduced the weight of continuous beams considerably and permitted spans and slenderness ratios unknown until then. At first open rib longitudinal stiffeners were used, later the closed stiffeners with a higher torsion stiffness were introduced [Gurney 1992].

As far is known, the first orthotropic steel bridge was the Kurpfalz Bridge over the river Neckar in Mannheim opened in 1950, while the first suspension bridge to have an orthotropic deck was the Cologne-Muelheim Bridge over the Rhine completed in 1951. The first major orthotropic structure in North America was the Port Mann Bridge at Vancouver, which was opened in 1964. In the Netherlands the Hartel Bridge and the Harmsen Bridge, opened in 1968 were the first to be built [Bosch, 2001]. Nowadays there are more than 1000 orthotropic steel bridges in Europe, out of which 86 are in the Netherlands. In Asia, there are several bridges that are built or being built, especially in China and Japan. In the USA, the number of orthotropic steel bridges is remarkably small as only 51 out of its 650,000 bridges are orthotropic steel deck bridges [Mangus, 1988].

Orthotropic steel deck bridges are relatively light in weight, which make them attractive where a high degree of pre-fabrication or rapid erection is required [Gurney, 1992]. They are also an important option for bridges in seismic zones, for movable bridges, for long-span bridges and for rehabilitation to reduce bridge mass [Mangus and Sun, 1999]. Moreover, they can be built in cold climates at any time of the year, [Mangus, 2002]. While concrete must be at or above 5o C to properly cure, it is physically possible to encapsulate and heat the concrete construction process; but this will of course add to the construction costs [Mangus 1988, 2002].

Pictures of some famous orthotropic steel deck bridges are shown in Figures 2.3-2.8.

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Figure 2.4: San Diego-Coronado Bridge, opened in 1969, USA

Figure 2.5: Van Brienenoord Bridge, opened in 1989, The Netherlands

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Orthotropic steel bridges 11 Figure 2.7: The Forth Road Bridge, built in 1964, Scotland

2.4 DECK SURFACINGS

Surfacings on orthotropic steel deck bridges serve three main purposes [Gurney, 1992]:

• provision of a running surface with suitable skid resistance, etc;

• provision of a flat running surface by varying its thickness to compensate for distortion of the steel deck plate;

• protection of the deck plate by providing a waterproofing layer.

Considering that these functions are generally not fulfilled or only partially fulfilled by one material, a functional division can be made for the layers constituting the surfacing of the steel deck. In general, the deck surfacing consists of a bonding layer, an isolation layer, an adhesion layer and a wearing course [Figure 2.8]:

Figure 2.8: Typical layers in a cross section of an orthotropic steel deck bridge • bonding layer: to assure sufficient adhesion between the steel deck and the

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• isolation layer: to protect the underlying steel deck against corrosion and to provide flexible transition between the wearing course and the steel deck; • adhesion layer: to assure sufficient adhesion between the isolation layer and the

wearing course;

• wearing course: to carry and transfer the traffic loading to the underlying structure and to provide the necessary skid resistance.

The above-mentioned division of functions is not always applicable. Sometimes one layer can achieve several functions [Kolstein, 1990].

2.4.1 General material requirements of different surfacing layers

The general requirements of surfacing materials on orthotropic steel bridge decks are: • the materials used for the wearing course should have sufficient resistance to

rutting;

• the wearing course has to provide sufficient skid resistance; • the materials should have sufficient resistance to fatigue cracking; • the adhesion between the different layers should remain intact.

At high temperatures, the wearing course has to satisfy the stiffness requirement and at low temperatures, it must not crack or loose adhesion to the steel deck. However, it is difficult to satisfy all these requirements.

Brants [1972] divided the requirements for the bonding layer, isolation layer and the wearing course. Because a clear distinction between the different layers is not always possible, some requirements are valid for more than one layer.

2.4.1.1 Bonding layer

This layer should be able to:

• give reliable protection against corrosion;

• assure sufficient bonding between the overlying layers and the steel deck, and sufficient shear strength to resist braking and other shear forces.

According to Kohler and Deters [1974], the bonding layer needs to possess a low viscosity to comply with the above requirements.

2.4.1.2 Isolation layer

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Orthotropic steel bridges 13 • extremely low permeability to prevent penetration of oil, water, minerals and

de-icing salts to the lower layer,

• sufficient resistance to fatigue cracking. Two types of isolation layers are in common use:

1. dense mastic layer and as a protective layer, a thin guss-asphalt layer. This system is in common use in Germany. The thickness of the layer is normally between 8-10 mm. Sometimes artificial material or rubber is added.

2. mastic coating layer with grit spread over it.

2.4.1.3 Adhesion layer

This layer has to provide strong adhesion between the wearing course and the underlying layers. Furthermore, the desired properties include durability, reliability and simplicity in construction.

According to Kohler and Deters [1974] and Kraft [1979], three types of adhesion layers can be distinguished based on:

1. bitumen (hot fluid bitumen),

2. bitumen emulsion (cold fluid bitumen),

3. artificial resin: these are adhesion layers, which consist of cold hardening epoxy resins, spread with gravel. This is necessary to obtain a good adhesion with the overlaying layers.

2.4.1.4 Wearing course

To ensure a safe and comfortable ride for the road users the wearing course needs to posses the following characteristics:

1. good skid resistance, 2. flat surface,

3. low sound levels,

To ensure durability the required characteristics of the wearing course includes: 1. sufficient resistance against fatigue cracking and rutting,

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2.5 STRESS REDUCTION IN STEEL DECK DUE TO THE

SURFACINGS

It is known that surfacings of orthotropic steel deck bridges reduce the stresses in the steel structure. This is thought to be in part due to the composite action in bending with the deck plate, and in part due to the thickness of the wearing course dispersing the load [Smith and Cullimore, 1988].

The composite action in bending of the wearing course with the steel deck causes reduction of stresses. By applying a surfacing on the steel plate, the moment of inertia of the structure increases, resulting in smaller deformations and thus lower stresses. The total reduction of stresses due to composite action depends mainly on the bond between the wearing course and the steel deck, and the stiffness of the wearing course. When there is no bond between the steel and the wearing course, the two layers are effectively separated, resulting in a lower moment of inertia of the structure and thus higher stresses in both the steel plate and the surfacing. When the two layers are completely bonded, the system acts as a composite structure, resulting in the lowest stresses in the two layers.

In case of asphaltic surfacings, the properties of the wearing course become temperature, stress and strain rate dependent and they may change as the asphalt ages. This makes the reduction of stresses in the steel plate due to the contribution of the surfacing variable. This is in fact one of the main reasons why its contribution is normally not taken into account for design purposes. Therefore, at present the surfacing provides a factor of safety of unknown magnitude [Kolstein, 1990].

The Department of Bridges and Steel Structures of the Ministry of Transport, Public Works and Water Management [1985] has conducted some measurements on a test section at the Kreekrak orthotropic bridge. The measurements have shown that the reduction of stresses in the steel plate due to surfacing is 80% in the winter and, of course, less in summer but still around 30%.

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2.6 WEARING COURSE MATERIALS

Different countries use different surfacing materials and different layer thicknesses. In general, the thickness of the wearing course ranges between 30 and 70 mm. In some countries there is preference for “poured” asphalt, e.g. mastic asphalt in the Netherlands and guss asphalt in Germany, and in others for asphalt concrete e.g. France [Kolstein, 1990]. In this research project three candidate wearing course materials were investigated [Figure 2.9], namely mastic asphalt (MA), which was used for resurfacing of the Moerdijk Bridge in the Netherlands in 2000, guss asphalt (GA) and an open synthetic wearing course (OS). For this reason, some background information on these mixes will be presented in the coming sections.

Figure 2.9: Specimens of the three surfacing materials

2.6.1 Mastic asphalt

Mastic asphalt mixes, e.g. ‘gietasfalt’ in the Netherlands and ‘gussasphalt’ in Germany, are considered overfilled mixes. This means that the pores in the aggregate skeleton are overfilled with the mastic, so the mineral aggregates do not form a matrix skeleton but float in the mastic. Thus, the stability has to be guaranteed by a stiff type of bitumen and a high percentage of filler. The amount of voids is extremely small (< 2.5 %). In general, mastic asphalt mixes have the following characteristics [Medani, 2001]:

• the voids are overfilled with mastic,

• due to the overfilled voids and the absence of a matrix skeleton, the mix does not need to be compacted,

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• the very low void content renders these types of mixes to be extremely impermeable,

• due to the large mortar stiffness, the production and processing take place at relative high temperatures (220-240ºC),

• because of the high processing temperature insulated transport is normally used,

• for the production of the mix a normal asphalt plant can be used,

• for processing, the use of a special spreading machine is required, but because of the small areas normally laid, mastic asphalt may also be hand-laid,

• the thickness of the layers varies between 30 mm and 70 mm [Whiteoak, 1990].

2.6.1.1 General requirements of mastic asphalt in the Netherlands

In the Netherlands, mastic asphalt is used for surfacing steel bridge decks, parking decks and industrial floors. Table 2.1 shows the requirements for gradation, bitumen and voids content of mastic asphalt [CROW, 2000].

Table 2.1: Some requirements of mastic asphalt in the Netherlands

Targeted Min. Max.

C8 C4 2 mm 63 µm - 25 44 83 0 20 39 80 5 35 52 86 Bitumen content by mass

(on 100% mineral aggregate) Voids content (%)

8.5 7.0 10.0 2.5

Furthermore, the maximum aggregate size is 8 mm. The mix consists of crushed stones 2/8, sand type A (aggregate size between 2 mm and 63 µm), filler and bitumen 40/60.

However, some qualitative measures were set after an extensive testing program carried out by NPC in 1995 and 1996 for selection of materials for the Hagestein and Ewijk Bridges:

• high resistance against fatigue cracking (flexural bending test), • high fracture strength (semi-circular bending test),

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Orthotropic steel bridges 17 • the top layer (in a 2-layer system) must have elastic properties (tough, rubbery

behaviour) to resist fatigue cracking,

• the sub layer (in a 2-layer system) must possess a high stiffness to limit permanent deformation and to contribute to the strength of the total structure. A typical cross-section of an orthotropic steel bridge in the Netherlands is shown in Figure 2.10 (all dimensions are in mm).

Figure 2.10: A typical cross section of an orthotropic steel bridge in the Netherlands

2.6.2 Guss asphalt

As have been mentioned before, guss asphalt is an overfilled asphaltic mix too. It is used in Germany for different purposes among which surfacing of orthotropic steel deck bridges.

2.6.2.1 General requirements of guss asphalt in Germany

In Germany, there are four main types of guss asphalt, varying in the maximum grain size, as shown in Table 2.2 [ARS, 1983].

Table 2.2: Some requirements of guss asphalt in Germany

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Table 2.3: Stability requirements for different guss asphalt mixes in Germany Maximum

aggregate size

Penetration after 30 min [0.1 mm] and temperature of 40o C

Penetration in the next 30 min [0.1mm] and temperature of 40o C 11 S 10 - 35 ≤ 4 11 10 – 50 ≤ 6 8 10 – 50 ≤ 6 5 10 – 50 ≤ 6

Often a bitumen type B65 is used in Germany with a penetration of 50-70 (at 25ºC, 0.1 mm).

The requirements for the bonding layer between the steel and the membrane [Huber, 1992] are:

• ball penetration [ZTV-BEL-ST 92, 1992] : ≤ 6 mm at 25°C and ≤ 8.5 mm at 40°C,

• temperature ring and ball : ≥ 70°C,

• after 1 million load repetitions, the bonding between the different layers must be intact (bending test).

A typical orthotropic steel bridge cross-section in Germany is shown in Figure 2.11 (all dimensions are in mm).

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2.6.3 Open synthetic wearing course

This system has been developed by Bolidt b.v. in the Netherlands, abbreviated in Dutch as ZOK (Zeer Open Kunststof) and may be translated in English to open synthetic wearing course (OS). The system was first applied on the Caland Bridge in Rotterdam, followed by trial sections on the Rama IX Bridge in Bangkok, Thailand and the West Gate Bridge in Melbourne, Australia. The system comprises of two layers of special membrane and a layer of open graded synthetic wearing layer with a thickness of 80 mm.

2.6.3.1 Application of the system

The steps used to apply the ZOK system can be summarised as follows:

• the steel deck has to be grit blasted and then a primer is applied. To prevent oxidation of the steel plate, the primer must be applied within one hour,

• after curing of the primer (4 hours at 30o _{C), the first membrane layer of 1.5} mm is applied,

• after curing of the first membrane layer (2 hours at 30o _{C), the second layer is} then applied in a thickness of 1.5 mm. To increase the adhesion of the wearing course to the membrane, Graziet or Basalt of size 5-8 mm is spread on the membrane,

• after curing of the second membrane layer (2 hours at 30o _{C), and just prior to} the application of the OS, the membrane is coated with a spray coat of Bolidt binder,

• the OS is mixed in a mortar pan mixer and transported from the mixer to a standard asphalt paver with 3-4 m width, and directly spread. The paver is not heated and must not vibrate,

• after application of the ZOK system, it is rolled with a low-line load smooth steel wheel roller. Rolling is limited in time and number of cycles,

• after curing of the ZOK system (minimum 24 hours), it is coated with a spray coat of Bolidt binder and Bauxite of size 0-1 mm.

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Figure 2.12: A cross section of the ZOK system

2.7 SOME DESIGN CONSIDERATIONS

2.7.1 Estimation of stresses/strains in the bridge section

In the so-called composite action theories, the complex geometry of the structure is simplified into a single span or, at most, 2-span beam model, with different support conditions. Engineers often use these theories for estimation of the stresses and strains in the bridge section. In most of these theories, it is assumed that the steel and the wearing course layers have the same radius of curvature and plane cross-sections before and after loading [Metcalf, 1967; Johnson, 1975]. Similar theories for bending of sandwich beams and plates have been offered by di Taranto [1965, 1973]. The analysis of a multi-layer elastic beam was tackled by first by Rao [1979] followed by Cullimore et al. [1983] using Airy stress functions. More recent theories have been suggested by Sedlacek and Blid [1985], Kolstein [1990] and Nakanishi and Kensetsu [2000].

However, all researchers based their work on linear elastic theory [Metcalf, 1967; Cullimore et al, 1983; Kolstein, 1990; Kolstein and Wardenier, 1997; Sedlacek, 1982; Sedlacek and Bild, 1985; Natanishi and Kensetsu, 2000]. Furthermore, they have adopted one or both of the following assumptions [Figure 2.13]:

• linear strain distribution in the asphalt and the steel.

• the gradients of the strain distribution along the depth of the asphalt and steel are equal.

The last assumption provided researchers with an extra equation, which was needed for the solution of the system of simultaneous equations.

first layer membrane D60 (1.5 mm)

second layer membrane D60 (1.5 mm) grit steel plate OS (80 mm) grit primer

first layer membrane D60 (1.5 mm)

second layer membrane D60 (1.5 mm) grit steel plate OS (80 mm) grit primer

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Orthotropic steel bridges 21 Assumed strain distribution (full-bond) Assumed strain distribution (no-bond)

Figure 2.13: Assumed strain distribution along the depth of the bridge section However, there is no theoretical and/or experimental background for such assumptions. In fact, experimental results have shown the contrary. Hameau et al. [1981] have executed an experimental program on a two-span beam model. The model was tested using a sinusoidal loading with an amplitude of 4000 N. The measured strain along the depth of the asphalt and the steel is shown in Figure 2.14.

Figure 2.14: Strain distribution in the asphalt and the steel [Hameau et al., 1981] Figure 2.14 shows clearly that the strain distribution along the depth of the asphalt is not linear. This non-linearity may be attributed to the non-linear response of the asphalt or the membrane and/or the geometry of the structure. However, this work indicates clearly that the assumptions upon which most composite theories are based might not be true.

Furthermore, measurements done by the Department of Bridges and Steel Structures of the Ministry of Transport, Public Works and Water Management [1985] on a test

wearing course

steel wearing course

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section at the Kreekrak orthotropic bridge, have shown that a strain level of around 1200 µm/m can occur at the top of asphaltic wearing course. This strain level is quite high when compared to what is normally measured in ordinary pavements (50 to 200 µm/m). With such high strain levels, it is doubtful that wearing course materials will still behave linearly!

2.7.2 Adhesion between the steel deck and the asphalt surfacing

In most of the design requirements, a vague statement like “sufficient bond between the steel deck and the surfacing should be provided” is often stated. However, what is a sufficient bond? In the literature, no absolute minimum bond strength (or shear stiffness) was set for the bonding layer. While the Dutch working group H2 [CROW, 1981] proposed that a bonding strength of 0.3 N/mm2 is adequate for sufficient bonding, Fondriest [1969] claimed a value of 1.38 N/mm2!

According to Harre [1972], the problem of loss of adhesion is associated with the stress and deformation criteria. The adhesion layer must be able to withstand shear stresses in the order of 0.8-1.0 N/mm2 at low temperatures. On the other hand, at high temperatures the adhesion layer should be able to withstand large shear stresses arising from the large deflection of the steel deck due to traffic loading and deformation due to an increase in temperature.

Work done by Medani et al. [2002] and Huurman et al. [2004], using the Finite Element Code CAPA-3D [Scarpas, 1992], has shown that the shear stiffness of the membrane material significantly influences the response of the structure.

2.8 DISTRESS IN SURFACINGS OF ORTHOTROPIC STEEL

BRIDGES

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Orthotropic steel bridges 23 Figure 2.15: Pictures show different types of damages of surfacing of two Chinese

orthotropic steel deck bridges

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0 10000 20000 30000 40000 50000 0 50 100 150 200 250 300 350 400

Number of days per year exceeds a given level

Cum ul ativ e dai ly traffi c 1995 2003 congestion level

Figure 2.16: Cumulative distribution of daily traffic (Northbound) of the Forth Road Bridge in Edinburgh, Scotland

First, it seems that such changes were not anticipated by the designers of “old” orthotropic steel bridges. Secondly, the assumption of linear elastic response of the bridge materials proves to be too simple to describe the response of the structure under increasing loads. Lastly, the oversimplification of the complex geometry of the 3D structure into a simple 2D single or 2-span beam model. Therefore, it is not surprising that inspection of the Moerdijk Bridge in the Netherlands [RHED, 1991] has revealed that 80% of the cracks were observed in the slow lanes (used mainly by trucks).

The distress types in orthotropic steel bridge surfacings include permanent deformation, cracking, loss of bond between the wearing course and the steel plate, formation of blisters and disintegration.

2.8.1 Permanent deformation

In general, permanent deformations are caused by high and/or repeated compressive or shear stresses. Because mastic asphalt has a large amount of bitumen, it is susceptible for rutting. However, with the use of modified bitumen, this problem was significantly reduced. Kohler and Deters [1974] indicate that “knobs” occur above and between the longitudinal stiffeners. Henneke [1964] explains that the knobs between the longitudinal stiffeners are caused by movement of the pavement, which is caused by traffic moving from the stiffer part of the plate (above the stiffeners) to the weaker parts (between the stiffeners).

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2.8.2 Cracking

Cracking is probably the most occurring type of damage in asphaltic surfacings of orthotropic steel bridges. Fatigue cracking is caused by repeated stresses (shear or tensile) induced by traffic, environment and poor construction. When the cracks reach the steel deck, it rusts and may lead to debonding. De Backer [1978] made a distinction between cracking caused by strain exceeding the failure strain in the asphalt after one load cycle and fatigue cracking caused by repeated loading. According to De Backer [1978], fatigue cracking dominates.

Visible cracks normally take place at the point where the stiffener is welded to the plate or between the stiffeners [Kohler and Deters, 1974]. Cracks at the point where the stiffener is welded to the plate, start at the top of the wearing course and propagate downwards to the underlying layers (point A in Figure 2.17). When the bond between the asphalt and steel is weak, tension forces may occur at the bottom of the wearing course, thus leading to cracks starting at the bottom of the wearing course and propagating to the top (point B in Figure 2.17). Nishizawa et al. [2004], who inspected several bridges in Japan, have reported that longitudinal cracks occurred in the middle of the stiffener (point C in Figure 2.17).

A B C A B C

Figure 2.17: Transversal stresses of an orthotropic steel bridge under a dual wheel axle [Huurman et al., 2004]

2.8.3 Loss of bond between the surfacing and the steel plate

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• The rather high application temperature of the mix (≈ 220o _{C) may result in} high strains at the top of the steel plate. The severity of the problem is dramatically increased when the deck plate is restrained from free expansion. • According to Fondriest [1969], the high shear stresses between the pavement

and the deck produced by accelerating or braking wheel loads weaken and hence destroy the bond.

• It has also been speculated, particularly by the German road authorities, that vibrations induced in the deck by fast moving traffic also weaken the bond. • The shear forces (both in the longitudinal and transverse directions) increase

with the increase of the slope of the bridge deck. These shear forces may result in cracks at the elevated part of the structure.

• Blight et al [1973] have reported that delamination occurred in Bridge 6 in South Africa. They also noticed that this separation occurred both under the roadway and under the untrafficked shoulder. They concluded that, traffic is not the reason behind the separation, or at least not the sole reason; and that the separation was caused by the differential thermal movement between the pavement and the steel deck.

From their work, they have concluded that asphalt proved to be anisotropic in its expansion behaviour, the coefficient of expansion in one principal direction being 2.5 times that in the other principal direction. Similar findings have been reported by Sedlacek and Bild [1985].

• For pavements on a subgrade it is known that dynamic axle loads can be up to 40% higher than the static ones [Divine, 1997]. For surfacings on orthotropic steel deck bridges, it is expected that vibrations may play even a more important role. That is because on these bridges not only the vehicles are vibrating, but also the bridge itself. The dynamic response of the bridge depends on bridge geometry, damping, natural frequencies, vehicle mass, frequencies of the vehicles dynamic wheel loads and the number of vehicles driving simultaneously on the bridge.

2.8.4 Blisters

Blisters occur when a waterproof layer is laid on a wet layer. When asphalt is laid on such a layer, water evaporates forming bubbles, which will be seen at the surface as “isolated lumps”.

2.8.5 Disintegration

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Orthotropic steel bridges 27 distress mechanisms. Ravelling can seriously reduce the skid resistance of the wearing course and hence endangers the safety of the road users.

2.9 DISTRESS MECHANISMS

Sometimes distress is characterised by means of the mechanisms causing the distress. Examples of distress mechanisms are:

• fatigue cracking of surfacing materials due to repeated stresses (shear or tensile) induced by traffic and environment;

• cracking due to exceptionally high stresses, induced by heavy traffic or freezing, exceeding the material’s tensile or shear strength;

• crack propagation because of high tensile stress intensities at the tip of a previously initiated crack;

• permanent deformation of surfacing materials due to exceptionally high and/or repeated stresses (compressive or shear);

• bleeding because of deformation of bituminous materials with a too high bitumen content and a too low voids content (typical characteristics for mastic asphalt);

• polishing of surfacing aggregates by tyres.

It is known that different types of distress are caused by different mechanisms, but there can be considerable interaction between the different distress types. As an example, the unevenness of the surface can cause high dynamic loads, which will in turn result in faster cracking of the pavement [Groenendijk, 1999]. However, it is sometimes difficult to determine the real cause or causes of an occurring damage.

2.10 SUMMARY

In this chapter, several topics related to surfacings on orthotropic steel decks were discussed including surfacing materials, stress reduction in steel due to the surfacing and some design issues related to orthotropic steel bridge decks. Furthermore, issues related to distress phenomena in surfacings of orthotropic steel bridges and their mechanisms were reviewed. The following are the most important conclusions.

• The light weight of orthotropic steel bridge decks make them an attractive option in cases where a high degree of pre-fabrication or rapid erection is required, in seismic zones, for movable bridges, for long-span bridges and for rehabilitation to reduce bridge mass. Furthermore, they can be built in cold climates at any time of the year.

Design principles of surfacings on orthotropic steel bridge decks