New_steel-concrete_connection_for_prefabricated_composite_bridges1

Fachthemen

Dimitrios PapastergiouJean-Paul Lebet

DOI: 10.1002/stab.201101493

New steel-concrete connection for prefabricatedcomposite bridges

Herrn Univ.-Prof. Dr.-Ing. Gerhard Hanswillezur Vollendung seines 60. Lebensjahres gewidmet

An investigation of a new type of connection between precastconcrete decks and steel girders, for composite steel-concretebridges, is being performed by the Steel Structures Laboratory,ICOM, of Ecole Polytechinque Fédéral de Lausanne, EPFL. Theconnection resists the loads by shear resistance between basematerials. Hence, a fundamental part of the research focuses onthe behaviour of the different confined interfaces of the connection,subjected to shear, static and fatigue loading. The confinement isa combined outcome of the kinematic law of the interfaces andthe section geometry. Experimental investigation through a seriesof direct shear tests on confined interfaces has resulted in thedevelopment of failure criteria and constitutive and kinematicmodels describing the behaviour of the different interfaces. Those

laws are used as an input for a model that simulates the connectionbehaviour. The model of the connection is validated by push-outtests on large-scale specimens. Finally, a composite beam hasbeen tested under constant amplitude for five million cycles. Astatic test, following the fatigue sequence, has shown that thecomposite beam reaches its plastic moment capacity due to thesufficient ductility of the connection.

Neue Stahl-Beton-Verbindung für vorgefertigte Stahlverbund -brücken.Im Labor für Stahlkonstruktionen (ICOM) der Eidgenössi-schen Polytechnischen Hochschule Lausanne werden Untersu-chungen für eine neue Verbindungsart zwischen Betonfertigteil-Fahrbahn und Stahlträgern bei Stahlverbundbrücken durchgeführt.Die Verbindung nimmt die Beanspruchungen über den Schubwider-stand zwischen den Grundmaterialien auf. Daher konzentriert sichein wesentlicher Teil der Forschung auf das Verhalten der verschie-denen, eingeschlossenen Verbundfugen, die Schubkräften und sta-tischen und ermüdungswirksamen Beanspruchungen ausgesetztsind. Die Zwängungsbeanspruchung ist ein kombiniertes Ergebnisder Kinematik der Verbundfugen und der Profilgeometrie. Experi-mentelle Forschung anhand einer Reihe von direkten Abscherver-suchen an eingeschlossenen Verbundfugen führte zur Entwicklungvon Versagenskriterien, Materialgesetzen und kinematischen Mo-dellen zur Beschreibung des Verhaltens der verschiedenen Ver-bundfugen. Diese Gesetze werden als Eingangsgrößen für ein Mo-dell verwendet, welches das Verhalten der unterschiedlichen Ver-bundfugen simuliert. Das Modell der Verbindung wird durch Push-Out-Versuche an großen Prüfkörpern validiert. Abschließend ist einVerbundträger unter konstanter Amplitude mit 5 Millionen Last-wechseln geprüft worden. Eine statische Prüfung der Resttragfä-higkeit hat gezeigt, dass der Verbundträger die vollplastische Mo-mententragfähigkeit aufgrund einer ausreichenden Duktilität derVerbindung erreicht.

1Introduction

When building new bridges or replacing existing ones, theduration of on-site work has a significant influence on thecost as well as on the potentially harmful effects (noise,traffic jams, diversions) of the construction work. More-over, rehabilitated structures nowadays have to complywith restrictions for extended life. Thus, it is of great inter-est to design bridges in such a way as to minimize the con-struction time and provide increased durability. Steel-con-crete composite bridges with precast decks are ideal forthis purpose [7]. However, to achievecomposite action, thecommonly used connections such as groups of headedstuds welded to the upper steel flange and connected tothe slab, when concreting the pockets in the slab on site,present several disadvantages. The numerous small quan-tities that need to be poured on site to fill the pockets,Figs.1a and 1b, slow down construction progress. Further-

b)

Fig.1.a) Deck before concreting the pockets, b) pocket detail (photos: ICOM archives)

Bild1.a) Fahrbahnplatte vor dem Betonieren der Aussparungen, b) Aussparungs-Detail (Fotos: ICOM-Archiv)

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D. Papastergiou/J.-P. Lebet · New steel-concrete connection for prefabricated composite bridges

more, cracks may develop in the corners of the pockets,increasing the risk of degradation by corrosion for boththe slab reinforcement and the connection itself, and thusdiminishing the structure’s service life. Consequently, newtypes of connection have to be developed for compositesteel-concrete structures.

Connections, by adherence and friction [8], resisting byshear between various interfaces, constitute a promisingsolution. Fig.2a illustrates a connection by adherence andfriction. An embossed steel plate is first welded longitudi-nally to the upper flange of the steel beam. The concretedeck elements are precast with a longitudinal rib in theunderside. During fabrication, a retarder agent is appliedto the concrete surface of this U-shaped rib which is thenroughened by the use of water-jetting and sandblasting(Fig.2b). On site, the precast deck elements are laid on thesteel girders. The transverse joints of the slabs are glued to-gether with an epoxy resin and the deck is then prestressedlongitudinally. No passive reinforcement crosses the joint.The gap between the embossed steel plate and the concretedeck is finally injected with a high-strength cement groutfrom one end of the bridge, in a way similar to that of apost-tensioning duct [2].

Fig.3a shows a section through the connection. It canbe seen that two different types of interface are formed:the ribbed steel-cement grout interface and the roughenedconcrete-cement grout interface. They are subjected to longi-tudinalshear and to compressive normal stresses, the lat-ter depending on the loads acting on the bridge, after theexecution of the injection, and on the transverse flexuralstiffness of the deck. These interfaces resist shear due totheir macro-roughness.

When longitudinal shear loading acts on the connec-tion, slip s will develop between the upper flange of thesteel beam and the concrete slab, and consequently upliftu will occur perpendicular to the interfaces. Fig.3b illus-trates the uplift u1that occurs between the embossed steelplate and the cement grout. Uplift u1is partially preventedboth by the concrete slab around the embossed plate andby the other interface between the roughened concrete andthe cement grout. Uplift causes a normal force N in thelower reinforcement of the concrete slab just over the em-bossed plate. By equilibrium, normal compressive stressesdevelop at the embossed steel plate-cement grout inter-face.

In order to understand how the connection works andwith the final aim being to produce dimensioning tools forthe application of the above connection system in prac-tice, significant research is being performed at the SteelStructures Laboratory of the Ecole Polytechnique Fédéral,Lausanne. The current work is described and analysed be-low.

a)b)

Fig.2.Connection by adherence and friction: a) global view, b) detail of U-shaped rib in slab

Bild2.Verbund durch Haftfestigkeit und Reibung: a) Überblick, b) Detail der U-förmigen Nut in der Platte

a)b)

Fig.3.a) Principle of the shear connection and loads acting on the slab, b) deformation and associated stresses

Bild3.a) Prinzip des Scherverbundes und auf die Platte wirkende Lasten, b) Verformung und damit verbundene Belastungen

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2Experimental study

The experimental study is divided into three main parts.The initial part of the research focuses on the behaviour ofthe interfaces that constitute the connection. For this rea-son, direct shear tests have been carried out for each typeof interface under monotonic and cyclic loading. The sec-ond part includes push-outs tests (with both static and cyclicloading) on large specimens fabricated with the new steel-concrete connection. Finally, a composite beam has beensubmitted to a five million cycle (pulsar) loading test fol-lowed by a static test up to failure.

2.1Experimental study of confined interfaces under

shear loading

The interface behaviour (i.e. the relationships between slip s,uplift u, shear stress τand normal stress σat an interface,Fig.4) were investigated through a number of direct sheartests conducted with the experimental setup shown in Fig.5.

Fig.5 illustrates the arrangement for the direct sheartests. The test setup is the same as used by Thomann[9].Specimens are made of two plates with a cement groutblock between them. During the test, the interfaces aresubjected to a constant normal stress σ. The shear force isapplied using displacement control, whereas slip is in-creased until failure and then until residual shear resis-tance is achieved. Load cells situated underneath each in-terface serve for the acquisition of the force that passesthrough them and consequently the mean shear stress τ.In addition to slip and shear force, the transverse separa-tion (uplift u) of each plate from the cement grout block ismeasured.

The parameters for these series of tests are normalstress σand type of interface. Three different types of in-terface are being investigated. Material1 is always a high-strength cement grout and material2 is each time one ofthe following (Fig.6): a) ribbed steel of type BRI8/10(height of ribs =1.4mm), b) roughened concrete, and c)ultra high-performance fibre-reinforced concrete (UHPFRC)with 8mm high conical studs.

2.1.1Direct shear tests on confined interfaces under

shear monotonic loading

Ten tests were carried out for the ribbed steel-cement groutinterface and for a confinement σvarying from 0.5 to 5MPawith the compression resistance of the cement grout be-tween 90 and 99.4MPa. Thirteen tests were carried outfor the roughened concrete-cement grout interface and for

Fig.4.Interface loaded in shear and compression

Bild4.

Verbundfuge unter Scher- und Druckbeanspruchung

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Fig.5.Test setup for small-scale specimens in biaxial loadingBild5. Versuchsaufbau für kleine Versuchskörper mit zwei-achsialer Beanspruchung

a confinement σvarying from 0.5MPa to 5MPa with thecompression resistance of the cement grout between 90and 102MPa. Similar tests were executed for the UHPFRC-cement grout interface. Figs.7, 8 and 9 illustrate some ofthe most important results of the direct shear tests. Fig.7illustrates, for two types of interfaces, the ultimate shearresistance as a function of the normal stress σat the inter-face.

Fig.8 illustrates the relationship between slip s andshear stress τfor different values of normal stress σfor twotypes of interface, as obtained from experiments, whereasFig.9 illustrates the relationship between slip s and upliftu for different values of normal stress σfor the same twotypes of interface.

2.1.2Confined interfaces under shear fatigue loading

The experimental setup (Fig.5) used for monotonic shearloading was also used for cyclic loading of the confined in-terfaces. A sinusoidal shear load is applied with a frequencyof 2Hz while confinement stress σ

is kept constant at a

D. Papastergiou/J.-P. Lebet · New steel-concrete connection for prefabricated composite bridges

value of 1MPa. This value was chosen bearing in mindthat in reality during fatigue loading stresses at the con-nection remain in the elastic domain and uplift, which isresponsible for the development of the confinement, isconsequently limited. Large values for the confinementstress σof 4–5MPa are expected to develop at ultimatestrength, as we can see in section3.2. The two assump-tions mentioned above have been verified by detailed fi-

a)b)c)

Fig.6.Surfaces of plates for specimens: a) ribbed steel, b) roughened concrete, c) UHPFRC with conical studs

Bild6. Plattenoberflächen für die Versuchskörper: a) gerippter Stahl, b) aufgerauter Beton, c) UHPFRC/UHFB (ultra-hoch-leistungsfähiger Faserbeton) mit konischen Noppen

a)b)

Fig.7.τmaxversus normal stress σ– failure criteria: a) ribbed steel-cement grout interface, b) roughened concrete-cementgrout interface

Bild7. τmaxin Abhängigkeit der Normalspannung σ— Versagenskriterien: a) Verbundfuge zwischen geripptem Stahl und

Zementmörtel, b) Verbundfuge zwischen aufgerautem Beton und Zementmörtel

a)b)

Fig.8.Shear stress-slip curves from tests and constitutive law: a) ribbed steel-cement grout interface, b) roughened concrete-cement grout interface

Bild8. Schubspannungs-Verformungs-Kurven aus den Versuchen und dem Materialgesetz: a) Verbundfuge zwischen geripptem

Stahl und Zementmörtel, b) Verbundfuge zwischen aufgerautem Beton und Zementmörtel

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a)b)

Fig.9.Uplift-slip curves from tests and kinematic law: a) ribbed steel-cement grout interface, b) roughened concrete-cementgrout interface

Bild9. Abhebe-Verformungs-Kurven aus den Versuchen und dem kinematischen Modell: a) Verbundfuge zwischen geripp-tem Stahl und Zementmörtel, b) Verbundfuge zwischen aufgerautem Beton und Zementmörtel

nite element analysis of the cross-section of the push-outspecimens using the Abaqus software. Table1 describesthe characteristics of the fatigue tests executed for a ribbedsteel-cement grout interface.

For specimen TS-C_10 a five million cycle loading wasexecuted with a loading range ΔV1equal to 30% of the es-timated failure load. Since no failure occurred, the test wasstopped and a static shear test to failure was conducted.The interface’s shear resistance was not affected by fatigueloading. It is worth mentioning that the ratio of maximumfatigue load versus estimated failure load V1max/Vu,est=0.60, smaller than the ratio τel/τmax(τel/τmax=0.75) of theinterface, implying that shear stresses were kept in thequasi elastic domain. That was not the case for specimenTS-C_13, where the loading range ΔV1/Vu,est=0.56, butmost importantly the ratio V1max/Vu,estrose to 0.86 and, asa consequence, failure was reached at 4208 cycles. Forspecimen TS-C_17, two loading histories were applied.Firstly, similarly to the first specimen, a five million cycleloading with the same characteristics was applied. No fail-ure occurred and a second fatigue loading sequence withtwo million cycles was applied with ΔV2/Vu,est=0.45 andV2max/Vu,est=0.73, almost on the limit of the quasi elasticdomain. Again, since no failure occurred, a static loadingto failure was executed. For specimen TS-C_18, the firstloading sequence was similar to that of specimens TS-C_10and TS-C_17. In the second sequence the loading rangereaches 59% of the estimated failure load and, as ex-pected, since the maximum load is inferior to the elasticlimit, no failure was observed for two million cycles. A fi-nal sequence was executed with the maximum load closeto the ultimate resistance and the specimen failed after1184 cycles.

Similar fatigue tests were also carried out for a rough-ened concrete-cement grout interface. Table2 describesthe characteristics of the fatigue tests executed for this in-terface.

As with the previous series of tests, it was observedthat as long as the maximum applied load is inferior to theelastic limit, the interface exhibits sufficient fatigue resis-tance. During the loading sequence of the fatigue tests, theshear stress-slip relationship was measured and the resid-ual slip was defined for both interfaces. The data was ac-quired by executing intermediate displacement-controlledstatic tests, up to Vmaxand complete unloading, once acertain number of cycles had been reached. It was foundthat the ultimate shear resistance and slip at failure arenot affected by repeated loading as long as stresses remainin the quasi elastic domain. Residual slip increases withrepeated loading; however, the increase is very limited af-ter one million cycles. Since residual slip is accumulatedand slip at failure is similar to that of static tests, whenperforming a static test after a fatigue loading history, theshear stress increases with high stiffness to reach the shearstress level of the monotonic loading (Fig.10). Those re-sults are similar to those found by Byung Hwan

[1] con-

Table1.Tests on fatigue shear loading for a ribbed steel-cement grout interface

Tabelle1. Versuche zur ermüdungswirksamen Scherbeanspruchung für eine Verbundfuge zwischen geripptem Stahl und Ze-mentmörtel

Test nameTS-C_10TS-C_13TS-C_17TS-C_18Units

V1min95959595kN

V1max190280190190kN

V2min––951515kN

V2max––235205285kN

–ΔV1/Vu,est

0.300.590.300.30

ΔV2/Vu,est

––0.450.590.84–

–V1max/Vu,est

0.600.860.600.60

V2max/Vu,est

––0.730.640.89–

–Cycles15E+6*42085E+6*5E+6*

Cycles2

––2E+6*2E+6*1184–

N/mm2

fc9093108102

*No failure occurred and the test was stopped to continue another loading sequence indicated by superscript 2

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