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AbstractIn this study, an experimental test on a full-scale model of a two-span continuous composite two-girder bridge with prefabricated slabs wascautiously conducted and observed in order to study crack control. In the testing, a first loading was applied up to 360 kN and then fatigue loadingtest proceeded. Lastly, static loading was applied up to 900 kN. From the test results, it is confirmed that the two-girder continuous compositebridge with loop joints prefabricated slabs shows composite section behaviour for both strength and stiffness under static and fatigue loads.Although, in negative moment regions, cracks concentrated at the cast interface of the joints between decks above the internal support and theinitial crack spacing is constituted by the distance between joints, crack widths can be controlled appropriately within an allowable crack widthin the decks and transverse joints of the composite bridge with prefabricated slabs on an interior support under service loads. Moment curvaturerelationship or the flexural stiffness by Eurocode 4-2 is still useful for the estimation of the effective stiffness considering tension stiffening effectsin the composite bridge with loop joints prefabricated slabs.c 2007 Published by Elsevier LtdKeywords: Crack control; Continuous composite two-girder bridge; Prefabricated slabs; Loop joint; Transverse joint; Static and fatigue loads; 61737
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Composite sectionbehaviour; Negative moment region; Crack width; Flexural stiffness of composite section
1. IntroductionSteel and concrete composite bridges are very attractivesolutions for short and medium span bridges. However, for steeland concrete composite continuous bridges, when a concreteslab is in tension and a lower flange of a steel girder is incompression under hogging moments, there are shortcomingsin view of durability and strength. Especially, concrete crackingaffects the durability and service life of bridges. Therefore,crack control is an important issue in steel and compositecontinuous bridges. There are two approaches for dealing withconcrete cracking in composite bridges: one is to preventcracking using prestressing methods and the other is to allowthe formation of cracks but limit their widths to acceptable values. Prestressing methods, however, are inconvenient anddoubtful due to prestress losses by the long-term behaviourof concrete. Therefore, it is considered that the control ofcrack width without prestressing is the more economical andinteresting solution.Some previous researchers have studied the cracking ofthe decks in composite bridges such as the influence ofreinforcement ratios, diameter and spacing on crack spacingand crack widths. Also, it is necessary to notify local weakeningof the tensile capacity of a concrete slab caused by shearconnectors or transverse reinforcement since these factors mayinfluence the crack spacing and widths [5,7,10,12].A precast concrete deck could be very attractive becausethe system can ensure the quality of concrete decks, improveworking environments for the workers, and reduce manhours outdoors and traffic disruption.
A shorter constructiontime could be an important factor in choosing precast deckbridges. A precast deck bridge has two types of connection:shear connection between steel girder and precast deck, andtransverse joint between precast panels. However, in order to apply precast decks to continuouscomposite bridges, the tensile behaviour of precast decks ortransverse joints between slabs in hogging moment regionsshould be confirmed in view of serviceability and durability.Particularly, stiffness of the composite section during crackingshould be evaluated precisely, because it is very important toestimate crack widths, deflection and stress ranges applied tostructural members under service loads.Recently, an experimental test on a full-scale model of asteel and concrete composite plate girder with prefabricatedslabs under hogging moments was cautiously conducted andobserved in order to study crack control [7]. From the study,it was concluded that initial crack spacing of the slab in thecomposite girder with prefabricated slabs can be wider thanthose of general RC beam structures. Also, it is consideredthat crack widths of the composite girder with prefabricatedslabs were more enlarged in weak surfaces of constructionjoints. Moreover, the crack spacing is decisively influenced bytransverse reinforcement spacing. Therefore, it is necessary toconsider the existence of construction joints and the influenceof transverse reinforcement spacing on the crack spacing in thecalculation of crack width.In this study, an experimental test on a full-scale model ofa continuous composite two-girder bridge with prefabricatedslabs was cautiously conducted and observed in order to studycrack control. The bridge is a 2-span continuous compositegirder with spans of 15 m for a total span of 30 m. The designof the deck has been conducted considering the real dimensionsapplied for highway bridges. Girder spacing of 2.5 m has beendesigned taking into regard the deck span of ordinary multi-girder composite bridges.Static loading test has been conducted to observe the elasticand cracking behaviour. After a survey of the crack widthdeveloped at the bottom of the decks located in the loadedsections and at the top of the decks near the internal supports,fatigue load was applied one million times, followed by staticloading test up to 900 kN, the maximum capacity of theactuator. Deflection, relative slip, crack widths and momentcurvature curve of the composite section under static andfatigue loadings were observed. Crack distributions and crackspacing were viewed. The composite section behaviour of theprecast deck with loop joints was confirmed. Test results wereanalyzed by finite element analysis and design methods. Theflexural stiffness of the composite section is compared with thatof the proposals in EUROCODE 4-2 [4] and discussed.2. Experimental works2.1. Bridge prototypeThe thickness of the deck has been determined so asto satisfy the minimum thickness specifications [9] andon the basis of previous research results [7,8] on jointdetails. Transverse pre-tension was introduced in the transversedirection of the deck. Pre-tension level was determined so thatthe deck resists the bending moment produced by the deadload during the transport and the transverse bending moment developed in the deck due to design live load. Twenty fourtendons of diameter 12.7 mm were arranged transversely ineach prefabricated slab. Transverse pre-tension appears to berequired especially in case of transverse long precast decks.Diameter of transverse reinforcement was 13 mm. The ratio oflongitudinal reinforcement was 1.28% of which diameter was16 mm.Vertical stiffeners have been disposed at regular intervalsin the girder in order to prevent shear buckling of the webplate, and cross beams have been installed at the supports.Except near the interior support, spacing of vertical stiffenersis three times the girder height because the tension-field actionis assured. Horizontal stiffeners are not installed anywhere. Thethickness of vertical stiffeners was 14 mm. The dimensions ofthe precast deck are illustrated in Fig. 1(c), and a total of 15decks with width of 2.04 m were installed transversely on thegirders. Three shear connectors per shear pocket were disposedand stud shear connectors were applied. Design was conductedso as to produce full shear connection in view of strength.The characteristics of the composite section are depicted inFig. 1(a). The bridge is a 2-span continuous composite girderwith spans of 15 m for a total span of 30 m (Fig. 1(b)). The fabrication of the composite girder prototype proceedsas follows. Firstly, the precast decks manufactured in thefactory are transported to the prototype bridge plant.