Cantilever Built Bridges with Prefabricated Segments
Bridges with cast-in-situ segments came up against certain limitations, particularly as far as large structures are concerned, or at least have certain drawbacks.
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R. Lacroix Freyssinet, Boulogne, Francia
G. Creazza et al. (eds.), Advanced Problems in Bridge Construction © Springer-Verlag Wien 1991
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R. Lacroix
1. llmrruTICJJ Bridges with cast-in-situ segm:nts care up against certain limitations, particularly as far as large structures are concerned , or at least have certain drawbacks. The first consists of a relatively lengthy construction schedule' caused by the casting cycle of segm:nts : the no:nml cycle lasts one week, in order to allow for the hardening of concrete on days off ; an average seg-a:mt leQgth of 2 •.::JJ m, WliliiEEIIl a construction of 5 m a week on each side of the pier, and for a very long bridge, the contractor nay rnve to increase the mnnber of carriage form travellers to nmrit:ain an . adequate cprogram.
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In certain cases, a cycle of two segm:nts a week could be attained, by IIE8IlS of either a high strength concrete, or a the:nml treat:Irent to increase the concrete's early strength. However, such achievaiE!lts are not easy, and the difficulty in controlling the arms' deflections is increased, due to the concrete being so recently cast at the tine of the prestress tensioning.
Fran a quality point of view, the cantilever built bridge with cast-in-situ segm:nts runs another risk due to the fact that the strength of the concrete is only koo.-m after 28 aays ; if the results of the control samples prove to be insufficient, one must then choose between either the ClaiDlition of all the concrete already cast, or the acceptance of a derogation , provided that a few extra precautions are taken. Fiually, statistically, a higher frequency of disorders has been noticed on bridges with cast-in-situ segm:nts because of early stressing of the concrete and the obviously difficult casting conditions. The design of prefabricated segm:nt bridges IIEkes these problans avoidable, and is therefore an actual 1mprovaiE!lt despite a few inconveniences which will be described below.
2. 'IHE DESIGN PRil'CIPlE RR PREFAPRICATED SEJJ1ENIS Prestressing is An excellent llB'lllS of assenbly, thanks to the canpression exerted on the faces in contact with the prefabricated pieces ; but to achieve this tightening without any problans, the nutual contact must be perfect. ·
The solution to dris problan was thought up in the late fifties by Campenon-Bemard, for the construction of the Oloisy-le-Roi Bridge on the Seine River tnear Paris).
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Cantilever built bridges
This solution consh;ts of casting the nth seerent with its cootact face with (n-1) segm:mt cast directly m contact with the latter·, A perfect similarity of the cootiguous faces is therefore achieved! ; these faces are said to be rratch-cast, and provided that incidental defamations do not appear between the casting of concrete and the assenbling of Beg!!El1ts, an excellent mutual contact at the joints is obtained. Generally, :ir.r.ediately prior to assaTJbling two contiguous segr'E11ts, the contact faces are crnted with a rrrulti-purpuse epoxy resin which ; - firstly guarantees the joint's watertightness,
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