Jännepunosvaurion vaikutus betonisen sillan rakenteelliseen turvallisuuteen: Esiselvitys

In this preliminary study common causes for tendon corrosion and wire breakages are
examined by literature research. The research focuses on risks of typical Finnish post
tensioned and post-grouted bridges due to wire breakage. The preliminary study
prefaces the research project in which the structural safety of post-tensioned bridge in
case of tendon wire breakages is assessed.



In Finland, the number of prestressed bridges is low in copmparison to other types of
bridges. Still, if comparison is made by comparing total lengths or deck areas, the post
tensioned bridges form a majority of bridge stock. The prestressed bridges can be
prefabricated with prestressing or cast in place and post-tensioned. This study focuses
on latter. In Finland a typical post-tensioned bridge is cast-in-place with post-grouted
interior tendons. A typical number of spans of this kind of bridge is two and span length
usually varies from 20 to 30 meters.



The deterioration mechanism for post tensioning tendons is mainly due to defects in
post grouting. The trapped air and water in grout in tendon duct causes a break on
corrosion protection of tendons and tendon corrosion in duct is therefore possible. In
anchoring zone at the end of the bridge, a leaking joints and poor detailing can allow
water and chlorides from de-icing salts to flow in tendon ducts and anchors. In
permanently moist conditions, corrosion initiates and progresses.



In favourable conditions the corrosion progresses, which leads the reduction of tendon
area and increase of tendon stress. Corrosion in tendons causes also crack initiation
on tendon wires. Some steel grades are prone to hydrogen embrittlement due to
corrosion, which means the ultimate behaviour of strand is brittle. As wire or strand
breaks, the forces redistribute to adjacent tendons or strands and, depending on
structure, to another load bearing structures.



The bond between the strand and postgrouting plays significant role in structural
robustness of bridge. In case of bondless tendon, the break anywhere along its length
causes the lost of the function of entire unit. With bond, the broken tendon can re-
anchor on grout and still function in other cross-section away from breaking point.
From point of structural robustness, it is beneficial that concrete structure cracks as
tendon area reduces. Still, the structure should have sufficient capacity left and ductile
behaviour after concrete cracking.



In this study, calculations are made by method with which the ductitility of cross-
section in case of tendon break was assessed. As a result of calculations was observed
that the bridges in this case have sufficient robustness and ductile behaviour in case of
tendon loss. The calculation method seems to be reliable, still calibration is needed for
adequate target reliability level for bridges.