Page 372 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 357

HYDROGEN-ASSISTED STRESS-CORROSION OF
PRESTRESSING WIRES IN A MOTORWAY VIADUCT

L. VEHOVAR*

Institute of Metals and Technology, Lepi pot 1 I, IO00 Ljubljana, Slovenia
V. KUHAR
National Building and Civil Enginering Institute, Dimikva 12, IO00 Ljubljana, Slovenia

and
A. VEHOVAR

University of Ljubljana, FNT-Department of Metallurgy and Material Science, 1000 Ljubljana, Slovenia
(Received 26 August 1997)
Abstract-This paper deals with the stress corrosion cracking of colddrawn stress relieved prestressing wire
in a prestressed reinforced-concrete structure due to the influence of atmospheric water and especially the
chlorides which were present during the winter salting of the road pavements of a motonvay viaduct in
Slovenia. Twenty-two years after the bridge was first opened for traffic, numerous wires in the prestressing
cables had become brittle, and had broken and many others were damaged, to a greater or lesser degree, by
corrosion and the effect of stress corrosion supported by the additional operation of hydrogen at the crack
tips. Investigations showed that as the degree of corrosion increased the mechanical properties of the steel,
particularly its toughness, were reduced drastically. As a result, such material is unable to prevent the initiation
or spreading of cracks in the case of the static and dynamic loadings occurring on the bridge. Systematic SEM
investigations of the morphology of the fracture surfaces of the wires confirmed the authors’ assumption that
stress corrosion and hydrogen embrittlement had made possible the occurrence of brittle areas with cleavage
fracture surfaces, the proportion of which increased with an increase in the degree of corrosion. 0 1998
Elsevier Science Ltd. All rights reserved.
Keywords: Bridge failures, brittle fracture, hydrogen-assisted cracking, stress-corrosion cracking.

1. INTRODUCTION
During the visual inspection of the main load-bearing longitudinal and transverse beams of a viaduct
measuring 500 m in length (Fig. l), a large number of cracks were observed in the surface layer of
the concrete. The cracks, up to 2 m long, extended the length of the external stirrup reinforcement,
and brown-coloured corrosion products of steel were observed leaking out of these cracks. Through
an analysis of the concrete and of the corrosion products, it was determined that aggressive
corrosion-causing chlorides were present, which had reached the concrete with the atmospheric
water filtering down from the inadequately protected pavement slabs. However, since the chloride
ions are fairly mobile and capable of diffusing deeper into the internal part of the beams up to the
prestressing wire of the main load-bearing cables (Fig. 2), these locations, too, were included in the
diagnostic investigations of the damaged beams. It was found that at certain locations the cables
had undergone heavy corrosion damage (Fig. 3). Samples of the prestressing wires were taken from
the prestressing cables at locations where the degree of the corrosion threat varied from lesser to
greater. Samples of concrete and the grouting mixture were also taken.

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*Author to whom correspondence should be addressed.

Reprinted from Engineering Failure Analysis 5 (l), 2 1-27 (1 998)