Group 17
Wexford Bridge
Wexford Bridge – Chloride Ingression
Group 17
Vincent Carey
Amy Fitzpatrick
Patrick Gibbs
Mick McCarthy
Fig. 1
Introduction
Wexford Bridge (fig. 1) spans across the river Slaney that flows diagonally
northwest to southwest dividing Wexford in two. The first bridge was wooden. It was
constructed in 1794 at a cost of £15, 000 (approximately one million euro). In 1959 a
seven span pre -stressed concrete balanced cantilever was constructed in its place. It
spans 400 meters with piers spaced 63 meters apart. The deck needed to be replaced
in 1997 due to effects of the marine environment. This will be discussed below.
Discussion
The original bridge was a seven span prestressed concrete balanced cantilever
bridge. On inspection of the reinforced concrete, it was found that there was an
excessive chloride ion content, which had resulted in serious corrosion of the
reinforcement and prestressing anchors together with lamination and the spalling
throughout the bridge. The corrosion had advanced to them point that the structural
stability of the bridge was no longer adequate. With no possibility of salvaging the
existing structure, which was irreparable, the only solution was to demolish it and
rebuild the bridge. Before we analyse the replacement bridge we must first examine
why the original bridge failed. To do this must answer the question what is chloride
ingress?
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Group 17
Wexford Bridge
What is chloride ingress?
Chloride ingress is one of the main causes of reinforcement corrosion in
reinforced concrete structures. It can be said to occur in two distinct two stages.
Firstly, the chloride ions must be transported from either within or without the
concrete to the areas containing the reinforcement. This usually occurs through the
chloride content of the water in the concrete. The water carrying the chloride ions
diffuses much quicker than the ions do themselves which leads to a build up or
accumulation. If they are in close proximity to steel reinforcement they will begin to
attack it. This is the second stage in the process. The chloride ions destroyed the
protective passivation layer, which surrounds the reinforcement bars in the concrete.
Then provided that oxygen is present, corrosion will begin on the bars themselves.
This type of corrosion is called pitting corrosion. Over time it can reduce the strength
of the reinforcement and thus its ability to resist mechanical stress, and if left
unchecked, will eventually lead to structural failure.
Why does it occur?
The main reason for chloride ingress is due to exposure to a saline
environment. Salt water carries chloride ions through its salt content. Thus, exposure
to a marine environment is a major threat to the durability of reinforced concrete
structures. Other factors influencing the time for corrosion initiation from chloride
ingress are the initial chloride level, the concrete quality, mix type and water-tocement ratio, concrete cover over reinforcement, consolidation, curing and the
environment. Penetration of the chloride ions is affected by the heterogeneity of the
material, so defects in the structure coupled with a saline marine environment makes
the situation extremely hazardous.
How can we prevent this?
Laboratory corrosion and chloride penetration studies on ordinary Portland
cement (OPC) and sulphate-resisting cement (SRPC) have been carried out and report
better performance of OPC cement over SRPC cement. Investigations into the
influence of different cement types on chloride-induced corrosion of reinforcing steel
show that OPC performs better than SRPC in terms of chloride binding capacity,
chloride diffusivity and reinforcement corrosion. In addition, the temperature of
concrete plays a significant role in the diffusion rate of chloride ions increasing with
the elevation of temperature.
Proper placement and consolidation of concrete also determines the final
quality and durability of concrete. With regard to diffusion of chloride in OPC
cement, investigations show that concretes of good (consolidated) and poor (nonconsolidated) qualities have significant differences in diffusion coefficients, from
which average values of non-consolidated concrete were more than two times the
values of consolidated concrete.
The specification and codes of practice have recommended threshold chloride
values varying in the range 0.l-0.4% (or more) by weight of cement depending on the
cement used and the moisture content of concrete. In general, there are three levels of
chloride concentrations, expressed by weight of cement. These are low chloride
content where the chloride content is up to 0.4%, medium chloride content where it is
in the range 0.4-l .0%, and high content which exceeds 1.0% of the cement content.
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Group 17
Wexford Bridge
Results
A new superstructure (fig. 2) has replaced the 383m long Wexford Bridge
during a 10-week road closure. The replacement bridge, designed by Barry &
Partners, is a continuous steel girder deck with a composite concrete slab. The girders
are supported off the reconstructed original piers and abutments. The design
philosophy was for maximum prefabrication of the structural elements to enable
replacement in the shortest time possible. Exposed steel was painted with a Marine A
20 year maintenance specification. The interior of the structure between the steel
flanges was enclosed by metal decking. Defective and laminated concrete in the
bridge piers as well as areas showing high levels of electrochemical potential were
broken out to expose corroding reinforcement. Repairs were carried out using repair
concrete or mortar including a corrosion-inhibiting additive.
Conclusion
In conclusion, once it was decided to take down the Wexford Bridge and
replace it due to the fact that it was in a state of disrepair, the best solution was to use
good quality, highly consolidated OPC cement in the replacement bridge. It was also
noted that due care and attention should be taken at the time of placement and
considerations should be made for the ambient temperature at the time of
construction, as these factors would further influence the performance of the new
bridge against further chloride ingress.
The replacement structure was built in 1997. It incorporates a continuous steel
girder deck with a composite concrete slab. Corrosion inhibitors where added along
with pozzolanic binders capable of immobilizing free chlorides to improve the
durability of the new structure. In addition, a hydrophobic surface treatment was
applied and the steel was painted to a 20-year marine specification.
Fig. 2
References:
www.iseroi.ie
www.asconltd.ie
www.barry&partners.com
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