Civil Engineering Reference
In-Depth Information
The cost of access and disruption associated with repairs makes it
important to be aware of the extent to which further deterioration may
develop over the next 20 years and its structural consequences (Wood, 2009).
Tuckton Bridge, built in 1905, is a good example of progressive cycles of
repair (Wood et al., 2008). Often deterioration has been aggravated by bad
local details creating ponding or channelling moisture and salt into joints.
Upgraded drainage and providing shelter to slow deterioration should be
considered in the prediction of future deterioration.
Extrapolation from the statistical analysis of cover depth related to chloride
and/or carbonation profiles, including the data from the deeper investigation
in the trial repair contract, should enable future trends to be predicted. The
likely cost of these future repairs, as well as the currently proposed repairs,
relative to the value of the structure must be considered before embarking on
extensive remedial work. The poor performance of many concrete repairs,
with 50% failures reported within 10 years (Matthews and Morlidge, 2006),
needs to be considered in the prediction.
Some concrete structures are highly stressed with thin members with little
margin for deterioration. Corrosion in exposed columns creates particular
difficulties as this can involve cutting out behind all stirrups and main bars
leaving just a central core of uncontained concrete, which makes removal
and full recasting a more reliable option than patching. With precast
construction, the joints may be badly constructed and poorly toleranced and
they are particularly vulnerable to deterioration.
Other forms of concrete structure, sometimes for architectural effect, use
large low-stressed members which can be cut into with little detrimental
effect. An evaluation of stress levels and reinforcement configuration in
a member will give a preliminary indication of the need for achieving a
structural repair. For cosmetic reasons and/or corrosion control the repair
material does not necessarily need to reinstate the load-carrying role of the
original concrete, but it is important that it stays safely attached.
Where the reinforcement is corroded the structural role of the bars needs
to be reconsidered relative to their current condition. Appraisal may have
shown inadequacies in the original design relative to current requirements for
shear strength, robustness and/or impact resistance. It may be appropriate,
and in other cases essential, to remedy these shortcoming as part of the
overall remedial works.
Large concrete members often contain reinforcement for early age crack
control. If these bars with low covers are corroding, it may be appropriate,
subject to a structural check, to cut some of them out to facilitate repair.
The three golden rules for design also apply to concrete repair. One must
balance the three key objectives of:
•
Fitness for purpose, including restoration of strength and control of
further deterioration
