Civil Engineering Reference
In-Depth Information
design codes, design errors and construction errors even before deterioration
sets in.
The easiest way to understand structural behaviour is to consider the
flow of forces through the structure from the applied forces to ground.
Never forget longitudinal and lateral forces from wind and considerations
of stability. Concrete and repairs can only reliably take compression. When
a member, or one side of it, is in tension from axial load, bending or shear
there must be adequate reinforcement to carry all that tension with particular
attention to connections and laps. Corrosion will weaken the tensile load
paths especially at connections.
Spalling and cutting out for repairs will reduce the compressive load
paths. Only if dimensional compatibility of the repair with the underlying
concrete is achieved and the reapplied load puts the repair into compression
will it contribute to the strength of the structure (Emberson and Mays, 1990
& 1996). Repair patches which shrink and creep in the long term will be
structurally ineffective and the strength will be that of the structure with the
hole cut out for the 'repair'. The bigger the cut out, the weaker the structure
becomes.
Some reinforcement is required by the reinforcement detailing rules, for
example, stirrups in columns. They are not stressed directly but are essential
to contain bursting and splitting of a column and to prevent buckling of the
main reinforcing bars. Shear steel in beams ensures a ductile failure mode.
When these links are corroded, the element becomes brittle with a risk of
sudden failure. When reinforcement area is lost by corrosion, ductility and
robustness must be considered as well as simple evaluation of stress.
Trying to fit new reinforcement into old concrete sections is never easy.
Sufficient cover must be achieved. Stainless steel cannot be used in contact
with normal reinforcement because of bimetallic corrosion. Lap connections
need long lengths and the structure must be unloaded for new reinforcement
to take its share of load.
Where deterioration is severe but localised, external strengthening (
Figure
3.8)
or the provision of an alternative load path around the weak part can be
considered, but it is not easy to do this elegantly.
A column with corrosion of the stirrups may be better treated by wrapping
with carbon fibre or steel bands to replace the containing function of the
stirrups. A more seriously damaged column can be reinforced with non-
structural patch repairs and steel plates bolted and epoxied on.
Since the 1980s the use of epoxy resins to attach stainless steel plates
and carbon fibre for flexural strengthening of deficient reinforced and
prestressed structures has become well established. However, when corrosion
is damaging a structure, these techniques are inappropriate unless testing has
established that there is no risk of corrosion delaminating the concrete cover
to which they adhere. They can also restrict oxygen access and can aggravate
'black rust' if there is a sufficiently large cathodic area elsewhere to drive the
corrosion cell.
