Civil Engineering Reference
In-Depth Information
situation such a mix will develop a residual compressive strain which will
offset subsequent shrinkage, particularly where damp conditions reduce
subsequent shrinkage. Where the hydrating mix is not well contained, as in
most repairs, the expansion occurs outwards so little compressive strain is
developed. In the dry conditions for many repairs, the long-term shrinkage
far exceeds the short-term residual compressive strain. Late formation of the
ettringite can damage the repair.
The inclusion of fibres in repair mixes can help keep the repair in one
piece. However as failure occurs around the boundary of the repair, fibres
give no advantage in preventing the edge and delamination cracking.
3.2.6 Placing the repair mix
The adhesion of the repair to the substrate is of great importance as this is
where most repairs fail. Pre-moistening the substrate will prevent it sucking
moisture out of the repair, but it is important to avoid over-wetting which
gives a weakened repair interface. A bond coat compatible with the repair mix
is beneficial, as long as it does not dry and harden before the repair is applied.
Placing the repair and keeping it in place as it bonds and gains strength
requires tailored shuttering. The cohesion and consistency should ideally
allow well-rammed filling, with care as it is applied, so that air is not trapped
at the interface. Shuttering can than be applied to hold and compress the
repair while it fully hydrates and gains strength. Early drying shrinkage and
incomplete hydration can lead to early failure.
Well-controlled shotcrete can be an effective way of placing concrete to
fully fill the cavity and key and bond to the back and edges. However, it is
only economical where large areas of repair are needed.
3.2.7 Finishing
In some locations an approximate matching of profile and colour is sufficient.
However where the concrete is an architectural feature and particularly
where texture, board marking or exposed aggregates are a special feature,
special measures are required. Often a two-layer approach is necessary,
with the main repair providing structural/corrosion control and a finishing
cosmetic layer matched to restore the original appearance.
3.3 Conclusion
Where deterioration or damage necessitates repair, there needs to be a
structural engineering consideration in investigation, appraisal, specification
and methodology of the repair. Cutting out for repair further weakens the
structure and patching will not restore strength unless rigorous procedures
are followed.
