Civil Engineering Reference
In-Depth Information
It should not be assumed that all proprietary products will conform to
this overview, as many variations are possible; in addition, a combination
of generic types may be used to provide a sealer/primer/intermediate and
top-coat system. Generic characteristics (e.g. 'all epoxies bond well and
have good chemical resistance') can also encompass significant variations,
depending upon the detailed composition (formulation) of a product.
Properties are not discussed here in detail, but a summary of characteristics
that may have to be considered for various applications is given in
Table
17.6.
Where corresponding information is not provided on data sheets,
suppliers should be consulted (on occasion, specific tests may need to be
commissioned).
17.5 Surface preparation and application
Concrete is inherently suitable as a substrate for coatings, although it can
present problems that are not met with steel, for example:
•
The (uncarbonated) surface has a high alkalinity.
•
Concrete surfaces are invariably rougher and often have partly open
air-voids ('blow-holes') and protrusions (e.g., fins/nibs or grout runs).
•
Concrete is absorbent to a varying degree and frequently has a relatively
high moisture content.
•
Surfaces may be dusty and friable and often have a thin, relatively weak
surface layer (laitance).
•
Contamination due to mould release agents, curing membranes and
fungal growth is common (note: curing membranes designed to degrade
may persist in the absence of sunlight; some specific membrane types
can be overcoated).
For a coating to perform satisfactorily, application to a clean, sound
surface is normally essential, thus enabling the development and retention
of the maximum bond strength. Certain demands may be relaxed somewhat
for many penetrants and sealers as surface adhesion is less critical, although
some preparatory work may still be necessary; contaminants that hinder
penetration, for example, should be removed.
The need to remove an existing coating can vary, depending upon
its condition, including adhesion, and its chemical/physical/adhesive
compatibility with the new system. Identification of the existing coating will
assist significantly in determining the best course of action; in-situ testing,
e.g. for adhesion, can also be advisable. Specific performance properties
required of the original system should be identified and maintained, where
necessary, by the new coating.
For new concrete, controlled permeability formwork can simplify surface
preparation as it provides a sound surface with minimal blow-holes and also
eliminates contamination by shutter release agents (McKenna, 1995).
