Architects memo no.
8: April 1981
painting concrete surfaces - specialised products for specialised jobs
The concept of fair-faced concrete is that concrete, carefully
placed, using high-quality formwork accurately constructed, will provide
a surface of sufficient standard that plastering will not be necessary.
This is an important aim as plastering is not only highly expensive
but also the number of skilled craftsmen available in New Zealand to
do this sort of work is not high.
These are two main factors affecting the successful outcome of fair-faced
concrete (a) the accuracy of the formwork and (b) the presence of 'bug-holes'.
With regard to the latter, many papers have been written on the formation
of bug-holes and opinion is that they are affected by (1) shape and
surface characteristics of the formwork, (2) ratio of fines to aggregate
in the concrete, (3) slump of the concrete, (4) type of form-release
agent used, (5) degree and type of vibration used.
Notwithstanding this however, the major problem is what to do with
them when you have got them? Standard paints won't fill them. One approach
has been to bag the surface with a fine sand/cement slurry. Bagging
leaves a very obvious surface that weathers differently to the base
concrete; it also provides a very weak surface for subsequent painting.
When the problem was considered by Resene in 1971, it soon
became obvious that a successful system would need to physically bridge
the bug-holes rather than attempt to fill them. This led to a
concept of fish-scales that was eventually achieved in practice
using very coarse mica flakes in the product Masonry Filler. When the
product is brushed onto a pitted surface, the mica flakes overlap one
another in one place forming a strong mineral bridge. The flakes
are cemented together by the acrylic binder used. Although coarse flake
reinforcement of films has become common overseas, Resene were first
with the concept and were granted New Zealand patent rights.
Although developed as a fix-it product, experience over the
years has shown that Masonry Filler has other advantages. The interleaved
plate structure provides a tremendous barrier to the ingress of moisture;
and the uv-opaque properties of the mica flake gives excellent protection
from UV light. Masonry Filler has been used to bring concrete block
surfaces up to a hygienically acceptable standard as well as waterproofing
them.
Cost, its inability to be sprayed, and the difficulty of finishing
to a clean line, eventually led to the product's demise.
Lack of accuracy in the formwork can result in a wall having within
it various planes. This is visually very obvious and not pleasing aesthetically.
One method of overcoming this is to trick the eye into not seeing it
by applying a textured coating to the surface. The texture diffuses
light and substantially reduces the obviousness of surface defects.
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