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| Categories
of Finish
Off-the-form
Surfaces
This category covers the concrete surfaces that are primarily
dependent on the formwork for texture and finish. As such
the most effective results are gained by attention to
detail before the concrete is cast.
The formwork dimensions, rigidity, joint tightness and
texture all become of increasing importance. It is commonly
accepted that time spent before casting to ensure the
exactness of all of the above is never wasted.
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Often short-cuts prove troublesome and more costly in the
long-term with significantly more hours and money being spent
attempting to remedy defects.
The six classifications (F1 to F6) cover all qualities of
formed finish. They range from hidden surfaces - (F1) (e.g.
foundations, rear of retaining walls, lined surfaces, underwater
dam faces), plaster surfaces (F2), exposed surfaces viewed
from afar (F3), to architectural and high quality panels (F6).
Table 1 outlines the range and requirements incorporated in
the standard specification.
Table
1 is accessible here
Although there is a grey scale included in the standard there
are no specific limit set as to the variations accepted under
each finish. It is up to the specifier to determine an acceptable
range and to monitor this with a sample reference panel (SRP).
Any finish that has nominated colour range restriction must
be designated with the X suffix and the range specified.
The typical control is the restriction to a range of 2 or
3 shades. For example: Using the shade chart NZS 3114. The
SRP sets the mean position of colour shade. In this example
let it be shade 3. A specification calling for a range of
reshapes would permit panels ranging in colour from shade
2 to shade 4. The greatest contrast for two adjoining panels
or areas is shade 2 compared to shade 4. Tighter control can
only be exercised with a range of 2 shades which is difficult
to administer since the concept is that the SRP sets a mean
about which there can be lighter and darker shades. Consequently
unless the SRP mean shade is literally 'between' two shades,
interpretation in a light/dark variation can be difficult
for a range of 2.
Exposed Aggregate Surfaces
As defined, surfaces enter this category if they have exposed
aggregate on their surface. The specification extends the
classification from Parts 1 and 3 with an 'E'' suffix. Thus
finish F5E is a formed finish to F5 tolerance limits with
exposed aggregate surface texture. The primal extension relates
to the selection and uniform end result of the aggregates
to be exposed. In all cases SRPs are required to provide a
means of compliance. When using exposed aggregate surfaces,
the specifier must consider and define the following:
The Aggregate
- Weathering and staining characteristics
- The colour and mineral type
- The source of the material
- Exposure depth
- Angular characteristics required
The Matrix
Because exposed aggregate finishes will only be used where
the surface is visible, the formed finishes F4, F5, F6 are
extended to accept the exposed aggregate standard of F4E,
F5E, and F6E. Exposed aggregate may also be used in unformed
surface, resulting in a shallow texture. Thus U7 finish
may be applied to exposed aggregate unformed surfaces as
U5E.
Unformed Spaces
This category of concrete surface finish extends to all floors,
pavings, slabs and inverts. These surfaces remain exposed
when concrete casting is completed. The surface results from
screed, float or trowel action, and texture sometimes provided
by additional measures such as brooming, raking, grinding
or scabbling. There are eleven standard finishes specified
in the code. They relate primarily to the texture required
for the surface to perform its intended function. The derivation
of a particular class of finish frequently requires the surface
to proceed through lesser classes (i.e. a U5 broomed finish
is usually screeded to Class U1 and floated to Class U2 prior
to the final texture being applied. The classifications for
unformed finishes are shown in Table 2.
| Class |
Finish |
Technique |
Examples |
| U1
|
Screeded |
Hand sawing
motion with straight-edge or mechanical vibrating
screed. |
Finishes
covered by backfill or concrete, footpaths, yards
and driveways. First stage for placement. |
| U2 |
Floated |
Wood or
bull float, or both. Generally manual but power
driven equipment may be used. |
As for U1
where a higher standard of finish is required.
Floors to receive carpets with underlay or similar
coverings, inverts of syphons, flumes, floors of
canal structures, spillways outlet works and
stilling basins. Surfaces which are intended for
use by ambulant disabled or wheelchair-bound
persons. |
| U3 |
Trowelled |
Manual or
mechanical steel trowelling of floated finish
after concrete is sufficiently hardened, to prevent
excess fine material and water being worked to the
surface, may be done in one or two stages
depending on degree of smoothness required. |
Direct
wearing floors such as in factories, warehouses
and processing plants, Floors to receive thin
sheet coverings, carpet and similar coverings.
Inverts of water, tunnels and tunnel spillways.
Not generally used for pedestrian or vehicular
traffic where a smooth finish should be dangerous
in icy or wet conditions. Is not suitable even
when dry, for surfaces which are intended for use by
ambulant disabled or wheelchair-bound persons. |
| U4 |
Machine |
Vibrating
or oscillating screed or vibrating plate, or both,
which may be supplemented by long handled metal,
wooden, or rubber floats. |
Used for durability
where resistance to erosion and cavitation under
action of high velocity water is especially
required; and as firsts and second stage finishing
for roads and airfield pavements prior to
texturing with U5, U6 and U8 finishes. |
| U5 |
Shallow
Textured |
Hard or
soft bristled brooms. |
Footpaths,
yards, driveways, roads, pavements for aircraft. |
| U6 |
Deep
Textured |
Wire broom
or rubber tyning. |
Surface to
receive a subsequent textured bonded concrete
topping. Roads and runways where greater
frictional resistances are required than can be obtained
by U5 finish. |
| U7 |
Grooved |
Saw cutting
or flailing by mechanical means. |
Treatment
to existing roads and runways to provide frictional
resistance and drainage paths for run-off to
minimise aquaplaning. |
| U8 |
Grooved |
Mechanical
grooving the fresh concrete surface after
compaction and surface screeding techniques. |
Roads and
runways. |
| U9 |
Scabbled |
Mechanical
hammering of hardened concrete. |
Can be used
on any pavement surface to produce a textured
effect or to reduce high surfaces to the correct
level or to rectify out-of-tolerance pavements. |
| U10 |
Special
Textured |
The use of
equipment to give special effects. |
Architectural
effects on pavements and slabs, produced by
rollers with drums of expanded mental, or profiled
tempers on screedboards, and the like. |
| U11 |
Ground
Finish |
Low speed
coarse stone grinding to remove thin weak surface
layers/minor ridges and to produce and even 'glasspaper'
textured surface, that is, not a polished surface.
Used as a second stage finish to U2, approximately
36-48 hours after laying. |
Direct
wearing floors such as in warehouses. |
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Table 2. Classes of floor, exterior pavement, and invert finishes
The durability required of the slab often dictates the finish
specified, with U3, U4 and U11 (trowelled, machine screeded
and ground finishes) increasing the toughness of the surface.
The specification tolerances with respect to colour and physical
irregularities are tabulated and vary somewhat depending on
location and end use. Abrupt deviations are to be less than
3mm in all finishes but should be avoided where carpets and
thin tiles are to be used for floor coverings. Gradual deviations
are within 5mm over 3m for most classes of finish. The problems
of plastic cracking and crazing are more common with the large
exposed surface areas involved. The specifier must stipulate
the spacing and requirements of joints to minimise these effects.
Download
IB33
- Specification and Production of Concrete Surface Finishes
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