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.
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:
- Weathering and staining characteristics
- The colour and mineral type
- The source of the material
- Exposure depth
- Angular characteristics required
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
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.
||Hand sawing motion with straight-edge or mechanical vibrating screed.
||Finishes covered by backfill or concrete, footpaths, yards and driveways. First stage for placement.
||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.
||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.
||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.
||Hard or soft bristled brooms.
||Footpaths, yards, driveways, roads, pavements for aircraft.
||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.
||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.
||Mechanical grooving the fresh concrete surface after compaction and surface screeding techniques.
||Roads and runways.
||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.
||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.
||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.
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.
For more information:
IB33 - Specification and Production of Concrete Surface Finishes