Page 27 - Build 165

classi cation from parts 1 and 3 with an E su x. For example, finish F5E is a formed finish to
F5 tolerance limits with exposed aggregate surface texture. In all cases, sample reference panels are required to provide a basis for qual- ity and finish assessment.
Materials and workmanship
The nish will depend on concrete grade, ce- ment content, workability, formwork, release agents, placement technique, compaction, curing methods, protection, nishing method and dressing.
Production of o -the-form nishes
A systematic approach and consistently good quality products are needed to achieve the required o -the-form nish.
The range of patterned and smooth textured nishes is considerable. A range of materials can be used to create di erent nishes.
Timber
Timber can be sandblasted to raise the grain or rough-cut to enhance a bold texture. For a smooth nish, use only dressed tongue and groove boards or grooved boards with loose tongues and plastic foam strips. To achieve uniformity of colour, pre- treat the timber by oiling before the rst use. Plywood
The advantages of exterior grade plywood include rapid xing, large surface areas without joints, resistance to impact and resistance to moisture shrinkage and swelling. Plywood can also be bent to form a curved surface.
Plywood’s strength depends on the direction of the grain of the outer plywood face. Panels should normally be used with the grain parallel to the span of the sheet.
Surface coatings on the plywood reduce the colour variations on the concrete face as well
as extending the plywood’s use. As the surface coating becomes more impermeable, there tend to be more blowholes.
Steel
Steel formwork produces concrete of uniform colour provided it is protected from rusting and form face vibration can be reduced. However, its impermeability can create blowholes.
Due to the exibility of sheet steel facing, pay attention to jointing to avoid leakage during vibration and ensure that the steel sheet is of su cient thickness to limit de ection.
Always select a mould oil with rust-inhibiting properties.
Oil-tempered hardboard
This provides concrete of reasonably uniform colour although some surface blowholes are inevitable as it is relatively impermeable. A recommended procedure is to wet the back of sheets and stack at for 48 hours before use. Preferably the sheets should be centre pinned and fully supported to reduce buckling.
The material should be oiled before use. Clean down using a sti brush and cold water before reoiling.
Rubber linings
Rubber linings may be used to create textured or pro led surfaces. The surface is of uniform colour.
For xing in vertical positions, linings can be glued or tacked to a backing form.
Mineral oil-based release agents must not be used since these soften the rubber. Castor oil or lanolin are suitable release agents. On shallow pro les, no release agent is necessary. The rubber liners should be cleaned down with water and then lightly oiled.
Select the right release agents
Most form materials require a release agent
on the surfaces that will contact the concrete. Along with their function of e ecting release, agents can in uence the surface appearance and durability. The release agent should be carefully selected so the speci ed nish can be achieved.
Striking or stripping the formwork
The quality of an o -the-form nish is also dictated by appropriate striking – stripping of the formwork.
The time of striking is normally speci ed, otherwise follow NZS 3109:1997 Concrete construction. Correct striking can also impact the reusability of the form, including any liners.
Quality requires care
Concrete should provide a long-term mainte- nance-free surface nish. However, excellent nishes do not just happen. A wide variety of aspects need to be considered and skilled profes- sionals involved in the process.
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