The prime purpose of formwork is to provide a temporary fixed shape for concrete to harden in like a jelly mould. If the requirements for the finish of the hardened concrete are important in terms of texture, pattern or general appearance, then these need to be properly specified and considered from the outset. With feature finishes comes additional work, not just at the design stage but with the concrete mix requirements as well as during the pour itself.
Classes of Finish
Typical classes of finish for formed surfaces are specified in:
- Specification for Highway Works (Volume I Structural Concrete (Clause 1708)
- Civil Engineering Specification for the Water Industry (Clause 4.22)
- National Building Specification Formed Finishes for In-Situ Concrete (Section E20 Clauses 610 to 710)
- National Structural Concrete Specification Edition
- BS EN 13670 Execution of Concrete Structures
|Specification for Highway Works||CESWI||National Building Specification||BS EN 13670 and NSCS||Typical applications|
|F1||Rough finish||Basic finish||Basic finish||Foundations, unseen|
|F2||Plain smooth||Ordinary finish||Back of retaining walls, car park beams/columns|
|Fine smooth finish||Plain finish||Visible surfaces but no ties. Parapet edges etc.|
|F4||Visible surface, bridge abutments, ties allowed|
|F5||Precast items, embedment of metal parts allowed|
|–||Fair worked finish||–||–||Aqueous liquid-retaining faces|
|Special finishes||Rough board finish
Finish for tooling
|Special finish||Functional or aesthetic finishes including enhanced durability|
Figure 1. Approximate comparison of the major UK specifications for concrete finishes.
The following factors need to be considered to achieve required classes of finish:
- The right concrete mix
- Conditions for transporting the concrete mix
- The correct choice of formwork including uniformity of face contact material
- The release agent
- Consistency of the concrete and its materials
- Quantitative and qualitative selection of the concrete mix materials and composition
- Use of a constant rate of placing
- Uniformity of compaction
- Geometry and dimensions of the element, spacing and sizes of the reinforcement and concrete cover
- Use of a baffle to avoid the direct flow of concrete against the form face
- Walls and columns should have a minimum number of joints and preferably be in one lift
- Features should be designed to be predominantly vertical (such as ribbing), and permit easy runoff of water and dirt, as horizontal or sloping features will often weather badly
- The period between completion of concreting and striking the formwork should be constant for all panels of a wall to minimize variations in colour
Trial Panels (Mock Up)
If a trial run is required, the mock up should be made using similar materials and placing techniques to those planned for the permanent works.
Figure 2. Examples of trial panels
Form liners are non-structural materials used to line the face of the structural forms. They are used to produce either a smoother or more profiled / patterned surface to the concrete. They can also be used to extend the life of old form faces or to enhance the durability of the concrete surface.
There are several different types of form liners, which vary based on the application. Single-use form liners are usually made of styrene plastic. Multi-use form liners are usually made of ABS plastic and range in number of uses from 2 to over 10. Multiple reuse form liners are usually made of polyurethane, a heavy rubber material known for its durability.
Other form liner materials used include polystyrene foam, fiberglass and even aluminum–styrene plastic. However ABS plastic and polyurethane are considered to be the industry standard, and are most often specified in plans by architects and engineers.
Figure 3. Examples of form liners use – sound walls
Fair Faced Concrete
In modern architecture concrete is not only used as a load-bearing material but increasingly for aesthetic reasons as well. Fair faced concrete is a concrete surface that is left exposed, without any covering applied.
As the surface of the concrete is visible, it is imperative that the design considers every tie hole position and ply layout, in order to achieve the best finish possible and adhere to the architect’s design.
Figure 4. details a design by IDH for a stair core where fair faced concrete was required. Figure 5. shows an example of the type of finish this design would achieve.
Figure 4. Tie holes and plywood arrangement to fair face concrete
Figure 5. Fair faced concrete typical finish – From www.doka.com