Exposed concrete surfaces

Fig. 22: Timber boards and honeycombing result in rough surfaces

Rudolf Olgiati: house for Dr G. Olgiati, Flims-Waldhaus (CH), 1964-65

Fig. 22: Timber boards and honeycombing result in rough surfaces

Rudolf Olgiati: house for Dr G. Olgiati, Flims-Waldhaus (CH), 1964-65

Exposed concrete

Basically, we distinguish between two types of exposed concrete depending on whether the outermost, thin layer of cement directly adjacent to the surface of the formwork is retained or removed.

Cement"skin" retained

The pattern of the f ormwork and the f ormwork ties determine the appearance. Joints in the formwork can be dealt with in various ways - from the simple "butt joint" to the "open j oint" to covering the joints with various strips and tapes.

The holes created by f ormwork ties are either filled with concrete subsequently, left open or plugged.

Cement"skin" removed

The outermost, thin layer of cement can be modified or completely removed by using various manual or technical treatments. The cement "skin", the surface layer, is worked or treated to reveal the aggregate.

Characteristics of concrete surfaces cast against formwork

The appearance of the struck concrete is determined mainly by the surface texture of the formwork material but also by the joints in the formwork and the formwork ties. This aspect calls for meticulous planning of all joints and ties plus subsequent rigorous inspections during the work on site, or a tolerant attitude towards the quality of the concrete surfaces.

Fig. 23: Courtyard wall in in situ concrete, constructed with formwork panels the size of tatami mats (91 x 182 cm), courtyard floor finished with precast concrete flags

Tadao Ando: Vitra conference pavilion, Weil am Rhein (D), 1993

Fig. 23: Courtyard wall in in situ concrete, constructed with formwork panels the size of tatami mats (91 x 182 cm), courtyard floor finished with precast concrete flags

Tadao Ando: Vitra conference pavilion, Weil am Rhein (D), 1993

Manual treatments

- Bossing

- Point tooling

- Bush hammering

- Comb chiselling

Technical treatments (exposing the grains of aggregate)

- Blasting (sand, steel shot, corundum, water/sand mixture)

- Flame cleaning

- Brushing and washing

- Acid etching

Mechanical treatments (surface only)

- Grinding

- Polishing

Characteristics of concrete surfaces not cast against formwork

These surfaces (floors and tops of walls) can also be worked with the above treatments once the concrete has hardened.

But before such surfaces have hardened, they can also be treated with a diverse range of tools.

Colour

The colour of the concrete is determined by the quality of the concrete mix (coarse aggregate and cement quality plus any pigments added) and the formwork (new or used formwork, also quality and quantity of release agent).

Surface characteristics of concrete cast against formwork

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