Materiality and surface
B 3.2
Timber formwork Timber formwork is the oldest kind of mould making in modern concreting. It uses the natural wooden surfaces of traditional con struction timber. Untreated softwood planks can be used for formwork twice and up to a maximum of four times. A distinction is made here between tongue and groove (match boards) formwork and butt-jointed timber form work without tongue-and-groove joints, which can be made with regular or irregular board widths. Timber formwork surface textures vary from rough-sawn to planed. Higher quality woods are also sometimes used in concrete formwork to produce a particular grained texture or joint pattern. Formwork made from butted-jointed softwood planking is hard to seal against fluid leaks. Leakage at joints between planks almost always results in an undesirable dark discol ouration on the finished concrete surface. This effect did not appear in traditional con struction with earth-moist tamped concretes because the concrete did not contain a fluid matrix. Butt-jointed timber formwork without tongue-and-groove joints was used until around 1950. Increasingly fluid concretes have been used since the 1960s however, and these have to be poured in a properly sealed formwork of precise dimensions. Very absorbent raw timber formwork skins imprint the concrete surface with their own rough texture while reliably preventing visible air pores from forming because the wood absorbs air and water bubbles close to the surface. This rough formwork does not gener ally produce structural components with pre cise edges but also rarely requires triangular fillets or similar measures to form edges. The growing technical mechanisation of timber processing in the 1950s meant that tongue-and-groove planks were increasingly used in formwork. This type of formwork, when used with timber with medium moisture content and carefully sealed joints, prevents significant cement paste leakage. Applying formwork oil as a preservative and mouldrelease agent makes plank surfaces smoother so they can be reused several times in form
work. Equipment for lifting and moving larger formwork elements was being developed during this period, making it more economi cal to join softwood planks to make large formwork elements and move them with cranes several times without first disassem bling them. Industrial timber processing was also developing at this time, so smoother, planed planks could be produced at lower cost. The rough-sawn butt-jointed timber form work that had been standard began giving way to very high quality tongue-and-groove planed timber formwork, which is still used in some areas of architecture and construction (e.g. bridge construction).
The following construction operations issues must be taken into account when using timber formwork: • Before being used for the first time, new formwork skin must be “aged” with cement slurry, concrete or by being sprayed twice with a 3 – 5 % caustic soda (sodium hydrox ide) solution because untreated wood can contain substances that can impede the fresh concrete’s hardening and result in washed-out concrete surfaces and defects. • When wood first comes into contact with fresh concrete, not only water, but also the cement matrix (water, cement and the finest aggregate) is absorbed by the void system
B 3.3
55