June_1970 — Page 30

boards against damage where only a single use is required.

Steel formwork linings. Steel forms produce very satisfactory finishes from the point of view of uniformity of colour, but since they are impermeable they encourage the formation of blow- holes. Vibration of steel formwork may dislodge entrapped air bubbles, but may also cause transparency dis- coloration.

Absorbent formwork linings. Ab- sorbent materials allow water to bleed from the concrete face, but the pores of these materials are too small to ad- mit the solid cement particles. The re- sult is a denser concrete surface, as the bleeding process brings the finest parti- cles to the surface.

A darker colour is also imparted to the concrete finish as a result of the great concentration of fine cement particles and reduced water content at the surface. Since the absorption of a lining decreases with each use, the colour of the concrete surface tends to become a shade lighter with each suc- cessive use of the lining.

The development of special highly absorptive formwork linings of soft cane-fibre is of both structural and aesthetic significance. Because of their

absorptive capacity, pitting of the sur- face is virtually eliminated, and the concrete receives a matt finish. In ad- dition, as a result of the removal of water from the concrete, a tougher and denser surface than is usual with normal formwork is obtained.

Moreover such a surface offers greater resistance to weathering and abrasion, and is also less likely to craze than is a normal concrete surface. Since the number of possible uses of a lining decreases with increasing rate of absorption, these special absorbent formwork linings should be used once only, and therefore they are costly when compared with other linings.

Texture rubber and plastic form- work linings. A large number of rubber and plastic materials can be used to give either textured or sculptured sur- faces. The cost of such linings is great, so that they should be treated with care in order to gain the maximum possible number of uses from them.

Experience has shown that it is ex- tremely difficult to produce com- plicated details in-situ in vertical planes, unless such patterns take the form of predominantly vertical rib- bing; hence textured linings should be used only after experimentation.

Conclusion

Considerable care and effort are re- quired in the production of off-the- form concrete finishes of such quality as to make them generally acceptable alternatives to stone or other conven- tional finishes.

It is fallacious to regard off-the- form concrete finishes as being low in initial cost. However, because of the minimal maintenance that is a charac- teristic of off-the-form finishes, com- pared with other finishes, there are likely to be significant savings in over- all costs during the life of a building.

References

1. BLAKE, L.S., KINNEAR, R.G. and MURPHY, W.E. Recent research into factors affecting the appearance of in situ concrete. C.A.C.A. Symposium on Surface Treatment of In Situ Concrete. Lond., 1964. 2. KINNEAR,

R.G. Concrete surface blemishes. C.A.C.A. Tech. Rep. TRA/ 380. Lond., 1964.

3. MORRISH, C.F. Review of techniques and possibilities in architectural off-form concrete finishes. C.A.C.A. Concr. Inf. C.32. Syd., 1964.

Far East Builder is grateful to the Com- monwealth Experimental Building Station, Chatsworth, New South Wales, for permis- sion to publish this paper,

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Far East BUILDER, June 1970

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