June_1970 — Page 29

for board markings are those of grain, degree of grain exposure, saw mark- ings, and the extent of impression re- quired. The absorbent characteristics of the boards directly affect the colour of the finished concrete.

Timber formwork with high ab- sorption produces a concrete surface darker in colour than that with low ab- sorption. It follows that where uni- formity of colour is desired the use of form linings with uniform absorptive characteristics is vital.

Dark lines will appear in the finish ed concrete on the lines of the joint of individual boards where mortar can escape, or where the boards open up as a result either of shrinkage or cupping. The darkening of colour results from a change in the physical characteristics of the concrete. Therefore it is essen- tial to keep the formwork constantly wet right up to the moment of strip- ping to counteract these effects. Simi- larly, discoloration will result if part of the form lining pulls free of the con- crete face while other parts remain in contact.

Where a rough finish is required to show the grain of the timber, or saw marks and the like, the boards are left as sawn, but for all other finishes it is preferable to have the boards dressed. Where a smooth finish is required, it is desirable to use dressed tongue-and-

groove boards painted if necessary in- side and outside with a good lacquer or paint.

Whatever finish is sought, it is un- desirable to use old and new boards in the same formwork, as they will ab- sorb water at different rates, and hence they will tend to produce a patchy appearance on the finished con- crete.

Joints can be made to project by chamfering the edges of the boards. and grooves or recessed joints may be produced by fixing timber fillets to the face of the formwork covering the joint between the boards. If the fillets are rectangular, the bottom face must be slightly bevelled to facilitate their removal, and to produce an outward slope in the concrete so as to shed rainwater.

Plywood formwork. The absorptive characteristics of plywood vary over its face, and need to be carefully con- trolled either by coating the plywood with a form oil or by painting it. Plywood that incorporates highly water-resistant adhesives is normally used in formwork, and the particular grade adopted is usually determined by the number of uses expected.

The thinner plywoods require to be stiffened with boarding, but the thick- er plywoods may be supported on studding or walings. The joints be-

tween sheets may be filled with putty, plaster, or white lead, and should be lightly sandpapered.

Plastic-faced plywood formwork. Plastic-faced plywood formwork over- comes the problem of variation in ab- sorptive characteristics, and produces a smooth finish. Glass-fibre reinforced polyester resin, polyvinyl chloride, po- lystyrene, and polyethylene film, have been used with varying degrees of suc- cess, depending on the nature of the work and the precautions required to be taken.

However, all such linings have a re- latively poor resistance to abrasion and for this reason processes which involve an impregnation or seal of phenol formaldehyde or polyurethane lac- quers on the inside face of the form- work are also used.

Hardboard formwork linings. Hard- boards which usually also require solid backing, vary in absorbency from sheet to sheet, and across the face of individual sheets. Though a consistent- ly satisfactory finish with few blow- holes can be obtained with standard hardboards, these linings should be used once only, since they become progressively clogged by mould oil and cement fines, and yield concrete sur- faces of progressively lighter shades at each use. Little difficulty is experi- enced in protecting the edges of such

Far East BUILDER, June 1970

Left: Off-the-form concrete finish at Macquarie Uni- versity, Sydney.

Above: Detail of rough finish showing grain of tim- ber form imparted to con- crete surface.

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