Off-the-form concrete finishes
UNDER most normal conditions of exposure and environment, concrete surfaces are durable and virtually free from recurring maintenance costs.
Despite these desirable characteris- tics, most concrete surfaces have in the past been faced with other materials such as brick, marble, and stone, be- cause of the difficulties of securing off-the-form concrete finishes com- patible with conventional aesthetic tastes. However, as a result of advances in technological capability, and changes in aesthetic and other con- siderations, this trend is now slowly being reversed, and exposed concrete finishes are being used increasingly.
But blemishes or defects can and do occur in cast-in-situ, off-the-form concrete finishes and architects should familiarise themselves with the means of reducing or eliminating these de- fects.
Off-the-form concrete finishes are usually considered to be those finishes that are obtained directly the form- work is removed, and without further surface treatment of the placed con- crete. The finish is usually smooth, board-marked, or textured, depending on the nature of the formwork or lining.
Blemishes in concrete surfaces are usually considered to result from in- discriminate discoloration or physical irregularities, or from a combination of both of these causes. However, not all types of blemishes are objection- able, and the degree to which some are objectionable depends on the size and position of the concrete surface.
For example, a discoloration in which the concrete has an overall mottled appearance and random varia- tions in shade can be pleasing, but un- controlled and isolated colour varia- tions, such as those that result from patching, are generally objectionable.
Similarly, small blow-holes are ob- viously of little significance in units viewed from a distance, but their eli- mination from positions close to eye level may be important.
Extreme attention to detail through
all stages of design, construction and supervision is essential in all aspects of off-the-form concrete finishes.
Mix design
Variation in the colour of the con- stituents, and in the physical proper- ties of the mix, have a marked effect on the colour of the concrete, on colour variations, and on the occur- rence of blow-holes. It follows that where uniformity is essential, the cement for a particular job should be obtained from one plant, and that fine and coarse aggregates should be free of deleterious impurities. The water- cement ratio and mix proportions, also, must be accurately controlled, preferably by weigh-batching.
Variations in the water or cement content can have considerable effect on the colour of the concrete, and thorough mixing is essential to obtain uniformity of colour. The most uni- formly coloured concrete surfaces are obtained when the cement and aggre- gates are of a similar colour, and the fine and coarse aggregates are used at constant moisture contents.
For a concrete surface of uniform colour, the mix should be rich in cement and have a high content of fine sand, and the coarse aggregate should be continuously graded. An air-en- training agent reduces the possibility of colour variations by reducing move- ment of moisture within the concrete.
The number and size of blow holes are reduced by the use of a cement- rich workable mix, which should in- corporate coarse aggregate of the greatest practicable size, but have the lowest content of fine aggregate for uniformity of colour. The mix should be carefully designed, and preferably be given trial use to gauge whether the objectives will be achieved.
Reinforcement
The placement of reinforcement must be suited to the grading and size of coarse aggregate to be used, and access for the proper placement and compaction of the concrete is essen-
tial. Care must be taken that there is adequate concrete cover over the rein- forcement, to ensure that moisture will not penetrate, and attention must be paid to such details as the use of the minimum number of spacing blocks, and to the careful compaction of concrete around the blocks.
Otherwise, rust stains may occur on the concrete and, later, the con- crete may spall. Protruding reinforce- ment should be protected by water- proof covers to prevent rust being washed by rain onto the face of con- crete below.
Mixing and placement
Mixers of sufficient capacity to permit continuous placement of con- crete should be used. Each member of the concreting team should be ex- perienced and, if necessary, rehearsed in his particular task. Thorough mixing is essential, and for site-mixed con- crete, the mixers should be located as close as possible to the formwork, to limit the possibility of segregation dur- ing transport.
Segregation of the mix should be prevented by ensuring that it is not subjected to excessive vibration, not allowed to fall freely down inclined slopes, or be obstructed during its fall.
The rate of placement of concrete strongly influences the pressure exert- ed by the concrete on the formwork. It therefore affects the occurrence of blow-holes and, with the use of ab- sorbent linings, influences the uni- formity of colour. There is evidence1 that pressures generated by rates of placement exceeding 6ft. per hour are sufficient to remove most air bubbles from vibrated concrete. This rate may easily be achieved in columns, but in walls and similar sections, a large mix- er may be required, or the vertical joints may have to be more closely spaced.
Whatever the rate of placement of concrete, it should be as uniform as possible, and in any lift the concrete should be placed as nearly continuous- ly as practicable. The volume of any
Far East BUILDER, June 1970
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