consideration must be given to the depth of penetration related to the size of the main aggregate and, tak- ing as a guide that at least two thirds of the main aggregate should remain embedded in the concrete mass, it is obvious that a depth of penetration of 1/16 in. to 1% in. from the shutter face for a 3% in. main aggregate would be desirable, for a 4 in. down aggregate in. to 3/16 in., with a much deeper penetration for the larger aggregates.
This method can be applied to full strength concrete provided the main aggregate is harder than the matrix. In the case of a soft aggregate, for ex- ample a limestone, it is possible that if the concrete has reached full strength the main aggregate would be abraded away at a similar rate to the fines.
With exposed aggregate finishes it is frequently desirable to leave an arris or to produce a patterned effect on the face and with the use of tem- plates the abrasive blasting technique can achieve these design effects easily and accurately.
In addition to the production of complex concrete shapes for which a glass-fibre mould would appear to be the most acceptable answer, it is fre- quently an advantage to have a deep relief or sculptured face to the con- crete and this can be achieved to very good effect by casting the concrete directly against a carved rigid expand- ed plastic block which will provide a satisfactory one-off casting, or, if a number of castings are required, the reinforced plastic can be applied directly to a sculptured polyurethane block enabling a considerable number of castings to be taken from this deep relief reinforced plastic mould.
Methacrylate Resins
Further improvements to exposed aggregate concrete, sculptured or fair- faced concrete can be obtained with the application of a sealing solution and here the methacrylate resins offer interesting possibilities, particularly in view of their outstanding weathering properties under all types of atmos- pheric conditions including marine and heavy industrial atmosphere.
Due to the water white nature of the clear resins, with the addition of selected dyes it is possible to provide a strong colour and still obtain suffi- cient transparency to allow the tex- ture of the substrate to be shown. With careful selection of dyes it is possible for these methacrylate lac- quers to have colours which are light fast and to obtain transparent opaque films.
or
Probably the most attractive use of concrete is in the Naturbetong (natural concrete) method of concrete con- struction (British Patent No. 788985) where the specially graded aggregate is packed into the formwork dry and a special cement grout is pumped through this aggregate, the final sur- face being finished by abrasive blast- ing. It is necessary to pay particular attention to the selection of aggregates in relation to colour, size and shape
Far East Architect & Builder December, 1967
Precast panels cast against plastic moulds (W. G. Mitcheli)
and also to the fine sand used in the cement grout.
The considerable experience gained in Norway can be taken as the cor- rect recommendation in this selection. In using this method it is possible for lift marks and joints to be completely eliminated, giving an outstanding finish to the face of the concrete.
The examples of this technique used in Oslo provide a quality of con- crete construction and final finish which would be difficult to criticise. The final effect of this surface finish- ing is such that it can be used for interior and exterior applications and provides a surface of pleasing appear- ance for a very considerable period of time. This system can undoubtedly fit satisfactorily into present methods of building construction and its results will blend in with the general environ- ment of old and modern buildings,
In the Cement and Concrete Association's journal the comment was made that
the sand blasting instrument in the hands of the right person has as many pos- sibilities as the pencil or graver in the hands of the artist". This com- ment was related to the method of finishing Naturbetong but it applies equally well to a wide range of other building materials where intricate carving is possible on such materials as brickwork, aerated concrete, timber and any other materials where the hardness of the surface is ap- preciably less than the abrasive
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material which is bombarding the surface at a very high velocity.
Further interesting effects can also be obtained by the etching of glass and
by the use of templates or specially prepared plastic-backed adhesive tapes. Intricate design effects can be obtained on glass, if necessary after the glass is in position presenting no limits to the size of glass sheet which is treated. If desirable these etched faces now represent an excellent surface for the bonding of other materials, for example the pigmented methacrylate resins, which would further provide interesting colour effects to the pat- terned areas.
Landscaping
Landscaping is now generally be- coming accepted as forming the linking element between buildings and the space around them and there is undoubtedly enormous scope in this field which probably provides the largest single contribution for im- proving the general environment in which we live.
With the increase in height of many buildings for offices or flats there are many cases where the space between these buildings consists of large paved areas which can be en- hanced by the choice of colour, tex- turing and patterning of the paving.
With the correct choice of aggre- gate in the paving, generally in con-
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