Architectural unpainted off-form finishes are just about the most difficult to manufacture and may be more expensive than sandblasted or other architectural finishes. This difficulty comes more from our customer's expectations than from the technical difficulties in complying with AS 3610-1995.
The basis for off-form surface finish documentation is often Australian Standard AS 3610-1995 Formwork for Concrete supported by AS 3610 Supplement 1 Blowhole and Colour Evaluation Charts and AS 3610 Supplement 2-1996 Formwork for Concrete, Commentary. These documents are frequently tempered by the use of various sample panels which are agreed upon between Hanson and our customers and their consultants. In any event Class 2 is the normal best standard that should be specified. Class 3 is adequate for many uses.
AS 3610 is a quality guidance document but no document could possibly eliminate the subjective component of surface finishes and colour control. Specifiers should read the Commentary (AS 3610 Supplement 2) to appreciate the concerns of the document's drafting committee.
In particular it should be noted that Class 1 finish should never be specified except for use "in very special features of buildings of a monumental nature." In particular AS 3610 points out that Class 1 finishes relate only to colour control within one panel - not from panel to panel.
What the codes cannot cover are all the exceptions. Different panels shapes and orientations when poured have a great influence over both the colour and number of blowholes in the finished product.
Prior to the introduction of AS 3610, the industry relied heavily upon the use of sample panels as the means of negotiating a contract and controlling production. This approach is still widely used by architects and precast manufacturers. Specifiers may have a requirement for, say, a Class 2 finish and still introduce the use of sample panels.
In the great majority of projects finish selection is a straightforward process where an initial selection is made from a 300 x 300 sample and then a larger scale unit is used to illustrate how this finish may vary on production panels, with various shapes and orientations of finished surfaces.
The great majority of problems come from unrealistic client expectations. It is really important to understand just what is possible with precast concrete, to set realistic criteria and then to monitor against that criteria.