Section 3 for Surface Finish and Colour Control, and Section 5 covering the Evaluation of Completed Work and Repairs, are the two important chapters for precast concrete in the Standard.

The class of surface finish specified for a project can range from Class 1 (the highest) to Class 5 (the lowest). A class of finish does not cover any surface that is either a hand finish, a power float finish, or is finished by any other means.

The vast majority of projects using precast elements will generally specify a Class 2 off form finish that will require no further treatment. However, Class 3 can be acceptable in some instances. If a class of finish is not specified in the project documentation then a Class 3 finish will be provided.

Class 1 is only used in rare applications such as single concrete pours on monumental buildings and shall not be specified for whole elevations. Table 3.2.1 in the Standard provides more information.

Colour

One of the most misunderstood aspects of precast concrete that can cause heartache to the specifier is the finished seen colour of precast concrete. For concrete that is grey in colour the specifier can control colour either within an individual element or over many units by using Clause 3.5.3 from the Standard.The suffix C should be used when colour control to AS3610 is required and can only be imposed on Class 1, Class 2 and Class 3.

An acceptable tonal colour range with precast must always be established before any precast manufacture commences. Table 3.5.3 in the Standard is for grey concrete but this concept can be adopted for concrete of any colour that is enriched by pigment to determine and record an acceptable tonal range.

Class 1 – 4 tones minimum range Class 2 – 5 tones minimum range Class 3 – 6 tones minimum range

Australian Standard AS3610.1-2010 Formwork for Concrete provides information on how to specify off form precast concrete that also includes acceptable tolerances.

Off Form FinishFACT SHEET

HANSONPRECAST.COM.AU (02) 9627 2666

Class 2C grey concrete. Sydney Adventist Hospital – Wahroonga, NSW.

Class 2 black oxide concrete + mould liner. Shell Cove - Shellharbour, NSW

KEY POINTS

• Surface finishes range from Class 1 (the highest) to Class 5 (the lowest) for all off form concrete, with Class 2 most commonly specified for precast concrete.

• The suffix C to be used when colour control is important with Figure A4 from AS3610 providing the range for selection.

• Samples should always be produced when colour control is required to show the acceptable tonal range.

• The specifier should be aware that casting table minor grinding nicks and scratches are always present in the manufacture of precast elements.

Quality of Surface Finish

The quality of the casting table, usually steel, is important in manufacturing off form precast concrete. In Table 3.3.2 the Standard provides acceptable tolerances, however, it does not acknowledge the minor grinding nicks or scratches that are always present. The specifier should make themselves aware of these minor imperfections when considering precast concrete for a project.

Vertical Faces

3D elements (e.g. L-shaped retaining walls) with an off form finish may be difficult to manufacture with a uniform finish. These elements should be discussed before manufacture to determine what is practical.

Tolerances

Table 3.3.3 in the Standard covers the allowable tolerances for an individual element. Chapter 4 of the Precast Concrete Handbook provides further information on reinforcement and building tolerances.

Painting

If an off form finish is to be painted then this needs to be advised by the specifier, prior to manufacture, to ensure compatibility of the mould release agent with the paint.

Storage

Precast elements maybe stored in an outside area and will be subject to weathering which is a natural process. Protection to stop this natural weathering is not usually provided.

Off Form FinishFACT SHEET

Class 2 grey concrete - Sydney Buses Leichhardt Depot

Class 2 grey concrete + mould liner. Windsor Rd – Rouse Hill, NSW

Class 2 grey concrete. Abbott Road Bridge – Seven Hills, NSW

HANSONPRECAST.COM.AU (02) 9627 2666 VE

RS

ION

1