Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres CAMPBELL ACT 2614 Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
Class A1 concrete finish
It is
firstly imperative that you understand that this type of formwork is normally
only specified for monumental surfaces of relatively small area. For just what
to expect and when it should be specified we suggest reading AS3610 and
commentary. We can fax you some extracts
if need be. The Cement and Concrete
Association also has some useful publications on the subject.
Some
information that you won’t find in the above literature but that you may find
useful is given below.
Rebates
It is
important to specify what materials shall be used to form rebates as standard
rebates used to be ripped from maple but are now often ripped from miranti or
other cheaper timber. The trouble with
some of these is that the surface left after ripping is often not smooth and
this has a deleterious effect on the final product. Consider specifying clear
strips of radiata pine but also check and approve all rebate timber prior to
being used in the forms.
Plastic
rebates leave a beautifully smooth finish but can cause serious problems where
you have high daily temperature variations. The thermal coefficient of plastic
can be as much as 20 times that of timber.
Nails are
generally used to hold the rebates in place and the specification may call for
these to be punched filled and sanded smooth.
Covering nail holes is also a little more difficult with plastic
rebates.
Curing
If you are
looking for colour control A, we do not recommend wetting intermittently or
covering with plastic or leaving the formwork on too long as all of these will
tend to lead to a colour variation.
Consider the use of a water based acrylic curing compound complying with
AS3799 such as Masterkure 404 but remember that some of these do leave a milky
film on the surface that can take a while to vanish especially if it is rolled
on rather than sprayed.
If your
concrete is coloured forget these water based acrylic and go for a solvent
based alternative such as Masterkure 402 and make sure you spay it. It does not quite meet the Australian
Standard for water retention but it will give you a superior finish.
If you are
doing coloured precast work we normally recommend using CCS same day sealer and
curing out of the sun and wind.
Bar
Chairs
Give
serious thought as to how you can keep the reinforcement in position without
the need for bar chairs and take extreme care during the placement and
vibration operations. Make the contactor
aware that you have a cover meter and will reject any elements found with
reinforcement cover outside the specification tolerances.
Formwork
For this
class of finish the formwork needs to be very stiff and often has a reduced
allowance for form tie holes or a detailed setout of the requirements. If you need to have the formwork specifically
designed or require an structural engineer’s certification in the Canberra ACT
just give Advanced Structural Designs a call on (02) 61612171.