Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres CAMPBELL ACT 2612 Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
Saw Cutting Joints in Slabs
You might think
that this is a relatively straightforward topic but the number of times it goes
wrong is beyond belief. Over the years
we have seen a number of pavements jack hammered up and redone because the saw
cuts were carried out too late and the client could not accept the random crack
pattern that resulted. The reason there
is a problem is normally because specification requirements are too loose or
poorly policed. The client is often told
that there was a problem with the mix design or that it was caused by high
shrink concrete, which is almost never the case.
The
contractor hired to do the saw cutting is normally paid by the lineal metre and
would prefer using a normal diamond tipped blade in normal working hours, which
is never what is required.
The concrete
may need to be cut as early as 4 hours after finishing to prevent shrinkage
cracks occurring. We have found
historically that 5 to 8 hours is normal in a Canberra summer and 8 to 15 in
winter. We normally specify that a small
test area be set aside for testing the saw before commencing the cutting so
that no work is damaged testing the timing.
Cutting should always be specified to be carried out with a “Green
Concrete” saw blade as this will eliminate fretting or tearing of the concrete
surface and allow the cutting to be carried out far earlier than with a normal
diamond blade. Check this on site, as
these blades cost twice as much to run as they wear more quickly than a normal
blade.
It is worth
noting that if you are saw cutting coloured paving you should specify a dry
cut. This again cost more money as it
again doubles blade wear but wet cutting the concrete when it is green has a
tendency to stain it.
You may
hear talk in the business of a “soft-cut” or “soff cut” saw that allows even earlier
cutting of the concrete. Our experience
with these saws is very disappointing.
While the results looked good at the time we were inevitable left with a
relatively weak edge that fretted over time.
The saw used on the project we were involved in was hand held and the
importer of the “Soff cut” saws says that they no longer import these saws due
to their poor performance. The importer
does however recommend the use of their larger saws which he suggests are being
used to great effect on a number of significant projects. Theselarger saws are naturally held steadier
during the cut and apparently have a guide that compresses the exposed edges
during cutting to reduce the tendency of the edge to tear. We have yet to see any definitive empirical
data supporting the use of these saws over normal good practice but we are as
ever open to comment and would be only to pleased to get some feedback from end
users.
When saw
cuts meet each other at acute angles drop the blade down deep and do not finish
short of the intersection or a small triangle will attach to the adjacent
slabs.
There are a
few tricks you can use to minimise the possibility and consequences of edge
fretting and if you have your structural engineering in Canberra carried out by
Advanced Structural designs we will make sure that the documents address any
potential problems.
For further
information on this problem in Canberra ACT and surrounding region call Mal
Wilson Ph (02) 61612171 he will be happy to help you out.