Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres CAMPBELL ACT
2612 Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
What are the potential consequences of my slab having been poured in the
rain?
The first thing to note about this,
is that pouring a slab when rain is forecast is a conscious decision to put the
quality of final product at risk. A
moderate chance of prolonged showers may pose a higher risk to the final
outcome than a high risk of intermittent light showers as the slab can be
protected for short intervals providing adequate covering and manpower is at
hand. We suggest that concreters read
their insurance policies carefully before making the final decision.
The risk with rain-affected slabs is
normally that the surface of the slab will collect too much water during the
finishing process and result in a relatively soft, weak surface that is prone
to dusting. The depth of this weak surface is often only 2 to 5mm but can be as
much as 10 mm in extreme cases.
This problem is extremely important
in industrial applications where surface wear is an important serviceability
consideration. A weak surface can also cause
problems in residential construction where direct stick parquetry or tiling can
delaminate the top 1 to 3 mm of the slabs surface. This problem is caused by differential
thermal, shrinkage or expansion stresses and can sometimes occur in a matter of
weeks rather than years.
The extent to which the surface of
the slab may be damaged during rain depends on
Intensity of the rain.
Timing of the rain relative to the initial and final setting of the
concrete.
Efforts made to remove excess water.
Efforts taken to protect the work.
Whether vibration or power toweling took place
The time
between pouring and initial setting of the concrete is generally around 2 to 3
hours and is the most critical in terms of ensuring a hard durable
surface. Every effort should be made to
remove any additional water from the surface before finishing of the slab by
rolling or dragging a hose over it. If
showers are intermittent it is sometimes possible to protect the work for a
period to enable finishing in dryer weather.
Under no circumstances should cement powder be used to soak up the
additional water.
More
superficial damage can be done in the following 4.5 to 5 hours between initial
and final set so protecting the surface before leaving the site is often a good
idea.
If a
builder wants to argue that the surface of the concrete is fit for purpose we
generally score it with a coal chisel and compare the mark with a hard concrete
surface. This might not sound very
scientific but there is no standard test for surface hardness. Old fashion Schmidt Hammers used to give some
indication if the problem was severe but over the last 10 years or so these
have more of an indication of the density of the top 100mm.
To achieve
high surface densities it is necessary to vibrate the concrete during
placing. Whilst this practice is
universally accepted throughout most sectors of the building industry,
residential builders often ignore this requirement as it is not expressly
required by AS2780 unless the site is class H or E or if the slab forms part of
the termite protection system. Further hardening of the surface is achieved
during the power trowelling operation and you can expect weaker surfaces where
this operation is not carried out.
How you fix
the problem depends on the severity of the damage and the likely traffic and or
coverings. For example with superficial
damage to a floor to be tiled we recommend water blasting at 3500 psi to remove
the weak laitance and treating with a concrete hardener such as Construction Chemicals
Concrete Hardener.
In similar
cases where linoleum is the finished surface we often recommend the same high
pressure cleaning followed by Adure K15 (self levelling floor underlayment)
over Ardion P51 (primer).
We cannot
responsibly recommend any treatment without first looking at the problem so for
a full structural engineering assessment in Canberra ACT or the surrounding
area call Mal Wilson from Advanced
Structural Designs on Ph 61612171 if you need a definite answer and full
specification.
PS If you
need advice on rain-affected coloured concrete please call and discuss, as
there are many more variables involved.