Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
How does
fire affect Concrete Structures?
When you first walk into the blackened remains of a fire ravaged
concrete framed building it can be difficult to see past the devastation and
look at what might be the framework of an entirely new building. The fact is that after a fire most concrete
buildings can have their structure repaired which can appreciably reduce the
costs and time lost in rebuilding.
We are experienced in assessing fires in the Canberra region and can
often quickly tell by the discolouration in the local aggregates what
temperatures the fires have reached at various depths in the concrete and what
the ramifications are for the long-term viability of the structure. The concrete is of course not the only
material that may have undergone change and we also assess the reinforcement or
prestressing in the concrete to establish any changes in yield stress or
effective prestress.
These effects can be quite variable as some of the steel may have been
rapidly cooled during the fire-fighting operation, which can result in loss of
ductility in certain areas.
Many other indicators such as the degree and type of spalling and
cracking need to be assessed in a systematic way before repair procedures are
established. A correlation between
surface hardness and underlying concrete strength may also prove a useful in
establishing the extent of the damage.
An experienced observer can often establish using burnt remains, an
indication of the maximum heat of the fire as well as the duration of the burn.
Obviously peak temperature is only a small part of a very complex
equation but to give some feel for what damage is being done as the temperature
at any point increases we have listed some effects below. Remember however that even when a fire with a
peak temperature 850 degrees on the face of the concrete it may do very little
damage if the duration is short.
Below 300 C
No appreciable damage done
300 to 500 C
Damage to concrete requires careful assessment (especially above
400C). Concrete will be weakened and
some loss of modulus will have occurred.
Prestressing will be down to 50% of its strength above 400C and cold
worked steel will be affected above 450C.
Above 500 C
At these temperature significant loss of strength occurs in the concrete
and the modulus of the concrete is significantly reduced. Hot rolled steel is
likely to recover its full yield strength even when heated to 600C but beyond
this some strength losses will occur.
The full storey is a long and involved one so if you do require advice
on a fire damaged structure in Canberra ACT or the surrounding region call Mal
Wilson from Advanced Structural Designs on 02 61612171.
Canberra Fires Jan 2003
(Update)
After the recent Canberra bushfires a number of people rang to ask what
damage has been done to their driveway slabs or house slabs and could they be
reused.
Generally speaking slabs on ground don’t fair all that badly due to the
fact that the maximum temperatures are above rather than below the fuel source
and if there is damage it is often confined to the top 3 or 4 mm of the
slab. If there is any damaged concrete
around from edge spalling we have a look at the top few mm for the colour
changes that often occur with some of local (iron rich) aggregates.
We suggest cleaning the slab with a heated high pressure water jet (over
3500 PSI) and see how it scrubs up. If
it is almost perfect you can use some Floorclean by MBT which should lift the
remainder with 1 or 2 applications.
Floorclean is $132 plus tax for a 20 litre drum.
If you think you have lost some surface hardness we can confirm your
suspicion with a Schmitt Hammer or you can go straight to the next step and
treat with a surface hardener which is around $20/m^2.
If you want a Rolls Royce solution (in terms of abrasion resistance) you
can get a spray on application like Adtex for around $30/m^2 that they used on
Magnet Mart in Gungahlin. This may need
a non slip additive though if you are doing it in an exposed driveway.
If you have existing shrinkage cracks they will reflect through whatever
you do.