INGENIEUR
1. On-site Inspection
As stated above, visual inspection of the concrete
surface is the primary on-site investigation
technique. Visual inspection consists of
documenting concrete spalling, loss of concrete
cover to reinforcements, colour changes, and
cracking. It can provide some indication of the
approximate temperature reached by certain
materials. Furthermore, by examining a cross-
section of the damaged concrete core, the
temperature-depth factor can also be deduced.
This is done by observing the colour change of the
aggregates and the extent of cracks.
It may be sufficient to take ‘soundings’ on
the damaged concrete to determine the degree
of deterioration. This is to obtain a qualitative
assessment of the surface hardness of the
concrete and an indication of the presence
of subsurface defects. The ‘ring’ sound of
concrete and the ‘dull thud’ of weak material
are readily distinguished, and this test can be
done successfully with a hammer and chisel.
An alternative to taking soundings is the drilling
resistance test, which uses a hammer drill to
determine the depth of weakened concrete.
As mentioned before, discolouration of
concrete can occur as a result of carbonation
during a fire. In addition, care needs to be taken
when investigating older concrete because
carbonation is commonly found in old buildings.
Carbonation depth can be identified by spraying
a freshly broken surface with a phenolphthalein
indicator. If the depth of visual discolouration is
beyond the layer shown by the phenolphthalein,
then it is clearly due to the effects of a fire.
If it coincides with the layer shown by the
phenolphthalein, then it may be due to ordinary
carbonation and not the fire. The boundary for
the pink/red zone may be taken as being on the
300°C temperature profile and hence the strength
loss and equivalent duration of the fire may be
determined.
2. Non-destructive Tests
Visual inspection, hammer tapping, coring and
breakouts are the principal on-site methods of fire
damage assessment. However, in certain situations
there may be benefits in supplementing the normal
on-site regime with some non-destructive tests
(NDT). The NDT methods that are commonly used
6
34
VOL
2018
VOL 76
55 OCTOBER-DECEMBER
JUNE 2013
for assessing the building’s concrete condition
are the rebound test (Schmidt hammer) and the
ultrasonic pulse velocity (UPV) test.
The rebound hammer measures the surface
hardness of the concrete by releasing a spring-
loaded plunger which impacts the concrete and
measures the rebound distance. It is a useful
test in a detailed investigation for assessing the
uniformity of an element at selected points, e.g.
coring. Furthermore, it can be used to estimate
strength if it is calibrated for the concrete being
tested; where the measurements of the damaged
and undamaged areas are taken to indicate the
area of where the surface strengths are relatively
lower compared with the undamaged ones. The
aim is to have a quick indication of the effect of
the fire on the impact hardness and thus indirectly
on the strength of concrete.
For evaluating concrete soundness, the UPV
test can be applied. The method is based on
determining the time for a vibration pulse at an
ultrasonic frequency to travel through the concrete
between two transducers. By knowing the direct
path length between the transducers, the pulse
velocity through the concrete is obtained. The
velocity of an ultrasonic sound in concrete is
related to the concrete quality, uniformity and
strength. Both the UPV values of the unaffected
and affected concrete can be compared to
estimate the severity of the damage.
3. Petrographic Examination
Petrography is a branch of petrology that focuses
on detailed descriptions of rocks. Petrographic
examination is a laboratory procedure that relies
highly on the visual inspection of the samples,
aided by microscopical equipment. It is a definitive
technique for determining the depth of fire
damage in concrete. It must be performed in the
laboratory by experienced concrete petrographers,
using optical microscopes. Concrete core or
lump samples are subjected to visual and low-
power microscopical examination. Following this,
samples are selected for thin-section preparation
and more detailed examination with a high-power
microscope. By using a microscope, one can
determine the composition of concrete, assess
its quality, and investigate causes and extent of
deterioration. The petrographic analysis is mainly
concerned with the damage and property change