fire protection of concrete structures exposed to fast fires

Fourth International Symposium on Tunnel Safety and Security, Frankfurt am Main, Germany, March 17-19, 2010
The assessment of the residual mechanical performance of concrete slabs has been carried out on
drilled cores. The cores were drilled at different locations of the slabs several months after the fire test
(see Figure 9). The first operation consisted in measuring the height of the core, allowing us to
determine the concrete spalling depth (operation in Figure 9). Small disks were then cut from the
cores; the distance "x" from the interface between concrete and thermal protection board to the upper
side of the cut disk was noted (operation in Figure 9). At last, a compression test was carried out in
a direction parallel to the main heat flow in the slab (operation in Figure 9). The obtained
compressive strength was assumed to be representative of the residual mechanical performance of the
concrete at a distance "x" from the interface.
Figure 9. Schematic representation of the testing procedure for the assessment of the residual compressive
strength on drilled cores from fire tested slabs (blue zones correspond to ejected concrete).
The different spalling depths that were measured on drilled cores (for a same slab, we present the
minimal, maximal and mean measured spalling depths) are presented in Figure 10. The results are
presented for different thermal protection board thicknesses. The main observations are as following:
- by comparing the measurements of concrete spalling that were done just after the fire tests
(red curves in Figure 10) and the measurements of concrete spalling that were done on drilled
cores (i.e. several month after fire test), we can observe that the departure of cover concrete
can continue even after a long period following a fire. This phenomenon is particularly true in
the case of the silico-calcareous concrete. This "post-fire" concrete departure can be explained
by different mechanisms (inverse thermal gradient during cooling down that induces tensile
stresses in the cover concrete zones, "re-hydration" or "re-humidification" of the concrete that
can result in swelling and falling of the material…) and appears as a main factor to take into
account for the post-fire assessing of a concrete structure. Indeed, this cover concrete
departure can play a role for the residual mechanical performance of the structure (decrease of
the resistive cross-section) as well for its durability (decrease of reinforcement protection
regarding to aggressive factors),
- the measurements confirm the tendencies observed just after the fire test (see § 4.1): the
absence, or the under sizing of the thermal protection (from 8 to 15 mm in our study),
involves and important spalling of cover concrete. Moreover, the measurements confirm the
fact that in the case of an under sized thermal protection, concrete spalling can be deeper than
for a slab without protection,
- a thermal protection that is correctly designed (20 and 25 mm in our study) allows avoiding
concrete spalling, even after a long period after the fire.
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