Moisture transport and clogging
of concrete under fire conditions
Introduction
Concrete is one of the most used building materials. In
the past decades it has evolved into a strong and durable
construction material. The problem of concrete is the behavior
in severe conditions that occur in a fire. The tensile strength
of concrete in a fire decreases due to dehydration and thermal
strain at high temperatures. Moreover, the moisture inside the
concrete will evaporate. This combined with the low permeability
of the High strength concrete will yield high pressures. The
pressure can be high enough to cause (violent) spalling. With
spalling pieces of concrete are chipped off, and in some cases
concrete members can even explode. A large fire occurred in the
Channel Tunnel in 1996. The thickness of the concrete lining was
greatly reduced (see also Figure 1). The tunnel would have
collapsed if the fire occurred in another part of the tunnel.
Figure 1: Two examples of fire
spalling of concrete: (left) the Mont Blanc tunnel and (right)
the Channel Tunnel
after the fire. The damage to
the concrete is clearly visible.
Model
A simple analytical model has been developed to describe the
spalling proces. In this 1D approximation of the problem x is taken
as the distance of the boiling front/drying front to the heated
surface. The heating will cause the temperature to rise everywhere
in the material. As a result moisture will start to boil and
evaporate and a drying front will develop in the material. The
drying front will recede into the material as moisture is boiling
there, and its speed will be governed by the vapour flux from the
drying front to the external surface. The pressure at the drying
front depends on the temperature at which the water boils.
Fig. 1. Schematic overview of
the model. The porous material is heated uniformly at its
surface (x=0). Due to the heating the temperature T(x,t) rises
(solid line), and a receding
drying front moves into the sample. This front is located at
x=u(t) and separates the material into a dry and a wet part. The
pressure (dotted line) will be
highest at the drying front.
Using this simple approach the speed of the boiling front can be
obtained by solving the following di fferential equation:
where k is the vapour permeability, Vm the molar volume of water,
μ the viscosity of water, R
the gas constant, T the temperature,
θ the porosity, the
moisture content, and p the pressure at the boiling front.
Using this simplified model calculation where made of the pressure
development in various materials. The results are given in
figure2a.As can be seen for concrete a high pressire build up. In
order to make some predictions regarding spalling behaviour, a
reference pressure of 50 bar (5 MPa) is chosen. This pressure is
chosen because it reflects the typical tensile strength of a HPC.
When this pressure is exceeded, possible cracking and explosive
spalling can occur. Fig. 2b shows the depth u* at which this
critical pressure is exceeded first, as a function of the heat flux
into the material for several permeabilities that are typical for
concrete. For high heat fluxes the spalling would occur close to the
surface, whereas lower heat fluxes would result in spalling deeper
into the material.
Figure
2:
(left) Calculated pressure at the drying front, for concrete (○)
and brick (◊). The dashed line marks atmospheric pressure (1
bar).
(right) Depth at which a
critical pressure of 50 bar is exceeded, as a function of the
heat flux into the material,
for various intrinsic
permeabilities: k = 10-17 m-2 (○), k = 10-18
m-2 (◊) and k = 10-19 m-2 (□).
Experimental setup
For the experiments in this study, a NMR scanner built used. This
instrument was especially designed for quantitative measurements of
moisture in porous materials with short transverse relaxation (T2)
times (unlike standard Magnetic Resonance Imaging (MRI), which is
generally used in a qualitative way). The machine makes use of the
magnet of a whole body MRI scanner (Gyroscan, Philips) which
operates at a main field of 1.5 Tesla. The setup is placed within
the scanner and a schematic diagram is given in Fig 3. The sample is
heated with a halogen lamp. The reflector of the lamp was gold
plated to ensure maximum reflection of the infra-red radiation. The
sample is placed inside the birdcage coil and is thermally insulated
using mineral wool in order to create a 1D experiment.
Figure 3: Schematic diagram of
the NMR setup. A 1.5 T MRI scanner provides the main magnetic
field B0. Two
Helmholtz coils provide a
gradient G of 86.5 mT m-1 in the x direction. A bird-cage coil
is used for both applying
RF pulses B1 and receiving the
NMR signal. A halogen lamp
Results
At the start of the experiment a concrete sample was equilibrilatd
with 97 % RH, corresponding to a free moisture content of 0.07 m3m-3.
The moisture and temperature profi les as measured are shown in Fig.
4.
Figure 4;
Moisture and temperature pro les of a concrete sample heated
from one side with 12 kWm2. At the start of the experiment the
sample was equilibrated
at 97 % RH which corresponds to
a moisture content of approximately 0.07 m3m-3. The pro les are shown every 8.5
minutes.
The temperature at the boiling front is indicated by 5 in the
temperature pro les. The positions of the thermocouples are
indicated in the temperature profi le.
The evolution of the profi les in time is indicated by arrows.
The moisture pro les are normalized with respect to the fi rst
moisture pro les (bold line). As the surface (0 mm) is heated the
temperature increases after 8.5 minutes to above 100 oC.
The free moisture at the surface will boil and we can observe a
boiling front developing at the surface. As the heating continues,
the boiling front moves further into the material. The measured
signal does not decrease to zero due to the chemically bound
moisture in the concrete. The plateau which is reached to the left
of the boiling front corresponds to a zero free moisture content. To
provide a complete overview, we have chosen to include the signal
originating from the chemically bound moisture. The temperature
pro les are not influenced by the boiling and dehydration processes.
In previous heating experiments on red-clay brick and calcium
silicate brick a clear inflection in the temperature pro les was
observed either caused by boiling and/or dehydration (van der
Heijden et al., 2007). In this experiment, the moisture content of
the concrete is relatively low and hence both the heat capacity, and
thermal conductivity of the sample are dominated by the concrete
parameters.
As the boiling front moves further into the sample we can observe a
signi ficant increase in moisture content to the right of the
boiling front. However, as the boiling front progresses the moisture
content does not increase above approximately 0.11 m3m-3. This moisture content
corresponds to the porosity of the concrete sample (indicated by the
dashed line). The moisture content in this region cannot increase
any further, because the material is saturated.
To our knowledge this is the first direct and quantitative proof of
so-called moisture clogging in concrete. A possible explanation for
the increase in moisture content can be found in the local vapour
pressure at the boiling front. In Fig. 4b) the temperature at the
boiling front is indicate. The temperature at the front increases
from approximately 160 oC to 195 oC. The
increase above 100 oC (atmospheric boiling point)
indicates that there is a local increased vapour pressure. The
vapour pressure can be calculated using the Clausius Clapeyron
equation for the saturated vapour pressure, and assuming a
thermodynamic equilibrium. The saturated vapour pressures
corresponding to these temperatures are approximately 0.7 and 1.4
MPa respectively, compared to 0.1 MPa atmospheric pressure. As a
result of these high vapour pressures the vapour released at the
boiling front is advected both towards the heated surface as well as
the back of the sample. A temperature gradient is present across the
sample, and as a result the saturated vapour pressure immediately to
the right of the boiling front will be lower. The vapour advected to
the back of the sample can condensate, resulting in an increase of
the free moisture content in the region to the right of the boiling
front. The vapour advected towards the surface is able to exit the
sample.
Model
In this experiment the
material is saturated due to the moisture clogging. Therefore, the
assumption that the vapour is only advected towards the surface
holds and the simple model can be used. The result is shown in Fig.
6b. The black dots indicate the measured front positions, and the
solid line is the model with a relative permeability of 1.10-16
m2.
Figure 5: a)
Front positions of the 100oC isotherm and
the moisture fronts of the four di fferent saturation
experiments. b) The boiling front and 100 oC isotherm
positions of the 97 % RH experiment
are plotted as a function of time. The front positions are
fi tted using Eq. 1.
We have included the model results for two other permeabilities:
1.10-15 and 1.10-17m2. It is clear
that the speed of the boiling front is very sensitive to the vapour
permeability. Therefore, the proposed model is a good method to
obtain thevapour permeability of a porous material.
- An extensive description can be found in:
L. Pel, G.H.A. van der Heijden and
H. Huinink, Spalling of concrete as studied by NMR, 2nd
International Symposium on Advances in Concrete through Science
and Engineering 11-13 September 2006, Quebec City, Canada
G.H.A. van der Heijden, R.M.W. van Bijnen, L. Pel and H.P.
Huinink, Moisture transport in heated concrete, as studied by NMR,
and its consequences for fire spalling,
Cement and Concrete Research 37,
894-901 (2007).
G.H.A. van der Heijden, R.M.W. van
Bijnen, L.Pel and H.P. Huinink, Moisture transport and pressure
build up at high temperature in concrete: a model of fire
spalling, Second International Conference on Porous Media and
its Applications in Science, Engineering and Industry, Kauai,
Hawaii, June 17-21, 2007
G.H.A. van der Heijden, L.Pel, H.P. Huinink and K.Kopinga.,
One-dimensional scanning of moisture in heated porous building
materials with NMR,
J. of
Magn. Reson 208
235-242 (2011)
GHA van der Heijden, NMR imaging of moisture inside heated porous
building materials, Eindhoven University of Technology (2011).
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