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 developement 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 normalised 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.


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). (Download 8 Mb)