ORIGINAL ARTICLE
Moisture transport properties of concrete with SCMs
– descriptions, test methods and some applications
Lars-Olof Nilsson1
Correspondence
Abstract
Prof. em. Lars-Olof Nilsson
Lund University, P O Box 118,
SE-221 00 Lund and
Moistenginst AB
Fru Alstad Byav. 396-0
SE-231 96 Trelleborg
Email:
Moisture transport properties of concrete are rarely determined, simply because it
is difficult and time-consuming. Most studies in literature are limited to cement
paste and mortar. Translation of these properties to concrete are not straight-forward. Recently, however, a few measurements are done, also on concretes with
supplementary cementitious materials.
An overview is given on various ways of theoretically describing the moisture
transport properties of concrete, with different “moisture transport coefficients” and
their interrelations. Methods of determining those properties are shown and their
advantages and drawbacks are discussed. Available data in literature are summarized and compared. A recently developed test method, the tin-can method, is described. New results are shown for concretes with SCMs. The method’s possibilities
to be used for short-term tests of aging concrete are discussed.
A few important applications where the moisture transport properties are decisive
are presented: applying moisture-sensitive flooring materials on concrete floor slabs
and carbonation-initiated reinforcement corrosion under varying climatic conditions.
[email protected]
[email protected]
1
Laboratory of Building Materials,
Lund University, Lund, Sweden
and
Moistenginst AB, Trelleborg, Sweden
Keywords
Moisture transport, concrete, SCM, test methods, flooring, carbonation
1
Introduction
Moisture transport properties of concrete are important for
being able to predict the moisture conditions in concrete
in many applications and the moisture transport properties
are frequently decisive for the moisture levels to be expected. Direct measurements of moisture transport properties of concrete are rare, especially for concretes with
SCM. We have no real consensus on how to describe the
moisture transport properties and we have no generally
accepted method to measure them. More than 50 years
ago, an empirical model for the diffusivity of concrete was
presented [1][2], see Figure 1.
This diffusivity was adopted by the fib Model Code and still
is the preferred description of the moisture transport properties of concrete [3]. The same diffusivity was used in
2018 [4] partly verified by >40 year old drying profiles [5]
without considering the simultaneous chemical binding of
water in hardening concrete. Many other researchers, e.g.
[5][6][7][8], have later shown very different moisture dependencies of the moisture diffusivity. Consequently,
there is an urgent need to clarify how these properties
should be determined and expressed. A recently started
RILEM Technical committee TC-MMS [9] may develop
methods and means to reach a better consensus.
Figure 1 Proposed moisture dependency 1971 of the moisture diffusivity Dw for concrete [1] with three parameters.
Here a brief over-view of the significance of moisture for
the performance of concrete is given. Then a few relevant
test methods are described and discussed. Theoretical
ways of describing the test results as “moisture transport
coefficients” are presented and the relationships between
different coefficients are given. Finally, measured
transport properties for concrete without and with SCMs
are presented, compared and discussed.
Prediction of these moisture variations are complicated.
The thickness of the carbonated part that has different
moisture transport properties will increase with time. This
must be considered in the moisture calculation, see e.g.,
the CEB micro model [10].
2
Predicting the moisture conditions close to the reinforcement, that is decisive for the corrosion rate, are to some
extent easier. These conditions are relevant first when carbonation has reached the reinforcement, i.e., the whole
cover has the same moisture transport properties. The
properties beyond the reinforcement maybe somewhat
different but will have a minor effect on the conditions at
the steel surface.
The significance of moisture
Moisture is important, and frequently decisive, for the performance of concrete in several applications. Examples
are:
- Diffusion of gases, like CO2 in carbonation and oxygen
to the cathode in a corrosion process; water-filled parts
of the pore system will block transport of gases.
- Diffusion of ions is limited if the pore system is not fully
saturated; the question is what part of the pore water
that can act as a solvent for the ions.
- Convection of ions, like chloride moving in and out
through a concrete cover at drying and wetting and following moisture flow in tunnel walls exposed to sea water.
- Chemical reactions will not necessarily be complete if
concrete is too dry, e.g. the carbonation reaction will
leave some unreacted CaO at a humidity below some
80 % RH.
- The corrosion process requires electrical conduction
between the anode and cathode; this is certainly moisture dependent.
2.1
Data on the moisture transport properties of carbonated
concrete is essentially lacking. Consequently, the moisture
variations shown in figure 2 are presently not predictable.
2.2
Example 2: Redistribution of excess moisture
beneath a flooring on a concrete floor slab
An important application for moisture transport calculations is the treatment of excess moisture after drying of a
concrete floor slab before applying a moisture sensitive
flooring material like a vinyl flooring [11]. The extent of
drying is quantified by an RH-measurement at a certain
depth x1, cf. figure 3, with an expected RH-profile as the
black dashed curve.
Example 1: Carbonation of the concrete cover
of the reinforcement in façades.
The moisture conditions beneath a concrete surface will
vary with time and depth depending on the surrounding
climatic conditions and the moisture transport properties
of the concrete, cf. figure 2.
Figure 3 Redistribution of excess moisture in a concrete floor slab beneath a vinyl flooring [11].
After applying the flooring material, that has a significant
resistance Z to moisture transport, the excess moisture
will redistribute.
Figure 2 Moisture variations in the concrete cover of the reinforcement.
Parts of the year the concrete surface is wet, and CO2diffusion is partly blocked. During the drier parts of the
year the pore system is partly emptied, and CO2-diffusion
is more rapid. These conditions are different from time to
time and different at different depths. The parts that are
mostly relevant for the progress of carbonation are the already carbonated parts. These parts have different moisture transport properties than the still uncarbonated parts
at larger depth.
The maximum RH underneath the flooring after redistribution will mainly depend on the relationship between the
moisture transport properties of the concrete and the resistance to moisture flow of the flooring material.
3
Test methods for moisture transport properties
To be able to quantify moisture transport properties we
have to be able to measure them, i.e. we need a test
method for this quantification. Different test methods will
give those properties in different ways. Here, available
methods are briefly described, and the test results are
used to theoretically describe moisture transport properties. The relationship between different ways of describing
moisture transport coefficients are pointed out.
The main principles of quantifying moisture transport
properties are two:
A.
Measuring the steady-state flux of moisture and
determining a moisture gradient.
B.
Measuring the non-steady-state weight changes
and curve-fit it to a theoretical model for moisture
changes.
A few other principles are available, for instance where the
moisture profile is measured at two occasions. From the
profiles the moisture flux and the moisture gradient at different depths are evaluated. Then the moisture transport
coefficients at different depths, i.e. at different moisture
levels, can be determined. This method gives, however, a
large scatter for concrete because of the aggregate making it difficult to measure moisture content profiles accurately.
3.1
Measuring the steady-state flux of moisture
and determining a moisture gradient
3.1.1
The cup method
The most simple and straight-forward method to determine moisture transport properties of materials is the so
called “cup method”, which is standardized as ISO
12572:2016 [12]. A slice of the material is put as a lid at
a “cup”, that can be of glass, aluminum etc., and the edges
are sealed to ensure one-dimensional moisture flow, see
figure 4.
conditions and the steady-state flux J [kg/(m2s)] can be
calculated. From this flux, the thickness d of the slice and
the two relative humidities inside and outside the cup, the
test results as the steady-state flux J can be expressed in
different ways. A general description would be
"!
𝐽 = −𝑘! ⋅ "# = 𝑘! ⋅
!! $!"
%
𝑜𝑟 𝐽 =
!! $!"
&#
(1)
where y is a moisture transport “potential”, ky is a moisture transport coefficient and Zy is a resistance to moisture
flow for this potential. Different potentials are chosen by
different researchers, testing institutes and even countries, such as vapour pressure p, vapour content v, moisture content w, relative humidity j, and the coefficients
vary of course with the selected potential.
One elegant description is the use of the “fundamental potential” Y, where the transport coefficient becomes zero
Ψ(𝑅𝐻$ , 𝑅𝐻% ) = 𝐽(𝑅𝐻$ , 𝑅𝐻% ) ∙ 𝑑
(2)
The use of the fundamental potential, the measured data
(=the measured flux) is used directly without any derivation that adds a lot of scatter.
By performing tests with the cup method in a series of RH
intervals, the moisture dependency of the transport coefficient can be determined in the whole RH-scale. A boundary condition can even be direct contact with water, which
is an “upside-down cup” to eliminate the resistance to
moisture flow of the air gap between the solution inside
the cup and the resistances at the surfaces.
For concrete the thickness of the slice cannot be too small,
because of the aggregate. A thickness of at least two-three
times the maximum size of the aggregate is recommended. This means that the required testing time is long
for concrete with low w/b; months or even years may be
required to reach the steady-state flux.
A large study [13] of the moisture transport properties of
concrete with silica fume and fly ash was done with upsidedown cups with RH = 65 % outside the cups. The results
were shown as a moisture transport coefficient d [m2/s]
with the vapour content v as the transport potential. The
results as the average d in the RH-interval 65-100 % are
shown in figure 5 as a function of the large porosity, including the air content.
Figure 4 The cup method and the RH profile during a test.
The moisture flux is derived from a difference in RH between inside the cup, usually created by a saturated salt
solution, and outside the cup, in a climate room or a climate chamber. The flux out of, or into the cup, is measured by determining the weight changes of the cup with
time.
Eventually, the weight changes will reach steady-state
The larger pores and the air voids can be seen as “shortcuts” for moisture transport. The results can be understood as if the main resistance to moisture flow is the cement gel with transport of adsorbed water as the decisive
transport process. The two concretes with a mix of silica
fume and fly ash have very slow moisture transport even
though the air content is around 6 %.
3.1.2
The van der Kooi method
Another, rarely used, steady-state flux method to determine the moisture transport properties could be called the
van der Kooi method after an excellent, early study [15]
in 1971 on autoclaved aerated concrete. The principle is
shown in figure 7.
Figure 5 The average moisture transport coefficient d(65,100) for a
series of concretes with silica fume and fly ash; data from [13].
A recent study [14] using the cup method of moisture
transport properties of cementitious materials with SCM
was performed on mortars in a series of RH-intervals always with 33 % RH outside the cups. First, the fundamental potential Y(33%, RH) was evaluated and data points
with their scatter were fitted to a simple equation. For
most of the mortars this equation was a straight line; the
scatter could not motivate another relationship. Finally,
the derivatives of the fundamental potential with respect
to the vapor content were evaluated. This procedure gives
the moisture transport coefficient d with the vapour content v as the transport potential.
"'
𝐽 = −𝛿(𝑅𝐻) ⋅ "#
(3)
The results from [14] are shown in figure 6.
Figure 7 The van der Kooi method for determining moisture
transport properties, with a steady-state flux and a moisture profile.
A thick specimen is exposed to wet conditions on one side
and dry conditions on the other. The one-dimensional flux
J through the specimen is measured. Once steady-state
flux is reached, for concrete after a very long time, a moisture profile through the specimen is determined. From the
gradients of this profile, at different depths and moisture
levels, the moisture transport coefficient and its moisture
dependency can be evaluated according to equation (1).
In the original study by van der Kooi the moisture profile
was measured as moisture contents on small samples of
autoclaved, aerated concrete from different depths in the
specimen. Consequently, van der Kooi’s moisture
transport potential was the moisture content w and he determined the moisture diffusivity Dw, cf. equation (4).
𝐽 = −𝐷( (𝑤) ⋅
"(
"#
(4)
A long-term study on concrete with the van der Kooi
method was published in 1993 [16] after waiting for
steady-state flux through thick concrete specimens for five
years. In this case the moisture profiles were determined
as RH in small holes at different depths.
From the steady-state fluxes and the gradients of the RHprofiles at different depths the moisture transport coefficients d(RH) could be determined in the whole RH range,
see figure 8.
Figure 6 The moisture dependency of the moisture transport coefficient d(RH) for a series of cement mortars with SCM; data from [14].
Note that five of the eight mortars have almost no measurable moisture dependency at all. Note also that all mortars, independent of type of SCM, have almost the same
moisture transport coefficient, d = 0.05-0.07×10-6 m2/s for
w/b =0.38 and d = 0.08-0.11×10-6 m2/s for w/b=0.53, at
RH below 70%.
conditions are created, preferably an even distribution of
moisture content and RH throughout a specimen. One-dimensional conditions are ensured by sealing all surfaces
but one or two. The specimen is then exposed to a known,
constant, climate and the average moisture content
changes are determined by measuring weight changes. Finally, the measured weight changes are curve-fitted to average moisture contents with time predicted by a theoretical model.
The theory is based on solving the mass balance equation,
neglecting chemical binding of water, with equation (4) as
the flux equation, see equation [5].
"(
")
"*
= − "#
𝑜𝑟
"(
")
"
= "# 𝐷(
"(
"#
(5)
General analytical solutions, or numerical solutions, to
equation [5] can be expressed as normalised average
moisture contents Um(F0), where
𝑈! =
"! ($)&""
"# &""
(6)
and
𝐹' =
($ ∙$
*%
(7)
where
wm(t) is the average moisture content at time t [kg/m3]
w0 is the initial moisture content [kg/m3]
w¥ is the moisture content at equilibrium [kg/m3]
F0
is the Fourier number [-]
Dw is the moisture diffusivity [m2/s] in equation (4)
L
is the “equivalent thickness” [m], half the thickness of a specimen drying two ways.
The analytical solution of (5) for a drying process between
Um = 1 and Um = 0 for a constant Dw is shown in figure 9
(blue curve) in a logarithmic timescale.
Figure 8 Moisture transport coefficient d(RH) for OPC-concretes with
w/c between 0.4 and 0.8. The figure at the bottom is a magnified portion of the bottom of the top figure. Data from [16].
There is a tremendously large moisture dependency of the
moisture transport coefficient for w/c = 0.6-0.8 and very
small for w/c = 0.4. The transport coefficient is almost the
same for all w/c up to some 70 % RH. The RH is not 100
%, even though the bottom of the specimens is standing
in water, because of the alkalis in the pores. The top RH
for w/c is 95-97 %.
3.2
Measuring
changes
3.2.1
Inverse analysis
the
non-steady-state
weight
A common, non-steady state method to determine moisture transport properties is the so called “inverse analysis”, see e.g. [5][6][17][18], where weight changes of
small specimens are followed with time. First, known initial
Figure 9 Example of experimental data (thin red curve with yellow
dots) fitted at Um=0.5 to the analytical solution to the mass balance
equation (5) for a constant Dw (thick blue curve) [19].
In figure 9 an example [19] of curve-fitting experimental
data on weight changes to the analytical solution is shown.
The curve-fitting is done in one point and from this fitting
a value of a constant diffusivity Dw can be derived. The fit
is, however, not very good and it is obvious that this experimental data does not support a constant diffusivity.
Now, new numerical solutions must be calculated by varying the moisture dependency of the diffusivity Dw(w).
This is not an easy task.
A better approach is to make experiments in a series of
narrow RH-intervals [6] and assume that the diffusivity is
constant in each interval. In such a way a series of moisture dependent average diffusivities are obtained.
The method of inverse analysis has several difficulties. The
main experimental problem is to obtain an even distribution of moisture content and RH throughout the specimen.
It is very time-consuming to reach a steady-state weight
in a certain climate by simply drying a wet specimen, or
humidify a dry specimen, in a controlled climate, especially
for concrete with large aggregate, i.e. with thick specimens. It is not good enough to dry a specimen to an intended weight, seal it and wait for the moisture to redistribute. In such a case the moisture content at the surfaces
will follow scanning curves to an equilibrium-RH and the
moisture content profile will not be even.
3.2.2
The tin-can method
A method developed to handle the main problem with the
inverse analysis is the “tin-can method” [20]. Here the
significant self-desiccation of low-w/b concrete is utilized
to achieve an even, initial distribution of moisture content,
and of RH, throughout the specimen (concrete cast in a
tin-can). After sealed curing for a certain period of time
the lid of the can is removed and drying starts. The drying
process is followed for a couple of weeks as weight
changes over time. The weight changes are plotted versus
square root of time, see figure 11.
A second difficulty with the inverse analysis is that the actual moisture dependency of the diffusivity Dw(w) is unknown; it is to be determined. Different shapes of the
moisture dependency must be tried but several different
ones may very well give an acceptable curve-fit.
The expected moisture dependency of the diffusivity
Dw(RH) can be calculated from equations (3) and (4). If
the fluxes in the two equations are equal one obtains
𝐷" (𝑅𝐻) = 𝛿+ (𝑅𝐻) ⋅
+&
'$
'(
(8)
where vs is the vapour content of air at saturation
[kg/m3] and dw/dj is the slope of the sorption isotherm
w(RH) [kg/m3].
Consequently, the moisture dependency of the diffusivity
Dw depends on the shape of the sorption isotherm. If the
sorption isotherm would be fully linear, the diffusivity
would look like the curves in figures 5 and 7, with a different scale on the vertical axis. Now, a sorption isotherm is
far from linear. Where the sorption isotherm is more flat,
the diffusivity would have a peak and where the sorption
isotherm is steep the diffusivity would be small. An example is shown in figure 10 [19].
The diffusivity in figure 10 is obviously very different from
the one in figure 1. It is, however, for a concrete with
w/c=0.5; it should be very different for concrete with another w/c, because of the shape of the sorption isotherm.
Figure 11 An example of weight changes versus square-root of time
in the tin-can method for four fly ash concretes with w/c 0.32-0.50,
during one month; data from [19]
The analytical solution to the mass balance equation in the
initial part of a drying process can be obtained from Crank
[21] as the weight loss DM(t) versus square root of time t,
see equation (9).
-& ∙)
∆𝑀(𝑡) = 2(𝑤+ − 𝑤, )4
/
= 𝑘 ∙ √𝑡
(9)
where
w0 is the initial moisture content (kg/m3)
w¥ is the equilibrium moisture content (kg/m3)
k is a proportionality parameter (kg/(m2Ös)
The main experimental difficulty with the inverse analysis
is here overcome by utilizing the moisture content after
self-desiccation as the initial moisture content w0(x) at all
depths. Instead of measuring the initial and equilibrium
moisture contents, which is difficult, the RH0 after selfdesiccation and RH¥ of the drying climate are used. The
diffusivity Dw can then be evaluated from equation
(10)[20]
𝐷" =
Figure 10 Calculated moisture dependency of the moisture diffusivity
Dw(RH) for an OPC-concrete with w/c = 0.5 based on the moisture
transport coefficient from [16], cf. figure [7], and a sorption isotherm
from [5].
,- %
./
%
'$
(01# &01" )2
'(
(10)
where dw/dj (kg/m3) is the “moisture capacity”, the slope
of the desorption isotherm, in the RH-interval (RH0, RH¥).
The diffusivity Dw can easily be translated to a moisture
transport coefficient d with equation (8).
Carbonation will reduce weight loss due to drying to some
extent. This effect was quantified [20] and found to be
small. The effect should, however, be analysed in more
detail.
The method has been verified [20] against measurements
with the cup method on the very same concretes with different binders and w/b, cf. figure 12.
4
Discussion and conclusions
4.1
Theoretical description of moisture transport
From the test methods described above, a moisture
transport property or a moisture transport coefficient can
be quantified. From the measured steady state flux a
moisture transport coefficient is easily quantified from eg.
the moisture flux equation (1) or (2).
The moisture transport potential in these flux equations
should be a parameter that is most relevant for moisture
transport. This is not an easy selection, however. For pure
vapour diffusion in air-filled pores the vapour pressure p
or the vapour concentration/content v would be natural
transport potentials.
Figure 12 Results from the tin-can method (orange) compared with
results from the cup method (blue) for four mature fly ash concretes
with w/b 0.32-0.50, during one month; data from [19]
In figure 12 the results from the cup method are for the
RH-interval (60,85). The results from the tin-can method
are from an RH-interval between 50 % and the RH after
self-desiccation, i.e very much higher for the concrete with
w/b=0.50 (96 % RH). The concrete with w/b=0.32-0.40
were self-desiccated to 88-92 % RH. Since the moisture
dependency of the moisture transport coefficient for concretes with w/b smaller than 0.4 is expected to be very
limited, cf. figure 6, the results from the two methods are
comparable.
3.3
The effects of anomalous moisture changes
A number of studies have documented anomalous moisture changes in cementitious materials, eg. [22][23]
[24][25][26][27]. Changes of the moisture content do not
follow common physical laws as eg. Fick’s laws. The sorption isotherm does not fully describe the relationship between the moisture content and the state of water. There
seems to be a time-effect that is not yet fully understood.
Sorption isotherms determined on small samples are different from sorption isotherms from long-term tests on
larger samples. These effects are significant in the RHinterval 20-90 %.
The anomalous moisture changes should be of great importance when using inverse analysis to determine moisture transport properties. Weight changes in small RHintervals where the whole or part of the drying process is
followed should be significantly affected by anaomalous
behaviour.
Methods where the steady-state flux is determined or
short-term non-steady state tests like the tin-can method,
should be less influenced by this anomalous behaviour.
The time to reach steady state flow, however, could be
longer than expected due to this effect.
For liquid flow in water-filled pores the pore-water pressure Pw would be the prefered transport potential. For
transport of adsorbed water in the cement gel the relative
humidity j, that can be regarded as a measure of the state
of water, would be a reasonable transport potential.
Since we cannot separate the vapour flux, the liquid flux
and the flux of adsorbed water in a measurement, and we
do not know how to combine them, we have to select one
of the transport potentials. This is theoretically sound for
isothermal conditions where all three fluxes of moisture
have the same direction and the different transport potentials are possible to express in each other.
For non-isothermal moisture transport with temperature
gradients liquid flow, vapour flow and flow of adsorbed water may very well have different directions under certain
conditions [28]. Then, a description of the moisture flux
must have at least two terms with different transport potentials. The two, or more, transport coefficients will not
be easy to determine in cases like that.
The use of the diffusivity Dw as the moisture transport coefficient is natural when using non-steady state methods.
The translation between the diffusivity and the coefficients
in the flux equations includes the moisture capacity, the
slope of the sorption isotherm. This parameter is directly
proportional to the binder content in the concrete, mortar
or paste and must be considered in all comparisons of
properties. The moisture capacity is different for a desorption isotherm compared to an absorption isotherm. It is
also very different for scanning curves, generally much
smaller, between the desorption and the absorption isotherm.
4.2
Moisture dependency
From the old data on OPC-concretes with different w/c
[16] an extreme moisture dependency is evident for w/c
= 0.50 and larger. Below some 70 % RH, however, the
moisture transport coefficient d is the same for all concretes, with w/c=0.4 to 0.8. Similar results were obtained
for OPC mortars [14] with w/c 0.38 and 0.53 as well as for
a mortar with 5 % silica fume and w/b = 0.53.
For mortars with larger amounts of SCM (GGBFS) the d
had no moisture dependency at all. These mortars, with
two w/b:s, had the same d in the whole RH range as the
OPC mortars at RH < 70 % and two w/b:s.
Since the sorption isotherm is almost a straight line between 50 % RH and 90 % RH for low w/b:s, the “moisture
capacity”, the slope of the sorption isotherm, is almost
constant in that interval. With a constant d, this means
that the diffusivity Dw is also almost constant, far away
from the moisture dependency shown in figure 1 and 10.
4.3
Effects of SCM
Several concretes, with and without SCM, have recently
been tested [29], with the cup method and the tin-can
method, see the results in Table I, that includes the data
in figure 12.
amount of SCM, but then limited to RH below 70 % RH.
Above 70 % RH there is a large moisture dependency because of capillary pores making liquid moisture transport
possible for OPC-concrete and concrete with a small
amount of SCM.
To better understand these differences a comparison with
the sorption isotherms is useful. For the moisture
transport data for the cement mortars in figure 6 the desorption isotherms were determined, cf. figure 14.
Table I Average moisture transport coefficients d in various RHintervals for concretes with and without SCM and air entrainment; data
from [29]
The data in Table I is displayed in figure 13, except the
OPC concrete with significant air entrainment and w/c
0.50. It seems as if air entrainment affected the moisture
transport only for the OPC concrete with a large w/c. In
the other concretes the effect of large air contents was not
clearly visible. This is in contradiction with the results in
figure 5 but that data is for very wet conditions.
Figure 14 The desorption isotherms for the cement mortars in figure
6, with different binders with and without SCM, at an age of 8 months;
w/b=0.38 (top) and w/b=0.53 (bottom). Data from [14].
Figure 13 The moisture transport coefficients for the concretes in Table I, except the CEM I concrete with air entrainment and w/c = 0.50.
There is no clear effect of the different SCMs in figure 13;
a small effect of the w/b is visible but the scatter is large.
It seems as if mature concrete, and cement mortar, with
larger amounts of SCM has a moisture transport coefficient
d of some 0.4-1.5·10-7 m2/s, for all RH:s, somewhat depending on the w/b. The same transport properties are
also valid for OPC concrete and concrete with smaller
For the mortars with w/b=0.38 only the OPC-mortar has a
moisture dependency of the moisture transport coefficient,
cf. figure 6. All four mortars have almost the same volume
of pores in the region RH=70-100%. The OPC-mortar has
much less pores at RH<70. A possible explanation is that
the secondary binder reaction in the mortars with SCM creates enough CSH to fill the capillary pores and thus prevent capillary transport. The CSH-gel has a “broad” poresize distribution that explains the larger pores at RH=70100%.
For the mortars with w/b=0.53 the OPC-mortar has the
larges amount of pores in the region RH=70-100%. These
pores are, most probably, capillary pores that promote capillary suction, and a large moisture dependency.
Strangely enough, the two mortars with a small amount of
silica fume and larger amount of GGBFS have almost identical desorption isotherms. In spite of this, they have very
different moisture transport properties, with a significant
moisture dependency for the silica fume mortar. A possible
explanation is that the secondary reaction products from
the slag have filled most of the capillary pores in the same
way as for low w/b-mortar.
4.4
Age dependency
The moisture transport properties of concrete at younger
ages are difficult to determine. Most methods are longterm tests, waiting for steady-state flux or equilibrium
conditions. Because of that, we do not have any data on
moisture transport coefficients for hardening concrete.
Most data are for concrete of an age of a year or more.
The new method, however, the tin-can method gives the
possibilities of determing the transport properties at ages
of a month or so, since the method only requires a testing
time of a week or two. Measurements like that is going on.
The challenge is that the desorption isotherm is required
at the same age and needs a longer testing time to avoid
the problems with anomalous moisture in small samples.
For younger ages a certain moisture dependency of the
moisture transport coefficient should be expected, also for
concretes with low w/b and large amounts of SCM.
References
[1] Bazant, Z.P.; Najjar, L.J. (1971) Drying of concrete
as a nonlinear diffusion problem. Cement and Concrete Research. Vol. I, pp. 461-473.
[2] Bazant, Z.P.; Najjar, L.J. (1972) Nonlinear water diffusion in nonsaturated concrete. Matériaux et Constructions, vol.5, No 25.
[3] fib (2010) fib Model Code for Concrete Structures
2010, fib, Lausanne.
[4] Bazant, .P.; Jirásek, M. (2018) Creep and Hygrothermal Effects in Concrete Structures. Solid Mechanics and Its Applications. Vol.225, Springer,
https://doi.org/10.1007/978-94-024-1138-6.
[5] Nilsson, L.-O. (1980) Hygroscopic moisture in concrete – drying, measurements and related material
properties. Report TVBM-1003, Laboratory of Building Materials, Lund University, Lund.
[6] Baroghel-Bouny, V. (2007) Water vapour sorption
experiments on hardened cementitious materials.
Part II: Essential tool for assessment of transport
properties
and for durability prediction. Cement and Concrete
Research Vol. 37, pp. 438–454.
[7] Zhang, Z.; Thiery, M.; Baroghel-Bouny, V. (2016) Investigation of moisture transport properties of cementitious materials, Cement and Concrete Research. Vol. 89, pp. 257–268.
[8] Wu, Z.; Wong, H.S.; Buenfeld, N.R. (2017) Transport
properties of concrete after drying-wetting regimes
to elucidate the effects of moisture content, hysteresis and microcracking. Cement and Concrete Research 98 (2017) 136–154.
[9] Zhang, Z. (2023) (chairman) RILEM TC-MMS on Modelling and experimental validation of moisture state
in bulk cementitious materials and at the steel-concrete interface.
[10] Nilsson, L.-O.; Rodhe, M. (1997) The CTH carbonation model - a micro leel model for carbonation in a
natural climate. Appendix 2 in CEB Bulletin No 238,
Lausanne 1997.
[11] Nilsson, L.-O. (2021) Vinyl flooring on concrete – a
moisture problem only in the Nordic countries? 1st International Conference on Moisture in Buildings
(ICMB21), UCL London 28-29 June 2021.
[12] ISO (2022) Hygrothermal performance of building
materials and products — Determination of water vapour transmission properties — Cup method. ISO
12572:2016.
[13] Nilsson, L.-O. (2002) Long-term moisture transport
in high performance concrete. Materials and Structures/Matériaux et Constructions, Vol. 35, December
2002, pp. 641-649.
[14] Olsson, N.; Nilsson, L.-O.; Åhs, M.; Baroghel-Bouny,
V. (2018) Moisture transport and sorption in cement
based materials containing slag or silica fume. Cement and Concrete Research, 106 (2018) pp. 23–32.
[15] van der Kooi, J. (1971) Moisture transport in cellular
concrete roofs. PhD Thesis, Eindhoven University of
Technology, https://doi.org/10.6100/IR101248.
[16] Hedenblad, G. (1993) Water vapour permeability of
cement-based materials. Report TVBM-1014, Laboratory of Building Materials, Lund University, Lund,
http://www.byggnadsmaterial.lth.se/english/publications/#0.
[17] Pihlajavaara, S.E. (1963) Notes on the Drying of Concrete. VTT – The State Institute for Technical Research, Helsinki.
[18] Zhang, Z.; Thiery, M.; Baroghel-Bouny, V. (2016) Investigation of moisture transport properties of cementitious materials. Cement and Concrete Research
Vol. 89, November 2016, pages 257-268.
[19] Nilsson, L.-O. (2013) A comparison between different
methods to determine moisture transport properties
of cementitious materials. Report TVBM-3168, Laboratory of Building Materials, Lund University, Lund.
[20] Nilsson, L.-O.; Bergström, K. (2020) The tin can
method for determining moisture transport properties of concrete. NSB 2020, E3S Web of Conferences
172:14005, DOI: 10.1051/e3sconf/202017214005.
[21] Crank, J. (1973) The Mathematics of Diffusion. Second edition, Oxford University Press.
[22] Grudemo (1976) Determination of desorption isotherms of cement paste, Private communication,
published in [5].
[23] Taylor, S.C.; Hoff, W.D.; Wilson, M.A.; Green, K.M.
(1993) Anomalous water transport properties of Portland and blended cement-based materials. Journal
of Materials Science Letters 18 (1999) 1925– 1927.
[24] Hall, C. (2006) Anomalous diffusion in unsaturated
flow: Fact or fiction? Cement and Concrete Research
37 (2007) 378–385.
[25] Saeidpour, M.; Wadsö, L. (2015) Evidence for anomalous water vapor sorption kinetics in cement based
materials. Cement and Concrete Research 70 (2015)
60–66.
[26] de Burgh, J.M.; Foster, S.J. (2017) Influence of temperature on water vapour sorption isotherms. Cement and Concrete Research 92 (2017) 37–55.
[27] Rymes, J.; Maruyama, I.; Aili, A. (2021) Time-dependent water vapor desorption isotherm model of
hardened cement paste. Cement and Concrete Research
150.
DOI:
10.1016/j.cemconres.2021.106612.
[28] Nilsson, L.-O. (2013) Moisture transport under a temperature gradient – some old and new studies. Invited lecture at the conference on Understanding the
Fundamental Properties of Concrete celebrating Professor Erik J. Sellevold on his 75th birthday, 2013.
Trondheim, April 2013.
[29] Nilsson, L.-O.; Bergström, K.; Carlström, S.; Gunnarsson, M.; Tannfors, J. (2020) The tin-can method
– for moisture transport properties of concrete (in
Swedish). Bygg & Teknik 6/20, www.byggteknikforlaget.se .
