Cement & Concrete Composites ! 2 (1990) 19- 27
Hygral and Thermal Expansion/Shrinkage
Properties of Asbestos-Free Fibre Cement
S. A. S. Akers & M. Partl
Ametex AG, CH-8867 Niederurnen, Switzerland
(Received 17 February 1989; accepted 21 September 1989)
Abstract
This paper presents a study of basic properties of
fibre cement composites related to expansion and
shrinkage (thermal and hygraO. It is aimed at providing a fundamental understanding of the material
in order to describe its behaviour when exposed to
natural weathering conditions. The migration of
pore water and interrelated shrinkage stress of the
products have been studied using model environmental chamber experiments. It is pointed out that
movement of water within the product can be very
complex and that a product exposed to the
environment can experience complex stress concentrations depending on the density and/or pore
size distributions and rate of absorption or loss of
water. The build-up of stresses in aged products
occurred at a slower rate than for non-aged products. Also the density of the product plays a very
important role in the rate and build-up of stresses
and this property appears to override the pore size
distribution of certain composites.
Key words: Fibre cement composites, cellulose
fibres, synthetic fibres, thermal expansion,
shrinkage, moisture content, composite materials,
environmental tests, stresses, density, porosity,
curing.
INTRODUCTION
Cement-based composites exposed to natural
weathering absorb water and may also give up
water to their surroundings via a very complex
inherent pore structure. The extent of pore water
exchange and the speed at which it occurs depend
on numerous factors, for example:
-- the type of application (roofing or cladding);
the shape of the product;
-
-
-- the temperature of the surrounding environment;
the relative humidity of the surrounding
environment;
-- rain, snow and ice conditions;
-- wind velocity;
- - t h e porosity and pore structure of the
material;
-- the orientation of the structure (anisotropy);
-- the type of fibre in the product (cellulose or
synthetic);
-- the type of coating or impregnation applied
to the product (coated on both sides and/or
one side, edge coatings, etc.);
the age of the material.
-
-
-
-
It is evident from the factors mentioned above
that it would be an extremely difficult task (if not
impossible) to describe or predict the exchange of
water in the pores of a cement-based fibre composite during exposure to natural weathering. In
fact much has been reported in the literature ~-4
regarding porosity and water absorption of
cement pastes.
Although the transportation of pore water in a
fibre cement composite is complex, it is important
to describe in general terms the behaviour of a
material under certain climatic conditions as the
exchange of water in the pores results in dimensional changes (expansion or shrinkage) within the
product. These changes would most certainly
influence the behaviour of the product in its application. For example, a roofing slate is partly
covered on its upper surface by a neighbouring
slate and at the same time rests on slates below.
When exposed to the elements only a portion of
the upper surface of the slate is directly exposed
to the natural environment. The portion of the
slate in direct contact with the environment would
obviously react differently from the covered portion or reverse side of the same slate. It is easy to
19
Cement & Concrete Composites 0958-9465/90/$3.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in
Great Britain
20
S. A. S. Akers, M. Pard
realise the very complex manner in which a slate
could absorb or lose water. Further, because of
the exchange of water through the pores of the
slate, very complex stress fields are set up due to
the non-uniform expansion and shrinkage. These
stress patterns could result in complex deformations of the product and, in the extreme case, lead
to localised cracking of the product. It is for this
reason that the effect of water movement within
the product should be studied. In addition, expansion and shrinkage of the product may also occur
at varied temperatures which could be independent of the pore water. Temperature and humidity
variations occur simultaneously during natural
weathering and therefore both variables are to be
superimposed when describing the behaviour of
the material. This paper is concerned with fundamental studies aimed at a general understanding
of the basic material properties which may be
used for extrapolation purposes applied to the
product under 'real' conditions.
EXPERIMENTAL DETAILS
Products
Fibre cement products were manufactured on a
standard Hatschek machine used in the asbestos
cement industry. The products manufactured and
used for the studies described in this paper may
be categorised as follows.
Product A -
An autoclaved product (density
1510 kg/m a) containing 8% cellulose fibres, cement and silica
(5o:5o).
Product B
A normally cured (ambient t e m and humidity) highdensity (1870 kg/m 3) product
containing 2% synthetic fibres, 4%
cellulose fibres and cement.
Product C -- A normally cured (ambient temperature and humidity), mediumdensity (1500 kg/m 3) product of
similar mix formulation to product
B.
Product D -- A normally cured (ambient temperature and humidity), lowdensity (1230 kg/m 3) product of
similar mix formulation to product
B.
expansion/shrinkage properties in varied climatic
conditions. The environmental chamber used
consisted of standard equipment manufactured by
K6ttermann capable of providing the varied
climates required for the investigations described
in this paper.
In order to obtain an insight into stresses which
could be set up in a product during non-uniform
shrinkage or expansion in natural weathering,
model experiments were designed. Simple test
apparatus designed for measuring restrained
shrinkages was used (Fig. 1). The test comprised a
tensile specimen, 400 m m x 35 mmx 6 mm, fixed
between the jaws of a rigid steel frame with an
attached cell used to measure tensile forces on the
specimen during drying shrinkage. Shrinkage
stresses were measured on the specimens fixed in
the steel jaws of the restrained shrinkage frame.
'Actual' shrinkage was measured directly from an
adjacent test specimen suspended freely in the
chamber. The independent signals from the load
cell and extensometer were recorded simultaneously with respect to time and stored on an IBM
(PC-XT 286) computer. The stored data may be
plotted in the form of stress versus time, and/or
shrinkage versus time, curves.
Products were pre-conditioned by submerging
them in water for 48 h at 20°(2 prior to the
environmental chamber exposure. The specimens
were fixed in the jaws of the restrained shrinkage
equipment and exposed to an environment of
20°C at 95% RI-I for four days, after which the
relative humidity was changed to 50%. The temperature was kept constant at 20°C throughout
the exposure.
--
perature
Environmental chamber studies
The products were subjected to environmental
chamber experiments in order to monitor their
LOADCELL FOR
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Fig. I. Diagrammatic representation of equipment for
restrained shrinkagemeasurements.
Expansion and shrinkage of asbestos-free fibre cements
In order to study the influence of product age
on restrained shrinkage, extreme laboratorysimulated experiments were conducted. For this
particular study, products of similar mix formulation and density to Product C were used. The product had a history of five years' natural weathering
exposure in Switzerland. For comparative
purposes a non-aged equivalent was manufactured. The aim of the investigation was to
assess the response of aged (naturally weathered)
products to restrained shrinkage tests, and to
compare these with the non-aged equivalent. An
extreme laboratory exposure condition was
chosen in order to test the limits of the stress
build-up within aged and non-aged material. This
test consisted of submerging the products in water
for one week at 20"C and thereafter exposing
them to an environment of 60"C and 10% RH.
In an additional investigation with different test
equipment, the loss of water of the products was
determined. For this purpose a water-saturated
specimen (Product C) was suspended from a
cantilever beam with a built-in load cell. In this
way the loss of water from the pre-saturated
specimen could be measured when it was exposed
to varied climatic conditions. Climatic conditions
consisted of 95% RH over a period of time and
thereafter 50% RH. Temperature was kept constant at 20"C. The change in mass of the specimen
was continuously plotted as a function of time.
This value could be directly translated into
moisture content of the specimen using the 'wet'
and 'dry' mass values. Using the data obtained
from restrained shrinkage specimens, the
moisture content of the specimen could be plotted
as a function of time and/or stress and/or shrinkage.
Determination of the coefficient of capillary water
conductivity
In order to characterise varied materials according to rate of water absorption by the pores, the
coefficient of capillary water conductivity was
found to be a useful criterion. This can be determined by measuring the electrical conductivity, a
method recently developed and successfully
applied on concrete at the ETH Zurich. 5 The
application of this method for fibre cement products is demonstrated here on the products A, B,
C and D already described above.
The schematic representation given in Fig. 2
indicates several couples of silver electric contacts
10 mm wide which are attached on both sides of
the specimens at a distance of 10 mm from each
21
other and parallel to the imposed humidity front.
All sides of the specimens apart from the upper
and lower edges were coated with a sealing
varnish. The specimens were placed in a chamber
having a high humidity (95%) and a temperature
of 20+ 5°C. The uncoated lower edge of the
specimen was submerged about 2 mm in water.
The water penetrating from the lower edge causes
a temporarily delayed change of the electrical
conductivity L between the opposite electrodes.
This conductivity is expressed as a function of
t ' - ~given in the equation:
; l'=,~
f
t. dL
L~j
where r represents the edge of a rectangle (Fig. 3)
with length AL which is equal in area to the horizontal integral of the conductivity curve. In order
to calculate r for each pair of electrodes using the
above equation it is useful to approximate the
Fig. 2. Diagrammatic representation of equipment for
water penetration measurements.
L.
dL
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Fig. 3. Determinationof the characteristic pressing time •
from the electricalconductivitycurve.
22
S. A. S. Akers, M. Patti
10
experimental curve L(t') to the maximum value of
t
t
conductivity, i.e. in the range between to and tw,
using polynomial regression analysis. Hence the
coefficient of capillary water conductivity is
derived from the slope of the linear regression
curve in the diagram where the contact times are
plotted against the vertical position of the
electrodes.
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RESULTS AND DISCUSSION
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Influence of ambient relative humidity
Stress/time and shrinkage/time curves obtained
from restrained shrinkage and freely suspended
specimens are given in Figs 4 and 5 respectively.
The pre-condidoned ('wet') specimens were 99%
water-saturated before they were placed in the
environmental chamber. In a surrounding climate
of 20°C and 95% RH, the water contained in the
pores of the 'wet' products migrate into the surrounding environment and the product begins to
'dry out'. This results in 'drying' shrinkage which
may be measured on the freely suspended specimen using an extensometer. Simultaneously, an
associated strain may be measured on a specimen
cut from the same product.
Comparing the various products tested (Fig. 4),
it may be seen that the materials respond quite
differently. For example, the autoclaved product
(Product A) had a rapid stress build-up between
10 h and 20 h after which an equilibrium was
achieved whereas the stress build-up in Product B
remained increasingly progressive up to 72 h. The
latter product did not achieve equilibrium at 72 h
and was also at a lower stress level than Product
A. The build-up of stresses in Products C and D
was negligible at the start and only after 10 h and
30 h respectively could an increase in stress be
detected. Very low stress levels were achieved in
Product D even after 72 h. Changing the RH from
95% to 50% at constant temperature resulted in a
further drying out of the products and in turn in
more advanced shrinkage which led to increases
in stress. It is again apparent that the build-up of
stresses in Product A occurs at an increased rate
and to greater stress levels than in other products
tested.
The stress levels achieved by the various products throughout this experiment are well below
the ultimate 'dry' tensile strengths (Table 1) and,
in some products, are close to the 'wet' ultimate
tensile strength value. The build-up of stresses in
equivalent-density Products A and C does not
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Stress/time curves for tested products: (a) Product
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Expansion and shrinkage of asbestos-freefibre cements
23
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Table 1. Properties of composites tested
Product
Tensile strength"
(MPa)
'Wet'
A
B
C
D
9'2 + 2.1
9.0 ± 0.7
7.0 ± 0.6
3.8 ± 1'2
Strait; to failure
(%)
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0.04
Density
(kg/nr~)
'Dry'
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0.08 ± 0.01
0.07 ± 0'03
0.12 ± 0.03
1510
1870
1500
1230
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± 10
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"Tensile tests conducted on (a) 'Wet', water-saturated specimens submerged in a water bath for 48 h and (b) 'Dry', dried in an
oven at 80"C for 48 h and subsequently tested at room temperature.
occur at an equivalent rate. It would appear,
therefore, that the migration of the pore water of
the products studied is rather complex, and not
necessarily related to the density alone but also to
other material properties as well.
Influence of product age
Products exposed to extreme laboratorysimulated conditions achieved shrinkage stresses
exceeding the ultimate tensile strength of the products, thus resulting in failure of the protract
during the test. The results are given in Figs 6 and
7 for the non-aged and aged products respectively. The stress levels at failure for both aged and
non-aged products were similar; however, the
length of time to failure was significantly longer
for the aged product. Also, the shrinkage value at
ultimate failure for the aged product was significantly less. This is to be expected as the product becomes more rigid with age due to
carbonation of the matrix, changing its pore
structure. The slope of the curves confirms this.
The relevance of this type of test has been discussed earlier. Slates are exposed on the roof g~th
S. A. S. Akers, M. Partl
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Fig. 6. Shrinkage effects of non-aged products subjected to
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Fig. 7. Shrinkage effects of aged products subjected to
60°C and 10% RH.
a covered and uncovered (exposed)portion. The
internal localised stress concentrations set up in
the roofing slate during the drying out or wetting
of the roof during weathering are transient and
governed by the rate of water penetration as the
material expands or shrinks. This would have an
obvious influence on the intensity of the stress
concentrations built up within the sheet and in
turn could govern the propensity for cracking. As
the rate of absorption for aged products is slower
than that for the unaged equivalent, the stress concentrations between the covered and exposed
portions of the slate could be smaller for aged
products.
Influence of ambient relative humidity
mental conditions from 95% RH to 50% RH at
constant temperature resulted in a further loss of
water from 18% to 14% over a period of 30 h.
Comparing the data obtained from this change in
humidity with the previous conditions, a more
significant increase in stress ( = 4 N/mm 2) and
shrinkage strain (=1.6%) may be detected.
Further, this occurred for a less significant loss of
water (from 18% to 14%) compared with 26%
and 18% under the former environmental conditions. This would suggest that a considerable loss
of water is required in a product before significant
shrinkage may occur. In other words, this could
explain why some products only crack in extreme
climatic variations and not in relatively mild
climates.
From the curves given in Fig. 8, it may be seen
that within 30 h the water content of Product C
decreases from 26% to an equilibrium value of
18% under the climatic conditions of 95% RH
and 20°C. However, this considerable loss of
water leads to relatively small increases in stress
due to the relatively small shrinkage occurring
under these conditions. Changing the environ-
The results so far have dealt with water absorption or loss of water in all directions of the product. Fibre cement products manufactured on a
standard Hatschek machine, however, have an
anisotropic behaviour and the rate of exchange of
Coefficient of capillary water conductivity
Expansion and shrinkage of asbestos-freefibre cements
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water within the product could depend very significantly on the orientation of the product.
From a practical point of view, water penetration from the edges of a product exposed to
natural weathering is an important factor to consider, particularly for roofs situated in alpine
regions where damming up of water may take
place under certain snow and ice conditions. To
characterise the material behaviour in such situations, the coefficient of capillary water conductivity is appropriate.
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Fig. 9. Results of lateral water penetration tests for products (a) A, (b) B, (c) C, (d) D.
S.A.S. Akers, M. Patti
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Pore size distributions of Products A, B, C, and D.
Figure 9 shows the relation between the position of the electrodes and the contact time for
Products A, B, C and D, and in Table 1 the coefficients of capillary water conductivity are given. It
can be seen that for Product D, with the lowest
density, the lateral water penetration occurs at the
highest rate whereas the relatively dense Product
B shows the smallest rate of absorption.
It is interesting that although Product A (autoclaved) is significantly different in mix formulations from Product C, which has an equivalent
density, the absorption rates of both products are
very similar. The results given in Fig. 10 shows
that the pore size distribution (measured using
standard mercury porosimetry equipment) of
Product A is different from that of Product C.
Thus it would appear that the density of the
product is more important in influencing the rate
of absorption than the pore structure.
CONCLUDING REMARKS
1. Migration of the pore water of fibre cement
products is complex and not entirely related
to the density of the material; this has a significant influence on shrinkage properties.
2. Restrained shrinkage experiments have
indicated that although the stress levels at
failure between aged and non-aged products
were similar, the rate of build-up of these
stresses is much slower in aged products.
Also, less shrinkage occurred in aged products.
3. A study of the migration of pore water
within a product exposed to various
environments has indicated that initial
losses in pore water do not necessarily lead
to greater internal stresses; more important
are the rate at which the loss of water takes
Expansion and shrinkage o f asbestos-free fibre cements
place and its range within the product.
4. Density and porosity are related; however,
for products of equivalent density having
differing pore size distributions, the rate of
absorption was similar. This would suggest
that the density of the products, rather than
the pore size distribution, dictates the
absorption rate.
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