International Conference on Durability of HPC and Final Workshop of CONLIFE
PROPOSAL OF THE METHOD FOR ASSESSMENT OF
USABLE DURABILITY OF COMPOSITE MATERIALS FROM
THE RESULTS OF ACCELERATED TESTS
J. Knězek1, S. Modrý2
1 Research Institute of Building Materials, Czech Republic
2 Czech Technical University - Klokner Institute, Czech Republic
Abstract
Presently, the standard procedures make possible only an assessment of common
climatic influences of the winter and summer period separately and moreover specific
climate characteristics of the given area are not taken into account.
However, durability of a new cement composite materials can be significantly influenced
by both cyclic temperature changing in the winter period and by changing rain and
sunshine during the year. Also, the capillary suction of materials is of a great importance
for an assessment of the value of “characteristic rain”.
Experiments performed on some composite materials confirmed the necessity to respect
climatic cycles during the year also in accelerated durability tests.
Methodology for an assessment of the influence of climatic characteristics was proposed
for conditions in the Czech Republic. The use of so called “real time” for designing of
building materials with defined “usable durability” is suggested. The term “real time” is
closely bound to the life-time of the realised building structures. For some composite
materials the term “real time” has similar practical meaning as e.g. an assessment of
carbonation characteristics to concrete.
Frequency, intensity and the time of the lasting of the rain are presented and frequency,
lasting and the level of sub-zero temperatures in the Czech Republic are also given.
Results of accelerated tests of cement-bonded particleboards, fibre-cement sheets with a
high content of ash from fluidised bed combustion are presented and discussed.
Methodology for an assessment of “usable durability” of the composite materials is
described.
1.
Introduction
Currently there are no test methods and procedures available, which would allow to
assess the durability of building materials in a way, which would for the given region
prognose the real time of usability of their qualitative properties when exposed to the
weather conditions. With the “classic” materials, usually practical experience serves as
the starting point.
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J. Knězek, S. Modrý
Problems with the definition of the real time of “usable durability”* of the qualitative
properties of interest start with practical implementing of new composite materials, whose
structure consists often of very variable components, when the character of the bonds
between them may be uncertain or even unstable in the environmental conditions of
changing temperature and humidity. Above all for these cases, but for the cases of nonstandard use of “classic” building materials as well, a new method of assessing of the
usable durability with the accelerated test is proposed.
Proposed test method is based mainly on the climatic data of Czech Republic from the
years 1901 - 1950. Their actualisation is supposed for the years 1951 - 2000, after getting
the needed climatic characeristics, which reflect the dynamics of the climatic changes in
the Middle Europe in the following time. [1, 2, 3]
2.
Assessment of the “usable durability” from the accelerated tests
results
The aim of proposed evaluation method is to determine - with defined climate
characteristics of the given geographic region and for intended use of building
composites (i.e. cladding, roofs, flat surfaces, such as pavements or terraces, etc.) - the
time of “usable durability” of the qualitative properties of the composites with a number of
“climatically average years”*.
Proposed method respects the possibility of gradual relative intensifying of effects of
summer and winter environmental conditions on the dynamics of the qualitative changes
of the controlled properties of some new composites, above all composites, made by
combination of organic and anorganic components.
The term “climatically average year”* for the given region means in the proposed method
the cyclic changing of simulated summer conditions (defined number of cycles of
“characteristic rains”* alternating with solar radiation) and winter conditions (defined
number of cycles of “characteristic frosts”*, alternating with defrosting in the water).
3.
Model winter conditions
Model winter conditions of the given geographic region are characterised by the number
of changes from the above-zero temperatures to the level of “characteristic frost” during
one “climatically average year”. The problem remaining to solve is still the uncertainity of
the humidity state of the composites - above all those, which are situated in the vertical
position in the construction - regarding the uncertainity of the frequency of the sufficient
water saturation level of the composites and “characteristic frost” combination occurence.
The proposed metods accepts the current praxis, which uses full water saturation of the
composite and level of freezing temperature in the given region as well.
Proposed method introduces for the given geographic region the average count of the
currently established sub-zero temperatures occurence. At the same time the method
uses the identical count of the occurence of the “characteristic frost” for assessing the
*
See definitions on the page 9.
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PROPOSAL OF THE METHOD FOR ASSESSMENT OF USABLE DURABILITY OF
COMPOSITE MATERIALS FROM THE RESULTS OF ACCELERATED TESTS
composites with the same application (for example cladding of the buildings). Thus the
condition for objective comparison of different composites for the same application is set.
For the formulation of winter conditions parameters there are no complete records
available, which would allow to define the “characteristic frost” for the area of the Czech
republic, in the proposed method. (Continuous temperature record on monitoring stations
is introduced unevenly in the last 10 years). The analysis of data from 3 stations in
Southern Moravia (from the years 1983 - 1993) gave results, which could be for the start
taken as acceptable for the area of Czech Republic up to cca. 600 m altitude. It was
proven that the connection between the level of sub-zero temperatures, their lasting time
and frequency of their occurence are possible to express in the temperature range -5°C -20°C by an equation of type
1/Nmt=A0+A1t+A2t2+A3t3+A4t+A5t2
where




(1)
Nmt
is the frequency of occurence of sub-zero temperature with level at
least t and time of duration  in the winter time of average year
t
is the value of sub-zero temperature of environment

is the duration of the sub-zero temperature of environment
471
J. Knězek, S. Modrý
Table 1: Enumerated counts of occurence for various values and times of the sub-zero
temperatures in one “climatically average year”
Freezing
time
[hrs]
Levels of sub-zero temperatures in °C
-2
-3
-4
-5
-6
-8
-10
-12
-15
-18
-20
1
18.91
10.59
7.79
6.32
5.33
3.88
2.73
1.86
1.02
0.58
0.11
2
16.21
9.31
6.88
5.58
4.71
3.46
2.48
1.72
0.97
0.56
0.10
3
14.19
8.31
6.16
5.00
4.22
3.42
2.26
1.60
0.92
0.54
0.10
4
12.62
7.50
5.57
4.52
3.82
2.84
2.08
1.49
0.88
0.53
0.10
5
11.36
6.83
5.09
4.13
3.49
2.60
1.93
1.40
0.84
0.51
0.10
6
10.33
6.28
4.68
3.80
3.22
2.40
1.80
1.32
0.81
0.49
0.10
7
9.47
5.81
4.34
3.52
2.98
2.23
1.68
1.25
0.77
0.48
0.10
8
8.74
5.40
4.04
3.28
2.78
2.09
1.58
1.18
0.74
0.47
0.10
9
8.11
5.05
3.78
3.07
2.60
1.96
1.49
1.12
0.72
0.46
0.10
10
7.57
4.74
3.55
2.89
2.44
1.84
1.41
1.07
0.69
0.45
0.10
12
6.68
4.22
3.17
2.58
2.18
1.64
1.27
0.98
0.64
0.42
0.10
16
5.41
3.46
2.61
2.12
1.79
1.36
1.07
0.83
0.57
0.39
0.09
24
3.92
2.55
1.92
1.56
1.33
0.99
0.80
0.64
0.46
0.33
0.09
48
2.15
1.42
1.08
0.88
0.74
0.57
0.46
0.38
0.29
0.22
0.08
Enumerated counts of occurence for various values and times of the sub-zero
temperatures in one “climatically average year” are given in Table 1, which also contains with regard to the evaluation of horizontal constructions and effects of chemical defrosting
agents - data for the temperature values from -2°C.
4.
Model summer conditions
Model summer conditions of geographic region are characterised by the mean frequency
of changing “characteristic rain” and sunshine, where the frequency and lasting of these
climatic conditions are during one “climatically average year” generally the same (if we
omit the influence of UV radiation). The level of sunshine is in the given stage considered
as constant, some specifications may be realised.
Proposed method of assessing the “usable durability” from the accelerated test results is
based, when defining the “characteristic rain” (in case of the cladding), on the standard
practice of rinsing with water for 170 minutes. Question of intensity of the rinsing remains
unanswered until now (current practice had shifted in the last 10 - 15 years from 2.5 litres
per minute and m2 to 1 liter per minute and m2). Even the latter level of rain intensity does
not confirm with the real conditions in Middle Europe.
That is why the proposed method also starts from the requirement to rinse all materials
used in cladding, for 170 minutes with an intensity, which sufficiently adds to capillary
imbibition of the rain into open pore structure of the material and also confirms with its
occurence with the reality in the geographic region of the Czech Republic.
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PROPOSAL OF THE METHOD FOR ASSESSMENT OF USABLE DURABILITY OF
COMPOSITE MATERIALS FROM THE RESULTS OF ACCELERATED TESTS
Within the scope of the project problems of the surface sorptivity of the composite was
studied by means of capillary rising using the gravimetric method. The tests were realised
in the time span of 1 to 104 minutes, which allowed to formulate the relationship between
capillary rising and time of its duration.
It is possible to present obtained results by means of the mean value of water absorption
by the capillary rise - zit - also for the cases, when the dynamics of surface absorption
change during the time, for example due to the particles with specific porosity, such as at
the given circumstances wooden particles etc., in the formula
ln zit = A0+A1ln t + A2 t ,
where:
(2)
Vit
z it 
Fi Pi
Vit is volume of water, absorbed by surface in the time of contact with water t
Fi is the size of contact wetted area
Pi is the part of open porosity in the total volume of the sample.
From the experiments on the capillary rise of composite materials realised until now
follows, that open porosity is not higher than 50% vol. For this level the rain intensity of
0.025 mm/min was set, which corresponds to 5 times mean rate of capillary rise, during
170 minutes and thus represents the rain total of 4.25 mm.
It is obvious that the materials with higher thickness may show the tendency to stagnation
of the qualitative parameters after the initial decline due to cyclic rinsing and drying. The
influence of cyclic rinsing and drying may be significantly slowed or even stopped.
The relationships of the rain intensity (i), time of rain duration (t) with frequency of
occurence (N), when respecting the altitude of the given place (Nv) and influence of
continentality, given as a degree of eastern longitude (V d) was formulated to define the
“characteristic rain” on the base of the data from more than 90 monitoring stations in
Czech Republic for the times of rain duration 60, 90 and 120 minutes. Expression of
these relationships are given in the formulas (3) and (4).
ln (i) = A + B ln (t) + C ln (N)
(3)
where the values of regression coefficients K=A, B and C are defined with a linear
relation, which is acceptable for technical application.
K = a + b Nv + c V d
(4)
The results of extrapolation of the frequency of occurence of 170 minutes rain 170Nsvd in
relation to the altitude of given place, total precipitation amount 4.25 mm and with respect
to the influence of continentality are shown on the Figure 1.
The low quantity of data for the altitude higher than 600 meters above the sea influences
the above mentioned frequency of occurence for the 170 minutes rains making it
obviously higher.
473
J. Knězek, S. Modrý
Until there are no results of corresponding observations available, it is advisable to
correct the data of the rain occurence for the altitudes above 600 m with the result of
confrontation with the rain occurence for total precipitation amount 4.25 mm, determined
by the logarithmic interpolation of the given place data for a total precipitation amount
>1.0 mm and >10.0 mm.
The mentioned procedures are proposed for use until more adequate data on rain
occurence and character are available for the Czech Republic. In future, statistic
definition of the rain occurence in a normal year for places in Czech Republic, in context
with intensity and duration of the rain, is presumed.
150
145
140
135
130
125
120
115
110
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
Degrees
eastern
longitude
18°
15°
16
00
15
00
14
00
13
00
12
00
11
00
90
0
10
00
80
0
70
0
60
0
50
0
40
0
30
0
12°
20
0
10
0
Occurence of 170 minutes rain
The knowledge of the relation between frequency of the rains of defined intensity and
their duration gives an assumption for prognosis of the “usable durability” of materials in
long-time perspective of the means of use from the point of view of summer time.
Altitude above sea level [meters ]
Figure 1: Extrapolation of the frequency of occurence of 170 minutes rain
relation to the altitude of given place
474
170N
svd
in
PROPOSAL OF THE METHOD FOR ASSESSMENT OF USABLE DURABILITY OF
COMPOSITE MATERIALS FROM THE RESULTS OF ACCELERATED TESTS
5.
Method of assessment of the “usable durability” of thin-walled
composite materials used for the outer cladding of buildings
The process of assessment:
1)
Definition of the geographic position of the place in Czech Republic, for which
the “usable durability” has to be determined. The position is either particular
place - in cases, which require special attention - or a climatically defined part of
the Republic. (In the future - after collecting and evaluating the required climatic
data, acceptable for the future - using of the special maps of frequency, intensity
and duration of the rains and frequency, level and duration of the sub-zero
temperatures is supposed in Czech Republic).
2)
determination of the frequency of “characteristic rain” and “characteristic frost”
for the given geographic position. The set of “characteristic rains”, followed by
the set of “characteristic frosts” represents one “climatically average year”. The
samples, with shapes and dimensions according to the standard conventions,
are exposed in this order.
3)
Laboratory realisation of the “characteristic frost”:
Alternative A:
Exposition of the standard samples, for which the required time of storage in the
temperature -15°C or other is already determined according to the existing
specifications.
Alternative B:
Determination of the temperature characteristics of the tested water saturated
material and following determination of the required time to reach the level of
“characteristic frost” in the whole volume of the samples or to the defined depth,
if it is required by particular specifications.
4)
The range of the sample series must be chosen with respect to variability of the
watched qualitative sign, which has a dominant role in assessing the “usable
durability”. The quantity of sets of samples must be chosen with respect to the
presumed time of “usable durability”.
5)
When exposing the samples to the “characteristic rain”, the rain must be applied
always on the same adequate surface.
6)
The choice of sufficent number of sample sets makes it possible to apply
regression analysis and to determine for a given composite the “usable
durability” as a guaranteed time of “usable durability” with defined level of
reliability.
7)
Continuous observation of the level of qualitative signs allows to find out the
number of “climatically average years”, when the observed qualitative signs are
reaching the lowest acceptable limit. This number of “climatically average years”
is the limit for average time of “usable durability”.
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J. Knězek, S. Modrý
Remark: In the case of flat components with polymer-based surface finish the radiant
heat transmission is combined with the UV radiation and the length of exposition time is
extended to the level corresponding in total to the time of direct sunshine of given position
per year in the time from April to September (different time of exposition to the
“characteristic rain” and radiated heat).
Main characteristics of the proposed method of assessing the “usable
durability” from the accelerated tests results
6.
Until new actualised climatic data are acquired for area of the Czech Republic, it is
recommended to use the proposed method of prognosing the “usable durability” from the
accelerated tests results at these conditions and requisitions:
-
The internationally standardised duration of “rain” 170 minutes is respected as a
marked representative characteristic of rains in connection to the nature of open
porosity of contemporary building materials, used in outer environment.
-
Processed specific data for the “characteristic rain” and “characteristic frost” are
respecting the influence of altitude and continentality.
-
For the altitudes higher than 600 meters above sea level it is advisable to correct the
occurence of “characteristic rain” with the mentioned method.
-
The frequence of the “characteristic rain” is valid for composite materials with total
porosity not higher than 50% vol.
-
Every rain rinsing is alternated with the exposition to the radiant heat.
-
The radiant heat induces on the surface of the tested materials in the humidity
equilibrium and the normal atmosphere the surface temperature (60 +10) °C.
-
For the cases of breaking the surface layers of the composites when freezing in the
water or water solutions of chemical defrosting agents the value of the sub-zero
temperatures with regard to the porosity of the structure so that the conditions for
freezing the water in at least 95% of the pores are made.
-
The materials with significant volume of macropores and open porosity deserve
special attention when the level of influence of the occurence and time of
temperatures around -3 to -5°C in given place and “climatically average year”.
-
For the connection of average frequency of the occurence of defined level and
duration of sub-zero temperatures are available only the data, given in Table 1,
assessed according to the formula (1) for Southern Moravia region and for places with
altitude up to 600 m.
7.
Assessment of the effect of cyclic changing of summer and winter
time on the flexural strength level in accelerated test of “usable
durability” of cement-bonded particleboard
The influence of changing the climate of summer and winter time according to the
proposed method was experimentally proven with the 10 mm thick cement-bonded
particleboards on flexural strength changes as well as impact strength using the IZOD
method.
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PROPOSAL OF THE METHOD FOR ASSESSMENT OF USABLE DURABILITY OF
COMPOSITE MATERIALS FROM THE RESULTS OF ACCELERATED TESTS
Realisation of 50 freezing cycles induced the flexural strength loss to the 74% of the
original state.
With 100 cycles of water rinsing and drying the flexural strength loss to the 84% of the
original state was induced.
Combination of 10 freezing cycles and 100 rinsing/drying cycles (as used above) induced
the loss of strength to the 68% of the original state.
8.
Expected main contributions of the proposed method for assessment
of the “usable durability”
Proposed method makes possible:
-
to define “usable durability” of composites in specific climatic conditions with a number
of years, after which a lowest acceptable level of qualitative parameters will be
reached
-
to describe the synergy of summer and winter environment effects on level of the
qualitative parameters of the composites.
-
to assess the “usable durability” of surface finishes by combination of radiating heat
and UV radiation
-
to assess the “usable durability” of composites with less stable bonds with a better
organisation of tests
-
to evaluate the “usable durability” of composites with embedded components, which
are not “environmentally friendly” as during the exposition it is possible to make
ecological tests of the partially deteriorated composites etc.
9.
Definitions of terms
Usable durability - the time of lasting on a set level for the qualitative sign of a
composite, indicated by the number of “climatically average years” in the geographic
position of the given area.
Climatically average year - the sum of “characteristic rains” in the summer, followed by
the sum of “characteristic frosts” in the winter in the geographic position of the given area.
Characteristic rain - the time of rain sufficent to make up for the capillary suction of the
open porosity of composite, which occurs with defined occurence in the given geographic
position in summer. In the proposal of the method the time of rain is set to 170 minutes
(modelled in laboratory conditions according to current standards for fibre-cement
composites used for cladding).
Characteristic frost - the level of sub-zero temperatures, which occurs in the geographic
position of the composite usage in winter time with defined occurence for a set time. In
the proposal of the method the level and time are set according to current standards for
fibre-cement composites, but with respect to their occurence in given geographic region.
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J. Knězek, S. Modrý
Notice 1: The occurence and time of characteristic rain of defined intensity will be
subsequently formulated with defined probability of appearance according to the results
of long-time statistic researches in dependence on the geographic position.
Notice 2: The occurence and time of characteristic frost will be subsequently formulated
in dependence on the sub-zero temperature levels of the given geographic position with
defined probability of occurence, based on the long-term statistic research.
10. Acknowledgement
It was possible to realise the work on this project with the support of the Grant Agency of
the Czech Republic within the frame of the grant project GAČR 103/01/0603 and grant
project CEZ: J04/98:2100000 30.
References
[1]
Knězek J. and Modrý S., ‘Starting point of the complex methodics of assessing
the composite materials and their surface finishes durability‘ (in Czech),
Proceedings ‘Nové stavební hmoty a výrobky’, I. Díl, (VUSH Brno, 21.11.2001)
69 - 75.
[2]
Knězek J., ‘Some aspects of organic fibre reinforced cement composite
durability‘, (in Czech) Proceedings ‘Technologie, provádění a kontrola
betonových konstrukcí 2002‘, (ČBS, Praha, 20.-21.3.2002) 110 - 115.
[3]
Knězek J., Modrý S. and Hradil M., ‘Preparation of methodics of accelerated
assessment of the organic fibre reinforced composite‘ (in Czech). Proceedings
‘VIL Vedecká konference s medzinárodnou účasťou’. 10. Sekcia: Stavebné
materiály, (Košice, 22. - 24.5.2002) 103 - 106.
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