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Sealants between elements of aerated concrete
The International Journal of Lightweight Concrete. Vol 2, No 1, pp 43-47, 1980 © The Construction Press Ltd.
0142-0968/80/02140043502.00
Sealants between elements of aerated concrete P. G. Burstr6m*
Synopsis Because of the l o w tensile strength of aerated concrete, the sealing of joints between such elements causes special problems. In this paper some main characteristics of sealants for use in aerated concrete joints are discussed. These properties are elongation resistance, elasticity, and ageing. The elongation resistance was examined at different rates and different temperatures. The elasticity was determined by measuring the residual deformation after a certain elongation. Storing in heat (40°C or 70°C) was used for accelerating ageing. Of ten sealants, only four were found to be suitable in aerated concrete joints, Many of the disqualified sealants showed defective ageing properties or high resistance to elongation, which in some cases caused cohesive failure within the aerated concrete.
The main characteristics to be examined were the elongation resistance at different temperatures, the elasticity, and the ageing properties. As mentioned above it is important to know the elongation resistance. As most sealants show a certain amount of thermoplasticity, it is also important to test them at low temperatures, as the joint usually reaches its greatest width at these temperatures. The elasticity reveals a sealant's ability to accommodate cyclic joint movements. However, a sealant should not be too elastic as this may lead to adhesive failure. An 'elastic sealant' is a sealant which exhibits predominantly elastic behaviour in use, stresses remaining in the sealant as a result of joint movement are almost proportional to the strain. The ageing properties are naturally of fundamental importance when the life of a sealant is to be estimated.
Keywords Aerated concrete, sealants, mastic, ageing, deformation characteristics, heat. Introduction Autoclaved aerated concrete has a considerably lower tensile strength than ordinary concrete. This makes it necessary to choose sealants which do not, when movements occur in the joint, transfer such high stresses that the edges of the units are broken. Research work regarding the ageing and deformation properties of sealants has been carried out at the Division of Building Materials, Lund Institute of Technology in Sweden [1].
Experimental programme Ten different sealants from five manufacturers were chosen. The nominal specimen dimensions were 12 x 12 x 50 mm. However, some sealants showed a considerable shrinkage. Therefore, the exact dimensions were measured before the specimens were elongated. The substrate was asbestos cement or aerated concrete with a special surface coating made of an acrylic-binder and mineral grains ('Preobas'). Primers were used for some sealanys as required by the manufacturers.
The purpose of an investigation made specially for the aerated concrete industry was to make recommendations regarding the types of sealants to be used with aerated concrete in this respect. Some of the work will be reviewed in this paper.
The specimens were stored for either 63 days at 20°C/50% RH or 7 days at 20°C/50% RH, followed by 56 days at 70°C. Storage at 70°C is frequently used to accelerate the ageing of sealants, cf for example [1]. The method is very simple to carry out, but one must be aware of its limitations; the method can be very onerous for some sealants.
*Per G. Burstr6m received an MSc in Civil Engineering at Lund Institute of Technology in 1969. He obtained the PhD degree from Lund Institute of Technology in 1979. He has been associated with the Division of Building Materials since 1969. His field of interest is the mechanical and durability properties of sealants. Apart from research in the field of sealants, he has had several years experience of teaching students studying to be architects and civil engineers in the subject of Building Materials.
When determining the stresses in the sealants during elongation it is important to choose a realistic testing rate. Many sealants are plastic to a great extent. Plastic sealants show considerably higher stresses at higher rates compared to the stresses arising at lower rates. Joint movements in practice are usually very slow. Therefore 0.001 mm/min was used
43
and compared to 1.5 mm/min which is a more convenient 'laboratory rate'. The temperatures for testing were 2°C and -25°C. Experimental
results
Some typical results of the tests are shown. Figure 1 shows the effect of temperature-ageing on a two-component polyurethane sealant. This material is almost unaffected by the temperatureageing. However, the testing rate is high compared to actual joint movements. Therefore, if this sealant is tested at the lower rate (0.001 mm/min) the stresses
arising are considerably lower (Figure 2). From this Figure the residual deformation at zero-stress is also revealed after two hours of stress relaxation at maximum deformation. Figures 3 and 4 show the corresponding results for another polyurethane sealant, comprising one-component. The temperature-ageing has had a great effect on this sealant and the stresses arising are rather high. This is also true when the material is elongated at the lower speed (Figure 4). Figures 5 and 6 show the results for an oil based, skin-forming sealant. This type of sealant is more complicated to test than the elastic sealants. The skin
Tensite stress
l"ensite stress
MPa
MPa 7* 56 days
j
0.3
t I / / - ~
~'~
1.0
? +56 days
" ~
/
0.8
0.2
//
o.6
//
. . . . 70°
~
0.4 I
I
t
25
50
75
)00 Strain, %
Figure I Stress-strain curves for a two-component polyurethane sealant after storing at different temperatures. Deformation rate: 1.5 ram~rain. Testing temperature: 2°C. Testing temperature
TensiLestress
MPa 7*56 days+ 20:C/50% RH ....
70
c
j , ~ "1"w'-
/.I<.~'~
/
t I
-25"£
, 2°C
I0
20
30
/ ~
///
: I
I
I
25
50
75
100 Strain, °/o
Figure 3 Stress-strain curves for a one-component polyurethane sealant after storing at different temperatures. Deformation rate: 1.5 mm/min. Testing temperature: 2°C cracks at a certain elongation. That is why the typical stress peak arises. The temperature-ageing has increased the stress, partly depending upon an increase in the thickness during heat-storage.
/
0.1
40 50 Strain, %
Figure 2 Stress-strain curves for the two-component polyurethane sealant at deformation rate 0.001 ram~rain
44
f
/
.2o°c15o% RH
!
0.3
/
/
//~'~ //
0.1
/
/
At a lower rate (Figure 6), the stresses are lowered, while at - 2 5 ° C the tensile stress is too high for aerated concrete. The ability of an oil-based sealant to function is, apart from the ductility of the skin, also greatly dependent on the fact that the thickness of the surface skin does not increase too quickly. In another investigation, the thickness of the skin was therefore determined at different climates. Figure 7 shows the skin thickness as a function of time. For material stored at 20°C/50% RH the relation is approximately a straight line in the log-log diagram.
Tensile stress MPa
Testing temperature
_25°C
7+56 days
0.5
/ ////1
+20°C/50°/=RH
-----
+70°C
'4 / .// ,, / / ~ - 2 5 " c / / ~
0.4
0.3
// / //
.,2%
.2%
29 + X . 2 3 6 = 8 5 0 X~3.5
Having recalculated the real times in the different climates to equivalent times, the values of the temperature-aged specimens have been plotted (40°C and 70°C) in Figure 7. However, the point of the example is a direct plot. Figure 7 also shows that a normal outdoor exposure is not more severe for the sealant than the laboratory climate 20°C/50% RH!
Y 10
I
I
20
30
I
40 50 Strain, °/o
Figure 4 Stress-strain curves for the one-component polyurethane sealant at deformation rate 0.001 mm/min Tensile stress MPa
/
The point S -= 0.85/t = 265 (29 days at 20°C/50% RH and 236 days at 40°C) has, according to the Figure, an equivalent hardening time of about 850 days at 20°C/50% RH. Suppose that the skin growth is x times faster at 40°C than at 20°C/50% RH.
The average value for the corresponding calculations for a number of other points was 4.0. Corresponding calculations for specimens stored at 70°C resulted in a growth factor of 22.
/
o.,
0.4
respective temperatures were recalculated to equivalent times at 20°C/50% RH. The calculations are shown in one example in the Figure.
f ~
Discussion
7,91 days
\
In Table 1 an attempt is made to sum up the differences between some of the sealants examined. The column 'Ageing characteristics" shows the quotient between the tensile stress at 25% elonga-
\ \
\
0.3
/-~.
i
When using 'Preobas' as a substrate, most cohesive failure within the aerated concrete arose at about 0.3 MPa. This is not too far from the tensile strength 0.5 MPa, which is often given for the density 500 kg/m 3. The lower value can be explained by cracks which may have arisen when the blocks were cut for specimens.
\
\
.2o°c/so'/o RH
\
....
\\ •
0.1 ~
I
- - ~
/.,O°C
Tensile stress MPa 1.0 /'-\
/
25
\
/
l
7 * 56 days
\
/
*70°C
Testing temperature
+ 20°C/500/o RH
\
/
50
75
100 Strain,%
Figure 5 Stress-strain curves for an oil-based, skin forming sealant after storing at different temperatures. Deformation rate: 1.5 mm/min. Testing temperature: 2oC When storing the sealant at an increased temperature oxidation naturally takes place quicker. Consequently the growth in thickness of the skin is also quicker. With the aid of the skin thickness, measured after storage at 40°C and 70°C, the real storage times at
0.5
\
/ I I I I I
---- +?0oc \ \ \
\
\
\
\ \ \ \
0.1 /
,~
i
20
3'0
-25% *2 -25 4'0 50,2 Strain, %
Figure 6 Stress-strain curves for the oil-based sealant at deformation rate 0.001 mm/min
45
Skin thickness mm 10.0
Example ,20°C/50% RH
0
X
,40°C
•
+?OOC
S=0.85 t = 29 d *200C/50% RH 236d *40°C
~7 UV storing * 20°C
ix Outdoor
1.0
o~,9"
~
exposure
_ _ ~ ~ <
z~
O-iioo oo X
/ f I"
0.1 f ,,-/"1
i
i
o ~ n 0.03 oo S .-,,
•
0t 4 8
f
f
f
T
o
i
I
III
I
I
I
I
10
I
I I I I
I
I
I
100
,
= i
II
i
I
I
!
I
IOOL,
I
I
I
10000 Time, days
Figure 7 Skin thickness as a function of storing time at 20°C/50% RH for the oil-based sealant. Example of calculating the equivalent time at 20°C/50% RH for material stored at 40°C and 70°C are calculated values to equivalent times at 20°C/50% RH according to the example in the Figure.
Table 1 Comparisons of essential properties for three different sealants. Testing rate: 0.001 mm/min
Sealant
Two-component. polyurethane One-cornponent polyurethane Oil based
'Ageing characteristics' °25; + 70/025; + 23
Elongation resistance at-25°C
2°C
-25°C
025; + 70°C (MPa)
0.9
1.0
0.25
23
1.3 ~4
1.5 >>1
0.32 ~ 1 at e ~ , 1 0 %
9 50
tion of heat-aged and normal stored specimens. The two-component polyurethane sealant is hardly affected at all. The oil-based sealant is affected to a high degree. However, this kind of accelerated ageing affects oilbased sealant more than both the others. Therefore, the method described above would be more suitable.
46
Remaining deformation a t - 2 5 ° C of heat-aged material (%)
The elongation resistance a t - 2 5 ° C determined at 0.001 mm/min is also shown in Table 1. Elastic sealants are often designed to accommodate 25% elongation. Therefore, the stress at this elongation is shown for the heat-aged sealants (o25; 70 ° ). However, the oil-based sealant shows a stress-peak at elongation. Therefore, the maximum stress and the elongation connected to this stress are shown.
The last column in Table 1 shows the remaining deformation after elongation a t - 2 5 ° C up to 50% followed by stress relaxation for 2 hours. As can be seen the one-component polyurethane sealant is highly elastic while the other polyurethane sealant has a fairly large plastic component.
Apart from the above mentioned examples, two other sealants were recommended, a two-component soft polysulphide and a one-component water-dispersed acrylic sealant.
The results in Table 1 indicate that the twocomponent polyurethane sealant should be suitable in joints between aerated concrete elements. The sealant is unaffected by heat ageing, shows a sufficiently low elongation resistance and has a certain plasticity which reduces the adhesion problems at elongation.
Conclusions
The one-component polyurethane sealant on the other hand, is sensitive to the heat ageing. The elongation resistance is low enough but the tendency to harden during accelerated ageing also reveals an increasing stress at normal ageing. Furthermore this sealant is highly elastic.
Great differences were found within the same group of sealants, ie sealants with approximately the same binder. For the examples given, one polyurethane sealant was found to be suitable while another proved to be unsuitable for the joints in question.
Because of these properties, the one-component polyurethane sealant is not suitable for use in joints between aerated concrete. According to Table 1 the oil-based sealant is sensitive to heat ageing, has a fairly high elongation resistance and is fully plastic. It should not be suitable for the joints considered here. However, practical experience is very positive. The surface of this sealant becomes wrinkled due to varying joint movements, but the joints remain tight. Therefore, the chosen test methods do not accurately reveal the properties of this sealant. Instead, the ageing characteristics were partly tested by the method shown in Figure 7. The measured elongation resistance is too high. However, the given value is obtained a t - 2 5 ° C . At this temperature the actual joint movements are probably zero. The movements occur at a higher temperature At 2°C the maximum elongation resistance for the oil-based sealant is about 0.1 MPa. Therefore, from these discussions and from the good practical experiences one is able to conclude that the oil-based sealant should be suitable in aerated concrete joints.
It has been shown how ten different sealants were tested for aerated concrete joint purposes and how the properties of ageing characteristics, elongation resistance, and elasticity were examined.
These results also correspond well with practical experience. For plastic sealants, which are normally more thermoplastic in nature compared to elastic ones, other methods may be necessary. For an oil-based, skin-forming sealant the rate of the skin-formation was determined as a supplementary test. This rate was about equal whether the sealant was in 20°(3/50% RH or in real joints outdoors. The rate could be accelerated by a factor of 22 if the sealant was heat aged at 70°C. Of the ten sealants tested, only four were found to be suitable in aerated concrete joints.
References 1. Burstr6m, P. G., 'Ageing and deformation properties of building joint sealants', Report TVBM1002, 1979, Division of Building Materials, Lund Institute of Technology, Lund, Sweden. 2. Engwall, A. and Mattsson, H. 'Joints between elements of aerated concrete', Norwegian Building Research Institute, Report 51 C, 1968, Weathertight joints for walls, pp. 1 5 0 - 1 5 2 .
47
0142-0968/80/02140043502.00
Sealants between elements of aerated concrete P. G. Burstr6m*
Synopsis Because of the l o w tensile strength of aerated concrete, the sealing of joints between such elements causes special problems. In this paper some main characteristics of sealants for use in aerated concrete joints are discussed. These properties are elongation resistance, elasticity, and ageing. The elongation resistance was examined at different rates and different temperatures. The elasticity was determined by measuring the residual deformation after a certain elongation. Storing in heat (40°C or 70°C) was used for accelerating ageing. Of ten sealants, only four were found to be suitable in aerated concrete joints, Many of the disqualified sealants showed defective ageing properties or high resistance to elongation, which in some cases caused cohesive failure within the aerated concrete.
The main characteristics to be examined were the elongation resistance at different temperatures, the elasticity, and the ageing properties. As mentioned above it is important to know the elongation resistance. As most sealants show a certain amount of thermoplasticity, it is also important to test them at low temperatures, as the joint usually reaches its greatest width at these temperatures. The elasticity reveals a sealant's ability to accommodate cyclic joint movements. However, a sealant should not be too elastic as this may lead to adhesive failure. An 'elastic sealant' is a sealant which exhibits predominantly elastic behaviour in use, stresses remaining in the sealant as a result of joint movement are almost proportional to the strain. The ageing properties are naturally of fundamental importance when the life of a sealant is to be estimated.
Keywords Aerated concrete, sealants, mastic, ageing, deformation characteristics, heat. Introduction Autoclaved aerated concrete has a considerably lower tensile strength than ordinary concrete. This makes it necessary to choose sealants which do not, when movements occur in the joint, transfer such high stresses that the edges of the units are broken. Research work regarding the ageing and deformation properties of sealants has been carried out at the Division of Building Materials, Lund Institute of Technology in Sweden [1].
Experimental programme Ten different sealants from five manufacturers were chosen. The nominal specimen dimensions were 12 x 12 x 50 mm. However, some sealants showed a considerable shrinkage. Therefore, the exact dimensions were measured before the specimens were elongated. The substrate was asbestos cement or aerated concrete with a special surface coating made of an acrylic-binder and mineral grains ('Preobas'). Primers were used for some sealanys as required by the manufacturers.
The purpose of an investigation made specially for the aerated concrete industry was to make recommendations regarding the types of sealants to be used with aerated concrete in this respect. Some of the work will be reviewed in this paper.
The specimens were stored for either 63 days at 20°C/50% RH or 7 days at 20°C/50% RH, followed by 56 days at 70°C. Storage at 70°C is frequently used to accelerate the ageing of sealants, cf for example [1]. The method is very simple to carry out, but one must be aware of its limitations; the method can be very onerous for some sealants.
*Per G. Burstr6m received an MSc in Civil Engineering at Lund Institute of Technology in 1969. He obtained the PhD degree from Lund Institute of Technology in 1979. He has been associated with the Division of Building Materials since 1969. His field of interest is the mechanical and durability properties of sealants. Apart from research in the field of sealants, he has had several years experience of teaching students studying to be architects and civil engineers in the subject of Building Materials.
When determining the stresses in the sealants during elongation it is important to choose a realistic testing rate. Many sealants are plastic to a great extent. Plastic sealants show considerably higher stresses at higher rates compared to the stresses arising at lower rates. Joint movements in practice are usually very slow. Therefore 0.001 mm/min was used
43
and compared to 1.5 mm/min which is a more convenient 'laboratory rate'. The temperatures for testing were 2°C and -25°C. Experimental
results
Some typical results of the tests are shown. Figure 1 shows the effect of temperature-ageing on a two-component polyurethane sealant. This material is almost unaffected by the temperatureageing. However, the testing rate is high compared to actual joint movements. Therefore, if this sealant is tested at the lower rate (0.001 mm/min) the stresses
arising are considerably lower (Figure 2). From this Figure the residual deformation at zero-stress is also revealed after two hours of stress relaxation at maximum deformation. Figures 3 and 4 show the corresponding results for another polyurethane sealant, comprising one-component. The temperature-ageing has had a great effect on this sealant and the stresses arising are rather high. This is also true when the material is elongated at the lower speed (Figure 4). Figures 5 and 6 show the results for an oil based, skin-forming sealant. This type of sealant is more complicated to test than the elastic sealants. The skin
Tensite stress
l"ensite stress
MPa
MPa 7* 56 days
j
0.3
t I / / - ~
~'~
1.0
? +56 days
" ~
/
0.8
0.2
//
o.6
//
. . . . 70°
~
0.4 I
I
t
25
50
75
)00 Strain, %
Figure I Stress-strain curves for a two-component polyurethane sealant after storing at different temperatures. Deformation rate: 1.5 ram~rain. Testing temperature: 2°C. Testing temperature
TensiLestress
MPa 7*56 days+ 20:C/50% RH ....
70
c
j , ~ "1"w'-
/.I<.~'~
/
t I
-25"£
, 2°C
I0
20
30
/ ~
///
: I
I
I
25
50
75
100 Strain, °/o
Figure 3 Stress-strain curves for a one-component polyurethane sealant after storing at different temperatures. Deformation rate: 1.5 mm/min. Testing temperature: 2°C cracks at a certain elongation. That is why the typical stress peak arises. The temperature-ageing has increased the stress, partly depending upon an increase in the thickness during heat-storage.
/
0.1
40 50 Strain, %
Figure 2 Stress-strain curves for the two-component polyurethane sealant at deformation rate 0.001 ram~rain
44
f
/
.2o°c15o% RH
!
0.3
/
/
//~'~ //
0.1
/
/
At a lower rate (Figure 6), the stresses are lowered, while at - 2 5 ° C the tensile stress is too high for aerated concrete. The ability of an oil-based sealant to function is, apart from the ductility of the skin, also greatly dependent on the fact that the thickness of the surface skin does not increase too quickly. In another investigation, the thickness of the skin was therefore determined at different climates. Figure 7 shows the skin thickness as a function of time. For material stored at 20°C/50% RH the relation is approximately a straight line in the log-log diagram.
Tensile stress MPa
Testing temperature
_25°C
7+56 days
0.5
/ ////1
+20°C/50°/=RH
-----
+70°C
'4 / .// ,, / / ~ - 2 5 " c / / ~
0.4
0.3
// / //
.,2%
.2%
29 + X . 2 3 6 = 8 5 0 X~3.5
Having recalculated the real times in the different climates to equivalent times, the values of the temperature-aged specimens have been plotted (40°C and 70°C) in Figure 7. However, the point of the example is a direct plot. Figure 7 also shows that a normal outdoor exposure is not more severe for the sealant than the laboratory climate 20°C/50% RH!
Y 10
I
I
20
30
I
40 50 Strain, °/o
Figure 4 Stress-strain curves for the one-component polyurethane sealant at deformation rate 0.001 mm/min Tensile stress MPa
/
The point S -= 0.85/t = 265 (29 days at 20°C/50% RH and 236 days at 40°C) has, according to the Figure, an equivalent hardening time of about 850 days at 20°C/50% RH. Suppose that the skin growth is x times faster at 40°C than at 20°C/50% RH.
The average value for the corresponding calculations for a number of other points was 4.0. Corresponding calculations for specimens stored at 70°C resulted in a growth factor of 22.
/
o.,
0.4
respective temperatures were recalculated to equivalent times at 20°C/50% RH. The calculations are shown in one example in the Figure.
f ~
Discussion
7,91 days
\
In Table 1 an attempt is made to sum up the differences between some of the sealants examined. The column 'Ageing characteristics" shows the quotient between the tensile stress at 25% elonga-
\ \
\
0.3
/-~.
i
When using 'Preobas' as a substrate, most cohesive failure within the aerated concrete arose at about 0.3 MPa. This is not too far from the tensile strength 0.5 MPa, which is often given for the density 500 kg/m 3. The lower value can be explained by cracks which may have arisen when the blocks were cut for specimens.
\
\
.2o°c/so'/o RH
\
....
\\ •
0.1 ~
I
- - ~
/.,O°C
Tensile stress MPa 1.0 /'-\
/
25
\
/
l
7 * 56 days
\
/
*70°C
Testing temperature
+ 20°C/500/o RH
\
/
50
75
100 Strain,%
Figure 5 Stress-strain curves for an oil-based, skin forming sealant after storing at different temperatures. Deformation rate: 1.5 mm/min. Testing temperature: 2oC When storing the sealant at an increased temperature oxidation naturally takes place quicker. Consequently the growth in thickness of the skin is also quicker. With the aid of the skin thickness, measured after storage at 40°C and 70°C, the real storage times at
0.5
\
/ I I I I I
---- +?0oc \ \ \
\
\
\
\ \ \ \
0.1 /
,~
i
20
3'0
-25% *2 -25 4'0 50,2 Strain, %
Figure 6 Stress-strain curves for the oil-based sealant at deformation rate 0.001 mm/min
45
Skin thickness mm 10.0
Example ,20°C/50% RH
0
X
,40°C
•
+?OOC
S=0.85 t = 29 d *200C/50% RH 236d *40°C
~7 UV storing * 20°C
ix Outdoor
1.0
o~,9"
~
exposure
_ _ ~ ~ <
z~
O-iioo oo X
/ f I"
0.1 f ,,-/"1
i
i
o ~ n 0.03 oo S .-,,
•
0t 4 8
f
f
f
T
o
i
I
III
I
I
I
I
10
I
I I I I
I
I
I
100
,
= i
II
i
I
I
!
I
IOOL,
I
I
I
10000 Time, days
Figure 7 Skin thickness as a function of storing time at 20°C/50% RH for the oil-based sealant. Example of calculating the equivalent time at 20°C/50% RH for material stored at 40°C and 70°C are calculated values to equivalent times at 20°C/50% RH according to the example in the Figure.
Table 1 Comparisons of essential properties for three different sealants. Testing rate: 0.001 mm/min
Sealant
Two-component. polyurethane One-cornponent polyurethane Oil based
'Ageing characteristics' °25; + 70/025; + 23
Elongation resistance at-25°C
2°C
-25°C
025; + 70°C (MPa)
0.9
1.0
0.25
23
1.3 ~4
1.5 >>1
0.32 ~ 1 at e ~ , 1 0 %
9 50
tion of heat-aged and normal stored specimens. The two-component polyurethane sealant is hardly affected at all. The oil-based sealant is affected to a high degree. However, this kind of accelerated ageing affects oilbased sealant more than both the others. Therefore, the method described above would be more suitable.
46
Remaining deformation a t - 2 5 ° C of heat-aged material (%)
The elongation resistance a t - 2 5 ° C determined at 0.001 mm/min is also shown in Table 1. Elastic sealants are often designed to accommodate 25% elongation. Therefore, the stress at this elongation is shown for the heat-aged sealants (o25; 70 ° ). However, the oil-based sealant shows a stress-peak at elongation. Therefore, the maximum stress and the elongation connected to this stress are shown.
The last column in Table 1 shows the remaining deformation after elongation a t - 2 5 ° C up to 50% followed by stress relaxation for 2 hours. As can be seen the one-component polyurethane sealant is highly elastic while the other polyurethane sealant has a fairly large plastic component.
Apart from the above mentioned examples, two other sealants were recommended, a two-component soft polysulphide and a one-component water-dispersed acrylic sealant.
The results in Table 1 indicate that the twocomponent polyurethane sealant should be suitable in joints between aerated concrete elements. The sealant is unaffected by heat ageing, shows a sufficiently low elongation resistance and has a certain plasticity which reduces the adhesion problems at elongation.
Conclusions
The one-component polyurethane sealant on the other hand, is sensitive to the heat ageing. The elongation resistance is low enough but the tendency to harden during accelerated ageing also reveals an increasing stress at normal ageing. Furthermore this sealant is highly elastic.
Great differences were found within the same group of sealants, ie sealants with approximately the same binder. For the examples given, one polyurethane sealant was found to be suitable while another proved to be unsuitable for the joints in question.
Because of these properties, the one-component polyurethane sealant is not suitable for use in joints between aerated concrete. According to Table 1 the oil-based sealant is sensitive to heat ageing, has a fairly high elongation resistance and is fully plastic. It should not be suitable for the joints considered here. However, practical experience is very positive. The surface of this sealant becomes wrinkled due to varying joint movements, but the joints remain tight. Therefore, the chosen test methods do not accurately reveal the properties of this sealant. Instead, the ageing characteristics were partly tested by the method shown in Figure 7. The measured elongation resistance is too high. However, the given value is obtained a t - 2 5 ° C . At this temperature the actual joint movements are probably zero. The movements occur at a higher temperature At 2°C the maximum elongation resistance for the oil-based sealant is about 0.1 MPa. Therefore, from these discussions and from the good practical experiences one is able to conclude that the oil-based sealant should be suitable in aerated concrete joints.
It has been shown how ten different sealants were tested for aerated concrete joint purposes and how the properties of ageing characteristics, elongation resistance, and elasticity were examined.
These results also correspond well with practical experience. For plastic sealants, which are normally more thermoplastic in nature compared to elastic ones, other methods may be necessary. For an oil-based, skin-forming sealant the rate of the skin-formation was determined as a supplementary test. This rate was about equal whether the sealant was in 20°(3/50% RH or in real joints outdoors. The rate could be accelerated by a factor of 22 if the sealant was heat aged at 70°C. Of the ten sealants tested, only four were found to be suitable in aerated concrete joints.
References 1. Burstr6m, P. G., 'Ageing and deformation properties of building joint sealants', Report TVBM1002, 1979, Division of Building Materials, Lund Institute of Technology, Lund, Sweden. 2. Engwall, A. and Mattsson, H. 'Joints between elements of aerated concrete', Norwegian Building Research Institute, Report 51 C, 1968, Weathertight joints for walls, pp. 1 5 0 - 1 5 2 .
47