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Durability of sound absorbing materials for highway noise barriers
Journal of Sound and Vibration (1980) 71(l), 33-54
DURABILITY
OF SOUND
FOR HIGHWAY A. BEHAR?
ABSORBING
MATERIALS
NOISE BARRIERS AND
D.N.MAYS
Acoustics Ofice, Research and Development Division, Ministry of Transportation and Communications, Downsview, Ontario M3M 138, Canada (Received 24 September 1979, and in revised form 26 January 1980) This paper presents the results of a research study into two properties of sound absorbing materials intended for highway noise barrier applications: their durability and their sound absorption coefficients before and after exposure to adverse weather. After surveying the products of 34 manufacturers, eight materials and one “absorption system” were tested in the field and the laboratory. Even though there is no single, accepted test for weather
endurance for these kinds of material, the results of the study provide information on their likely future behaviour when installed on a barrier. The results apply at least partially to other outdoor situations too, and augment the limited information presently available on sound absorptive materials for outdoor use.
1. INTRODUCTION
There is considerable interest in the use of sound absorbing materials to improve the performance of highway noise barriers. Some studies have resulted in claims that they improve barrier attenuation. Without entering into a critical analysis of these studies, there are accompanying questions to be raised about the outdoor use of these materials, which is a relatively new application for them. When sound absorptive materials are used indoors, they have to comply with requirements related more to the danger of fire than to durability. For outdoor applications this situation is reversed since, in highway applications for example, these materials must be resistant to (a) weather conditions, which are particularly severe when there are freezethaw cycles, (b) acts of vandalism, (c) impacts from vehicles, even though barriers are usually protected by guide rails, and (d) the presence of chlorides from snow ploughing operations and spray. From an acoustical point of view a relatively high sound absorption would be desired in the range 500 to 2000 Hz, where the A-weighted traffic noise contains most of its energy. This paper presents results of a study of the durability and sound absorptive properties of the available sound absorbing materials for highway noise barrier applications. An inquiry letter was sent to the 57 manufacturers listed in references [l] and [2]. Thirty-four replies were received, resulting in the selection for testing of seven materials and one “absorption system”. Materials were not selected if they were to control sound transmission or vibration or if they were for indoor applications exclusively. It was also found that many manufacturers were offering similar products, in which case only one material of a particular type was selected for test. A description of the materials is given in Table 1, and some of their characteristics are listed in Table 2. t Now at Ontario Hydro, 700 University Avenue, Toronto, Ontario M5G 1X6, Canada. $ Now at Wyle Laboratories, 128 Maryland Street, El Segundo, California 90245, U.S.A. 33 0022-460X/80/130033 +22 $02.00/O @ 1980 Academic Press Inc. (London) Limited
A. BEHAR AND D. N. MAY
34
TABLE
1
Description of the materials and absorbing system Material or system no. 1
2
3 4 5 6 7 8 9
Description Low density cellular glass, not containing organic binders. Supplied in 0.30 m X 0.46 m (12 in x 18 in) and 0.46 m X 0.61 m (18 in X 24 in) units in both O-05 m and 0.10 m (2 in and 4 in) thicknesses. See/Figures 3 and 11. Lightweight building material, made of chemically mineralized and neutralized organic softwood shavings, bonded together under pressure with portland cement. It is normally supplied with a hard backing, consisting of 30 mm (1.18 in) thick concrete. All tests were done without the backing. See Figures 3 and 12. Sound absorption system consisting of aluminium cage, enclosing glass wool boards, wrapped in polyethylene. See Figures 4 and 13. Glass fibres bonded in a thermosetting resin. See Figure 5. Same as 4, protected by glass cloth. See Figure 6. Chemically treated long northern aspen wood fibres bonded with portland cement, moulded under pressure. See Figure 7. Homogeneous, 100% recycled, all wood fibre material, processed with moisture resistant ingredients, compressed into a high density structural panel. See Figure 8. Flexible polyurethane
foam. See Figure 9.
Porous, random textured material made of polyester resin, glass fibres, aggregates filters. See Figure
and
10.
Six different tests were applied, some of them on all materials and on the absorption system, and some only on some of the materials and not on the system. They were as follows: (1) the samples were attached for 9 months (through winter) to a wooden noise barrier erected just behind the guide rail of the Queensway freeway in Ottawa; (2) the sound absorption coefficients of most of the materials were measured before and after the above-mentioned weather exposure, in order to see if there were any significant changes in their values; (3) four accelerated durability tests were run in the laboratory. All these tests are described in detail in section 2.
TABLE 2 Density and thickness of the materials
Material no. 1 2 4 and 59 6 7 8 9 t Absorption glass cloth.
Density (kg/m?
Thickness (mm)
120 1000 604:
50 76$ and 50$ so 34 50 25 5Ot. and 34f
470 31 650 test; $ freeze-thaw
tests; 1 without
ABSORPTIVE
NOISE
BARRIER
DURABILITY
35
2. DESCRIPTION OF THE TESTS 2.1.
HIGHWAY
EXPOSURE
All the materials and the absorption system were attached for the nine months between November 1977 and July 1978 to the highway-facing side of a noise barrier, which is approximately 4 m (12 ft) from the edge-of-pavement. Figures 1 and 2 give general views of the samples, and Figures 3-13 show close-ups. The samples were tested for their endurance to the weather, and to the handling involved in their mounting and demounting. Parts of the samples were covered by snow during the winter. They went through freeze-thaw cycles, and were also splashed with a salt-snow-water mixture by passing vehicles. The Average Annual Daily Traffic on the near lane of this highway is estimated to be 15 000 (and is relatively unseasonal). Typical truck mix is 8% on the highway as a whole; it will be greater on the near lane.
Figure 1. General view of the barrier and samples.
Figure 2. General view of the barrier and samples.
A. BEHAR
Figure 3. Materials
AND
D. N. MAY
1 and 2, and the absorption
Figure 4. Absorption
system 3.
system 3.
No attempt was made during the test period to clean, dry or alter the environment of the samples in any way, with only one exception: the sample of material 4 suffered considerable mechanical damage to its surface (see Figure 5). Wind and rain tore away its outer skin and, because of its then ugly appearance (which drew complaints), the sample was covered with glass cloth similar to that provided with material 5, and the test was continued to see if there was to be further deterioration. (It was found later that there was none.)
ABSORPTIVE
NOISE BARRIER
DURABILITY
Figure 5. Material 4 damaged by wind and rain after 4 months of exposure.
Figure 6. Material 5.
At the end of the test period, the materials were removed from the barrier and examined visually for damage. 2.2. ACOUSTICAL TEST The sound absorption coefficients were measured on samfiles of materials 2, 4,6, 7, 8 and 9, by using the test method of ASTM Standard C384-72 [3], chosen because of its simplicity and the small size of sample required.
38
A. BEHAR
AND D. N. MAY
Figure 7. Material 6.
Figure 8. Material 7.
Three sets of measurements were done in all. The first was performed before the materials were exposed, and the second after exposure (see section 2.1), the side tested being obviously the side facing the road. A third set of measurements was also performed, also after exposure, on the side facing the barrier. It was not possible to cut an absorption test sample from material 1 because of its fragility, and so its absorption coefficient was not measured. Because of its size, the
ABSORPTIVE
NOISE
BARRIER
DURABILITY
39
absorption coefficient of system 3 could also not be measured by the test employed here. Being a system and not a material, a small sample would not be expected to perform like a complete unit. The cost of this system was high enough to probably preclude it from serious consideration in purchasing for noise barrier applications; therefore, it was not considered worthwhile to measure absorption by an alternative (reverberation room) method. The after-exposure measurement of material 4 was not performed because of the severe
Figure 9. Material 8.
Figure 10. Material 9.
40
A.
BEtIAR
AND
D. N. MAY
Figure 11. Material 1 damaged during transportation and installation on the barrier (see section 3 .l).
Figure 12. Material 2 after highway exposure showing surface crumbling and loss of material (see sectioIn3.1).
deterioration of its surface, as mentioned in section 2.1. Material 5 was measured w:ithout the glass cloth, which is provided for mechanical protection and should not affect acoustical performance. 2.3. The
FREEZING
AND
THAWING
following two tests on the effects of freeze-thaw
cycles were carried out.
ABSORPTIVE
NOISE
BARRIER
DURABILITY
41
Figure 13. System 3 after highway exposure showing twisted lower cage, presumably from snow clearing operations (see section 3.1).
Total immersion. A specimen of each product was immersed in a 3% NaCl solution and subjected to 50 freeze-thaw cycles. It was felt that this test simulated a very severe condition for these materials and would give some indication of their durability in a saturated state. Partial immersion. A specimen of each product was held in a vertical position in a tray of 3% NaCl solution. The level of the solution was maintained between 25 mm and 50 mm above the bottom of the specimen. Each specimen was splashed above the water line twice a day before each freezing cycle. The specimens were subjected to 50 freeze-thaw cycles. This test was intended to simulate a field situation where panels of these materials might be standing in water and subjected to freezing and thawing. This test also gave some indication of the damage that might be expected because of capillary action. (In some situations capillary action might increase the area saturated and cause physical destruction above the original water line by freeze-thaw cycles.)
2.4.
ACCELERATED
WEATHERING
AND
SALT
SPRAY
EXPOSURE
The materials were exposed for 2500 hours in an Atlas Twin-Arc weatherometer operating at 55°C. The weathering cycle used consisted of 102 minutes of light and 18 minutes of light and water (2500 hours of exposure in the weatherometer is approximately equivalent to about seven years of exterior exposure in southern Ontario for structural steel coatings). In the salt spray cabinet, the materials were exposed for 1000 hours in accordance with ASTM B117 Standard Method of Salt Spray Testing [4].
42
A. BEHAR
AND D. N. MAY
3. EXPERIMENTAL RESULTS 3.1. HIGHWAY
EXPOSURE
Material 1 was very fragile and several blocks either chipped or fell apart during installation on the barrier (see Figure 11). Even though this damage was not due to the exposure itself, the fragility of this material seems to be a very serious handicap for its use on noise barriers, unless it is structurally protected and very carefully handled. Material 2 showed a small degree of deterioration: a loss of material at its edges. The deterioration was more severe on the lower part which was often buried in snow (see Figure 12). System 3 showed no visible effects due to weather. One side of its cage was found to be twisted as if from the pressure of snow during clearing operations (see Figure 13); however the source of damage was not directly observed. Material 4 suffered severe mechanical damage found to be progressive with time. Successive layers of the material were virtually torn apart by cumulative weather effects, as can be observed in Figure 5. Material 5 showed no signs of damage after exposure. This suggested that the glass cloth could protect material 4 adequately, since it differs only in having no such covering. Materials 6-9 did not show signs of damage after exposure. 3.2. ACOUSTICAL TEST Table 3 presents the sound absorption coefficients, (Y,of materials 2,4,5,6,7,8 and 9, measured before and after exposure on the barrier. Figures 14-19 show the variation of CY with frequency. All these results are for the highway-facing side of the samples, and indicate very little change in cx over the period of exposure; this was also true for the barrier-facing sides of the samples. TABLE
3
Sound absorption coefficients of the material before (I) and after (II) exposure
Sound absorption coefficient Test frequency (I-W
IS
115
I
II
I
II
I
II
I
II
I
II
200 250 315 400 500 630 800 1000 1250 1600 2000
0.22 0.32 0.52 0.79 0.94 0.71 0.52 0.44 0.58 0.83 0.57
0.25 0.36 0.55 0.84 0.93 0.70 0.48 0.39 0.45 0.77 0.49
0.10 0.16 0.20 0.30 0.33 0.46 0.59 0.70 0.79 0.89 0.92
0.13 0.19 0.23 0.30 0.40 0.52 0.64 0.72 0.84 0.91 0.92
0.12 0.13 0.12 0.14 0.17 0.21 0.33 0.52 0.81 0.83 0.53
o-14 0.15 0.14 0.15 0.17 0.21 0.35 0.54 0.81 0.82 0.53
0.05 0.07 0.06 0.08 0.12 0.13 0.22 0.23 0.15 0.11 0.08
0.05 0.05 0.07 0.08 0.15 0.13 0.10 0.13 0.10 0.09 0.06
0.21 0.12 0.13 0.16 0.16 0.22 0.27 0.34 0.49 0.68 0.84
0.05 0.06 0.08 0.11 0.08 0.16 0.25 0.33 0.49 0.70 0.85
0.04 0.07 0.10 0.15 0.21 0.31 0.57 0.81 0.95 0.75 0.53
0.15 0,14 0.12 0.15 0.17 0.23 0.37 0.60 0.89 0.95 0.70
NRC11
0.55
0.55
0.55
0.55
0.35
0.35
0.15
0.10
0.35
0.35
0.40
0.40
2t
4 and 5
6
7
8
9
t Material number. $ Value of (I before exposure. 8 Value of a after exposure. oise reduction coefficient, equal to the mean value of a at the frequencies of 250,500,lOOO ro!!nNdedto the nearest 0.05.
and 2000 Hz,
ABSORPTIVE
NOISE
BARRIER
DURABILITY
43
It should be noted that all these results are from standing wave tube measurements, in which the material sample is placed against a thick steel plate without an intervening air gap. The results therefore apply for only a hard-backed mounting, and the values of (Yfor these materials may change in other circumstances [5]. In general, absorptive materials can behave in three ways depending on whether they are (a) mounted freely without a backing, (b) mounted on a hard surface (such as steel or concrete) without an air gap, or (c) mounted on a hard surface, but with an intervening air gap. With the first mounting, absorption coefficients are usually very low at low frequencies but rise steadily as frequency increases. Therefore they may absorb high frequency (e.g., tyre) noise better than low frequency (e.g., engine) noise. The second of these mountings often results in a resonant absorptive peak in the mid-frequency range which improves the overall behaviour of the material. With the third mounting, it is sometimes possible to shift the resonance peak towards the lower end of the frequency range by increasing the air gap, thus increasing absorption at traffic noise frequencies. Furthermore, if mechanical protection such as a perforated metallic covering is provided, more changes are likely to occur in the absorption coefficient such as an increase in the middle frequency range and a decrease at high frequencies [S]. These are general trends in the behaviour of absorptive materials. Standing wave tube measurements are a useful means to compare the absorption of some classes of these materials on a common basis, and to assess the change in (Ywith time. However, the actual sound absorption of a material mounted in a particular way can only be found by a proper measurement of a test sample mounted in the same way as it will be mounted in situ. (Further discussion on this subject appears in the Appendix.) The following are observations regarding the absorption coefficients of each of the materials, tested in a standing wave tube, with a hard backing and without an air gap.
0
I 250
I
,
1,
500 Frequency
I
I
1000
I
I 2ocQ
(Hz)
Figure 14. Sound absorption coefficient of material 2 before (0) and after (0) exposure.
Material 2 (see Figure 14). Only the absorptive part of the material was tested because of the difficulties involved in cutting the concrete backing for the standing wave tube sample. Nevertheless, since the measurement was done with the hard backing provided by the measurement device, the results should be similar to those obtained from a concretebacked sample of the material (see the Appendix and Figure A3). Two resonant peaks can be observed in Figure 14.
A. BEHAR
44
01
I
’
250
’
AND D. N. MAY
1
’
500
Frequency
’
1
’
loo0
’
a
1
MOO
b-k)
Figure 15. Sound absorption coefficient of material 5 before (a) and after (0) exposure. (Measurements were performed without the glass cloth.)
Materials 5 and 8 (Figures 15 and 18 respectively). Both these materials are typical, acoustically porous materials, whose absorption coefficients increase steadily with frequency (see also Table 3). The change in (Y of material 5 with exposure cannot be considered significant. This is also true of material 8, where a somewhat larger difference between the before and after exposure values of (Ycan be observed in the low frequency range due to the relatively low accuracy of the measuring method for values of a! lower than about 0.2.
0.
IO t
0’
b
’
250
I
2
’
Frequency
’
’
’
1000
’
’
1
2000
(Hz)
Figure 16. Sound absorption coefficient of material 6 before (0) and after (0) exposure.
Material 6 (Figure 16). The absorption coefficient of material 6 has a resonant peak between 1000 and 2000 Hz, and relatively low values at other frequencies. It may therefore absorb tyre noise more than engine noise. Material 7 (Figure 17). Material 7 cannot be considered sound absorbing, because of its very low value of (Y.The apparent changes in the values of the coefficient before and after exposure, which can be observed in Figure 17, are probably due to the previously mentioned inaccuracy in measurements of low values of CL
ABSORPTIVE
0’
I
I 250
NOISE
BARRIER
1
45
DURABILITY
I 500
I
Frequency
(Hz)
1000
2000
Figure 17. Sound absorption coefficient of material 7 before (0) and after (0) exposure.
Frequency
(Hz)
Figure 18. Sound absorption coefficient of material 8 before (0) and after (0) exposure.
Material 9 (Figure 19). The absorption coefficient of material 9 has a resonant peak between 1250 and 1600 Hz, coincident with much of the A-weighted highway noise spectrum. No explanation could be found for the apparent shift of the resonant frequency after exposure.
0’
7
’
’
’
’
’
250
’
’ 000
Frequency
’
’
I 2000
(Hz)
Figure 19. Sound absorption coefficient of material 9 before (0) and after (0) exposure.
46
A.
3.3.
FREEZING
AND
BEHAR
AND
D. N. MAY
THAWING
As the freezing and thawing tests provided no numerical data, the results are given as a table of observations of the condition of each material for each test at every tenth cycle. These results are shown in Tables 4 and 5, corresponding to tests (A) and (B) respectively. (A) and (B) refer to total immersion and partial immersion, respectively. The following are general comments on the results of the different tests. Material 2. (A) This product disintegrated when exposed to freezing and thawing in a saturated condition. The damage is the result of a combination of the swelling of the saturated wood particles and the pressure generated by the solution expanding when freezing. (B) After 50 cycles of freezing and thawing, this material was wet and had swelled for approximately 50 mm above the level of the salt solution. This wetting and swelling was probably due to capillary action. The material above this area wetted by capillary action showed no effects from the exposure conditions. Material 4. This material showed damage in both partial and total immersion tests. Because of its softness and low strength, most of the damage was probably from handling while saturated with water. Material 6. (A) This product softened and lost strength during the exposure to freezing in the salt solution. Because the wood in this product was in the form of long thin strips, the specimens tested only softened and swelled rather than disintegrated. If the exposure had TABLE
4
Total immersion in salt solution-observations
Freeze-thaw cycle no. Material no.
10
20
30
40
Top 15 mm quite friable
Quite friable separating from mortar coated rebar
Continued deterioration top 20 mm breaks easily
Continued deterioration, mortar covered bars can be lifted out
4
No change
No change
Slight softening
No change
No significant change
6
Softening
Softness increasing
Softness increasing
Spongy, can be pulled apart
Spongy
7
Slight softening
Softness increasing
Softness increasing, swelling
Further softening, edges can be pulled apart
Quite soft, swollen, still retained shape
8
No change
No change
No change
No change
No visible sign of deterioration
9
No change
No change
Very slight deterioration at top edge
No change
Material still hard, brittle, with only a suggestion of deterioration at top
2
30 TOP
completely disintegrated, area below in slightly better condition
10
Immersed section swelling
Absorbed solution approx. 20 mm above liquid
Portion immersed swelling to approx. 25 mm above surface of solution
Submerged portion soft, absorbed solution approx. 25 mm above liquid level
Absorbed solution approx. 20 mm above liquid
Absorbed solution approx. 50 mm above liquid
Material no.
2
4
6
7
8
9
of
No change
No change
Immersed portion quite soft and swelling
Immersed portion starting to break up
Slight deterioration submerged portion
Immersed portion starting to break up, quite friable
20
30
cycle no.
Slight deterioration submerged edges
No change on
Swelling increasing slowly, moving upwards
Swelling moving upwards
No change
Swelling moving upwards above submerged section; immersed portion more friable
Freeze-thaw 40
No major change
No change
Bottom 75 mm can be pulled apart, swelling increasing
Top still hard, bottom continues to soften, swelling moving up
Deterioration on bottom from handling and freezing, on top from handling
Swelling approx. 50 mm above solution
Partial immersion in salt solution-observations
TABLE 5
No change
No change
Slight increase in swelling, starting to separate into layers
Slight increase in swelling, crumbles readily at bottom
No major change
Increased swelling, crumbling of submerged portion
50
No.
Good (NRC = O-55)
Little loss of material, more severe on the lower part of the sample
Visually undamaged
Material torn apart by cumulative weather effects
Visually undamaged
Visually undamaged
Wood shavings bonded with Portland cement
Aluminium cage enclosing glass wool wrapped in polyethylene
Glass fibres bonded in a thermosetting resin
Same as 4, protected by glass cloth
Wood fibres bonded with Portland cement
None
Not tested
None
Not tested
Fair (NRC = 0.35)
Not tested
None
Not tested
Change in sound absorption after highway exposure
Good (NRC = 0.55)
Not tested
Not tested
Sound absorptioni’
Very fragile, fell apart during installation
Highway exposure (9 months)
Low density cellular glass
Description
Material
Softened and swelled
Softened and swelled up to 50 mm above water level
Not tested
Forms a mass of loose fibres
Forms a mass of loose fibres
Not tested
Not tested
Not tested
Swelled up 50 mm above the water level
Not tested
Not tested
Disintegrated
Partial immersion
Total immersion
Tests and results
Overall summary of tests and results
TABLE 6
Adhesive properties of the cement destroyed
Not tested
Loses rigidity and forms a mass of glass fibres
Not tested
Adhesive properties of the cement destroyed
Not tested
Accelerated weathering
No reaction to salt
Not tested
Loses rigidity and forms a mass of glass fibres
Not tested
No reaction to salt
Not tested
Salt spray exposure
E
$ u P z
8 ? P
?
Visually undamaged
Random textured glass fibres and polyester resin
Good (NRC = 0.40)
Fair (NRC = 0.35)
Poor (NRC = O-15)
Softened and swelled
Undamaged
Undamaged
None
None
None
Undamaged
Surface degradation and 50% thickness shrinkage
Delaminated completely
Undamaged
No reaction to salt
Delaminated completely
poor NRC< 0.20; tested according to ASTM C384-72 Hz, rounded to the nearest 0.05.
Undamaged
Undamaged
Softened and swelled up to 75 mm above water level
t Basis for ratings: Excellent NIXa 0.80; very good NRC 0.60-0.75; good NRC 0.40-0.55; fair NRC 0.20-0.35; [3]; NRC (noise reduction coefficient) = mean value of the sound absorption coefficients at 250, 500, 1000 and 2000
Visually undamaged
Visually undamaged
Flexible polyurethane foam
material compressed into a high density structural panel
Wood fibre
; i: q 4
3
g
$ 2
2
zz
iz
3
4
$ “0
A. BEHAR
50
AND
D. N. MAY
been continued it is expected that the material would have shown deterioration similar to that of material 2. (B) As with material 2 this specimen softened and swelled for approximately 50 mm above the solution. Material above this area wetted by capillary action showed no effects from exposure conditions. Material 7. (A) Over the length of the test this material softened, absorbed the solution, and swelled. The swelling, softening and absorbing were continuing when the test was stopped. (B) The material softened and swelled in and above the solution to a height of 75 mm. The material above the wetted portion showed no effect from the exposure conditions. Material 8. This material showed no evidence of change in either of the two tests. Material 9. This material started out as a hard, brittle, porous material in each test. Although each specimen lost a small amount of material on some corners, the specimens remained essentially unchanged at the end of each test. 3.4.
ACCELERATING
WEATHERING
AND
SALT
SPRAY
EXPOSURE
Material 2. After 2500 hours of accelerate&weathering
test, particles of wood chips crumbled or flaked off the face and edges of the test pieces, probably caused by the expansion and contraction of the wood chips during the wetting and drying cycles in the weatherometer, which appears to destroy the adhesive properties of the cement. Black discoloration (possibly algae) appeared to be forming on the wood chips. The salt spray test indicates that the material did not react with salt, but it did become saturated and heavy with absorbed water. Material 4. Due to the absorption of water by the fibreglass panels in both the weatherometer and the salt spray test, this material rapidly lost rigidity by separation of layers of fibreglass and sagging to form a mass of loose fibres. Material 6. Similar to material 2, the adhesive holding the straw-like fibres together appeared to be washed out, leaving loose fibres at the surface of the accelerated weathering panels. Due to the porous construction, a large amount of water was absorbed by this material. In addition a considerable amount of black discoloration (possibly algae) formed on the surface and extended into the interior of the panel. Other than water absorption by the fibres, there appeared to be no reaction with salt water. Material 7. In both the accelerated weathering and salt spray tests, this material absorbed and retained large amounts of water and when subjected to accelerated weathering and salt spray exposure, the four layers of compressed paper expanded and completely delaminated. Material 8. The foam sponge showed surface degradation and 50% shrinkage in thickness during accelerated weathering causing black carbon formation on the surface. Other than absorption of water by the foam, no change in the sample was noted in the salt spray exposure. Material 9. Little or no degradation was apparent after exposure in the weathering and salt spray cabinet. 4. CONCLUSIONS The conclusions below are based on the results of the tests described in this report. These tests, while typical of those conducted for acoustical materials on the one hand and concrete highway-use products on the other, are not necessarily the best or only tests for this application. Note particularly that noise barrier installations are designed for adequate drainage, and that freeze-thaw cycles while the barrier material is immersed in water constitute an extremely harsh and not entirely representative ,test environment. The
ABSORPTIVE
NOISE
BARRIER
DURABILITY
51
authors recommend that the results be treated with caution and viewed as resource material for decision-making. Table 6 contains a summary of the results of each test for the various materials. Conclusions are also given below.
4.1.
HIGHWAY
EXPOSURE
AND
SOUND
ABSORPTION
COEFFICIENT
various tests provide tentative conclusions about the ability of the materials and the absorption system to withstand weather exposure while still possessing sound absorptive properties. Material 1 cannot be recommended for highway barrier use. Its extreme fragility makes its handling difficult. Its sound absorption coefficient was not tested. After exposure, material 2 showed a small degree of surface crumbling and loss of material mostly from the lower part of the sample. Its sound absorption coefficient was “good” and did not change with highway exposure. The sound absorption coefficient of system 3 was not tested because of the impossibility of cutting a sample for the tube measurement. The system was visually undamaged by highway exposure. Material 4 clearly showed its inadequacy for outdoor use without protection. However material 5, which was undamaged by highway exposure and had presumably the same “good” sound absorption before and after highway exposure, showed that the protection provided by glass cloth might cause material 4 to be equally suitable for highway barrier applications. Material 7 cannot be described as acoustically absorbent because of the low value of its absorption coefficient over the frequency range of interest. Its good resistance to highway exposure is not, therefore, of much interest. The sound absorption of materials 6 and 8 was “fair” with a relatively good absorption of high frequency (i.e., tyre) noise and relatively low absorption of low (i.e., engine) noise, both of which were unaltered by highway exposure. Finally, the sound absorption coefficient of material 9, unaltered by highway exposure, was “good”. The material was found to be visually undamaged by the exposure. The
AND THAWING 4.2. FREEZING Two products, materials 2 and 6, softened and swelled badly due to saturation and freezing. Based on the results of the tests performed, these two products cannot be recommended for exposed locations where they will be subjected to freeze-thaw cycles while sitting in a pond of salt solution. The results quoted for material 2 are, however, those for the less dense side of the sample, which is generally bonded to concrete to form the noise barrier panel. The other side of the sample, which is the side more exposed to the weather, is more dense and deteriorated much more slowly. However, a faster rate of deterioration can be expected once its outer “skin” has been penetrated. Material 4 showed a lot of damage due to handling. Due to its frail nature, it cannot be recommended for exposed surfaces. Material 7 swelled and softened considerably over the course of these tests. As deterioration was still continuing at the end of the tests, this product is not recommended for exposed locations. Two materials, 8 and 9, showed essentially no deterioration in either test. Based on the tests performed, material 7 should be a more durable product for an exposed surface because of its rigidity.
52
A. BEHAR
AND
D. N. MAY
4.3.ACCELERATED WEATHERING AND SALT SPRAY EXPOSURE Both of the wood-cement products, materials 2 and 6, tended to disintegrate slowly upon weathering and may be subject to biological degradation. Material 7 is unsatisfactory because of complete delamination upon weathering. Both materials 4 and 8 did not have satisfactory rigidity. Without additional protective cover and support, these materials shrank and deformed so that they did not appear to be satisfactory for use as self-supporting noise absorptive materials. Of the six sound absorption materials tested only material 9 appeared to be unaffected by accelerated weathering and salt spray. It has sharp rough edges and very coarse surface texture so that care should be exercised in handling it.
ACKNOWLEDGMENTS The sound absorption measurements were performed at the MTC Research Laboratory by G. Giles and A. Maio. Freezing and thawing tests were done at the Concrete Unit (T. Church), Materials and Laboratory Services Section, and the accelerated weathering and salt spray exposure tests at the Chemicals and Metals Unit (R. Fujii) of the same Section. Thanks are also due to the MTC District Office, Ottawa, for help over installation of the samples of the weather exposure test. We also wish to express gratitude to Dr T. D. Northwood and W. T. Chu of the National Research Council, Ottawa, for assistance with the work described in the Appendix. Finally we want to thank the manufacturers of materials 1, 2, 3,6, 7, 8 and 9 for kindly supplying samples and supporting literature.
REFERENCES July 1. Buyer’s Guide to Materials for Noise and Vibration Control 1977 Sound and Vibration 11,7, 1977. 2. A. THUMANN and R. MILLER 1974 Secrets of Noise Control. Atlanta: Fairmont Press. 3. Standard Test Method for Impedance and Absorption ofAcoustical Materials by the Tube Method. ASTM C384-72, American Society for Testing and Materials, 1972. 4. Standard Method of Salt Spray (Fog) Testing. ASTM B-117-73, American Society for Testing and Materials, 1973. 5. W. FURRER 1964 Room and Building Acoustics and Noise Abatement. London: Butterworths. See chapter 2.
APPENDIX: THE SOUND ABSORPTIVE QUALITIES OF MATERIAL 2 FOR VARIOUS METHODS OF INSTALLATION To test the influence of the mounting of material 2 upon its sound absorption coefficient, several tests were performed at the Division of Building Research, National Research Council, Ottawa. The sample area was 8*34m2 (89.8 ft’) and its thickness 75mm (2.95 in). The measurement was done according to the ASTM C423-72 standard. The sample was tested in two different ways: laid on the floor (mounting 4 according to the standard) and in an upright, free-standing position. The results of the measurements are shown in Figure Al. When the material is free-standing (curve B) it behaves as acoustically porous. Its absorption coefficient is low at low frequencies, and increases steadily as frequency rises. When, on the other hand, there is a hard material behind, as appears when the material is laid on the floor, a resonant peak appears (curve A) which quite radically changes the absorption coefficient values at the
ABSORPTIVE
NOISE BARRIER
500
53
DURABILITY
1000
2000
4cim
8cco
Frequency(Hz)
Figure Al. Sound absorption coefficient of material 2 in reverberation standing. Noise reduction coefficient (NRC): A, 0.85; B, 0.54.
room: A, laid on floor; B, free-
mid-frequencies. The same material is then quite highly absorbent. Its noise reduction coefficient (NRC) rises from 0.54 to 0.85. Figure A2 shows the results of two similar measurements performed on a sample of material 2 as used in the field. This sample is composed of 50 mm sound absorbing material with a 30 mm steel reinforced concrete backing. The differences between the two curves are minor, thus allowing one to conclude that the hard backing is responsible for the beneficial peak in (Yaround 500 Hz evidenced in Figure Al. ,
I
,
,
I
(
I
I
q
1.1 I.0 0.9 0.8 @70.6 0.5 D4-
01
’ 125
I
I
’ 250
4
“‘I
500
” loo0
I
”
2ocO
”4000
“I
.9m
Frequency (Hz)
Figure A2. Sound absorption coefficient of material 2 as used in practice (with concrete backing) in reverberation room: A, laid on floor; B, free-standing. Noise reduction coefficient (NRC): A, 0.71; B, 0.66.
Finally, Figure A3 shows the results of the measurement of material 2 in the reverberant room (same as curve A, Figure Al) and by the standing wave measuring method (also measured at National Research Council in this case) according to ASTM C384-72 standard. The absolute values of LYcannot be compared for two reasons: the sound field is in one case diffuse (in the reverberant room), and in the other it is normal (in the tube). Also the units are in the first case sabine (S), and in the other a sound pressure ratio. What can be compared is the variation of (Ywith frequency in the two curves. In this case they are similar, meaning that both the floor of the room and the bottom of the tube are
A. BEHAR
54
O;; 125
250
AND D. N. MAY
2000
1000
500 Frequency
4Oco
8000
(HZ)
Figure A3. Sound absorption coefficient of material 2: A, laid on floor in reverberation wave tube.
room; B, in standing
acting in the same way. This permits the conclusion that the tube method gives similar results to the conventional reverberant method in which the material is laid on the floor, and that the general trend of the sound absorption coefficients obtained from the tube measurements is only valid if the material is mounted in practice against a hard wall (or barrier).
DURABILITY
OF SOUND
FOR HIGHWAY A. BEHAR?
ABSORBING
MATERIALS
NOISE BARRIERS AND
D.N.MAYS
Acoustics Ofice, Research and Development Division, Ministry of Transportation and Communications, Downsview, Ontario M3M 138, Canada (Received 24 September 1979, and in revised form 26 January 1980) This paper presents the results of a research study into two properties of sound absorbing materials intended for highway noise barrier applications: their durability and their sound absorption coefficients before and after exposure to adverse weather. After surveying the products of 34 manufacturers, eight materials and one “absorption system” were tested in the field and the laboratory. Even though there is no single, accepted test for weather
endurance for these kinds of material, the results of the study provide information on their likely future behaviour when installed on a barrier. The results apply at least partially to other outdoor situations too, and augment the limited information presently available on sound absorptive materials for outdoor use.
1. INTRODUCTION
There is considerable interest in the use of sound absorbing materials to improve the performance of highway noise barriers. Some studies have resulted in claims that they improve barrier attenuation. Without entering into a critical analysis of these studies, there are accompanying questions to be raised about the outdoor use of these materials, which is a relatively new application for them. When sound absorptive materials are used indoors, they have to comply with requirements related more to the danger of fire than to durability. For outdoor applications this situation is reversed since, in highway applications for example, these materials must be resistant to (a) weather conditions, which are particularly severe when there are freezethaw cycles, (b) acts of vandalism, (c) impacts from vehicles, even though barriers are usually protected by guide rails, and (d) the presence of chlorides from snow ploughing operations and spray. From an acoustical point of view a relatively high sound absorption would be desired in the range 500 to 2000 Hz, where the A-weighted traffic noise contains most of its energy. This paper presents results of a study of the durability and sound absorptive properties of the available sound absorbing materials for highway noise barrier applications. An inquiry letter was sent to the 57 manufacturers listed in references [l] and [2]. Thirty-four replies were received, resulting in the selection for testing of seven materials and one “absorption system”. Materials were not selected if they were to control sound transmission or vibration or if they were for indoor applications exclusively. It was also found that many manufacturers were offering similar products, in which case only one material of a particular type was selected for test. A description of the materials is given in Table 1, and some of their characteristics are listed in Table 2. t Now at Ontario Hydro, 700 University Avenue, Toronto, Ontario M5G 1X6, Canada. $ Now at Wyle Laboratories, 128 Maryland Street, El Segundo, California 90245, U.S.A. 33 0022-460X/80/130033 +22 $02.00/O @ 1980 Academic Press Inc. (London) Limited
A. BEHAR AND D. N. MAY
34
TABLE
1
Description of the materials and absorbing system Material or system no. 1
2
3 4 5 6 7 8 9
Description Low density cellular glass, not containing organic binders. Supplied in 0.30 m X 0.46 m (12 in x 18 in) and 0.46 m X 0.61 m (18 in X 24 in) units in both O-05 m and 0.10 m (2 in and 4 in) thicknesses. See/Figures 3 and 11. Lightweight building material, made of chemically mineralized and neutralized organic softwood shavings, bonded together under pressure with portland cement. It is normally supplied with a hard backing, consisting of 30 mm (1.18 in) thick concrete. All tests were done without the backing. See Figures 3 and 12. Sound absorption system consisting of aluminium cage, enclosing glass wool boards, wrapped in polyethylene. See Figures 4 and 13. Glass fibres bonded in a thermosetting resin. See Figure 5. Same as 4, protected by glass cloth. See Figure 6. Chemically treated long northern aspen wood fibres bonded with portland cement, moulded under pressure. See Figure 7. Homogeneous, 100% recycled, all wood fibre material, processed with moisture resistant ingredients, compressed into a high density structural panel. See Figure 8. Flexible polyurethane
foam. See Figure 9.
Porous, random textured material made of polyester resin, glass fibres, aggregates filters. See Figure
and
10.
Six different tests were applied, some of them on all materials and on the absorption system, and some only on some of the materials and not on the system. They were as follows: (1) the samples were attached for 9 months (through winter) to a wooden noise barrier erected just behind the guide rail of the Queensway freeway in Ottawa; (2) the sound absorption coefficients of most of the materials were measured before and after the above-mentioned weather exposure, in order to see if there were any significant changes in their values; (3) four accelerated durability tests were run in the laboratory. All these tests are described in detail in section 2.
TABLE 2 Density and thickness of the materials
Material no. 1 2 4 and 59 6 7 8 9 t Absorption glass cloth.
Density (kg/m?
Thickness (mm)
120 1000 604:
50 76$ and 50$ so 34 50 25 5Ot. and 34f
470 31 650 test; $ freeze-thaw
tests; 1 without
ABSORPTIVE
NOISE
BARRIER
DURABILITY
35
2. DESCRIPTION OF THE TESTS 2.1.
HIGHWAY
EXPOSURE
All the materials and the absorption system were attached for the nine months between November 1977 and July 1978 to the highway-facing side of a noise barrier, which is approximately 4 m (12 ft) from the edge-of-pavement. Figures 1 and 2 give general views of the samples, and Figures 3-13 show close-ups. The samples were tested for their endurance to the weather, and to the handling involved in their mounting and demounting. Parts of the samples were covered by snow during the winter. They went through freeze-thaw cycles, and were also splashed with a salt-snow-water mixture by passing vehicles. The Average Annual Daily Traffic on the near lane of this highway is estimated to be 15 000 (and is relatively unseasonal). Typical truck mix is 8% on the highway as a whole; it will be greater on the near lane.
Figure 1. General view of the barrier and samples.
Figure 2. General view of the barrier and samples.
A. BEHAR
Figure 3. Materials
AND
D. N. MAY
1 and 2, and the absorption
Figure 4. Absorption
system 3.
system 3.
No attempt was made during the test period to clean, dry or alter the environment of the samples in any way, with only one exception: the sample of material 4 suffered considerable mechanical damage to its surface (see Figure 5). Wind and rain tore away its outer skin and, because of its then ugly appearance (which drew complaints), the sample was covered with glass cloth similar to that provided with material 5, and the test was continued to see if there was to be further deterioration. (It was found later that there was none.)
ABSORPTIVE
NOISE BARRIER
DURABILITY
Figure 5. Material 4 damaged by wind and rain after 4 months of exposure.
Figure 6. Material 5.
At the end of the test period, the materials were removed from the barrier and examined visually for damage. 2.2. ACOUSTICAL TEST The sound absorption coefficients were measured on samfiles of materials 2, 4,6, 7, 8 and 9, by using the test method of ASTM Standard C384-72 [3], chosen because of its simplicity and the small size of sample required.
38
A. BEHAR
AND D. N. MAY
Figure 7. Material 6.
Figure 8. Material 7.
Three sets of measurements were done in all. The first was performed before the materials were exposed, and the second after exposure (see section 2.1), the side tested being obviously the side facing the road. A third set of measurements was also performed, also after exposure, on the side facing the barrier. It was not possible to cut an absorption test sample from material 1 because of its fragility, and so its absorption coefficient was not measured. Because of its size, the
ABSORPTIVE
NOISE
BARRIER
DURABILITY
39
absorption coefficient of system 3 could also not be measured by the test employed here. Being a system and not a material, a small sample would not be expected to perform like a complete unit. The cost of this system was high enough to probably preclude it from serious consideration in purchasing for noise barrier applications; therefore, it was not considered worthwhile to measure absorption by an alternative (reverberation room) method. The after-exposure measurement of material 4 was not performed because of the severe
Figure 9. Material 8.
Figure 10. Material 9.
40
A.
BEtIAR
AND
D. N. MAY
Figure 11. Material 1 damaged during transportation and installation on the barrier (see section 3 .l).
Figure 12. Material 2 after highway exposure showing surface crumbling and loss of material (see sectioIn3.1).
deterioration of its surface, as mentioned in section 2.1. Material 5 was measured w:ithout the glass cloth, which is provided for mechanical protection and should not affect acoustical performance. 2.3. The
FREEZING
AND
THAWING
following two tests on the effects of freeze-thaw
cycles were carried out.
ABSORPTIVE
NOISE
BARRIER
DURABILITY
41
Figure 13. System 3 after highway exposure showing twisted lower cage, presumably from snow clearing operations (see section 3.1).
Total immersion. A specimen of each product was immersed in a 3% NaCl solution and subjected to 50 freeze-thaw cycles. It was felt that this test simulated a very severe condition for these materials and would give some indication of their durability in a saturated state. Partial immersion. A specimen of each product was held in a vertical position in a tray of 3% NaCl solution. The level of the solution was maintained between 25 mm and 50 mm above the bottom of the specimen. Each specimen was splashed above the water line twice a day before each freezing cycle. The specimens were subjected to 50 freeze-thaw cycles. This test was intended to simulate a field situation where panels of these materials might be standing in water and subjected to freezing and thawing. This test also gave some indication of the damage that might be expected because of capillary action. (In some situations capillary action might increase the area saturated and cause physical destruction above the original water line by freeze-thaw cycles.)
2.4.
ACCELERATED
WEATHERING
AND
SALT
SPRAY
EXPOSURE
The materials were exposed for 2500 hours in an Atlas Twin-Arc weatherometer operating at 55°C. The weathering cycle used consisted of 102 minutes of light and 18 minutes of light and water (2500 hours of exposure in the weatherometer is approximately equivalent to about seven years of exterior exposure in southern Ontario for structural steel coatings). In the salt spray cabinet, the materials were exposed for 1000 hours in accordance with ASTM B117 Standard Method of Salt Spray Testing [4].
42
A. BEHAR
AND D. N. MAY
3. EXPERIMENTAL RESULTS 3.1. HIGHWAY
EXPOSURE
Material 1 was very fragile and several blocks either chipped or fell apart during installation on the barrier (see Figure 11). Even though this damage was not due to the exposure itself, the fragility of this material seems to be a very serious handicap for its use on noise barriers, unless it is structurally protected and very carefully handled. Material 2 showed a small degree of deterioration: a loss of material at its edges. The deterioration was more severe on the lower part which was often buried in snow (see Figure 12). System 3 showed no visible effects due to weather. One side of its cage was found to be twisted as if from the pressure of snow during clearing operations (see Figure 13); however the source of damage was not directly observed. Material 4 suffered severe mechanical damage found to be progressive with time. Successive layers of the material were virtually torn apart by cumulative weather effects, as can be observed in Figure 5. Material 5 showed no signs of damage after exposure. This suggested that the glass cloth could protect material 4 adequately, since it differs only in having no such covering. Materials 6-9 did not show signs of damage after exposure. 3.2. ACOUSTICAL TEST Table 3 presents the sound absorption coefficients, (Y,of materials 2,4,5,6,7,8 and 9, measured before and after exposure on the barrier. Figures 14-19 show the variation of CY with frequency. All these results are for the highway-facing side of the samples, and indicate very little change in cx over the period of exposure; this was also true for the barrier-facing sides of the samples. TABLE
3
Sound absorption coefficients of the material before (I) and after (II) exposure
Sound absorption coefficient Test frequency (I-W
IS
115
I
II
I
II
I
II
I
II
I
II
200 250 315 400 500 630 800 1000 1250 1600 2000
0.22 0.32 0.52 0.79 0.94 0.71 0.52 0.44 0.58 0.83 0.57
0.25 0.36 0.55 0.84 0.93 0.70 0.48 0.39 0.45 0.77 0.49
0.10 0.16 0.20 0.30 0.33 0.46 0.59 0.70 0.79 0.89 0.92
0.13 0.19 0.23 0.30 0.40 0.52 0.64 0.72 0.84 0.91 0.92
0.12 0.13 0.12 0.14 0.17 0.21 0.33 0.52 0.81 0.83 0.53
o-14 0.15 0.14 0.15 0.17 0.21 0.35 0.54 0.81 0.82 0.53
0.05 0.07 0.06 0.08 0.12 0.13 0.22 0.23 0.15 0.11 0.08
0.05 0.05 0.07 0.08 0.15 0.13 0.10 0.13 0.10 0.09 0.06
0.21 0.12 0.13 0.16 0.16 0.22 0.27 0.34 0.49 0.68 0.84
0.05 0.06 0.08 0.11 0.08 0.16 0.25 0.33 0.49 0.70 0.85
0.04 0.07 0.10 0.15 0.21 0.31 0.57 0.81 0.95 0.75 0.53
0.15 0,14 0.12 0.15 0.17 0.23 0.37 0.60 0.89 0.95 0.70
NRC11
0.55
0.55
0.55
0.55
0.35
0.35
0.15
0.10
0.35
0.35
0.40
0.40
2t
4 and 5
6
7
8
9
t Material number. $ Value of (I before exposure. 8 Value of a after exposure. oise reduction coefficient, equal to the mean value of a at the frequencies of 250,500,lOOO ro!!nNdedto the nearest 0.05.
and 2000 Hz,
ABSORPTIVE
NOISE
BARRIER
DURABILITY
43
It should be noted that all these results are from standing wave tube measurements, in which the material sample is placed against a thick steel plate without an intervening air gap. The results therefore apply for only a hard-backed mounting, and the values of (Yfor these materials may change in other circumstances [5]. In general, absorptive materials can behave in three ways depending on whether they are (a) mounted freely without a backing, (b) mounted on a hard surface (such as steel or concrete) without an air gap, or (c) mounted on a hard surface, but with an intervening air gap. With the first mounting, absorption coefficients are usually very low at low frequencies but rise steadily as frequency increases. Therefore they may absorb high frequency (e.g., tyre) noise better than low frequency (e.g., engine) noise. The second of these mountings often results in a resonant absorptive peak in the mid-frequency range which improves the overall behaviour of the material. With the third mounting, it is sometimes possible to shift the resonance peak towards the lower end of the frequency range by increasing the air gap, thus increasing absorption at traffic noise frequencies. Furthermore, if mechanical protection such as a perforated metallic covering is provided, more changes are likely to occur in the absorption coefficient such as an increase in the middle frequency range and a decrease at high frequencies [S]. These are general trends in the behaviour of absorptive materials. Standing wave tube measurements are a useful means to compare the absorption of some classes of these materials on a common basis, and to assess the change in (Ywith time. However, the actual sound absorption of a material mounted in a particular way can only be found by a proper measurement of a test sample mounted in the same way as it will be mounted in situ. (Further discussion on this subject appears in the Appendix.) The following are observations regarding the absorption coefficients of each of the materials, tested in a standing wave tube, with a hard backing and without an air gap.
0
I 250
I
,
1,
500 Frequency
I
I
1000
I
I 2ocQ
(Hz)
Figure 14. Sound absorption coefficient of material 2 before (0) and after (0) exposure.
Material 2 (see Figure 14). Only the absorptive part of the material was tested because of the difficulties involved in cutting the concrete backing for the standing wave tube sample. Nevertheless, since the measurement was done with the hard backing provided by the measurement device, the results should be similar to those obtained from a concretebacked sample of the material (see the Appendix and Figure A3). Two resonant peaks can be observed in Figure 14.
A. BEHAR
44
01
I
’
250
’
AND D. N. MAY
1
’
500
Frequency
’
1
’
loo0
’
a
1
MOO
b-k)
Figure 15. Sound absorption coefficient of material 5 before (a) and after (0) exposure. (Measurements were performed without the glass cloth.)
Materials 5 and 8 (Figures 15 and 18 respectively). Both these materials are typical, acoustically porous materials, whose absorption coefficients increase steadily with frequency (see also Table 3). The change in (Y of material 5 with exposure cannot be considered significant. This is also true of material 8, where a somewhat larger difference between the before and after exposure values of (Ycan be observed in the low frequency range due to the relatively low accuracy of the measuring method for values of a! lower than about 0.2.
0.
IO t
0’
b
’
250
I
2
’
Frequency
’
’
’
1000
’
’
1
2000
(Hz)
Figure 16. Sound absorption coefficient of material 6 before (0) and after (0) exposure.
Material 6 (Figure 16). The absorption coefficient of material 6 has a resonant peak between 1000 and 2000 Hz, and relatively low values at other frequencies. It may therefore absorb tyre noise more than engine noise. Material 7 (Figure 17). Material 7 cannot be considered sound absorbing, because of its very low value of (Y.The apparent changes in the values of the coefficient before and after exposure, which can be observed in Figure 17, are probably due to the previously mentioned inaccuracy in measurements of low values of CL
ABSORPTIVE
0’
I
I 250
NOISE
BARRIER
1
45
DURABILITY
I 500
I
Frequency
(Hz)
1000
2000
Figure 17. Sound absorption coefficient of material 7 before (0) and after (0) exposure.
Frequency
(Hz)
Figure 18. Sound absorption coefficient of material 8 before (0) and after (0) exposure.
Material 9 (Figure 19). The absorption coefficient of material 9 has a resonant peak between 1250 and 1600 Hz, coincident with much of the A-weighted highway noise spectrum. No explanation could be found for the apparent shift of the resonant frequency after exposure.
0’
7
’
’
’
’
’
250
’
’ 000
Frequency
’
’
I 2000
(Hz)
Figure 19. Sound absorption coefficient of material 9 before (0) and after (0) exposure.
46
A.
3.3.
FREEZING
AND
BEHAR
AND
D. N. MAY
THAWING
As the freezing and thawing tests provided no numerical data, the results are given as a table of observations of the condition of each material for each test at every tenth cycle. These results are shown in Tables 4 and 5, corresponding to tests (A) and (B) respectively. (A) and (B) refer to total immersion and partial immersion, respectively. The following are general comments on the results of the different tests. Material 2. (A) This product disintegrated when exposed to freezing and thawing in a saturated condition. The damage is the result of a combination of the swelling of the saturated wood particles and the pressure generated by the solution expanding when freezing. (B) After 50 cycles of freezing and thawing, this material was wet and had swelled for approximately 50 mm above the level of the salt solution. This wetting and swelling was probably due to capillary action. The material above this area wetted by capillary action showed no effects from the exposure conditions. Material 4. This material showed damage in both partial and total immersion tests. Because of its softness and low strength, most of the damage was probably from handling while saturated with water. Material 6. (A) This product softened and lost strength during the exposure to freezing in the salt solution. Because the wood in this product was in the form of long thin strips, the specimens tested only softened and swelled rather than disintegrated. If the exposure had TABLE
4
Total immersion in salt solution-observations
Freeze-thaw cycle no. Material no.
10
20
30
40
Top 15 mm quite friable
Quite friable separating from mortar coated rebar
Continued deterioration top 20 mm breaks easily
Continued deterioration, mortar covered bars can be lifted out
4
No change
No change
Slight softening
No change
No significant change
6
Softening
Softness increasing
Softness increasing
Spongy, can be pulled apart
Spongy
7
Slight softening
Softness increasing
Softness increasing, swelling
Further softening, edges can be pulled apart
Quite soft, swollen, still retained shape
8
No change
No change
No change
No change
No visible sign of deterioration
9
No change
No change
Very slight deterioration at top edge
No change
Material still hard, brittle, with only a suggestion of deterioration at top
2
30 TOP
completely disintegrated, area below in slightly better condition
10
Immersed section swelling
Absorbed solution approx. 20 mm above liquid
Portion immersed swelling to approx. 25 mm above surface of solution
Submerged portion soft, absorbed solution approx. 25 mm above liquid level
Absorbed solution approx. 20 mm above liquid
Absorbed solution approx. 50 mm above liquid
Material no.
2
4
6
7
8
9
of
No change
No change
Immersed portion quite soft and swelling
Immersed portion starting to break up
Slight deterioration submerged portion
Immersed portion starting to break up, quite friable
20
30
cycle no.
Slight deterioration submerged edges
No change on
Swelling increasing slowly, moving upwards
Swelling moving upwards
No change
Swelling moving upwards above submerged section; immersed portion more friable
Freeze-thaw 40
No major change
No change
Bottom 75 mm can be pulled apart, swelling increasing
Top still hard, bottom continues to soften, swelling moving up
Deterioration on bottom from handling and freezing, on top from handling
Swelling approx. 50 mm above solution
Partial immersion in salt solution-observations
TABLE 5
No change
No change
Slight increase in swelling, starting to separate into layers
Slight increase in swelling, crumbles readily at bottom
No major change
Increased swelling, crumbling of submerged portion
50
No.
Good (NRC = O-55)
Little loss of material, more severe on the lower part of the sample
Visually undamaged
Material torn apart by cumulative weather effects
Visually undamaged
Visually undamaged
Wood shavings bonded with Portland cement
Aluminium cage enclosing glass wool wrapped in polyethylene
Glass fibres bonded in a thermosetting resin
Same as 4, protected by glass cloth
Wood fibres bonded with Portland cement
None
Not tested
None
Not tested
Fair (NRC = 0.35)
Not tested
None
Not tested
Change in sound absorption after highway exposure
Good (NRC = 0.55)
Not tested
Not tested
Sound absorptioni’
Very fragile, fell apart during installation
Highway exposure (9 months)
Low density cellular glass
Description
Material
Softened and swelled
Softened and swelled up to 50 mm above water level
Not tested
Forms a mass of loose fibres
Forms a mass of loose fibres
Not tested
Not tested
Not tested
Swelled up 50 mm above the water level
Not tested
Not tested
Disintegrated
Partial immersion
Total immersion
Tests and results
Overall summary of tests and results
TABLE 6
Adhesive properties of the cement destroyed
Not tested
Loses rigidity and forms a mass of glass fibres
Not tested
Adhesive properties of the cement destroyed
Not tested
Accelerated weathering
No reaction to salt
Not tested
Loses rigidity and forms a mass of glass fibres
Not tested
No reaction to salt
Not tested
Salt spray exposure
E
$ u P z
8 ? P
?
Visually undamaged
Random textured glass fibres and polyester resin
Good (NRC = 0.40)
Fair (NRC = 0.35)
Poor (NRC = O-15)
Softened and swelled
Undamaged
Undamaged
None
None
None
Undamaged
Surface degradation and 50% thickness shrinkage
Delaminated completely
Undamaged
No reaction to salt
Delaminated completely
poor NRC< 0.20; tested according to ASTM C384-72 Hz, rounded to the nearest 0.05.
Undamaged
Undamaged
Softened and swelled up to 75 mm above water level
t Basis for ratings: Excellent NIXa 0.80; very good NRC 0.60-0.75; good NRC 0.40-0.55; fair NRC 0.20-0.35; [3]; NRC (noise reduction coefficient) = mean value of the sound absorption coefficients at 250, 500, 1000 and 2000
Visually undamaged
Visually undamaged
Flexible polyurethane foam
material compressed into a high density structural panel
Wood fibre
; i: q 4
3
g
$ 2
2
zz
iz
3
4
$ “0
A. BEHAR
50
AND
D. N. MAY
been continued it is expected that the material would have shown deterioration similar to that of material 2. (B) As with material 2 this specimen softened and swelled for approximately 50 mm above the solution. Material above this area wetted by capillary action showed no effects from exposure conditions. Material 7. (A) Over the length of the test this material softened, absorbed the solution, and swelled. The swelling, softening and absorbing were continuing when the test was stopped. (B) The material softened and swelled in and above the solution to a height of 75 mm. The material above the wetted portion showed no effect from the exposure conditions. Material 8. This material showed no evidence of change in either of the two tests. Material 9. This material started out as a hard, brittle, porous material in each test. Although each specimen lost a small amount of material on some corners, the specimens remained essentially unchanged at the end of each test. 3.4.
ACCELERATING
WEATHERING
AND
SALT
SPRAY
EXPOSURE
Material 2. After 2500 hours of accelerate&weathering
test, particles of wood chips crumbled or flaked off the face and edges of the test pieces, probably caused by the expansion and contraction of the wood chips during the wetting and drying cycles in the weatherometer, which appears to destroy the adhesive properties of the cement. Black discoloration (possibly algae) appeared to be forming on the wood chips. The salt spray test indicates that the material did not react with salt, but it did become saturated and heavy with absorbed water. Material 4. Due to the absorption of water by the fibreglass panels in both the weatherometer and the salt spray test, this material rapidly lost rigidity by separation of layers of fibreglass and sagging to form a mass of loose fibres. Material 6. Similar to material 2, the adhesive holding the straw-like fibres together appeared to be washed out, leaving loose fibres at the surface of the accelerated weathering panels. Due to the porous construction, a large amount of water was absorbed by this material. In addition a considerable amount of black discoloration (possibly algae) formed on the surface and extended into the interior of the panel. Other than water absorption by the fibres, there appeared to be no reaction with salt water. Material 7. In both the accelerated weathering and salt spray tests, this material absorbed and retained large amounts of water and when subjected to accelerated weathering and salt spray exposure, the four layers of compressed paper expanded and completely delaminated. Material 8. The foam sponge showed surface degradation and 50% shrinkage in thickness during accelerated weathering causing black carbon formation on the surface. Other than absorption of water by the foam, no change in the sample was noted in the salt spray exposure. Material 9. Little or no degradation was apparent after exposure in the weathering and salt spray cabinet. 4. CONCLUSIONS The conclusions below are based on the results of the tests described in this report. These tests, while typical of those conducted for acoustical materials on the one hand and concrete highway-use products on the other, are not necessarily the best or only tests for this application. Note particularly that noise barrier installations are designed for adequate drainage, and that freeze-thaw cycles while the barrier material is immersed in water constitute an extremely harsh and not entirely representative ,test environment. The
ABSORPTIVE
NOISE
BARRIER
DURABILITY
51
authors recommend that the results be treated with caution and viewed as resource material for decision-making. Table 6 contains a summary of the results of each test for the various materials. Conclusions are also given below.
4.1.
HIGHWAY
EXPOSURE
AND
SOUND
ABSORPTION
COEFFICIENT
various tests provide tentative conclusions about the ability of the materials and the absorption system to withstand weather exposure while still possessing sound absorptive properties. Material 1 cannot be recommended for highway barrier use. Its extreme fragility makes its handling difficult. Its sound absorption coefficient was not tested. After exposure, material 2 showed a small degree of surface crumbling and loss of material mostly from the lower part of the sample. Its sound absorption coefficient was “good” and did not change with highway exposure. The sound absorption coefficient of system 3 was not tested because of the impossibility of cutting a sample for the tube measurement. The system was visually undamaged by highway exposure. Material 4 clearly showed its inadequacy for outdoor use without protection. However material 5, which was undamaged by highway exposure and had presumably the same “good” sound absorption before and after highway exposure, showed that the protection provided by glass cloth might cause material 4 to be equally suitable for highway barrier applications. Material 7 cannot be described as acoustically absorbent because of the low value of its absorption coefficient over the frequency range of interest. Its good resistance to highway exposure is not, therefore, of much interest. The sound absorption of materials 6 and 8 was “fair” with a relatively good absorption of high frequency (i.e., tyre) noise and relatively low absorption of low (i.e., engine) noise, both of which were unaltered by highway exposure. Finally, the sound absorption coefficient of material 9, unaltered by highway exposure, was “good”. The material was found to be visually undamaged by the exposure. The
AND THAWING 4.2. FREEZING Two products, materials 2 and 6, softened and swelled badly due to saturation and freezing. Based on the results of the tests performed, these two products cannot be recommended for exposed locations where they will be subjected to freeze-thaw cycles while sitting in a pond of salt solution. The results quoted for material 2 are, however, those for the less dense side of the sample, which is generally bonded to concrete to form the noise barrier panel. The other side of the sample, which is the side more exposed to the weather, is more dense and deteriorated much more slowly. However, a faster rate of deterioration can be expected once its outer “skin” has been penetrated. Material 4 showed a lot of damage due to handling. Due to its frail nature, it cannot be recommended for exposed surfaces. Material 7 swelled and softened considerably over the course of these tests. As deterioration was still continuing at the end of the tests, this product is not recommended for exposed locations. Two materials, 8 and 9, showed essentially no deterioration in either test. Based on the tests performed, material 7 should be a more durable product for an exposed surface because of its rigidity.
52
A. BEHAR
AND
D. N. MAY
4.3.ACCELERATED WEATHERING AND SALT SPRAY EXPOSURE Both of the wood-cement products, materials 2 and 6, tended to disintegrate slowly upon weathering and may be subject to biological degradation. Material 7 is unsatisfactory because of complete delamination upon weathering. Both materials 4 and 8 did not have satisfactory rigidity. Without additional protective cover and support, these materials shrank and deformed so that they did not appear to be satisfactory for use as self-supporting noise absorptive materials. Of the six sound absorption materials tested only material 9 appeared to be unaffected by accelerated weathering and salt spray. It has sharp rough edges and very coarse surface texture so that care should be exercised in handling it.
ACKNOWLEDGMENTS The sound absorption measurements were performed at the MTC Research Laboratory by G. Giles and A. Maio. Freezing and thawing tests were done at the Concrete Unit (T. Church), Materials and Laboratory Services Section, and the accelerated weathering and salt spray exposure tests at the Chemicals and Metals Unit (R. Fujii) of the same Section. Thanks are also due to the MTC District Office, Ottawa, for help over installation of the samples of the weather exposure test. We also wish to express gratitude to Dr T. D. Northwood and W. T. Chu of the National Research Council, Ottawa, for assistance with the work described in the Appendix. Finally we want to thank the manufacturers of materials 1, 2, 3,6, 7, 8 and 9 for kindly supplying samples and supporting literature.
REFERENCES July 1. Buyer’s Guide to Materials for Noise and Vibration Control 1977 Sound and Vibration 11,7, 1977. 2. A. THUMANN and R. MILLER 1974 Secrets of Noise Control. Atlanta: Fairmont Press. 3. Standard Test Method for Impedance and Absorption ofAcoustical Materials by the Tube Method. ASTM C384-72, American Society for Testing and Materials, 1972. 4. Standard Method of Salt Spray (Fog) Testing. ASTM B-117-73, American Society for Testing and Materials, 1973. 5. W. FURRER 1964 Room and Building Acoustics and Noise Abatement. London: Butterworths. See chapter 2.
APPENDIX: THE SOUND ABSORPTIVE QUALITIES OF MATERIAL 2 FOR VARIOUS METHODS OF INSTALLATION To test the influence of the mounting of material 2 upon its sound absorption coefficient, several tests were performed at the Division of Building Research, National Research Council, Ottawa. The sample area was 8*34m2 (89.8 ft’) and its thickness 75mm (2.95 in). The measurement was done according to the ASTM C423-72 standard. The sample was tested in two different ways: laid on the floor (mounting 4 according to the standard) and in an upright, free-standing position. The results of the measurements are shown in Figure Al. When the material is free-standing (curve B) it behaves as acoustically porous. Its absorption coefficient is low at low frequencies, and increases steadily as frequency rises. When, on the other hand, there is a hard material behind, as appears when the material is laid on the floor, a resonant peak appears (curve A) which quite radically changes the absorption coefficient values at the
ABSORPTIVE
NOISE BARRIER
500
53
DURABILITY
1000
2000
4cim
8cco
Frequency(Hz)
Figure Al. Sound absorption coefficient of material 2 in reverberation standing. Noise reduction coefficient (NRC): A, 0.85; B, 0.54.
room: A, laid on floor; B, free-
mid-frequencies. The same material is then quite highly absorbent. Its noise reduction coefficient (NRC) rises from 0.54 to 0.85. Figure A2 shows the results of two similar measurements performed on a sample of material 2 as used in the field. This sample is composed of 50 mm sound absorbing material with a 30 mm steel reinforced concrete backing. The differences between the two curves are minor, thus allowing one to conclude that the hard backing is responsible for the beneficial peak in (Yaround 500 Hz evidenced in Figure Al. ,
I
,
,
I
(
I
I
q
1.1 I.0 0.9 0.8 @70.6 0.5 D4-
01
’ 125
I
I
’ 250
4
“‘I
500
” loo0
I
”
2ocO
”4000
“I
.9m
Frequency (Hz)
Figure A2. Sound absorption coefficient of material 2 as used in practice (with concrete backing) in reverberation room: A, laid on floor; B, free-standing. Noise reduction coefficient (NRC): A, 0.71; B, 0.66.
Finally, Figure A3 shows the results of the measurement of material 2 in the reverberant room (same as curve A, Figure Al) and by the standing wave measuring method (also measured at National Research Council in this case) according to ASTM C384-72 standard. The absolute values of LYcannot be compared for two reasons: the sound field is in one case diffuse (in the reverberant room), and in the other it is normal (in the tube). Also the units are in the first case sabine (S), and in the other a sound pressure ratio. What can be compared is the variation of (Ywith frequency in the two curves. In this case they are similar, meaning that both the floor of the room and the bottom of the tube are
A. BEHAR
54
O;; 125
250
AND D. N. MAY
2000
1000
500 Frequency
4Oco
8000
(HZ)
Figure A3. Sound absorption coefficient of material 2: A, laid on floor in reverberation wave tube.
room; B, in standing
acting in the same way. This permits the conclusion that the tube method gives similar results to the conventional reverberant method in which the material is laid on the floor, and that the general trend of the sound absorption coefficients obtained from the tube measurements is only valid if the material is mounted in practice against a hard wall (or barrier).