Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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POROUS SOUND-ABSORBING CONCRETE FOR
BUILDING ACOUSTICALLY INSULATING STRUCTURES
BACKGROUND OF THE INVENTION
1. Field of the invention:
The present invention relates to a sound-absorbing
concrete presenting a rigid structure, and made of materials
conventionally used in the fabrication of concrete.
2. Brief description of the prior art:
Human activity has created a level of noise pollution that
has become a real nuisance. This noise is generated inside and outside
buildings in places such as swimming pools, community centres,
factories, subway stations, etc. Additionally with the expansion of
highways and increased air traffic the level of noise pollution from these
sources has increased dramatically. As a consequence the need to
reduce and control the noise level has become an important issue in
many urban areas. In many regions the authorities in charge of highway
maintenance have set noise level standards. Thus the control of sound
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pollution today has become imperative for achieving a desired quality of
life. Many prior art solutions have been proposed to meet these needs
and standards.
As a first example, US patent 4,325,457 (Docherty et al.)
issued on April 20, 1982 relates to an acoustic panel having two layers.
The first layer constitutes a sound-absorbing layer, comprises chemically
mineralized fibrous material blended with Portland cement, and is
commercialized under the trademark DURISOL. The second layer is
more dense and acts as a transmission loss layer; this second layer is
comprised of fine aggregate non-porous concrete. The first and second
layers are reinforced by a wire mesh.
US patent 4,899,498 granted to Grieb on February 13,
1990 relates to sound-absorbing and reflecting panels comprising a
cellular foam core reinforced with a thin fibreglass coating on the outer
surface, the whole being covered with cementitious material.
US patent 5,314,744 (Walter et al.) issued on May 24,
1994 relates to a process for encasing free aggregate of individual wood
chips within an inorganic mineralizing coating.
US Patent 5,324,469 granted to Walter et al., on June
28, 1994 is concerned with a single layer sound absorption panel. This
panel is a wood-concrete layer comprised of kaolin mineralized organic
fiber chips encased in Portland cement and reinforced with steel.
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US patent 5,504,281 granted to Whitney et al., on April
2, 1996 is concerned with the fabrication of perforated sound attenuators
The function of these sound attenuators is to reduce noise produced by
interior and office equipments. The sound attenuator panels consist of
sound absorbing material formed of non fibrous particles adhered to each
other at their points of contact to leave interstitial void spaces between
the particles. The particles are solid or hollow, of inorganic or polymeric
origin: glass-ceramic particles, glass microbubbles, polyethylene,
polypropylene, nylon, etc. The most efficient particles have an external
diameter situated between 20 Nm and 100 Nm. The characteristics of the
obtained porous material are the following:
- diameter of the pores situated between 5 Nm and 280 Nm (preferably
between 25 Nm and 50 Nm);
- porosity situated between 20% and 60% (preferably between 35 % and
40%); and
- a tortuosity located between 1.25 and 2.5.
The binder used for binding the particles is of inorganic or organic origin,
such as ceramic, polymeric, silicone and elastomeric binders. The
particles can also be thermally treated for adhering them to each other.
The volume weight of the porous material is situated between 80 and
1000 kg/m3, preferably lower than 650 kg/m3.
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US patent 5,564,241 (Ogorchock et al.) issued on
October 15, 1996 relates to a sound barrier panel composed of two
layers; one is a sound-absorbing layer and the other is a structural
concrete layer. The two layers are bonded together by either mechanical
or chemical means. The sound-absorbing layer includes cementitious
material, chipped tire fiber and rubber, light weight coarse aggregate,
vermiculite, water, a water-reducing agent and an air-entraining
admixture. This composition results in a cement that has internal,
interconnected cavities.
The above review of the prior art demonstrates that the
prior art sound-absorbing panels are a composite of cementitious material
and some other aggregate such as wood fibres and chipped tire rubber
that are not used in the preparation of conventional concrete.
OBJECT OF THE INVENTION
An object of the invention is therefore to overcome this
deficiency of the prior art by providing an effective porous sound-
absorbing concrete composition which is economical and easy to make,
and comprises ingredients that are easily available and conventionally
used in the preparation of concrete.
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Another object of this invention is to provide a method
for making a sound-absorbing concrete, both easily and economically)
using ingredients that are easily available and conventionally used in the
preparation of concrete.
SUMMARY OF THE INVENTION
More specifically, according to the present invention,
there is provided a porous, sound-absorbing concrete comprising an
aggregate formed of particles and a binder for binding these particles of
aggregate together in view of forming that porous sound-absorbing
concrete. The binder defines between the particles of aggregate empty
spaces forming a sound-absorbing network of interconnected pores and
cavities having dimensions determined by the dimension of the particles
of aggregate and the quantity of binder relative to the quantity of
aggregate. The interconnected pores and cavities of the network are
dimensioned to provide a porous sound-absorbing concrete having an
acoustic impedance situated between 5 000 and 100 000 Rayls/meter
(mks).
The dimension of the particles of aggregate is
advantageously situated between 50 Nm to 12.5 mm, more preferably
between 105 Nm to 1.2 mm, and most preferably between 215 Nm to 1.7
mm.
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According to a preferred embodiment, the aggregate is
selected from the group consisting of silica, quartz, limestone, granite, or
a combination thereof.
According to another preferred embodiment, the
aggregate is a manufactured aggregate selected from the group
consisting of pearlite, vermiculite, polymeric material, or a combination
thereof.
The binder comprises ingredients selected from the
group consisting of cement, epoxy or a combination thereof. The binder
may further comprise an adjuvant selected from the group consisting of
a superplasticizer, a water reducer, an air-entraining agent, or a
combination thereof. Furthermore, the binder may further comprise at
least one substance selected from the group consisting of a concrete
setting accelerator, a concrete setting retardant, a mineral material
selected from the group consisting of silica fume, fly ash, slag, a
calcareous filler, a tinting color, or a combination thereof, a latex, or a
combination thereof.
Also in accordance with the present invention, there is
provided a method of fabricating a porous, sound-absorbing concrete
including an aggregate formed of particles and a binder for binding these
particles of aggregate together in view of forming that porous sound-
absorbing concrete, comprising the steps of:
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predetermining the dimension of the particles of
aggregate and the quantity of binder relative to the quantity of aggregate
to cause the binder to define between the particles of aggregate empty
spaces forming a sound-absorbing network of interconnected pores and
cavities dimensioned to provide a porous sound-absorbing concrete
having an acoustic impedance situated between 5 000 and 100 000
Rayls/meter (mks) ;
mixing the aggregate and binder into a wet concrete
mixture;
separating the particles of aggregate covered with wet
binder of the wet concrete mixture; and
dropping the separated aggregate particles one above
the other with substantially no compaction energy to form the porous
sound-absorbing concrete having the above network of interconnected
pores and cavities dimensioned to provide the porous sound-absorbing
concrete with an acoustic impedance situated between 5 000 and 100
000 Rayls/meter (mks).
Preferably, the separating and dropping steps comprise
disposing the wet concrete mixture structure onto a generally horizontal
sieve having openings of predetermined dimension, and passing the
particles of aggregate covered with wet binder through the openings of
the sieve in order to separate the particles of aggregate and sprinkle the
separated aggregate particles into a mold placed underneath the sieve.
Passing of the particles of aggregate covered with wet binder through the
openings of the sieve may comprise vibrating the sieve, pushing the wet
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concrete mixture through the openings of the sieve, or, in combination,
vibrating the sieve and pushing the wet concrete mixture through the
openings of the sieve.
The objects, advantages and other features of the
present invention will become more apparent upon reading of the
following non restrictive description of preferred embodiments thereof,
given by way of example only with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the appended drawings:
Figure 1 is a perspective view of a first system for
pouring and molding the sound-absorbing concrete according to the
present invention, with substantially no compaction energy;
Figure 2 is a perspective view of a second system for
pouring and molding the sound-absorbing concrete according to the
present invention, with substantially no compaction energy;
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Figure 3 is a graph showing the curve of the sound
absorption coefficient "versus" frequency of a first panel made of porous
sound-absorbing concrete (example 1 ) according to the present invention;
Figure 4 is a graph showing the curve of the sound
absorption coefficient "versus" frequency of a second panel made of
porous sound-absorbing concrete (example 2) according to the present
invention;
Figure 5 is a graph showing the curve of the sound
absorption coefficient "versus" frequency of a third panel made of porous
sound-absorbing concrete (example 3) according to the present invention;
and
Figure 6 is a graph showing the curve of the sound
absorption coefficient "versus" frequency of a fourth panel made of
porous sound-absorbing concrete (example 4) according to the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The sound-absorbing concrete according to the present
invention is porous and defines a network of interconnected air pores and
cavities. More specifically, the sound-absorbing concrete of the present
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invention is composed of an uniform graded aggregate and a binder. The
size of the particles of aggregate as well as the respective proportions of
aggregate and binder are precisely predetermined in view of binding the
aggregate without filling all the space between the particles of aggregate.
The aggregate can be a natural aggregate or a
manufactured aggregate. It can also be a mineral aggregate, and/or
presents various compositions. Examples of natural aggregates are
silica, quartz, limestone, granite, etc. Examples of manufactured
aggregates comprise pearlite, vermiculite, polymeric material such as
polystyrene, etc. The particles of the aggregate can further present
various shapes, although a more or less cubic shape appears to give
better results.
As indicated in the foregoing description, the aggregate
has an uniform gradation; the particles of aggregate have very similar and
uniform sizes. Particles having a diameter situated between 5 pm (sieve
270) and 12.5 mm ('/2"). Best results have been obtained with (a) an
aggregate in which 87% of the particles have a diameter situated
between 300 Nm (sieve 50) and 850 Nm (sieve 20), and (b) with an
aggregate in which 89% of the particles have a diameter situated
between 600 Nm (sieve 30) and 1,2 mm (sieve 16). It has been noticed
that modifying the particle-size distribution of the aggregate modifies the
concrete absorption characteristics.
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The binder is preferably a mixture of cement, in
particular but not exclusively Portland cement, water and adjuvants such
as a superplasticizer, a water reducer and/or an air-entraining agent. The
binder can further comprise a concrete setting accelerator or a concrete
setting retardant. Mineral materials such as silica fume, fly ash, slag,
calcareous filler, tinting color, etc., and chemical substances such as latex
can further be used in the fabrication of the binder. Of course, the
different types of Portland cement available on the market can be used.
It is further within the scope of the present invention to use an aluminous
cement or a mixture of aluminous cement and Portland cement.
An epoxy binder can also be used. However, the cost
of an epoxy binder is high if compared to the cost of a Portland cement
binder. An alternative is to mix a certain quantity of epoxy binder with the
cement binder.
Referring to Figure 1 of the appended drawings, a panel
4 of sound-absorbing concrete according to the invention can be
fabricated as follows. The ingredients (aggregate and binder) are
minimally mixed with water to form a wet concrete 2. The wet concrete
2 is disposed on the top surface of a generally horizontal sieve 1 having
openings of predetermined dimension. To pass the particles of aggregate
covered with wet binder through the openings of the sieve 1 in order to
separate these particles and sprinkle the separated aggregate particles
into the mold 3 placed underneath the sieve 1, the sieve 1 is vibrated.
Alternatively, as shown in Figure 2, the wet concrete mixture may be
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pushed through the openings of the sieve 1 by scraper blades 5 mounted
transversally onto a longitudinally moved belt 6, the scraper blades 5
scraping the wet concrete mixture 2 onto the top surface of the sieve 1.
Finally, in combination, the sieve 1 is vibrated and the wet concrete
mixture is mechanically pushed through the openings of the sieve 1 by
the blades 5 or by other mechanical means.
The sieve 1 separates the particles of aggregates
covered with binder and drops these particles one above the other in a
mold 3 to form a non-compacted concrete panel. The wet concrete 2 is
therefore sieved directly into the mold 3 with no compaction energy, and
is left to set and gain strength with no further handling, thus minimizing
compaction of the concrete.
Avoiding as much as possible compaction of the
concrete is essential since compaction will reduce porosity and
accordingly acoustic impedance of the concrete panel.
Of course, the panel 4 can be fabricated through pouring
of successive layers of concrete 2 in the mold 3 through the sieve 1. This
will allow a lower layer to gain sufficient strength to withstand compaction
by a top layer. Also, it is possible to fabricate a multi-layer panel 4
comprising a layer of sound-absorbing concrete according to the
invention and a layer of conventional non-absorbing concrete. In these
two cases, it is important to pour a subsequent layer of concrete
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sufficiently rapidly to create between that subsequent layer and the
former concrete layer a chemical cement bound.
The present invention will be further described and
illustrated by means of the following examples. Although specific
materials, quantities and proportions, and other conditions and details are
described in these examples, these should not be interpreted as limiting
the present invention.
Examples:
The following four representative examples provide
formulation for a sound-absorbing concrete according to the present
invention. The use of different ingredients that are known in the art in the
same or different proportions and resulting in a sound-absorbing concrete
having similar characteristics are within the scope of this invention. More
specifically, Table 1 represents the general characteristics of four sound-
absorbing concrete formulations (concrete 1, concrete 2, concrete 3 and
concrete 4) described herein.
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TABLE 1
EXAMPLE CHARACTERISTICS
Concrete 1 50% porosity, average thickness
10.5 cm
Concrete 2 60% porosity, average thickness
10.5 cm
Concrete 3 double layer, 70% porosity, average
thickness 9 cm
Concrete 4 80% porosity, average thickness
13 cm
The proportions, by weight of the different ingredients of the four
representative examples (concrete 1, concrete 2) concrete 3 and concrete
4) of sound-absorbing concrete are summarised in Table 2. The binder
used in these examples is either Portland cement of type 30 with high
initial resistance, a pre-mixed cement comprising silica fume (HSF) or
epoxide resin, or a combination thereof. The superplasticizer is
polynaphtalene sulfonate. Table 3 represents the particle-size distribution
of the different aggregates used. The examples provided herein are
crystalline silica aggregates of grade 2010 and 2075 sold by the company
UNIMIN, and natural granite.
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TABLE 2
concreteconcrete concreteconcrete4
1 2 3
Ingredients ProportionsProportionsProportionsProportions
Binder Cement 1 - - -
type
30
Cement - 1 1 -
HSF
Epoxy resin- - - 1
Water (total) 0,25 0,25 0,025 -
*
Aggregate Unimin - 3,9 - 7,7
2010
Unimin 3,35 - - -
2075
Granite - - 4,7 -
Superplasticizer"* 0,025 0,025 0,025 _
* Includes the water of the superplasticizer
** Dry extract content
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TABLE 3
SIEVE APPROX WEIGHT
(ASTM*) METRIC % RETAINED
BY EACH
SIEVE
SIEVE Unimin Unimin Granite Granite
(mm) 2010 2075 0.85 1.7
6 3.4 - - - -
8 2.4 - Trace - -
12 1.7 Trace 2 - 100
16 1.2 1 27 - -
20 0.850 10 43 100 -
30 0.600 28 19 - -
40 0.425 30 6 - -
50 0.300 19 2 - -
70 0.215 7 1 - -
100 0.150 3 - - -
140 0.105 1 - - -
* American Society of Testing Materials
These four examples demonstrate that it is possible to
produce the sound-absorbing concrete of the present invention
comprising different types of binders, different types of aggregates having
different sizes and different proportions of binder/aggregate. As well,
when the concrete is comprised of more than one layer the layers may be
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the same or different, as shown in the concrete of Example 3 where the
two layers are comprised of two different aggregate (Granite 0.85 and
granite 1.7, respectively). More specifically, in Example 3, one layer is
comprised with a coarser aggregate (granite 1.7) and the other layer is
comprised with a finer aggregate (granite 0.85).
When the concrete comprises a cement binder, the
aggregate is first mixed with a portion of the mixing water. This portion
of the mixing water is mostly absorbed by the aggregate. The cement is
then added to this water logged aggregate and the whole is mixed to
homogeneity. Finally the rest of the water, mixed in with the
superplasticizer, is added. Again, the whole is mixed to homogeneity.
In the instance when the concrete comprises an epoxy
resin binder, the pre-mixed epoxy binder is simply added to the aggregate
during mixing.
The pouring of the concrete of this invention is a critical
step in optimizing its acoustic absorption properties. The manner of
pouring firstly ensures maximizing the formation of the network of
interconnected pores and cavities and secondly minimizing the
compaction of the concrete layer. To that end, the above described wet
concrete is disposed on the sieve 1 of Figure 1. The sieve 1 has a mesh
size of 5 mm and the wet concrete is caused to pass through the sieve by
vibration and by pushing the concrete through the sieve by hand, though
any other means known in the art that improves the passage of the
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concrete through the sieve such as scraper blades 5 can be used. The
wet concrete particles of aggregate covered with wet binder that have
been separated from each other upon passage through the sieve 1 are
left to drop directly into the mold 3, with no other handling or manipulation
and with no supply of energy of compaction of the concrete. The sieved
wet concrete in the mold 3 is then water cured for a predetermined period
of time.
When, to complete the panel or other product, an
additional layer of non porous conventional concrete has to be poured in
the mold 3 of Figure 1 on the top of the porous sound-absorbing concrete
according to the invention, the layer of conventional concrete is poured
after the porous sound-absorbing layer has developed a sufficient
resistance to prevent the weight of the conventional concrete to compact
the porous sound-absorbing concrete.
The physical and mechanical characteristics obtained
with the concrete 1 from Table 2 are presented in the following Table 4.
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TABLE 4
Flow resistivity Rayls/meter
(mks)
((meter-kilogram-second)
unit of
specific acoustic impedance)35 000
Porosity, % 40%
Density, kg/m3 1000
Compression resistance, 1.0
MPa
Finally, Figures 3 to 6 are the graphs showing the curve
of acoustic absorption versus frequency of the concretes 1, 2, 3 and 4
respectively for an acoustic wave having an angle of incidence normal to
the surface of the concrete.
For example, as can be seen in Figure 3, the absorption
coefficient of concrete 1, for a sample 12.7 cm thick, is approximately 25
at 250 Hz, and raises to reach 80% at 500 Hz. Over 500 Hz, the
absorption coefficient will not lower under 60%. These sound absorption
values correspond to a fibreglass wool of 1.5 inches thick. As any other
sound-absorbing material, sound absorption can be substantially
improved by increasing the thickness of the porous sound-absorbing
concrete material. For example, a thickness of 15 cm will give an
absorption coefficient of the order of 50% at a frequency of 250 Hz and
an absorption coefficient of the order of 90% at a frequency of 500 Hz
and higher. As indicated in the foregoing description, this quality of sound
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absorption is the result of the flow resistivity of the concrete material,
which is a physical parameter characterizing the resistance of the porous
sound-absorbing concrete to the passage of air. This flow resistivity or
acoustic impedance is the principal parameter determining the sound
absorption properties of a given porous sound-absorbing material.
The variation of the acoustic absorption coefficients of
concrete 2, 3 and 4 in relation to frequency can also be observed in
Figures 4, 5 and 6, respectively.
To efficiently absorb sound, the porous concrete
according to the present invention presents an acoustic impedance
situated befinreen 5 000 and 100 000 Rayls (mks).
The porous sound-absorbing concrete according to the
invention presents, amongst others, the following advantages:
- as conventional concrete, the porous sound-absorbing concrete can be
molded as desired to form a sound-absorbing concrete structure of any
form, relief and thickness to meet with the requirements of any situation;
- since the pores and cavities of the sound-absorbing concrete according
to the invention are open and its pores and cavities are interconnected,
fluids such as water can drip rapidly through the concrete material
whereby the concrete will resist to freeze/thaw conditions; the effect of
freeze/thaw is detrimental to water-saturated materials only;
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- as conventional concrete, the porous sound-absorbing concrete can be
colored as desired;
- as conventional concrete, the porous sound-absorbing concrete of the
present invention adapt to various conventional assembly methods and
structures;
- the porous sound-absorbing concrete is made of usual concrete
ingredients (aggregate and binder) and accordingly presents an excellent
sound absorption coefficient while offering the advantages of classical
concretes in terms of durability in the presence of bad weather conditions,
resistance to fire, ease of installation, appearance (molding and addition
of coloring matters), etc.;
- Since it does not requires the addition of special particles or aggregates,
or a special formwork, the porous sound-absorbing concrete according
to the invention is economical when compared to other types of sound-
absorbing materials available on the market;
Applications of the porous sound-absorbing concrete
according to the invention comprise, in particular but not exclusively, the
following outdoors and/or indoors applications in which durability of the
concrete is as important as its sound-absorbing properties:
- fabrication of barriers, panels, blocks, tiles and any sound-absorbing
structures;
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- vertical siding of buildings;
- walls of subway stations;
- pools; and
- highway noise barriers.
The porous sound-absorbing concrete can compose the
entirety of the barriers, panels, blocks, and other sound-absorbing
structures. As indicated in the foregoing description, two or more layers
of porous sound-absorbing concrete according to the invention can be
superposed to meet with requirements related for example to the
performance and appearance. The bond between the different layers can
be a cementitious bond, and /or it can be ensured by a reinforcement wire
mesh and/or a metallic reinforcement and/or a non-metallic reinforcement
consisting of fibres of various nature, shape and dimension.
In the application to highway noise barriers, one of the
faces of a layer of porous sound-absorbing concrete according to the
invention can be covered with a layer of conventional concrete in order
to construct noise barrier rigid panels presenting both an excellent sound
absorption coefficient (capacity of the material to absorb an acoustic
wave upon reflexion, on the highway side), and excellent transmission
losses (capacity of the material to minimize the transmission of acoustic
waves therethrough). Also, the layer of porous sound-absorbing concrete
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can be applied to a layer of other non-porous material such as wood,
steel or other material to prevent non-dissipated acoustic energy to pass
through the noise barrier.
Although the present invention has been described
hereinabove by way of a preferred embodiment thereof, this embodiment
can be modified at will, within the scope of the appended claims without
departing from the spirit and nature of the subject invention.
