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 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.

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.

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.

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.

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:

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 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;

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 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.

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 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 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.

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.

concreteconcrete concreteconcrete4 Ingredients ProportionsProportionsProportionsProportions Binder Cement 1 - - -type Cement - 1 1 -HSF

Epoxy resin- - - 1 Water (total) 0,25 0,25 0,025 -*

Aggregate Unimin - 3,9 - 7,7 Unimin 3,35 - - -Granite - - 4,7 -Superplasticizer"* 0,025 0,025 0,025 _ * Includes the water of the superplasticizer ** Dry extract content 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 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 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.

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 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;

- 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;

- 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 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.