Patent Publication Number: US-11040463-B2

Title: System and method for spraying lightweight insulating concrete

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the U.S. National Stage of PCT/FR2016/000109, filed Jun. 29, 2016, which in turn claims priority to French Patent Application No. 1501393, filed Jun. 30, 2015, the entire contents of all applications are incorporated herein by reference in their entireties. 
     FIELD OF THE INVENTION 
     The invention relates to the field of constructions, and more specifically to the thermal or acoustic insulation of buildings by spraying a lightweight shotcrete. 
     BACKGROUND OF THE INVENTION 
     Shotcrete has been known for a long time, as shown by French Patent FR 578 421 (1924), disclosing a shotcrete spraying device using pressurized air. 
     Among the shotcrete spraying techniques, there exist the wet-mix and the dry-mix versions. In the wet-mix version, water is added to the mixture including the aggregate and the binder to obtain the shotcrete before its spraying on the site to cover. In the dry-mix version, water is added to the mixture at the very moment of the spraying. The wet-mix version has long been (and still is) used for mineral concretes and mortars. 
     Recently, techniques were imagined for spraying lightweight insulating shotcretes, and more specifically shotcretes in which the aggregate is a plant aggregate (e.g. hemp shive, which is the fragmented lower part of the hemp stem). One may e.g. refer to French Patent Application No. FR 2 923 242, which discloses a low-density shotcrete spraying method wherein a dry mixture is obtained by homogenizing a lightweight plant aggregate (such as hemp shive) with a binder; that dry mixture is then displaced in a pneumatic way by means of a blower; the dry mixture is humidified during its displacement by water dispersion; then the such obtained concrete is sprayed onto a surface. 
     That method, which is in-between the wet-mix version (allegedly unsuitable to compressible shotcretes, according to FR 2 923 242) and the dry-mix version (the important volume of which would lead, according to FR 2 923 242, to jamming and obstructions in the metering devices), is however not exempt of drawbacks. 
     Firstly, the dosage of the shotcrete is complex, especially because it is difficult to master the flow rate of the dry mixture (moreover if the pressure generated by the blower is not settable). 
     Secondly, as humidification is provided immediately before the spraying, the water only partly impregnates the dry mixture. The fraction of mixture which remains dry is nevertheless sprayed but does not aggregate to the surface to cover and, bouncing back, scatters around, resulting in material loss. In order to minimize the loss, fully admitted by FR 2 923 242, this document proposes to recycle the scattered material. This solution is theoretically satisfactory; however, in practice, recycling the scattered material takes time, and the recycled material requires a dedicated metering device. In addition, the dry-mix scattering generates dust, which, given the presence of fibers and binder (cement or lime), may attack the respiratory system. One may increase the water rate but this solution is excluded because of the risk of humidifying the aggregate (this is specifically forbidden by FR 2 923 242). 
     One object is to propose a method and a system for spraying wet-mix shotcrete, which allow, either separately or together:
         To enhance actual yields   To minimize loss   To minimize dust   To optimize water consumption   To spray a lightweight shotcrete including a fast set binder.       

     SUMMARY OF THE INVENTION 
     To that end, it is proposed, in a first aspect, a method for coating a support by means of a lightweight insulating shotcrete, including the following operations:
         Preparing the shotcrete by mixing, in predetermined proportions, at least a fibrous aggregate, a binder and water;   Pour the prepared shotcrete in a tank having at least one outlet;   Continuously move the wet shotcrete in the tank until it reaches the or each outlet;   Suck the wet shotcrete along a feeding pipe linking the outlet of the tank to a spraying gun, by means of a Venturi hose fed with pressurized air, included in the spraying gun;   Spraying the sucked shotcrete on the support by means of the spraying gun.       

     This wet-mix spraying method allows to maximize the output (and hence the yield) while optimizing the water consumption and minimizing dust. 
     Various additional features of this method may be provided, either alone or in combination:
         The aggregate is hemp shive;   The binder is natural quick-setting cement;   The shotcrete contains a water retainer adjuvant.   The shotcrete contains a set retarder.   The pressure inside the spraying gun next to the Venturi hose is lower than 0.3 bar.       

     It is proposed, in a second aspect, a spraying system for insulating lightweight shotcrete, comprising:
         A container provided with a tank and at least one forth worm screw rotatably mounted in the tank, the tank having at least one primary outlet facing the forth worm screw;   A spraying gun provided with a barrel, with a Venturi hose opening in the barrel and with a primary inlet duct opening in the barrel next to the Venturi hose;   A pressurized air source connected to the Venturi hose;   At least one primary feeding pipe connecting the primary outlet of the tank to the primary inlet duct of the spraying gun.       

     Various additional features may be provided, either alone or in combination:
         The container comprises a return worm screw mounted parallel to the forth worm screw and driven in rotation in a opposite direction;   The tank is provided with a secondary outlet next to the forth worm screw, the spraying gun is provided with a secondary inlet duct opening in the barrel, and the system comprises a secondary feeding pipe connecting the secondary outlet of the tank to the secondary inlet duct;   The secondary inlet duct opens next to the primary inlet duct.       

     The above and further objects and advantages of the invention will become apparent from the detailed description of preferred embodiments, considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a perspective view showing a construction site for outer coating a building with a lightweight shotcrete spraying system. 
         FIG. 2  is a detailed view of the construction site of  FIG. 1 , at a larger scale. 
         FIG. 3  is a detailed view, at a larger scale, of  FIG. 1 . 
         FIG. 4  is a top view of a container within the spraying system; in the magnifying circles, two enlarged details are shown. 
         FIG. 5  is a perspective view, partly cut out, of the container of  FIG. 4 . 
         FIG. 6  is a partial cut view of the container of  FIG. 4 , taken along the cut plane VI-VI. 
         FIG. 7  is a detailed cut view of the container of  FIG. 6 , taken along the cut plane VI-VI. 
         FIG. 8  is a perspective view of a spraying gun within the spraying system, in a first embodiment. 
         FIG. 9  is a cut view of the spraying gun of  FIG. 8 , taken along the cut plane IX. 
         FIG. 10  is a detailed view of the spraying gun of  FIG. 9 , taken in the magnifying circle IX; 
         FIG. 11  is a perspective view of a spraying gun within the spraying system, in a second embodiment. 
         FIG. 12  is a cut view of the spraying gun of  FIG. 11 , taken along cut plane XII. 
         FIG. 13  is a detailed view of the spraying gun of  FIG. 12 , taken in the magnifying circle XIII. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a construction site for insulating a building  1 . The nature of the building  1  is irrelevant; here it is a house, but it might be a block building, an outbuilding, a garage, a shelter. 
     The building  1  comprises, in a known manner, masonries  2  (including frontage, side walls, floors, slabs), covered with a roofing. Here, the constructions site aims at coating the masonry  2  (e.g. the side wall facing the prevailing wind) with a sprayed layer of a lightweight insulating shotcrete  3 , composing examples of which shall be provided hereinafter. 
     The coating is provided by means of a spraying system  4  which may be transported on site (as illustrated). This spraying system  4  comprises, firstly, a container  5  provided with a tank  6  in which the ready shotcrete  3  is poured, and at least one forth worm screw  7 . The tank  6  has at least one primary outlet  8 A, located next to the forth worm screw  7 . 
     The forth worm screw  7  is rotatably mounted in the tank  6  to continuously move the shotcrete  3  until it reaches the primary outlet  8 A. The tank  6  comprises a pair of inclined longitudinal walls  9 , connected through two transverse ending walls, i.e. an upstream transverse wall  10  and a downstream transverse wall  11 . The tank  6  has an opening  12  through which the shotcrete  3  is poured and, opposite the opening  12 , a gutter-shaped bottom  13 . 
     The forth worm screw  7  is an Archimedes (preferably stainless) steel screw, including a shaft  14  mounted between the transverse walls  10 ,  11 , and a propeller  15  fixed to the shaft  14 . The outer diameter of the propeller  15 , substantially equal (possibly with a few millimeters gap) to the inner diameter of the bottom  13  of the tank  6 , is comprised between 100 mm and 200 mm, and preferably of about 125 mm. The pitch of the propeller  15  is comprised between 100 mm and 200 mm, and preferably of about 125 mm. The diameter of the shaft  14  is comprised between 20 mm and 40 mm, and e.g. of about 30 mm. 
     The forth worm screw  7  is rotatably mounted with respect to the tank  6  (and more precisely with respect to the transverse walls  10 ,  11  by means of bearings, preferably ball bearings. At an upstream end, the shaft  14  has a cantilevered section  16  which protrudes from the tank  6  and to which a wheel  17  (pulley or gear wheel) is fixed. 
     As depicted on  FIG. 3 , the container  5  is provided with a motor  18  (thermal or electrical) which drives in rotation the shaft  14  through the wheel  17 , by means of a belt or a chain transmission  19  meshed with the wheel  17 . In a preferred embodiment depicted on  FIG. 3 - FIG. 5 , the container  5  comprises a mobile cart  20  provided with a chassis  21 , on which the tank  6  and the motor  18  are mounted, and a wheel train  22  rotatably mounted to the chassis  21 . The chassis  21  is preferably provided with a latch  23  of the trailer type, whereby the container  5  may be towed by a suitable coupling. 
     As already stated, the shotcrete  3  is already prepared (and hence wet) when it is poured in the tank  6 . The shotcrete  3  may be handmade, but, in a preferred embodiment, the spraying system  4  comprises, to this end, a mixer  24  in which the ingredients are poured, and which prepares the shotcrete  3  therewith. 
     In the depicted example, the mixer  24  is of the vertical axis-type; it includes a chassis  25 , a tank  26  mounted onto the chassis  25 , a rotor  27  with blades  28 , and a motor  29  coupled to the rotor  27  to drive it in rotation around the axis in order to mix the ingredients with a view to obtaining a homogeneous shotcrete  3 . 
     As may be seen on  FIG. 1 - FIG. 3 , the mixer  24  may be mounted on a utility vehicle  30 , such as a pick-up truck. Then, the container  5  is placed below the mixer  24 , which is preferably provided with a hatch and a hopper  31  through which, when the hatch opens, the wet shotcrete  3  is poured into the tank  6  of the container  5 . 
     The ingredients of the shotcrete  3  include at least a plant aggregate, a binder and water. 
     The aggregate is e.g. hemp shive, the volumetric mass (for dry hemp shive) of which is of about 100 kg/m 3  when abounded (i.e. not cupped). The binder is e.g. natural quick-setting cement. 
     The following ranges of mass proportions should be respected:
         Aggregate (e.g. hemp shive): 21%-34%   Binder (e.g. natural quick-setting cement): 21%-40%   Water: 35%-44%       

     Composition example (in mass proportion) for a roof-insulating shotcrete:
         Aggregate (e.g. hemp shive): 34.8%   Binder (e.g. natural quick-setting cement): 21.7%   Water: 43.5%       

     Hence, for 100 kg (or 1000 l) hemp shive, 62.5 kg quick-setting cement and 125 l water. 
     Composition example (in mass proportion) for a wall-insulating shotcrete:
         Aggregate (e.g. hemp shive): 28.6%   Binder (e.g. natural quick-setting cement): 35.7%   Water: 37.5%       

     Hence, for 100 kg (or 1000 l) hemp shive, 125 kg quick-setting cement and 125 l water. 
     Composition example (in mass proportion) for a floor-insulating shotcrete:
         Aggregate (e.g. hemp shive): 21%   Binder (e.g. natural quick-setting cement): 39.5%   Water: 39.5%       

     Hence, for 100 kg (or 1000 l) hemp shive, 187.5 kg quick-setting cement and 187.5 l water. 
     It is better to add to the mixture a set retarder, to prevent the shotcrete  3  from hardening before it has been sprayed, especially when the weather is hot. Ordinarily, the set retarder is e.g. citric acid, e.g. of food quality. The quantity is negligible with respect of that of the main components (aggregate, binder, water); one may respect the ordinary recommendations, which propose to use 80 g of citric acid for one 25 kg sack of natural quick-setting cement (hence 320 g for 100 kg of natural quick-setting cement). Such a quantity is sufficient to delay by a half-hour the setting of the shotcrete, whichever the chosen composition among the three disclosed hereinbefore. 
     Furthermore, it is preferable to add to the mixture a cohesion agent in order to maintain the shotcrete cohesion during spraying. The cohesion agent may act as a water-retention adjuvant, aiming at enhancing the stability and homogeneity of the shotcrete. Methylcellulose is most recommended, since it provides both functions. The quantity added to the mixture is negligible with respect to those of the main components. 
     Trials have shown that the quantity of cohesion agent/water-retention agent is preferably proportional to the aggregate mass. In the case of hemp shive, a quantity of water-retention agent (e.g. methylcellulose) of 2% in mass (i.e. 2 kg for 100 kg or 1000 l hemp shive) provides good results, whichever the proportions of binder and water. 
     One may take advantage, for preparing the shotcrete, of using the following method, which has proven excellent. The dry hemp shive is first poured into the mixer  24 ; then the methylcellulose, with a mass ratio of 2% with respect of the hemp shive, is added to it. One then leaves the mixer  24  rotate for a few seconds in order for the methylcellulose to coat the hemp shive, and one then adds 1 l water per kg hemp shive. One then adds the binder (e.g. quick-setting cement), and then 0.5 l water per kg binder. 
     As already stated, the tank  6  of the container  5  is provided with at least one primary outlet  8 A, positioned next to the forth worm screw  7 . More precisely, the primary outlet  8 A opens in or in the vicinity of the bottom  13 . 
     The primary outlet  8 A may be formed by a drilling made in the tank  6  (possibly directly in the bottom  13 ) or, as depicted, by an added tube  32 , fixed to a longitudinal wall  9  e.g. by welding. That tube  32  preferably has an inner diameter of 50 mm. 
     The primary outlet  8 A is preferably positioned in the vicinity of the downstream transverse wall  11 , at low distance therefrom. As may be seen on  FIG. 4 , the forth worm screw  7  has a right-handed thread; in such case, it is counter-clockwise driven in rotation by the motor  18  in order to move the poured shotcrete  3  towards the primary outlet  8 A. Between the primary outlet  8 A and the downstream transverse wall  11 , the shaft  14  of the forth worm screw  7  has a downstream end section  33  has no propeller (the propeller  15  stops slightly downstream the primary outlet  8 A), but it has a clod breaker  34 , here under the form of a series of cylindrical blades  35 . The function of this clod breaker  34  is to break up the shotcrete  3  that accumulates around the downstream end section  33 . 
     The tank  6  is preferably provided with a secondary outlet  8 B, also positioned next to the forth worm screw  7 . The secondary outlet  8 B opens in or in the vicinity of the bottom  13 , next to the primary outlet  8 A. The secondary outlet  8 B may be formed by an added tube fixed to a longitudinal wall or, as depicted, directly to the bottom  13  of the tank  6 . The secondary outlet  8 B is advantageously extended by an added manifold  37 , welded to the tank  6 . That manifold  37  preferably has an inner diameter of 50 mm. 
     In a preferred depicted embodiment, the container  5  is provided with a return worm screw  38 , mounted parallel to the forth worm screw  7 , and driven in rotation in the opposite direction. The return worm screw  38  may be of identical design of the forth worm screw  7 , and also comprises a shaft  14  and a propeller  15 . The return worm screw  38  is however mounted head to tail with respect to the forth worm screw  7 , as shown on  FIG. 4 . In the depicted example, the return worm screw  38  is located above the forth worm screw  7 , and slightly offset transversely, with respect thereto. The propeller  14  of the return worm screw  38  extends from the downstream transverse wall  11  of the tank  6  to a short distance from the upstream transverse wall  10 , and has, in the vicinity thereof, a clod breaker  34  of similar design to that of the forth worm screw  7 . 
     The driving of the return worm screw  38  may be achieved by means of the motor  18 ; inversion of the rotation direction of the return worm screw  38  with respect of the forth worm screw  7  may be achieved by means of a pair of gear wheels  39 , mounted on the downstream ends of the screws  7 ,  38 , which protrude from the downstream transverse wall  11 . 
     In consequence, the shotcrete  3  which has not been evacuated by the outlet  8 A (or the outlets  8 A,  8 B) is broken up by the clod breaker  34  of the forth worm screw  7  and moved back upstream by the return worm screw  38 . When it reaches the vicinity of the upstream transverse wall  10 , the such moved back shotcrete  3  is broken up again by the clod breaker  34  of the return worm screw  38  and falls to the bottom  13  before it is again moved downstream by the forth worm screw  7 , to be evacuated by the outlet  8 A (or the outlets  8 A,  8 B). Such recirculation of the shotcrete  3  is repeated until all of it is evacuated. 
     The spraying system  4  further comprises:
         A spraying gun  40  provided with a barrel  41 , a Venturi hose  42  opening in the barrel  41  and at least one primary inlet duct  43 A opening in the barrel  41  next to the Venturi hose  42 ;   A pressurized air source  44  connected to the Venturi hose  42 ;   At least one primary feeding pipe  45 A which connects the primary outlet  8 A of the tank  6  to the primary inlet duct  43 A of the spraying gun  40 .       

     The (or each) feeding pipe  45 A is preferably a flexible pipe, possibly reinforced with a helical thread, and preferably has a smooth inner wall and an outer diameter equal (with a possible clearance) to the inner diameter of the tube  32  forming the primary outlet  8 A (respectively of the manifold  37  extending the secondary outlet  8 B)—hence about 500 mm in the depicted example. In a preferred embodiment, the reinforcing thread is made of metal, in order to conduct electricity. Connecting such thread to the ground then eliminates the static electricity generated by the friction of the material flow against the inner wall of the feeding pipe  45 A. 
     The pressurized air source  44  is preferably under the form of a compressor. That compressor  44  is e.g. mounted onto a sleigh  46  provided with a wheel train  47  and a latch  48  of the trailer type, whereby the compressor  44  may be towed to be transported on the construction site by a vehicle provided with a suitable coupling. 
     The compressor  44  is connected to the Venturi hose  42  by means of a flexible pipe  49  made of rubber or any other pressure resistant elastomer. The spraying gun  40  is provided with a connector  50  to which the flexible pipe  49  is sealingly connected. The spraying gun  40  is preferably further provided with a valve  51  mounted between the connector  50  and the Venturi hose  42 . That valve  51  is e.g. of the quarter-turn type and has a ball  52  rotatably fixed to a handle  53 , displacement of which moves the ball  52  to an opening position (depicted on  FIG. 10  and  FIG. 13 ) wherein the ball  52  lets the air from the compressor  44  pass, or to a closing position (not shown) wherein the ball  52  seals the air passage. 
     The spraying gun  40  has a duct  54  which links the connector  50  to the barrel  41 ; the Venturi hose  42  is under the form of a throttle made in that duct  54  next to the barrel  41 . More precisely, the Venturi hose  42  comprises a throttled section  55  (i.e. of lower diameter than that of the duct  54  downstream the ball  52 ), followed by a diverging section  56  by which the Venturi hose  42  opens into the barrel  41 , which has a diameter much larger than that of the duct  54 , and more specifically that of the throttled section  55 . In a preferred embodiment, the duct  54  has an average diameter D 1  comprised between 12 mm and 20 mm, and e.g. of about 14 mm, the throttled section  55  has a diameter D 2  comprised between 5 mm and 15 mm, and of about 10 mm, the diverging section  56  (taken at the largest) an outlet diameter D 3  of about 20 mm, and the barrel  41  an inner diameter D 4  of 50 mm. Such configuration allows, with a work pressure provided by the compressor  44  of about 7-8 bar (for an output flow of about 3000 l/min to 5000 l/min), to obtain in the barrel  41 , next to the Venturi hose  42 , a depression greater than 0.5 bar (i.e. a pressure lower than 0.5 bar), e.g. comprised between 0.5 bar and 0.99 bar (i.e. a pressure comprised between 0.01 bar and 0.5 bar). The depression is preferably greater than 0.7 bar (i.e. the pressure is lower than 0.3 bar). 
     Such depression transmits, through the primary inlet duct  43 A and the primary feeding pipe  45 A, to the primary outlet  8 A by which the circulating shotcrete  3  is then sucked. 
     When, as in the depicted example, the tank  6  comprises two outlets  8 A,  8 B, i.e. a primary outlet  8 A and a secondary outlet  8 B, the spraying gun  40  is then provided with a secondary inlet duct  43 B opening to the barrel  41 , and the spraying system  4  comprises a secondary feeding pipe  45 B connecting the secondary outlet  8 B to the secondary inlet duct  43 B. In such case, the depression in the barrel  41  transmits, through the secondary inlet duct  43 B and the secondary feeding pipe  45 B, to the secondary outlet  8 B by which the circulating shotcrete  3  is then sucked. 
     In a preferred embodiment depicted on  FIG. 5  and  FIG. 6 , the or each feeding pipe  45 A,  45 B is provided with one or more slot(s)  57  which protrude from the tube  32  (respectively from the manifold  37 ) and extend to the outlet  8 A (respectively  8 B). This (these) slot(s)  57  avoid obstruction of the outlet  8 A (respectively  8 B) by making air circulation easier under the depression generated by the Venturi hose  42 , and hence make the sucking of the shotcrete circulating in the tank  6  easier. In the example depicted on  FIG. 7 , each feeding pipe  45 A,  45 B is provided with three slots  57  distributed at 120°. In the depicted example, and e.g. on  FIG. 5 ,  FIG. 6  and  FIG. 7 , each slot  57  goes all through the wall of the feeding pipe  45 A,  45 B, and the feeding pipe  45 A,  45 B is mounted on a tube  58  which extends from the respective outlet  8 A,  8 B to beyond the slot  57 . In an alternate embodiment, each slot  57  is dug in the feeding pipe  45 A,  45 B without going through it. 
     In a first embodiment shown on  FIG. 8 ,  FIG. 9  and  FIG. 10 , the spraying gun  40  comprises superposed primary inlet duct  43 A and secondary inlet duct  43 B, which both open in the barrel  41  through a collector manifold  59  preferably inclined by an angle A of about 45° with respect to the barrel  41 . The barrel  41  has a main section  60  of constant diameter D 4  (i.e. of about 50 mm in the depicted example), and it also preferably has an end section  61  of greater diameter (that diameter, D 5 , is preferably comprised between 60 mm and 90 mm, e.g. of about 70 mm) the function of which is to reduce the flow speed of the shotcrete  3  to minimize bouncing against the surface to coat, while forming a diverging spray allowing for the coated surface to be increased. 
     In a second embodiment shown on  FIG. 12  and  FIG. 13 , the spraying gun  40  comprises a primary inlet duct  43 A and a secondary inlet duct  43 B which are symmetrical with respect to the barrel  41 , in which they both open next to the Venturi hose  42 , preferably with an angle B of about 45° in the depicted example. 
     Downstream the junction between the inlet ducts  43 A,  43 B and the barrel  41 , the latter comprises a lower diameter section  62 , (that diameter D 6  is preferably comprised between 30 mm and 45 mm, and e.g. of about 40 mm). That lower section  62  includes a straight section  63 , the length of which is comprised between 150 mm and 300 mm (and e.g. of about 200 mm), and a bent portion  64  which extends the straight section  63  and forms therewith an angle C which is preferably comprised between 30° and 50°, and e.g. of about 45°. The straight portion  63  may be formed by fitting a low diameter tube (between 30 mm and 50 mm, and e.g. of about 40 mm) within an outer ending tube of greater diameter (e.g. of about 50 mm) with a filling and air-sealing product  65  therebetween, such as a closed cell polymer foam. As depicted on  FIG. 12 , the lower diameter section  62  opens in a greater diameter section  66  (that diameter D 7  is preferably greater than 90 mm, e.g. of about 120 mm), with an aim to decrease the flow speed of the shotcrete  3  to minimize the bouncing on the surface to coat, while forming a diverging spray allowing for the coated surface to be increased. 
     In addition, in the example depicted on  FIG. 12 , the greater diameter section  66  partly overlaps the lower diameter section  62 , and has a bent outer section  67  which connects thereto upstream its opening, in order to create around the bent portion  64  a decompression chamber  68  aiming at generating turbulences in the shotcrete flow thereby lowering its speed and allowing its spraying in small clods instead of large clods and its spreading on the surface to coat. 
     An outer diameter ending section  61  may be added to the barrel  41 , such ending section  61  allowing, through pressure loss, to slow the shotcrete flow down, thus lowering the risk of the material bouncing onto the surface to coat. 
     To coat the support  2  (e.g. a wall) by means of the lightweight insulating shotcrete  3 , the composition of which is provided hereinbefore, one may proceed as follows. 
     A first phase consists in preparing the shotcrete  3  by mixing predetermined proportions (see above) the fibrous plant aggregate (here the hemp shive), the binder (here the fast setting cement) and the water, possibly with addition of the set retarder (such as citric acid) and the water retainer adjuvant such as methylcellulose). That mixture may be hand-made but it is preferable to make it with the mixer  24 . 
     After having obtained a homogeneous shotcrete  3 , a second phase consists of pouring the prepared wet shotcrete (in grey on  FIG. 2 ) in the container  5 , and more precisely in the tank  6 , the worm screws  7 ,  68  having been actuated. To pour the wet shotcrete  3  from the mixer  24  into the tank  6 , one opens the hatch. The shotcrete  3  falls in the tank  6  through the hopper  31 . 
     A third phase consists in continuously circulating the wet shotcrete  3  in the tank  6  until it reaches the outlet  8 A (or the outlets  8 A,  8 B). Such circulation is achieved by means of the worm screw  7  (or the worm screws  7 ,  38 ), commonly (and reversely) driven in rotation by the motor  18 . The clod breaker(s)  34  break up the aggregates that possibly form in the vicinity of the downstream transverse wall  11  (respectively the upstream transverse wall  10 ). 
     A fourth phase consists in sucking the wet shotcrete  3  along the feeding pipe  45 A (or feeding pipes  45 A,  45 B) linking the outlet  8 A (respectively the outlets  8 A,  8 B) of the tank  6  to the spraying gun  40 , by means of the Venturi hose  42  fed with pressurized air by the compressor  44 , and spraying the sucked wet shotcrete  3  onto the support  2 , by means of the spraying gun  40 . 
     The slots  57  formed in the feeding pipe  45 A (or the feeding pipes  45 A,  45 B) make the air circulation easier (as illustrated by the arrows in the magnifying details of  FIG. 6 ) and avoid clogging of the outlet  8 A (or outlets  8 A,  8 B) by the shotcrete  3 . The shotcrete  3  is sucked in the feeding pipe  45 A (or the feeding pipes  45 A,  45 B) in small sized clods (of several mm 3  to several cm 3 ) which hence do not clog either the feeding pipe  45 A (or the feeding pipes  45 A,  45 B) or the spraying gun  40  and are sprayed onto the surface to coat at a speed which, as suggested above, may be varied by setting the diameter of the barrel  41 . The architecture of the spraying system  4  allows to obtain a spraying flow rate greater than 2 m 3 /h, and possibly up to 3 m 3 /h. To coat a wall  2  having a length of 10 m and a height of 3.3 m with a lightweight insulating shotcrete  3  of a 15-cm thickness (i.e. an approximate volume of 5 m 3 ), 2-3 hours spraying are sufficient (with a lone operator  69  having a single spraying gun  40 ). If the operator is alone on the construction site, he should add the time for preparing the shotcrete  3  (e.g. by successive rounds of 100 l), unless the shotcrete  3  is continuously prepared, e.g. by a second operator dedicated to it, whereby the preparation time of the shotcrete is not taken into account. 
     It may be preferred, for the insulating of the walls, to prepare guides  70  (such as boards) aiming at ensuring that the surface of sprayed shotcrete is planar and constant in thickness. After having filled the space between guides  70 , the operator may smooth over the sprayed shotcrete  3  with a mason&#39;s ruler. If the shotcrete is still wet enough, the ruled shotcrete may be recycled and poured in the container  5  again. 
     The spraying system  4  and method disclosed hereinbefore provide several benefits. 
     First, being able to spray the wet-mix shotcrete  3  (the shotcrete is prepared and wet before it is sucked) allows for optimizing water consumption, as the water rate of the shotcrete  3  is lower than that of a handcrafted concrete. 
     Secondly, as the shotcrete  3  is wet when sprayed, it generates no dust while being sprayed, unlike the dry-mix shotcretes. Dust may be produced when the aggregate and the cement are poured into the mixer  24 , but they may be contained by means of a lid or a fabric covering the tank  26 . 
     Thirdly, as the spraying speed may be adjusted, and as the shotcrete  3  is sprayed in a wet-mix version after having been sufficiently mixed (i.e. there is no more aggregate unimpregnated with binder and water), its adherence to the support is good, whereby the bouncing (and hence the loss) is minimized. 
     Finally, the efficiency of the sucking generated by the Venturi hose  42 , the dimensions of the feeding pipe  45 A (or the feeding pipes  45 A,  45 B), the structure of the container  5  and the spraying gun  40 , together allow for good spraying rates, and hence for enhancing the actual production yield. One may observe that the presence of hemp in the barrel  41  reduces the passage section and hence increases the depression at the Venturi hose  42 , thereby increasing its sucking force. 
     One may note that the spraying system  4  may be used for spraying dry-mix shotcrete, as the Venturi hose  42  is capable of sucking lone aggregate, which may be wet and added with binder as it exits the spraying gun  40 .