CROSS-REFERENCE

[001] The present application claims priority from Australian Provisional Patent Application No. 2019902154 the contents of which should be understood to be incorporated into this specification by this reference.

TECHNICAL FIELD

[002] The present invention relates to a printer nozzle, a printer assembly including a printer nozzle, and a method of printing, such as for example, for 3D printing.

BACKGROUND TO THE INVENTION

[003] The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

[004] Additive manufacturing (AM) is a process involving the addition of successive layers of print material to create a structure, often referred to as 3D (three dimensional) printing. One of the primary benefits of AM is the ability to create custom, complex geometric shapes that would not be possible with traditional subtractive machining and construction methods.

[005] 3D concrete printing (3DCP) is a technology that can be applied in the construction industry, involving the application of AM technologies to the field of concrete construction.

[006] The 3DCP process is similar in nature to fused deposition modelling (FDM) found on desktop 3D printers. Typically, a stiff, visco-plastic, print material (such as a cement or geopolymer) is extruded from a nozzle. The nozzle is autonomously controlled to provide continuous movement, while the deposited print material is built up layer by layer. [007] Cementitious materials have high compressive strength, but typically have weak tensile strength. As such, structures made from such materials often have to be reinforced to provide the necessary tensile strength.

[008] One method of reinforcing the formed structure is by manual incorporation of a reinforcing material into the formed structure. For example, some large scale 3DCP structures consist of a 3DCP-formed outer shell, with conventional concrete poured inside. Typically, steel reinforcing bars are manually placed within the shell prior to concrete pouring (Diggs-McGee, B., et al., Print time vs. elapsed time: A temporal analysis of a continuous printing operation for additive constructed concrete. Additive Manufacturing, 2019).

[009] Another method of reinforcing 3DCP structures involves the use of printable fibre-reinforced concrete. In this method, fibres are added in the mixing stage of concrete, such that when hardened, tensile stresses in the binder matrix are borne by the fibres (Nam, Y.J., et al., Fiber-Reinforced Cementitious Composite Design with Controlled Distribution and Orientation of Fibers Using Three-Dimensional Printing Technology, in 3D Concrete Printing Technology. 2019, Elsevier p. 59-72).

[010] Such existing techniques for reinforcing 3DCP structures have a number of disadvantages including challenges in introducing the reinforcing material during the printing process and providing reinforcing material that imparts an acceptable tensile strength to the printed structure.

[01 1] The present invention has been devised with these shortcomings in mind.

SUMMARY OF THE INVENTION

[012] The present invention provides an improved method of reinforcing 3DCP structures and articles. In one embodiment, the present invention incorporates the use of overlapping reinforcing members to provide reinforcement between adjacent layers of print material. [013] In this specification, reference to deposition of print material or a reinforcing member onto a substrate encompasses the deposition of print material or a reinforcing member on an existing print material layer on the substrate.

[014] In this specification, the term“layer” refers to the arrangement of print material and/or reinforcing members determined from an elevation view of the printed structure. A layer is substantially defined as print material lying along a horizontal plane. Two layers are formed when a second layer is deposited adjacent to a first layer along the vertical plane, regardless of whether the layers are formed by print material and/or reinforcing member deposited in a continuous printing path, multiple printing paths, or in discrete layers.

[015] In this specification, reference to the first and second print material layers and first and second reinforcing members encompass overlapping layers of print material or reinforcing members formed from a continuous printing path, multiple printing paths, or in discrete layers.

[016] In this specification, the expression“first path” refers to the route the printer assembly takes to form a layer of the printed structure. The first path can include any number of directional changes necessary to form the layer. The first path may terminate/end at the start of the second layer of the printed structure.

[017] In this specification, the expression “second path” refers to the route the printer assembly takes after the formation of a layer of the printed structure to either form a successive layer or to position the nozzle in preparation for the formation of a successive layer of the printed structure. The second path can include any number of directional changes.

[018] In this specification, the term“adjacent” used in relation to the positioning of the print material layers refers to the positioning of the print material layers on top of each other.

[019] A first aspect of the present invention provides a nozzle for a printhead of a 3D printer, the nozzle comprising: a body having an inlet end adapted to feed print material into the nozzle and an outlet end adapted to deposit the print material onto a substrate, the outlet end including an opening that is adapted to receive a reinforcing member to be deposited on the substrate, and a shielding element located within the body that is adapted to shield a portion of the reinforcing member positioned within the opening from the print material during deposition of the print material.

[020] The nozzle allows the placement of the reinforcing member and the deposition of print material onto the substrate.

[021] The nozzle may be adapted to change the flow of print material from the inlet end towards the outlet end, such as by changing the direction of flow of the print material from the inlet end to the outlet end. Suitably, the nozzle is adapted to change an axial flow of print material from the inlet end to a transversal flow of print material at the outlet end.

[022] In one embodiment, the nozzle may be adapted to direct print material from the inlet end onto either side of a reinforcing member located within the opening. In another embodiment, the nozzle may be adapted to direct print material from the inlet end onto opposing faces of a reinforcing member located within the opening. This allows an effective bond to be formed between the print material and the reinforcing member.

[023] The nozzle may have an inner profile that is shaped to direct flow of print material on either side of the shielding element towards the reinforcing member. Suitably, the nozzle includes at least one convex inner surface. More suitably, the nozzle includes opposing convex inner surfaces. The inner surfaces may facilitate the flow of print material towards the outlet end and/or control the height of print material layer.

[024] The body may be elongate. Suitably, the body is substantially cylindrical in shape. [025] The body may have a cross-sectional profile of any shape, including polygonal, square, rectangular and circular. The cross-sectional profile may determine the surface profile of the formed structure. Suitably, the body has a circular cross-sectional profile.

[026] The shielding element shields a portion of the reinforcing member positioned within the opening of the nozzle from the print material during deposition of the print material.

[027] The shielding element may shield an upper portion of the reinforcing member from the print material. Accordingly, the shielding element can block part of the reinforcing member positioned within the opening from being exposed to the print material flowing out of the opening such that when the print material is deposited onto the reinforcing member to form a print material layer, the part of the reinforcing member within the shielding element protrudes from the first print material layer.

[028] The shielding element may be adapted to direct print material onto opposing faces of the unshielded part of the reinforcing member. The shielding element may have a generally convex outer profile. The shielding element may have a generally elliptical shape in axial cross section which splits the flow of print material onto either side of the shielding element, before converging the print material on the unshielded part of the reinforcing member. Suitably, the shielding element has a generally oval shape.

[029] The shielding element may extend transversely across the body. Suitably, the shielding element extends parallel along the printing direction. This allows the shielding element to cover part of the opening to shield a portion of the reinforcing member positioned within the opening from print material during the printing process. In particular, the shielding element shields the portion of reinforcing member within the shielding element. [030] Suitably, the shielding element includes an inverted channel that extends transversely across the body. More suitably, the inverted channel covers part of the opening to block print material from being deposited on a portion of the reinforcing member positioned within the opening. Even more suitably, the inverted channel covers a side of the opening parallel to the printing direction.

[031] The inverted channel may comprise a transverse wall that extends across the body and two side walls extending from opposing lengths of the transverse wall towards the outlet end. Suitably, the side walls comprise planar inner surfaces. More suitably, the planar inner surfaces are parallel to each other.

[032] The opening may be a gap or a slot extending in an axial direction from the outlet end towards the inlet end.

[033] The opening may have a width that can receive two reinforcing members. This allows the opening to orientate two reinforcing members in an overlapping arrangement. The width of the opening is preferably slightly wider than the combined thicknesses of two reinforcing members such that the two reinforcing members are snugly received in the opening when overlapped.

[034] The opening may have a closed side in the axial direction of the body. Suitably, the opening comprises a slot having a closed side and a pair of apertures located on the wall of the body and extending from the closed side. More suitably, the closed side is an inverted channel and the pair of apertures extend from ends of the inverted channel towards the outlet end.

[035] Suitably, the inverted channel has a transverse wall that extends across the body and two side walls extending from opposing lengths of the transverse wall towards the outlet end. The inverted channel forms a closed side of the opening and the pair of apertures form an open side of the opening to deposit print material onto a substrate and to receive a reinforcing member.

[036] The pair of apertures may be diametrically opposed. [037] The apertures may be of any shape. Suitably, the apertures are rectangular in shape.

[038] In some embodiments, the nozzle may be configured to have a variable width opening. In these embodiments, the outlet end of the nozzle comprises two movable outlet bodies each including a portion of the opening. Each outlet body being movable relative to each other to vary the width of the opening about the reinforcing member. Each outlet body preferably including a nozzle outlet adapted to deposit the print material onto the substrate, preferably on transversely opposite sides of the reinforcing member that extends axially between each nozzle outlet and outlet body.

[039] Each outlet body also preferably includes part of the shielding element in the form of inverted channel and form between each outlet body a pair of diametrically opposed apertures and which extend from the ends of the inverted channel to the outlet end. The width/ size of these apertures as well as inverted channel preferably vary through relative transverse movement of the outlet body, either spacing these bodies. In such embodiments, the opening of the outlet end of the nozzle has now been modified to articulate to have a variable width about the reinforcing member, and preferably move between open and closed positions. This allows the nozzle to more efficiently feed over the reinforcing member without collision, while also allow it to clamp or otherwise hold onto and along the reinforcing member while printing.

[040] A second aspect of the present invention provides a printing assembly of a 3D printer comprising the nozzle of the first aspect of the present invention connected to a printhead, and a reinforcement dispenser adapted to dispense the reinforcing member during printing, wherein in use, the printhead trails the reinforcement dispenser in a printing direction.

[041] A third aspect of the present invention provides a method of printing a structure using a 3D printer including a printing assembly according to the second aspect of the present invention, the method including: receiving a first reinforcing member within the opening of the nozzle;

moving the printhead along a first path substantially defined by the first reinforcing member; and

depositing print material from the nozzle onto the first reinforcing member to form a first print material layer that forms at least part of the structure wherein the first reinforcing member is at least partially embedded in the first print material layer.

[042] The method may include feeding the reinforcing member through the opening of the nozzle.

[043] The method may include dispensing the reinforcing member onto the substrate as the nozzle moves along the first path.

[044] The method may include simultaneously dispensing the reinforcing member onto the substrate and depositing the print material onto at least part of the reinforcing member.

[045] The method may include depositing a layer of print material having a thickness that is less than the height of the reinforcing member. This results in a portion of the reinforcing member protruding from the layer.

[046] The method may include depositing a second layer of print material onto the first reinforcing member such that the first reinforcing member extends from the first print material layer into the second print material layer. Suitably, the second layer of print material is deposited onto the exposed portion of the first reinforcing member. This provides reinforcement between the first and second print material layers.

[047] The method may include positioning a second reinforcing member in the second print layer such that the second reinforcing member overlaps with at least part of the first reinforcing member embedded in the second layer.

[048] The overlapping first and second reinforcing members provide continuous reinforcement between the print material layers to enhance the structural integrity of the structure. Advantageously, the arrangement of the reinforcing members in the printed structure enables the reinforcing members to be incorporated into printed structure during printing.

[049] Alternatively, the reinforcing member in the first layer and the reinforcing member in the second layer can be weaved together to provide continuous reinforcement. Weaving can reduce the amount of overlapping required and provide additional reinforcement.

[050] The method may include moving the reinforcement dispenser containing the reinforcing member along the first path to position the reinforcing member on the substrate.

[051] The method may include moving the printhead along the first path to position the second reinforcing member in an overlapping arrangement with the first reinforcing member.

[052] The method may include cutting the reinforcing member after the reinforcement dispenser reaches an end of the first path.

[053] The method may include folding or bending the reinforcing member during the printing of a layer.

[054] The method may include folding or bending the reinforcing member prior to forming the second print material layer. Suitably, the folding or bending occurs after the printhead completes the first print material layer.

[055] The method may include moving the printhead along a second path in preparation to form a second print material layer adjacent to the first print material layer. [056] The method may include moving the printhead along the second path to position a second reinforcing member in an overlapping arrangement with the first reinforcing member.

[057] The second path may retrace the first path taken by the printhead along a horizontal plane.

[058] The reinforcement dispenser may be positioned in front of the nozzle in the printing direction. This arrangement allows the print material to be deposited onto the reinforcing member as the reinforcing member is being dispensed by the dispenser.

[059] The reinforcement dispenser may be fixably attached to the printhead.

[060] The reinforcement dispenser may be a spool or a reel.

[061] The reinforcement dispenser may store the reinforcing member in a coil.

[062] The layers of print material may be arranged on top of each other.

[063] The reinforcing member may be porous.

[064] The reinforcing member may be a mesh, grid, wire or bar. The mesh/grid may comprise wires or bars in two directions with different diameters. The diameters of the wires ranging from 0.2 to 3 mm. The diameters of bars ranging from 3 mm to 30 mm. The mesh/grid spacing, i.e., the holes or apertures between the wires or bars, ranging from 3 mm to 500 mm. Suitably, the mesh/grid is made from wire of diameter 0.5mm to form a mesh/grid having a 6 mm spacing.

[065] The reinforcing member may be made from any one or more of a metallic, polymeric, or fibrous material.

[066] Each reinforcing member may extend across two print material layers only. [067] Each print material layer may contain one overlapping portion of the reinforcing members only.

[068] The print material may comprise a mixture of binder, water, sand and aggregates, activators, admixtures and/or minerals.

[069] The binder may comprise a mix of many cementitious materials. Suitably, the cementitious materials may be Ordinary Portland Cement, fly ash, slag, metakaolin, silica fume or clay.

[070] A fourth aspect of the present invention provides a printed structure comprising: at least three layers of print material, wherein a first reinforcing member is located between a first print material layer and second adjacent print material layer and which overlaps with a portion of a second reinforcing member located between the second print material layer and a third adjacent print material layer.

[071] It should be appreciated that this fourth aspect of the present invention can incorporate any of the features described above in relation to the first, second and third aspects of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

[072] The present invention will now be described by way of example only, with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

[073] Figure 1 shows a side view of a printhead including a nozzle according to a first embodiment of the invention.

[074] Figure 2 shows an isometric view of the printhead of Figure 1.

[075] Figure 3 shows a cross-sectional front view of the nozzle of Figure 1 illustrating the flow path of the print material; [076] Figure 4 shows a cross-sectional side view of the nozzle of Figure 3 along the A-A axis illustrating the flow path of the print material.

[077] Figure 5 shows a side view of a printer assembly according to a second embodiment of the invention, including the nozzle of Figure 1 and a reinforcement dispenser.

[078] Figure 6a is a schematic representation of a partially formed 3DCP structure comprising a first print material layer and a first reinforcing member.

[079] Figure 6b shows a schematic representation of the structure of Figure 6a, further comprising a second print material layer and a second reinforcing member in which the first and second reinforcing members are overlapping within the second print material layer.

[080] Figure 6c shows a schematic representation of the structure of Figure 6a, further comprising a third print material layer and a third reinforcing member in which the second and third reinforcing members overlap within the third print material layer.

[081 ] Figures 6d, 6e, 6f and 6g illustrates an embodiment where the nozzle is configured to have a variable width opening.

[082] Figure 7 is a graph showing the particle size distribution of the silica sands, OPC and densified silica fume from the experimental test procedure.

[083] Figure 8 shows the 26 mm strip of the galvanised steel wire mesh used for reinforcement in the Experimental section.

[084] Figure 9 is a graph showing a box plot of the tensile strength results of single wire strands under uniaxial tensile tests from the Experimental section.

[085] Figure 10 shows A) hardened wall section being cut into 45 mm to 50 mm beam sections; B) cut face of a beam section next to schematic cross-sectional view of the beam section illustrating the location of mesh; C) a magnified view of a section of the beam section of B) from the Experimental section.

[086] Figure 1 1 is a schematic illustration of the loading directions and dimensions of the 3D printed samples and cast samples for compressive testing from the Experimental section.

[087] Figure 12 shows a schematic representation of the three-point bending test set-up from the Experimental section.

[088] Figure 13 shows a schematic of the derivation of the calculated ultimate bending (Mult.cal) in relation to the 3D printed sample section and stress block from the Experimental section.

[089] Figure 14 is a graph showing the compressive strength results for 3D printed and cast cube samples from the Experimental section.

[090] Figure 15 is a graph showing the moment strength results for the reinforced samples from the Experimental section.

[091] Figure 16 is a graph showing the moment strength results for the un reinforced samples from the Experimental section.

[092] Figure 17 shows images of cracking on both sides of the print material layer, as well as horizontal cracking across the print material layer of sample R3 from the Experimental section.

[093] Figure 18 shows images of cracking on both sides of the print material layer, as well as horizontal cracking across the print material layer of sample R5 from the Experimental section. DETAILED DESCRIPTION

[094] In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings may be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

[095] One aspect of the nozzle for a printhead of a 3D printer as defined by the invention is marked as 10 in Figure 1. The nozzle 10 comprises an elongate body 12 having an inlet end 14 adapted to feed print material into the nozzle and an opposing outlet end 16 adapted to deposit the print material onto a substrate 18.

[096] The elongate body 12 is substantially cylindrical in shape and further includes an inner profile that comprises a sigmoidal surface 20 and a convex surface 22 along the axis of the nozzle that facilitate the flow of print material towards the outlet end. The sigmoidal surface 20 terminates at the base of the outlet end while the second convex surface 22 terminates with the shielded section of the nozzle. The terminating ends of surfaces 20 and 22 control the height of the print material layer deposited on the substrate.

[097] The outlet end 16 includes an opening in the form of closed sided slot 24, in which the closed side is transverse to the body 12 and parallel to the printing direction.

[098] Slot 24 includes a shielding element in the form of an inverted channel 26 that is connected to opposing walls of the body 12 and a pair of diametrically opposed apertures 28, 30 located on the wall of the body 12 and which extend from the ends of the inverted channel 26 to the outlet end.

[099] The inverted channel 26 forms a closed side of the slot 24 while the pair of diametrically opposed apertures 28, 30 form an open region of the slot 24 for receiving a reinforcing member 32 and for depositing print material onto the substrate 18.

[100] The apertures in Figures 1 and 2 have a substantially rectangular shape and are aligned parallel to the printing direction.

[101] The apertures 28, 30 direct print material onto opposing faces of the reinforcing member 32 to embed the reinforcing member 32 within the print material (see Figures 1 and 2).

[102] As shown in Figure 3, the inverted channel 26 comprises a transverse wall 34 that extends across the diameter of the outlet end and two substantially parallel planar inner side walls 36, 38 extending from a length of the transverse wall 34 to the outlet end 16.

[103] The inverted channel 26 has a convex outer profile 40 in the form of an ellipse which splits the flow of print material onto either side of the inverted channel, before converging the print material towards opposing sides of the reinforcing member located within the slot 24.

[104] Together with the convex outer profile 40 of inverted channel 26, the inner profile of the nozzle facilitates a change in direction of the print material flow from an axial direction along the elongate body to a substantially transverse direction across the elongate body 12 onto opposing faces of the reinforcing member 32. This forms a print material layer in which the reinforcing member 32 and the print material are strongly bonded together. The effectiveness of this bond between the print material and the reinforcing member 32 enhances the physical properties and hence performance of the composite structure 42. The directional flow of the print material is indicated in Figures 3 and 4 by the arrows.

[105] Whilst the embodiment illustrated in Figure 1 is substantially cylindrical in shape, it is understood that other geometries of the body 12 are also possible. The outlet end 16 may also have a variety of shapes. The shape of the outlet end 16 may be used to control the surface profile of the formed structure 42.

[106] One aspect of the printer assembly 44 of a 3D printer according to the invention is illustrated in Figure 5. The printer assembly 44 comprises nozzle 10 connected to printhead 46, and additionally includes a reinforcement dispenser 48 in the form of a spool or a reel. The reinforcement dispenser 48 is fixably attached to the printhead 46 by a mounting bracket 50 and adapted to dispense the reinforcing member 32 during printing. The reinforcement dispenser 48 is positioned in front of the nozzle 10 in the direction of travel of the printhead 46 such that the printhead trails the reinforcement dispenser in a printing direction X.

[107] The method of printing a structure using a 3D printer according to the invention can vary depending on whether the printing path taken by the printer assembly is continuous or discontinuous.

[108] According to a first method of printing a structure using a 3D printer including a printer assembly 44 according to an embodiment of the invention, the printhead 46 moves along a first pre-programmed continuous pathway which defines the cross- sectional profile of the first print material layer of the structure 42.

[109] In operation, the spool 52 feeds a reinforcing member 32 through the slot 24 as the printhead 46 moves along the first path. The reinforcing member 32 is in the form of a porous mesh made from a metallic, polymeric or fibrous material. The mesh has a diameter of about 0.5 mm, however it is understood that the diameter of the mesh may range between about 0.2 and 0.7 mm. The mesh spacing is large enough to allow the print material to flow through and provide adequate bonding and anchoring of the reinforcing member 32 in the print material layer. The diameter of the holes is selected to minimize any hinderance to the mobility of the nozzle 10 during the printing process and to allow the reinforcing member to form fine curvatures, for example, when printing around a bend. It is understood that the mesh as described above is suitable for small scale laboratory trials. When applied to construction applications, the sizing of the mesh may have to be scaled up depending on the size, shape and load bearing requirements of the formed structure.

[1 10] The print material comprises of a mixture of a sand and a binder. Preferably, the binder comprises of Ordinary Portland Cement and densified amorphous fumed silica. The print material is selected to allow flowability around and between the reinforcing member to create a strong bond, whilst also providing suitable buildability, wherein the transition between the fresh and hardened state of the print material enables an effective build rate without premature collapse or deformation of the formed print material layers.

[1 1 1] The reinforcing member 32 is unrolled from the dispenser 48 at a rate equal to the rate of the movement of the nozzle 10. This arrangement allows for the print material to be uniformly deposited onto the reinforcing member 32 as the reinforcing member 32 is being dispensed by the dispenser 48. As illustrated in Figure 5, the dispenser 48 takes the form of a spool or a reel, with the reinforcing member 32 being stored on the dispenser 48 in a coil. The printer assembly 44 is also fitted with a cutter (not shown) to enable the reinforcing member to be cut.

[1 12] The reinforcing effect provided to the structure 42 is enhanced by a partial overlapping of the reinforcing member 32 during the deposition of each successive print material layer.

[1 13] When forming the first print material layer 54, the nozzle 10 moves along a path substantially defined by the first reinforcing member 32A and deposits print material onto both faces of reinforcing member 32A. During the deposition, print material flows from the inlet end 14 towards the inverted channel 26. The convex outer profile of the inverted channel 26 causes the flow of the print material to split to the sides of the inverted channel and converge after passing the inverted channel. The inner profile of the nozzle 10 facilitates a change in the generally axial flow of the print material to a generally transversal direction towards apertures 28, 30 such that the print material is deposited onto opposing faces of the first reinforcing member 32A to embed the reinforcing member in the first print material layer 54 (see Figure 4). Suitably, the reinforcing member 32A is positioned in the middle of the print material layer 54. As such, the nozzle 10 allows for the simultaneous deposition of the print material and reinforcing member 32A onto the substrate 18 to form a reinforced print material layer (see Figure 6).

[1 14] Because the upper region of the reinforcing member 32A is located within inverted channel 26 of the nozzle 10, this region is shielded from the print material being discharged through the opening (see Figure 3). As such, the reinforcing member 32A is partially embedded into the print material layer to form the first print material layer 54. The exposed portion of the reinforcing member 32A above the print material layer would receive the next print material layer and overlap with a section of the next reinforcing member.

[1 15] When the spool 52 reaches the end point of the first print material layer, the printer assembly 44 continues to deposit print material onto the reinforcing member until the printhead 46 reaches the same end point. This allows the first print material layer 54 to terminate at the same point as the first reinforcing member 32A to form a first print material layer 54 with a first reinforcing member 32A partially embedded in the print material layer (see Figure 6a).

[1 16] The height of the first reinforcing member 32A is selected to ensure that it does not extend beyond the adjacent second print material layer to minimize interference with the printing process.

[1 17] Upon formation of the first print material layer 54, the printhead 46 moves along an upwardly inclined path to continue along the first path to form the second print material layer 56. During this process, the printhead 46 positions a second reinforcing member in an overlapping arrangement with the first reinforcing member 32A and deposits print material onto the exposed portions of the first reinforcing member 32A to form a second print material layer 56 in which the first reinforcing member 32A is located between the first and second print material layers 54 and 56. [1 18] The reinforcing member 32 may be bent or folded as the printhead 46 continues along the first path to form further print material layers. When forming the final print material layer, a cutter cuts the reinforcing member 32 when the spool 52 reaches the end point to terminate the deposition of the reinforcing member. The printer assembly continues to deposit print material onto the reinforcing member until it reaches the same end point to complete the structure 42.

[1 19] The reinforcing members are arranged in a partial overlapping configuration between the layers of print materials of the structure 42 (i.e. in an interlayer direction). This enhances the reinforcement provided to the formed structure 42 by providing reinforcement between the layers of print materials of the structure 42 (i.e. in an interlayer direction). The overlapping arrangement of the reinforcement layers also provides support to the structure 42 during the printing process. In contrast, typical 3DCP structures formed by layer by layer deposition are inherently weak under tensile forces due to the lack of reinforcement between layers of the structure.

[120] In this first method, it can be appreciated that the first and second print material layers and first and second reinforcing members are laid continuously and are formed as the printhead 46 travels along the first path.

[121] In a second method of printing a structure using a 3D printer according to an embodiment of the invention in which the printer assembly takes a discontinuous path to form structure 42, when the spool 52 reaches the end point of the first print material layer 54, a cutter cuts the reinforcing member 32 and the printer assembly continues to deposit print material onto the reinforcing member until the printhead 46 reaches the same end point. This completes the first print material layer 54 having a reinforcing member 32A partially embedded in the print material layer (see Figure 6a).

[122] Upon formation of the first print material layer 54, the printhead 46 is raised to accommodate the height of the first print material layer 54 and moves along a second path which may be in the same direction as the first path or which may be in a different direction to the first path to deposit a second reinforcing member onto the first print material layer such that the exposed portion of the first reinforcing member overlaps with part of the second reinforcing member. At the same time, print material is deposited onto the exposed portion of the second reinforcing member to form a second print material layer 56 with the first reinforcing member located between the first and second print material layers with the second reinforcing member having a partially exposed portion (see Figure 6b). If the second path is in a different direction to the first path, the printer assembly may rotate to position the spool 52 in front of the printhead 46.

[123] Upon formation of the second print material layer 56, the printhead 46 is raised again to accommodate the combined height of the first and second print material layers, and moves along a third path which may be or may not be in the same direction as the second path to position a third reinforcing member 32C on the second layer and to deposit print material onto the exposed portion of the second reinforcing member. This forms a third print material layer 58 in which the exposed portion of the second reinforcing member is embedded within the third print material layer and which overlaps with part of the third reinforcing member (see Figure 6c).

[124] This process is repeated to form a multi-layered structure having a reinforcing member extending from one print material layer to an adjacent print material layer.

[125] In a third method of printing a structure using a 3D printer according to an embodiment of the invention in which the printer assembly takes a discontinuous path, when the spool 52 reaches the end point of the first path as outlined in the first method, a cutter cuts the reinforcing member 32 and the printer assembly continues to deposit print material onto the reinforcing member until the printhead 46 reaches the same end point. This forms a first print material layer 54 with a first reinforcing member 32A partially embedded in the first print material layer (see Figure 6a).

[126] The printhead 46 is then raised to accommodate the height of the first print material layer and moved into position in preparation to form the second layer of the printed structure 42. In this method, the reinforcing member and print material are deposited when the printer assembly continues along the first path after formation of the first print material layer as opposed to the second method described above which may involve the deposition of the reinforcing member and print material to form the second print material in a second path that is in the opposite or reverse direction to the first path.

[127] Figures 6a-6c show the arrangement of the reinforcing members 32A-C with respect to the print material layers 54-58.

[128] The first print material layer 54 comprises a reinforcing member 32A embedded in the first print material layer in which the height of a first reinforcing member 32A is greater than the height of the first print material layer 54 such that a portion of the first reinforcing member 32A protrudes from the first print material layer 54.

[129] A second reinforcing member 32B is positioned in an overlapping arrangement to the exposed part of the first reinforcing member and print material is applied to the overlapping sections of the first and second reinforcing members to form the second print material 56.

[130] The reinforcement provided to the structure 42 is enhanced by the partial overlapping of the reinforcing members during formation of each successive print material layer. This provides reinforcing member structures between the layers of print materials of the structure 42 (i.e. in an interlayer direction) that afford continuous structural support across the layers.

[131] Alternatively, the first reinforcing member 32A and the second reinforcing member 32B can be weaved together to provide continuous reinforcement. Weaving can reduce the amount of overlapping required and provide additional reinforcement to the structure 42.

[132] One aspect of the printed structure according to the invention comprises at least three layers of print material, in which a first reinforcing member is located between a first print material layer and second adjacent print material layer and which overlaps with a portion of a second reinforcing member located between the second print material layer and a third adjacent print material layer (see Figure 6).

[133] Figures 6d, 6e, 6f and 6g illustrates an embodiment where the nozzle 10B may be configured to have a variable width opening 24B. Like the previous embodiments (for example Figures 1 and 2) the nozzle 10B comprises an elongate body 12B having an inlet end (not shown) adapted to feed print material into the nozzle 10B and an opposing outlet end 16B adapted to deposit the print material onto a substrate 18B (Figure 6f).

[134] Like the previously described embodiments, the elongate body 12B is substantially cylindrical in shape and further includes an inner profile as described. The outlet end 16B includes an opening in the form of closed sided slot 24B, in which the closed side is transverse to the body 12B and parallel to the printing direction. In this embodiment, the body 12B comprises two movable outlet bodies 12C and 12D each including half of the opening 24B. As shown in Figure 6d and 6e, each outlet body 12C and 12D is transversely movable relative to each other to vary the width of the opening 24B about the reinforcing member 32B (Figure 6f).

[135] Each outlet body 12C and 12D includes a nozzle outlet 55B (Figure 6g) adapted to deposit the print material onto the substrate 18B (Figure 6f). Each outlet body 12C and 12D also includes part of the shielding element in the form of inverted channel 26 that is connected to opposing walls of the respective outlet bodies 12C and 12D and form between them a pair of diametrically opposed apertures 28B, 30B and which extend from the ends of the inverted channel 26B to the outlet end. Again, the width/ size of these apertures 28B, 30B as well as inverted channel 26B vary through relative transverse movement of the outlet body 12C and 12D, either spacing these bodies 12C and 12D apart (in an open position) as shown in Figure 6e or closing this gap to bring these outlet bodies 12C and 12D proximate together (closed position) as shown in Figure 6d.

[136] This nozzle 10B operates in a similar manner as described for previous embodiments, with the inverted channel 26B forming a closed side of the slot 24B while the pair of diametrically opposed apertures 28B, 30B form an open region of the slot 24B for receiving a reinforcing member 32B and for depositing print material onto the substrate 18B. The apertures 28B, 30B direct print material onto opposing faces of the reinforcing member 32B to embed the reinforcing member 32B within the print material (see Figure 6f). Here the slot 24B is able to articulate to have a variable width about the reinforcing member 32B, allowing the nozzle 10B to more efficiently feed over the reinforcing member 32B without collision, while also allow it to clamp or otherwise hold onto and along the reinforcing member 32B while printing.

[137] Figure 6g shows a cross-sectional front view of the nozzle of Figure 6a to 6c illustrating the flow path of the print material in each outlet body 12C and 12D. As shown in Figure 6g, the nozzle body 10B is split into the separate and movable outlet bodies 12C and 12D. Each outlet body 12C and 12D include nozzle outlets 55B which deposit each layer 54 onto the substrate and reinforcing material 32B. Again, the inverted channel 26 comprises a transverse wall 34B that extends across the diameter of the outlet end and on each opposing outlet body 12C and 12D. Each outlet body 12C and 12D includes the two substantially parallel planar inner side walls 36B, 38B extending from a length of the transverse wall 34B to the outlet end 16B.

[138] The directional flow of the print material is indicated in Figure 6g by the arrows. As described above for the previous embodiments, together with the profile 40B of inverted channel 26B, the inner profile of the nozzle 10B facilitates a change in direction of the print material flow from an axial direction along the elongate body to a substantially transverse direction across the elongate body 12B onto opposing faces of the reinforcing member 32B (Figure 6f). This forms a print material layer in which the reinforcing member 32B and the print material are strongly bonded together.

EXAMPLE - LABORATORY TRIALS

[139] Small scale laboratory trials were conducted to characterise the properties of the structure and to determine the effectiveness of the reinforcement provided by the reinforcing member. It should be appreciated that the invention is intended to be used in large scale in precast concrete factories or construction sites. The invention is not limited to the materials, mesh or layer sizes used in the trials.

[140] Details of the sample preparation and tests performed on the samples can be found in the Experimental section.

[141] These trials utilized a mesh reinforcing member inserted into 3D printed layers of cementitious material. The process involved the progressive insertion of 6 mm x 6 mm galvanised steel reinforcing mesh within single extruded layers, which partially overlapped in subsequent layers (see Figure 10). A wall section was printed using this process and cut into sections to form samples before being tested in three-point bending.

[142] The following observations were made after conclusion of the tests:

[143] The samples failed by steel yielding and fracturing rather than bond failure between the mesh and print material. This demonstrated the nozzle design to be effective in creating sufficient bond between the mesh and print material.

[144] The tests and calculations indicated the overlapping arrangement of the mesh was more effective as a continuous mesh.

[145] Steel reinforcing members deposited and arranged in this manner provided continuity and increased moment strength in flexure by 170% - 290%.

[146] Initial cracking and failure occurred primarily through the layer interfaces, with reinforcement rupture as the final failure mode.

[147] Bond strength was on average, 42% of the estimated tensile strength of the cementitious mix.

[148] The results and analyses of the various tests performed on the samples are detailed below. Compression Tests

[149] Compression test results are displayed in Figure 14 comparing the 3D printed samples tested in perpendicular and parallel directions, against the mould cast samples at 7-day strength. Samples tested in the perpendicular direction displayed and average compressive strength of 22.6 MPa, while the samples tested in the parallel direction display an average compressive strength only slightly lower of 21.5 MPa. The two samples from this study are closely within standard deviations of each other highlighting minimal if any anisotropic behaviour.

[150] Mould cast samples as expected due to the form filling ability, flat surfaces and vibration, minimising voids and unevenness, displayed the largest average compressive strength of 43.5 MPa.

Three-Point Bending

Flexural Moment Strength

[151] Three-point bending tests were performed at 7-day strength for reinforced and unreinforced samples of a wall section. Results for the six reinforced and two unreinforced samples are plotted in moment - displacement graphs in Figure 15 and Figure 16 respectively. Data regarding sample specifications and moment strength from the graphs are also tabulated in Table 1. Table 1 also provides flexural bond tensile data and calculated moment strength. Cracking moments (Mcr) are the point at which cracking initiates, represented in Figure 15 as the first drop in moment strength. Ultimate moment strength (Mult) was taken as the highest moment attained by the sample before failure. Sample C-2 was subjected to a 120 mm span three point bending test rather than the 140 mm span, although no variation was noticed, and results are calculated accordingly.

[152] Observed in Figure 15 the reinforced samples immediately display the effectiveness of the reinforcement. Mcr are observed to take place within the region of approximately 6.88 x 103 Nmm - 1 1.23 x103 Nmm. After cracking all reinforced samples sustained further progressive loading until failure within a tight range of averaging approximately 19.5 x 103 Nmm, excluding sample R-4. As sample R-4 was wider and contained an extra strand of reinforcement, the moment strength was able to reach the largest of the grouping at 22.72 x 103 Nmm.

[153] For the sake of completeness, three point bending test results for unreinforced samples (C-1 and C-2) are shown in Figure 16. The unreinforced samples display failure when cracking initiates between 9.69 x 103 Nmm and 1 1.33 x 103 Nmm for samples C-2 and C-1 respectively. Bond strength is considered highly influential and variable in concrete especially by processing parameters. On average the inclusion of reinforcement in this study has provided 170% to 290% increase in flexural strength, compared to unreinforced samples.

Failure mode

[154] All reinforced samples failed via breakage of the reinforcing steel wire. As can be observed Figure 15, there is minimal, if any ductility in the section, confirming sections as under reinforced. Typically cracking of the concrete section occurred through a single layer interface, closest to the location of the applied point load. Point load positioning was at the centre span, in the proximity of the 5 ^th layer, or either side on the interlayer joints. Positioning of the point load had minor influence on crack location, as typically the weaker interface (left or right) of the 5 ^th layer would crack first.

[155] Noticeably what was observed in samples R-3 and R-5 was abnormal cracking on both sides of the layer, as well as horizontal cracking across the layer, at the level of the reinforcement depicted in Figures 17 and 18 respectively. This phenomenon is also acknowledged in Figure 15, where the samples R-3 and R-5 both experience a large sharp drop and rise in moment strength, before failure. This fluctuation is indicative of the horizontal cracking. This phenomenon was independent of the loading position on the interlayer or in the centre of the layer. Sample R-5 was loaded directly on the interlayer while R-3 was loaded within the layer, as seen in Figures 17 and 18. This cracking mode can only be attributed to weaknesses in the bond interface of the local area, and tension of the reinforcing mesh strips. Although this abnormal fracture occurred, moment strength was not hindered, remaining consistent with the other samples. [156] Table 1. Tabulated specifications and results of three point bending tests

Control Samples (Unreinforced)

C-1 50 31 60 1.42 1 1.33

C-2 ^* 47 30 58 1.37 9.69

Note: All samples were subject to 140 mm span under three point bending except those marked with ^* were subject to 120 mm.

The following values have been quantified and used in calculations for this table

f’ _c = 22.6 MPa (perpendicular direction)

f’t.conc = 2.85 MPa

F _{u st} = 174 N

d =D/2

Flexural Bond Strength

[157] From the three-point bending tests, flexural bond strength (ft bond) was calculated for the samples using equation 3 (see experimental section below) with results tabulated in Table 1. Flexural bond strength is analogous to the cracking moment, and universally accepted, as having high variability. As previously confirmed by the variability in failure mode and crack location in this study, not all layer joints are of even strength, therefore cracking will occur typically on the weaker joint. Consequently, the bond is not necessarily representative of the whole sample but rather the weakest bond in respect to the loading location.

[158] Reinforced samples displayed a range of ft bond from 0.9 MPa to 1.43 MPa. The unreinforced samples C-1 and C-2 displayed a ft bond at the higher end range of the reinforced samples at 1.42 MPa and 1.37 MPa respectively. Considering both data sets, an average ft bond of 1.19 MPa can be quantified.

[159] Using equation 4 (see experimental section below), which was extracted from AS3600, an estimation of the concrete tensile strength (ft.conc) from the compressive strength (f _c ) can be determined. The average compressive strength in the perpendicular direction, 22.6 MPa, as outlined in the compression tests, is analogous to the direction of the tensile forces. Equating equation 4, we find f t.conc equals 2.85 MPa. Comparing ft.conc to the average ft.bond, bond strength is approximately only 42% of the tensile capacity of the concrete.

Steel contribution

[160] Using equation 5 (see experimental section below) to equate the ultimate moment strength as a comparison to the measured moment strength, effectiveness of the reinforcing mesh against slippage can be determined. Calculations involved the use of the average ultimate tensile strength of the reinforcing steel (Fu st) of 174 N per wire. To determine the length of the internal lever arm, calculating the depth of neutral axis (dn) equation 6 (see experimental section below) was required. Calculating equation 6, f _c was taken in the perpendicular direction (22.6 MPa), as this was the relative direction of the compressive force in the section under three- point bending. The results of dn and Muit.cai for the reinforced sections are found in Table 1. Calculating the internal lever arm from dn and d provided a ratio that averages at 0.95 of d, contrary to 0.9 of d commonly taken in concrete sections.

[161] The final column of Table 1 presents the percentage difference of Muit.cai and Muit. All samples achieved a higher M _u it compared to the Muit.cai, with the exception of sample R-2. R-2 although only minor and can be considered negligible, Muit.cai had a calculated strength 0.03 x10 ³ Nmm larger than the Muit. Generally, all samples were within 10% of the Muit. Accuracy of the calculations relative to each specimen are justified, as only the average Fu st was used. Values of Fu st greater than the average is obtainable in the reinforcing wires described in the Experimental section below. Therefore, the likelihood of stronger strands within sections is a definite possibility. Variability can also be accounted for due to geometric deviations and depth to the reinforcing member. Overarchingly, this comparison confirms full utilisation of the reinforcement, with no indication of reinforcement slippage in the concrete section.

EXPERIMENTAL

Materials

a. Binders

[162] Two cementitious materials were used as the binder matrix for the print material. An Ordinary Portland Cement (OPC), conforming to Australian Standard, AS 3972, type general purpose (GP) cement was used in this study as the primary binder for the 3D print material. The OPC constituents C3S, C2S, C3A and C4AF were 57.59%, 14.87%, 4.10% and 13.94%, respectively. Densified Amorphous Fumed Silica (SF), was added as a supplementary cementitious material to enhance bonding, and extrusion properties. The SF was supplied by Ecotec Silica Fume Pty Ltd. and compliant to AS/NZ 3582.3:2002. An average particle size of approximately 84 pm was found for SF while an average particle size of 14 pm was found for OPC. Particle size distributions (PSD) of these cementitious materials were determined by a CILAS 1 190 and shown in Figure 7. It should be noted that the particle size is mentioned for the densified silica fume which re-disperses during concrete mixing. b. Sand

[163] Three sieve graded high silica sands were used in the printed mix. The finest silica sand denoted as“TGS” with an average particle size of 170 pm was supplied by TGS Industrial Sand Ltd., Australia. Two larger size sands supplied by Sibelco Pty Ltd. were also used in this study. The mid-sized silica sand denoted as“30/60” has an average particle size of approximately 500 pm. The coarser silica sand denoted as“16/30” has an average particle size of approximately 840 pm. The sieve analysis results for the sands are also displayed in Figure 7.

c. Mix Proportions

[164] A mixture comprising a cementitious binder, tap water, sand and concrete admixtures were used. The mix proportions are shown in Table 2. All dry constituents were mixed in a Hobart mixer first, with progressive water addition during mixing. [165] Two liquid concrete admixtures supplied by BASF Australia were used to fundamentally control the rheological properties. MasterGlenium® SKY 8379, a carboxylic ether polymer hyperplasticiser (HP) was used to induce cement dispersion and reduce water demand. MasterGlenium® SKY 8379 complies with AS 1478.1 - 2000 Type HWR and ASTM C494 Types A and F. As the extruded mix is exposed to friction and higher pressures than conventional cast concrete, MasterMatrix® 362, a viscosity modifying admixture (VMA), was used to prevent segregation and provide enhanced viscosity and consolidation. MasterMatrix® 362 complies with AS 1478 for Type SN admixtures. Admixtures were both added with the initial mixing water.

[166] Table 2. Mix proportions of the print material

0.90 0.10 0.47 0.70 0.88 0.28 6.70 ml 8.00 ml

Note: All numbers are mass ratios of the cementitious binder weight except the admixtures which are volume ratios of cementitious binders.

a Hyperplasticiser (MasterGlenium® SKY 8379)

b Viscosity Modifying Admixture (MasterMatrix® 362) d. Steel Reinforcing Mesh

[167] A steel mesh was chosen as the support material for reinforcing across layer interfaces. A galvanised wire mesh having an average diameter of 0.5 mm and an aperture of approximately 6 x 6 mm welded squares was used. For experimental purposes, 26 mm strips of the mesh were used, shown in Figure 8.

[168] Uniaxial tensile tests were performed longitudinally on single strands of the wire mesh, having the perpendicular wires cut off. Due to the very small size of the welded grids, the results were found to have high variability. The results of the steel wire tensile tests are represented as a box plot in Figure 9. [169] As can be seen in Figure 9, there is large distribution in tensile strength ranging from 143 N to 220 N. Fracture of the wire typically occurred at the weld, suggesting that the properties of the steel, were changed at that location due to the heat generation of the welding process, causing a local strength reduction. This is not atypical for such a light gauge steel with such a small aperture. Therefore, a true representation of the tensile strength in the wire, where potential print material bond may be more active is not represented by the average ultimate strength of 174 N. Attention to the tensile strength‘range’ should be taken into consideration, and not solely the average tensile strength from these tests.

2 Methods

[170] A lab scale ram type extrusion 3D printer system was used, with stepper motor control. The ram extruder works by means of a sealed piston within an 050 mm x 400 mm cylinder where the cementitious mix is loaded. The mix is then extruded out of a custom designed nozzle where the orifice reduces to a 30 mm x 17 mm rectangle compressing and profiling the dimensions of the printed layer.

2.1 Sample Preparation

[171 ] A vertical wall consisting of 9 laminated layers was printed with 26 mm high strips of the steel reinforcing mesh at each layer. Reinforcement was positioned on alternating sides of overlap, providing continuity of reinforcement. Each layer was laminated with an 8-minute time interval between layer depositions. The printed wall was left to cure at ambient temperature (23 ± 3 °C) for 7 days in a conditioning room before testing was performed on the 7 ^th day. Prior to testing the wall section was cut vertically into 6 approximate 45 mm wide strips (Figure 10(A)). In this case, the lap ratio a is 0.76, shown in Figure 10(B). After visual inspection of the cut samples, observations of the steel to concrete bond surface indicated that the quality of the bond was flawless (Error! Reference source not found. 10(C)).

[172] Along with the reinforced sample, another smaller unreinforced section was made as a control sample for comparison. The unreinforced sample used the same nozzle as the reinforced sample except without the insertion of the steel, therefore the flow path for layer formations remained the same between all samples. 2.2 Compression Tests

[173] To measure the compressive strength of the concrete, two sample types were made and tested in compression. Six cube specimens cast in 25 mm x 25 mm x 25 mm moulds were made with vibration applied while filling the moulds.

[174] Printed samples were also fabricated and tested to give a true representation of compressive strength. A 3D printed strip consisting of two layers was ground and cut into approximate 30 mm x 30 mm x 30 mm cubes, to closely resemble the cast variants. 12 cubes were made from the printed sample with six tested perpendicular to the print direction and six tested parallel to the print direction, as illustrated in Figure 1 1.

[175] Both cast and printed samples were prepared on the same day. The samples were cured at ambient temperature (23 ± 3 °C) for 7 days. Samples were subject to a compressive load rate of 20 ± 2 MPa/min with rubber pads top and bottom to minimise uneven surface effects, conforming to AS1012.9:2014.

2.3 Three-Point Bending

[176] The three-point bending test was performed on the 45 mm to 50mm wide reinforced and unreinforced specimens. Specimens were placed over a 140 mm span with the loading direction lateral to the deposition of layers. Testing in this direction enables the measurement of flexural strength (indirect bond strength) of the concrete and ultimate strength of the section due to the reinforcement across layer interfaces conforming to ASTMC-293. Rubber pads were placed on each support and under the load contact area to mitigate uneven surface induced stresses. Testing was performed on 7-day strength samples at a displacement control rate of 0.5 mm/min using an MTS testing machine. Due to the layer by layer deposition and inherently lower strength at the bond interface, cracking may not typically occur at the centre point under the load. Therefore, measurements were taken at a distance from the closest support to the flexural crack (Li) after three-point bending tests to calculate strength shown in Figure 12. 2.4 Calculation of Flexural Moment Strength

[177] To quantify the moment at which concrete section cracking occurs (Mcr), and the ultimate moment at failure (M _uit ), from the respective loads recorded by the MTS machine, the following equations were used.

Mult = ^ (2) wherein

Mcr, Muit = Cracking moment; Ultimate moment (Nmm)

Li = Length from support to flexural crack (mm)

Per, Puit = Load when cracking occurs; Ultimate failure load (N)

2.5 Calculation of Flexural Bond Strength

[178] An indirect measurement of the bond strength can be derived due to flexural cracking occurring at the bond interfaces. Flexural strength can be calculated from the cracking moment (M _C r) and second moment of area. The following equation was used to determine the flexural bond strength.

r M _cr d

t bond ^~ T (3)

wherein

ft bond = Flexural bond strength in tension (MPa)

Mcr = Cracking moment (Nmm)

d = Depth from top fibre of specimen to the centroid of reinforcement (mm)

lg = Gross second moment of area of specimen (mm ⁴ )

[179] A comparison using the calculated tensile strength of the concrete from Australian concrete standard AS3600, was used to determine the difference in flexural bond strength to the concrete strength.

f, cone = 0.(, Tc W

wherein

ft cone = Concrete tensile strength (MPa)

fc = Concrete compressive strength (MPa) 2.5 Calculation of Moment Capacity

[180] The printed specimens are relatively thin in comparison to many conventional reinforced concrete sections. Therefore, when calculating the ultimate moment strength (Muit.cai) an equilibrium equation assuming the concrete section is still in its elastic state was derived. The concrete is assumed to be in its elastic state as the concrete stresses do not reach compressive crushing limits as the section is severely under-reinforced. The derivation of Muit.cai ignores the effect of the concrete tensile strength as calculations have shown very minor influence on the overall moment strength. A schematic of the derivation of the Muit.cai from a stress block analysis of the 3D printed sample section is illustrated in Figure 13.

Muit.cai = F _u .st N _st Internal Lever arm = F _{u st} N _st (d - ) (5) Fu.st N _s t

1 0.5 f' _c - d -b (6) wherein

Muit. cai = Calculated ultimate moment strength (Nmm)

Fu st = Ultimate tensile strength of one steel reinforcing wire (N)

Nst = Number of steel wire strands in section

d = Depth to the centroid of reinforcing steel from top of concrete compressive fibre (mm)

dn = Depth to the neutral axis from the top of the concrete compressive fibre (mm) fc = Concrete compressive strength (perpendicular direction) (MPa)

b = Width of the concrete section (mm)

[181] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

[182] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.