Field of the invention

The present invention relates to a scaffold for forming a light weight concrete composition, a light weight concrete composite, and a method of forming a light weight concrete composite.

Background of the invention

Light weight concrete or light cementitious materials are widely used in buildings bricks, and mining to bear loads, and/or improve insulation. However, the weak strength of concrete in tension compromises the compressive strength of lightweight concrete. Light weight concrete, also known as foam concrete, is a human-made cellular material fabricated from either a cement paste or motor by introducing air-voids or pores using a suitable foaming agent. Lightweight concrete usually has the properties of low self-weight, minimal consumption of aggregate, and excellent thermal insulation properties.

The specimen size and shape, the method of pore formation, direction of loading, age, water content, cement/sand and water/cement ratios, curing regime and type of foam agent used are reported to affect the strength of the lightweight concrete. In particular, the compressive strength will decrease with the increasing of void diameter when the dry density of lightweight concrete is between 500 to 1000 kg/m ³ . When the density is larger than 1000 kg/m ³ , the air-voids or pores are sufficiently far apart from each other that they have minimal influence on the strength of the concrete, and the strength is instead mainly due to the properties of composite concrete material.

Most natural and engineered cellular solids with random porosity exhibit a quadratic or stronger scaling relationship between strength and density. That is, the compressive strength decreases exponentially with a reduction in density of lightweight concrete. This decrease in strength with porosity is due, in part, to the lightweight concrete behaving in a bending-dominated manner where it is subjected to tensile forces. Due to the low strength of concrete in tension and the failure of light weight concrete behaving in a bending-dominated manner, the resultant compressive strength of the lightweight concrete is very low. This low strength limits the application of light weight concrete as a structural material.

Notwithstanding the above, a concrete material exhibiting both light weight and high strength is desirable as a structural material. Before the year 2000, the density of lightweight concrete was generally above 1000 kg/m ³ with a compressive strength lower than 30 MPa. It was found that by introducing diversified constituent materials and high- efficiency chemical additives, lightweight concrete with a density lower than 500 kg/m ³ could be produced. However, this low density light weight concrete had very low compressive strength. The effect of advanced manufacturing processes and curing methods was also assessed, which led to a high strength lightweight concrete with rapid hardening Portland cement and fly ashes (graded or unclassified) in 2002. This composition was found to have a compressive strength of almost 20 MPa after curing for 365 days with a density of 1050 kg/m ³ . However, reducing the density of the concrete to 800 kg/m ³ led to a drop in strength to about 8 MPa. Over the last five years, research has focused on ultralight (<300 kg/m ³ ) concrete which presently has a compressive strength of only about 0.5 MPa.

There is an ongoing desire for a light weight concrete having a combination of high compressive strength and low density.

Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

Summary of the invention

In one aspect of the invention, there is provided a scaffold for forming a light weight concrete composite, the scaffold including: an upper surface and an under surface parallel with the upper surface; a plurality of hollow cell structures, each hollow cell structure including: a central pore having an axis that extends perpendicularly from a first opening in the upper surface to a second opening in the under surface, and a wall having an external surface and an internal surface bounding the central pore and forming the first and second openings, the wall adapted to receive and retain a cementitious composition thereon as a substantially continuous film; wherein the plurality of hollow cell structures are arranged adjacent to each other in an array to form the scaffold. The light weight concrete composite is also referred to as a controlled-air void foam concrete (CAFC), because the hollow-cell structure can be designed and fabricated with different central pore sizes, allowing the sizes and quantities of voids (defined by the central pores) to be controlled. This also allows the density and strength of the light weight concrete composite to be tailored to an application.

In an embodiment, the upper surface and the under surface are planar surfaces.

In an embodiment, the scaffold is configured to receive a cementitious composition to coat the external and internal surfaces of the wall with the substantially continuous film of the cementitious composition. In a preferred embodiment, the scaffold is configured to be sprayed with, poured with, or dip-coated into a cementitious composition to coat the external and internal surfaces of the wall with the substantially continuous film of cementitious composition. In an embodiment, the scaffold is adapted to form a light weight concrete composition having a stretching dominated structure. "Stretching dominated structure" means that bending of the cementitious composition is minimized.

Generally, it is preferred that both the first and second openings exhibit the same planar geometry in the upper and under surfaces respectively, and the central pore has a constant cross-sectional area co-planar with the first and second openings along the axis. Preferably, the cross-sectional area of the central pore is from about 2 mm ² to about 400 mm ² . Preferably, the cross-sectional area of the central pore is from about 10 mm ² to about 300 mm ² . More preferably, the cross-sectional area of the central pore is from about 20 mm ² to about 200 mm ² . Most preferably, the cross-sectional area is about 100 mm ² .

The skilled person will appreciate that a wide variety of cross-sectional geometries can be employed. For example, the central pore may have a polygonal (such as 3-, 4-, 5-, 6-, 7-, or 8- sided polygon), circular, oval, or elliptical cross-sectional area, wherein each side of the polygonal geometry may be straight and/or curved. Alternatively, the central pore may have a cross-sectional area that is a combination of these shapes. Where the central pore has a polygonal cross-sectional area, the wall further comprises a plurality of wall sides extending from the first opening to the second opening and bounding the central pore to provide the polygonal cross-sectional area. For example, the cross-sectional area may be triangular, square, rectangular, pentagonal, hexagonal, and so on. In such cases, the wall comprises three, four, five, six, etc. wall sides. Some or all of these wall portions may be concave (curved inward toward the axis of the central pore) or convex (curved outward away from the axis of the central pore) such that the cross-sectional area of the central pore is a triangle, square, rectangle, pentagon, hexagon, and so on, with concave sides, convex sides, or a combination of concave and convex curved sides.

In certain embodiments, the wall has an even number of wall sides, where all of the wall sides are straight, concave, convex, or the wall has alternating convex and concave sides, alternating convex and straight sides, or alternating concave and straight sides. In alternative embodiments, the wall has an odd number of wall sides, where all of the wall sides are straight, concave, convex, or a mixture thereof.

While a range of different geometries are contemplated, it is preferred that the cross-sectional area of the central pore is hexagonal, and more preferably, hexagonal with alternating convex and concave sides. More preferably, the wall includes six wall sides bounding the central pore to provide a hexagonal cross-section, the wall sides being a combination of alternating convex and concave sides.

As discussed above, the wall is adapted to receive and retain a cementitious composition thereon as a substantially continuous film. As such, the wall needs to be of sufficient strength to support the weight of the cementitious composition without collapsing or buckling. This strength may be as a result of the materials that are used to form the wall, or due to the physical design of the wall itself. It is preferred that the compressive strength of the material used to form the scaffold is at least 6 MPa. Preferably, the strength is at least 8 MPa. Most preferably, the strength is at least 10 MPa. It is expected that where the light weight concrete composite is used to form a structure more than 5 m in height, the material used to form the scaffold is greater than 10 MPa.

Preferably, the scaffold has a strength that is from about 1 to about 5% of the total strength of the resultant light weight concrete composite that is formed after the concrete composition is applied to the scaffold. Thus, the scaffold preferably has a strength that is from about 0.06 MPa to about 0.5 MPa.

Preferred materials for forming the wall include plastics, natural or synthetic fiber, metal. Thermoplastics are particularly advantageous as they allow the scaffold to be formed using three-dimensional (3D) rapid prototyping methods such as 3D printing and ultraviolet (UV) projection waveguide systems. Preferred thermoplastics include ABS, polycarbonate, a variety of blends, as well as engineered thermoplastic such as photocured polymer. However, in alternative forms of the invention, the wall may be formed from metals. Preferred metals include those which are relatively inexpensive and light, such as steel or aluminium. It is preferred that the wall has a minimum wall thickness of at least 0.02 mm. More preferably the wall has a minimum wall thickness of at least 0.1 mm. Even more preferably the wall has a minimum wall thickness of at least 0.2 mm. Most preferably, the wall has a minimum wall thickness of at least 0.35 mm. It is preferred that the wall has a maximum wall thickness of no more than 5 mm. It is also preferred that the wall has a constant thickness between its' external and internal surfaces.

In one or more embodiments, the external and internal surfaces of the wall extend perpendicularly between the upper surface and the under surface.

In one or more embodiments, the wall includes a plurality of openings from the external surface to the internal surface, the openings configured to receive and retain the cementitious composition as a substantially continuous film over the internal and external surfaces of the wall. It is preferred that each of the openings in the wall is in a plane that is perpendicular with the upper surface and the under surface. The presence of these openings is advantageous as they reduce the quantity of material required to form the wall, and assist with the adherence of the cementitious composition to the wall to form the substantially continuous film. In one form, the wall is a mesh wall having a plurality of openings to form the mesh.

The size of the openings is dependent on the overall size and surface area of each hollow cell structure. Notwithstanding this, it is preferred that each of the openings have an area of from no less than 0.25 mm ² . Preferably the area of the openings is no less than 0.5 mm ² . More preferably the area of the openings is no less than 1 mm ² . Even more preferably, the area of the openings is no less than 2 mm ² . Most preferably, the area of the openings is no less than 2.25 mm ² . Additionally, it is preferred that the area of the openings is no more than 9 mm ² . It is also preferred that the plurality of openings is in a patterned arrangement, such as in a spaced apart arrangement where the wall spacing between adjacent openings is at least 0.02 mm and at most 5 mm More preferably, the wall spacing between adjacent openings is from about 0.1 mm to 4.5 mm. Even more preferably, the spacing is about 0.3 mm to about 0.4 mm. Most preferably, the spacing is about 0.35 mm.

The wall may include a plurality of openings of different shapes and sizes. However, openings of the same size and shape are preferred. The openings may be of any geometric shape, such as 3-, 4-, 5-, or 6- sided polygon, such as triangular, square, rectangular, pentagonal, hexagonal; or circular, elliptical, oval, etc.; or a combination thereof. Preferably, the openings are of the form of a 4-sided polygon, and in particular square openings.

In an embodiment, the openings are arranged as a series of columns and rows, where the columns extend parallel with the axis, and the rows are arranged in a plane that is transverse to the axis, such as in an axial plane. This arrangement advantageously provides a safe failure mechanism for the scaffold (such as when concrete has been applied to the scaffold to form a light weight concrete composite), as the light weight concrete composite can collapse in a row-by-row manner if an excessive compressive stress is applied.

In an embodiment, the wall has an open frame-like structure. Preferably, the wall is formed from a plurality of support beams that extend between the upper surface to the under surface from the first opening to the second opening, and the plurality of support beams are interconnected with a plurality of lateral beams. In this case, the openings in the wall correspond to the gaps in the open frame-like structure. It is further preferred that the plurality of support beams are parallel with the axis of the central pore. Additionally, or alternatively, the plurality of lateral beams is substantially perpendicular to the plurality of support beams.

It is preferred that each of the support beams has a thickness and/or width of at least 0.02 mm. More preferably, the support beams have a thickness and/or width of at least 0.1 mm. Even more preferably, the support beams have a thickness and/or width of at least 0.3 mm. Most preferably, the support beams have a thickness and/or width of at least 0.35 mm. Additionally, it is preferred that each of the support beams has a thickness and/or width of no more than 5 mm.

It is further preferred that at least some of the support beams are strengthened elements, having a thickness that is greater than the thickness of the support beams. Typically these strengthened elements will have a thickness of at least 50% greater than the thickness of the support beams.

It is preferred that each of the lateral beams has a thickness and/or width of at least 0.02 mm. More preferably, the lateral beams have a thickness and/or width of at least 0.1 mm. Even more preferably, the lateral beams have a thickness and/or width of at least 0.3 mm. Most preferably, the lateral beams have a thickness and/or width of at least 0.35 mm. Additionally, it is preferred that each of the support beams has a thickness and/or width of no more than 5 mm.

It is preferred that the spacing between adjacent support beams is from about 0.5 to about 3 mm and/or the spacing between adjacent lateral beams is from about 0.5 to 3 mm. More preferably, the spacing between adjacent support beams is from about 1 to about 2.5 mm and/or the spacing between adjacent lateral beams is from about 1 to about 2.5 mm. Most preferably, the spacing between adjacent support beams is from about 1.75 to about 2 mm and/or the spacing between adjacent lateral beams is from about 1.75 to about 2 mm. As discussed previously, the plurality of hollow cell structures are arranged adjacent to each other in an array to form the scaffold. It will be appreciated that the hollow cell structures may be arranged in a number of different patterns depending on the geometry of the hollow cell structures. The scaffold may be formed from a plurality of hollow cell structures that are the same or different and arranged in an array aligned such that the central pores of the hollow cell structures are parallel. By way of example, the scaffold may be formed from hollow cell structures having different cross-sectional geometries, for example a plurality of hollow cell structures having square and triangular cross-sections, or a plurality of hollow cell structures having circular and concave triangular cross-sections. Alternatively, the scaffold may be formed from a single type of hollow cell structures exhibiting the same cross-sectional area, such as only cell structures having a triangular cross-section, or only a square cross-section, or only a hexagonal cross-section. In an embodiment, the plurality of hollow cell structures is arranged in a manner such that at least a portion of the wall, such as a side wall, is shared with an adjacent hollow cell structure. Thus, in the case of a hollow cell structures with a hexagonal cross-section, the wall includes six wall sides of which at least one of those wall sides forms a wall of an adjacent hollow cell structure. In a preferred embodiment, the plurality of hollow cell structures is arranged in a hexagonal honeycomb array.

In an embodiment, the scaffold has been cut from a larger scaffold-structure to provide a scaffold of a desired size or shape.

In another aspect of the invention, there is provided a light weight concrete composition including a scaffold, and a continuous concrete layer formed on a wall or walls of the scaffold, wherein the light weight concrete composition has a stretching dominated structure.

In another aspect of the invention, there is provided a light weight concrete composite including: the scaffold as previously defined and a continuous concrete layer formed on the internal and external surface of walls of the plurality of hollow cell structures of the scaffold.

The thickness of the continuous concrete layer is dependent on the overall size and surface area of each hollow cell structure. Notwithstanding this, the invention can accommodate a range of different thicknesses of the continuous concrete layer. In an embodiment, the thickness of the continuous concrete layer is from about 0.02 to about 9 mm. Preferably, the thickness of the continuous concrete layer is from about 0.1 to about 5 mm. In practice, particularly for structural applications, the thickness of the continuous layer is most preferably in the range of about 0.35 mm to about 0.6 mm.

In an embodiment, the upper surface and the under surface are compressive load bearing surfaces, and the central pores are oriented parallel to the direction of compression.

In a further aspect of the invention, there is provided a method of forming a light weight concrete composite including: dip-coating the scaffold as previously defined into a cementitious composition for a time sufficient to form a substantially continuous film of cement over internal and external surfaces of the wall; and curing the cementitious composition into concrete.

In an embodiment, the cementitious composition includes cement powder, water, and a surfactant or polymeric superplasticiser, and wherein the ratio of water to cement powder is from about 0.2: 1 to about 0.6: 1 , and the surfactant or polymeric superplasticiser is present in the cementitious composition in an amount of from 0.1 to 1.5 wt% based on the total weight of the cement powder.

The skilled person will appreciate that a variety of different cement powders may be used in the method of forming the light weight concrete composite, and that the amount of water and superplastizer that is used will depend, in part, on the cement powder that is selected. Preferred cement powders include: Portland cement, Pozzolan- lime cement, slag-lime cement, supersulfated cements, calcium sulfoaluminate cement, geopolymer/alkaline activated cement, sulphate aluminium cement, Ferrous aluminate cement, Fluoraluminate cement, self-stressing cement, expansion cement or magnesium phosphate cement, calcium phosphate cement and cement blends thereof.

In some cases, the water is mixed with a liquid alkaline activator, such as for a geopolymer/alkaline activated cement powder.

The skilled person will appreciate that a variety of different cement additives may be used in the method of forming the light weight concrete composite. The cementitious composition may further include one or more cement additives. Preferred additives include: fine aggregates; retarders; water reducers; reaction accelerators; thickeners; silica fume; slag; lime; clay; fly ash; natural additives such as rice husk, cellulose; nano- silica; nano-aluminium; micro-reinforcement fibers; nano-reinforcing fibers; and nano- reinforcing sheets; and combinations thereof. In an embodiment, the step of curing the cementitious composition includes curing in an environment with a relative high humidity, where the relative humidity is >75%, and preferably >95% for the first 24 hours of curing.

Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings. Brief description of the drawings

Figure 1 : is a 3D scaffold according to the present invention having a cell size of 6.68 mm

Figure 2: is a 3D scaffold according to the present invention having a cell size of 5 mm

Figure 3: is a 3D scaffold according to the present invention having a cell size of 3.342 mm

Figure 4: is a photograph showing the cells of Figures 1 , 2, and 3 coated in concrete. Figure 5: is a design of a 3D scaffold for a cell size R = 6.68 mm. Strengthened element - C represents the strengthened element at the corner of cell, which has a dimension of 0.5 mmx0.5 mm. Strengthened element - M with a dimension of 0.4 mmx0.4 mm is in the middle of each curve.

Figure 6: is a graph showing a comparison of compressive strengths between known lightweight concretes and the Controlled-air void foam concrete (CAFC) of the present invention.

Figure 7: is a graph showing compressive strength of the CAFCs of the present invention.

Figure 8: is a graph showing the dry density of CAFCs of the present invention. Figure 9: is a graph showing the loading curves of CAFC of the present invention.

The dashed line indicates the pseudo-yielding plateau for R3.342 0.25w/c obtained by low pass filtering.

Figure 10: is a photo showing failure of the first layer modules can be seen in this picture. Figure 1 1 : is schematic showing examples of fabrication method of (a) a corrugated triangular shaped aluminium 3D scaffold, and (b) a honeycomb shaped aluminium 3D scaffold. Figure 12: is a photo showing the casting of an aluminium mesh 3D scaffold based CAFC.

Figure 13: is a photo of an aluminium mesh scaffold based CAFC.

Figure 14: is a loading curve graph of an aluminium mesh scaffold based CAFC Figure 15: is a photo of an aluminium mesh scaffold based CAFC during strength testing.

Detailed description of the embodiments

The invention will now be described below in terms of a preferred embodiment, in which there is provided a new type of light weight concrete with controlled air voids (central pores) abbreviated as CAFC (Controlled-air void foam concrete), having a stretching-dominated structure.

The lightweight concrete composition includes a scaffold, such as a 3D scaffold, as a base to which cement is applied to form a continuous cement film on a surface of that 3D scaffold which results in a composition having a high compressive load bearing strength and low density (such as below) 1000 kg/m ³ . The compressive load bearing strength of this composition is higher than that of a standard foamed concrete composition having the same density.

The high strength of the CAFC is particularly apparent at densities of lower than 500 kg/m ³ , at which density the strength of CAFC is almost two times greater than the strength of known lightweight concrete compositions. As generally discussed below, the inventors have found that the compressive strength and density of the CAFC is, in part, a function of the flowability of cement paste and the cell size of 3D scaffold.

Example 1

Materials and instrumentation A 3D printer was used to produce the 3D scaffold. The minimum build layer of this 3D printer is 16 microns. A photocured polymer is used as the building material while another photocured water soluble polymer is used as the supporting material when printing free standing structures. The supporting material is be dissolved after printing.

A Constant Speed Mixer from Cement Test Equipment was used to mix the fresh cement paste. A polyacrylate based superplasticizer, is used to adjust the workability of the fresh cement. A general purpose cement was used which is a Portland cement. A loading frame was used to test the compressive strength of samples. An extensometer with a 50 mm gauge length and ± 1 mm measurement range was used to measure the strain of CAFC.

Forming CAFC using surface tension and 3D micro frame Examples of the 3D-printed scaffold are shown in Figures 1 , 2, and 3. In each of these cases, the 3D printed scaffold is formed from a number of hollow cells exhibiting a curved hexagonal cross-section. This particular shape is advantageous as it helps militate against buckling failure. The hollow cells are formed into an array having a shape-optimized honeycomb design with the central pores being vertically aligned. By aligning the pores parallel to the direction of compression, the failure mode of the structure is compression rather than bending. This is advantageous for a structure having high compressive strength.

When the scaffold is wetted with a cementitious composition, such as a cement paste, the cement paste adheres to the external and internal surfaces of the walls of the cells due to surface tension, which adherence is enhanced by openings in the wall. This is illustrated in Figure 4, which shows a continuous concrete layer formed on the walls of the scaffolds of Figures 1 , 2, and 3.

Figure 5 provides a schematic of a single hollow cell.

As shown in Figure 5, the void (shape-optimized hexagon) in the horizontal surface is called "cell" and the grid (rectangular) in the vertical surface is called "opening". The cell size represents the radius of the cell curves. The element size refers to the dimension of the cross-section of elements. To increase the strength and integrity of the frames, a part of the vertical elements' side is increased, called "strengthened element". Strengthened element - C with a dimension of 0.5 mmx0.5 mm is inserted at each corner of the cell. For frames with a cell size 6.68 mm, strengthened element - M with a dimension of 0.4 mmx0.4 mm is also used in the middle of curves. The dimension of cells, openings and elements for three different cell sizes are summarized in Table 1 . By altering the size of the cells, the density of the CAFC can be changed. After printing using the 3D printer, the frames are immersed in 7 wt% NaOH solution for 24 hours until the supporting material is completely removed. Then the frames are carefully rinsed with distilled water to remove residual NaOH solution. The frames are then dried in a room environment.

Table 1 Dimensions of 3D frames

cell size (mm) o dimension (mm) element size (mm)

Figure 1 6.68 175x2 0.35x0.35

Figure 2 5 1.75x2 0.35x0.35

Figure 3 3.342 1.75x2 0.35x0.35

The fresh cement paste was made according to the procedure as specified in ASTM Standard C1738. Superplasticizer was added to increase the workability of cement paste. Mini slump tests were done after mixing to measure the workability of the ordinary Portland cement (OPC) paste and control variations between different batches of samples.

The 3D scaffolds were immersed into the cement paste, by dipping the scaffold into cement paste or spraying/pouring the cement paste onto the scaffold. For the dipping method, the scaffold was withdrawn after about 10-30 second time to ensure stable films were formed in the vertical surfaces. For the spray/pour method, the wet cement paste was sprayed/poured onto a scaffold and the wet cement paste flowed through the scaffold. This achieved a similar effect to dipping the scaffolds into the cement paste. The cement paste may be sprayed/poured on to the scaffold 2 to 5 times to ensure that the cement paste flows through the central pore and wall openings of the walls on scaffolds to ensure stable cementitious films are formed on the vertical surfaces of the walls. The samples were then kept in a high humidity environment (ideal: RH>95%, minimum RH>75%) for curing over the first 24 hours. The samples were then subsequently cured with saturated Ca(OH) ₂ solution until test. The use of Ca(OH) ₂ solution is generally for lab based testing in order to obtain reliable and repeatable results. In field applications, any method to prevent water loss from the cement can be used such as wrapping the cement with plastic sheets or spraying chemicals to prevent water evaporation. Before testing, the samples were dried in room temperature for 24 hours and in a 45°C oven for 3 hours. Apparent dry densities are calculated before testing.

Mix design ADVA 210 was added as a surfactant to increase the flowability of OPC paste thus the cement film could form in the openings in vertical surface but do not clog the cells. Mix design for two different water cement ratios (w/c ratios) is given in Table 2. Different proportions of ADVA were added for the different w/c ratios to ensure a similar flowability of the mixes.

Table 2 Mix design

w/c 0.25 0.4 water 150g 240g cement 600g 600g

1.40% 0.55%

ADVA

8.4g 3.3g

slump test 130mm 120mm

Mechanical testing

After curing and drying, the compressive strength of the samples was tested using an loading frame. A 50 kN loading cell was used for the compressive test. An extensometer was attached to the loading plate to measure the strain of CAFC during loading. For CAFCs, the loading continued after the failure of the first layer and it stopped when the extension reached 10 mm. Four specimens were prepared and tested for each batch of samples.

Strength improvement of CAFC The test results demonstrate that the strength of the CAFC is higher than currently available lightweight concrete composition having the same density. Figure 6 provides a comparison of the compressive strengths of the CAFC against the strengths and densities of lightweight concrete, including that formed from cement mortar, cement paste, cement fly ash mortar, cement with fly ash replacement etc. It can be seen that the strength of the CAFC is much higher than the strength of all lightweight concretes having the same densities. This is particularly the case when the density is around 500 kg/m ³ , where the compressive strength of the CAFC is more than twice that of known lightweight concretes.

The strength of the 3D scaffold was also tested and was found to have a compressive strength that is less than 5% of the strength of CAFC. The 3D scaffold is designed to bear minimal strength since it is used mainly as a frame for retaining the cement/concrete. Based on this, it can be seen that the resultant strength of the CAFC is primarily derived from the concrete.

The strength of the CAFC mainly depends on the flowability of cement paste and cell size of the frame. The flowability of the cement paste has a significant influence on the density and compressive strength of the CAFC. Figure 7 illustrates the test results of compressive strengths with different mix design and Figure 8 provides the corresponding dry densities. With a slight decrease in flowability, from 130 mm to 120 mm (shown in Table 2 for two different mix designs), the density of the CAFC increased from 450 kg/m ³ to 800 kg/m ³ for the same frame scaffold structure (with cell size 5 mm). This increase in density can be attributed to increasing the average thickness of cement film on the wall surfaces. As a result of this, more cement is arranged in the load direction which has the effect of increasing the average compressive strength from 6 MPa to more than 10 MPa.

The cell size (or cross-sectional area of the central pore of the hollow cell) is another important factor that affects the density and compressive strength of the CAFC. Figure 6 illustrates that for the same mix design (0.25w/c), the frames with smaller cell size exhibit a larger density and stronger compressive strength. A smaller cell size means that the structure has a larger vertical surface area (wall area) for attachment of cement which then results in greater density and compressive strength.

Other factors such as curing age and water to cement (w/c) ratio are usually critical factors that contribute to the strength of cement bulk material. However, in contrast with this, the test results indicate that for the CAFC, there is no conclusive evidence showing that they have a great influence on the compressive strength. Periodic load bearing and pseudo-yielding of CAFC

Typical loading curves of CAFCs with different w/c ratio and cell sizes are given in Figure 9. The curing age of these CAFCs is 28 days. It can be seen that there are several periodically occurring peaks in the curve and the period of two close peaks is around 2 mm which is the height of each opening. As observed in the loading process, the frame collapses layer by layer from top to bottom.

Every peak represents a failure of a layer of the scaffold. Usually the first peak corresponds to the largest loading force and the compressive strength of the CAFC. After the first peak is arrived, the load decreases dramatically until an extension of around 2 mm is achieved thus the loading plate acts on the second layer directly. After that the load starts to increase again until the second layer fails. Figure 10 shows the failure of the first layer in loading test.

The residual strength after the failure of first layer is still significant of around 60- 80% of the compressive strength while the strain of the CAFC is greater than 50%. This is not observed for normal lightweight concrete. Thus, this can be considered a "yielding" response when the periodic variation in the curve is filtered (as shown in Figure 9). This pseudo-yielding behaviour is an important advantage to be used in buildings and infrastructures since it provides extra safety and resilience instead of instant brittle failure. In addition, the application of using the CAFC as an energy absorbing material can be studied in the future due to its unique compressive behavior.

The CAFC disclosed here is a stretching-dominated structure and it is much more weight efficient than lightweight concrete which is considered a bending- dominated structure. In addition, cement has better performance in compression than tension or bending. Thus the CAFC has a higher strength to density ratio than known lightweight concrete especially when the density is lower than 500 kg/m ³

Example 2

The following example relates to a production method of a 3D scaffold using aluminium mesh sheets.

Figure 1 1 (a) illustrates a 3D scaffold 1 100 produced by stacking alternating sheets of flat mesh sheets 1 102 and corrugated mesh sheets 1 104. In this case, the flat mesh sheets 1 102 and the corrugated mesh sheets 1 104 are formed from aluminium. To form the 3D scaffold, the flat mesh sheets 1 102 are sandwiched between corrugated aluminium mesh sheets 1 104. It will be appreciated that this method can be applied to produce scaffolds exhibiting a variety of different shapes, see for example Figure 11 (b) illustrates a honeycomb shaped 3D scaffold 1 1 10 formed using two corrugated mesh sheets 1 1 12 each having a profile corresponding to half of the honey comb structure.

Returning to the 3D scaffold illustrated in Figure 1 1 , this 3D scaffold 1 100 may be manufactured according to the process illustrated in Figure 12. In this case, a 3D scaffold 1 100 is formed from aluminium wire mesh sheets 1 102 and 1 104 that are stacked and held together 1 100. The mesh sheets 1 102 and 1 104 may be held together by a variety of different means available to the person skilled in the art. Such means may include application of a holding force, the use of a frame or jig, or by adherence of the stacked meshed sheets together (for example by gluing or welding). In the present example, a compressive force is applied by rubber bands 1202 to hold the mesh sheets together to form the 3D scaffold 1 100. After forming the 3D scaffold from the stacked mesh sheets, cement paste 1204 was poured onto the 3D scaffold which subsequently resulted in a layer of cement paste 1302 on the surfaces of the 3D scaffold 1 100 (as shown in Figure 13). The resultant aluminium mesh based CAFC 1300 shown in Figure 13 has dimensions of 10 cm x 8.6 cm x 8.2 cm. The apparent dry density was calculated as 320 kg/m ³ .

The aluminium mesh based CAFC was then subjected to compression testing. For this testing, the top and bottom surfaces of the CAFC were grinded flat and parallel. The compressive strength of this aluminium mesh CAFC was found to be 2.25MPa.

The loading curve of this aluminium mesh made CAFC is shown in Figure 14 and a photograph of the CAFC 1300 in the testing rig 1500 during testing is illustrated in Figure 15. As can be seen from Figure 14, the ductility was increased in comparison to the 3D printing based CAFC. The first peak occurred (around 19kN, 2.25MPa) when the extension is about 1.4mm and no periodic peaks can be found compared to the loading curves of 3D printing based CAFC. However, the remaining strength was maintained above 70% of the compressive strength until 50mm extension (strain over 0.6). A strain hardening process can also be identified after the first peak in Figure 14. With the increasing of extension and plastic deformation, the load was increased gradually. The load even exceeded the compressive strength after 43mm extension. This special failure behaviour suggests that the CAFC is suitable for energy absorption purposes. Figure 15 shows the CAFC failure started from bottom surface 1502.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.