• Ball, Richard J.
• Dams, Barrie
• Shepherd, Paul

Abstract

Additive Manufacturing (AM), better known as ‘3D printing’ is being increasingly investigated as a method of constructing buildings using both cementitious material for structural purposes with polymer materials, typically foam, for insulation. AM offers potential advantages to construction – reduced material wastage, reduced on-site labour, increased productivity and crucially a reduction in the opportunity for accidents or fatalities on-site. However, there are challenges to overcome such as the high initial costs of materials and material development, the dimensions of the deposition printing equipment and the absence of established specialised international standards. Typically, deposition platforms involve large ground-based gantries or robotic arms, or alternatively multiple smaller coordinated grounded agents. Inherently, the dimensions of the building to be realised will be governed by the dimensions or reach capabilities of the deposition devices. An alternative approach to ground based methods is to bring an aerial capability to the AM process, releasing operations from ground-based constraints. The Aerial Additive Manufacturing (AAM) project is the world’s first project to demonstrate the feasibility of multiple self-powered untethered drones extruding material in flight to construct multiple layers of material suitable for construction. The use of drones requires the miniaturisation of the additive manufacturing process and the use of lightweight cementitious material. The deposition device consisted of a cartridge through which cementitious material was pushed. A length of tubing was required to connect the cartridge to the ultimate nozzle. The nozzle was manipulated by a stabilising delta robot attached to the base of the flying drone which minimised lateral deviation in trajectory and maintained lateral tolerances below 4mm. Extruded material was resistant to any effects of drone propellor downwash. Material development needs to consider workability – the ability of a material to pass through deposition equipment while in the fresh state, and buildability – the ability of a material to resist deformation once extruded from either self-weight or the weight of subsequently deposited layers. A structural material must also possess viable structural strength in the cured state. For AAM, material in the fresh state needs to be pseudoplastic – exhibit shear thinning behaviour – therefore reducing viscosity while the material is under stress in the deposition system and increasing by orders of magnitude once deposited to minimise deformation. Hydroxyethyl Methyl Cellulose (HEMC) and Xanthan gum were used in combination as rheology-modifying admixtures to promote pseudoplastic behaviour. Fly ash and smooth-particle sand were used to aid workability, with rounded and sub-rounded particles preferable to angular, although mixes without sand were also successfully trialled. Typical material properties of a fine aggregate-free pseudoplastic cementitious material for AAM, successfully deposited by a flying self-powered untethered drone in a programmed trajectory, were a cured compressive strength of 25 MPa, a complex modulus of 4-9 MPa within a two-hour open time, a yield stress of 1.1 KPa and material densities of approximately 1650 kg/m3. Material extrusion during autonomous flight is demonstrated with test flights in a range of trajectories through to a printed 28-layer object featuring a complex Peano curve design. The 4mm maximum lateral printing tolerances enabled by the delta robot stabilising nozzle positions are demonstrated. Fibrous reinforcement can be added as part of cementitious mixes, with both natural (banana) and synthetic (PVA, Kevlar) fibres trialled with mixes. A further issue is the acceleration of curing, which is desired to happen as soon as possible following extrusion. Further work will encompass investigation into accelerating agents and the use of Calcium Aluminate Cement to trail suitability for AAM. In conclusion, AAM brings an aerial capability to AM in construction which is envisaged to enable work in elevated or challenging site conditions, reducing the danger to human life, and promoting architectural freedom in design.

Topics

• Deposition
• impedance spectroscopy
• polymer
• extrusion
• strength
• cement
• viscosity
• positron annihilation lifetime spectroscopy
• Photoacoustic spectroscopy
• Calcium
• cellulose
• curing
• material extrusion
• complex modulus