# Development of Cementitious Mortars for Aerial Additive Manufacturing
## 1 Introduction  
The construction industry has traditionally used slow and energy-intensive methods to build structures [1], often using formative or subtractive techniques [2] with the latter approach involving the machining of a bulk quantity of material down to a required dimension [3]. By contrast, additive manufacturing (AM), often referred to as '3D printing', creates objects by depositing one layer of material at a time [4], using only the specific amount of material required during construction. The use of AM in the construction industry, a sector regarded as traditionally fragmented, risk-averse [5] and having low levels of innovation [6], is on the increase with projects realised by the use of AM methods on a construction-sized scale. AM offers considerable benefits to the construction industry by reducing material wastage, labour costs and delays resulting from health and safety-related issues along with increasing the scope for bespoke architecture and design [7]. Initial project outlays on machinery and materials can be high, but these costs can be mitigated by integrating the 3D printing of services and structural elements [2]. AM techniques under investigation in the construction industry have involved either large ground-based, fixed position approaches such as frames [2; 7; 8; 9], rotating compound robotic arm systems [1], robotic frames moving upon dual rails either side of the printing envelope [10] or the use of autonomous mobile coordinated, grounded robots using simultaneous localisation and mapping algorithms [11]. The dimensions and design of the buildings are restricted by the size and manoeuvrability of ground-based deposition instruments [12]. Ground-based research projects have included the concrete printing method developed at the University of Loughborough, UK [7; 13], contour crafting, developed at the University of Southern California, USA [10; 14], the digital construction platform project being developed at the Massachusetts Institute of Technology, USA [1] and D-shape printing, created by [PERSON] and D-shape Enterprises [15]. Typically (though not entirely), cementitious-based AM construction related projects are based upon the AM principle of fused deposition modelling (FDM), where suitably viscous materials such as thermoplastic polymers [16], mortars and concretes can be extruded through a nozzle and deposited one layer at a time to create a solidifying object or structure [2]. In contrast, other methods can spray cementitious material on to formwork [17]. The aerial additive manufacturing (AAM) project is investigating a new, transformative approach to the autonomous construction of buildings using unmanned aerial vehicles (UAVs) to deposit cementitious materials of suitable viscosity to create or repair structures in situ [18; 19]. It has been demonstrated by the AAM project that coordinated flying, untethered UAVs can extrude a cementitious material with suitable rheological properties and combination of workability and buildability to create a multi-layered object with a complex geometry requiring high levels of precision [12]. To the authors knowledge, the AAM project is the world's first published demonstration of a successful extrusion of cementitious material by a UAV 'on the fly' using an on-board power source, the UAV being neither tethered, landed, nor powered by an external ground-based source. Figure 1 illustrates a programmed, untethered UAV extruding multiple layers of cementitious material in a Peano curve design 'on the fly' [12]. FDM is a suitable principle for AAM and the feasibility of UAVs 3D extrusion-printing other materials in addition to cementitious-based formulas such as polyurethane foam during controlled flight has also been demonstrated [18]. UAVs have become established in numerous sectors including military, civilian and emergency services [20]. The use of aerial robots in the construction industry has grown to the extent that the industry is now a leading sector in aerial robotic use [21].