Fresh Properties and Autonomous Deposition of Pseudoplastic Cementitious Mortars for Aerial Additive Manufacturing

Additive Manufacturing (AM) in relation to the construction industry is an emerging technology. However, ground-based AM on construction scales may be limited by the dimensions, reach and weight of the ground-based deposition platform. Aerial additive manufacturing (AAM) can revolutionise construction-based AM by employing multiple untethered unmanned aerial vehicles (UAV, known as ‘drones’) depositing material using miniature deposition devices. This study investigates aerial platform and cementitious material requirements for AAM and details development of structurally viable cementitious composite material with suitable rheological properties to demonstrate AAM as a novel aerial approach to complement ground-based activities. A synergistic combination of natural hydrophilic and partially synthetic hygroscopic polymeric hydrocolloids was developed in cementitious material to achieve optimal rheology properties in the fresh state. Analysis involved oscillation and flow tests, calorimetry, microscopy, computed tomography and mechanical tests. AAM application considerations focused on technical characteristics of UAV platforms, flight times, payloads and developed extrusion systems with optimal nozzle dimensions. Results demonstrate critical material parameters of 1700 kg/m3 density, 4° phase angle, 1.1 kPa yield stress, $ < 10$ MPa complex modulus, and the ability to be processed through miniature deposition devices with 500 N force and 250 mA current. Material extrusions were realised using a custom-designed miniature deposition system which a UAV can carry and power. AAM will significantly impact automated construction by enabling new advances in aerial platform applications featuring multiple coordinated agents depositing bespoke material. This is particularly relevant to elevated or challenging construction conditions where an automated aerial approach can crucially reduce safety risks.


I. INTRODUCTION
Additive manufacturing (AM) has revolutionised automated production in sectors such as the medical, automotive dimensions and ensuing building envelope of the deposition system [33].This is an issue when considering the height of a typical structure, with parts for multi-storey buildings requiring off-site prefabrication [32].However, prefabrication also has drawbacks regarding the cost and logistical issues in creating and transporting customised components to the site [31].
An approach to addressing these issues would be introducing an aerial capability to automated in-situ construction, thus freeing a building project from ground and labour-based constraints.The aerial additive manufacturing (AAM) project proposes an innovative solution to bring aerial capability to in-situ AM by using a coordinated, communicating group of unmanned aerial vehicles (UAV).Each UAV is designed to carry an automated lightweight miniature deposition device, replete with a structural material, to create or repair structures in diverse and challenging environments [34], [35], [36], [37], [38].AAM material development required considerable modification of traditional mortar mixes and different mix proportions to those featured in ground-based AM studies such as contour crafting [39], and concrete printing [22], [29].
The extrusion of structural material during controlled flight represents a paradigm shift in the use of UAVs in the construction industry, which previously had been limited to surveillance work [40].Early studies of aerial robot deployment in construction have covered mainly the on-site assembly of prefabricated [41] or specifically designed components [42], ropes for tensile structuring [43], [44], [45], [46], and polystyrene prisms [41].Recent studies have demonstrated real-world applications of discrete aerial additive manufacturing by assembling concrete blocks [47] and a reconfigurable structure of cyber-physical modules with onboard sensing and computing [48].Even though these studies indicate novelties and improvements in scale, structural viability, and flexibility, the design of those particular elements necessitates significant labour, cost, and a certain amount of lead time for the final assembly.These deficiencies heavily lessen the power of discrete AM and orient the research direction towards continuous AM.Fig. 1 illustrates the conceptual vision of AAM with a small swarm of UAVs extruding a pseudoplastic cementitious material.It has been demonstrated that a cementitious mortar with suitable rheological properties and an appropriate balance between workability and buildability can be extruded by multiple coordinated flying UAVs in a complex trajectory and to a high level of precision [38].The aerial approach would be particularly advantageous when working at height or in a post-disaster reconstruction environment with difficult ground conditions [35].
This study builds upon AAM project work [37] by examining the differences between on-site aerial and off-site ground-based AM requirements, identifying suitable aerial platforms while detailing the refined development of a novel pseudoplastic cement-based composite material with suitable rheological properties for AAM.With material development, there is an emphasis upon the addition of polymeric rheology modifying admixtures (RMA) to enhance cohesion, stability and water retention [49] within fresh mix open-times.The importance of identifying a UAV platform with appropriate technical characteristics and miniaturising the deposition process for AAM in relation to ground-based methods is highlighted.In addition, extrudability and pumpability are amalgamated into the encompassing term 'workability'.Crucial to material development suitable for AAM is recognising the inherent trade-off between workability and buildability (the ability of an extruded material to retain shape and structure while in the fresh state), which requires contrasting rheological characteristics.The former requires low viscosities and liquid-like behaviour, while the latter requires high viscosities and solid-like behaviour to resist deformation from subsequently deposited layers.
Freshly mixed material is required to pass through a light, miniaturised deposition system appropriate for carriage on a flying UAV.The deposition system must process the material without adversely interfering with power delivery capabilities or the lateral precision of a UAV while following an architecturally informed programmed trajectory.Extruded material should also be sufficiently rigid to resist downwash effects resulting from UAV propeller rotation.
A two-stage material formulation strategy is presented.
In this study, two mixes first focus on buildability, and subsequently, three bespoke mixes focus on workability in conjunction with the development of a miniature deposition device and nozzle design.UAV platform options are evaluated for technical suitability with AAM material extrusion.Material 164 tests encompass a wide range of experiments to ascertain an 165 indication of suitable material properties for on-site AAM 166 in accordance with the capabilities of the aerial platform.167 Tests in this study include material settlement, rheology, 168 calorimetry and microstructure, along with optimisation of 169 nozzle design and dimensions for material extrusion from 170 miniature deposition devices.

A. AERIAL PLATFORM CONSTRAINTS EVALUATION
Aerial systems represent a class of open-loop unstable 174 systems that present unique control challenges.These 175 challenges are further compounded by the underactuated 176 nature of the system, where the number of states exceeds 177 the available control inputs.To tackle these complexities, 178 considering cascaded or hierarchical control architectures 179 becomes necessary, enabling the implementation of control 180 loops operating at different frequencies and facilitating 181 controller designs tailored to specific cycles.For applications 182 such as AAM, which involve close flight proximity to 183 objects and the environment, the system must maintain 184 stability amidst reaction forces, aerodynamic reflections, and 185 potential friction effects.Furthermore, the dynamics of the 186 propellers change when operating in close proximity to the 187 surroundings [50].This proximity results in an increased 188 rotor wake, leading to elevated propeller velocities and the 189 emergence of the ground effect, which generates additional 190 repulsion forces from the ground.Additionally, material deposition during mid-flight introduces variations in the mass distribution, further emphasizing the need for adaptability in the overall system design to address these challenges.
Recent investigations have explored the utilisation of surface friction to enhance the precision of printing [51], yielding promising results with reported position accuracy in the range of 4 mm and printing precision of 1 cm.However, the relatively lower printing precision, particularly in corners, can be attributed to the flight dynamics of the aerial platform for the given trajectories, which result in material accumulation at these corner points.The AAM platform is a coupled six-degrees-of-freedom system that is under-actuated using four propellers.To mitigate possible yaw moments induced by the propellers, pairs of motors on each axis rotate in opposite directions with equal power.This configuration allows quad-rotors to adjust their position along the Z-axis easily by powering up all the motors.However, movement in the other two axes necessitates the speeding up of one set of rotors while simultaneously slowing down the other set [52].Fig. 2 showcases the dynamics of UAV flight and illustrates the frame and movements of a quadrotor UAV.
Combining insights from existing literature and conducting experimental UAV flights, a comprehensive evaluation of the differences between ground-based and aerial-based platforms is undertaken, highlighting the constraints specific to aerial systems.

B. DEPOSITION DEVICES
This study used two deposition device designs suitable for AAM as illustrated in Fig. 3 (adapted from the AAM project deposition device and delta stabilising robot design [38]).The 60 ml cartridge design accommodated two cartridges and was initially developed for systems requiring two liquid components, such as polyurethane foam [53].The system could also function using one cartridge powered by a 6 V DC 298:1 micro metal gear motor and was used during initial pseudoplastic cementitious material development focusing upon mix buildability (Fig. 3o-u).The larger device in the principal image (Fig. 3a-n) employed a 310 ml cartridge powered by a 12 V motor and was developed to provide an upscaled, more powerful deposition system capable of holding more material while being appropriate for the power and payload capabilities of the UAV platform [38].Both designs used a powered descending plunger to push the material out of the cartridge (rather than an auger-based design) due to the rheology modifying admixtures used in material mixes.
During the study, the 60 ml capacity device was manoeuvred in three-dimensional space during laboratory experimentation by a Dobot Magician multi-functional robotic arm, with four degrees of freedom, and also by hand.The 310 ml capacity device nozzle was manoeuvred by hand.The tip of the 310 ml cartridge is connected by a length of 8 mm diameter flexible plastic tubing to the nozzle, which is located between universal joints at the base of a delta arm robot which attaches to a flying UAV.An additional tapering 3D-printed 245 plastic component is placed into the 310 ml cartridge (Fig. 3l) 246 to provide a sloping plane for the material to pass through the 247 cartridge tip and into the tubing.

248
Deposition device specifications are shown in Table 1.249 When full of material, the total mass of both devices is 250 within the 1 kg payload limit of a typical flying UAV.251 310 ml cartridges were considered to have a volume of 252 202 ml in practice to allow for inserting a 3D-printed tapered 253 component at the base of the cartridge and plunger insertion 254 at the top.Similarly, 60 ml cartridges were considered to have 255 a practical capacity of 50 ml due to the drilling of a hole in the 256 side of the cartridge to allow injection of re-filling material by 257 a supply cartridge (as seen in Fig. 3r,t).

258
Two nozzle designs were used during this study.An 259 8 mm diameter circular outlet was used with the automated 260 deposition devices due to the current lateral stability levels 261 of the yaw of the flying UAV platform rotating about its 262 axis.For manually controlled extrusion, 3D-printed plastic 263 components with 20 mm x 5 mm and 15 mm x 5 mm 264 rectangular apertures were attached to the tip of a 60 ml 265 cartridge (Fig. 3s) to compare ease of deposition with circular 266 nozzle extrusion.

267
The volumetric flow rate Q within deposition devices can 268 be calculated using the equation 269 where V is the mean material flow velocity and A is the 271 cross-sectional area of the cartridge.microfibres for AAM mixes have been investigated by the authors [54].This study focuses on developing pseudoplastic hydrocolloids in AAM mortars and does not include fibres in the fresh mixes.Pseudoplastic (shear-thinning) material is highly appropriate for a small, lightweight deposition system.Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.FIGURE 3. Two deposition devices developed for AAM -310 ml capacity device (principal image) and 60 ml device (bottom-right partition): a) 310 ml cartridge casing and seal.b) Delta robot servomechanism.c) Threaded rod attached to the plunger.d) Gearbox casing.e) 8 mm diameter flexible tubing connecting cartridge and nozzle.f) Delta stabilising robot.g) 8 mm diameter circular nozzle.h) Multiple layer extrusion.i) Gearbox.j) 12 V metal gearmotor.k) Metal components securing tubing to the cartridge.l) 3D printed component with tapering interior.m) 310 ml capacity cartridge.n) Plunger.o) Threaded rod attached to the plunger.p) Gearbox and casing.q) 6 V micro metal gear-motor.r) 60 ml capacity cartridge.s) 3D printed rectangular nozzle attachment.t) Refilling cartridge.u) 8 mm diameter nozzle.(Principle image adapted from the AAM deposition device and delta robot design [38].with a bulk density of 200 kg/m 3 and mean particle size of 342 0.2 µm.PFA, a by-product of the coal industry [56], was expected to aid workability, possessing a microstructure of smooth, rounded particles [26], in addition to contributing to the strength of mixes [56].Constituents which contribute to higher-performance strength, such as silica fume and silica flour [56], were expected to contribute to buildability, with the small (generally below 0.1 µm) particles filling voids in material such as ordinary Portland cement type 1 (CEM I) and sand [26].

272
Coarse aggregate can be considered unsuitable for miniature AAM deposition devices, but fine aggregate with particle sizes of <2 mm diameter is feasible.The fine aggregate used in this study consisted of angular-particle and smoothparticle sand.Angular-particle sand (supplied by Jewsons, UK, product number AGSTB003) was kiln dried at a temperature of 105°C for twenty-four hours prior to sieving to modify the rheology of the mix and assess potential synergistic effects.Mixes required a binding, water-retaining agent to prevent bleeding and the ensuing build-up of fineaggregate zones around the tip of the deposition cartridge as the material passed through, potentially causing blockage [57].Mix formulation was informed by the behaviour of pseudoplastic materials such as paint, which requires low viscosity during application and high viscosity once applied and at rest [58].
Albumen-based foam was trialled alongside a cellular lightweight concrete foaming agent manufactured by EAB Associates, with the latter being more effective.When mixed at a concentration of 3% agent to 97% water, this product produced a foam of stiff-peak consistency in 15 seconds, which can be added to slurries.The foaming agent could not be combined with silicone oil, as the latter possesses antifoaming properties [59].During trial mix formulation, it was discovered that EAB Associates foaming agent needed to be used in much smaller quantities to achieve the same effect on workability as silicone oil.
It is noted for viscosity modification [65], contribution to mechanical strength [66] and particularly water retention, via the mechanism of water sorption and the formation of water-retaining polymer networks within cementitious matrices [64].
The chosen plasticiser to provide further pseudo-plasticity was Adomix Adoflow S. This is a lignin-based plasticiser, working via the mechanism of electrostatic repulsion, where the polymeric molecule chains cover the cementitious binder particles and impart a repelling negative charge.This is the same mechanism used by naphthalene-based superplasticisers [67], and it has been noted that these exhibit shear thinning properties [68].Conversely, polycarboxylatebased superplasticisers, working by the mechanism of steric stabilisation [69], can impart shear thickening properties into material [68].

E. MIX MANUFACTURE
Mixes were created in the laboratory using the following method: 1) Dry constituents -cementitious binder of CEM I and PFA, fine aggregate and hydrocolloids -were hand-mixed.
2) Water and plasticiser were mixed and poured evenly over dry constituents.
3) An automated mixer beater was activated for three periods of 30 seconds of planetary motion at high speed.
4) Separately, the foaming agent was added to water and mixed to a stiff-peak consistency.
5) The foam was then added to the mix and underwent two mixing periods for 10 seconds on a slow setting.
Mixes not containing foaming agents followed steps 1 -3 only.
The temperature of the laboratory environment during mix-manufacturing was 20°C ±3°C, and the water added to dry constituents was 16.5°C ±2°C.

F. AXIAL FORCE AND POWER REQUIREMENTS
To discover the axial force required for a deposition device plunger to push a fresh mix through a deposition system, a rig was constructed as shown in Fig. 5b.Displacementcontrolled force was applied at a constant rate of 5 mm/minute upon a plunger using a 50 kN Instron Universal 2630-120/305632 device.
Compressive stresses experienced by the fresh mixes while 443 passing through a cartridge may be calculated using: where F is the axial force required and A is the cross-sectional 446 area of the cartridge (as shown in Table 1).

447
To obtain the power required to process the mixes through 448 miniature deposition systems, freshly mixed material was 449 loaded into a cartridge and extruded, with the location of 450 the nozzle in a three-dimensional space controlled by a 451 robotic arm to simulate automated UAV movement (the 60 ml 452 capacity device as shown in Fig. 5a) and by hand (the 310 ml 453 capacity device).The power supply voltage was maintained 454 at a constant 6 V for the 60 ml device and 12 V for the 455 310 ml device.A more buildable, viscous mix was expected 456 to require greater current to be drawn from the power supply 457 for successful extrusion.

459
Following extrusion, the deformation scenario of layer 460 settlement affects the structure of fresh material.Layer 461 settlement, which can be tested to quantify the stability 462 of extruded material [70].To investigate layer settlement, 463 explicitly defined in this study as the extent to which a 464 freshly extruded bead of material might compress under the 465 weight of subsequently added layers, 8 mm diameter beads 466 of mixed material were extruded onto steel plates to a length 467 of 100 mm at 5 minute intervals.They were compressed 468 at a 2 mm/minute rate by an upper steel plate fixed to the 469 Instron Universal device, as shown in Fig. 5c-d where G ′ is the solid-like behaviour component, storage mod-491 ulus (recoverable elastic deformation as a result of energy 492 storage), and G ′′ is the liquid-like behaviour component, loss 493

J. MATERIAL MICROSTRUCTURE
The particle sizes and surfaces of the constituents and microstructure of 28-day cured mixes were examined using scanning electron microscopy (SEM).Samples were coated in a 10 nm layer of gold to prevent charging and increase signal-to-noise ratio and subsequently analysed using a JEOL SEM6480LV microscope.

K. X-RAY COMPUTED TOMOGRAPHY (CT)
The 3D structures of three selected printing trajectory designs, a wall (adjacent lines), an alternating 'ruffle' design and a continuous curve design, were investigated using x-ray computed tomography (CT).Previous tests by the AAM project had used these trajectory designs to demonstrate the versatility of the developed pseudoplastic material and the lateral precision capabilities of AAM material extrusion during flight [38].The CT scans were measured using a Nikon XT H 225 ST model machine and conducted using 65 kV, an exposure rate of 1.5 seconds and 50 µA x-ray beam output.The obtained data files were subsequently analysed by using VGStudioMAX software.

A. EVALUATION OF A SUITABLE AERIAL PLATFORM
Among AM research studies, AAM brings a different perspective by deploying aerial vehicles with a robotic manipulator to produce large-scale structures with additive manufacturing methods.This novel production method facilitates multi-agent parallel additive manufacturing with an unrestrained build envelope in hard-to-reach zones.This will allow maintenance tasks such as crack repair [71] to be performed at height without scaffolding or supporting infrastructure and free-form construction.AAM complements the limitations of ground-based systems and holds enormous potential and promise for robotic construction.
Table 3 summarises the advantages and disadvantages of comparing ground-based robotic systems and aerial platforms in construction tasks.
Furthermore, the design of the aerial platform and extrusion mechanism is as important as the flight dynamics.
The design difficulties related to the use of an aerial platform in continuous additive manufacturing tasks cover the positioning of the extrusion mechanism and nozzle, potent interaction with the construction surface, minimisation of structural vibrations and aerodynamic perturbations on the built structure caused by the aerial flow and propellers' downwash, and the scale optimisation of the overall system.
To clarify, flight dynamics can easily be disrupted by any change in the alignment of the centre of gravity.
For that reason, to achieve higher printing accuracy, the positioning of the extrusion mechanism and nozzle should be in balance with the aerial platform's centre of gravity (CoG).
Moreover, a certain distance between the nozzle and the propellers' level should be secured to decrease the downwash effect, which may cause the extruded material to scatter around.The general approach against these perturbations is using a manipulator [72], [73].However, a unique way of re-compensating the negative effect of the aerial platform can be the deployment of multi-directional thrust systems [74].
In the current AAM framework, a parallel manipulator is used, which is added to the drone body to isolate vibrations and oscillations caused by the aerial platform's behaviour and minimise the effect of the downwash generated by the propellers.Another critical aspect of AAM is the dimensions and properties of the nozzle.The narrower the nozzle diameter, the greater the print length and resolution that can be achieved with each cartridge of fresh material.However, this will decrease the precision tolerance of the overall system and place extra importance on the lateral stability of the extrusion device while depositing fresh material.
After a certain threshold, as the system cannot provide that clarity, errors such as breaking during the printing will occur.Furthermore, in an AAM application, a few practical aspects could be considered.An example of a 596 hardware-based approach would be covering the area around 597 the nozzle with a sheet of material to minimise the effect 598 of the downwash generated by the propellers.The need to 599 address downwash would also be reflected in the material 600 development strategy, with extra emphasis being placed 601 upon cementitious material possessing suitable rheology 602 parameters and a yield stress sufficient to provide resistance 603 to deformation due to downwash while in the fresh state.

604
Another significant constraint of the aerial platforms is 605 their heavily bounded flight times and payload capaci-606 ties [34].This strictly defines the maximum amount of 607 material printed within a single flight.While it is possible 608 to solve the problem of carrying capacity and limited 609 energy by scaling up the aerial vehicle, it should not be 610 forgotten that this will compromise safety and mobility.618 [75], specially designed for aerial construction and repair 619 operations.In addition, there is further interest in aerial 620 perching to extend operation time [76], battery-tethered aerial 621 vehicles [77], more efficient battery technologies [78] and 622 efficient mission planning [79] in this context.This scale optimisation is also a highly significant topic 644 for collective robotic construction [87].The use of a Sub-millimetre precision is a hard-to-reach range for aerial platforms and it poses a significant challenge for aerial construction tasks.Therefore, aerial construction literature heavily uses motion capture systems that can manage this precision level.However, these systems are still only stepping stones towards the potential promise of AAM for on-site tasks.To overcome this problem, multi-sensor fusions, for example, a GPS module with a SLAM camera or LIDAR, can be used for more precision in localisation [89], [90], [91].
After handling the localisation, mission planning should be dealt with for multi-agent AAM.This has been investigated [92]; however, another challenge of lack of physical reference is additionally introduced in the case of aerial platforms.Moreover, the time aspect is an extra dimension in mission planning over ground-based systems' two or three dimensions.This planning may further be complex by bringing multiple tasks simultaneously [93], [94].The overall aim related to mission planning covers time efficiency, maximising material extrusion precision, and energy use efficiency.Future research will explore this optimisation type of mission planning solutions further [95].
While this study focuses upon pseudoplastic cementitious material development strategies and extrusion platform and nozzle considerations, in the current state of the AAM, two main materials have been printed using unmanned aerial vehicles (UAV) in self-powered, untethered flight within a laboratory environment: (i) cement-based mortar and paste; (ii) polyurethane foam-based material.For images and details of printed cement and foam structures using UAVs, the reader is referred to the AAM projects' UAV flight extrusion publication [38].
Considering future work, autonomy, end-effector precision and collective behaviour are the research nodes that should be undertaken for further advancement in aerial robotics.A swarm of UAVs should be able to coordinate work packages and flight paths without any collision or interference with a global digital twin, which is updated along with the material and built structure information and should be able to adapt and correct on the way for the most optimum and close result from the intended design.This necessitates a high level of autonomy with real-time scanning in the loop, higher precision at the tooltip, low platform vibration, greater payload capacity and flight endurance, and reduced disturbance from the flight dynamics with further software development for multi-agent coordination.

B. PRIORITISATION OF BUILDABILITY PHASE-DESIGN, RESULTS, AND DISCUSSION
Material extrusion experiments tested with the first 60 ml capacity deposition device focused upon buildability and used both an 8 mm diameter circular nozzle, an aperture flush 708 with the cartridge base (Fig. 3u) and 3D printed rectangular 709 nozzles, fitted over the base of the cartridge (Fig. 3s) to

746
Fig. 8 shows calorimetric (Fig. 8a,b) and rheological results 747 (Fig. 8c,d).It can be seen in the calorimetry images that 759 Fig. 9 shows SEM microstructural images of more angular, rough-surfaced sand particles (a), smoothed sand particles (Fig. 9b) and HEMC particles (Fig. 9c) (along with xanthan gum particles -Fig.9d -used with the 310 ml capacity device and discussed further in section 7.3.5).Images illustrate how the surface of the smoother sand particles would aid workability (Fig. 9b), as opposed to the rougher, more uneven surface of the more typical building sand (Fig. 9a).The HEMC image (Fig. 9c) reveals highly irregular particle sizes and long polymer chains.HEMC performed successfully both in binding the constituents together (with segregation and compaction of material not in evidence) and increasing viscosity.

4) BUILDABILITY DISCUSSION
Although mix B contains less cement than mix A, it is suggested that Fig. 8a,b may also display confirmation that HEMC possesses secondary hydration-retarding properties, with mix B showing both a reduction in the energy transferred during the 48 hours following mixing (a) and the rate of transfer (b).The initial C 3 A reaction leading to ettringite formation (Fig. 8b 1) appears to be unaffected, but the dormant period (Fig. 8b 2) is clearly extended.The rate of the C 3 S reaction-led acceleration period (Fig. 8b 3), leading to the primary hydration products C-S-H gel and Ca(OH) 2 , is reduced and formation of further ettringite and monosulfates from C 3 A (Fig. 8b 4) appears less defined in mix B.
Three parameters affect the chemical structure of HEMCthe molecular weight, the presence of the hydroxyethyl group and the presence of the methoxyl group [66].Hydroxyethyl cellulose (HEC) -without the methoxyl group -has been shown to retard both C 3 A [96] and C 3 S [97] hydration reactions.HEC reduces the rate of C 3 A dissolution, ettringite precipitation and calcium hydroaluminate precipitation, with HEC particles adsorbed onto calcium hydroaluminate surfaces observed [96].
The presence of HEC leads to slower C 3 S dissolution rates (dissolution is limited by the ionic composition of the liquid phase induced by the cellulose ether), strongly modifying the growth rate of the C-S-H gel phase.Through adsorption, cellulose ether restricts the nucleation and growth of C-S-H particles on surfaces of C 3 S particles, which results in ultimately thicker, more permeable C-S-H shells [97].HEMC has further been shown to retard the precipitation of calcium hydroxide (portlandite) [66].
Following the calorimetry results, further oscillation tests took place on the rheometer to assess the effectiveness of two accelerating admixtures in combating the retardation effects of HEMC: BASF Master X-seed 100 and a 1:1 laboratoryformulated combination of aluminium lactate and diethanolamine each added to mixes at a dosage of 3.25% by weight.
Master X-seed consists of a suspension of nano-sized crystalline C-S-H seeds and is designed to promote the rapid nucleation and growth of C-S-H crystals, primarily targeting The workability-buildability combination of mix B was appropriate for extrusion immediately out of the cartridge of this deposition device design.However, in readiness for fully testing mixes with flying UAVs, further experimentation was required, with workability being the primary parameter informing mix design, using a newly-developed, upscaled 310 ml cartridge device.The attachment of a deposition device to a UAV required a 560 mm length of flexible plastic tubing to connect the cartridge tip to a nozzle at the base of the UAV-attached delta robot, which controls the nozzle trajectory and stabilises movement.Mix B requires 800 N -900 N of force to process mixes through the deposition devices' length of tubing.This proved too challenging for the power capabilities of the UAV batteries, which have to power both the UAV and the deposition device with a stabilising delta robot.Materials strategy, therefore, evolved to place extra importance on developing the pseudoplastic properties of the mixes, as viscosity is required to decrease by orders of magnitude while material passes through the deposition system, yet increase once deposited.While needing to exhibit liquid-like behaviour while in the deposition system, the Fine aggregate should, therefore, consist of smoother particles of sand in a more workable mix.Rougher and more angular particles, along with wide variation in particle size, lead to increased viscosity as particles lock together in the fresh mix -an asset once extruded but a drawback preextrusion.Fine aggregate was also used in a reduced quantity, with increased use of pseudoplastic hydrocolloids to provide buildability.

C. PRIORITISATION OF WORKABILITY PHASE-DESIGN, RESULTS, AND DISCUSSION
During a further phase of experimentation focusing on workability, all mix designs were tested with the developed larger deposition device accommodating a larger 310 ml capacity cartridge.The flexible tubing which passes from 889 the UAV deposition device cartridge through the stabilising 890 delta robot arms was manipulated by hand during the material 891 deposition tests detailed in this study.Mix formulation involved increased use of smooth-particles 894 PFA, decreased use of sand and investigation into whether 895 alternative hydrocolloid constituents were superior, compati-896 ble or synergistic with HEMC.Table 5 lists the hydrocolloids 897 investigated during the study to evaluate their effectiveness 898 as an RMA suitable for AAM cementitious mixes.All 899 hydrocolloids listed in Table 5 were trialled individually and 900 with HEMC.Mix densities remained above 1700 kg/m 3 .901 Diutan gum is established as an RMA in concrete 902 and cement [49], [102], [103].However, during hydration, 903 extruded mix formulations featuring diutan gum exhibited 904 the behaviour of adsorbing water on the external surface of 905 the material, giving a moist veneer to cured specimens -906 a behaviour not observed with the remaining hydrocolloids 907 listed in Table 5.The anionic nature of diutan gum requires a 908 polycarboxylate-based superplasticiser to prevent this surface 909 adsorption [49]; therefore, diutan gum appeared to be 910 In combination with HEMC, xanthan gum provided superior buildability in relation to the quantity used during trial formulations.Coupled with suitable workability, it was therefore decided that the most effective and AAM-appropriate rheological-modifying hydrocolloid was a combination of HEMC and xanthan gum, a hydrophilic native bio-polysaccharide derived from the bacteria xanthomonas campestris [104] following an aerobic fermentation process [60].
Three new mixes, termed C -E, were formulated 922 (Fig. 10).Plasticiser content was maintained at 1% by 923 weight of the binder.Constituents that promoted build-924 ability, such as silica fume, were discontinued in the mix 925 formulation.Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.Force and current requirements increased as the material passed through the tubing and plateaued after extrusion had commenced.Fig. 13a shows the relationship between force and current for the mixes.Using equation (2) stresses experienced by the material are between 0.2 MPa -0.4 MPa while in the cartridge, rising to 6 MPa -13 MPa while in the tubing.Mixes C -E required less force to process than mixes B (which was ≈800 N) and A (≈900 N).Pa.s while at rest to 10 2 Pa.s while moving through the length of tubing.
The current dimensions of the 310 ml capacity device tubing are indicated in Fig. 13c,d.The resistance profile (Fig. 13c) changes linearly with length yet begins to increase dramatically once radius values fall below 3 mm.With viscosity (Fig. 13d), increasing the tube radius beyond 4 mm sees the rate of viscosity increase significantly (with increases in orders of magnitude beyond 12 mm).Fig. 14 shows the yield stress (a) and viscosity (b) flow curves for mixes C and D. Mix E was very similar to mix D and was omitted for clarity.It can be observed that the most suitable mix in this study, mix D, possesses a yield stress of 1.1 kPa, with mix C lacking sufficient buildability and displaying a lower yield stress.Viscosity decreases by orders of magnitude in all mixes, reducing to below 10 Pa.s as the shear rate increases.Fig. 15 shows the 3D reconstruction of three wall trajectory designs -wall, ruffle and continuous curve.The multi-layered extruded specimens shown in the images were coated in a layer of dental plaster for the convenience of handling and having been subjected to mechanical tests.Extruded specimens can be viewed in 2D images from three planes, namely xy-plane, yz-plane, and zx-plane.The bright areas, which are grey colour, show the dense material in the part, such as cement and plaster, while the dark areas (black colour) represent pores and gaps in the extruded filaments.Pores may have originated from air bubbles during mix preparation.As shown in Fig. 15a, the wall structure deposited by mix D exhibit several pores and gaps in comparison with the other two structures.The three dark lines shown in Fig. 15a (1) suggest compromised layer-boundary bonding.However, it is surprising that no obvious layer-boundary gaps can be observed in the continuous curve specimen (shown in Fig. 15c), which was also deposited by using mix D. Thus, considering the inherent trajectory variation in hand-controlled extrusion, it is suggested that the different trajectory designs would affect the quality of the internal structures of deposited filaments.In terms of the alternating ruffle design (shown in Fig. 15b), the alternating layers of parallel lines and the ruffle design can be identified, which means the deposition structure was well maintained.A contributing factor to this is that mix E contained sand, which helps to provide a mix with buildability.

4) WORKABILITY DISCUSSION
A pseudoplastic material should possess low viscosity while in a miniature deposition system and experience as little flow resistance imparted by confining walls as possible.The results of this study have shown that tube radius is the key dimensional parameter when considering how a pseudoplastic material may pass through a miniature  In the trial formulation, xanthan gum did not possess water-retentive and constituent binding qualities that were comparable to the standards exhibited by HEMC.It was also observed during trial mixes that HEMC in isolation did not impart such a strong influence over viscosity and yield stress compared to being combined with xanthan gum in equivalent quantities, though the effect was still pronounced.The two hydrocolloids proved synergistic in mixes, resulting in a cementitious-polymeric composite material suitable for AAM.With mix D exhibiting a yield stress of 1.1 kPa and possessing the most suitable workability-buildability combination in this study, it can be obtained that a material   or eliminate the requirement for fine aggregate in a mix 1072 suitable for AAM.Therefore, the justification for using fine aggregate in these circumstances would be based on cost and carbon reductions rather than the necessity for buildability.
The HEMC micro-structural image (Fig. 9c) shows water-absorbing particles consisting of long polymeric chains capable of wrapping around water molecules, adsorbing and expanding, reducing segregation and bleeding in the fresh mix.Water-retaining HEMC particles also adsorb onto the surface of both C 3 S and C 3 A particles [97].By contrast, the xanthan gum micro-structural image (Fig. 9d), shows a greater particle distribution, with a greater quantity of smaller and more angular particles in comparison to HEMC, suggesting the ability to lock together, with smaller particles filling voids and increasing viscosity and buildability at low shear rates.
The two products affect viscosity by differing mechanisms -xanthan gum by adsorption onto cement particles, increasing inter-particle attraction, whereas HEMC molecules increase the viscosity of the water in the mix by adsorbing onto water molecules, expanding and attracting molecules in adjacent chains.Cellulose ether molecules entangle and intertwine amongst themselves at low shear rates, but at high shear rates, disentanglement and subsequent alignment parallel to flow direction occurs [61] -this pseudoplastic behaviour is desirable for AAM.Cellulose ether molecules additionally readily absorb moisture from the air [61].
HEMC and xanthan gum, a semi-synthetic hygroscopic polymer and a natural hydrophilic polymeric gum, respectively, are reasoned to be compatible and synergistic in fresh cementitious mixes suitable for AAM.This dual approach to increasing viscosity (at rest following extrusion) and decreasing viscosity (under stress within the deposition device) is particularly critical for a miniaturised deposition The reader is referred to [38] for the demonstration that cementitious mixes can be extruded by a flying, self-powered untethered UAV to a lateral precision within 4 mm and cured material 28-day compressive strengths are shown to be in the region of ≈25 MPa; therefore AAM cementitious material is structurally viable.Considering the suitable rheological and structural properties of mixes containing a synergistic combination of pseudoplastic hydrocolloids, if the lateral in-flight trajectory deviation of the UAV is kept within 4 mm (set to decrease further through continuing iterative development), AAM with a miniature deposition device would be particularly suitable for precision repair work, 1139 especially at height.Considering the inherent dangers of 1140 working at height and on structures subjected to high lateral 1141 wind loading, this would be a prime application for AAM.1142 UAVs are capable of landing upon vertical surfaces in 1143 addition to horizontal surfaces and an attached delta arm robot 1144 is capable of directing the nozzle administering the material 1145 in addition to stabilising UAV trajectories during flight.
1146 Future work for novel AAM cementitious material devel-1147 opment could examine whether any additional constituents 1148 may be added to further improve the compressive strength 1149 of cured material and minimise lateral deformation of 1150 extruded layers.However, additives and admixtures should 1151 not excessively compromise the workability or pseudoplastic 1152 properties of the material while in a fresh state and flowing 1153 through the deposition system.Alternative methods of 1154 accelerating fresh material may also be explored, such as 1155 CAC or another agent which may promote a flash-setting 1156 at the appropriate timescale shortly following deposition.1157 Power capabilities of UAVs and the force imparted by 1158 custom extrusion mechanisms are also areas to continually 1159 evolve.These workflows should consider low energy usage, 1160 lightweight hardware characteristics, and versatility for 1161 different precision application scenarios while processing 1162 more viscous, denser cementitious material.1163 Additionally, AAM using UAVs would be an appropriate 1164 solution for repairing infrastructure cracks and potholes, such 1165 as those in roads and pavements, reducing the requirement 1166 for expensive labour and ground-based machinery in a sector 1167 where, in the developed world, repair expenditure can outstrip 1168 that of new infrastructure construction [105].

IV. CONCLUSION 1170
This study demonstrated the feasibility of aerial additive 1171 manufacturing (AAM) and the development of a pseudo-1172 plastic cementitious-polymeric composite structurally viable 1173 material specifically for AAM.The material can be extruded 1174 by a lightweight miniature deposition system suitable to 1175 be carried and powered by an untethered unmanned aerial 1176 vehicle (UAV) in flight.

1177
The study evaluated aerial platform considerations and 1178 identified differences between off-site ground-based additive 1179 manufacturing (AM) platforms and on-site aerial platforms, 1180 which highlighted the importance of maintaining stability 1181 and required the miniaturisation of the deposition process 1182 and development of pseudoplastic cementitious material for 1183 AAM, which were less dense than traditional or ground-based 1184 AM mortar mixes.Material approaches focused first on 1185 buildability, and as aerial platforms, deposition devices, 1186 extrusion tubing and nozzle requirements evolved, the need 1187 to ultimately focus on workability was emphasised.Important properties of fresh material suitable for AAM can be identified as 1.1 kPa yield stress, <10 MPa complex modulus, 4°phase angle, and requiring 500 N force and 250 mA current to be processed through the miniature deposition system.A parameter of key importance in the miniaturised deposition system is the circular cross-sectional area of tubing connecting a nozzle to the reservoir cartridge tip, with a 4 mm radius being identified as optimal for the miniature deposition device designed for on-site AAM deposition of cementitious material.

FIGURE 1 .
FIGURE 1.The conceptual representation of aerial additive building manufacturing (AAM) with multiple coordinated unmanned aerial vehicles (UAV) extruding a suitable pseudoplastic mortar in a customised continuous curve printing path.

FIGURE 2 .
FIGURE 2. Quadrotors' flight dynamics along a movement in the X or Y axis (above) and an example of quadrotor frame (below).

FIGURE 4 .
FIGURE 4. Constituents with particle distribution properties investigated for AAM.a) Schematic contribution to material properties' workability, buildability, and strength.b) Particle size gradation of fine aggregates.

FIGURE 5 .
FIGURE 5. Robotic arm, axial force and material deformation tests.a) Robotic arm manipulating a 60 ml capacity device printing a fresh mix.b) Axial force test rig with direction of force indicated.c,d) Material deformation test rig shown with direction of uniformly distributed load indicated (c) and evaluating the settlement of an 8 mm diameter bead of extruded fresh material (d).

611
The negotiation between these two necessitates an effective 612 scale optimisation.Two basic approaches are presented 613 here.The first is developing and producing a platform 614 suitable for the target task, and the second is choosing and 615 adapting the most suitable off-the-shelf platform.Recent 616 work on the first approach of aerial platform optimisation 617 is the compact coaxial tri-rotors developed by Orr et al.

623
If the project time is restricted, selecting an 'off-the-shelf' 624 aerial platform might be a better way to proceed.For the 625 research presented herein, the unmanned aerial vehicle will 626 be required to withstand the weight of the deposition device 627 under 1 kg during flight.In addition, according to the figures 628 in Table 4, the total printing time of a singular layer of 629 material may be under 10 minutes.Therefore, the endurance 630 of the flying vehicle can be kept within 20 to 30-minute 631 intervals to allow for small-distance flying and printing time.632 Table 4 displays the technical characteristics of seven 633 off-the-shelf UAVs to be considered for aerial additive 634 manufacturing using the deposition devices mentioned above.635 It is worth noting the maximum flight time of a flying device 636 is determined with no payload; as the mass added onto the 637 main body increases, the endurance will decrease.From this, 638 the vehicles that meet both payload and endurance criteria set 639 above and therefore suitable for the mission mentioned above 640 are Aurelia X6 standard, Hercules 10 or Hercules 20 with a 641 flight time while loaded with 1 kg of a payload of over 20, 642 30 and 37 minutes, respectively.643 TABLE 4. Technical characteristics of off-the-shelf UAV's.
645 swarm of UAVs to manufacture buildings enables greater 646 scalability, increase the speed of production, and improve 647 the robustness of the methodology since the loss of an 648 agent won't affect production and can be easily replaced.649 Bigger UAVs will lead to less task parallelization.Even 650 though the UAVs would have a higher payload and flight 651 endurance, there should be an optimum number of agents 652 with an optimum scale to get the full potential of AAM realised.Zhang et al.[38] demonstrated a dry flight of three UAVs working collaboratively to build the light trace of a dome structure.However, the collective robotic construction software framework for UAVs is a topic of ongoing wider research endeavours[88].
748 less energy is transferred during the first 48 hours of the 749 hydration process for mix B in relation to mix A. A time 750 differential can also be observed in Fig. 8b, with a longer 751 dormant period (Fig. 8b 2) and delayed calcium silicate 752 hydrate (C-S-H) gel phase, and calcium hydroxide formation 753 from C 3 S, clearly occurring later in mix B (Fig. 8b 3).There 754 is little difference observed in the later diffusion-limited 755 reaction period (Fig. 8b 5).The rheology results reveal 756 complex moduli for mix B to be higher than cement paste A, 757 illustrating the buildability qualities of silica fume, sand and 758 HEMC influencing the rigidity of mix B.

FIGURE 7 .
FIGURE 7. Hand and robotic arm-driven mix B extrusions using the 60 ml capacity device, using rectangular and circular nozzles, respectively.a) Partial sine wave with five layers.b) Sine wave, which shows variation in alternate layer trajectory.c) 20 circular layers deposited.d-f) Rectangular nozzle extrusions by hand.g) Sine wave extrusions using the robotic arm.Images a-c and g have 8 mm diameter circular extrusions.Images d and f feature a 15 mm wide and 5 mm high rectangular layer, with e having wider layers at 20 mm.

FIGURE 8 .
FIGURE 8. Rheology and calorimetry results for fresh 60 ml device mixes.a) Calorimetry -energy transferred during hydration for mixes A and B. b) Rate of heat evolution during hydration.1: Initial C 3 A reaction.2: Dormant period.3: Main C 3 S reaction forming C-S-H gel and Ca(OH) 2 .4: Continuing C 3 A reaction forming ettringite and monosulfates.5: Diffusion limited reaction period.c) Oscillatory test results for mixes A, B and C showing elastic modulus G' and storage modulus G''.d) Complex modulus G* and phase angle δ for mixes A and B.

926 2 )
Fig.11illustrates extrusions with different trajectory designs 928 using the hand-controlled 310 ml capacity device to demon-929 strate design possibilities using AAM.930

FIGURE 11 .
FIGURE 11.Extrusions using the 310 ml capacity device.a) Circular column element with alternating layers of three concentric lines and ruffle design featuring mixes D and E with the deposition device moved by hand.b) Examples with alternating layers of parallel lines and the ruffle design using mix D, with the deposition device moved by hand.c) Three designs printed by hand using mix D: Four adjacent beads forming a wall (c1), an alternating layer design using three straight lines alternating with a ruffle design (c2) and a continuous curve design (c3).
Fig.12shows how mix C (workable paste) and mix D

Fig. 13b depicts
Fig. 13b depicts the two-hour oscillation test profile of the most suitable mix in this study for AAM deposition, mix D, showing how the elastic modulus G' dominates over the viscous modulus G'' for the pseudoplastic mortar mixes.Moduli values initially increase with the initial dissolution of

FIGURE 12 .
FIGURE 12. Deformation results for fresh 310 ml device mixes; settlement of mixes C and D under compressive loading (mix E was similar to mix D and omitted for clarity).
the C 3 A phase and then broadly plateau for the remainder 971 of the mix open time, within the dormant hydration period.972 Mixes in this study possessed a phase angle δ within the 973 range of 3°-10°and applying equation 4, complex moduli 974 G* can be calculated as 10 6 -10 7 Pa.Therefore, 10 MPa can 975 be considered a quantitative upper limit for AAM.976 To quantitatively assess the optimisation of the 310 ml 977 capacity device tubing dimensions concerning the resistance 978 to flow imparted by the deposition device R and material 979 viscosity η, Fig. 13c and Fig. 13d illustrate how the viscosity and resistance profiles for mix D would change in accordance with tubing dimension variation.Fig. 13c,d uses the viscosity profile of mix D, which shows viscosity reducing from 10 7

FIGURE 13 .
FIGURE 13. force, current required flow resistance and viscosity (in relation to deposition device cartridge and tubing dimensions) results for fresh 310 ml device mixes.a) Axial force and current required to process mixes through the tubing.b) Rheology oscillation test for mix D, which possessed the best workability-buildability combination, showing elastic modulus G', viscous modulus G'' and phase angle δ. c,d) Flow test for mix D showing the impact upon resistance to flow R and viscosity η that would arise from varying the tubing dimensions, demonstrating the suitability of the 560 mm length and 4 mm radius used in the extrusions.
deposition system.Tubing required to connect reservoir 1035 cartridge tips to extrusion nozzles is the component that 1036 exerts the most influence over material flow, and dimensional 1037 optimisation of tubing is of primary importance.1038 With a radius of 4 mm, resistance remains comparable 1039 to that imparted by a larger radius, and it is reasoned that 1040 the radius should not be reduced further.Increasing the 1041 radius beyond 4 mm would increase viscosity to a greater 1042 extent than reducing resistance.Therefore, a tubing radius of 1043 4 mm is suggested to be optimal for a miniature deposition 1044 device suitable for AAM.Tubing length in this study is 1045 based on operational needs and the logistical necessity for 1046 the delta arm to function optimally; therefore, it cannot be 1047 reduced.Although length reduction would be beneficial, the 1048 results confirm length as the secondary parameter concerning pseudoplastic material flow within the device.

FIGURE 14 .
FIGURE 14. Yield stress (a) and viscosity (b) flow profiles shown for mixes C and D. (Mix E was very similar to mix D and is omitted for clarity).

FIGURE 15 .
FIGURE 15.Micro-CT 3D images of a) a wall structure using mix D, b) a linear structure with alternating layers of parallel lines and the ruffle design using mix E, and c) a continuous curve using mix D. Their 2D image views are in xy-plane (1), yz-plane (2), and zx-plane (3), respectively.
suitable for a miniature automated deposition device should 1063 aim to possess neither significantly less (due to inadequate 1064 buildability) or significantly more, as mix E proved challeng-1065 ing for the deposition device and possessed only a marginal 1066 increased yield stress to mix D. 1067 Although the SEM images (Fig. 9) support the choice 1068 of smooth-particle sand (rather than angular and sub-1069 angular), the level of buildability provided by a sufficient 1070 quantity of the hydrocolloid combination can serve to reduce 1071 system.The deformation results emphasise the importance of keeping spans to a minimum in trajectory design when working with mixes which adhere to the consideration of workability as being the primary parameter.A further course of action to address extruded bead deformation and promote hydration would be to investigate calcium aluminate cement (CAC) and calcium sulphate (CS) augmented mixes.Along with suitable plasticiser and alternative accelerating or retarding agents, this approach would be a means of controlling and promoting ettringite formation which promotes early rigidity (thus buildability) and strength.The criteria of success for such an approach would be ideally to firstly provide sufficient open time for deposition device cartridge loading and subsequent UAV attachment and flight, plus a small buffer in case of a technical issue with the UAV operation.Following the expiration of the desired open time, which can be identified as a function of combined mix manufacture, deposition device loading and UAV flight time, a successful CAC/CS augmented system should promote rapid hydration, unhindered by the established retardation effects of HEMC.

1188
Cementitious binders were CEM I-based, augmented 1189 by PFA and lignin-based plasticiser to aid workability.1190 An effective rheology-modifying admixture was formed 1191 by combining hydroxyethyl methyl cellulose and xanthan 1192 gum.This combination is capable of mitigating constituent 1193 segregation and providing sufficient material buildability for multiple-layer extrusion.Fine aggregate can be used in low ratios and should consist of sand particles with a smoothed surface and may be accompanied by a foaming agent to maintain sufficient workability.Material of 1700 kg/m 3 density is lightweight compared to traditional mortars.
Any future work for material development could examine whether any further additives may be used to increase compressive strength of cured material or explore alternative methods of accelerating fresh material such as CAC.Continuing deposition device custom development could add increased power capabilities which in turn would allow an increase in viscosity and density of material.For aerial robotics, autonomy, end-effector precision and collective behaviour are areas identified for further advancement.Improvements in flight coordination necessitates continuing software development, a high level of autonomy with real-time scanning in the loop, higher precision at the tooltip, low platform vibration, increased payload capacity and flight endurance, and reduced disturbance from the flight dynamics.AAM is a highly interdisciplinary fabrication technology.It comprehends aerial robotics, architectural design, and material science.The creation of cohesive cementitious structures with defined layers using AAM demonstrates a significant advancement towards bringing a high-precision on-site, multiple-agent, untethered aerial capability to AM in the construction industry.s Seconds V Material flow velocity (mean) V Volts δ Phase angle η Viscosity of the material σ Compressive stress

TABLE 2 .
Rietveld quantitative phase analysis of the chemical composition of Dragon Alfa CEM I 42.5 R Portland cement shown as a percentage by weight.
. The tests 470 were conducted over the material open period of two hours.performed to quantify the pseu-473 doplastic and viscoelastic properties of the mixes.Tests 474 were conducted on a TA Instruments DHR2 rheometer 475 at a constant temperature of 25°C.Oscillatory tests used 476 disposable aluminium flat plates with a 40 mm base plate 477 and 25 mm diameter upper plate.Flow tests used steel cross-478 hatched 40 mm base plate and upper plate to minimise 479 slippage at greater shear rates.A 1000 µm geometry gap was 480 used in all rheology tests, and material was placed upon the 481 base plate immediately following mixing.

TABLE 3 .
Advantages and disadvantages of different robotic construction methods.