What next for 3D-Printing of Structures? (Part 1)

The Future is Additive

3D-Printing is everywhere. Every trade show, technology fair, engineering conference, consumer article and science journal is incomplete without a discussion on the use of 3D-Printing (Additive Manufacturing), a review of recent technological developments or a prediction of its impending impact on a particular market or sector. The technology is tipped to change the way we design, manufacture, construct, prepare food and even repair our bodies; transforming our traditional concepts of production, supply and consumption.

This change has come incredibly quickly and continues to accelerate with the development of software, additive manufacturing technology and materials science. Only a few years ago the concept of additive manufacturing had yet to venture beyond the realm of futurists and specialist researchers, and yet now its potential promises to transform every aspect of our lives.

Soon we will have the ability to fabricate a wide range of products, digitally downloaded from the internet and comprising multiple raw materials, in the comfort of our own home. With this will come a huge range of environmental benefits, consumer-oriented value, commercial opportunity and disruption, and as with most technologies, the potential for nefarious applications (e.g. 3D-Printed guns).

This is a monumental shift in thinking. The internet transformed our lives by bringing a previously unimaginable information resource into our homes, allowing us to search and benefit from knowledge that was practically impossible for the vast majority of the population to access, and it now offers the potential to do the same for physical objects; effectively allowing the material replication of any man-made object from one location to any other location – on or off planet.

"The internet transformed our lives by bringing a previously unimaginable information resource into our homes......and now it offers the potential to do the same for physical objects"

So, is it conceivable that whilst we are downloading and fabricating products in the comfort of our own homes, we will be living in buildings that are themselves constructed using 3D-Printing technology?

In Part 1 of this article I will be exploring the potential benefits of 3D-Printing structures and the challenges that it faces currently in providing cost-effective and practical solutions. In Part 2 I will be taking the shackles off and looking at what the future might hold….

3D-Printing in Construction

As I have discussed before, the construction industry is not like other industries; focused on the delivery of one-off projects using procurement methods and transactional supply chains that create barriers, encourage short-termism, favour lowest cost and demand risk aversion. This continues to create an industry environment in which a blinkered ‘false-truth’ pervades; making strategic research and development and collaborative innovation far more difficult than in other product-driven industries.

"3D-Printing is emblematic of the issues surrounding innovation in the industry"

3D-Printing is emblematic of the issues surrounding innovation in the industry. The commercial application of materials and technology research carried out by a few academic institutions and innovative start-ups has led to several proof-of-concept examples that can be found on the internet, demonstrating some of the opportunities (and challenges) that 3D-Printing presents for the industry. Whilst some progress has been made, 3D-Printing is having a difficult time getting the investment and opportunity it needs and deserves to develop further. Clearly greater collaboration and long-term strategic investment is needed to enable the technology to upscale from limited trial projects to providing a tangible value proposition for clients and contractors. I will come back to this point in Part 2, but let’s first focus on the technical challenges and opportunities of 3D-Printing itself; highlighting the specific areas requiring further research and development.

3D-Printing of Structures

Firstly, it is necessary to point out that 3D-Printing in construction covers a very large range of materials and technologies which can potentially be used for the manufacture of almost any building component, from individual parts through to contour-crafting of whole structural systems. Whilst the former potentially offers greater immediate returns to the industry across the disciplines of architecture, interior design and MEP, the focus of this article is the 3D-Printing of structures; in particular the 3D-Printing of cementitious materials.

The concept of cementitious 3D-Printing is similar to that of other extrusion-based additive manufacturing technologies. The material is mixed and delivered via a pump to a nozzle which deposits the material in layers. A computer-controlled robot manoeuvres the nozzle through a predefined path ensuring the layers are deposited at the correct location and at a specific rate. The design of the material, nozzle and speed/timing of the deposition is carefully managed to ensure the material achieves the temporary properties required so that each layer is able to support the next and, on curing, can resist the permanent forces.

Several types of robotic systems currently exist to facilitate the placement of the cementitious material however these broadly divide into two categories; namely gantry systems and robotic arm systems.

Gantry systems comprise a nozzle that is fixed in orientation (usually vertically) but is maneuvered on a 3-Dimensional axis system to position the nozzle in the required location. These systems can either be cartesian (i.e. moving on orthogonal axes, x, y, z) or polar (i.e. moving on a rotational axis, r, θ, z). Examples of a cartesian gantry system are COBOD whereas Apis Cor is based on a rotary gantry system.

Robotic arm systems are based on systems used in manufacturing (e.g. car assembly lines) and feature articulation about 6-axes which facilitates orientation of the nozzle (in theory) in any direction and at any location in 3-Dimensional space. An example of robotic arm systems are CyBe and XTreeE.

These technologies and their application to 3D-Printing are still evolving, however there are generalised pros and cons to these types of systems, depending on the specific application, that need to be considered. This is important, as the method of robotic delivery significantly impacts the design, programming and the logistical planning of any 3D-Printed project. This is a detailed topic, and not one I intend to cover here, however for one perspective please see here.

Why 3D-Printing?

This question cannot be answered in simple terms based on today’s technology and its position in the construction market. Given the current proof-of-concept status of 3D-Printing, the true potential of 3D-Printing has yet to be fully established or measured. Based on an extrapolation of the technology as it stands, and some assumptions regarding future development and cost-effectiveness (i.e. the track record of other technologies on the Gartner Hype Cycle), we can reasonably hypothesise that 3D-Printing may offer the following opportunities:

• Reduction in wastage. There is a significant amount of material waste generated in the process of traditional insitu concrete. If 3D-Printing can truly offer elimination, or significant reduction, in formwork then there are obvious benefits to be gained environmentally. It also doesn’t end there. Concrete over-supply, over-pouring and construction errors all contribute to wastage. 3D-Printing offers concrete delivered precisely only where it is required, and using only the specific amount required. It should be noted however that an assumption of a net environmental benefit only holds true however if the structure itself is of comparable efficiency. It has yet to be seen if 3D-Printing structures can rival traditional forms of construction for efficiency and practicality. Equally though, for certain applications 3D-Printing could potentially offer a more efficient structure than traditional construction techniques can provide (based on traditional design techniques). More on this later.
• Speed of construction. One of the biggest attractions of 3D-Printing is the potential to build faster. This benefit is obvious for certain applications and not so obvious for others. Repetitive, low rise construction that requires significant manual labour (e.g. single storey blockwork) could be implemented much faster at scale with the smart adoption of 3D-Printing technology. This assumption is very dependent on the integration of 3D-Printing into the design, the scale of the development and the availability of raw materials. Of course, it also depends on our benchmark. Other technologies are evolving that will rival 3D-Printing in terms of speed (and other benefits) for certain forms of structure. For example, Fast Brick Robotics has the potential to be able to build a blockwork house much faster than traditional techniques.
• Reduction of labour. Along with the increased speed of construction comes an associated reduction in personnel on site. Putting aside the ‘robots are taking our jobs’ discussion, from a pure capitalist perspective, humans are expensive. Sure, labour costs vary significantly throughout the world, however over time these gaps will close due to international market forces. Even in places where labour is relatively cheap, the rise of (much needed) regulation regarding worker welfare, will also serve to increase costs to employers whilst the issues of quality and consistency of production will remain. 3D-Printing potentially offers to replace gangs of labourers with just one or two, technically proficient, supervisors and finishers. This argument for 3D-Printing will only become more persuasive with time.
• Health and safety. Hand-in-hand with the reduction of labour are benefits to health and safety. It’s quite obvious that less labour = lower risk exposure = less accidents. For the labour still on-site however 3D-Printing has the additional benefit of offering less hazards due to the elimination of formwork and associated craneage and manual handling.
• Quality and consistency. By replacing manual tasks with those controlled and implemented by precision technology human error is potentially reduced. Of course, ‘rubbish in = rubbish out’ still applies, so if the human operator gets something wrong then the system has the potential to execute the error with even greater speed and efficiency. The outcomes are also more predictable and consistent. A 3D-Printer can produce the same, identical structure over and over with utter reliability. Compare this to traditional construction where it is inevitable that even the smallest human error will result in an inconsistent final deliverable (by way of example, I have lived in two, supposedly identical houses in the same development, but found that furniture that fitted in one will not fit in the other!). Note that the term ‘quality’ here is focused on consistency of the outcome with respect to expectations. This is different to an interpretation of quality as intrinsically better (e.g. can a 3D-Printer produce a ‘better’ wall than a bricklayer?). Read Zen and The Art of Motorcycle Maintenance if you are interested in this philosophical debate.
• Plant simplification. 3D-Printing offers simplification of the construction process, and with it, simplification of the plant required. Elimination of formwork and blockwork means that less material has to be physically delivered and moved, either by crane or vehicle. The equipment components are also generic, and often scaleable, meaning that it can be used repeatedly in the same fashion across sites and from site-to-site. As technology develops and our paradigms for providing reinforcement also change, then it is possible we could see many construction sites with no craneage (especially as heavy-lifting drones come of age).
• Remote locations. All the above benefits make 3D-Printing an attractive solution in remote locations, often where low-cost, or emergency housing is required and access is difficult. With locally-sourced materials and a supply of water, in principle the construction of low-cost housing could be done almost anywhere in the world, with minimal use of imported binder material and the printer itself. Indeed, this is already being recognised, and it is not by chance that this is a potential market that is already being pursued.
• Architectural freedom and flexibility. An oft-lauded benefit of 3D-Printing is its ability to produce free-flowing, unhindered geometrical forms which can give greater architectural freedom and expression. Examining these claims more closely, and in the context of current technical constraints (discussed further below) we can see that these benefits, whilst not necessarily untrue in terms of potential, are certainly tempered. We need to be careful with our expectations here. We do see examples in manufacturing and bio-mechanical industries of 3D-Printing being used to manufacture complex components that would be impossible, or very difficult, to manufacture using subtractive methods or from casting in moulds. The ability for 3D-Printing to produce something geometrically complex of significant size is limited by both physical scale and material science. For example, at present cementitious 3D-Printing can produce in-plan curvature relatively easily, however anything more than subtle non-verticality is still a significant challenge – both in terms of layer-to-layer stability and also overall structural stability. Nevertheless, interesting geometrical forms can be created that would otherwise be impossible to create using traditional techniques.
• Bespoke design. In many respects 3D-Printing is the antithesis of modularisation. As the industry moves closer towards off-site fabrication and DfMA (Design for Manufacture and Assembly) there is a natural trend towards repetition, rationalisation, simplification and a copy-paste mentality. In parallel with increased digitisation, there is a natural place for this, however it can lead to uninspiring architecture and ultimately materially inefficient construction. 3D-Printing offers, in tandem with (albeit currently limited) geometrical freedom, an opportunity for uniqueness, for one-offs and for efficient bespoke design. Couple this with an ability to prepare 3D-Printed designs that are, at a fundamental level, entirely integrated with made-to fit components and geometrical arrangements, then the attraction becomes clear. Interestingly perhaps there is a spectrum of solutions that have yet to appear; blending the benefits of modularisation with 3D-Printed freedom of expression…..

"Interestingly perhaps there is a spectrum of solutions that have yet to appear; blending the benefits of modularisation with 3D-Printed freedom of expression...."

 What are the Challenges?

3D-Printing tends to get wrapped up into one amorphous entity by the uninitiated, and this makes discussion of its limitations and challenges too generic and hence difficult to untangle. It’s like a teacher writing one report card for a whole class – it tells us nothing about the performance of the individuals. So, its on this point that we need to be quite specific and look at individual technologies and applications. For the purposes of this article I’m going to keep the following discussion limited to 3D-Printing of cementitious structures, because this is where I believe there is the most potential for change, and also where there is the greatest need for an improved understanding;

• Material rheology. If you ask most in the business, they will tell you the greatest challenge for 3D-Printing is in the material itself. The primary issue here is the ability of the material to tread the fine line between extrudability (i.e. the ability to push the material through the nozzle) and strength and stiffness development; namely that sufficient to provide support to the next layer. This is not a simple equation, but comprises a plethora of parameters that interact in a way that only a few experts in the industry truly understand. Nozzle design, layer width, layer thickness, print speed, length of continuous print, ambient temperature, water temperature and mix design all play their part in this black-art. The industry is a very long way from providing standard mixes or guidance on these aspects, and much of the current knowledge is locked behind patents and NDAs.
• Reinforcement. I have for a while been using reinforced concrete as an example of how our industry has failed to develop new paradigms. This becomes patently apparent when we start to examine how a 3D-Printed concrete structure can incorporate the reinforcement needed to resist the actions imposed upon it. Current proof-of-concept examples approach this in one of two ways;

1. They ignore it, i.e. the buildings are not reinforced. Whilst this may prove to be a reasonable, practical approach for the simplest forms of construction in some geographies, we do not yet have an understanding of thin-walled, layered construction, in the same way we do blockwork or brickwork which has been used for hundreds, or even thousands, of years. We are therefore someway from developing a rule-based, codified approach to the use of this form of construction to provide strength and stability under load.
2. Discrete reinforcement is included. The 3D-Printed structure provides permanent formwork within which a traditional reinforced concrete structure can be created. This method has the benefit of potentially complying with current building standards but impairs the 3D-Printing process as it has to be coordinated with the placement of discrete reinforcement and the pouring of in-situ concrete. Additional formwork and temporary support must be provided in key areas such as lintels and ring-beams. In addition, unless the 3D-Printed elements can be shown to be acting compositely with the in-situ structure then the outcome is inevitably less efficient than a traditionally formed equivalent.

• Temporary stability. As mentioned before, whilst 3D-Printing offers the ability to place layers of concrete in unique and interesting geometries, if the vertical inclination exceeds the ability of one layer to overhang the next then the structure will become unstable whilst still gaining strength and the layers can ‘collapse’ during or after printing. Some systems include plastic or metal ‘ties’ between the inner and outer walls to mitigate stability issues of the individual walls, however this still does not prevent overall instability of the system where vertical inclinations are required. The degree to which vertical inclination can be accommodated therefore depends on the extruded properties of the layer being printed and the stiffened properties of the previous layers (all of which are tied to the rheology and mix parameters discussed above). Obviously, an assessment of wall stability is required to ensure the hardened wall section is stable during construction and temporary works may be necessary to provide support until any permanent structure proving support is completed - something that will detract from 3D-Printing's attractiveness.
• Interfaces, joints and connections. When we think about 3D-Printing it is convenient to imagine a structure that is 3D-Printed in its entirety; a structure that grows before our eyes, eventually providing a finished article complete with roof, windows, doors and all the other trimmings. Let’s not forget that buildings are made from thousands of components, structural and non-structural, all of which must be installed and connected. At present 3D-Printed structures are incomplete. They are part of the whole and must be constructed whilst interacting and connecting with these other components. This creates difficulty, as it requires the 3D-Printing process, and built form, to accommodate these interfaces. Simple examples might be connections to roof elements, lintels, door frames, electrical conduits and of course foundations. Each of these interfaces requires new details and will impact on the continuity and complexity of the printing process.
• Environmental performance. 3D-Printing is often hailed as less environmentally damaging that traditional construction (see earlier discussion on wastage), however one aspect that needs careful consideration is the energy and thermal performance of 3D-Printed structures. The ‘double-walled’ approach favoured by many of the 3D-Printing systems offers the potential for insulation to be included in between the cavity, however many systems actually connect the two thin walls together for stability, which creates thermal bridging issues. Of course, insulation can be provided on the exterior, however this hides the 3D-Printed structure (desirable or undesirable??) and may present its own problems if the geometry is irregular.
• Printing horizontal elements. Perhaps one of the most significant challenges for 3D-Printing is that it is naturally suited to the printing of vertical elements only. Printing of horizontal elements presents practical difficulties for the technology, for example printing a floor slab or a beam in-situ is simply not currently practicable. Alternative approaches exist, for example printing horizontal elements vertically in sections and then lifting and rotating them into place. Obviously horizontal elements generally work in flexure therefore prestressing is needed to overcome tension forces. Structures have been built using this technique, and other methods have been used however none are as practical or as simple to implement as vertical elements, and all require an additional degree of technical complication over a simple cast-in-situ element. Of course, some schemes look to overcome this limitation by creating domed structures, however the application of these seem limited for today’s diverse building stock.
• Codes and standards. As mentioned previously, there is currently an absence of 3D-Printing standards for construction, and this means that anything more than a ‘proof-of-concept’ (i.e. fully functional and compliant with building/municipal standards) requires demonstration using first principles and testing. This is often prohibitively expensive and time-consuming for consultants wishing to explore the technology, or developers looking to apply it to real projects. Thankfully the American Concrete Institute has recently set up ACI Committee 564 to develop and publish guidance, however it may be some time before we see a codified approach. Until then 3D-Printing will only take confident strides forward if the stakeholders take time to understand what is needed to bring a new technology to the market and give consultants and 3D-Printing companies the commercial space they need to deliver. 

So, I've highlighted the potential benefits of 3D-Printing and also had a sober look at some of the challenges and limitations that currently exist. I do not see this examination of the current challenges and limitations as a negative reflection on the potential of 3D-Printing. This is because I do not believe innovation comes through blind faith, moreover it comes about from a constructive appraisal of the challenges ahead, and looking with open eyes at how they may be overcome. I'll explore this further in Part 2.