The Application of 3D-Printing Techniques in the Manufacturing of Cement-Based Construction Products and Experiences Based on the Assessment of Such Products

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Article The Application of 3D-Printing Techniques in the Manufacturing of Cement-Based Construction Products and Experiences Based on the Assessment of Such Products

Guillermo Sotorrío Ortega 1,*, Javier Alonso Madrid 2 , Nils O. E. Olsson 3 and José Antonio Tenorio Ríos 1 1 Eduardo Torroja Institute for Construction Science, Spanish National Research Council (CSIC), 28033 Madrid, Spain; [email protected] 2 ATANGA Arquitectura e Ingeniería, 28692 Madrid, Spain; [email protected] 3 Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway; [email protected] * Correspondence: [email protected]

Received: 29 June 2020; Accepted: 15 August 2020; Published: 19 August 2020

Abstract: The construction industry has embraced digitisation and industrialisation in response to the need to increase its productivity, optimise material consumption and improve workmanship. Additive manufacturing (AM), more widely known as 3D printing, has driven substantial progress in these respects in other industries, and a number of national and international projects have helped to introduce the technique to the construction industry. As with other innovative processes not covered by uniform standards, appropriate assessments and testing methodologies to control the quality of the 3D-printed end products, while not obligatory, are advisable. This article shows that regulation is not an obstacle to the use of an innovative product, such as 3D printing, by proposing quality-control tests and an assessment methodology, in the understanding that standardisation ensures the viability of a technology. The information, including the methods and results, is based on the authors’ experiences in the development of three research projects pertaining to 3D printing. This paper also discusses whether the performance of the materials used in 3D printing could be superior to traditional ones.

Keywords: assessment; standards; concrete; cement; 3D printing; testing; additive manufacturing; construction

1. Introduction Both building and civil construction have traditionally been more reluctant to adopt innovative procedures than have other industries. However, this has been changing in recent years as the industry has modernised and adapted to the conditions imposed by the fourth industrial revolution, also known as Industry 4.0 and, in construction in particular, as Construction 4.0. This may herald a significant change in the construction industry [1]. The fourth industrial revolution builds upon digitalisation and includes a synthesis of production, management and governance systems [2]. Examples of currently emerging technologies include materials science, artificial intelligence (AI), the Internet of Things (IoT), autonomous vehicles, robotics, 3D printing and so forth [2]. These technologies can contribute to maintaining the value-adding activities in building on construction sites, as industrialisation in construction has traditionally focused on moving towards oﬀsite production and prefabrication [3]. The technological transformation will aﬀect economies, businesses, societies and politics [4]. Schwab [5] demonstrated that areas such as real estate and infrastructure will need to change design and construction processes to operate and maintain built assets.

Buildings 2020, 10, 144; doi:10.3390/buildings10090144 www.mdpi.com/journal/buildings Buildings 2020, 10, 144 2 of 18

The digitisation of the construction process and industrialisation are the two main foundations for the modernisation of the industry, with the aim of improving performance, optimising the spend on materials, decreasing costs, reducing energy consumption and enhancing the quality of workmanship. Additive manufacturing (AM), usually referred to as 3D printing, is a new technology that is indisputably linked to such objectives. It took other industries by storm years ago, and construction is no longer a stranger to this technique, which is being applied in many projects in Spain, Norway and other countries. Schwab and Davis [6] claimed that construction companies need to prepare for the disruption created by the widespread use of 3D printing and other new technologies and business models. AM technology includes the production of components based on a computer-aided design (CAD) model in which various materials can be used. In construction, building information modelling (BIM) models are typically used, and serve the purpose of CAD models in other industries. Three-dimensional printing is the process of making an object via a 3D CAD model by slicing the model into several 2D layers, with each layer being produced at a time [7]. Three-dimensional printing can be used to produce objects with complex geometries with low material waste [8]. The use of 3D-printing technology in the construction industry is still at an early stage of development. Paris and Mandil [9] pointed out some limitations and issues, such as high costs, resolution problems (meaning poor surface quality with rough and chunky outputs, resulting in reduced dimensional accuracy), and a long production time because printing speed is compromised by scale. A key question related to the implementation of this new technology is whether existing legislations are an obstacle to its introduction. The immediate answer is no. Legislation should not be an obstacle, although certain factors may be viewed as such in practice. The UK National Strategy for Additive Manufacturing [10] conducted a survey among a broad sample of private and public organisations to identify the factors most commonly perceived to constitute obstacles to 3D printing. These obstacles include:

the availability, standardisation and certiﬁcation of printing materials, • proper or suitable design methods to overcome the constraints on AM, • the lack of skilled and trained operators of 3D-printing equipment, • the high initial investment compared to other production methods, and • existing legislation and the lack of a comprehensive set of standards for 3D printing, with certification • and standardisation deemed to be key elements.

The aforementioned obstacles are also highly relevant to the general acceptance of 3D concrete printing to a greater or lesser extent, as the lack of uniform analyses and comparable results have generated doubt in the marketplace and amongst investors. The testing of 3D-printed concrete needs to be addressed for the technology to be developed. There is ongoing research, such as that conducted by Jayathilakage, Sanjayan and Rajeev [11], who described trials involving shear tests of 3D-printed concrete. They found that the shear rates had little effect on the cohesion values, and that the mixtures used followed the Mohr–Coulomb model. Wolfs, Bos and Salet [12] developed a numerical model to analyse the behaviour of 3D-printed concrete. The model was validated via a comparison to printing experiments. Jayathilakage, Rajeev and Sanjayan [13] also conducted laboratory experiments to study the properties of 3D-printed concrete. We provide a brief overview of the relevant regulations. CE marking is compulsory for construction products subject to a harmonised European standard (hEN), but not for innovative products in general. Inasmuch as standardisation always refers to existing products, a series of organisations have been entrusted with the technical assessment of new and innovative products. The basis for assessing innovative materials is a document published by the OECD in 1997 entitled The Measurement of Scientific and Technological Activities. Oslo Manual: Guidelines for Collecting and Interpreting Innovation Data [14]. It defines innovation as ‘the implementation of a new or significantly improved product (good or service), or process, a new marketing method, or a new organisational Buildings 2020, 10, x FOR PEER REVIEW 3 of 19 Buildings 2020, 10, x FOR PEER REVIEW 3 of 19 Buildingsnew organisational2020, 10, 144 method in business practices, workplace organisation or external relations’.3 ofThe 18 manualnew organisational distinguishes method amongst in business four practices,types of woinnovation:rkplace organisation product, orprocess, external marketing relations’. andThe organisation. Several of these are involved in AM or 3D printing, as the materials used, the methodmanual indistinguishes business practices, amongst workplace four organisationtypes of innovation: or external relations’.product, process, The manual marketing distinguishes and construction processes and the end products are all innovative. amongstorganisation. four typesSeveral of innovation:of these are product, involved process, in AM marketing or 3D printing, and organisation. as the materials Several of used, these arethe The European Union of Agrément (UEAtc) [15] is a network of European and neighbouring involvedconstruction in AM processes or 3D printing, and the asend the products materials are used, all innovative. the construction processes and the end products country institutes, centres and organisations engaged in the issuing of technical approvals. It aims to are allThe innovative. European Union of Agrément (UEAtc) [15] is a network of European and neighbouring lower the barriers to all types of innovation, and promotes a scientific approach towards the approval countryThe institutes, European centres Union and of Agr organisationsément (UEAtc) engaged [15] in is the a network issuing of of technical European approvals. and neighbouring It aims to of innovation in construction. The European Organisation for Technical Assessment of Construction countrylower the institutes, barriers to centres all types and of organisations innovation, and engaged promotes in the a scientific issuing of approach technical towards approvals. the Itapproval aims to Products (EOTA) coordinates the application of the procedures established for applying for a lowerof innovation the barriers in construction. to all types ofThe innovation, European and Organi promotessation afor scientific Technical approach Assessment towards of Construction the approval European Technical Assessment (ETA) and develops and adopts European Assessment Documents ofProducts innovation (EOTA) in construction. coordinates The the European application Organisation of the procedures for Technical established Assessment for ofapplying Construction for a (EAD). ProductsEuropean (EOTA) Technical coordinates Assessment the application(ETA) and ofdevel theops procedures and adopts established European for Assessment applying for Documents a European While the respective regulations are incumbent upon the member states, the European Technical(EAD). Assessment (ETA) and develops and adopts European Assessment Documents (EAD). Commission sets out minimum, Europe-wide safety and suitability requirements. Safety provisions While thethe respective respective regulations regulations are incumbentare incumb uponent theupon member the states,member the Europeanstates, the Commission European are laid down in non-mandatory standards, the Eurocodes, which co-exist with national legislation. setsCommission out minimum, sets out Europe-wide minimum, safety Europe-wide and suitability safety requirements.and suitability Safety requirements. provisions Safety are laid provisions down in Since Eurocodes are generally based on performance, they do not constitute an obstacle to the use of non-mandatoryare laid down in standards, non-mandatory the Eurocodes, standards, which the co-existEurocodes, with which national co-exist legislation. with national Since Eurocodes legislation. are innovative products, processes or designs, such as 3D printing. generallySince Eurocodes based on are performance, generally based they on do performance, not constitute they an obstacledo not constitute to the use an of obstacle innovative to the products, use of The member states do not have a common legislation for safety in the event of fire, or for processesinnovative or products, designs, suchprocesses as 3D or printing. designs, such as 3D printing. hygiene, health and the environment, safety of use, noise control, energy economics, or heat retention. The member states do not have a common legislationlegislation for safety in the event of fire,fire, or for Each member state sets its own national standards. hygiene, health and the environment, safety of use, noise control, control, energy energy economics, economics, or or heat heat retention. retention. As an innovative product, 3D concrete printing might be covered by an ETA, the alternative for Each member state sets its own nationalnational standards.standards. construction products that are not fully subject to a harmonised standard, although this would not As an innovative product, 3D concrete printing might be covered by an ETA, the alternative for be mandatory. ETAs are based on an EAD adopted by the EOTA, as illustrated in Figures 1 and 2. construction productsproducts thatthat areare not not fully fully subject subject to to a harmoniseda harmonised standard, standard, although although this this would would not not be mandatory.be mandatory. ETAs ETAs are are based based on anon EADan EAD adopted adopted by theby the EOTA, EOTA, as illustrated as illustrated in Figures in Figures1 and 1 2and. 2.

Figure 1. European Technical Assessment (ETA) and European Assessment Document (EAD). Figure 1. European Technical Assessment (ETA) and European Assessment Document (EAD). Figure 1. European Technical Assessment (ETA) and European Assessment Document (EAD).

Figure 2. IllustrativeIllustrative flowchart ﬂowchart for ETA. Figure 2. Illustrative flowchart for ETA. The purpose of this paper is to study the assessment of 3D-printed concrete construction objects. The research questions are: Buildings 2020, 10, x FOR PEER REVIEW 4 of 19

BuildingsThe2020 purpose, 10, 144 of this paper is to study the assessment of 3D-printed concrete construction objects.4 of 18 The research questions are: • How cancan 3D-printing3D-printing technologies, technologies, which which are are not coverednot covered by any by harmonised any harmonised standard, standard, be eligible be • foreligible some for type some of markingtype of marking to confirm to confirm quality andquality safety? and safety? • How can a 3D-printed object made of concrete bebe assessed?assessed? • What parts of the production process and the finished object should be included in the What parts of the production process and the finished object should be included in the assessment? • assessment? What special considerations need to be taken into account when assessing 3D-printed • What special considerations need to be taken into account when assessing 3D-printed concrete concrete objects? objects? 2. Materials and Methods 2. Materials and Methods A key characteristic of 3D printing of concrete is the interdependency of the design, material, processA key and characteristic product properties. of 3D printing The setting of concrete reaction ofis the concrete interdependency affects the printing of the design, speed, thematerial, pump pressure,process and filament product stacking properties. and otherThe setting factors reaction [12]. of concrete affects the printing speed, the pump pressure, filament stacking and other factors [12]. 2.1. New 3D-Printing Materials 2.1. New 3D-Printing Materials Two types of cement-, concrete- or mortar-based materials are used in AM: those developed explicitlyTwo fortypes a given of cement-, project, concrete- and those or available mortar-bas previouslyed materials on the are market. used Irrespective in AM: those of theirdeveloped origin, explicitlythese materials for a given must project, meet the and following those available technical previously requirements: on the market. Irrespective of their origin, these materials must meet the following technical requirements: They must be printable; that is, they must conserve fresh-state workability for as long as required • toThey be pumpedmust be printable; easily. that is, they must conserve fresh-state workability for as long as required Atto be the pumped same time, easily. upon extrusion, the materials must be sufficiently firm to maintain their shape • underAt the theirsame owntime, weight, upon extrusion, either by the setting materials rapidly must or as be a sufficiently result of their firm thixotropy. to maintain their shape under their own weight, either by setting rapidly or as a result of their thixotropy. They must develop strength in a short period to support the weight of the successive layers • They must develop strength in a short period to support the weight of the successive layers applied during printing. applied during printing. They must not harden too rapidly; however, ensuring cohesion between successive layers can • They must not harden too rapidly; however, ensuring cohesion between successive layers can only be guaranteed if both the lower and upper layers are fresh. only be guaranteed if both the lower and upper layers are fresh. Their mechanical strength must be in keeping with their envisaged use. • Their mechanical strength must be in keeping with their envisaged use. Figure3 3 shows shows anan exampleexample ofof aa verticalvertical 3D-printing3D-printing test.test.

FigureFigure 3. VerticalVertical printing test with material developed within the 3DCONS project.

The options adopted for materials vary, from commercial products that are mixed and pumped into the printing facility to plant materials prepared explicitly for 3D printing, or others admixtured at Buildings 2020, 10, x FOR PEER REVIEW 5 of 19

The options adopted for materials vary, from commercial products that are mixed and pumped Buildingsinto the2020 printing, 10, 144 facility to plant materials prepared explicitly for 3D printing, or others admixtured5 of 18 at the printing nozzle. The use of those three types of materials is exemplified in the present description of three recent projects: Print ‘n’ Build (commercial), 3DCONS (customised) and the printing nozzle. The use of those three types of materials is exempliﬁed in the present description of HINDCON (admixtured). three recent projects: Print ‘n’ Build (commercial), 3DCONS (customised) and HINDCON (admixtured).

2.2.2.2. New WorkflowWorkflow Design AsAs partpart ofof thethe completecomplete process,process, newnew formsforms areare neededneeded toto meetmeet thethe specialspecial requirementsrequirements ofof thethe materialisationmaterialisation process of overlappingoverlapping additive layers; but also, to take advantageadvantage of thethe specialspecial materialmaterial andand processprocess characteristics.characteristics. TheThe relationshiprelationship between form and function, which is a classic dilemma in architectural design, isis eveneven moremore relevantrelevant forfor 3D3D printing,printing, asas thethe geometricgeometric volumevolume mustmust fulfilfulfil thethe functionfunction ofof thethe partpart oror object,object, andand needsneeds toto be constructed viavia newnew processesprocesses inin continuouscontinuous evolution.evolution. In addition, materials cancan determinedetermine form,form, asas this is related strongly to their technical capacities, andand maymay even have special configurationsconfigurations thatthat can can change change during during the the deposition depositio process,n process, as a variableas a variable additive additive or even or the even natural the hardeningnatural hardening of concrete of concrete in the mix in the recipient mix recipient or lower or layers.lower layers. Therefore, Therefore, design design is also is relatedalso related to and to aandffected affected by aspects by aspects such assuch the as angle the betweenangle between layers, thelayers, widening the widening of line fabrication of line fabrication from the extruder,from the theextruder, end diameter the end and diameter the thickness and th ofe thickness the extruder. of the The extruder. general relationshipThe general of relationship design, materials of design, and machinesmaterials isand illustrated machines in is Figure illustrated4. in Figure 4.

FigureFigure 4.4. Relationship of design, materials and machines.

InIn thisthis process,process, parametricparametric designdesign isis essential,essential, asas parametrising thethe valuesvalues ofof geometric partsparts can allowallow designersdesigners to to personalise personalise designs designs for for diff erentdifferent environments, environments, needs, needs, material material capacities capacities or simple or aestheticsimple aesthetic appeal withappeal ease. with ease. 2.3. New 3D-Printing Processes 2.3. New 3D-Printing Processes Construction projects involving AM have been implemented via different approaches, including Construction projects involving AM have been implemented via different approaches, including the in situ printing of complete buildings or building elements, or printing over existing elements, the in situ printing of complete buildings or building elements, or printing over existing elements, such as in façade restoration or precasting. such as in façade restoration or precasting. A 3D-printing system for concrete has three major components, in which different parameters A 3D-printing system for concrete has three major components, in which different parameters and variables are grouped under each category [16]: and variables are grouped under each category [16]: • Printable concrete: composition, aggregate size, additives, admixtures and open time. • Printable concrete: composition, aggregate size, additives, admixtures and open time. • Three-dimensional printer: pump pressure, flow, robot speed, acceleration, system length, system • Three-dimensional printer: pump pressure, flow, robot speed, acceleration, system length, friction,system friction, nozzle geometry, nozzle geometry, temperature temperature and humidity. and humidity. • Print geometry: filament, overall shape, dimensions, curvatures, strength and stiffness. In the 3D • Print geometry: filament, overall shape, dimensions, curvatures, strength and stiffness. In the printing3D printing of concrete, of concrete, it is importantit is important that the that concrete the concrete retains retains its shape its aftershape extrusion. after extrusion. It should It alsoshould be ablealso to be carry able subsequent to carry subsequent layers without layers deformation. without deformation. It is difficult toIt stackis difficult concrete to layersstack becauseconcrete of layers the material’s because of setting the material’s time, slump setting and time, flow slump behaviour and [flow17]. behaviour [17]. As in the case of the materials, very different construction processes have been developed. New construction processes are dictated by the type of printer used. The Print ‘n’ Build (Figure5), Buildings 2020, 10, x FOR PEER REVIEW 6 of 19

As in the case of the materials, very different construction processes have been developed. New Buildingsconstruction 2020, 10, xprocesses FOR PEER areREVIEW dictated by the type of printer used. The Print ‘n’ Build (Figure 5), and6 of 19 Buildings3DCONS2020, 10(D-Shape, 144 and TU Eindhoven) projects used bridge crane printers, while the HINDCON6 of 18 projectAs in (Figure the case 6) of used the materials,a cable robot. very Adifferent third type construction of facility, processes robot arms have such been as developed.Apis-Cor and New constructionCOBOD, is processesalso available. are dictated by the type of printer used. The Print ‘n’ Build (Figure 5), and 3DCONSand 3DCONS (D-Shape (D-Shape and and TU TUEindhoven) Eindhoven) projects projects used used bridge bridge crane crane printers, printers, while while the the HINDCON HINDCON project (Figure(Figure6 )6) used used a cablea cable robot. robot. A third A third type oftype facility, of facility, robot armsrobot such arms as Apis-Corsuch as Apis-Cor and COBOD, and COBOD,is also available. is also available.

Figure 5. Print ‘n’ Build project printer.

Of particular note is the HINDCON project, in which additive and subtractive manufacturing were integrated in a single robot (Figure 6). In other words, parts of the printed piece were cut away using subtractive methods for greaterFigure freedom5. PrintPrint ‘n’ ‘n’ of Build Builddesign. project project printer.

(a) (b)

FigureFigure 6. 6.HINDCON HINDCON project project ‘all-in-one’‘all-in-one’ printer;printer; ( a)) additive additive system system and and (b (b) subtractive) subtractive system. system.

OfA particular number of note stages is that the HINDCONform part of the project, construc in whichtion process, additive such and as data subtractive collection manufacturing and design, wereprecede integrated printing in aper single se. Data robot only (Figure need 6be). gathered In other words,when the parts surface of the to printedbe printed piece over were is irregular, cut away usingas in subtractive the case of methodsan existing for structure.( greatera) freedomBuilding ofa construction design. element may therefore(b) be divided into fourA stages, numberFigure 6.all ofHINDCON of stages which that must project form be ‘all-in-one’assessed part of the as printer; parts construction of(a )the additive whole: process, system such and as (b data) subtractive collection system. and design, precede• Three-dimensional printing per se. Data data onlycollection, need beprocessing gathered and when interpretation: the surface depending to be printed on overthe needs is irregular, and as inA the numberthe case scope of of an ofstages existing the thatintervention, structure. form part Buildingdataof the may construc a be construction actionquired process, via element different such may as datamethods, therefore collection such be divided and as design,laser into precedefour stages,photogrammetry printing all of per which se. orData must thermo only be assessedgraphic need be photography. as gathered parts of when the whole: the surface to be printed over is irregular, as in Three-dimensionalthe case of an existing data structure. collection, Building processing a construction and interpretation: element may dependingtherefore be on divided the needs into • four andstages, the all scope of which of the must intervention, be assessed data as parts may of be the acquired whole: via diﬀerent methods, such as laser • photogrammetryThree-dimensional or data thermographic collection, processing photography. and interpretation: depending on the needs and Printthe scope design: of thethe printer’s intervention, path, speed,data may ﬂow andbe ac soquired on must via be different designed tomethods, accommodate such as the laser data • gatheredphotogrammetry to optimise or thermo timinggraphic and cost. photography. Buildings 2020, 10, 144 7 of 18 Buildings 2020, 10, x FOR PEER REVIEW 7 of 19

• PrintingPrint perdesign: se: the printer’s extruder path, paths, speed, tolerances, flow and speed,so on must and be amount designed of to material accommodate applied the must • data gathered to optimise timing and cost. be monitored. • Printing per se: the extruder paths, tolerances, speed, and amount of material applied must be Quality control: upon completion, an established protocol must be applied to ensure the process • monitored. was• performedQuality control: according upon completion, to the speciﬁcations an established and theprotocol quality must of thebe applied end product to ensure must the be process conﬁrmed. was performed according to the specifications and the quality of the end product must be Depending on the 3D-printing process, it may be necessary to establish and verify the values of confirmed. factors such as the printing speed. Depending on the 3D-printing process, it may be necessary to establish and verify the values of 2.4. Newfactors 3D-Printed such as the Construction printing speed. Products

A2.4. challenge New 3D-Printed of 3D Construction printing is Products slower build time than that of cast-based manufacturing due to the layered printing approach. The reduction in build time is linked to build complexity. Printing A challenge of 3D printing is slower build time than that of cast-based manufacturing due to the resolutionlayered and printing the level approach. of detail The depend reduction on the in thicknessbuild time of is the linked layers. to Acceleratingbuild complexity. the printingPrinting time can compromiseresolution and the the level level of of detaildetail depend in the printed on the thickness object [18 of]. the layers. Accelerating the printing time Dicanff erentcompromise types ofthe elements level of detail are subject in the printed to diff erentobjectrequirements [18]. depending on the envisaged use. Oher conditionsDifferent are types imposed of elements by the formalare subject and to functional different designrequirements of the objectdepending to be on printed, the envisaged including its geometryuse. Oher (thickness, conditions height are andimposed maximum by the dimensions),formal and functional equipment design and of process the object parameters to be printed, (extruder diameterincluding and maximumits geometry aggregate (thickness, size) height and an thed characteristicsmaximum dimensions), of the constituent equipment materialand process (fluidity, thixotropy,parameters temperature, (extruder density,diameter setting and maximum times and soaggregate on). size) and the characteristics of the constituent material (fluidity, thixotropy, temperature, density, setting times and so on). 2.5. Examples of 3D-Printed Construction Products 2.5. Examples of 3D-Printed Construction Products Figure7 shows the design for a 3D printed column using the HINDCON prototype and the final Figure 7 shows the design for a 3D printed column using the HINDCON prototype and the final product.product. The The types types of objectsof objects that that were were considered considered and how how they they were were evaluated evaluated is also is also shown. shown.

FigureFigure 7. HINDCON7. HINDCON model model forfor a 3D-printed column column by byAtanga. Atanga.

In theIn HINDCON the HINDCON project, project, the following the following items items were analysedwere analysed for the for additive the additive/subtractive/subtractive processes: processes: 1. 1.Wall Wall

a. a. Entrance:Entrance: personalised personalised entrance numbering numbering or orname name decoration decoration

b. b.Technical:Technical: hospital hospital wallwall withwith technic technic MEP MEP inner inner items items c. Lattice: holed wall c. Lattice: holed wall d. Organic: freeform design based on nature 2. Stairs 3. Roof Buildings 2020, 10, 144 8 of 18 Buildings 2020, 10, x FOR PEER REVIEW 8 of 19

4. Structurald. Organic: freeform design based on nature 5. 2. OtherStairs 3. Roof 4.The Structural following parameters were included for product determination (Figure8), ranked from 1 (less) to 35. (more): Other Popular:The following number parameters of items existing were included in conventional for product buildings determination (Figure 8), ranked from 1 • (less)Complexity: to 3 (more): diffi culty of fabrication • •Cost: Popular: monetary number amount of items to purchase existing in conventional buildings • •Coordination: Complexity: need difficulty for relationships of fabrication among different agents • • Materials: Cost: monetary difficulty amount of adapting to purchase/developing materials for additive processes • • Coordination: need for relationships among different agents Geometry: complexity of the geometric definition • • Materials: difficulty of adapting/developing materials for additive processes • Geometry: complexity of the geometric definition

popular complexity cost coordination materials geometry

1. WALL

1.1. Entrance

1.1.1. Outside 2 2 2 2 2 1

1.1.2. Inside 3 1 1 2 2 1

1.2. Technical

1.2.1. Outside 2 2 3 3 3 2

1.2.2. Inside 3 1 1 2 2 1

1.3. Lattice

1.3.1. Outside 2 3 2 2 2 3

1.3.2. Inside 1 1 1 1 1 2

1.4. Organic

1.4.1. Outside 1 3 2 2 2 3

1.4.2. Inside 1 1 1 2 1 2

2. STAIR

2.1. Outside 2 3 3 3 2 2

2.2. Inside 3 3 3 3 2 1

3. ROOF

3.1. Parts 3 2 3 2 2 1

3.2. Ceiling floor 2 1 1 1 1 1

3.3. Joints 2 2 2 2 2 2

4. STRUCTURAL

4.1. Joints 2 3 3 2 3 3

4.2. Coordination 2 3 2 2 3 2

5. OTHERS

FigureFigure 8. 8.HINDCON HINDCON product analysis analysis..

2.6.2.6. AM AM Products: Products: Requirements Requirements and and Characteristics Characteristics As notedAs noted previously, previously, legislative legislative requirements requirements shouldshould be be related related to to the the envisaged envisaged use, use, and and must must thereforetherefore be speciﬁedbe specified separately separately for for each each application. application. The The European European Commission Commission sets sets out out minimum minimum safetysafety and and suitability suitability requirements, requirements, the the Eurocodes, Eurocodes, which co-exist co-exist with with national national building building codes codes that that lay down the safety requirements. No common safety legislation is in place for ﬁre prevention, or for hygiene, health and the environment, safe use, noise control, energy savings or heat retention. Buildings 2020, 10, 144 9 of 18

Building regulations can be seen as a barrier [3]. The main reason is that traditional products are regulated by standardisation processes, and building regulations are often supported by standardisation schemes. In order to solve this problem when introducing new products, certain European countries created the UEAtc organisation. The European Union for technical approval in construction is a de facto partnership that brings together national institutes, centres or organisations that are engaged in the issuing of technical approval, on a voluntary basis. The purpose of the UEAtc is to reduce barriers of any kind and to increase and promote a scientific approach to the approval of innovation in the field of construction. Institutions that are able to develop the assessments of the previous organisation are integrated at the national level. Building regulations are put in place on many levels for different reasons. In construction, CE marking is mandatory for any product covered by a hEN, but not for innovative products in general. Regulations should not be barriers to new construction products or processes. However, as the stated values of material and product properties that accompany CE marking are used as input for the calculations needed to design a product structure according to the Eurocodes, the development of a technical assessment for new and innovative products is necessary. Three-dimensionally printed concrete, as an innovative product, should be covered by an ETA as an alternative for construction products not fully covered by a harmonised standard. The ETA will be developed based on an EAD adopted by the EOTA.

2.7. Assessment Methods Assessment methods to ensure compliance with the quality standards in different domains should vary depending on the envisaged use. While materials may be laboratory tested to assess their structural properties, the assessment of the construction process as a whole requires information about the printing technology and the optimal values of parameters such as the printing speed, volume, temperature and so on. Cement-based mortar and concrete should be subject to laboratory tests to determine their chemical composition, consistency, air content, density, setting times, bonding, hardened mechanical strength and durability. The present paper is based on the extensive testing of materials obtained from the Print ń’ Build, 3DCONS and HINDCON projects. These materials can be described more precisely as mortars rather than as concrete, as concrete is supposed to contain aggregates that have a larger maximum size. The mechanical properties can be divided into long-term and short-term properties. Long-term properties refer the strength of the material once hardened; this is mainly characterised by the compression and the flexion strength after one, seven and 28 days using protocols that are employed widely in the construction industry, according to standard EN 1015-11. In these regards, the properties of 3D-printable concrete do not differ significantly from the properties of regular cementitious materials. Short-term properties refer to the mechanical properties displayed in the first few hours, in which the material must first hold its own weight, then absorb the weight of the subsequent layers. In order to maintain the shape of the structure, experience has proven that a target value of 2 kPa in shear resistance should be achieved. The strength acquisition over time during the first few hours depends on the processing scenario, including the geometry of the item being printed, the speed of processing and so forth. Tests are not a definitive answer. Many of the measurements of strength acquisition were obtained from samples produced in the laboratory; others were obtained directly from the 3D printer (Figure9). Buildings 2020, 10, x FOR PEER REVIEW 10 of 19

• The nozzle additives are added and incorporated using a mild mixing stage to emulate the conditions in the extrusion nozzle (30 s of mixing at a low speed). The “t = 0” reference, which is extremely important for characterisation in the early stages, is set as the instant at which the nozzle additive is added to the mix (before 30 s of secondary mixing). There is no guarantee that this procedure safely reproduces the conditions in the extruder. This isBuildings particularly2020, 10, critical 144 in the shortest time range when sensitivity to mixing and processing is10 more of 18 acute.

(a) (b)

Figure 9. HINDCONHINDCON project; project; ( aa)) taking taking samples samples and and ( (b)) material samples.

TheIn both choice cases, of thesetting procedure the waiting aims totime reproduce to a fixed the value conditions of 10 min in an is actual also likely 3D-printing to have system, some impacts.as follows: A batch that has had more time to mature prior to the addition of the accelerator is likely to display a faster strength acquisition than relatively recent batches tested via the standard method. The mortar is prepared in batches of one to two litres in standard laboratory concrete mixers. Both• of these factors will be the subject of further scrutiny in the future. The material is left to rest for 10 min. • Special protocols were needed to determine the short-term strength acquisition (in the first few The nozzle additives are added and incorporated using a mild mixing stage to emulate the hours),• as few standards to assess performance at these levels of strength and maturity exist. Two methodsconditions were employed: in the extrusion nozzle (30 s of mixing at a low speed). • AThe scissometer, “t = 0” reference, or shear which vane, is was extremely used to important measure forthe characterisation material in the inearliest the early stages. stages, A cross- is set as theshaped instant pin at whichis inserted the nozzle into additivethe mortar. is added The operat to theor mix rotates (before it 30manually s of secondary until the mixing). torque is sufficientThere is no to guarantee shear a cylindrical that this procedure portion of safely the material reproduces from the the conditions bulk. The in maximum the extruder. torque This is particularlymeasured critical by an in apparatus, the shortest and time the range maximum when sensitivityshear stress to can mixing be assessed. and processing is more acute. • AThe special choice compression of setting the test waiting was developed time to a fixed for valueslightly of more 10 min mature is also likelysamples. to have Cubes some of 5 impacts. × 5 × 5 A batchcm thatwere has prepared had more using time fresh to mature concrete, prior and to were the additiontested using of the a low-force accelerator press. is likely The todevice display and a fasterthe strength method acquisition of sample than preparation relatively allowed recent batchesfor the measurement tested via the of standard compression method. strength Both of values these factorsas willlow beas the10 subjectkPa for of maturities further scrutiny as short in theas future.15 min. The system also allowed for the large displacementSpecial protocols (greater were than needed 10%) tonecessary determine for thecharacterising short-term the strength material, acquisition while it still (in theexhibits first few hours),highly plastic as few behaviour. standards to assess performance at these levels of strength and maturity exist. Two methods were employed: Each test has specific advantages and time frames for use. Shear testing is a robust field test that can beA scissometer,adapted for oronsite shear testing, vane, was and used it provides to measure almost the materialimmediate in the results. earliest However, stages. A it cross-shaped has limited • precisionpin is and inserted can prove into the to mortar.be operator The operator dependant rotates; moreover, it manually it isuntil not theoptimal torque when is su thefficient shear to resistanceshear exceeds a cylindrical 100 kPa. portion of the material from the bulk. The maximum torque is measured by an Compressionapparatus, and testing the maximum is more precise shear stressand is cannot belimi assessed.ted in terms of the higher ranges of resistance, but itA cannot special provide compression results test for was very developed short time for ranges slightly (sample more mature preparation samples. takes Cubes at least of 5 15 5min),5cm or • × × if thewere material prepared is not using sufficiently fresh concrete, solid to andform were well-defined tested using cubes a low-force (below 10 press. kPa in The compression device and strength).the method The data of are sample also preparationnot directly allowedcomparable for, the as one measurement set is measured of compression in terms of strength shear, and values the as low as 10 kPa for maturities as short as 15 min. The system also allowed for the large displacement (greater than 10%) necessary for characterising the material, while it still exhibits highly plastic behaviour.

Each test has specific advantages and time frames for use. Shear testing is a robust field test that can be adapted for onsite testing, and it provides almost immediate results. However, it has limited Buildings 2020, 10, 144 11 of 18 precision and can prove to be operator dependant; moreover, it is not optimal when the shear resistance exceeds 100 kPa. Compression testing is more precise and is not limited in terms of the higher ranges of resistance, but it cannot provide results for very short time ranges (sample preparation takes at least 15 min), or if theBuildings material 2020, is10,not x FOR su ffiPEERciently REVIEW solid to form well-defined cubes (below 10 kPa in compression strength).11 of 19 The data are also not directly comparable, as one set is measured in terms of shear, and the other in termsother in of terms compression. of compression. However, However, the range the of range both techniques of both techniques overlaps overlaps in a suffi cientlyin a sufficiently wide range wide to allowrange forto allow extrapolation for extrapolation to the compression to the compression strength in strength the short-term in the range,short-term andto range, shear and resistance to shear in theresistance long term in the (Figure long 10term). (Figure 10).

Figure 10. Schematic of the young age characterisation procedure.

The ‘pull-o‘pull-off’ﬀ’ adhesion adhesion test test is ais tension a tension test thattest isth conductedat is conducted in the laboratory.in the laboratory. It involves It involves printing theprinting mortar the onto mortar the surface onto the of asurface large-format of a large-fo brick andrmat using brick epoxy and using resin toepoxy glue severalresin to metal glue discsseveral to thismetal layer discs of to impression this layer material;of impression tensile material; force is thentensile applied force is to then the discsapplied once to thethe timediscs needed once the for time the resinneeded or for adhesive the resin material or adhesive to harden material has elapsed. to harden has elapsed. The testtest isis carriedcarried out out with with two two main main objectives: objectives: the the ﬁrst first is tois to estimate estimate the the surface surface resistance resistance of the of impressionthe impression material, material, and the and other the isother to evaluate is to evaluate the strength the ofstrength the connection of the connection between the between impression the materialimpression and material the support; and the in thissupport; case, in ceramic. this case, ceramic. The general procedure procedure followed followed in in this this trial trial can can be besummarised summarised according according to the to following the following four fourphases: phases:

• Step 1: Marking and preparing the test area, whichwhich must be perfectly devoid of any residue that • wouldwould preventprevent thethe goodgood adhesionadhesion ofof thethe testtest disc.disc. • StepStep 2:2: Cutting thethe testtest area area to to which which the the test test disc disc will will later later be be fixed fixed using using a diamond a diamond crown crown drill drill bit. • Stepbit. 3: Placing the disc on the surface of the impression material using an epoxy resin. This bonding • materialStep 3: Placing is highly the resistant, disc on setsthe surface quickly, of and the reaches impression tensile material strength using values an ofepoxy around resin. 10 MPaThis whenbonding fully material cured. is Hardening highly resistant, usually sets takes quickly, two to and five reaches minutes. tensile strength values of around Step10 MPa 4: Execution when fully of cured. the pull-o Hardeningff test, in usually which thetakes direct two tensionto five minutes. needed to be applied to the disc • toStep detach 4: Execution it from the of the element pull-off to whichtest, in it which had been the direct attached tension is quantified. needed to be applied to the disc to detach it from the element to which it had been attached is quantified. In general, the direct tension or adhesion stress can be obtained as the quotient between the In general, the direct tension or adhesion stress can be obtained as the quotient between the maximum force obtained in the test and the area of the interface; this is correct if the fault occurs maximum force obtained in the test and the area of the interface; this is correct if the fault occurs entirely in this zone. Peel strength and the failure pattern of the coating are important properties, entirely in this zone. Peel strength and the failure pattern of the coating are important properties, and and both parameters should be taken into account when evaluating the quality of the results of one of both parameters should be taken into account when evaluating the quality of the results of one of these tests, as well as to establish the general specifications of a project. these tests, as well as to establish the general specifications of a project. Failure between print layers indicates that the interface resistance is greater than the tensile strength between the layers. Failure within the impression material indicates that the bond strength is greater than is the tensile strength of the overlay material. Two important elements to consider are the magnitude of the failure with respect to the tensile strength of the material, and the preparation of the surface prior to the printing process. The pull-off test determines the highest perpendicular (direct) tension that the surface can withstand before it is separated from the printing surface. Failure will occur along the weakest plane Buildings 2020, 10, 144 12 of 18

Failure between print layers indicates that the interface resistance is greater than the tensile strength between the layers. Failure within the impression material indicates that the bond strength is greater than is the tensile strength of the overlay material. Two important elements to consider are the magnitude of the failure with respect to the tensile strength of the material, and the preparation of the surface prior to the printing process. The pull-oﬀ test determines the highest perpendicular (direct) tension that the surface can withstandBuildings before 2020, 10 it, xis FOR separated PEER REVIEW from the printing surface. Failure will occur along the weakest12 of 19 plane withinwithin the system the system consisting consisting of the of the test test equipment, equipmen thet, the adhesive adhesive material, material, the the impression impression materialmaterial and the interfaceand the interface of the union of the with union the with support, the suppo leavingrt, leaving the fracturethe fracture surface surface exposed. exposed.

3. Results3. Results FigureFigure 11 below 11 below shows shows the the results results of of initial initial setting-timesetting-time tests tests conducted conducted via viaa Vicat a Vicat apparatus apparatus (UNE-EN(UNE-EN 196-3) 196-3) [19 ][19] on oneon one of of the the 3DCONS 3DCONS project project mortars.mortars. The The graph graph shows shows that that the thesetting setting time time increasedincreased in accordance in accordance with with the the water water/cement-based/cement-based material ratio ratio (w/m). (w/m). This This finding, ﬁnding, along along with with otherother parameters, parameters, may may be appliedbe applied to to determine determine thethe mostmost suitable suitable ratio. ratio.

Figure 11. Results of the initial setting-time test for 3DCONS project mortars. Figure 11. Results of the initial setting-time test for 3DCONS project mortars. Freeze/thaw cycling is one of the durability tests that can be conducted on printing materials. Freeze/thaw cycling is one of the durability tests that can be conducted on printing materials. The The existence (or non-existence) of reinforcements that may be subject to corrosion determines the existence (or non-existence) of reinforcements that may be subject to corrosion determines the need for need for other tests, such as chloride ingress and carbonation resistance. Bonding after freeze/thaw other tests, such as chloride ingress and carbonation resistance. Bonding after freeze/thaw cycles (see cyclesFigure (see Figure12) is another 12) is anotherdurability durability trial that may trial be that advisable, may be depending advisable, on depending the expected on use. the expected use. Figure 13 shows the results of tests to determine the strength of the bond to the substrate (ceramic in this case) in the 3DCONS project, in which a mean of 0.63 kN was recorded. At 4.57 kN, the mean inter-layer strength observed in the HINDCON project (Figure 14) was much greater. Although these findings refer to different materials and different types of bonding, the recommended values were attained in both cases. Buildings 2020, 10, x FOR PEER REVIEW 13 of 19

Buildings 2020, 10, 144 13 of 18 Buildings 2020, 10, x FOR PEER REVIEW 13 of 19

Figure 12. Pull-off adhesion test between layers (left) and to the substrate (right); HINDCON and 3DCONS projects.

Figure 13 shows the results of tests to determine the strength of the bond to the substrate (ceramic in this case) in the 3DCONS project, in which a mean of 0.63 kN was recorded. At 4.57 kN, the mean inter-layer strength observed in the HINDCON project (Figure 14) was much greater. Although these findings refer to different materials and different types of bonding, the recommended FigureFigure 12. Pull-o12. Pull-offﬀ adhesion adhesion test test between between layerslayers ( leftleft)) and and to to the the substrate substrate (right (right); HINDCON); HINDCON and and values were attained in both cases. 3DCONS3DCONS projects. projects.

Buildings 2020, 10Figure, x FOR 13. PEERResults REVIEW of the material-substrate pull-oﬀ adhesion tests: 3DCONS project. 14 of 19 Figure 13. Results of the material-substrate pull-off adhesion tests: 3DCONS project.

Figure 13. Results of the material-substrate pull-off adhesion tests: 3DCONS project.

FigureFigure 14. 14. ResultsResults of of the the inter-layer inter-layer pull-off pull-oﬀ adhesionadhesion tests: tests: HINDCON HINDCON project project (sample (sample 1–5 1–5 ( (leftleft)) and and 6–96–9 ( (rightright)).)).

TheThe compressive compressive and and flexural ﬂexural strength strength tests tests consis consistedted of of two two stages. In In the the first ﬁrst stage, stage, the the tests were conducted on specimens prepared using materi materialal mixed mixed in in the the laboratory laboratory or or sampled sampled from from the the printer. In the second stage, the tests were conducted on the printed element itself. In the former, which is used to test for very early age strength, the values for later ages could not be determined. Nonetheless, a comparison of the findings for the laboratory mixes and printer samples would reveal whether extrusion affects strength. The graphs in Figure 15 show the compressive strength development in ultra-high-performance concrete (UHPC) from 30 to 120 min, and Figure 16 for flexural/tensile strength development from 2 to 30 min. As the figures show, the strength increased in accordance with the temperature. With regard to the graphs showing compressive and flexural strength in Figure 17, the HINDCON project material developed an initial compressive strength of over 60 MPa and a 28-day strength of nearly 100 MPa; these values were much higher than they were for the other two materials. The difference in flexural strength (Figure 18) was less.

Figure 15. Early age compressive strength at 10, 20 and 30 °C (HINDCON project). Buildings 2020, 10, x FOR PEER REVIEW 14 of 19

Figure 14. Results of the inter-layer pull-off adhesion tests: HINDCON project (sample 1–5 (left) and 6–9 (right)).

The compressive and flexural strength tests consisted of two stages. In the first stage, the tests were conducted on specimens prepared using material mixed in the laboratory or sampled from the printer. In the second stage, the tests were conducted on the printed element itself. In the former, Buildingswhich2020 is, 10 used, 144 to test for very early age strength, the values for later ages could not be determined.14 of 18 Nonetheless, a comparison of the findings for the laboratory mixes and printer samples would reveal whether extrusion affects strength. printer. InThe the graphs second in Figure stage, 15 the show tests the were compressive conducted strength on thedevelopment printed element in ultra-high-performance itself. In the former, whichconcrete is used (UHPC) to test from for very30 to early120 min, age and strength, Figure 16 the for values flexural/tensile for later strength ages could development not be determined. from 2 Nonetheless,to 30 min. aAs comparison the figures show, of the the findings strength for increased the laboratory in accordance mixes andwith printerthe temperature. samples would reveal whether extrusionWith regard aff ectsto the strength. graphs showing compressive and flexural strength in Figure 17, the HINDCONThe graphs project in Figure material 15 show developed the compressive an initial compressive strength development strength of over in 60 ultra-high-performance MPa and a 28-day concretestrength (UHPC) of nearly from 100 30 to MPa; 120 min,these and values Figure were 16 muchfor flexural higher/tensile than they strength were developmentfor the other fromtwo 2 to materials. The difference in flexural strength (Figure 18) was less. 30 min. As the figures show, the strength increased in accordance with the temperature.

Buildings 2020, 10, x FOR PEER REVIEW 15 of 19 FigureFigure 15. 15.Early Early age age compressive compressive strengthstrength at 10, 10, 20 20 and and 30 30 °C◦ C(HINDCON (HINDCON project). project).

Buildings 2020, 10, x FOR PEER REVIEW 15 of 19

FigureFigure 16. 16.Early Early age age ﬂexural flexural strengthstrength at at 10, 10, 20 20 and and 30 30 °C◦ C(HINDCON (HINDCON project). project).

With regard to the graphs showing compressive and flexural strength in Figure 17, the HINDCON project material developed an initial compressive strength of over 60 MPa and a 28-day strength of nearly 100 MPa; these values were much higher than they were for the other two materials. The difference in flexuralFigure 16. strength Early age (Figure flexural 18strength) was at less. 10, 20 and 30 °C (HINDCON project).

Figure 17. Compressive strength of Print ‘n’ Build, 3DCONS and HINDCON mortars.

FigureFigure 17. 17.Compressive Compressive strength strength of of Print Print ‘n’‘n’ Build, 3DCONS 3DCONS and and HINDCON HINDCON mortars. mortars.

Figure 18. Flexural strength of Print ’n’ Build, 3DCONS and HINDCON mortars.

The tests of the constituent materials should be followed by an assessment of the end product parameters, including the following (also see Figure 19):

• Dimensions and tolerances: the length, width and height and their tolerances provided by the manufacturerFigure are 18. to Flexural be specified strength on of the Print assessment ’n’ Build, 3DCONSsheet. and HINDCON mortars. • Thermal and acoustic insulation: compliance with the provisions for these parameters set out in The tests of the constituent materials should be followed by an assessment of the end product existing legislation must be determined. parameters, including the following (also see Figure 19): • Dimensions and tolerances: the length, width and height and their tolerances provided by the manufacturer are to be specified on the assessment sheet. • Thermal and acoustic insulation: compliance with the provisions for these parameters set out in existing legislation must be determined. Buildings 2020, 10, x FOR PEER REVIEW 15 of 19

Figure 16. Early age flexural strength at 10, 20 and 30 °C (HINDCON project).

Buildings 2020, 10, 144 15 of 18 Figure 17. Compressive strength of Print ‘n’ Build, 3DCONS and HINDCON mortars.

Figure 18. Flexural strength of Print ’n’ Build, 3DCONS and HINDCON mortars. Figure 18. Flexural strength of Print ’n’ Build, 3DCONS and HINDCON mortars. The tests of the constituent materials should be followed by an assessment of the end product The tests of the constituent materials should be followed by an assessment of the end product parameters,parameters, including including the the following following (also (also see see FigureFigure 1919):): •Dimensions Dimensions and and tolerances: tolerances: the the length, length, widthwidth and and height height and and their their tolerances tolerances provided provided by the by the • manufacturermanufacturer are are to to be be speciﬁed specified on on the the assessment assessment sheet. •Thermal Thermal and and acoustic acoustic insulation: insulation: compliance compliance with the the provisions provisions for for these these parameters parameters set out set outin in • existingexisting legislation legislation must must be be determined. determined. Buildings 2020, 10, x FOR PEER REVIEW 16 of 19 Fire resistance: the 3D-printed products should be assessed for this parameter and rated in • • accordanceFire resistance: with the the 3D-printed respective regulatoryproducts should provisions. be assessed for this parameter and rated in Veriﬁcation:accordance with depending the respective on the 3D-printing regulatory provisions. process, it may be necessary to establish and verify the • • valuesVerification: of factors depending such as on printing the 3D-printing speed. process, it may be necessary to establish and verify Impactthe values resistance: of factors depending such as printing on the speed. use envisaged for the element, its impact resistance to soft • • andImpact hard resistance: blows may depending need to be on determined. the use envisaged for the element, its impact resistance to soft Mechanicaland hard blows strength may of need the to printed be determined. element: the recommended values for the proposed use must •• beMechanical ensured. strength of the printed element: the recommended values for the proposed use must be ensured.

Figure 19. 19. StructureStructure of of assessment. assessment.

TheThe results results ofof thethe mechanicalmechanical strengthstrength tests conductedconducted on on a a printed printed HINDCON HINDCON piece, piece, for for which which a compressivea compressive strength strength of of 444.19 444.19 kN kN was was recorded,recorded, are shown in in Figures Figures 20 20 and and 21. 21 Although. Although it was it was brittle,brittle, the the elementelement exhibitedexhibited greater than standard standard stability stability during during the the test. test. Figure 20 shows a ﬁnite element simulation of the thermal behaviour of a façade element. These tests are conducted with the aim of estimating the behaviour of the element and conﬁrming that it meets the requirements before printing. As noted earlier, the requirements to be met by AM products are determined by the envisaged use. By way of an example, residential façade elements are not assessed in the same manner as are plot enclosure elements. The former are subject to mechanical strength, thermal and acoustic requirements, whereas the latter may only need to comply with structural safety conditions.

Figure 20. Finite element simulation of the thermal behaviour of a façade element.

Figure 20 shows a finite element simulation of the thermal behaviour of a façade element. These tests are conducted with the aim of estimating the behaviour of the element and confirming that it meets the requirements before printing. As noted earlier, the requirements to be met by AM products are determined by the envisaged use. By way of an example, residential façade elements are not assessed in the same manner as are plot enclosure elements. The former are subject to mechanical strength, thermal and acoustic requirements, whereas the latter may only need to comply with structural safety conditions. Buildings 2020, 10, x FOR PEER REVIEW 16 of 19

• Fire resistance: the 3D-printed products should be assessed for this parameter and rated in accordance with the respective regulatory provisions. • Verification: depending on the 3D-printing process, it may be necessary to establish and verify the values of factors such as printing speed. • Impact resistance: depending on the use envisaged for the element, its impact resistance to soft and hard blows may need to be determined. • Mechanical strength of the printed element: the recommended values for the proposed use must be ensured.

Figure 19. Structure of assessment.

The results of the mechanical strength tests conducted on a printed HINDCON piece, for which aBuildings compressive2020, 10, 144strength of 444.19 kN was recorded, are shown in Figures 20 and 21. Although 16it was of 18 brittle, the element exhibited greater than standard stability during the test.

Buildings 2020, 10, x FOR PEER REVIEW 17 of 19 Figure 20. FiniteFinite element element simulation of the th thermalermal behaviour of a façade element.

Figure 21. Mechanical testing of a HINDCON project 3D-printed element.element. 4. Discussion 4. Discussion The first research question in this paper addressed how 3D-printing technologies that are not The first research question in this paper addressed how 3D-printing technologies that are not covered by any harmonised standard could be eligible for some type of marking to confirm quality and covered by any harmonised standard could be eligible for some type of marking to confirm quality safety. The first conclusion to be drawn is that, inasmuch as 3D-printing technologies are not covered and safety. The first conclusion to be drawn is that, inasmuch as 3D-printing technologies are not by any harmonised standard, the products thereof are eligible for a Declaration of Performance (DoP) covered by any harmonised standard, the products thereof are eligible for a Declaration of award and CE marking on the basis of the ETA criteria and an EAD. Although CE marking is not Performance (DoP) award and CE marking on the basis of the ETA criteria and an EAD. Although compulsory, it promotes the commercialisation of both innovative and conventional products. CE marking is not compulsory, it promotes the commercialisation of both innovative and The second research question was related to how 3D-printed objects could be assessed. conventional products. The assessment of 3D-printed products is based mainly on the envisaged use and the technology The second research question was related to how 3D-printed objects could be assessed. The involved (the printing system and materials). In other words, the assessment covers a given AM assessment of 3D-printed products is based mainly on the envisaged use and the technology involved product for a particular use. (the printing system and materials). In other words, the assessment covers a given AM product for a The third research question queried the parts of the production process and finished object that particular use. should be included in the assessment. If a material is developed specifically for the printing method, The third research question queried the parts of the production process and finished object that the material must also be assessed and be shown to meet the needs of the designed printing process. should be included in the assessment. If a material is developed specifically for the printing method, the material must also be assessed and be shown to meet the needs of the designed printing process. Such needs may be structural or of any other nature as laid down by the national legislation. Depending on the type of printing, factors such as printing speed, inter-layer bonding, material– substrate bonding, layer thickness and so on may also need to be assessed. This is a new technology; therefore, it requires the definition of processes from scratch, as well as the definition of the methods used to evaluate the results. However, the present tests and trials were based on current experience and testing methods.

Such needs may be structural or of any other nature as laid down by the national legislation. Depending on the type of printing, factors such as printing speed, inter-layer bonding, material–substrate bonding, layer thickness and so on may also need to be assessed. This is a new technology; therefore, it requires the deﬁnition of processes from scratch, as well as the deﬁnition of the methods used to evaluate the results. However, the present tests and trials were based on current experience and testing methods.

5. Conclusions Assessing AM 3D-printed products entails establishing a series of universal tests to ensure compliance with quality standards. The type of test to which each product is subject will depend on the application envisaged, the printing system employed, and the materials used. Three-dimensional printing (additive and subtractive manufacturing) is a novel technology; thus, it is not covered by existing standards. This is not an obstacle to proposing an assessment methodology to ensure the quality of such products. Some of the current standards can be used to evaluate the performance of the new materials, but it is not clear how these standards can be applied, thus requiring the development of an appropriate methodology, as well as new tests and procedures. There is a close relationship amongst materials, machines and design. Assessments should cover all the stages and processes involved, not simply the resulting products. The methods and results presented in this article are based on the experience of the authors in the development of three research projects involving 3D printing which, despite being a ﬁrst step, indicate how much remains to be done.

Author Contributions: Conceptualization, G.S.O., J.A.M., N.O.E.O. and J.A.T.R.; methodology, G.S.O. and J.A.T.R.; software, G.S.O. and J.A.M.; validation, J.A.T.R.; formal analysis, G.S.O.; investigation, G.S.O.; resources, J.A.T.R.; data curation, Sotorrio; writing—original draft preparation, Sotorrio and Olsson; writing—review and editing, G.S.O., J.A.M., N.O.E.O. and J.A.T.R.; visualization, G.S.O. and J.A.M.; supervision, J.A.T.R.; project administration, G.S.O. and N.O.E.O.; funding acquisition, J.A.T.R. All authors have read and agreed to the published version of the manuscript. Funding: This research has been partially funded by the European Commission under the Horizon 2020 Framework Programme, Grant Agreement No. 723611. Acknowledgments: The authors would like to thank Hybrid INDustrial CONstruction through a 3D printing “all-in-one” machine for a large-scale, advanced manufacturing and building processes project. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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