ADC : Assessment of Additively Manufactured Concrete Materials and Structures
Deputy Chair: Dr. Freek BOS
The development of digital construction technologies is a decisive step towards solving the current problems in the construction industry: stagnating productivity and diminishing skilled workforces, which are major limiting factors for healthy development, not only in the so-called developed industrial countries, but also in most emerging economies. While the job profile of the construction worker has been steadily losing its appeal, increasing the degree of automation and digitalization in construction appears to be the only feasible option in mastering the current and future challenges confronting the building industry. The technological and economic potentials of digital concrete construction have now been recognized by many industrial stakeholders; the extent of innovation in this field is increasing month by month. While tools of digital design and planning (CAD, BIM, etc.) are already relatively well developed and have been increasingly commercialized, the means of digital, automated fabrication are less developed. But activities in this field have been rapidly gaining speed over the last few years, and technologies have been maturing ever more. Obviously, the seamless data flow from digital planning into fully automated construction, often described as Construction Industry 4.0, would mark this qualitatively new level of technology.
The increasing interest in the new technology and the need for intensive scientific exchange among research groups led in 2016 to the initiation of the RILEM Technical Committee 276-DFC “Digital fabrication with cement-based materials”. The committee organized two international RILEM conferences, in Zurich (2018) and in Eindhoven (2020), and prepared two special issues of Concrete and Cement Research on digital concrete technologies. The main product of the committee’s work is a State-of-the-Art Report which summarizes the available information and knowledge in the area and provided as well a solid basis and a good reference for further research. The report is nearly finished and will be published by the end of 2020. It serves as the major reference for activities of the new technical committee.
Most work on 3D-concrete-printing and other digital concrete technologies has been focusing on concrete properties in its fresh and transition states relevant for understanding and mastering the process chain as well as stability of freshly fabricated elements. Less attention has been dedicated as yet to prediction and characterisation of properties of hardening and hardened concrete. In this, mostly the quality of the interlayer bond for extrusion-based additive manufacturing has been addressed. Indeed, it depends on various parameters such as the time interval between the layers’ deposition, the evaporation rate, the rheological properties of the concrete, roughness, etc. Nearly always interface regions will be more porous than core material, thus creating weak links. As a result, printed concrete is, as a rule, both more anisotropic and less homogenous in comparison to cast concrete. Thus, weak interfaces may affect both the mechanical properties and the durability of printed elements, the latter due to easier ingress of aggressive agents into the concrete and to its reinforcement. This needs to be considered in the material testing, in the structural design of elements to be printed, and in respect of their durability design. However, there are many more open questions related to the properties print material and structural elements which need to be answered to facilitate the transfer of new digital fabrication technologies into practise of construction. The new TC will deal with this challenge.
In particular, this TC will cover products realized from additive manufacturing by material extrusion, material jetting, and particle-bed binding, with an overall target of establishing
Terms of reference
There are promising pilot applications which encourage companies and scientists to move forward the use of the digital concrete technologies in construction. However, the step from the research lab into the practice of construction requires considerable efforts. At the present stage of development, fine-grained concretes (or mortars) with compositions not fitting into the existing concrete codes (e.g., due to very high fines’ content) are used as “ink” in additive manufacturing processes. For a given compressive strength, such materials may exhibit very different tensile strength, modulus of elasticity, density, shrinkage behaviour, creep, and durability in comparison to conventional concrete. Additionally, the quality of interface (or rather interphase) between subsequent layers may pronouncedly influence the material properties and introduce anisotropy. Scientific base for predicting and testing properties of hardening and hardened print concrete yields many gaps which need to be closed. This holds true for the use of the tested properties for purposes of structural design. Integration of reinforcement poses further questions, such as the quality of its bond towards or its corrosion protection by print concrete.
The main objectives of the committee are:
(1) to collect and analyze information on specification and testing of the mechanical properties of print concrete in hardening and hardened states which are relevant to structural behaviour. These properties such as compressive and tensile strengths, modulus of elasticity, creep and shrinkage need to be considered under consideration of their statistical distributions as well as the anisotropy of print material, resulting primary from week interlayer joints. Furthermore, the interaction between cementitious matrix and reinforcement is in the focus of interest.
(2) to initiate necessary research activities for clarifying open questions related to characterisation of properties of printable and print concrete under consideration of reinforcement, which may consist of:
- interlaboratory studies on such properties as strength,
- round-robin tests on durability characteristics,
- applying for joint research projects in international calls.
With the results from objectives (1) and (2), the TC aims to establish the inter-applicability of testing methods, so as:
(3) to develop practical recommendations, based on the principle of performance-based design
- on procedures for the definition, setting and testing of the required material properties of hardening and hardened concrete;
- for adequate solutions for reinforcement integration;
- for technology-compliant or technology-promoting design principles and design rules.
The conclusions of the TC are expected within 4 to 5 working years.
The members to be recruited for the TC will be recognized experts in the domain. In order to take into account the different practices and standards in Europe, Asia and North America as well as their respective spheres of influence, including ISO, EN and ASTM standards, the TC will include representatives from the aforementioned world regions.
Since the main goal of the new TC is the elaboration/generation of recommendations for engineers and practitioners, their participation is necessary. Actions are expected in the different geographical areas covered by the members of the TC to initiate local (national or multi-national) groups involving practitioners. These groups will work complimentarily to the TC. The close information exchange between the TC and the practitioners’ groups will enable recognizing the most relevant and current practical problems on one hand, and implementation of the recent findings and recommendations of the TC in practice on the other hand.
Detailed working programme
The work of the TC will focus on bringing together the expertise and knowledge of all participating members. The first synthesis of expertise will be done at the inaugural TC meeting.
On this base, the TC will simultaneously address the following items:
- Recommendations for mechanical testing of printable and printed concrete
- Recommendations for estimation and characterisation of shrinkage and creep behaviour of printed elements
- Recommendation on assessment of durability of printed concrete and its protective function for steel reinforcement
- Recommendation on integration of reinforcement and testing the bond towards printed concrete.
The working program also included the literature reviews, interlaboratory studies, round-robin tests and other activities related to preparation of the above listed recommendations. Furthermore, activities of the committee will include (co-)organizing international RILEM conferences on digital concrete construction.
The working program will be further developed in detail according to the priorities which will be defined by the TC members, particularly considering input from industry representatives.
The proposed TC is a follow-up of TC 276-DFC “Digital fabrication with cement-based materials” lead by Prof. N. Roussel. The state-of-the-art report of the RILEM TC 276-DFC and other publications resulting from its work gave an account of the opportunities and challenges related to the use of new technology. On the TC meeting in January 2020 it was decided to initiate two follow-up RILEM technical committees: one focusing of processing chain and characterisation of fresh concrete in the context of digital fabrication, and another one, namely this one, dealing with properties of hardening and hardened print concrete and structural elements. Both committees will collaborate closely to ensure alignment of recommendations and other deliverables.
The work of the committee is related to a number of other technical committees of RILEM. The following RILEM committees are identified to be relevant for the present project:
- Technical Committee 262-SCI : Characteristics of the steel/concrete interface and their effect on initiation of chloride-induced reinforcement corrosion
- Technical Committee 281-CCC: Carbonation of concrete with supplementary cementitious materials
- Technical Committee 286-GDP: Test Methods for Gas Diffusion in Porous Media
- Technical Committee 287-CCS: Early age and long-term crack width analysis in RC Structures
- Technical Committee MCC: Mechanical Characterization and Structural design of Textile Reinforced Concrete
- Technical Committee MPA: Mechanical properties of alkali-activated concrete
The TC will furthermore closely collaborate with the fib TG 2.11 Structures made by digital fabrication.
The work on recommendations will provide input to relevant standardization and pre-standardization work within ACI, ISO, fib, ASTM, NIST etc.
The work will be carried out in connection with national or multi-national local groups involving practitioners as mentioned in the proposed terms of reference.
- Recommendations for the practitioner
- Reports on results of interlaboratory and round robin tests
- International RILEM conferences incl. proceedings
- RILEM PhD courses
- Joint publications by TC members
- During the tenure of the TC: publication and presentations of the ongoing results by their respective authors (in scientific journals and conferences)
- Reports in “Materials and Structures”
- Proceedings of the RILEM Conference
- At the end of the TC tenure: Publication of Recommendations.
Group of users
The targeted users are researchers, practitioners and standardization bodies in the field of concrete technology.
Specific use of the results
The recommendations to be developed by TC should facilitate the introduction of this new technology into the practice of construction. The digital concrete fabrication offers new possibilities with regard to increasing productivity for concrete construction and enhancing its flexibility and sustainability. A considerable economic impact can be expected due to the benefits connected with its industrial utilization.
Through the local national or multi-national groups specified in the proposed terms of reference, the practitioners are envisaged to implement the TC’s recommendations in their respective countries until the adoption of national or international standards.