Hydration and rheology control of concrete for digital fabrication: Potential admixtures and cement chemistry
Introduction
The use of concrete in construction has primarily been limited to static and standardized formwork casting, where fresh concrete is poured into formwork, often vibrated to allow sufficient placement, and left to set. In standard practice, it is expected that concrete reaches sufficient strength a day later, at which time the formwork is stripped and the hardening concrete is cured to sustain hydration. The properties of the fresh mix, which are workability, compactibility, and resistance to segregation, are generally controlled or fine-tuned through the use of admixtures to facilitate the casting process. On the other hand, admixtures also allow the control of the hydration kinetics of the mix: first to avoid premature setting during transport or placement, and second for the formwork to be removed on time. In general, admixtures have contributed to numerous essential developments in concrete technology [1] and will continue to do so in novel approaches that are part of the emerging concrete digital fabrication [2,3].
Digital fabrication with concrete is a family of novel fabrication processes that offers greater freedom of shape by combining computer-aided design with additive, subtractive or forming manufacturing [[3], [4], [5], [6], [7]]. Many approaches exist, but this paper focuses on those that allow structures to be built additively and thereby eliminate the need for formwork. This mainly concerns extrusion printing, although other techniques, such as slip casting or spraying, could also benefit from the concepts presented in this analysis.
Building without formworks introduces a number of advantages, namely savings in cost, time, and materials associated with formwork construction. However, at the same time, it implies some significant materials engineering challenges as all the requirements that are normally fulfilled by the formwork are now directly imposed on the mix design of the concrete and the way it is deposited. Therefore, in absence of formwork, controlling the hydration and rheological properties becomes even more critical for successful execution [4]. Hydration kinetics must be delayed and accelerated in relatively extreme manners so that the material does not set during the printing process but, instead, right after deposition in order to support its own weight and that of subsequently deposited layers of material [8,9]. Concerning rheology, there must be a balance between flowability during printing and rate of structuration immediately after deposition. In addition, given the time-sensitive nature of the technique due to the continuous progression of hydration, prediction of flow rate is important to control the printing speed.
The aim of this paper is to discuss how various admixtures can be employed to achieve the desired properties for successful additive manufacturing, from the pumping and deposition characteristics to the hardening and curing stage. The key rheological and hydration properties for overcoming the challenges of form-free casting are presented along with required performance properties and potential use of admixtures. This includes the description of suitable chemical and mineral admixtures, their attributes and potential side effects with respect to cement hydration, as well as their possible incompatible combinations. While this paper mainly focuses on chemical aspects and working mechanisms on the physico-chemical level, complementary information with a focus on the control of structural build-up is offered in another paper of this special issue [8]. Rheological questions are also then detailed by Roussel [10].
Section snippets
Rheological aspects of hydrating cementitious systems
Controlling the rheological properties, expressed in practical terms as workability, of fresh concretes is important as it will determine the efficiency of the casting process during construction. In the case of novel applications like digital fabrication, it will determine its success or failure. A standard among field-friendly methods used to quantify concrete workability is the slump test [11]. However, workability is better described through fundamental rheological parameters, primarily
Properties evolution for printable concrete
The aim of this section is to describe the main stages of the rheological state of concretes used for digital fabrication. The focus is mainly related to extrusion printing, which represents the most investigated and implemented digital fabrication technique [4]. However, as stated by others [4,8,10], the fundamental questions about the control of the material properties are shared by other approaches, e.g. slip forming [4]. Fig. 6 schematically represents the expected evolution of the yield
Potential alternative candidates for the binding matrix
The previous sections describe the properties expected from a concrete used in some of the new technologies of digital fabrication and potential admixtures that can be used to target them. Another aspect to consider is the choice of the binding material to easily reach the expected properties. As an example, for the transition from stages 2 to 3 (Fig. 6), OPC can show limitations in terms of rate of surface development for fast structuration. The seemingly slow structuration build-up of OPC
Conclusion
Nowadays, chemical admixtures are essential for the production of modern concretes, for which good initial fluidity, workability retention and relative easiness of placement are some of the fundamental requirements. For digital fabrication, in addition to the same requirements, a precise control of the fluidity and of the structuration building rate are the new challenges that mainly come from eliminating or substantially reducing the use of formwork. In this paper, we outlined some chemical
Acknowledgements
Funding for D. Marchon was provided by an Early Postdoc.Mobility grant (no. P2EZP2-172177) from the Swiss National Science Foundation (SNSF). Funding for S. Mantellato was provided by the SNSF project no. 140615 and the NCCR Digital Fabrication, funded by the SNSF (NCCR Digital Fabrication, Agreement #51NF40-141853). The authors would like to thank Prof. Robert J. Flatt and Dr. Nicolas Roussel for their constructive comments on the manuscript.
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