Synopsis: It is well-known that cooling of fresh concrete to a subfreezing temperature interrupts the structure formation and can lead to serious damages of constructions. Most of the existing antifreeze additives reduce this destructive effect, however it should be acknowledged that the processes of cement hydration is still interrupted to an extent that the strength gain in these conditions is simply negligible. When using these admixtures, it is merely expected that concrete will not lose its integrity during the phase of cooling and that strength will be gained after the ambient temperature will reach positive values. However, in our work we aim at proving the possibility of rapid strength gain of UHPC with reduced water-cement ratio even at subfreezing temperatures. The following article presents analysis of the influence of various in-house developed admixtures on kinetics of strength gain of UHPC at negative temperatures.

This research project asks how we can efficiently solve structural problems with much smaller masses of concrete while maintaining the same level of cement consumption through the use of Ultra-High Performance Concrete (UHPC), degradable ice formwork, and through an active involvement of digital simulation. A survey undertaken in the recent advancements of concrete-related technologies has revealed an opportunity to propose a new concept of casting. The new method of fabrication for UHPC elements employs ice as the main material of the temporary formwork construction, involves automation of the fabrication process and solves the problems of waste material and manual labour at the demoulding stage. This casting method is being developed for cases in which unique and customized concrete elements of complex geometry are needed, e.g. topology-optimized structural elements. To impose a desirable geometry onto ice, the concept considers the use of 2.5D CNC milling operations. To minimize machining time, milling is thought to be combined with controllable melting of ice that allows achievement of a high-quality surface finish on a large scale in a very simple way. The main hypothesis of the research is that the control over melting could be possible through digital simulation, that is, the melting deformations can be pre-calculated if parameters of the material system are known.

This article describes an alternative material system for the fabrication of concrete precast elements using ice-based CNC-milled formwork. The use of ice as formwork resolves several systematic problems related to sustainability which are associated with the production of free-form prefabricated concrete elements, including the issue of material waste produced with existing models of fabrication. On the one hand, automation of formwork production by means of CNC machinery allows for the production of complex and precise geometries; on the other hand, as ice naturally melts away once the concrete has structurally set, the manual labor involved in the whole cycle of production is reduced dramatically. The Ice Formwork system is based on the utilization of a frost-resistant design of high-performance concrete (HPCfr). Numerous tests have proven that the Ice Formwork method of casting provides a high quality geometric transfer from ice to concrete, ensuring that no deformation of formwork occurs during the casting process. A key question is whether the trade-off in energy spent on refrigeration to maintain subfreezing temperature throughout the process of fabrication will negate Ice Formworks validity as an alternative to traditional concrete formwork. A preliminary assessment of that question shows that the energy required to produce Ice Formwork won't outweigh the benefits of the proposed fabrication process, enabling a cost efficient and environmentally sustainable solution to casting concrete for a variety of uses.

The following paper introduces a design implementation of an innovative fabrication method that aims at enabling an environmental and automated production of geometrically challenging cast concrete elements. The fabrication method is based on the use of ice as the moulding material for cast concrete. Empirical testing of ice CNC-processing, and a concrete mix capable of hardening at sub-zero temperatures was undertaken during previous research stages. The current paper illustrates a practical application of ice formwork. A façade rain screen has been developed using algorithmic modelling to illustrate a common case in which a non-repetitive geometrical pattern requires individual formwork to be produced for each element. Existing industrial methods capable of delivering such a project for formidable costs are based on CNC-processed expanded polystyrene (EPS), wood-based materials, or industrial wax formwork. These materials have been found to be either difficult to recycle, expensive, insufficiently strong, or energy- or labour-intensive to produce. Preliminary evaluation has shown that ice, used in their place, facilitates a much cleaner, economic, and an even more energy-efficient process. Moreover, a very gentle demoulding process through ice-thawing eliminates any shock stresses inflicted on newly cast concrete and provides optimal curing conditions. As a result, the thickness of façade elements can be reduced while still fulfilling all structural requirements.

This chapter discusses the material systems commonly employed in the production of precast concrete elements. In particular, it presents an alternative to expanded polystyrene (EPS), a material that is often used in moulding production for complex-shaped concrete elements. Although EPS plays an important role in the production of an energy-efficient built environment, the ecological implications of its growing global use, and inevitably growing waste, raise serious environmental concerns. Putting the industrial policies of EPS use under question, the author proposes the concept of an alternative ice-based material system for concrete manufacturing. This method provides a waste-free, closed-loop recycling manufacturing process. It enables the production of intricate and formally rich structural formations in concrete, for example mesoscale trabecular concrete structures – spatial material organisations that exceed the scale of concrete microstructures, but are much smaller than the design detailing of the concrete elements. Initially intended to merely eliminate production waste, ice-based concrete manufacturing is a relatively little-explored field in material organisation. This novel opportunity to produce reduced-weight concrete elements with differential local physical properties and expressive formal language is a welcome side-effect of stepping outside standard material practices.

The present article communicates knowledge in sustainability of a method of digital fabrication in precast concrete production. The method, known as Ice Formwork, serves in production of ultrathin lightweight architectural elements – a novel design typology that has emerged with the development of High-Performance Fibre-Reinforced Concrete (HPFRC). This class of prefabricated concrete structures is expected to reduce cement consumption, adopt digital fabrication processes, and increase design flexibility. In practice however, this production employs suboptimal methods of mould-making, using petrochemical or engineered wood products for single-use moulds. Thus, the excessive amount of non-recyclable waste raises the question of qualities of the advanced digital precast concrete production. The current work builds on the research and development of a novel production method Ice Formwork facilitated through single-use moulds made of artificially frozen water. Previously published work on the subject has covered topics such as cement hydration at subfreezing temperature (Sitnikov and Sitnikov 2018), subtractive digital processing of ice (Sitnikov 2019), and a case of practical application illustrated with the production of a prototype of a lightweight façade rain screen (Sitnikov et al. 2019). Yet, since artificial refrigeration and processing of large amounts of water is essential for Ice Formwork production, the environmental performance of this method requires a thorough examination. Thus, the current piece of work attempts to address two major issues, namely:

1. Energy consumption of industrial refrigeration in production of Ice Formwork.
2. Changes in water mineral content after a cycle of Ice Formwork concrete production.

For both topics, the research work required development of an original experimental methodology. This aspect has been given great attention and required development of new technical solutions on many levels. Further research and development is needed to come to a comprehensive environmental assessment, and the results of the current study merely provide a preparatory overview that can help to orient and properly set the objectives for the following research stages.