Large Concrete Rubble as a New Structural Construction Material: Opportunities and Digital Processes for Load-Bearing Walls

Abstract

Concrete is amongst the most wasted materials on earth, mainly due to building demolitions. Currently, after a building’s end of life, concrete is crushed to be used as replacement gravel in new concrete mixes or for backfilling. Aiming to increase the circularity of the construction industry, this article presents design explorations and a design-to-construction process for building single-leaf masonry walls from large flat demolition concrete rubble, thus avoiding the need for further crushing after initial demolition. The proposed process augments the capabilities of conventional construction machinery with new digital control and sensing devices that are widely available on the market and at low cost. The design-to-construction process is implemented through methods of physical prototyping and load testing of a full-scale demonstrator to benchmark the construction precision and the structural, environmental, and productivity performances. The results highlight the viability and scalability of the approach, calling for a more systematic reuse of concrete rubble as it allows for the construction of low-carbon masonry structures while diverging part of concrete waste from downcycling and landfilling.

1. Introduction

Concrete is, volume-wise, the most wasted material worldwide after excavated soils [1], propelling the construction industry as the biggest waste generator [2]. Concrete is also the most used material worldwide, second to water [3]. Its numerous qualities, such as availability, low cost, versatility, workability, strength, and durability, make it a suitable choice for building structures. However, producing and pouring new concrete is the biggest contributor to the detrimental environmental impact of the construction industry [4]. The production of cement, a constituent of concrete, is responsible for 8–9% of global anthropogenic CO₂ emissions [5]. Linking the need for CO₂-intensive concrete with the availability of disused concrete rubble, several circular strategies propose to use a part of concrete waste in new concrete structures. While the use of crushed concrete as a replacement for natural aggregates in new mixes, i.e., recycling concrete, is gradually adopted in the construction industry, it does not significantly diminish the embedded CO₂ of these mixes due to the use of similar amounts of cement as in regular mixes [6]. This recycling process, however, lowers the need for new natural aggregates by crushing concrete rubble into gravel. To decrease this crushing and the remaining need for cement in recycled concrete structures, a promising circular strategy is the reuse of concrete elements reclaimed from obsolete structures [7]. Reuse, distinguished from recycling, avoids the energy-intensive retransformation of materials to recreate a product of similar properties. Reusing concrete elements is a little-implemented strategy that can save up to 94% of the upfront carbon emission of concrete construction [8].

A distinction is made here between reusing elements that originate from the careful dismantling of structures, e.g., using diamond saws, and reusing rubble from conventional demolition, e.g., produced by demolition excavators. Reusing concrete rubble remains little explored in academia and in the construction industry while being a widely available material source. Reuse is considered here as an upcycling strategy, in contrast with concrete recycling, which is considered downcycling due to the reduction of size and bond between aggregates. This paper thus explores the upcycling reuse of large concrete rubble into structures, as shown in Figure 1.

This strategy is an additional circular method for concrete construction, whose cycle in the concrete value chain is detailed in Figure 2. This strategy provides no disturbance to current demolition dynamics and constant availability of source material in recycling centres and landfills.

As concrete waste is composed of variable and irregular rubble pieces, digital tools are here envisaged to facilitate the assembly of these pieces to develop a new structure. To maximise the prospective adoption of the proposed process in the construction industry, i.e., scalability, this study devotes special care to employing widely available construction machinery in combination with digital tools while considering the existing skills of construction workers. Moreover, design options are presented in this research to broaden the range of constructive possibilities for the upcycling of concrete rubble into structures. The proposed process also addresses resilience, i.e., the reliability of the process and tools in case of sudden changes in geometry, condition, or size of rubble units, assemblies, or stock.

1.1. State of the Art

1.1.1. Concrete Rubble Occurrence

Concrete waste is predominantly composed of rubble stemming from the demolition of buildings and infrastructures, while the rest comes from construction waste. Technical deficiency, e.g., weathering degradation, material ageing, or insufficient structural capacity, is not the primary reason for the demolition of concrete structures. Instead, healthy structures are often prematurely demolished due to functional obsolescence, i.e., change in use requirements [9].

Depending on the demolition method and execution, concrete rubble pieces have distinctive geometries and dimensions. Concrete rubble in Europe is mostly produced by the demolition of concrete structures using hydraulic excavators equipped with crusher or hammer attachments (see Figure 3) [10,11]. Once transformed into rubble, the potential use of concrete waste depends on nearby construction needs and the existing supply to meet these needs.

Current waste streams for concrete rubble are well streamlined, optimised, and beneficial from an economic point of view. Concrete rubble is currently recycled or landfilled, while its reuse is underestimated. Concrete piles of recycling centres, as shown in Figure 4, highlight the diversity of rubble sizes. A portion of concrete rubble pieces from demolition, usually smaller than 100 mm in diameter, do not possess distinguishable geometric features and are irregular on all sides. Larger rubble pieces tend to have distinctive geometric features: at least one recognisable flat surface or straight edge and often two parallel flat faces.

1.1.2. Reusing Concrete Rubble

Some of the few uses of unaltered concrete rubble in the construction industry are small landscaping walls (including retaining walls) and pathways, e.g., on the West Coast of the United States of America (USA), constructed by DIYers and landscaping companies [12]. Broken concrete, called ‘urbanite’ (neologism probably coining urban and granite), is mostly stacked on the flat faces of rubble pieces. This practice, visible in Figure 5 is also advocated in Europe, yet it has not been widely implemented [13].

Using large rubble pieces to upcycle concrete rubble in self-standing high retaining walls is considered constructively inefficient by some authors due to material heterogeneity and the increased need for material volume [15]. In industry, gabion cages made of crushed concrete aggregates are commercially available [16]. Another of the few existing examples of concrete rubble reuse without crushing is through the coastal shoreline protection at Gaza Beach, where some large concrete debris is laid flat on the slopes of endangered shores [17]. These scattered applications prompt the need for research to analyse the opportunities and limitations offered by the reuse of irregular concrete rubble as a base material for the construction of new structures.

In academia, a research project at EPFL in Switzerland explores the mechanical properties and environmental impact of masonry walls with small irregular concrete rubble pieces as substitutes for stone [18]. This research is limited to rubble units that are light enough to be lifted and positioned by hand and do not diminish mortar use compared to wet irregular stone masonry.

Other research projects address the reuse of concrete rubble using digital tools or processes. Digital technologies can assist design by harnessing the geometrical complexity of concrete rubble fragments. Clifford et al. [19,20] use design methods inspired by the cyclopean masonry of the Incas to build a self-standing undulating wall with rubble fragments robotically carved out of concrete and stone. Another study by Marshall et al. [21] proposes to stack flat rectangular concrete rubble vertically in supersized masonry. Marshall and Grangeot [22] also propose machine learning tools to match irregular flat rubble pieces for making prefabricated rectangular panels. These three solutions use the original thickness of existing structures in single-leaf walls, thus achieving similar slenderness and, therefore, space-saving for building applications. In these projects reusing large flat concrete rubble, the energy consumption for construction and structural performance of the single-leaf masonry walls are not known, and details on the environmental impacts and construction precision of such structures are lacking.

Other studies from ETH researchers propose the use of as-found rubble for the dry assembly of small-scale wall mock-ups using robotic arms [23,24]. Such assemblies using desktop-size robotic arms have been explored by Grangeot et al. [25] and Li et al. [26]. Marshall suggests rearrangements of irregular concrete debris for the construction of an arch bridge by filling the remaining voids with 3D-printed concrete [27]. Relatedly, 3D printing is considered a solution for bespoke connectors between concrete rubble fragments in a teaching context [28], under the initiative “Design with Debris” [29]. Connecting irregular concrete rubble pieces can also be achieved by using cables or jamming within tensile lattices, as developed by Khalil Yaqoob Al Khayat [30]. The reprocessing of rubble pieces through stereotomy-like approach has also been explored by Siefert [31] to design a pedestrian bridge. The connections with negative wood pieces through milling have also been proposed by [32]. The jamming of fine demolition rubble has been proposed by Blödel [33]. Taxonomies of geometries and tools have been proposed by Grangeot et al. [34] and Wyller et al. [35]. These digital design strategies that consider concrete rubble as building material lack full-scale implementations, thus reducing their constructive feasibility and applicability.

At full scale, Johns et al. [36] constructed three prototypes using large demolition concrete rubble and stone boulders: two freestanding walls and a retaining wall. By digitally augmenting a high-payload four-legged excavator with high-end sensors and computation capabilities [37], the projects demonstrate the autonomous scanning, planning and dry-stacking of multiple stone boulders and some concrete pieces with an on-the-fly supply.

All digital projects reusing concrete rubble described above rely on advanced tools, such as 3D scanning, digital fabrication, and/or robotic arms, which require significant expertise, financial investment, and highly skilled supervision. Thus, the existing methodologies for digitally planning the reuse of irregular concrete rubble are limited by the complexity and cost of 3D acquisition and manipulation processes, posing challenges for accessibility, scalability, and the handling of unique, non-replaceable geometries.

1.2. Problem Statement

1.2.1. Research Gaps

The review of the literature highlighted the following research gaps:

• There are missing insights on the functional, economic, environmental, structural and architectural opportunities and limitations offered by the reuse of irregular concrete rubble as a base material for the construction of new structures.
• The productivity and structural performance of single-leaf masonry walls from large flat concrete rubble are unknown. Moreover, details about their construction precision, energy consumption, and environmental impact are missing.
• There is a lack of accessible, scalable and resilient digital construction processes for reusing concrete rubble with unique geometries in new structures.

1.2.2. Industry Challenges

These gaps encompass critical challenges that must be considered for the successful implementation of a relevant design-to-fabrication process in the current construction industry using large concrete rubble from demolition:

• Value deficiency. Currently, the construction industry sees marginal value in building structures using large, flat, unaltered concrete rubble from demolition, especially because of the lack of productivity data from physical experimentations. Thus, the challenge is to convince the industry to embrace a new method without precedents.
• Workers’ safety. The non-standard geometry, the heterogeneity and the weight of concrete rubbles render their handling unergonomic and dangerous. Risks of accidents are accentuated as current lifting tools are not perfectly suited. The challenge is thus to implement a safe and ergonomic construction process to handle such pieces.
• Tools complexity. The current state of the art suggests the need for complex and expensive machines operated by experts. The challenge lies in the development of innovative tools that can be easily understood and maintained by non-expert construction workers.
• Space limitation. Logistically, the use of irregular pieces requires space for storage, analysis and sorting in anticipation of their assembly, while construction sites can be constrained in space. The challenge is, hence, to construct in a small space despite the need for extensive space. Additionally, constructing thick building structures implies less internal space for similar external boundaries, in turn negatively affecting the economic value of the building. The related challenge is to construct slender structures from a multiplicity of elements.
• Stock-based uncertainty. There is no upfront guarantee of feasible construction based on a small stock of concrete rubble or on-the-fly sourcing. In addition, rubble pieces can deteriorate or break, potentially compromising construction. Additionally, the structural performance of such applications depends on concrete rubble from demolition, whose geometrical and mechanical properties are also uncertain. The challenge is thus to design and construct load-bearing structures with such unknowns.
• Competitiveness. For the proposed process to be scalable, it must offer advantages, including speed and environmental impact, which is notably driven by mortar quantity.

1.3. Objectives

This study addresses the research gaps and challenges detailed above through the following objectives:

• Assess the opportunities and limitations of reusing concrete rubble in the construction of new structures (addressing research gap 1).
• Develop, implement, and test a safe, accessible, scalable, and resilient design-to-construction process by leveraging the benefits of digital tools to make low-carbon and sound structures from heavy, variable, and non-standard concrete rubble (addressing research gap 3 and industry challenges).
• Provide insights into the construction precision, productivity, and energy consumption of the developed process (addressing research gap 2).
• Benchmark the environmental saving potential, assess the structural potential of the resulting structure, and identify pathways to increase them (addressing research gap 2).

1.4. Scope

This study explores the reuse of concrete rubble sourced from demolition sites or recycling centers. It is further narrowed to the use of large flat concrete rubble of irregular bounding geometry—which cannot be conveniently placed by hand (i.e., the mass of a piece is typically greater than 25 kg)—as primary material, while smaller rubble pieces can be used as secondary material. The construction tools considered for their upcycling, along with the added digital hardware and software, are based on availability in industrialised regions. The structural performances are meant to comply with Swiss codes for stone masonry [38]. The environmental performances are addressed in a Swiss context.

1.5. Content Organisation

For the exploration, development, application and benchmarking of a relevant upcycling construction process using concrete rubble as the main material, this research and the corresponding sections in this paper follow the methodology illustrated in Figure 6. It is organised around three main steps. First, design exploration is performed in Section 2 to define the design brief and design strategies, based on the identification of opportunities and barriers. Then, methods related to the design process, the construction process, and performance assessments are developed in Section 3. The results of their application to a full-scale case study are laid out in Section 4, which follows the same sub-structure as Section 3. A discussion overarching these three sections is provided in Section 5.

2. Design Exploration

Design exploration is the initial step. It aims at framing the problem by listing opportunities and limitations related to: designing with concrete rubble (Section 2.1); precising aspects of the design brief related to the structural typology (Section 2.2), architectural parameters (Section 2.3) and rubble connections (Section 2.4); and identifying key design strategies (Section 2.5).

2.1. Opportunities and Limitations for Concrete Rubble

Based on Section 1.1, the following opportunities are identified for the design of new structures using concrete rubble:

• Cheap. Concrete rubble is a waste offered to recycling centres, with regular compensation to evade the high cost of landfilling. Consequently, the remaining costs to obtain concrete waste are exclusively related to logistics, which can be optimized.
• Low carbon. Any upcycling approach of concrete waste prior to recycling it by crushing leverages embedded emissions and avoids new ones.
• Local. Recycling centres are evenly distributed to reduce transport distance from demolition sites, providing local, abundant, and permanent sources of concrete rubble. Reusing rubble on or near demolition sites further reduces transport emissions.
• Reliable. Reusing the outcome of traditional demolition avoids the costly disturbance of deconstruction sites for sourcing reclaimed materials, the major drawback of component reuse.
• Sturdy. Concrete fragments benefit from the high isotropic compressive resistance of old concrete.
• Compliant. The reconfiguration of small concrete pieces into large structural elements allows a large diversity and freedom in new positioning and structure dimensions.
• Stone substitute. The compressive resistance and the mineral composition of small concrete rubble enable its direct use as stone, alleviating raw material extraction.
• Massive. Due to the preservation of density, concrete rubble pieces retain the thermal and acoustic attributes of concrete, valued for its mass.
• Distinctive. Despite heterogeneity, the similar geometric features of concrete rubble can be leveraged in the design: a significant part of concrete rubble possesses flat faces, often two or more, and straight edges.

Despite these opportunities, concrete rubble is currently not widely reused in new construction activities. The following limitations are identified:

• Compressive only. Demolition causes sectioning of rebars if any, resulting in degradation of tensile, shear, and bending capacity.
• Irregularity. The non-standard geometry of concrete rubble (broken edges and faces) is challenging to be described using primitives.
• Variable. Recycling centres collect rubble pieces from various sites, providing non-uniform properties and geometries. Using debris from a unique demolition site partly alleviates this drawback.
• Heavy. Due to the high density of concrete, manipulating rubble units by hand limits the maximum usable size of rubble pieces to a small proportion of total waste.
• Hard. The strength of concrete rubble pieces also makes it long to cut or drill.
• Uncertain. Assessing the properties of concrete rubble for reuse can be challenging due to uncertainties in fabrication, use, and dismantlement. These uncertainties include concrete composition and strength, wearing, previous exposition, and dismantlement method and care.

2.2. Identification of Relevant Structural Typology

To identify the possible applications of such material for the construction industry, basic structural typologies are gathered from Muttoni and Engel [39,40] and grouped by the dominant form of internal forces: compression, tension, and both. Concrete rubble is considered as the main construction material for each cluster of structural typologies. Combining concrete rubble with tensile elements allows a greater diversity of structural typologies, visible in Figure 7. The concrete rubble units are considered flat, reinforced, 17–30 cm thick, 50–200 cm long, 27–140 cm wide. Based on the opportunities and limitations offered by concrete rubble pieces (see previous subsection), relevant structures to be constructed from such material are identified as compression-based structures. These structures encompass walls, arches, vaults, columns, and foundations. They do not require a tensile capacity, which is suited for the assembly of distinct concrete rubble pieces. Although less optimum, other structural typologies could nonetheless be built, for sculptural, formal, or technical reasons. In this study, the construction of walls is explored due to their prominence in buildings and infrastructures.

2.3. Exploration of Architectural Parameters

Load-bearing walls made of large concrete rubble are further explored to identify key architectural parameters and specific design options to form the basis for the design-to-fabrication process. Architectural parameters, including flatness, bounding geometry, airtightness, acoustic performance, thermal performance and complementarity with technical components, are assigned to example design options organised into categories visible in Figure 8, to define the structure’s requirements.

The category “constructing geometry” determines structural, spatial and aesthetic performances by aligning the rubble with brick masonry orientations (i.e., header, rowlock, stretcher, shiner, sailor, and soldier) along geometric design constraints (i.e., arched openings, rectangular boundary) and by varying the number of masonry layers. The “void-filling strategies” explore options related to flatness and airtightness, while “layers and systems” anticipate the improvement of visual, thermal, structural, technical and acoustic properties.

To comply with standard practices of wall construction, the following parameters are prioritized as design requirements to be optimized:

• Slenderness. For space-saving structures in new buildings.
• Flatness on both sides. For ease of integration of other building layers or the fixation of technical systems.
• Rectangular bounding geometry. For ease of connection with other structural components.
• Compressive resistance. The load-bearing capacity should be sufficient for mid-rise residential buildings.
• Low environmental impact.
• Visual harmony. Aesthetically pleasing composition.
• Airtightness. For thermal performance in building applications.

The design option meeting all requirements is the vertical stacking of large concrete rubble pieces along their “flat direction” in single-leaf masonry walls, as shown in Figure 9. Doing so enables the construction of walls as thin as the thickest rubble composing them. It also highlights the geometric specificities offered by concrete rubble compared to natural stone, such as the distinctive parallel flat sides, contributing to the flatness requirement. Thus, an agile arrangement of the flat elements can contribute to all requirements, while airtightness can be satisfied by filling masonry gaps with smaller rubble pieces and mortar. For the scope of this study, no openings nor connections with other structural components (e.g., beams, slabs) are considered.

2.4. Exploration of Rubble Connections

Various processing and assembling strategies are considered for connecting flat rubble pieces in their vertical configuration with 3D irregularity on the interfaces. The alteration of concrete rubble through grinding, cutting, carving, chiselling, hydro-jetting, or hammering offers a wide range of constructive solutions. Such modification would induce re-processing steps that are expected to be time-consuming and/or more constraining (e.g., with additional tools, water sprinkling and treatment), limiting their market scalability. Keeping the existing geometry of concrete rubble pieces is expected to be faster, cheaper and easier to construct while compromising between material addition and structural performance. Thus, the exploration of processing for connections is narrowed to drilling and pouring. It does not consider the processing of emerging rebars beyond cutting (e.g., straightened, bent, welded, threaded). The categories of applicable connections to assemble irregularly shaped concrete rubble, shown in Figure 10, are analysed through five lenses—safety, scalability, resilience, rapidity, and precision—with a score of low (x), medium (xx) and high (xxx), and summarized in

Table 1. Comparison of advantages and drawbacks of various connection strategies.

	Safe	Scalable	Resilient	Rapid	Precise
Dry fit/wedges	x	xxx	x	xxx	xxx
3D-printed connectors [41]	x	x	x	xx	xxx
Pouring flat connections	xx	xxx	xx	x	xx
Mesh/Jamming	x	xxx	xxx	xxx	xx
Dowels	xx	xx	xx	xx	xxx
Tie	xxx	x	x	xx	xxx
Climbing formwork	x	xx	xxx	xx	xx

. This analysis, detailed hereafter, allows for the identification of the most relevant connection strategy for construction.

Dry fit connection for slender vertical assembly (Figure 10a), although largely scalable and fast is considered highly unsafe, fragile and, even with the use of wedges (Figure 10b), due to the need of a perfect triangle of support and thus precise positioning. Moreover, it does not fulfil the airtightness requirement without additional layers and concentrates loads on singular points.

Leveraging 3D printing, bespoke connections can ease the assembly of large irregular rubble, as demonstrated at a small scale by Wibranek and Tessmann [41]. These connectors (Figure 10c) enable a distributed transfer of loads between the rubble pieces. This could be best achieved with concrete printing, which requires high expertise and expensive machinery despite its recent popularization. The limited scalability of this method is also due to the need for upfront 3D scanning of an entire stock, 3D printing and keeping track of rubble pieces and connectors inventory, which can be space-demanding. Resilience is particularly weak in case of unexpected rubble geometry alteration. This construction strategy is also considered unsafe due to the heavy weight of each masonry unit.

Regularizing rubble pieces by material addition through pouring in peripheral formworks allows the transformation of irregular pieces into more simple geometries (Figure 10d). While this strategy remains unexplored, it appears more resilient than all other methods previously described, thanks to the ease of reproducing a specific geometry. The resilience also comes from the controlled (flat) geometry of interfaces. However, this strategy requires large quantities of materials for formwork and new concrete pouring and appears time-consuming. Since the heavy pieces can incorporate safe lifting connections, it is considered non-threatening for fatal injuries. If no complex digital processes are involved in the making of formworks, such as 3D scanning and CNC cutting, this unexplored strategy is considered as easily scalable.

Assembling rubble pieces using dowels is a strategy that allows precise positioning and interlocking through the drilling of rubble fragments at precisely aligned locations (Figure 10e). To ensure good vertical contact between rubble pieces, fast-setting mortar can be applied. Consequently, the method is considered rapid but requires drilling precision, mostly achievable with drills spaced at adjustable and repeatable distances, a task suited for industrial machinery.

Using a wire mesh similar to gabion cages to secure or jamming rubble pieces (Figure 10f) appears to be a rapid, resilient and scalable connection method. However, the structural behaviour of slender structures highly depends on the properties of the mesh, thus neglecting the relevance of rubble pieces. Moreover, such connection method does not fulfil airtightness requirement without an additional layer.

Drilling all the way through rubble units and inserting strands is another connection strategy (Figure 10g), providing cohesion throughout the built structure and can even provide bending or shear resistance to the structure. However, precise concrete drilling over such distances is even more arduous, making this connection strategy not easily scalable nor resilient despite being safer than the alternative methods.

Another considered connection method is to use a climbing formwork around two layers of rubble pieces to fill the voids with smaller rubble and mortar (Figure 10h). This allows for good contact surfaces between rubble pieces and airtightness and flatness on both sides of walls, despite being more time-consuming. It is inspired by cyclopean concrete [42], and the principle of “pierre banchée” employed by Fernand Pouillon in social housing in La Tourette housing complex in France [43] and by Franck Lloyd Wright in his early experiments with desert concrete in the United States of America [44]. Since the rubble placements can be adjusted and mortar joints resorb tolerances, this connection strategy is deemed resilient and thus chosen for construction. However, manipulating heavy rubble pieces in leaning against the climbing formwork until mortar hardens poses a risk to the safety of construction workers. Since this risk can be mitigated by developing robust formwork, such a connection strategy is adopted for the development of single-leaf walls from large concrete rubble pieces.

2.5. Design Strategies

For construction rationality, the design process of single-leaf masonry walls made of large concrete rubble pieces is developed based on self-imposed requirements:

• Efficiency. Stable structural configuration should be found using irregular pieces.
• Integrity. The geometry of each rubble unit must remain largely unaltered.
• Resilience. New design options must be possible if the assembly of a concrete fragment is compromised.
• Flexibility. The stock size, diversity and renewal must not prevent design solutions.
• Constriction. Space needed for scanning, handling, and storing must be minimized.

2.5.1. Masonry Rules

Designing single-leaf walls from large, flat irregular concrete rubble pieces can draw lessons from cyclopean masonry, “opus incertum”, irregular stone masonry in literature [20,45,46] and related norms [38]. Principles of stability in such literature indicate that the structure’s compressive resistance augments along the following parameters: wall size to stone size ratio, coarse horizontal joints, small joint size compared to stone size, vertical interlocking and mortar resistance [38]. These recommendations to achieve structurally sound walls are also well suited for large concrete rubble pieces and inform the design process. Moreover, minimising joint size per rubble height also decreases the quantity of added material, contributing to reducing the environmental impact of prospected walls.

2.5.2. Relevance of Digital Tools

Designing masonry walls using heavy, unaltered, irregular pieces while following the stability parameters addressed in Section 2.5.1 is complex since it requires recording the geometry of individual pieces and iteratively attempting to position them sequentially until a satisfactory complete layout solution is found. Describing, grouping and managing numerous complex shapes, their positioning, and their sequence is a cumbersome task for humans, but it is well-suited and more rapid for algorithms. Using advanced digital tools and processes, such as 3D acquisition through LiDAR scanning and digital stacking [19,47] through machine learning [48,49] or 3D mesh physics [36], can provide dense packing solutions. However, these digital stacking strategies require expensive tools, large stocks of 3D rubble meshes, and intensive computation. Such technical needs render this strategy possible mostly for prefabrication applications, where centralized production can compensate for the equipment and expertise costs. For in situ design, simple and affordable digital tools, such as cameras, smartphones, and mainstream computers are ideal for maximizing market scalability through 2D stock, design and manipulations.

In addition to the design of irregular shape aggregation, a preliminary structural assessment of masonry performances can be performed visually by observing masonry patterns. However, to objectively and quantitatively compare stacking solutions, digital tools can perform complex calculations effortlessly. Integrating digital analysis of the structural behaviour of stacking solutions under compression is considered relevant to estimating the load-bearing capacity of each digital solution. Such structural analysis can be achieved with one of the following digital methods: (1) implementing finite element method (FEM) simulations coupled with rigid block equilibrium with accurate 3D contact surfaces; (2) similarly describing the structural behaviour through discrete element modelling (DEM), or (3) geometrically analysing the regularity of the masonry pattern and correlating it to known performances of walls with similar pattern quality. To identify the potential structural capacity of each solution fast enough for in situ design and construction, FEM-based and DEM-based analyses are discarded since they are computationally intensive.

3. Methods

Methods are organized into three groups: design process methods (Section 3.1), construction process methods (Section 3.2), and performance assessment methods (Section 3.3). Results (Section 4) will follow the same organization.

3.1. Design Process

3.1.1. Geometry Acquisition

Due to the flat property of the large pieces of rubble considered for upcycling, and to provide design options with accessible tools, their geometry is approximated as 2.5D elements (a 2D silhouette and a thickness). Working with 2D elements allows digital acquisition with simple cameras through edge detection and a drastic reduction of image acquisition and computation costs and times compared to 3D handling. Through computer vision, the 2D detection of the irregular geometry of concrete rubble only based on pictures is considered affordable and accessible. This “scanning” method is particularly valuable in estimating the available stock from the topmost layer of concrete waste piles in recycling centres without the cumbersome individual 3D scanning developed in other research projects [19,50]. Moreover, stock characterisation can be incremental when unloading bins of concrete waste on re-construction sites and synchronised to masonry solutions increments.

When considered for fabrication, individual rubble pieces are extracted from the pile or bin using any lifting equipment and slings and placed on its flat side. Placing each rubble unit on a pallet eases stock logistics. The geometry acquisition uses a picture of the rubble unit from above, from a parallel plane. An algorithm for detecting irregular flat pieces from raster files exists [51], but it requires a video feed from a calibrated industrial camera, which can be rather expensive. To be applicable to any camera and with low digital processing costs, a lightweight edge detection script has been developed based on OpenCV [52], the popular library used in projects requiring such 2D detection. The scanning conditions and output are shown in Figure 11. Each scanned rubble unit is added to a dataset as a binary raster file. The thickness of each rubble unit is manually or digitally recorded (using a distance sensor).

3.1.2. Stacking Design

To design single-leaf unreinforced masonry walls using 2D outlines of concrete rubble, several irregular stone stacking algorithms exist in the literature [48,53,54,55]. While they all aim for tight packing, some algorithms do not seek straight horizontal courses nor vertical interlocking, reducing the potential compressive resistance of output stacking solutions. Moreover, most are computationally intensive, or even require reinforcement learning. The stacking algorithm by Wang et al., visually summarized in Figure 12, minimizes computational costs through regular matrix operations, while ensuring horizontal courses, interlocking, lateral stability, and minimum void area [53]. This is notably achieved by constraining each rubble piece’s position based on its minimum Object-Oriented Bounding Box orientation (OOBB). Additionally, this stacking algorithm sequentially places rubble units from the bottom, potentially allowing updating the stock dataset on the fly. The valuable sequential feature could improve resilience in case of unexpected alteration of geometry, e.g., due to rubble breaking, and can accommodate any size of stock. This includes the incremental documentation of a pile or bin, and thus minimizes storage and handling space. This coveted resilience through on-the-fly updates is crucial as the specific non-standard geometries of concrete rubble considered for stacking are difficult (and suboptimal) to replace. For these reasons, the stacking algorithm by Wang et al. [53] is used to generate stacking options from the dataset of 2D rubble outlines and by inputting the desired wall dimensions [56]. Its detailed principles are shown in Figure 12.

Pre-generating numerous stacking solutions increases the chances of obtaining a stable layout solution that reaches the desired height and gives more choices for well-performing solutions. To objectively compare pre-generated stacking solutions and select the best one for construction, a separate geometric analysis is performed to inform the quality of the stacking solution and the volume of added materials. This analysis through geometry is achievable as the compressive resistance of masonry is influenced by its regularity, which is quantifiable by the intensity of horizontal courses, vertical interlocking, and rubble rectangularity. These features are geometrically assessable using indices in percentage based on the shortest horizontal path (F_AH) and shortest vertical paths (F_AV), and shape factor (F_SP) as defined by Almeida et al. [57] and Wang et al. [53]. Such pure geometric analysis of the 2D masonry layout is used as a predictive tool to depict load-bearing capacity in comparison to historical stone masonry layouts [58].

The area of void within the bounding rectangle of the placed pieces in each stacking solution (F_VR) is also a quantifiable metric of masonry quality analysis and is directly correlated to the volume of added material. Such void factor is thus also important for comparing the environmental impacts of each layout solution. To best complement the geometry analysis and identify best-performing solutions specific to concrete rubble wall designs, a new indicator is considered: the achieved height ratio within the given boundary (F_HR). Moreover, a unique geometric index named masonry quality (F_MQ) is implemented, combining all these geometrical indicators through Equation (1), to rank all pre-generated solutions. The importance multipliers of i_VR = 0.5 and i_MR = 0.125 are defined here to prioritize the reduction of void since it has the most influence on material quantities, and hence on environmental impact and labour related to void infill. This combination of multipliers also ensures that masonry quality F_MQ can also be expressed in percentages through a sum of individually weighted indices that are smaller or equal to one. When the highest-ranked stacking options have a similar global index, the stacking option to be constructed is chosen for its visual appeal among the best-performing ones.

F_MQ = 𝐢_VR(1 − F_VR) + 𝐢_MR(1 − F_AH + F_AV + F_SF + F_HR)

(1)

Equation (1). Heuristics for ranking the best stacking option.

3.2. Construction Process

3.2.1. Construction Principles

With the known position and sequence of each rubble unit provided by the digital design, placing them accordingly to construct in situ walls is challenging. Among the most important challenges and constraints are: the small floor space available to handle the pieces, the needed precision of execution to fit the planned wall geometry, the heavy weight of the debris, hence dangerous lifting and placement operation, the risk of rubble pieces breaking during handling making the planned layout unfeasible, the pace of construction to remain economically competitive in areas of high labour costs, the rudimentary tools available on construction sites, and the constantly changing conditions of construction sites. To simultaneously tackle such challenges, the developed construction process for upcycling concrete rubble into single-leaf masonry walls leverages the digital information available from the design process to handle the heterogeneity and weight of such material safely and uses existing tools of construction sites. This assembly is based on the simple and effective principle of “orientation through gravity”, inspired by historical examples [20]. In this method, heavy, irregular pieces are oriented to their desired orientation in elevation by lifting each rubble laid flat on the ground from a specific point on its thinner sides. This anchor point is computed by drawing the intersection between the rubble outline and the vertical line starting from its centre of mass when oriented according to its stacking solution. The centre of mass of each rubble unit is approximated as the centroid of its 2D silhouette. The lifting point is drilled using a hammer drill at half the thickness of the thinner sides. For large rubble pieces, two lifting points are computed for redundancy, hence safety. When two lifting points are used, the geometric principle is identical but based on local centres of mass of the equally divided 2D outline areas, as pictured in Figure 13.

Knowing the position of the anchor point(s), simply lifting each rubble from its horizontal to its vertical positions provides accurate 3D orientation using gravity. This method thus enables the custom orientation of irregular geometries with any hoisting machinery, which is represented in its fabrication context in Figure 14.

The critical steps of this upcycling process are summarized in Figure 15.

The complete procedure for the processing and assembly of individual rubble pieces is detailed hereafter:

• Acquiring the irregular geometry of accessible rubble pieces. The top-down pictures are analysed using edge detection or image segmentation.
• Choosing a stacking solution and manually improving it if necessary. Improvements include decreasing void area or adjusting orientation to maximize stability during rubble placement.
• Positioning the (next) rubble unit in the processing area and registering its location using another top-view photo. Position the concrete hammer drill in front of the lifting point(s) location(s).
• Checking for rebar collision at the drilling point with a metal detector. In case of collision, slightly adapt the position of the lifting points and check collisions until it is satisfactory.
• Drilling of the lifting points. Such drilling operations can also be achieved through augmented drilling, laser-guided drilling, or robotic drilling (more details available at the end of the subsection).
• Inserting lifting eyes in drilled holes.
• Lifting and positioning using any hoisting machinery. Based on digital position instruction from the stacking solution and the position of the rubble units on the ground, the placement of each rubble can be achieved onsite with a tower crane, or offsite with an overhead crane of the prefabrication facility.
• Breaking off any protruding part using a jackhammer or bolt cutter. As this can only be executed when the rubble is flat on the ground for safety concerns, it is best to perform such adjustments before lifting. When the protruding parts remain undetected before placement, delicate manoeuvres to reposition the rubble unit on the ground are needed.
• Cleaning the rubble unit with water to remove dust and saturate the surface to ensure optimal mortar bond, as broken concrete absorbs more water than crushed stone [59].
• Aligning rubble front faces to formwork. A localized climbing formwork of the desired planar dimensions of the wall is used to align rubble units against the front face manually. It ensures the visual quality of the final structure and optimizes flatness and planarity.
• Controlling horizontal positioning using a ruler on the formwork and coordinates as the digital intersection between the rubble outlines and the formwork.
• Grounding using fast-setting mortar and wedges from concrete debris. To reach out of plane stability which is critical when stacking flat pieces on their thin faces, special care is taken by the human operator to ensure complete stability and grounding in mortar before releasing the lifting hooks. The tolerance of mortar discards the need for altimetric and rotational position control. Moreover, smaller mortar joints lead to better compressive resistance, but must remain thicker than 10 mm for them to play a structural role [38]. This is visually ensured while laying the concrete rubble pieces.
• Filling masonry gaps and backside voids. The large flat rubble pieces have variable thicknesses when sourced from recycling centres, leading to variable voids in the backside of the considered wall. Lighter rubble pieces and concrete gravel are thus manually placed in the remaining void to reduce its volume. The localized formwork encapsulating the contact area between two courses of rubble masonry acts as the boundary of the voids to be filled. When sourced from demolition sites directly, concrete rubble pieces have more similar thicknesses, alleviating the problem of filling the void in the back.
• Pouring fluid mortar. Liquid mortar is poured into remaining interstitial voids to obtain airtightness and joints with flat sides along the planes of the wall. Adding 3 mm gravel in the mix further minimizes mortar needs but degrades mortar viscosity and reach of small voids. Vibrating mortar is critical to fill voids. The mortar is poured below the top side of the large rubble pieces, keeping a rough surface to improve adhesion with the next course above.
• Rising climbing formwork. Once a course is completed and the mortar has hardened, the lightweight formwork is raised around the next horizontal joint and the procedure is repeated. To obtain a flat top, mortar is poured over the rubble pieces only in the last course.
• Ensuring airtightness. Once the localised formwork is removed, void might remain in between rubble pieces. These are filled again with smaller rubble pieces and mortar to obtain airtightness.

The rubble pieces are laid within a climbing formwork of custom dimensions and hence cannot stick out of the desired planar boundary. Vertically, the lack of altimetric positioning can, however, affect the effective built height and could be verified using the vertical distance sensor of the crane or video feedback with planning overlay. The drilling operation requires some degree of precision to achieve the desired orientation and positioning, to match the planned geometry and stability predictions. However, this needed precision is relative and currently not quantified.

This proposed upcycling process provides certain and predictable outcomes without the need for digital information about the 3D geometry of broken surfaces. Overall, the process is not prone to failure in the case of imprecise processing or positioning, an additional resilience to this industry-ready assembly process.

3.2.2. Construction Tools

The construction process for assembling large concrete rubble in single-leaf masonry walls is developed around existing construction tools and machinery (concrete drills and cranes) to ease the scalability in this construction industry. To improve the existing capabilities of available tools, various augmentations are used. As no commercially available reversible lifting anchor is available in Switzerland, custom ones are fabricated from regular drop-in anchors, as shown in Figure 16. Such a lifting system allows for the removal of the anchors and thus minimizes the environmental footprint of the walls and reduces construction costs.

The overhead crane of the prototyping facility is augmented with a sensing unit, as shown in Figure 17. An off-the-shelf concrete hammer drill is mounted on an industrial arm (Figure 18). Thus, the prototyping setup resembles conditions and processes of offsite prefabrication facilities while drawing learnings for onsite construction. In this prefabrication context, lifting points are robotically drilled in the side faces of each rubble unit positioned on a pallet. The position of the wall to be assembled is also referenced relative to the augmented crane. The interaction of all digitally augmented tools is centralised in Grasshopper for Rhino.

3.2.3. Demonstrator Construction

To verify the applicability of the developed design and construction processes, a full-scale demonstrator is constructed in an academic prototyping facility. The concrete rubble is sourced from the nearest concrete recycling centre, which benefits from a continuous supply, hence the used rubble pieces best represent current practices of the demolition industry. The largest rubble fragments able to fit within a 10 m³ bin are picked from a pile of concrete rubble in the recycling centre using a mobile hydraulic material handler equipped with an orange peel grab attachment. The gathered rubble pieces are used as the source stock for the design and construction of the demonstrator. During construction, material quantities (mortar by type, rubble weight) are recorded to compare the mortar ratio with academic literature and current construction practices, and to provide data for the environmental impact assessment.

3.3. Performance Assessment

3.3.1. Productivity Assessment

To quantify the productivity of the proposed construction process, the construction time of the demonstrator is recorded and categorized by action by using analysing video timeframes and manual labelling. The surface of the constructed structure divided by the total construction time provides productivity in m²/h. It is compared to industry and academic examples. When available, the masonry unit weight is also provided, along with the computed density, expressed in unit/m³. Such metrics can thus help provide early estimates of construction costs.

The design process, the installation of machinery, its initialisation, cleaning, and other side tasks are not included in the construction time.

3.3.2. Construction Precision Assessment

The precision of the construction process is quantified by measuring the divergence between planned and built geometries. To obtain the geometry of the built demonstrator, it is 3D scanned with a millimetre-precision stationary LiDAR scanner, and orthophotos are generated. The external dimensions of the wall are measured from the reconstructed mesh.

While the tolerances of masonry walls are to be specified on a case by case manner, the boundary geometry of the wall is thus compared to its design dimensions and the deviations checked against the tolerances of concrete casting from Swiss norms: + 12 and –8 mm for walls between 200 and 400 mm thick [60].

Additionally, the position of individual rubble pieces is analysed in elevation through the manual repositioning of their 2D outlines based on the frontal orthophoto. The biggest deviation compared to their digital design position is quantified in planar rotation and position.

3.3.3. Structural Assessment

The load-bearing capacity of the structures designed and constructed from reclaimed concrete rubble pieces using the developed upcycling process is evaluated by analysing design options before fabrication, performing material tests on representative rubble pieces, and load testing the demonstrator.

The material tests are achieved on three representative rubble pieces by sampling three concrete cores per rubble unit and performing a destructive compressive load test with elasticity measurements on each of the nine cores, following the Swiss norm [61].

The demonstrator is submitted to a uniaxial compression load test in an academic testing facility at the construction location, during which the load and deformations are recorded. During loading (2 kN/s) and unloading, displacement measurements are recorded using two sets of tools, represented in Figure 19. Linear variable distance transducers measure the displacement of the load-spreading beam. Global deformations of the structure are recorded through stereo-digital image correlation [62]. Photos of the demonstrator’s surface sprinkled with a speckle pattern, taken from two different points at regular intervals, help monitor deformations. This allows the analysis of strain correlated with the applied stress.

3.3.4. Energy Consumption Assessment

The energy use of fabrication and construction tools for the demonstrator is benchmarked through measurements of their electric consumption during construction using IoT devices. When measuring is not possible, the consumption is computed from tool wattage in technical documentation, with pessimist assumptions, and recorded use time.

The electric consumption per constructed area is also compared with manual masonry and with alternatives from the literature on digital wall construction and 3D printing.

3.3.5. Environmental Impact Assessment

To assess the environmental impacts of the upcycling process, a comparative process-based Life-Cycle Assessment (LCA) is conducted. The construction of the demonstrator serves as the basis for benchmarking the structures resulting from the developed upcycling process. For the developed upcycling process, three transportation options are considered: onsite construction with delivery from the demolition site (OP1), onsite construction with delivery for the recycling centre (OP2), or built on the demolition site (OP3). Five other structures variants with similar requirements for technical performance are assessed for the comparison:

• an irregular stone masonry structure made of mortar and local sandstone (VA1);
• a reinforced concrete structure with 50% of recycled aggregate (VA2);
• a 3D-printed hollow concrete structure (VA3) [63];
• a structure made of hollow concrete blocks and mortar (VA4);
• a structure made of hollow clay bricks and mortar (VA5).

The design of these alternative structures reflects standard practices in the European construction industry and are detailed in Appendix A.

The metrics used for the comparative LCA are the Global Warming Potential (GWP) expressed in kilogram of carbon dioxide equivalent unitized by square meters meter of structure (kg CO_2-eq/m²), and the ecological scarcity expressed in eco-points unitized by square meter of structure (UBP21/m²). The carbon dioxide emissions for the global warming potential are commonly used in the construction industry and are based on ISO standards [64,65]. The ecological scarcity measured in eco-points 2021 quantifies the environmental damages due to the use of material and energy resources, land and fresh water, emissions into the air, water and soil, waste disposal and traffic noise [66]. It is a metric based on Swiss eco-factors representing detrimental environmental impacts in relation to national targets.

The system boundary of the LCA is shown in Figure 20 and is based on life stage modules defined by European norms [67]. The studied system considers the steps following the demolition of a concrete structure (after C1) until the construction of a new structure (before B1), using a cut-off approach [68]. Thus, the assessment includes transportation (C2, A2), waste processing (C3), and disposal of unused concrete waste (C4). It also includes the product and construction process stages (A1–5) of all additional materials and processes used in the options and variants.

A Swiss federal database is used as source of data for the carbon dioxide emissions and eco-points of manufacturing, transportation, and disposal of individual construction materials and energy needs [69]. For the demonstrator, the use of newly-added mortars is measured during construction, and environmentally analysed using their Environmental Product Declaration [70]. The carbon dioxide coefficients used in the LCA are gathered in

Table A1. Carbon coefficient used in the life-cycle assessment.

Stage	Unit	kgCO2e/Unit	Source
Material production
Sandstone	kg	0.149	[69]
Masonry mortar	kg	0.393	[69]
Infill mortar	kg	0.6436	[89]
Fast setting mortar	kg	0.6436	assumption from [69,89]
Levelling mortar	kg	0.341	[90]
Crushed gravel	kg	0.005	[69]
Round gravel	kg	0.003	[69]
Sand	kg	0.003	[69]
Cement (CEM I, CAN B)	m3	249	[91]
Aggregates (50% RCA)	m3	3	[91]
Other for recycled concrete	m3	9	[91]
Concrete with 50% RCA	m3	193	[91]
Reinforcement steel	kg	0.773	[69]
Laminated board	kg	0.942	[69]
Construction process
Electricity	kWh	0.125	Swiss consumer mix [69]
Gasoil for machinery	kWh	0.324	[69]
Upcycling process	m3	0.722	Calculated from demonstrator
Pumping	m3	1.00	[92]
Robotic 3D printing	m2	46.12	Unreinforced 20 cm thick [63]
Transport
Truck (7.5–16 t capacity)	tkm	0.230	[69]
End of life
Crushing (0–40 mm)	t	1.733	Calculated from industry data
Fines	kg	0.013	[69]

of Appendix B.

The environmental impact of crushing is based on the average machine consumption provided by recycling centres summarized in

Table A5. Parameters used in the life-cycle assessment.

Name	Unit	qte	Source/Comment
Transport
Demolition site—Recycling centre	km	30	assumption
Demolition site—Construction site	km	30	assumption
Concrete plant—Construction site	km	9
Recycling centre—Construction site	km	3.1
Stone quarry—Construction site	km	60
Brick and mortar supplier—Construction site	km	4.7
Construction
Utilisation rate of available waste	%	40
Assembly of robotic dry-stone masonry	kgCO2eq/m3	49.2	[73] Incl. material
Reutilisation of formwork	x	10
Rebar content	kg/m2	16.44	comp. from ∅10, 150 mm
End of life
Concrete crushing consumption	litres/ton	0.5	Range is 0.25–0.5, technical doc + interview
Concrete hammering consumption	litters/ton	0.5	Range is 0.25–0.5 technical doc. +interviews
Oversized rubble pieces to be hammered	%	4.8	by weight, conservative
Yield of RCA production by crushing	%	60	[96] + interviews

. The wear and tear of tools is not considered as it is assumed to be marginal in a repeatedly used fabrication process [71].

The transportation distance is based on the demonstrator construction and given in Appendix B, in

Table A4. Materials quantities of the demonstrator for the options of the life-cycle analysis.

Material	Unit	qte	Source	Comment
Infill mortar used	kg	390	measured
Fast setting mortar used	kg	100.5	measured
Levelling mortar used	kg	80	measured
Total weight of wall	kg	3493	measured
Large rubble (crane-placed)	kg	2283	measured
Small rubble (manually placed)	kg	1024	assumed
8 mm gravel	kg	165.3	measured	To decrease mortar needs. Could be RCA

. Although the construction of the demonstrator consumed around 25% of the delivered waste due to prototyping constraints, an upcycling rate of 80% of the delivered concrete waste is considered in the environmental assessment. This assumption considers the application of the upcycling process as a system for a large portion of the structures considered for construction rather than for a single prototype. Scale optimisation is thus considered, minimizing unused concrete waste, which is not crushed after the end of construction, but stored in recycling centres. Other relevant parameters used in the life-cycle analysis are detailed in Appendix B.

For the stone masonry variants, three different mortar ratios are considered: lower and upper bound from literature, and the mortar ratio measured in the prototype.

4. Results

4.1. Demonstrator Design

From a bin of 10 m³ filled with 15 tons of concrete rubble, a total of 36 rubble pieces are scanned and pictured in Figure 21.

This digital stock is used to pre-generate 138 stacking options (most of which are visible in Figure 22) with 2.7 × 2.5 m boundaries, using the algorithm by Wang et al. [53].

For selecting the design geometry to be constructed for the demonstrator, the stacking options are ordered by void ratio (F_VR). The first eight are then ranked using the heuristics described in Section 3.1.2, using the shortest vertical and horizontal paths shown in Figure 23.

The results of the geometrical masonry analysis of the best eight design options are given in

Table 2. Geometric indicators of masonry irregularity.

Solution	Void Area [%]	FFP [%]	FAH [%]	FAV [%]
SO-1	22.35	7.97	3.94	28.53
SO-2	24.80	7.36	2.83	28.38
SO-3	24.88	7.32	4.39	5.12
SO-4	25.20	9.19	1.94	24.19
SO-5	25.46	8.18	7.41	4.59
SO-6	25.52	7.22	1.67	29.10
SO-7	26.61	8.22	14.11	6.58
SO-8	26.71	9.25	4.65	30.09

. The stacking solution SO-1 is the layout option chosen for the construction of the demonstrator due to its smallest void area and visual harmony. The rubble units of the chosen solution cover an area of 270 cm (length) by 225 cm (height). To further improve the stacking solution and decrease the void area, one rubble unit is manually added to the selected solution layout. This rubble visibly fits the geometry of a large void at the top, but such placement is not found by the stacking algorithm. Moreover, a rubble piece in the chosen stacking solution is rotated 180° around the axis normal to the wall surface. This rotation is necessary to ensure the stability of the piece before mortar hardening and thus safety of workers. Without this rotation, the stability is jeopardized by the 3D irregularity of the bottom face of the rubble piece, which is not recorded in the 2D scanning process. Nonetheless, the design solution demonstrates the performance and added value of the stacking algorithm by Wang et al. [53] through the successful generation of uniform horizontal courses, vertical interlocking joints, and minimum void area.

4.2. Demonstrator Construction

The thickest rubble considered for assembly is 31 cm. By planning a 5 mm tolerance on each side, the wall thickness of 32 cm is considered for construction and formwork dimensions. Thinner rubble pieces, when placed against the front face of the formwork, leave a void in the back, which is filled manually with smaller rubble pieces and liquid mortar. The planned height of 225 cm is reached and finished with a flat top surface using levelling mortar. The prototype, therefore, achieved a slenderness of 7:1, which is considered thinner, or equal to contemporary stone walls. Thus, such walls from reclaimed concrete can be considered for building structures while saving precious floor area. The construction and the resulting wall are visible in Figure 24.

The recorded quantities of concrete rubble and mortar are given in

Table 3. Comparison of mortar quantities.

	Ratio of Mortar [vol%]
Almeida et al. [57] 1	13–23
Norms [38] 2	15–24
Saloustros et al. [72]	42
Oreb et al. [18]	29–46
Historical renovation 3	34–38
Grangeot et al. (this study)	19

¹ Metrics from surface observation of roughly coursed masonry walls made of unsquared, or roughly squared stones. Volume of mortar is likely higher than visible 2D area. ² Estimations by a professional in renovation of historical stone masonry and range for double leaf roughly coursed masonry walls (type C in Swiss norms [38]). ³ Estimation provided by an experienced mason in renovation of historical stone masonry based on a case study.

and compared to other reference walls made of concrete rubble, or natural stone. Due to the large size of the masonry elements compared to wall size, the mortar content of the prototype is significantly lower than most irregular stone masonry walls. This is due to the optimization of the 2D void ratio in the design phase thanks to the digital stacking algorithm, and the manual filling of remaining voids with smaller rubble pieces.

4.3. Demonstrator Performances

4.3.1. Productivity

The construction of the demonstrator is carried out in 39.7 h, yielding a productivity of 0.15 m²/h with a single human worker. The most time-consuming part of the construction process is the manual filling of voids using smaller rubble pieces (23.6 h, 63% of the total time). This is followed by material handling (7.9 h, 21%) and the pouring of interstitial voids (3.8 h, 10%). The assembly of the 14 rubble pieces is achieved in 1.6 h (4%), while their drilling is executed in 0.2 h (1%), and the overall digital planning and monitoring after design selection is achieved in 0.3 h (1%). A comparison of productivity with literature and practice is provided in

Table 4. Comparison of masonry size and productivity between various wall construction systems.

	Unit Weight (Average) [kg]	Productivity [m2/h]	Density [unit/m3]
Robotic retaining wall 1 [73]	997	1.23	1.6
Excavator boulder wall [73]	-	1.82	-
Robotic mesh mould wall [74]	-	0.10	-
Concrete 3D printing 4 [75]	-	1.194	-
Dry stone masonry [76]	5–15 6	0.33	100–300 6
Irregular stone masonry 2 [76]	5–15 6	0.28	100–300 6
Concrete casting 3 [76]	-	0.46	-
Concrete Block Masonry 5 [76]	20	0.98	55
Clay Brick Masonry 5 [76]	8	0.57	97
Small concrete rubble wall [18]	2–11 6	0.04.-0.08	150–700
Large concrete rubble wall (this study)	163	0.15	7.3

¹ Construction time is computed from the average placement time and number of rubble pieces. ² in mortar bed, 9.4 h/m³, wall thickness equal to the demonstrator (32 cm). ³ Concrete walls cast in place, job-built formwork with one use. ⁴ Computed from printing speed (100 mm/s, 400 mm² nozzle) without setup and cleaning time. ⁵ See Figure A1. ⁶ Assumption from separate measurements.

. It must be noted that compiling data from various sources in different contexts and conditions can lead to large variability of productivity metrics.

Compared to other wall systems, the construction of single-leaf masonry walls made of large concrete rubble appears generally slower. Reducing the need to fill the voids with lighter rubble pieces manually can drastically improve productivity. In literature, productivity seems inversely correlated to the weight of the rubble units. Johns et al. have superior productivity compared to the process presented in this article because their rubble units are around six times bigger on average. Moreover, their voids are not filled because the expected uses differ, leading to large time savings.

4.3.2. Construction Precision

The boundary of the volume of the built wall respects the tolerances of concrete walls from Swiss norms, with an average deviation on all sides of +3 mm.

The local deviations of the position and rotation of individual rubble pieces are shown in Figure 25.

Excluding the intentional 180° rotation of rubble piece #02, the maximum deviation is 4.8 cm and 4.5° for rubble piece #17. Such maximum deviation at the top of the wall is due to a gradual build-up of imprecisions, as only the horizontal position of the rubble piece is verified.

4.3.3. Structural Performance

To verify the load-bearing capacity of the upcycling construction method, the design options are analysed, material tests are performed, and the structural behaviour of the proof-of-concept demonstrator is verified through a compression load test.

Horizontal courses are successfully achieved, which is the most important parameter, as pattern regularity influences compressive behaviour [58]. The masonry of the demonstrator and stacking solutions have some degree of regularity and no end-to-end horizontal joints. Therefore, the compressive resistance is conservatively expected to be 3–8 MPa based on Swiss norms detailing the influence of these parameters for irregular stone masonry [77]. The high compressive resistance of the mortar used in the fabrication process is not considered. However, the second most important parameter for compression resistance and stiffness is the contact between rubble and mortar [78]. For the demonstrator, the rubble pieces are not cleaned nor saturated in water, leading to the absorption of the mortar water and worsening bonding at contact surfaces [79]. The shapes of rubble pieces also matter since uneven surfaces can concentrate stresses at contact areas [78]. Such structures made of irregular elements inevitably contain voids. If voids cannot be filled during construction, injecting mortar afterwards can improve strength and stiffness [80]. For out-of-plane stability, the slenderness ratio of 7:1 respects the Swiss code of natural stone masonry [38].

The material characterization of the concrete rubble pieces yields an average compressive resistance of 58.2 ± 9.7 MPa and an elasticity of 37.3 ± 7.0 GPa.

The uniaxial compression load test did not result in failure of the demonstrator wall despite reaching the maximum testing capacity (1 765 kN, 2.04 MPa). The stereo-DIC monitoring shows a maximum plastic deformation of 0.1 mm vertically, and a secant modulus of elasticity of E_s50% =14.1 GPa during first loading. The Swiss norm [81] recommends measuring the secant modulus at one-third of the maximum stress. Since maximum stress could not be achieved but is expected at 3–8 MPa, the secant modulus is measured at 1 MPa, which is 50% of the maximum loading during testing. The major principal strains obtained through stereo-DIC at maximum load are shown in Figure 26.

Upon visual inspection, the areas with the maximum major principal strain have cracks of maximum 0.1 mm in the joints visible on one side. No cracks are detected on the surfaces of the rubble pieces. While the compression resistance of the wall could not be reached, this test result can be used as a lower bound.

To identify structural use cases, the expected compressive resistance of the developed walls is compared with residential buildings using the geometry and loads shown in Figure 27.

The Swiss security factors of γQ = 1.5 for live loads and γG = 1.35 for dead loads are used for dimensioning the maximum load [82]. For walls 32 cm thick (the demonstrator wall thickness), the resulting maximum load at the base of the ground floor wall is Qd = 312 kN/m. This is equal to a maximum stress of 0.98 MPa, which is below the maximum stress of 2.04 MPa reached without failure during the load test of the demonstrator. In Swiss building codes, a material security factor γM = 2.5 for irregular stone masonry in compression perpendicular to horizontal joints should be considered [38]. This lowers the maximum allowed stress to 0.82 MPa. As the 2.04 MPa compressive resistance of the wall is considered a lower bound, and 3–8 MPa could be expected from such walls, the walls are considered as sufficiently load bearing in compressive application up to 3 storeys above ground floor for residential buildings with spans of 4 m. To increase the number of stories while considering the same resistance, the deadweight due to the walls could be reduced by decreasing the wall thickness on each floor. This can be achieved by sorting the rubble pieces by descending thickness.

4.3.4. Energy Consumption

The energy demand and efficiency of the proposed upcycling process compared to literature and practice are given in

Table 5. Energy consumption of all tools used in the fabrication of masonry walls.

	Energy Demand [kWh/m2]
Robotic boulder wall 1 [73]	72–110
Engineered boulder wall 2	49–75
Robotic mesh wall [74]	17
3D-printed concrete wall [83]	1.87
Manual stone masonry 3	0.17
Large concrete rubble wall (this study)	1.83

¹ Based on 6.2–9.5 L/m³ of diesel fuel of placed stone and 10.7 kWh/L conversion factor. ² Using the ratio of productivity between automatic and manual operation of excavator detailed in Table 4. ³ Considering only the consumption for mixing mortar, based on electric measurements during mixing for the demonstrator, and considering twice the quantity of mortar. It excludes transportation.

The biggest energy demands of the proposed upcycling process measured during the construction of the demonstrator are the idle state of the robotic arm, at 3.96 kWh (36%), followed by the crane usage, at 3.62 kWh (33%) and its digital augmentation components, at 1.32 kWh (12%). The drilling is responsible for 0.8 kWh (7%), the concrete mixer for 0.5 kWh (5%), the jackhammering for 0.4 kWh (4%). The power consumption of the concrete drill itself is 0.28 kWh (3%). Splitting the consumption measurement of the drill and the robot allows to predict better the energy needs for onsite construction where no robotic processes would be involved. The consumption of other small handheld tools is negligible: 0.09 kWh combined (below 1%). Compared to other digital construction processes using mineral materials, the fabrication of walls, as described in this paper, appears to require less energy. While in the same order of magnitude as 3D printing of concrete, the energy needed for the proposed process is drastically lower than for boulder walls and concrete walls produced using robotic welding of rebars. Understandably, it still requires more energy than the manual construction of traditional stone masonry. Drilling the lifting anchors by hand while following digital instructions is expected to decrease the energy demand of the complete process by at least a third.

4.3.5. Environmental Impact

The results of the comparative life cycle assessment are shown in Figure 28 and Figure 29. The comparative LCA highlights that the construction of rubble masonry walls, have similar environmental impact regardless of transport variation. The OP2 option of rubble masonry wall with the most transport provides a reduction of 38.7% of global warming potential compared to newly poured concrete made with 50% of Recycled Concrete Aggregates (RCA), and of 57.4% compared to traditional irregular stone masonry with identical mortar ratio (19%). Conversely, OP2 has an upfront carbon emission 36.2% superior to walls built in hollow concrete blocks and 13.0% superior to walls built in hollow clay bricks. Despite the heavy weight of concrete, transport has less impact than the production of mortar, concrete, or bricks, considering transport distances detailed in Table A5.

When comparing wall variants using eco-points 2021 as the functional unit (Figure 29), the fabrication process detailed in Section 3.2 produces walls with the least ecological scarcity. Thus, OP2—the option with most transport—is expected to have an ecological scarcity reduction of 70.7% compared to stone masonry with equal mortar ratio (VA1), 51.4% compared to cast recycled concrete (VA2), 52.1% compared to 3D-printed concrete (VA3), 15.4% compared to hollow concrete blocks (VA4) and 21.2% compared to hollow clay bricks (VA5).

The comparative LCA highlights intriguing differences of performances of the hollow concrete blocks depending on the scope of analysis. The hollow concrete block has a lower environmental impact than the concrete rubble walls when considering the Global warming potential while the exact opposite is observed in the EcoPoint comparison. This is certainly due to the consideration of additional environmental indicators in Eco-points, such as resource use and waste impact, which is naturally lower for reuse solutions.

The reduction of the detrimental environmental impact of concrete rubble masonry, as described in this article, compared to traditional irregular stone masonry is mostly due to the avoidance of stone extraction, which is located further away than the closest concrete recycling centre, and the avoidance of concrete waste processing.

Traditional irregular stone masonry usually has a higher mortar ratio than the 19% measured during the construction of the demonstrator, further increasing the related savings. This is due to two main reasons: by using larger rubble pieces compared to wall size and by minimizing internal joints through the making of single-leaf masonry rather than multiple-leaf masonry. Making single-leaf masonry with flat, irregular pieces is rare because it requires extreme planning and the availability of heavy-lifting machinery, both made more streamlined in this research.

5. Discussion

5.1. Contributions

This study highlighted the opportunities of reusing concrete rubble from demolition for the construction of new structures. Through the exploration of possible structural typologies, architectural parameters and connections, the study identified relevant design solutions for structural rubble reuse. The design option meeting all requirements was built as a full-scale demonstrator. This study thus contributes to the increase of the value of concrete rubble from demolition, which is currently undervalued as waste and further downcycled.

Additionally, the contribution of this research lies in the holistic development of an accessible digital design-to-construction process of walls from such undervalued material. The developed process optimises construction time, storage space, and mortar quantities, thus reducing construction costs and reducing environmental impact. This construction process generates structural walls with a reduced thickness compared to multi-leaf masonry structures, thus allowing more usable floor area within a constrained bounding footprint.

The accessibility of the construction process is considered higher than pre-existing digital solutions, in particular thanks to the 2D acquisition and manipulation of irregular geometries. Thus, scanning of complex geometries can be achieved with low-cost cameras.

Moreover, by using a 2D stacking algorithm, the research allows the design of masonry structures while minimizing voids between blocks and ensuring overall stability. Combined with a low rubble size to wall size ratio, the mortar proportions are low—up to two times lower—compared to irregular stone masonry, or masonry with small concrete rubble as base material [18,72].

In this research, digital planning integrates the computation of pick points to achieve the 3D orientation of irregular heavy pieces during their lifting. Thus, digital planning avoids the iterative manual positioning of heavy concrete pieces and prevents site hazards, or injuries. Most important, this accessible construction method allow the complex 3D orientation of irregular pieces with simple lifting tools, while existing methods rely on high-end digital tools, such as robotic arms [23,24,26], and automated excavators [36,50].

Through the implementation of adequate stacking parameters, 2D geometrical benchmarking against built examples, and physical load testing of a full-scale demonstrator, this research provides promising early results for constructing slender walls expected to withstand compression forces typical of low-rise residential buildings.

The study also shows that the developed process provides alternatives to the detrimental environmental impact of the demolition of concrete structures and the construction of new ones.

5.2. Limitations

5.2.1. Research Limitations

The construction of the demonstrator reflects an unrealistic mix of prefabrication tools and on-site processes, which thus hinders their immediate applicability in the construction industry. Moreover, this construction process does not benefit from all state-of-the-art routines of prefabrication and on-site construction that are optimized for productivity.

Additionally, due to the large rubble-to-wall size ratio of the proposed structures, the state-of-the-art structural analysis of uniform masonry can hardly apply. Complementary analysis methods and tools are thus needed.

Lastly, the construction of the demonstrator contained two construction errors compared to state-of-the-art masonry construction principles. The first error concerns the choice of a mortar with high resistance (60–80 MPa), most likely higher than the resistance of rubble units. The second error is the application of this mortar on uncleaned rubble pieces.

5.2.2. Industry Limitations

The assembly of heavy concrete rubble, as presented in this article, relies on intensive crane usage, which is expensive for on-site assembly. To avoid a disproportionally high crane usage on construction sites compared to the generated value, the proposed upcycling process can be implemented during the downtime of large cranes, or using dedicated small hoisting equipment.

Moreover, acquiring the geometry, drilling, and assembling the concrete rubble pieces is more appropriate when done at the same location to avoid damage during transport and handling, thereby reducing the need for design adjustments. Lastly, being made of concrete and arranged as masonry, the walls are prone to the same problems as these two construction systems, whose vulnerabilities to outdoor exposure are important and known [84].

Depending on local dynamics, large flat pieces might not be the most predominantly available geometry of concrete rubble. Thus, substituting stone with small concrete rubble in irregular stone masonry, as developed by Oreb et al. [18], can adequately complement the options for rubble reuse for the accessible construction of walls for buildings.

5.3. Future Works

This study must be extended to improve the chances of adoption of the proposed construction process in the building industry and provide a wider range of constructive solutions using concrete rubble.

The potential for widespread use of prefabrication concrete rubble walls by the industry may be extended by laying the pieces flat non-sequentially and subsequently rotating the wall upward using a tilting table. This strategy is expected to increase safety and yield better productivity due to the avoidance of formwork repositioning and the improved accessibility of voids for filling.

Leveraging the flatness on both sides of the proposed walls, additional building layers, such as thermal insulation, or water protection can be added to obtain desired properties. Studying the implications of these new interactions within building layers through building physics is needed to validate this possibility.

Other design and construction features must be tackled, such as the integration of openings, the consideration of elongated reinforced rubble pieces as lintels, the design of interlocking rubble pieces in corners, and connections with slabs and building layers.

The structural study conducted on the built demonstrator only focused on compression behaviour without eccentricity of loads. More research is needed to qualify the walls’ structural performances under compression loads and other load cases, such as lateral loading.

The thickness and rebar layout of each rubble unit are registered during the construction of the demonstrator, but they are not used as an input of the stacking algorithm. In future works, integrating thicknesses in the design could reduce construction time and mortar quantities dedicated to filling the back void while improving stability. This would require either more selective stacking options, or the sorting of rubble per thickness. Ideally, such sorting would occur in recycling centres where rubble pieces larger than the crusher intake are already dispatched from the rest. Additionally, sieving small rubble pieces by size could improve productivity as this work is currently done by hand upon filling the voids and is time-consuming.

The mortar used in the construction of the demonstrator wall has a high strength and, thus, a high environmental impact. Since such high strength is not needed, mortars with lower environmental impact should be used in future prototypes.

This study uses a specific subset of concrete rubble, namely large flat pieces for the most part. Hence, the proportion of applicability of this research to existing material dynamics is not fully known. As no known study exists on the statistical distribution of the geometric and mechanical properties of concrete rubble from demolition, a stock characterization is conducted to answer this gap and inform the current share of applicability of multiple upcycling processes, and thus the market size for each category of rubble.

Lastly, as the presented structural typology relies on specific rubble shapes which depend on deconstruction practices, indicative guidelines for operators of hydraulic excavators for demolition could be drafted while considering the optimization of transported waste. Similarly, guidelines for masons could be issued since most of them are not trained for constructing such masonry.

6. Conclusions

Material circularity is a critical pathway to sustainable concrete construction. Given the lack of environmental savings offered by concrete recycling, alternative strategies are urgently needed to valorise concrete rubble better. This study unveils new opportunities offered when considering concrete rubble as an untapped construction material rather than a waste. In this research, this material is identified as local, low-carbon, cheap, and capable for use in compression-based structures of desired dimensions. Through their aggregation, concrete rubble pieces offer freedom of new structural geometry, which is explored to expand the design possibilities of walls using concrete rubble. The main advantage of reusing concrete as rubble lies in its sourcing, which does not impact current habits on demolition sites, contrary to other reclamation strategies.

Building on pre-existing construction machinery and current construction skills, an innovative and accessible upcycling process is developed for constructing single-leaf masonry walls from concrete rubble. This safe, scalable, and resilient upcycling process provides affordable tool augmentations for the design and fabrication of space-saving structures aimed at low-rise and mid-rise buildings. The accessible design tools (i.e., picture-based scanning, 2D layout planning) developed in this research minimize space on assembly sites and the addition of other materials, thus minimizing embodied emissions. The 3D orientation of irregular geometries is achieved through gravity by lifting from a specific point. Compared to manual irregular stone masonry, the construction process detailed in the article is expected to be faster to build, have sufficient compressive resistance, reach more slender outcomes, and generate a less detrimental environmental impact than conventional concrete walls while diverging concrete waste from energy-intensive recycling processes.

The rapid and widespread implementation of optimum upcycling applications in the construction industry is possible without altering current construction logistics. The construction of a demonstrator confirms the feasibility, accessibility, and environmental saving potential of the upcycling process. Research is still needed to develop the extent of possibilities and the prospect of industry adoption.