Industrializing precast productions Adaptive modularized constructions made in a flux

Building in heavy rain is seldom beneficial, but common practice on site. It promotes inaccuracies and impairs the use of modern but sensible high-performance materials and costs time, since disruption in construction frequently causes complicated returns to the planning process. Nevertheless, a handcrafted production process is still consid-ered the one and only alternative since all buildings are unique and thus must be manu-ally constructed on site. Indeed? The priority program entitled “ Adaptive modularized constructions made in a flux ” funded by the German Research Foundation follows a completely new approach. Buildings are divided into similar modular precast concrete elements, prefabricated in flow production, quality-assured, and just-in-time assembled on site. Comparable to puzzles with many pieces, the uniqueness of the structure is maintained. The motto is: “ Individuality on a large scale-similarity on a small scale ” . The contribution presents approaches of modularization, production concepts, and linking digital models. Serial, stationary prefabrication enables short production times and resource-efficient modules that are assembled to load-bearing structures with low geometrical deviations. Stringent digitalization ensures high quality of all intermediate steps. These comprise fabrication, assembly, and the whole service life of the structure. The result is a lean production process.

free shapeability, worldwide availability, and low material costs.
The amount of concrete used each year adds up to almost four tons per person. The associated GWP is correspondingly high. The production of the basic material cement alone is responsible for~5% to 10% of the worldwide CO 2 and is thus the highest single emitter. 2,5,6 The natural resources for concrete, such as sand or water, are becoming short. 5 Europe's challenge is the replacement of constructions. 3 After the building boom in the 1960s to 1980s, schools, bridges, and industrial plants are now to be replaced by new ones. Their service life is simply over. However, it is not only motorists at the countless (permanent) construction sites who realize every day that slow building activities cannot be reconciled with our sensitively networked flows of goods and traffic. The replacement of constructions is stalling. The "arteries" of our industrial base are clogged up. Time and human labor are wasted, and the environment is polluted.
Here, the research concept of the priority program starts from Reference 7. The aim is to consistently minimize waste (lean production 8 ) while maintaining individual, durable, and esthetic building structures. The aim is to use significantly less material, avoid errors, introduce consistent prefabrication with quality assurance, and thus achieve the fastest possible construction activity on site. The key is to break down concrete structures into similar individual modules which are mass-produced in a digitized production facility.
2 | THE PRIORITY PROGRAM 2187 " ADAPTIVE MODULARIZED CONSTRUCTIONS" The program "Adaptive modularized constructions made in a flux" (SPP 2187) was established in 2020 by the German Research Foundation (DFG). It is interdisciplinary and fundamental-oriented. Around 50 researchers from institutes of structural engineering, mechanical engineering, computation in engineering, and mathematics are working together to build as quickly and precisely as possible on-site using stationary serial prefabrication. Figure 1 shows the research team on the left in February 2020 and on the right the seven participating universities across Germany.
The possibility of serial production arises from the segmentation of load-bearing structures into modules, which can be plane or trusslike components. Figure 2 illustrates the basic principle using the example of a shell. The segmentation is not motivated by statical systems, for example, like modules of columns, beams, or plates, since F I G U R E 1 Team members and collaborating universities (photo: Julia Lippmann, graphic: Patrick Forman) F I G U R E 2 Segmentation of a shell into scalable modules along with characteristics for fabrication, assembly, and use (graphic: Patrick Forman) this hardly leads to a significant degree of repetition, but rather too extensive individual production (manufacturing) and high weights.
Besides, complicated nodes are created. Instead, the manufacturing and joining process controls the pitch with the goal of high quantities and simple connections that are tolerable to errors. The modules are similar-not the same-and scalable in basic sizes. Scaling means that side dimensions, thicknesses, reinforcement quantities, or materials of the modules remain changeable (adaptive). The individuality of the overall structure is preserved like a mosaic or a puzzle with hundreds of individual pieces. A positive side effect of the modular concept is that modules can be exchanged during use (alterable), that is, the supporting structures can be locally repaired, reinforced, or adapted to changed utilization.
Stationary prefabrication eliminates the unavoidable inaccuracy of the construction site in favor of the quality of industrial flow production, as known from the automotive industry. Production speed, geometric and material precision, durability, and resource conservation through component optimization [9][10][11] are improved multiple times.
The modularization starts retrospectively from the bearing structure. The production of the modules in the factory and the rapid assembly yield the design of the modules. The rule is: process controls designand not vice versa. Figure 3 shows this at the top. Production is automated in a linear flow principle (left side) with the individual steps of formwork, reinforcement, controlled temperature treatment for hardening, 12 qualification of the individual modules, and sensorbased labeling. The sensors are used for seamless tracking, assembly controlled by a digital twin (center right), and as indicators for assessing the load-bearing capacity or serviceability properties of modules 11 over their service life (right). A digital model controls all processes and interactions. In doing so, each module "knows" its properties (e.g., strength, geometry, position within the structure) and monitors them over their service life. Production can thus take place just-in-time without the need for additional storage space on the construction site.
The research program is divided into 12 individual projects, which are connected in their developments by means of three working groups and a central project. The working groups cover the three central research topics, namely: • Design and detailing for modularization, • Systems and concepts of production, • Digital models. Figure 4 shows the three thematic areas as colored circles and the projects with their short titles in their assignment. All projects are interdisciplinary and involve at least two subject areas.
F I G U R E 3 Robot-assisted flow production of concrete modules with rapid assembly using sensors and continuous digital modeling as well as quality control (graphic: Patrick Forman) F I G U R E 4 Project topics and classification into the three research fields (circles) (graphic: Patrick Forman) In the following article, the three topics are discussed in detail and the individual developments are presented in terms of their goals and initial results.

| DESIGN AND DETAILING FOR MODULARIZATION
Today, modular construction is characterized by the prefabrication of entire structures or partial structures, such as garages, subsystems of residential buildings or elements of entire high-rise buildings. Prefabricated single-family houses consisting of a few massive modules are already being erected within just one day. 13 Timber construction modules, which form functionally fully equipped residential cells, are "stacked" to form the overall supporting structure. 14 Concrete ceilings as classic precast concrete elements are also already prefabricated with integrated building technology. 15 The driving force here is the saving of costs and time. According to Reference 16, modular construction in this form already offers time savings of up to 50% as well as a cost reduction of about 20% compared to conventional construction methods. Nevertheless, these modules are massive (several tons) and are mostly prefabricated by hand. The design is determined by the later function.
In contrast, design and detailing in SPP 2187 is subject to a clear paradigm: precise fast-track construction of the future can only succeed with stationary fabrication and flow production methods.
Therefore, the subdivision of structures and components into transportable modules, which are ready for a plain assembly on the construction site, is mandatory. The design approach must be focused on modularization itselfand this must be done with a holistic standard.
However, modularization here does not mean a modular system with large and heavy "prefabricated parts", such as in industrial hall construction. The modules are not "ready-made," but they are adaptive during production, that is, they can be adapted to the respective requirements "on the fly" within previously defined limits (mass customization). 17 Only such adaptive modules can meet the demand to create individual and esthetic building structures. For this innovation, two essential factors enable a quantum leap compared to the developments of the 1960s: The use of Ultra-High Performance Concrete (UHPC) with corrosion-free reinforcements and the continuous digitization of the processes under the buzzword Industry 4.0. Thanks to the new and precisely adjustable material, slimmer components and novel joints without corrosion protection can be realized. 18 Digitization, on the other hand, enables the individualization of the components, the management of production with continuous quality control, and interactive feedback to the design in an overall planning model. 19 The implementation of Building Information Modeling (BIM) also provides a tool for monitoring the entire life cycle of the structure, including operation and recycling. 20 In principle, two approaches to modularization can be distinguished ( Figure 5). Either a set of known modules is the starting point for the overall structure (bottom-up) or a given structure is broken down into sufficiently small modules (top-down). In both cases, the manufacturing process, namely the limits of production, transport, and assembly, determines the design space.
Several subprojects have identified assembly on the construction site as a critical design factor and consequently place the last step of the production at the beginning of their considerations (design for assembly). 21 Technically, joints, especially dry joints, are weak points in traditional concrete construction and should be avoided if possible.
In modular construction, they are the ubiquitous standard case and must be included into the design process with regard to scalability, precision, and tolerance compensation. 22 The challenge is to use quality controls and measurements not only randomly, but to integrate them continuously into the production process and to feed the results back into the planning. The goal is, for example, an automation in which deviations in the dimensions of individual modules can be compensated by adjusting the manufacturing parameters of subsequent modules. 23 If successful, this procedure is "tolerance-free," which means that within the completed structure, all deviations from production will neutralize each other. In this respect, the design task is comprehensive since it must also shape the layout of the overall process. Of course, this can only succeed if the design itself is also subjected to a certain modularization. A key to this is the parametric modeling of the components with corresponding optimization routines.
As the interface between the modules, the formation of the joints, whether dry or bonded, requires special attention both functionally and in terms of design. Depending on the modularization method, the contact points must meet different requirements for force transmission, such as normal forces, shear forces, moments, and combinations of these. Suitable joining principles and joint designs have to be developed for this purpose, which also may place previously unimaginable demands on precision. 22 In this context, non-corrosive tendons made of carbon can be used for force transmission and allow dry joints without further corrosion protection. 24 F I G U R E 5 Merging of certain basic modules to form a structure (bottom-up) or modularization of a structure into new modules (topdown) (graphics: Patrick Forman) If we now consider the modules themselves, which have to be moved quickly, precisely, and automatically in serial flow production, the load-bearing capacity of the industrial robots required for this purpose sets an upper limit on the modules' mass of~1 ton. This limitation does not mean that the modules should be planned as small as possible. However, it is imperative to designing in a force-flow-oriented, light, effective, and thus material-saving way. 25 Voluminous and block-like modules have little chance of meeting the quest for resource-saving constructions. Planar or truss-like structures are sought after. Because of the defined and reproducible production conditions, it is also perfectly possible to increase the geometric complexity of the individual components according to the requirements.
It is well known that one of the biggest impediments in traditional concrete construction is the need to produce the formwork for the casting molds. It, therefore, makes sense to consider in particular processes that do not require any formwork at all, such as extrusion-based selective material deposition (additive concrete construction).
In this process, the fresh concrete is deposited in a geometrically defined manner as a so-called filament with the aid of an extruder nozzle. 26 If, in a first assumption, the deposition is made on a flat base surface, a modularization of double curved shell structures into planar facets is necessaryactually a domain of so-called gridshells made of steel and glass. Obviously, the discretization of free forms with the methods of discrete differential geometry belongs to the canon of topics. Equally promising is a link to the work of the BLOCK Research Group (BRG) at the ETH Zurich, where essentially compressionstressed vaults made of relatively small individual parts demonstrate the performance of modular constructions ( Figure 6). 27,28 The basis for such faceting, for example with planar quadrilateral facets (PQ-mesh), can be force-adaptive concrete shells similar to those of Felix CANDELA, Ulrich MÜTHER, Heinz ISLER and others (Figure 7).
The procedure can be easily transferred from roof structures to socalled shell bridges. 29 For example, a subproject is dedicated to a graphbased decomposition of bridge structures into surface elements. 30 In the end, in addition to the modularization (outer module geometry), all designs also have to define and parameterize the individual module elements (inner module geometry) as well as the coupling of the module elements with each another. A "digital construction kit" is desirable, in which static-mechanical aspects as well as simulation (order of assembly) and sensitivity analysis of the modules and the overall system can be mapped. 31

| SYSTEMS AND CONCEPTS OF PRODUCTION
Building is a highly individual process today, just as it was a few decades ago. Buildings and structures are planned as one-offs and A construction kit is defined in Reference 33 as an abstract construct that contains all those subsystems (modules) from which different systems (structures) can be configured (Figure 8). In addition to the modules and their variants, the construction kit also includes an associated set of rules that describes the nature of the subsystemswith particular attention to the interfacesand thus ensures compatibility between the systems. 33 In order to ensure the exchange of individual module variants (e.g., different module geometries), clearly defined and standardized interfaces are absolutely essential, especially for complex systems.
A platform design is given, as described in Reference 33, if the subsystems can be differentiated into "platform" and "hat." In this context, the platform comprises all subsystems that are used repeatedly and unmodified across different systems. However, the individual subsystems do not necessarily have to be physically connected to each other. The hat includes the remaining subsystems, which can differ across systems and thus create a range of variants. The concept of platform and hat is illustrated in Figure 8 (below) using the example of a column.
The fractal character of standardization methods in particular allows a high degree of flexibility in the design of systems with controllable complexity of the components to be provided. For example, the modules within a modular system can be constructed according to the platform design, which in turn can be variably designed in different forms. 34 The example of a box girder bridge in Figure 9 serves as an example of the fractal nature: The hollow box can be realized by interconnecting the segment module variants existing in the modular system, as is common practice in the match cast process. For modularization in the sense of the priority program, however, the individual modules have too high dead loads and are not suitable for serial production using the flow production principle.
By transferring the two methods to civil engineering and developing them further, the goal of individualized structures (external diversity) with a low number of module variants (internal diversity) at the same time is being pursued. By limiting the amount of module variants to a small number, high repetition rates can be achieved for each module, enabling highly productive series production, as it is, for example, F I G U R E 9 Modular structure of a box girder bridge (graphic: Agemar Manny) common in the automotive industry. The design of series production in mechanical engineering today largely follows the approach of holistic production systems. This is a set of rules for the design of production processes 35 that is based on the principle of lean production, that is, production that avoids waste such as unnecessary transport operations, inventories, or rework. The use of lean production is associated with numerous advantages such as, in particular, higher productivity and shorter lead times. 36 A typical design principle of holistic production systems that can be applied to the production of concrete components is the flow principle. During production, the component is moved in a line through fixed work stations arranged according to the operations to be performed (Figure 10, above). This principle of process organization contrasts with the usual stationary or site production in the construction industry, in which a stationary component is manufactured using moving production equipment ( Figure 10, bottom). Efficiency increases by a factor of >2 when changing to the flow principle which can for example, be shown for the SYNCHRO production system at the company Trumpf. 37 Eliminating waste improves quality as well as production time and costs. The introduction of the zero defect principle as part of holistic production systems also helps to create an awareness of defect prevention. 35 Eliminating defects in processes and products is the best way to reduce costs and improve lead times and customer satisfaction. Quality assurance becomes an integral part and is closely linked to production planning. 37 The modular design places particularly high demands on the dimensional accuracy of the components to be manufactured. If a modular structure is subjected to a tolerance analysis, the dimensional deviation of the entire system results from a superposition of the dimensional deviations of its individual components. 38 The more components the structure comprises, the more sensible this effect is.
Quality assurance is therefore an essential part of the processes which have to be (further) developed for the production of precast concrete components. The aim here is to achieve the shortest possible quality control loops by measuring and feeding back quality data inline. Specifically, off-process and off-machine quality control loops are to be used, that is, the measurement data are either collected after the processing operation in the same processing station (off-process) or in subsequent measuring stations (off-machine) and fed back into the process control 39 (Figure 11).
To avoid rejects of "inaccurate" components during production and assembly, the concept of so-called selective assembly, which is well known from mechanical engineering, can be used. This concept is used, for example, in the production of assemblies in the automotive industry with particularly high dimensional accuracy requirements, such as diesel injectors. Selective assembly is used to compensate for variations in actual dimensions occurring in the production process, for example with adaptive manufacturing in conjunction with individual assembly. 40 Here, components to be assembled of type A are manufactured in such a way that, statistically, they produce the best possible fit with previously manufactured parts of type B. For example, a shaft with oversize reduces the fit clearance to a hole with oversize. In modular structure construction, the modules are placed in the overall structure in such a way that dimensional deviations known In this way, novel approaches for the efficient production of modular load-bearing structures are created by adapting methods of industrial series production.

| DIGITAL MODELS
Digital design using concrete has enabled the viable construction of complex structures which would otherwise only have been possible with a great amount of craftsmanship and correspondingly high costs.
Digital design provides the basis for additive methods such as 3D concrete printing (eg, see . Also, formwork construction, 44 51 In the field of design, production, and assembly of precast concrete elements, digital building models are already being used in several ways. In the area of planning the focus is, on the one hand, how to describe precast concrete elements geometrically as simply and reusable as possible 52 and, on the other hand, how to implement production processes optimally, taking into account variations in production technology. Initial approaches to the integration of external information for the control of the production process have also already been conceptually considered. 53 Industry 4.0 is characterized by the interaction of products, services, processes, and organizational structures using innovations from the fields of information and communication technology. 54 The aim is to enable the manufacture of highly individualized products tailored to customer requirements without having to compromise automation or efficiency. The products and production systems that cause this para-  F I G U R E 1 2 Illustration of concrete modules as administration shells for construction digital twins. Source: Detlef Ger-hard & Markus König production system variants are generated with the help of parametric approaches, and also the digital control of the production of precision concrete elements is realized. Through the uniform use of data and the development of digital interaction chains, completely new methods for the production of adaptive modules from high-performance concrete in industrial flow production can be developed. Figure 13 illustrates two approaches to a module definition based on graph theory. In the top-down approach, a given design geometry is analyzed and transformed into a digital graph-based representation.
Subsequently, shape finding can be initiated by formally decomposing the design geometry using formal graph transformations. In the bottom-up approach, on the other hand, the concept is pursued by combining existing, parametrically described modules in such a manner that they come as close as possible to the design geometry. Here, too, graph transformations are used to assemble the entire system.

| CONCLUSION
Time becomes the decisive factor of building in existing context in Germany and Europerapidness is the guiding principle. It is necessary to drastically reduce restrictions on the traffic flows of infrastructures due to long construction periods. Therefore, the modular construction presented here relies on consistent quality-assured prefabrication with serial character and rapid construction on site lasting only a few days, controlled by a digital twin. In essence, it is about transferring and implementing the methods of lean production and Industry 4.0 to the construction industry. Costly human labor, unnecessary material consumption, waiting times, and traffic jams as well as inaccuracies and errors are prevented. The result is a holistic, lowwaste construction process, which is only possible through the methods of digitization. The main conclusions are: • The advantages of serial production such as weather independence, precision in geometry and material, production speed, or seamless quality control can be used for any supporting structure made from concrete. The key is to segment the structure into many similar modules.
• The fabrication principle can be used for modularized structures as well as for a priori mass production ready (repetitive) components such as segmental linings 56 or solar thermal collectors. 57 • In contrast to classical prefabricated building, the individual components (modules) are smaller, much lighter, and have a hundredfold repetition.
• Established concepts in mechanical engineering, such as lean production and construction kit methodology, enable to ensure the required quality and geometrical accuracy.
• Individualized serial production is only possible with complete digitization (digital twin) of all process steps. Only consistent, end-toend digitization ensures the quality and interoperability of the individual steps from production through assembly to the time of use and, if necessary, deconstruction and recycling.
First benchmarks and interaction chains to quantify possible savings in costs, time, material, and CO 2 emissions as well as accuracy limits are currently under development while first demonstrators have already been built up.