Device and method for determining the hardening state of concrete, method for 3D printing of concrete

Abstract

Device (1) for the non-destructive determination of the hardening state of concrete, characterized in that a detector (2, 2') for spectral analysis of light reflected from a concrete surface is connected to a control device (6) so that data transmission can take place, wherein the control device (6) is further connected to a database (8) in which at least a series of hardening state-spectrum pairs are stored, from which a spectral profile results, wherein a hardening state can be determined from a detected light spectrum by comparison with a corresponding light spectrum determined in experiments from the database (8).

Description

The invention relates to a device and a method for the non-destructive determination of the hardening state of concrete. The invention further relates to a method for 3D printing concrete. Concrete is a very important building material in Germany and worldwide. The ready-mix concrete industry currently produces over 50 million cubic meters of concrete annually in Germany. Assessing the hydration progress during the hardening process of this material is also dependent on external conditions. However, knowledge of the hardening state of concrete components is crucial for the operation of a construction site. Therefore, specific formwork removal times are currently observed for the respective installation situations. In addition, there are options such as conducting compression tests on specimens or installing sensors before concreting to determine the hardening of the concrete after pouring. Detecting the hardening state at any surface point of the concrete component is not possible, especially without contact. Another application where determining the hardening state of concrete is crucial is concrete 3D printing. Concrete 3D printing represents one of the future technologies in concrete construction. In concrete 3D printing, concrete strands are applied along a defined path using a print nozzle. Layer by layer, a concrete component, or even a building, is created. It is essential that the concrete strands are applied in such a way that the layer bond is "wet-on-wet," and that the existing layer(s) possess sufficient strength to support the subsequent layer without deformation. This results in a specific time window that must be adhered to. Non-contact determination of the hardening state is fundamental for automating the entire process.Currently, automated, especially non-contact, determination of the hardening state of the already applied concrete immediately before the subsequent filament is laid is not possible here either. Devices and methods for determining the hardening state of concrete are known from the prior art. Patent DE 100 55 099 C2 describes a method and a device for the non-destructive, automated determination of the strength of test specimens, in particular mortar prisms. The compressive strength is determined non-destructively using an improved ultrasonic test, ensuring unambiguous specimen marking and identification, especially through the use of RFID systems. A key aspect is the correlation between the resonance frequency and the flexural and compressive strength, which is determined by exciting the test specimens with ultrasound. The test specimens are placed in water tanks by a robotic system and removed for measurement. The coordinates of the storage locations are recorded along with the specimen identification. Before measurement, the test specimens are cleaned of any adhering water, and their mass and dimensions are precisely determined. The device comprises a storage area for the test specimens, a mobile robot with a gripper arm, intermediate storage locations, and measuring stations for determining mass, dimensions, and resonance frequencies. It enables high measurement accuracy for both mass determination (±0.1 g) and volume determination (±0.01 mm) and measures resonance frequencies with an accuracy of 0.01%. The measured values ​​are stored in a computer system and used to calculate compressive and flexural strength, based on the dynamic moduli of elasticity and other formulaic relationships. The method offers several advantages, including a reduction in the number of test specimens required, as one set can be used throughout the entire testing period, as well as significant time and material savings, particularly with standard sand. Furthermore, the method increases process reliability by allowing frequent and accurate measurements, which improves the documentation of the strength development and enables the prediction of the 28-day strength. This method can be applied to various building materials such as concrete, lime, and gypsum. Overall, the described method and apparatus provide a precise and automated approach for the non-destructive determination of the strength of mortar and concrete samples, thereby significantly improving the efficiency and accuracy of quality control in the building materials industry. A device for 3D printing concrete is known from publication CN 1 17 885 178 A. In this device, the hardening state of the concrete is determined by measuring a temperature profile using infrared imaging. Measurement of reflected light spectra and spectral analysis are not known from this publication and are not suggested by it. The publication DE LEÓN MARTINEZ, HA [et al.]: Optical evaluation on the setting of cement paste. In: Journal of physics: conference series, Vol. 582, 2015: Article no. 12021, 5 pp. ISSN 1742-6596. http: / / doi.org / 10.1088 / 1742-6596 / 582 / 11012021 [accessed on 2025-06-02] describes a device for determining the hydration of cement based on diffuse optical reflection. Spectral analysis is neither described nor suggested in the publication. Currently, there is no comprehensive technical solution to this problem. For concrete components cast in formwork, an empirical approach is used, adhering to specific stripping times. However, at low ambient temperatures, the necessary stripping times can increase significantly. Therefore, test specimens are also produced during the concreting process. Conducting compression tests on these specimens provides an indirect determination of the compressive strength of the installed concrete, as it is assumed that the hardening process proceeds at the same rate. In concrete 3D printing, the operators of the 3D concrete printer must manually or empirically determine the hardening state as the printing progresses and adjust the printing process accordingly. Current methods for determining the hardening state of fresh concrete are very imprecise and time-consuming. A non-contact detection method for the hardening state at any point on the surface of the concrete component is not currently available. For concrete 3D printing, there is currently no automated control loop for controlling the printer based on the hydration progress. Assessing the suitability of the concrete strands for receiving the subsequent layer(s) is subjective. Complete documentation of the hydration process throughout the entire printing process is not possible with current technology. The object of the invention is to offer a simplified and reliable method for determining the hardening state of concrete, which in particular does not require test specimens. The problem is solved by a device for the non-destructive determination of the hardening state of concrete. According to the invention, a detector for the spectral analysis of light reflected from a concrete surface is connected to a control unit, enabling data transmission to the control unit. The control unit processes the data from the spectral analysis. Furthermore, the control unit is connected to a database containing a series of hardening state-spectrum pairs, at least for the given concrete mix, resulting in a spectral profile. By comparing the detected light spectrum with the corresponding light spectrum from the database determined in previous tests, the hardening state of the concrete whose surface was detected can be determined. It has been surprisingly shown that the composition of light reflected from concrete surfaces changes characteristically during hardening. Also scientifically novel is the finding that spectral analysis of light reflected from fresh concrete surfaces can be used to infer the progress of hydration as the concrete hardens. During hydration, crystal growth occurs. This alters the light absorption and reflection behavior at the concrete surface. This can be measured by analyzing the reflected light. To account for the wide variety of concrete compositions (particle size distribution, water-cement ratio, admixtures), a comprehensive database must be compiled as a comparative value during preliminary investigations. First, the material-specific principles of the hardening process are determined to create this database. Since the hydration process of fresh concrete depends on external conditions (e.g., temperature), the compressive strength of the concrete surface should serve as an objective measure of the hydration progress. Preliminary tests determine the time course of concrete hardening through compression tests. These tests are conducted with a wide variety of concrete mixes. In parallel, the samples are analyzed spectrally, resulting in a comprehensive database of hardening state-spectrum pairs, preferably also including other boundary conditions such as the temperature of the concrete and / or the environment. The result of this series of tests is a database of concrete mix-specific, time-dependent compressive strength curves, for which the spectral profiles have also been determined. In a first embodiment, the detector is designed as a handheld device. This makes the device according to the invention universally applicable to any type of concrete component, because the detector can be used by the operator at any easily accessible location on the surface to be examined. Alternatively, the detector can be designed as an attachment. It is mounted on a machine and guided by it. For example, the detector can be simply attached to a frame, and concrete elements in formwork move past or under the detector. The detector could also be suspended from the ceiling. Another possibility is that the detector is mounted on a drone, which is then flown over a large construction site, for example. It has proven advantageous to mount the detector freely on a CNC machine, allowing it to move independently or in conjunction with other tools. In particular, the detector is mounted on a concrete 3D printer, a device for concrete 3D printing, and is arranged to move automatically. It can therefore either be controlled by an operator or automatically trace the last deposited concrete filament to determine its hardening state. For this purpose, it can also be positioned on the print head of the concrete 3D printer, as this too can be moved flexibly above the work surface. The detector is then located in the area of ​​the print head and is designed to be movable with it across a build surface, e.g. by means of a gantry system or a robot arm. Preferably, the detector comprises a light source, e.g., an LED light, directed at the surface to be examined. The light from the light source strikes the fresh concrete surface, from which light is reflected to the detector. On the fresh concrete surface, parts of the light spectrum are absorbed and other parts are reflected. The reflected light is separated into its components and analyzed (spectral analysis) using a suitable device (e.g., a diffraction grating). Standardized spectral profiles are recorded at different curing times using reference samples, as described above. It has proven advantageous to maintain at least one display unit connected to the control unit. This unit can display the results of the spectral analysis or directly indicate the hardening status, or translate these results into instructions for the operator. Such instructions might, for example, instruct the demolding process, i.e., the removal of the concrete component from its formwork. A particular advantage is that the spectral analysis of light reflected from the surface of a freshly poured concrete element is contactless. This allows conclusions to be drawn about the hydration progress without touching the concrete. Even if the detector is integrated into a housing and the housing is placed on a concrete surface, the detector remains inside the housing and analyzes the light reflected from the concrete surface without contact. Further advantages arise if at least one storage device remains connected to the control unit. This allows the concreting process, including the achieved hardening states, to be documented and used, for example, for quality assurance. The problem is further solved by a device for 3D printing concrete, a concrete 3D printer. According to the invention, the concrete 3D printer comprises a device for the non-destructive determination of the hardening state of concrete according to one of claims 1 to 7 or as described above. A concrete 3D printer equipped in this way exhibits optimal function and high efficiency in the production of structures, because the subsequent concrete filament is applied neither too early, when the previous filament could not yet bear the load, nor too late, when a wet-on-wet concreting process would no longer be guaranteed. Thus, the construction process is not only safe from a material perspective, but also takes place with maximum possible speed. The problem is further solved by a method for the non-destructive determination of the hardening state of concrete, which comprises the following procedural steps. In a first step, which can precede the actual testing procedure considerably in time, the time course of the concrete hardening of at least one concrete mix is ​​determined by a series of compression tests and simultaneously an analysis of the light reflected from the concrete surface. The determined values ​​are entered into a database, which is later used for comparison and evaluation during the actual testing procedure. The analysis of the light reflected from the concrete surface is carried out by spectral analysis (e.g., using a diffraction grating), which is optically recorded and analyzed.The spectral analysis during the preliminary investigation is carried out for each compression test, so that a concrete mix-specific, time-dependent compressive strength curve with the associated time-dependent spectral profile is determined and stored in a database as a data basis for the later evaluation of the test results on the investigated concrete component. In a second step, a spectral analysis of the light reflected from the surface of a freshly poured concrete component is performed, preferably by recording it with a detector. This marks the beginning of the actual investigation of a concrete component whose hardening is to be determined. In a third step, the analyzed spectral profile is compared with the spectral profile stored in the database from at least one test series. For this purpose, the stored test series from the database is selected whose test specimen composition corresponds to that of the concrete component, or where the greatest similarity exists. In a fourth step, the hardening state of the freshly poured concrete component is determined by comparing the light spectrum stored in the database with the light spectrum measured on the concrete component. During the production of concrete components, the hardening state now known from the database can be used to decide whether formwork removal is possible. Similarly, for concrete 3D printing, the compressive strength can be determined through spectral analysis and comparison with the baseline data, thus allowing conclusions to be drawn about the suitability of the concrete layer of the last concrete filament for the absorption of subsequent filaments. In order to achieve a broad applicability of the method according to the invention, it has proven advantageous to determine and store in a database time-dependent compressive strength curves with the associated time-dependent spectral profiles for a large number of different concrete mixes according to the first step of the process. The casting of concrete components or concrete 3D printing is carried out using a specific concrete mix design. The data sets from the preliminary investigation of a concrete mix design that most closely resemble the actual mix design are used as a basis. Therefore, the hardening behavior of the concrete mix design is known based on the preliminary investigations and the environmental conditions. In addition to the material-specific properties, the specific photophysical properties of the concrete surface are also known throughout the entire hardening process based on the data sets. During the production of concrete components or concrete 3D printing, a spectral analysis of the light reflected from the surface can be performed. The spectral profile is compared with that of the database, and conclusions are drawn about the hardening state from this comparison. In a further advantageous development of the method, a multitude of different temperatures, preferably of both the environment and the concrete, or at least of the concrete, are recorded during the concrete hardening process. Optionally, further boundary conditions are recorded and stored in the database for each individual, simultaneously recorded data set. Preferably, the analysis results are evaluated, presented to the user or machine operator, and documented and stored along with other information, such as coordinates. This allows a user or operator to utilize the method even without precise knowledge of the hydration process and the chemical-physical principles of crystallization. The problem is further solved by a method for 3D printing concrete or the operation of a concrete 3D printer. This method is characterized by the following process steps: a. A concrete filament is applied by a concrete 3D printer. b. The hardening state of the concrete is determined according to a method according to one of claims 9 to 12 or as described above. c. After reaching the required hardening state, the 3D concrete printing continues with step a. An advantageous embodiment of the method provides that the achieved hardening state is transmitted via an interface to a machine control of the device for 3D printing, the concrete 3D printer, and the 3D concrete printing is continued automatically. In the production of concrete components, the now-known hardening state and the specific installation situation allow for decisions regarding the possibility of demolding. For concrete 3D printing, the compressive strength can be determined via spectral analysis and comparison with the base data, thus allowing conclusions to be drawn about the suitability of the concrete layer for subsequent filaments. The result is displayed to the operator of the concrete 3D printer and documented. The invention addresses the need, described at the outset, to know the hardening state of fresh concrete. It allows for the automated, non-contact, in-situ detection of the concrete's hardening state. This enables the precise demolding of concrete elements. The non-contact detection is unique, as it allows conclusions to be drawn about the hydration progress without touching the concrete. While the measurement itself is non-contact, the detector could, of course, be integrated into a housing, which is then simply placed on a concrete surface. This ensures a defined distance to the surface being examined, maintaining reproducible boundary conditions. However, the housing then stands with its legs on the concrete part, the detector is arranged inside the housing and analyzes the light reflected from the concrete surface without contact. For concrete 3D printing, the result can have a regulatory effect on the printing process. If the concrete of the existing structure has not cured sufficiently, the printing speed can be reduced. If it turns out that the concrete hydration is already advanced, the printing speed can be increased. The machine operator is informed in each case. In addition, the measurement results should be logged and serve as a complete documentation of the printing process. Further advantages include increased decision-making certainty, time savings, and a simple method for documenting the manufacturing process of concrete components. Concrete 3D printing is a new way of constructing buildings. Besides ensuring adherence to critical time windows, the invention offers the additional advantage that the entire printing process, with regard to layer bonding, is metrologically recorded "wet-on-wet" and automatically documented for quality assurance purposes. This documentation would not be possible without the invention. The invention is explained in more detail below with reference to the description of exemplary embodiments and their illustration in the accompanying drawings. Figure 1 shows a schematic view of an embodiment of a device according to the invention for the non-destructive determination of the hardening state, in which the detector is designed as a handheld device; and Figure 2 shows a schematic view of an embodiment of a device according to the invention for the non-destructive determination of the hardening state, in which the detector is designed as an attachment to a 3D print head. Fig. 1 schematically shows an embodiment of a device 1 according to the invention for the non-destructive determination of the hardening state, in which detector 2 is designed as a handheld device. The detector 2 is grasped by the operator and held over the surface of the concrete component 20. The reflection properties of the surface of the concrete component 20 are thereby recorded by means of a spectral analysis and transmitted to the control unit 6 via the data line 4. There, a spectral profile is generated from the measured reflection spectra or the spectral analysis. The control unit 6 transmits data from the database 8 via another data line 4. The database 8 contains, at least for the concrete mix of the concrete component 20, a series of previously recorded test results, which provide the corresponding time-dependent spectral profiles for at least several degrees of hydration (hardening levels). By comparing the recorded spectral profile with the matching spectral profile from database 8, the hardening state in the concrete component 20 can be determined. The result of the comparison is sent from the control unit 6 to a display unit 10 via further data lines 4 and stored in a memory 12 for documentation. The operator can then decide, by interpreting the display 10, whether, for example, the concrete component 20 can already be switched off. This prevents premature stripping of the formwork and also ensures optimal use of the formwork capacity (not shown here), as the concrete component 20 does not remain in the formwork longer than necessary. Fig. 2 schematically shows an embodiment of a device 1 according to the invention for the non-destructive determination of the hardening state, in which detector 2' is designed as an attachment on a printhead 42. This allows the detector 2' to be moved by the portal arrangement 44 together with the printhead 42 to any desired location above the concrete structure 30. This makes it possible to determine the hardening state across the entire surface of the most recently applied concrete filament 32, in order to apply the next concrete filament 33 as soon as possible without damaging the existing structure. Early application ensures a secure bond between the superimposed concrete filaments. The determination of the hardening state of the individual concrete filaments 32 is carried out analogously to the procedure described for Fig. 1. In contrast to Fig. 1, however, the determined results are transmitted to a human-machine interface 14 and a quality assurance module 16, which in particular performs documentation. Reference symbol list 1 Device for non-destructive determination of the hardening state 2 Detector (handheld device) 2' Detector (attachment device) 4 Data line 6 Control unit 8 Database 10 Display unit 12 Storage unit 14 Human-machine interface (HMI) 16 Quality assurance 20 Concrete component 30 Concrete structure 32 Concrete filament 40 Concrete 3D printer 42 Print head 44 Gantry assembly

Claims

Device (1) for the non-destructive determination of the hardening state of concrete, characterized in that a detector (2, 2') for spectral analysis of light reflected from a concrete surface is connected to a control device (6) so that data transmission can take place, wherein the control device (6) is further connected to a database (8) in which at least a series of hardening state-spectrum pairs are stored, from which a spectral profile results, wherein a hardening state can be determined from a detected light spectrum by comparison with a corresponding light spectrum determined in experiments from the database (8). Device according to claim 1, wherein the detector (2, 2') is designed as a handheld device or as an attachment device, being attached to a machine and guided through it. Device according to claim 2, wherein the detector (2') is arranged to be automatically movable on a concrete 3D printer (40) comprising a print head (42). Device according to claim 3, wherein the detector (2') is arranged in the area of ​​the print head (42) and is designed to be movable with it over a building surface on which a concrete structure (30) is formed. Device according to one of the preceding claims, wherein the detector (2, 2') comprises a light source directed towards the surface to be examined, from which light is to be reflected to the detector (2, 2'). Device according to one of the preceding claims, wherein at least one display device (10) is further connected to the control device (6). Device according to one of the preceding claims, wherein at least one storage device (12) is further connected to the control device (6). Device for 3D printing of concrete, characterized in that a device (1) for non-destructive determination of the hardening state of concrete according to one of claims 1 to 7 is included. Method for the non-destructive determination of the hardening state of concrete, characterized in that a. in a first step, the time course of the concrete hardening of at least one concrete mix is ​​determined by a series of compression tests and simultaneously an analysis of the light reflected from the concrete surface is performed, wherein the analysis of the light reflected from the concrete surface is carried out by spectral analysis, wherein the spectral analysis is carried out during the preliminary investigation for the time of each compression test, so that a concrete mix-specific, time-dependent compressive strength curve with the associated time-dependent spectral profile is determined and stored in a database (8); wherein b. in a second step, a spectral analysis of the light reflected from the surface of a freshly cast concrete component (20) is carried out; wherein c.In a third step, the analyzed spectral profile is compared with the spectral profile stored in the database (8) from at least one test series; wherein, in a fourth step, the hardening state of the freshly cast concrete component (20) is determined from the comparison. Method according to claim 9, wherein time-dependent compressive strength curves with the associated time-dependent spectral profiles are determined for a plurality of different concrete mixes according to step a) and stored in a database (8). Method according to claim 9 or 10, wherein a plurality of different temperatures are recorded during the concrete hardening and stored in the database (8) for each individual, simultaneously recorded data set. Method according to one of claims 9 to 11, wherein the analysis results are evaluated, displayed for the user or the machine operator in a display device (10) or an HMI (14) and documented together with further information and stored in a storage device (12). Method according to one of claims 9 to 12, wherein the spectral analysis of the light reflected from the surface of a freshly poured concrete component (20) is carried out without contact. Method for operating a concrete 3D printer (40), characterized in that a. a concrete filament (32) is applied to a building surface or a concrete structure (30) by a concrete 3D printer (40), b. the hardening state of the concrete is determined according to a method according to one of claims 9 to 13, and c. after reaching the required hardening state, the 3D concrete printing is continued with step a). Method according to claim 14, wherein the achieved hardening state is transmitted via an interface to a machine control of the concrete 3D printer (40) and the 3D concrete printing is continued automatically.

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