Introduction

The handling and management of construction and demolition (C&D) materials is currently playing an increasingly important role, and will continue to do so in the future, due to an increase in raw material problem worldwide. Over 60% of building in stock account for a considerable share of the total German construction volume and will continue to gain in importance in the future due to limited building land in urban areas (Dorffmeister, 2019). This is usually associated with the deconstruction of buildings. Due to changes in planning and faulty construction, deconstruction work is also required for new buildings (replacement of faulty concrete structures, subsequent construction of openings, geometric correction of existing building components). In both cases, deconstruction methods are to be applied that can be used within existing or newly built structures. This sometimes results in very strict requirements for the type and method of deconstruction technology, especially with regard to reduced emission values for noise, vibration and dust generation.

At present, two basic demolition methods are available for this purpose: mechanical destruction of the structure by external application of force (caulking, milling, drilling, sawing) or media-transporting blasting methods (high-pressure water jetting, solid-state blasting). The vibrations of the mechanical demolition methods also damage, above all, the surrounding building fabric to be preserved. Another problem is health hazards for workers (noise, dust, vibration) and thus occupational health and safety concerns. Blasting methods, on the other hand, are relatively low in vibration and wear, but especially in the case of high-pressure water jetting, they introduce a large amount of water into the building fabric. Both demolition methods are therefore only suitable to a limited extent for construction in sensitive environments. Furthermore, some dismantling processes are operated with an internal combustion engine, which releases pollutants.

Another perspective of future developments in the German construction market focuses on the quality of recycling reclaimed building materials. The supply and flow of raw materials in Germany have been the subject of political, scientific and corporate activities for several years. The first raw materials strategy of the Federal Republic of Germany was published in 2010 with a focus on raw materials supply and has since been updated at regular intervals and reviewed by a raw materials policy compass of the Federal Government (Hartz et al., 2018). The extraction of raw materials in Germany was 1041 million tons in 2015 (Löschel et al., 2020). With 517 million tons, construction materials represent the largest group of raw materials (Löschel et al., 2020).

In this context, the extraction of raw materials for new building materials is accompanied by significant land consumption and high energy consumption. Consequently, the recovery of existing building materials and the recycling of mineral construction waste have become significantly more important. These are produced in large quantities, for example, during demolition, renovation and remodeling work on existing building structures. Results of investigations have shown that in the Federal Republic of Germany, the amount of material bound in buildings can be estimated at about 15 billion tons (Vogdt et al., 2016). It will, therefore, be one of the most important tasks of the future to make systematic use of this secondary raw material reservoir, the so-called anthropogenic material store (Schiller et al., 2015). In the German construction industry, more than 200 million tons of construction and demolition (C&D) waste are generated annually (Nelles et al., 2016; Umweltbundesamt, 2022). With a total waste volume of just over 400 million tons in Germany, this represents approximately half (Nelles et al., 2016). Statistically, 90% of this waste is recycled, but most of it is "downcycled", i.e., the material is made available again in reduced quality and functionality (Zhang et al., 2020). An example of this is C&D waste, which is used as low-quality secondary material in the unbound layers of road construction. The background to this fact is usually the lack of single-variety processing of the demolition material as well as the mechanical destruction of the building materials or their individual components during deconstruction (Müller, 2018). In addition to the points mentioned above, there are a number of other aspects that motivate individual stakeholders not to use recycled C&D waste. An interesting overview of the perceptions, decisions and motivations of various stakeholders regarding the use of recycled C&D waste is provided in Shooshtarian et al. (2020) in general and in Shooshtarian et al. (2022) specifically for Australia.

A targeted and material-selective demolition as well as a downstream sorting of mixed construction waste could ensure that the materials are reused in an equivalent manner. The aim should, therefore, be to produce new recycled building materials in significant quantities through targeted processing of mineral construction waste. The importance of the circular economy and its effect on the entire construction industry is elaborated in detail in Hosseini (2021) and Shooshtarian et al. (2022). The basis of the circular economy is to win building materials separately after their use and to use them as equally as possible. To ensure this, suitable processes must be developed for new construction, conversion and demolition of buildings and the raw materials they contain. A separate extraction of the individual building materials is of great importance (Hillebrandt, 2021). The circular economy will continue to gain in importance and is already playing an important role for investors.

In most cases, there is a lack of machine-based demolition technologies that meet these selective requirements for demolition and processing in an urban context. Common demolition and separation methods are characterized by high emissions (noise, vibration, dust) and low selectivity. Third parties are negatively affected to a high degree by acoustic pollution, which emanates both from the equipment and from the mechanical destruction of the structure. Often, users / tenants / residents in the immediate vicinity are disturbed by demolition procedures to such an extent that simultaneous use of neighboring premises is not possible or only possible to a limited extent. The mechanical structural destruction of structural facilities by means of demolition procedures also leads to notable vibrations in addition to acoustic impairments. These often lead to unintentional damage to neighboring building components or neighboring buildings. In addition, exposure to dust not only affects the occupational safety and health of workers but also causes a nuisance to residents of neighboring properties, particularly in inner-city areas. In most demolition work, dust generation is unavoidable and is usually caused by the breaking apart of mineral building materials.

Only a controlled interaction of new selective demolition technologies, separation of the construction waste and a modern and efficient processing technology will make it possible in the future to demolish buildings in the urban environment, obtain high-quality recycling products and reduce the land requirement as well as the energy consumption in the extraction of raw materials (Basten, 2018; Stroh et al., 2020).

Research aim and objectives

This is the starting point for the EIT (Electric-Impulse-Technology) described here, which can separate mineral materials with the aid of high-voltage pulses without significant vibrations, dust generation and noise (Meetz et al., 2015; Müller, 2018). Existing demolition processes often have one or more negative emissions and do not dissolve the material by type, which does not favor the circular economy of building materials. The goal of EIT-Technology is to fill the aforementioned gaps of established deconstruction technologies and thus promote the circular economy. The aim of the investigations carried out is the utilization of new technology for the low-emission and selective deconstruction of structural facilities. The study demonstrates by a large number of tests that selective deconstruction in sensitive areas can be implemented with low emissions by means of deconstruction using EIT. The EIT operates on a power connection, therefor, no pollutants are released. Compared to established demolition methods, which often run on diesel, this is a significant advantage of the EIT. The aim of the research project was to investigate the extent to which the EIT can be adapted for use in the construction industry. In addition to extensive literature research for a comparison of the EIV with the established demolition methods, the EIT was modified for use in the construction industry. Jens Otto (Institute of Construction Management) and Frank Will (Chair of Construction Machinery) played a key role in supervising the research project through interdisciplinary cooperation at Technische Universität Dresden, Germany (TUD). The “EIV-Bau” research project (Otto et al., 2021) was completed at the beginning of 2021, and the main results are summarized in this article. Here, some of the results of the research project, such as the study of the electrode gap, the number of pulses and the pulse energy are presented and then discussed.

Literature review: basic applications of EIT-technology

The EIT was researched and developed in the former Soviet Union in the middle of the twentieth century. The potential of the technology for hard rock processing was discovered and research in this field was intensified (Kunze et al., 2007).

EIT can be used to economically pre-damage, remove and crush particularly hard materials, such as granite, ore or high-strength concrete, in drilling, processing and construction applications (Anders, 2021; Lämmerer, 2017; Lehmann, 2017; Lehmann, 2021; Voigt et al., 2017). The destructive effect is not based on the application of mechanical force, but on high-voltage pulses that are applied to the material via an electrode arrangement. Contrary to conventional methods, the impulse causes the material to be subjected to tension instead of compression. An electric field is built up between the two electrodes, which leads to ionization of the process environment, i.e., the rock and the surrounding dielectric. Ionization is the release of charge carriers from their bond by a strong electric field. The force effect within the field ensures that the free charge carriers are stimulated to move, collide with other particles and release new charge carriers from their bond (Küchler, 2009). The resulting abrupt increase in temperature and pressure in the breakdown channel causes the tensile strength of the rock to be overcome, resulting in a brittle fracture with associated material removal. Under normal conditions, the tensile strength can be overcome with very low specific energy, since it is only about 10 percent of the compressive strength (Biela, 2009). Structural building materials in civil engineering have significantly lower tensile strength compared to their compressive strength. EIT makes use of this property of building materials, but this property is not a basic requirement for the process. The prerequisite for material removal is discharged within the rock to be dissolved. For this, it is necessary that the two electrodes rest directly on the rock and that the process space above the rock is enclosed by a medium that has higher electrical strength (dielectric strength) than the rock. Electrical strength defines the electric field strength that a material can withstand without electrical breakdown occurrence. In general, the electrical strength of solids is greater than that of liquids, which in turn is greater than that of gaseous substances. However, the electrical strength of substances changes depending on the loading time. In general, substances have higher electrical strength at short loading times. This behavior is more pronounced for liquids than for solids. If the voltage exposure occurs in sufficiently small times, i.e., in the nanosecond range, the electrical strength of the liquid increases rapidly and exceeds that of solids. In this way, a discharge in the solid can be realized. Since there is only a loose contact between the electrodes and the solid, no mechanical forces are necessary for the EIT compared to conventional methods.

To ensure a targeted breakthrough in the material, the following basic requirements must be met (Fig. 1a):

  • two electrodes with different potentials,

  • the rock itself and

  • an ambient fluid (dielectric) that acts as an insulator.

Fig. 1
figure 1

Basic principle of the EIT (1 High voltage electrode, 2 Grounding electrode, 3 Dielectric, 4 Rock, 5 Electric field, 6 Plasma channel, 7 Dissolved materials, 8 Cracks) (a basic requirements, b electric field, c electric field exceeds, d dissolving process)

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One of the two electrodes is grounded and, therefore, has a potential of zero. A high pulse voltage is applied to the other electrode. As a result of the potential difference that now prevails, an electric field is formed (Fig. 1b). If the strength of the electric field exceeds the dielectric strength of the rock, a breakdown channel is formed (Fig. 1c). This expands suddenly and tears the rock from the inside out. Rock fragments are blasted from the formation. Small cracks and previous damage form in the surrounding formation (Fig. 1d).

In order for a discharge to occur in the material and not in the surrounding fluid, the dielectric strength of the material must be significantly lower than that of the fluid. As already mentioned, the dielectric strength is not a constant value, but depends on the rise time. Figure 2 shows schematically the characteristics of the dielectric strength as a function of the rise time. It can be seen that, in general, the dielectric strength of rock and water has an intersection at 500 ns. This means that the dielectric strength of rock is lower than that of water when rise times of less than 500 ns are achieved. In this case, common industrial water is suitable as a dielectric for the EIT. If, on the other hand, it was possible to use oil as the ambient medium, breakdown would occur even with comparatively long rise times in the rock. Figure 2 also shows that as soon as air or air bubbles are in the vicinity of the electrodes, the breakdown always occurs through the air and thus there is no breakdown in the rock anymore.

Fig. 2
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Electric strength of various substances according to Bluhm et al., (2000)

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The EIT has been researched and developed for several years at the Chair for Construction Machinery at the Technische Universität Dresden (TUD) (Anders, 2021; Lämmerer and Flachberger, 2017; Lehmann et al., 2017; Lehmann, 2021; Voigt et al., 2017). Based on the investigations, various fields of application have already been developed. In some research projects, the EIT was developed for use in deep drilling in hard rock for geothermal energy (Anders, 2021; Lehmann et al., 2017, Lehmann, 2021). The researchers at the TUD succeeded for the first time in the consistent implementation of the electrodynamic EIT under practical conditions of mining and special civil engineering. High dissolution rates with low energy input and high selectivity were demonstrated. In further projects, the pulse voltage generators and drill heads for shallow drilling were also developed. Another field of application of the EIT is the processing, damaging and crushing of ores to support energy-intensive mechanical processes (Anders, 2021; Lehmann, 2021).

The interdisciplinary research project "Basic Investigation for the Adaptation of an Innovative Demolition Process from Mining (EIT) as a New Construction Technology for Selective Deconstruction in Sensitive Areas" of the Chair of Construction Machinery and the Institute of Construction Management (TUD) deals with the adaptation of EIT as an alternative deconstruction technology in the construction industry (Otto et al., 2021). The environmental conditions of a construction site and the application scenarios differ significantly from the aforementioned application scenarios. For example, while the process chamber in a vertical deep borehole is closed by the surrounding solid rock, a separate process chamber seal must be developed for use on the construction site for selective dismantling. If the remaining technological prerequisites are solved, EIT promises highly interesting areas of application for the construction industry as well.

Methodology

Aims of testing

The large number of tests carried out as part of the project aimed to be able to assess the influences of individual parameters on the performance of the EIT and the selectivity of the process. The calculated specific energy for dissolving the material as well as the measurement of the test samples before and after the tests further enable comparability with other demolition and separation processes. With regard to selectivity, the nature of the detached material as well as the nature of the remaining surface play a role. With the electrodynamic EIT, the deconstruction process should ultimately be carried out in such a way that selective loosening of mineral rock is possible with geometric precision at very low emissions. After a brief description of the test rig, important results of the individual tests are described below.

Methodology and basic experiments

The basic tests were intended to investigate the influence of essential parameters of the EIT and to evaluate them with regard to typical construction-specific conditions. For this purpose, the parameters electrode geometry, electrode spacing, pulse energy and number of pulses were varied singularly at the existing test rig of the TU Dresden and the effects on the type and size of the dissolved mineral substances were investigated.

In a first step, selective tests were carried out on test samples made of sand-lime brick using the EIT. The aim was to maximize the measurements of dissolved solid rock per pulse by varying the electrode spacing, electrode geometry, number of pulses as well as pulse energy of the electrodynamic EIT. Three geometrically different electrode pairs A, B and C were designed for the basic experiments (see Fig. 3).

Fig. 3
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Electrode pairs A, B and C with different geometries

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In the application of EIT in construction, the electrodes are to be guided continuously over a component, ensuring contact between the component and the electrodes. The electrode geometry selected was used to investigate the extent to which the contact surface has an influence on the dissolution process. The pulse parameters are adjusted by designing the electrodes and the pulse voltage generator. For this purpose, tests were carried out on samples made of sand-lime bricks as well as on samples made of concrete. Due to the industrial production and the resulting homogeneous nature, the first basic tests were carried out on sand-lime bricks. Subsequently, it was investigated whether the findings obtained could be transferred to concrete test samples.

The tests were carried out using a simple experimental setup consisting of an encapsulated pulse voltage generator, a sample container with a tub of water and the sample holder for the pair of electrodes (see Fig. 4). Figure 5 shows a typical result image of the achievable breakout area of a sand-lime brick sample.

Fig. 4
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Experimental setup for basic experiments (1—Pulse voltage generator in the pressure vessel, 2—Sample holder, 3—Container for water bath, 4—Sand-lime bricks sample, 5—Grounding)

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Fig. 5
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Typical breakout area (sand-lime brick)

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As to the experimental procedure: the samples were placed in the sample holder, and the pulse and ground electrodes were placed on the sample and fixed. The sample holder and sample were then placed in the water container. After the grounding was removed, the experiments could be started. Based on the tests from mining and special civil engineering, it was determined that a dissolving process can be realized with 5 pulses, which is why this determination was also made for this experimental design. The entire test procedure with the associated evaluation can be seen in Fig. 6 below. All steps that were necessary for a successful and complete test implementation are summarized on the basis of the figure. Steps 1–14 concern the preparation and execution of the experiments and steps 15–22 concern the follow-up of the experiments for a comprehensive evaluation of the experiments. To generate reliable results, each test was repeated at least 3 times and the measured values were subsequently evaluated.

Fig. 6
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Scheme of the experimental design

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Based on the fundamental tests, the influence of site-specific influencing criteria was also investigated, such as the influence of the degree of contamination from the dielectric (service water) on the dissolution process or the effect of interfering points made of plastics (e.g., spacers) or steel (e.g., reinforcing steel). In the following, the results of the variation of the electrode spacing, the number of pulses and the horizontal repositioningFootnote 1 are presented as a part of the series of experiments within the research project.

Results

In this section, some results are presented. The parameters electrode gap, number of pulses, pulse energy and electrode guidance are addressed. A variation of these parameters results in a change of the solvable volume and the required specific energy. The results are illustrated graphically.

Electrode gap

In a first series of experiments, we investigated to extent to which electrode geometry and electrode spacing affect the dissolved measurements as well as the required specific energy. The inner distance between the electrodes (EA) were varied between 8 and 45 mm (see Fig. 3). This determination was based on results from previous research with hard rocks such as granite (Anders, 2021; Lehmann, 2021). The following graphs (see Fig. 7) show the experimentally determined relationship between electrode spacing (EA) and dissolved mass (in cm3) as well as between electrode spacing (EA) and required specific energy (in J/cm3) on samples made of sand-lime brick. The experiments were carried out with three geometrically different electrodes (EG A, EG B and EG C, see Fig. 3).

Fig. 7
figure 7

Sand-lime brick: dissolved mass (left) and specific energy (right) depending on the spacing between the electrodes (EA) and the electrode geometry (EG) (Otto et al., 2021)

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Already at the beginning of the tests, it became clear that the electrode gap on a sand-lime brick could be selected significantly higher than on a natural brick. This is due to the lower material strength of industrially manufactured sand-lime bricks. The results in Fig. 7 represent in each case the mean value of the test results as well as their minimum and maximum values. For all three electrodes, you could see that the dissolved mass increases significantly up to an electrode spacing of about 25 mm and then remains approximately constant. You can also see that the dispersion of the dissolved mass up to an electrode spacing of approximately 25 mm is significantly smaller than for larger electrode spacing. The specific energy is also almost constant from an electrode spacing of 20 mm.

Number of impulses

In a second series of tests, the effects of the number of pulses on the dissolved mass and the required specific energy were investigated. Here, a series of tests were carried out to allow extrapolation of the demolition performance. The electrodes were not moved during the dissolving process itself, but were displaced by a distance of 10 mm on the sample only after the pulses of different numbers had been released. Varying the number of pulses showed that the most efficient result could be obtained in tests with five pulses. The following Fig. 8 show the results of the tests.

Fig. 8
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Sand-lime brick: dissolved mass (left) and specific energy (right) depending on the number of impulses and the electrode geometry (EG) (Only a few test were carried out with the electrode geometry A, therefore only individual values are shown in the diagrams, no progression.) (Otto et al., 2021)

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While a constant increase in the dissolved measurements could be recorded up to a number of five pulses, the increase in the dissolved mass is only very slight for more than five pulses. When considering the specific energy required per number of pulses, an increase in specific energy could be observed from five pulses onwards. This also confirmed experimental results from other research areas of the EIT. During the experiments it was also observed that the acoustics of the pulse changed with an increasing number of pulses. This gave further reason to assume that after a certain number of pulses the material is no longer dissolved from the composite, but the already dissolved material is merely further comminuted. Due to the required number of pulses for loosening the material, a first approach for the prediction of the working speed for the EIT can be derived.

Impulse energy

With a third series of experiments, the influence of different pulse energies on the dissolved measurements and the specific energy was investigated. The pulse energy is of great interest for selective deconstruction. If the energy is too low, the material is not dissolved from the composite. If the pulse energy is too high, on the other hand, there is a risk that the material will not only be dissolved from the composite along the grain boundaries, but also that the individual components of the already dissolved material will be crushed, which would have a negative effect on energy efficiency.

The generator used for the experiments has 5 stages, each with 10 capacitors. The charging voltage is about 40 kV. This results in a discharge voltage of about (5 × 40 kV =) 200 kV. Depending on the number of stages in connection with the capacitors, the charging voltage and thus also the discharging voltage can be changed. The pulse energy was reduced by removing one capacitor from each stage. For the generator used (about 80 J), this is about 8 J (10%) less per capacitor removed. The charging voltage was kept constant (Fig. 9).

Fig. 9
figure 9

Sand-lime brick: dissolved mass (left) and specific energy (right) depending on the impulse energy and the electrode geometry (EG), electrode spacing EA = 25 mm (Otto et al., 2021)

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It could be seen that the dissolved mass decreases with the reduction of the pulse energy. The specific energy, on the other hand, is not influenced by the pulse energy. It follows that the dissolved measurements and the pulse energy have a constant relationship to each other. Furthermore, it can be confirmed by the experiments that a reduction of the impulse energy reduces the probability of dissolving rock. Furthermore, it can be concluded that reducing the pulse energy decreases the probability of successful breakthrough. For the configuration considered, it is, therefore, recommended to work with an impulse energy of 80 J. Figure 10 illustrates this fact.

Fig. 10
figure 10

Sand-lime brick: proportion of attempts with a successful breakdown for different impulse energies (Otto et al., 2021)

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Transient electrode guidance

In addition to the previously described tests, a number of other investigations were carried out on various parameters of the EIT. Among other, tests were carried out on the vertical and horizontal repositioning of the electrodes during the process to determine the release performance of the EIT with a continuously moving mold. For the horizontal repositioning of the electrodes, the best experimental results were achieved by the following parameters:

  • Electrode spacing of 15 mm,

  • pulses per position and

  • offset of the electrodes by 10 mm.

The electrodes of the EIT were not moved over the samples in an ideally continuous manner during the tests. Rather, they were placed on the sample, subjected to 5 electrical pulses, and then placed on the sample again offset horizontally by 10 mm. The procedure was repeated a total of 7 times, so that a total of (7 × 5 =) 35 electrical pulses were applied to the samples. A distance of 60 mm was realized from the first position to the last position. The following Fig. 11 illustrates the test sequence with horizontal repositioning.

Fig. 11
figure 11

Schematic test procedure with horizontal repositioning of the electrode pair (Otto et al., 2021)

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The tests with horizontal repositioning were carried out with electrode geometry B. The preliminary series of tests showed that electrode geometry B is better suited for mass removal on the surface than electrode geometry A and C. The electrode geometry C, on the other hand, is better suited for mass removal in depth by vertical repositioning. The results of the tests are summarized in the following section. The tests carried out form the basis for determining the demolition performance of the EIT when removing mineral building materials over a large area.

Discussion

Extrapolation

The results obtained so far, are to be transferred to practical construction application scenarios by scaling. The aim of the extrapolation is to determine a comparative value of the demolition performance to the established demolition and separation methods for the surface removal, the surface removal and the separation of mineral building materials. For this purpose, the previously experimentally determined demolition performance of the EIT in m3/h and the separation performance in m2 [separation area]/h must be extrapolated. The extrapolation of the demolition and separation performance is carried out on the basis of the test configuration listed in Sect.  3.3.4 and the test results listed in Table 1.

Table 1 Test results horizontal repositioning concrete (Electrode geometry B, Offset by 10 mm after 5 pulses) (Otto et al., 2021)
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With this test setup, it was possible to dislodge 26.4 cm3 from an unreinforced concrete sample. The dimensions of the dislodged cavity/cone were approximately 4.1 cm × 8.1 cm × 0.83 cm (width × length × depth). By multiplying the length of the excavation cone by the mean depth, the separation area after 35 pulses of (8.00 cm × 0.83 cm =) 6.6 cm2 can be calculated.

As a result of the technical properties of the testing facility, a maximum pulse frequency of 25 Hz can be achieved. A higher frequency cannot be realized so far, because the capacitors of the testing facility heat up too much and thus a constant use of the system is not possible. The following extrapolation was, therefore, carried out at a frequency of 10 Hz and 25 Hz.

Depending on the frequency with which the high-voltage pulses are conducted into the building material, a different demolition performance can be realized. At a frequency of 10 Hz, 3.5 s are required for the 35 pulses, and only 1.4 s at 25 Hz. At 10 Hz 1028 pulses are generated in one hour and at 25 Hz 2571 pulses and a separation area of 6788 and 16,971 cm2 per hour (= 0.679 and 1.697 m2 per hour, respectively) can be realized. Table 2 summarizes the results of the extrapolation for a frequency of 10 Hz and Table 3 the results of the extrapolation for a frequency of 25 Hz.

Table 2 Extrapolation concrete; repositioning electrode B; 10 Hz; 25 Hz (Otto et al., 2021)
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Table 3 Extrapolation of the demolition performance based on the laboratory test by repositioning electrode B; hand-held EIT device; 10 Hz and 25 Hz (Otto et al., 2021)
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The demolition performance demonstrated experimentally with the EIT on an unreinforced concrete sample must be reduced for the demolition of reinforced concrete. The degree of reduction depends on the degree of reinforcement of the reinforced concrete. Demolition of reinforced concrete with the EIT requires specific positioning of the electrodes along the reinforcement. If the grounding electrode comes into contact with the reinforcement, the function of the process is hardly affected. However, when the pulse electrode contacts the reinforcement, a short circuit occurs and no material is dissolved. Tests on reinforced concrete test samples were only carried out qualitatively as part of the research project. It was investigated whether the reinforcement can be detached from the concrete matrix by the EIT. In the absence of concrete test results, it is assumed for the following consideration that the demolition performance of the EIT is 20–50% lower for reinforced concrete than for unreinforced concrete. Table 4 summarizes the results and assumptions for the demolition performance corresponding to the device used in the laboratory. This device could be the starting point for the development of a hand-held EIT device in the future.

Table 4 Extrapolation of the demolition performance based on the Laboratory test by repositioning electrode B; EIT attachment; 10 Hz and 25 Hz (Otto et al., 2021)
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By increasing the contact area between the electrodes and the component to be demolished by several pairs of electrodes (e.g., 4 pieces), the demolition performance of an EIT attachment can be increased by four times the demolition performance determined for one pair of electrodes if the arrangement is optimal. Since the adaptation of the EIT leads to an increase in the total weight of the EIT device, such an EIT device is no longer suitable for hand-held use, but only as an attachment to a carrier device. Table 5 summarizes the results of the extrapolation for an EIT attachment consisting of a total of 4 pairs of electrodes.

Table 5 Extrapolation of the separation performance based on the laboratory test by repositioning electrode B; 10 Hz and 25 Hz (Otto et al., 2021)
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As a result, it can be stated that the determined performance values of the EIT are significantly lower than the performance values of established hand- or machine-guided demolition processes. The advantageousness of the EIT can be explained here exclusively by the selectivity and the reduced emissions of the process. It must be taken into account that the extrapolation is based on assumptions and must be substantiated by concrete tests.

In addition to the extrapolation of the demolition performance of the EIT, the pure separation performance of the EIT for concrete is summarized in Table 6. A performance specification for the demolition of reinforced concrete components is not possible, since the EIT cannot cut through the reinforcement located in reinforced concrete, but can only expose it without damage. The comparison with established cutting methods is given in the following section.

Table 6 Cutting performance diamond saws (Schröder et al., 2015)
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Comparison of the separation performance of EIT with established separation processes

Basics

The basis of the comparative investigations was a full-scale analysis of the established demolition and cutting methods for selective deconstruction in sensitive areas. In the following, the established cutting and drilling methods are presented and their cutting and separation performance is clarified.

In general, cutting (= sawing) is used for the execution of partial demolitions or as a preparatory measure for a subsequent demolition technology. Sawing methods are suitable for demolition in existing structures, as they are accurate and low in vibration. Process water is required as rinsing or cooling water and for dust binding. Emissions from this cutting process are relatively low except for high noise levels, which is why it is commonly used in sensitive areas. Sawing allows for high precision. Saws of different sizes and types can be used. A basic distinction is made between circular blade sawing, chain sawing and wire sawing (Schröder et al., 2015). Table 7 summarizes the most important performance values of these three sawing methods.

Table 7 Drilling performance diamond-set drill bit by core drilling (Schröder et al., 2015)
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In drilling, a general distinction is made between core drilling and solid drilling methods. As with sawing, the use of water must be planned for this cutting method. This method has proven successfully in renovation and construction in existing structures, since the components to be preserved in the immediate vicinity remain undamaged and the vibrations introduced into existing structures are usually negligible. Noise pollution is significant with this method and must, therefore, be taken into account during planning. Table 8 summarizes the most important performance values for core drilling.

Table 8 Drilling performance hand-held drill hammer (Schröder et al., 2015)
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With a solid drill rig, a rotating and percussive motion transfers the force into the borehole via a drill head (Schröder et al., 2015). Due to this power transmission, a large part of the material in the borehole is crushed and discharged in coarse and fine components compared to the diamond core drill. Solid core drill rigs are used as hand-held rotary hammers (up to 20 kg) or as machine-guided tracked rotary hammers. Table 8 summarizes the most important performance values for manually executed solid drilling. It must be noted that drilling operations are usually associated with massive noise emissions, vibrations and dust generation.

Comparison with the cutting methods

When comparing the cutting methods with the EIT, the pure cutting performance is considered without taking secondary operations into account. Table 9 compares the cutting performance of the common cutting methods with the extrapolated cutting performance of the EIT.

Table 9 Comparison of cutting performance
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The results show that the EIT in particular produces a cutting performance at 25 Hz that is comparable to that of the established cutting methods. The quality of the cut surface is flat for the established cutting methods and rough for the EIT.

Comparison with the drilling methods

In the case of the drilling processes, the separation is not achieved by a continuous cut in the part or component, but by the complete or partial detachment of parts of the part or component.

The results of the extrapolation of the EIT do not include any pre- or post-processing. The aim is to use the process immediately after connection to the power supply and ensuring a sufficient amount of dielectric. Consequently, a very small amount of preparatory and finishing work, comparable to the use of a hammer drill, can be assumed. Table 10 compares the drilling performance of the EIT with the drilling performance of the established drilling methods.

Table 10 Drilling performance of established drilling methods and the EIT (Otto et al., 2021)
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Due to the process, a higher drilling performance can be achieved with a hammer drill for vertical work than for horizontal work as a result of the higher contact pressure. With the EIT, the same cutting performance can be achieved for both vertical and horizontal work, since the dead weight of the device has no influence on the cutting performance. Table 10 shows that at a frequency of 25 Hz, 45% of the drilling performance of a vertical hammer drill and even 90% of a horizontal hammer drill can be achieved. If the drilling performance of the EIT is compared with that of core drilling, the clear comparability of the two methods can be seen. In some cases, significantly higher drilling rates can be achieved with the EIT (25 Hz). Several pairs of electrodes can also be connected to a Marx generator, but only one active electrode is reached with each electrical pulse. The pulse repetition rate for the Marx generator should be limited to 25 Hz to ensure continuous use on a construction site.

Emissions

Compared to mechanical demolition and cutting processes, the acoustic impact of the EIT is only caused by the electrical flashover in the material and the generation of the voltage pulse in the pulse voltage generator. Other emission sources, such as water pump or voltage supply, are classified as low. The electrical flashover is acoustically shielded by the process chamber sealing with a dielectric located around the electrodes. As a result, it can be stated that the acoustic impact of the EIT is comparatively quiet and significantly lower than that of established dismantling processes. There is no transmission of sound waves over the entire building structure analogous to mechanical deconstruction methods.

The same applies to vibrations in comparison to mechanical demolition methods, since the material is not subjected to compression but to tensile stress during the EIT. This means that the process is virtually vibration-free. Consequently, in terms of emissions, EIT is comparable to the established drilling and sawing processes.

With regard to dust generation, it can be stated that the use of water as a liquid dielectric means that the dissolved material is bound directly in the process chamber. As a result, a virtually dust-free demolition process can be assumed.

Electricity consumption at EIT

A sufficient supply of electricity must be ensured for the operation of the Marx generator. To determine the actual power requirement of the EIT, the measurements of the laboratory tests were used and extrapolated for continuous use on a construction site. Based on the laboratory tests it became clear that with 5 pulses 4.13 cm3 of a sand-lime brick sample and 3.77 cm3 of a concrete sample can be dissolved with one pair of electrodes. Assuming a continuous dissolving process, a dissolving capacity of 1237.5 cm3 of a sandstone and 1131.0 cm3 of a concrete can thus be achieved in 1 min (cf. Tables 11, 12) (Otto et al., 2021). The specific energy for the dissolving power is 120 J/cm3 for the sand-lime brick sample and 272 J/cm3 for the concrete sample. These results from the basic tests were used to extrapolate the electricity consumption. Table 11 extrapolates the specific energy and electricity consumption of the EIT for dissolving sand-lime brick and Table 12 for dissolving concrete.

Table 11 Extrapolation specific energy and electricity consumption of the EIT by dissolving sand-lime samples (Otto et al., 2021)
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Table 12 Extrapolation specific energy and electricity consumption of the EIT by dissolving concrete sample (Otto et al., 2021)
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As the frequency of the EIT increases, so does the specific energy required. Due to the higher strength of concrete, a higher specific energy is required for deconstruction compared to sand-lime bricks.

Safety and health conditions

Safety and health protection are of immense importance, especially in dismantling operations, as the field of activity is one of the most hazardous in the construction industry. It has already been made clear that EIT can be considered advantageous in terms of emissions compared to established demolition and separation methods. In addition, the effects of the electrical current and high-voltage pulses must be considered in terms of safety and health.

All electrical components of a hand-held EIT device or an EIT attachment for a mini-excavator or demolition robot must be EMC-tested by the manufacturer (EMC = electromagnetic compatibility). A corresponding certificate must be available for each electrical component. The EMC and EMCU measurements (EMCU = electromagnetic incompatibility) carried out as part of the research project have shown that in the direct near field (distance of 1 m) to the pulse electrode, the electric and magnetic fields are not critical for humans. Nevertheless, the operating personnel of a hand-held EIT unit as well as an EIT attachment should always maintain a minimum distance of 1 m from the electrodes. The distance can be ensured by a design solution on the device.

Conclusions and outlook

Within the framework of the research project, it was possible to demonstrate on the basis of basic tests that the EIT can be used in the construction industry. The limits of the technology for use in the construction industry were also made clear. When considering the results, it is important to keep in mind that they only address a limited scope of investigation. The experiments were limited to the laboratory environment. In addition, when considering the results, it should be noted that they only cover a limited scope of the investigation. The experiments were performed in a laboratory setting. The boundary conditions and the building materials used were limited as a result. However, it is becoming clear that EIT technology still has great research potential.

The subsequent findings form an important basis for further research projects and a concomitant concretization of the EIT technology for applications in the construction industry:

  • The construction site environment, especially for building projects in existing structures, as well as the cost aspect only allow the use of industrial water as a dielectric. Service water is available in sufficient quantities on construction sites, a closed water circuit allows only a small amount of water to escape, and subsequent treatment of the service water is easily possible using established processes.

  • According to previous investigations, the electrode spacing should be 25 mm. This distance ensures that the electrical flashover takes place in the mineral building material. With a larger electrode spacing, the probability of electrical flashover in the material decreases (based on punctual tests with 5 pulses). A lower electrode spacing increases the specific energy.

  • The maximum number of pulses at one point should not exceed five pulses for an efficient dissolution process. A higher number of pulses increases the specific energy requirement. With a lower number of pulses, the overcut,Footnote 2 which is important for repositioning the electrodes (horizontally as well as vertically), cannot be ensured.

  • For the most energy-efficient removal of mineral building materials, the pulse energy should be between 72 and 80 J (based on the selected test setup). At lower pulse energy, no dissolution process of mineral building materials can be ensured.

Overall, the basic tests made it clear that EIT can be used as a deconstruction method in the construction industry. The economic field of application of the process does not lie in large-measurement removal (total demolition, mass removal), but in drilling and cutting processes as well as in surface treatment.

Automation of the EIT is conceivable for uniform surface removal. Conventional demolition methods often do not allow for automation, because the mechanical demolition causes vibrations in the carrier. With the EIT, the material is only subjected to tensile stress and only a low contact pressure on the material has to be ensured by the carrier device or a frame.

The results of the research project provide many starting points for further research and development. On the one hand, the further development of a hand-guided device for small applications in the field of sawing, slitting or also damage-free exposure of reinforcement is recommended. On the other hand, the use of higher pulse energies, pulse voltages and the simultaneous parallel use of a large number of electrode pairs can create the possibility of significantly increasing the productivity of the process and approaching the performance values of competing processes. Further research is needed at this point with regard to the technical implementation of the equipment. The dissolving performance can be improved through a technological improvement, but it must also be taken into account that use on the construction site and the prevailing conditions there can have a negative effect.

A third aspect is the utilization of the selectivity of the EIT. Here, the focus is on the selective removal of mineral layers, e.g., plaster layers, or the separation of already broken-off rock according to the individual components, e.g., masonry and plaster. Thus, the subsequent effort for separating and recycling the building material is reduced and can be automated.

Finally, it can be summarized that the results of the research project show interesting potentials of EIT for use in the construction industry and form the basis for further research and development for the elaboration of application-ready solution concepts in different areas of the construction industry. EIT will not be a fundamental substitute for established demolition methods for massive mass structures, but will rather be considered as a demolition technology that can be used for special boundary conditions. The continuation of the research work is considered to be promising.