Introduction

With the rapid development of urban construction and the increasing demand for demolition of old building projects year by year. approximately 460 million square meters of buildings are demolished every year1. The annual amount of building demolitions in the UK is about 120 million tons2, while in Japan it is 76 million tons3. Concrete is the most widely used material in civil construction facilities and buildings, and the mainly used concrete breaking modes nowadays are mechanical breaking, ejection shock wave breaking, and high-pressure water jet breaking4,5,6,7. However, the intensive progress in civil engineering and the pursuit of controllable and environmentally friendly technics essentially require the improvements of the current concrete breakage technologies or the development of the new ones. The technology of breaking concrete by high voltage pulse discharge (HVPD) was developed and implemented in recent twenty years. HVPD fragmentation is a novel technology that turns electrical energy into shock waves, which may successfully break concrete in aquatic conditions. It entails generating a pulse voltage with a rising edge of less than 500ns through a high-voltage pulse discharge device, injecting it into the interior of the concrete through an electrode rod in contact with the concrete surface, and requiring the concrete to be completely submerged in the aqueous medium. When supplied energy generates an ionization effect within the concrete, the number of charge carriers rapidly increases, forming a discharge channel. At this point, the high temperature and high voltage environment created by the high-voltage pulse discharge device encourage discharge. The channel rapidly widens, causing an explosion. The resulting shock wave forces the concrete in the water to break8,9,10. Due to the complexity of the concrete breaking process by HVPD and lots of affecting factors, the mechanism of concrete broken by HVPD is still not clear.

To gain insights into the mechanism of concrete breakage by HVPD, several studies have been done. Generally, the influence of single factor on the effectiveness of concrete crushing by HVPD has been studied in the literature, like the size, strength, composition and nature of the sample11,12,13,14,15, its structure and porosity16,17, discharge voltage parameters18,19, structure, material and the position of the electrodes20,21,22, and destruction media influence23. It is shown that under different conditions, the influence of these factors can be positive or negative24,25,26.

Simulation studies successfully allow predicting the optimal parameters of the electrodes for the fragmentation of hard rock15, and it was also revealed an electric field distortion existing in the rock due to the naturally occurring air gaps, which can enhance the internal electric field strength22.It can be assumed that the joint simultaneous change of two or more factors can lead to the process improvement, as well as to adverse consequences11,20,23. However, there are currently no works investigating the effect of a joint change in these factors.

The dynamic elastic modulus is often utilized as the damage variable to characterize the deterioration degree of concrete under several varied loads27,28. Some researchers have taken the loss of the relative dynamic modulus of elasticity as the damage variable when investigating the deterioration of concrete under different conditions29,30. However, There is almost no research on HVPD crushing concrete based on dynamic elastic modulus. Therefore, this paper uses the dynamic elastic modulus loss (DEML) as an index to examine the non-destructive effect of concrete material. Based on the orthogonal scheme, the effects of different applied voltage, pulse number, and discharge electrodes gap on the DEML of concrete broken by HVPD were experimentally analyzed. The cumulative effect of different factors on the DEML of crushing concrete was obtained through the method of mathematical modeling. Consequently, to improve the breaking mechanism of rock breakage by HVPD, we provide theoretical and practical guidance for the selection of fragmentation parameters, promoting the progress of breaking concrete materials' technology and contribute the city's sustainable development.

Materials and methods

Experimental system

The schematic of the experimental system of breaking concrete by HVPD is shown in Fig. 1a. It contains the high voltage pulse power supply, output electrodes, crushing container, experimental sample, and insulating medium, which was water. The high voltage pulse power supply based on ten stages impulse generator (Shenyang Ligong University, China) was used for further experiments.

Fig. 1
figure 1

Schematic of (a) the HVPD crushing concrete experimental system and (b) the experimental system for concrete dynamic elastic modulus measurements.

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During the process, a DC (Direct Current) source slowly charged the capacitor until the spherical spark switch closed, then switched to the self-breakdown mode. The high voltage pulse power supply was discharged once per experiment; the capacity was 5uF, the maximum output voltage was up to 450kV, and the maximum energy output of a single electric pulse was 100J. Output electrodes were composed of two stainless steel rods with an implemented needle-needle structure. One of the output electrodes was connected to the high voltage pulse power supply by positive output pole and the other by negative one. To avoid the breakdown outside the hard rock, a ceramic sleeve insulated the electrodes’ outer surface, and the electrodes/hard rock contact was constantly kept. The electrode gap ranged from 1 to 10cm. The crushing cuboid container was made of plexiglass to observe the experiment flow. The experimental sample used was a C45 concrete standard block which has a compressive strength grade of 45MPa. Insulating medium was tap water. Before the experiment, the concrete surface was cleaned and dried to avoid the dust affection on the crushing test results. During the experiment, the concrete sample was completely submerged by water to avoid any breakdown and breakage in the air.

The experimental system (Tianjin Sansitrang Test Equipment Manufacturing, China) used to measure the dynamic elastic modulus of concrete is shown in Fig. 1b. It contained two parts: dynamic elastic modulus tester (on the right) and the concrete target holder with two test probes (on the left). The dynamic elastic modulus tester consisted of tester host, launcher, receiver support frame and processing software of dynamic elasticity tester of concrete. The DEML of concrete crushed by HVPD was calculated as the difference between the dynamic elastic modulus value measured before and after crushing.

Experimental preparation

For this research, the self-made C45 cubic concrete sample with a side length of 150mm was made by mixing cement (PO42.5, Xuzhou Fengdu material Trade Co., Ltd, China), fly ash (Class F II, China Railway 15th Bureau Group Materials Co., Ltd, China), sand (dav = 0.5–0.25mm), spalls (30% of dav = 5–10mm, 20% of dav = 10–20mm, and 50% of da = 20–31.5mm), additives (Polycarboxylic acid, Shanxi Sangmusi Building Materials Chemical Co., Ltd, China) and water at the proportion of 1 : 0.43 : 2.12 : 3.93 : 0.01 : 0.62, respectively. Samples were cured up to 28 days under standard conditions (20 ± 2 ºC, relative humidity > 95%). According to the standards for mechanical testing methods of ordinary concrete (GB/T50081-2002), the strength of the concrete sample was analyzed by such parameters as mass, density, compressive strength, and elastic modulus. The experiment contained 27 cubic-shaped concrete samples. The mean values of relevant parameters are presented in Table 1, each parameter was calculated from the measurements of five individual samples. These data are consistent with concrete classification by compressive strength31.

Table 1 Relevant parameters of concrete samples.
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Experimental scheme

In the light of the literature32, the parameters of applied voltage, pulse number, and electrode spacing three factors affecting the DEML of concrete broken by HVPD, and the selected experimental conditions were as follows in Table 2. The L9(33) orthogonal table was selected for experimental analysis of the DEML. All the experiments were repeated three times at different levels of each factor. In the following discussion, we chose factors as A-applied voltage, B-pulse number, and C-discharge electrodes gap.

Table 2 L9(33) orthogonal table for experimental factors for DEML of concrete broken by HVPD.
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Significance analysis

To precisely estimate the variance scope of the experiment's results of the DEML of concrete fractured through HVPD, along with properly distinguish data fluctuation caused by experimental errors and variations of the experimental conditions, a significance analysis of the impact of the three variables considered in the tests on the DEML of concrete crushed through HVPD is carried out. Due to the orthogonal design used in this experiment, there are only three influencing factors, namely applied voltage, number of pulses, and electrode spacing, and each combination is only repeated 3 times, resulting in limited sample data. Therefore, the significance level of 0.1 is chosen in this article to increase the significance. According to the ANOVA (Analysis of Variance) statistics model, the degree of freedom is equal to the factor level number minus 1, which is 2 for the current experiments; f0.1 is the critical value of the F test when the significant level is 0.1; f0.1 can be obtained by querying the upper sub-table of the F distribution. Test statistic F was determined as the ratio of inter-group to intra-group variation33.

After generating the test statistic F, the significance of each factor is determined by comparing it to the test critical value f0.1. When the F value is bigger than f0.1 = 9, this factor has a considerable effect on the experimental results; on the contrary, the influence is insignificant.

Results and discussion

The resulting DEML of breaking concrete is shown in Table 3. It can be seen that the various combinations of applied voltage, pulse number, and discharge electrodes gap have a certain impact on the DEML of the concrete broken by HVPD: maximum of DEML can be observed in experiment #1, and that under #6 is minimum. For the further study of the factors’ effect on the loss of DEML of broken concrete, the following range and significance analysis are done.

Table 3 The experimental DEML of crushing concrete.
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Range analysis

Firstly, compute the sum of the DEML for each factor based on its level, then the average value of DEML for each factor and level was found, and the results are presented in Table 4. Secondly, the range of the DEML of concrete for each factor according to its level was calculated as the difference between the maximum and minimum average DEML value under the certain factor (Table 4).

Table 4 Sum and average of DEML of concrete at different factors’ levels.
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The range indicates the change of the DEML of concrete under the impact of a certain factor, which characterizes the influence of this factor on the dynamic elastic modulus loss. Taking the maximum range as 1, it can be seen from the obtained data that the largest impact on the average value of the DEML is in the raw of the pulse number, discharge electrodes gap and applied voltage with the rate of 1, 0.8 and 0.41. According to the relationship between energy and voltage, when the capacitance is constant, the output energy of high voltage pulse power supply is determined by the output voltage. In high-voltage pulse discharge crushing, when the input voltage is not large enough, one discharge cannot break, so it needs multiple pulse discharges to break it. For the discharge electrode gap, when the input voltage is constant, the value directly determines the electric field strength between the two electrodes, and then determines the breaking performance.

A comparison of the DEML under different levels demonstrates that applied voltage impact on the DEML exhibits the direct dependency (92.2192.21 ± 4.47MPa, 93.79 ± 6.27MPa and 105.85 ± 7.40MPa), pulse number—reverse (118.73 ± 5.05MPa, 87.00 ± 5.10MPa and 86.12 ± 7.82MPa), and for electrodes gap—the DEML decreases from the maximum at the gap of 3cm—108.70 ± 5.23MPa—to a minimum value at the 5cm—82.41 ± 5.15MPa, within the subsequent growth at 7cm—100.73 ± 7.77MPa (Table 4). The maximum average value of the DEML of concrete in the tests of the individual influence of factors are achieved at applied voltage 415 kV (A3), pulse number factor of 1 time (B1), and discharge electrodes gap of 3cm (C1), and equal to 105.85 ± 7.40MPa, 118.73 ± 5.05MPa, and 108.70 ± 5.23MPa, respectively.

Considering the above classification, the DEML of crushing concrete under the combination of factors A3B1C1 is expected to be the most significant. The impact of A3B1C1 factors combination was experiment-ally proven, and the resulting DEML of concrete broken by HVPD was obtained as 219.73 ± 9.58MPa, which is 25.19% higher than the maximum of the DEML of concrete broken by HVPD in the orthogonal experiment under various individual factors (Table 3).

Based on the analysis of Table 4, we can identify primary and secondary factors affecting the DEML of concrete broken by HVPD. If the factor has a great influence on the DEML of crushing concrete, the difference of the DEML under different levels of this factor will be significant, and the factor is considered to be the primary. Otherwise, this is the secondary factor. According to the above definition, the pulse number is the primary factor affecting the DEML, inter-electrode gap and applied voltage are considered to be secondary factors. The order of impact for these three factors on the DEML of concrete broken by HVPD is: pulse number—> discharge electrodes gap—> applied voltage. In the point of this finding, the DEML of concrete can be increased by adjusting sensitive factors, and the damaging of concrete building materials' problem can be further improved. In the experimental system of concrete crushed by HVPD, if the discharge electrodes gap is fixed, the distance between the electrodes can be regarded as a fixed value. Therefore, in the design of demolition of concrete building materials, the DEML of broken by HVPD concrete can be controlled by adjusting the applied voltage and the pulse number. Under these two factors, the maximum DEML of concrete is at the factor levels of A3B1.

The change in the DEML of concrete-broken by HVPD with different applied voltage under discharge electrodes gap of 3cm, 5cm, and 7cm and maintained pulse number is shown in Fig. 2a. The DEML increases with the increase of the applied voltage, and this is consistent with Wang's research result34. Within the increase of voltage from 360 to 415kV, the DEML of concrete broken by HVPD increases by 15.4%, 12.9% and 12.8% for electrode gap of 3cm, 5cm and 7cm, respectively. The change of overall average compressive strength of concrete increases with the increase of the applied voltage. This is because as the applied voltage increases, so does the amount of energy released into the interior of the concrete samples per unit time via the electrodes, resulting in a greater crushing force of shock waves on the concrete samples, and then contributes to a growth of the volume of voids, cracks, and micropores in concrete samples.

Fig. 2
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Variation curves of the DEML of concrete under different (a) applied voltages and (b) pulse numbers.

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The variation curves of the DEML of concrete broken by HVPD with different pulse numbers under the condition that of discharge electrodes gap of 3cm, 5cm, and 7cm, and fixed applied voltage of 360kV are shown in Fig. 2b. It can be seen that the DEML of concrete broken decreases with the increase of the pulse number, and the loss of dynamic elastic modulus decreases significantly when the pulse number increases. When the pulse number changes from one to five times, the DEML of concrete decreases by 26.7%. When the electrode spacing is constant, with the increase of pulse number, the particle size of broken concrete decreases, and the influence on un-crushed concrete decreases11. This is because when high-voltage pulse discharge breaks concrete, the energy effect is mainly concentrated between the two electrodes35. Therefore, as the number of pulse number increases, when the concrete between the electrodes is completely broken, if the spacing and position of the electrodes do not change, the effect on the concrete will be very small.

Significance analysis

In this experiment, according to mathematical and statistical methods, it can be calculated that the obvious impact of the pulse number on the DEML of concrete broken by HVPD can be seen with pulse number changing (FB = 9.8 > f0.1), it has a decisive role, then followed by the discharge electrodes gap (FC = 5.6 < f0.1), while the effect of the applied voltage is weak (FA = 1.4 <  < f0.1).

Conclusions

The orthogonal scheme experiment showed that the studied parameters have an obvious effect on the DEML of concrete broken by HVPD at the order from the highest impact to the lowest as: pulse number, discharge electrodes gap, and applied voltage. Because the distance of discharge electrodes is fixed during the breaking process, the DEML can be controlled more easily by changing the applied voltage and pulse number. Under the varying of these two factors, the combination of A3B1 is the most significant. Adjusting the applied voltage and pulse value could increase the DEML by 12.9% and 26.7%, respectively. The F-test results showed that the impact of the pulse number on the DEML of concrete broken by HVPD is the most significant. Thus, the crushing effect of concrete building materials can be improved by increasing the pulse number of HVPD power supply, and finely controlled the applied voltage, which provides data support for the optimal design and engineering application of a HVPD concrete crushing experimental system. The factors affecting the DEML of crushed concrete are not only the applied voltage, pulse number, and electrode spacing, but also include concrete strength and composition, output electrode material, rise time of applied voltage, insulation liquid properties, and other requires further research.