Introduction

As carriers of historical and cultural information, masonry relics present great historical, artistic, and economic value. However, accompanied by long-term natural deterioration, most masonry relics suffer from different degrees of deterioration and even threaten structural stability. Accurately evaluating the deterioration degree and consolidation effectiveness of masonry relics is highly important for disease investigation and restoration work.

To evaluate the deterioration degree of masonry relics, visual assessment of deterioration is the most intuitive method. Some researchers have proposed methods for evaluating the deterioration degree based on the clarity and legibility of inscriptions1,2. The generalized visual assessment method enables comprehensive evaluation of numerous masonry relics through a simple and efficient process, but there is still room for improvement in terms of accuracy and precision. In addition, researchers have proposed semiquantitative evaluation indices for the deterioration degree of masonry relics. For instance, Fitzner et al. proposed a damage index to assess limestone deterioration that uses planimetric data in conjunction with weathering forms and damage categories3. Warke et al. proposed a unit, area, and spread (UAS) staging system model to assess the deterioration degree, which involves controlling factors, including structural and mineralogical properties, inheritance effects, contaminant loading, and natural change4. According to the photobased and site-specific weathering forms, Thornbush proposed a weathering index (S-E index) to assess the deterioration degree5. However, because such schemes involve detailed surveying, there may be considerable demands on operator time and expertise.

In addition, the mechanical and physical properties of masonry relics, including microfracture and porosity6, as well as compressive and flexural strength7, can also reflect the deterioration degree. Therefore, mechanical and physical indices obtained from laboratory accelerated deterioration processes can be used in quantitative evaluation. As a result of the preciousness and uniqueness of masonry relics, more researchers have suggested the use of nondestructive testing methods to assess the deterioration degree in situ. There are many available studies, for example, deterioration assessments based on ultrasonic wave velocities8,9, Schmidt hammer rebound10,11, hardness testers12,13, penetration resistance testers14,15, ultrasonic CT16, and laser scanners17. However, ultrasonic, rebound, and hardness methods require the surface of the measured material to be as flat as possible. The applicable strength range of the penetration resistance method is from 0.4 MPa to 16 MPa, which is not recommended for hard rock18. Ultrasonic CT and laser scanners may be cumbersome to use in data processing and place considerable demands on operator time and expertise.

The majority of studies have concentrated on the variations in the physical and mechanical properties of masonry materials before and after consolidation to evaluate the consolidation effectiveness. These include pore size distributions, dynamic elastic moduli, and tensile strengths19,20,21. In addition, nondestructive test methods have been used to evaluate the consolidation effectiveness of masonry relics. The most commonly used method is the comparison of ultrasonic wave velocity before and after consolidation22,23. However, in most field situations, the material properties tend to vary with depth in deteriorated and consolidated masonry relics. The above methods make it difficult to directly and accurately reflect the mechanical properties versus with depth of the material surface layer before and after consolidation.

Drilling resistance measurement system (DRMS) is an instrument that can continuously measure the resistance of a material to a drill bit under constant drilling conditions. In contrast with other nondestructive measuring instruments, DRMS, which has high sensitivity, can directly and accurately reflect the variation in material properties from the surface to the interior24. Therefore, the DRMS has been applied to evaluate the deterioration degree of masonry relics. By analysing the variation in drilling resistance with drilling depth, the surface deterioration depth and the thickness of the deterioration layer can be obtained25,26. Fonseca et al. proposed a classification scheme for the deterioration of marble based on drilling resistance values to quantitatively classify the deterioration degree27,28. DRMS is also commonly used to evaluate the range and magnitude of variations in drilling resistance-depth profiles before and after consolidation and is one of the most suitable methods for assessing the consolidation effectiveness of masonry relics. Especially in soft rocks, the difference in drilling resistance before and after consolidation appears to be particularly pronounced29,30. The consolidation depth of different types and dosages of consolidants can be evaluated based on the change in drilling resistance31,32. According to the testing and comparison of rock before and after consolidation with a scanning electron microscope and DRMS, Ban et al. confirmed the reliability of assessing the consolidation effectiveness from drilling resistance-depth profiles33. In addition, DRMS has also been used to evaluate the consolidation effectiveness of microbially induced carbonate precipitation techniques34,35. However, the current application of DRMS in the evaluation of the deterioration degree and consolidation effectiveness of masonry relics is commonly used for qualitative or semiquantitative measurements of the deterioration layer thickness and deterioration depth, as well as mostly for qualitative comparisons of the differences in drilling resistance before and after consolidation. There is still a lack of quantitative indices for evaluating the deterioration degree and consolidation effectiveness of masonry relics in combination with nondestructive methods.

To study the nondestructive quantitative evaluation method of the deterioration degree and consolidation effectiveness in masonry relics, sandstone and clay bricks, which are common among masonry relics, are used as study objects. Five types of samples with different deterioration degrees were prepared by artificially accelerated deterioration tests for both sandstone and clay brick, and three types of consolidants were used to consolidate the deteriorated samples. Drilling resistance tests were conducted for deteriorated and consolidated samples, and the calculation method for the average drilling resistance was determined based on the range and magnitude of the variations in the drilling resistance-depth profiles. The variations in deterioration depth and average drilling resistance for samples with different numbers of deterioration cycles were analysed, while the differences in consolidation depth and average drilling resistance for samples with different consolidant types and dosages were compared. Moreover, deterioration degree indices (\(K\)) and consolidation effectiveness indices (\({R}_{c}\)), which are based on the average drilling resistance, are proposed. Finally, the results were compared with the evaluation indices in the relevant standardization (BS EN 12,371:2010; WW/T 0063–2015)36,37 to verify the accuracy and reliability of the \(K\) and \({R}_{c}\).

Materials and methods

Materials

Sandstone sample and clay brick sample

Sandstone samples were purchased from Yuze Stone Industry Co., Ltd. (Jining, China). The lithology is red fine-grained feldspar sandstone with blocky formations. According to the results of the rock thin-section analysis and identification (as shown in Fig. 1a), the sandstone is composed mainly of quartz (70–75%), potassium feldspar (5–10%), plagioclase (less than 5%), clasts (10–15%), and filler material (5–10%). The clasts are predominantly chlorite and white mica. The filler material contains reddish-brown ferruginous cement, which is commonly found in thin films and banded structures. The sandstone grains are mostly rounded and subangular in shape and consist mostly of fine sand (0.06–0.25 mm) and a small amount of medium sand (0.25–0.5 mm), with good sorting and rounding and a haphazard distribution. The sandstone samples were sliced from the same fine-grained sandstone. These samples have almost the same dimensions and mass. Samples with similar wave velocities were selected by ultrasonic wave velocity tests to ensure that there were no significant fissures within the experimental samples. A total of 8 sandstone samples (S1-S8) were obtained and each sample was a cylinder with a diameter of 50 mm and a height of 100 mm. Table 1 shows the bulk density, particle density, total porosity, free water absorption, forced water absorption, and uniaxial compressive strength of the sandstone.

Fig. 1
figure 1

The results of petrographical examination and X-ray diffraction analysis: (a) microstructure of the sandstone sample in thin section; (b) X-ray diffraction analysis result of the clay brick sample.

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Table 1 The test results of the physical and mechanical properties of the sandstone and clay brick samples.
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Clay brick samples were purchased from Dukai Ancient Brick Industry Co., Ltd. (Handan, China), which are blue bricks. The manufacturing process of the blue bricks is as follows: The clay was first soaked and cleaned with water and then dried to a constant mass. Subsequently, the clay was mashed and sieved through a 1 mm sieve. The sieved clay particles were mixed with water and put into moulds. The shaped clay blocks were removed from the moulds and left to dry naturally indoors for 15 days. After that, the clay blocks were fired in a high-temperature furnace for 10 days, maintaining the temperature at 1100 ℃. Finally, the fired clay bricks were cooled by the addition of water in a confined space. The clay brick sample was pulverized into powder for X-ray diffraction analysis (as shown in Fig. 1b). The X-ray diffraction pattern calculations were performed using the Clayquan program (version 2020) with Rietveld refinement methods. The components of the different minerals were calculated from the cumulative peak area. The results show that the main mineral components of the clay brick sample are quartz (62.0%), dolomite (7.8%), clay minerals (19.4%), potassium feldspar (4.5%), plagioclase (3.7%), and clasts (1.2%). A total of 8 clay brick samples (B1-B8) with similar ultrasonic wave speeds were obtained from the same batch of bricks, all of which were cubic with a length of 40 mm. Table 1 shows the bulk density, particle density, total porosity, free water absorption, forced water absorption, and uniaxial compressive strength of the clay brick.

Materials for deterioration and consolidation experiments

Sandstone and clay brick deterioration samples are obtained through laboratory accelerated dry and wet cycling processes. The instrument used for drying the samples was an electrothermal blast drying oven (produced by Shanghai Meiyu Instrument Co., Ltd., Shanghai, China). Sodium sulfate (Na2SO4) is one of the most frequently found salts and the most damaging to masonry artifacts38,39; hence, Na2SO4 solution was selected as the immersion fluid with a mass fraction of 14%. Three commonly known consolidants for masonry relics were used to consolidate the sandstone sample after two dry and wet cycles and the clay brick sample after three dry and wet cycles. The three types of consolidants used were Paraloid B-72 (B-72), Tetraethyl orthosilicate (TEOS), and PS solution (PS). Consolidation with B-72 and TEOS are widely used in the restoration of architectural and cultural heritage, and their performance in this application is quite excellent40,41. PS is one of the most used consolidants for natural stones in the restoration of cultural heritage in China and the literature concerning its performance is quite abundant42,43. This research work builds upon previous studies that have examined the optimum ratio of consolidants44, and the properties of consolidants are shown in Table 2.

Table 2 The three types of consolidants used in the consolidation experiment.
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Methods

Deterioration method

Literature indicates that salt can produce irreversible damage to masonry artifacts45. In this research work the accelerating salt weathering test was performed on sandstone and clay brick samples. This was done in order to study the drilling resistance for different deterioration degrees.

Sandstone and clay brick deterioration samples are obtained through laboratory accelerated dry and wet cycling processes, and samples of the same group are obtained at approximately the same ultrasonic velocity. The dry and wet cycle experiments were carried out according to BS EN 12,370:202046. The specific steps for a cycle are as follows (as shown in Fig. 2): (1) All the samples were first dried at 105°C to a constant weight (until the difference in mass within 24 h did not exceed 0.1% of the first weight). (2) After drying, the samples were cooled at room temperature for 2 h and then put into a Na2SO4 solution at 20°C for immersion. The distance between each sample was at least 10 mm, the distance between the sample and the container wall was at least 20 mm, and the liquid level of the solution was at least 8mm above the upper surface of the sample. In addition, the container was sealed with parafilm to reduce evaporation of the solution. (3) After immersion in Na2SO4 solution for 2 h, the test block was removed and put into a drying oven for 16 h. Before drying, the evaporating dish containing water was put into a drying oven and heated for 30 min in advance to maintain high humidity.

Fig. 2
figure 2

Dry and wet cycling process for the sandstone and clay brick samples.

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A total of 8 samples each from sandstones (S1-S8) and clay bricks (B1-B8) were taken for dry and wet cycle experiments. After a certain number of dry and wet cycles were reached, the sandstone and clay brick samples were removed, washed with distilled water, and dried. The maximum number of dry and wet cycles is 8 for the sandstone samples and 15 for the clay brick samples.

Consolidation method

After two dry and wet cycles, three sandstone samples (S6, S7, and S8) were taken for consolidation tests, and three clay brick samples (B6, B7, and B8) were taken for consolidation tests after three dry and wet cycles. Since the samples in the laboratory were quite small (the sandstone sample was 50 mm in diameter and the clay brick sample had a side length of 40 mm), to accurately control the uniform distribution of the consolidant on the surface of the consolidated materials, the consolidation method of dropwise infiltration with a dropper was used in this paper. The dosage of the consolidant is distributed evenly over the surface of the consolidated material. The consolidation steps were as follows: (1) A dropper was used to add 1 ml of consolidant uniformly to the sample surface, and then 1 ml was added again after all the consolidant had penetrated into the sample. The first consolidation was completed after the consolidated samples were placed in a room temperature environment for 3 days. (2) Subsequently, 2 ml of consolidant was added to the same surface again in the same way as in the first round of consolidation. The second consolidation was also completed after the consolidated samples were placed in a room temperature environment for 3 days. The drilling resistance was tested before consolidation and after completion of each consolidation, as shown in Fig. 3.

Fig. 3
figure 3

Deteriorated sample consolidation process and drilling resistance test procedure.

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Testing methods

Micro-drilling resistance testing method

The operation principles of the DRMS (produced by SINT Technology Co. Ltd., Italy) used in this experiment are shown in Fig. 4. Before drilling resistance testing starts, the instrument needs to be connected to the computer via a data cable with the penetration rate (\(v\)), revolution speed (\(\omega\)), and drilling depth (\(h\)) set in the corresponding "DRMS Cordless" software. During the drilling process, the instrument maintains a constant penetration rate and revolution speed to continuously measure the drilling resistance. The DRMS can visualize the output of real-time drilling resistance data and the drilling resistance-depth profile.

Fig. 4
figure 4

The components, operation principles, and operation processes of DRMS.

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A carbide drill bit (BOSCH, CYL-2, produced by BOSCH, Co. Ltd., Germany) was used in this experiment, and its structure is shown in Fig. 5. In addition, the DRMS is a very sensitive instrument, and its measurement data are affected by drilling parameter settings, drill diameter, etc47,48,49,50. To control variables, based on studies correlating drilling resistance values with drilling parameters and bit parameters24,44, carbide drill bits with a diameter of 5 mm are selected, and the instrument settings are \(v\)=10 mm/min, \(\omega\)=600 rpm, and \(h\)=10 mm. The drilling resistance data were acquired every 0.1 s. At the same time, to avoid the influence of drill bit wear on the drilling resistance, a new carbide drill bit was used for each drill hole in all the experiments. The samples were dried before drilling resistance testing.

Fig. 5
figure 5

DRMS and carbide drill bit used in the experiment: (a) Schematic of DRMS instrument; (b) carbide drill bit; (c) schematic structure of the carbide drill bit.

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Moreover, to avoid the influence of neighboring drill holes and sample edges, drill holes were selected at a distance greater than 1 cm from the sample edge, and the distance between neighboring drill holes was not less than 1 cm. In addition, to assess the variability between the samples, drilling resistance tests were performed before deterioration and consolidation experiments. The deteriorated sample was tested only twice, before deterioration and after a specified number of deterioration cycles. The consolidated samples were tested only thrice, before consolidation, after the first consolidation, and after the second consolidation. Three parallel drillings for each test, each drilling can be obtained with 100 data points of drilling resistance versus drilling depth. The drilling resistance was averaged for three parallel drillings at the same drilling depth. Hence, each data of the drilling resistance-depth profile is the mean value obtained from three parallel drillings at the same drilling parameters.

Ultrasonic wave velocity testing method

The literature indicates that there is a correlation between the ultrasonic wave velocity and drilling resistance8,9. In this regard, primary wave and shear wave velocities were measured by an ultrasonic detector (Proceq Pundit PL-200, Proceq Trading Shanghai Co. Ltd., Shanghai, China) with input signals at frequencies of 54 and 250 kHz, respectively. The samples of sandstone (S1-S8) and clay brick (B1-B8) were subjected to testing for primary wave and shear wave velocities in a direction parallel to the drilling. Sandstone and clay brick samples were tested for ultrasonic wave velocity before each drilling resistance test. The testing steps are as follows: The transducer is uniformly coated with couplant and tightly attached to both ends of the sandstone or clay brick samples. The transmission time of the ultrasonic waves through the waveform graph on the ultrasonic detector was obtained and recorded as t, accurate to 0.1 \(\mu s\), and 5 times in parallel to take the average value. According to the length (\(l\)) of each sample measured, the ultrasonic wave velocity can be calculated according to the ratio of length (\(l\)) to transmission time (t).

Results

Deterioration experiment

Figures 6 and 7 show the drilling resistance-depth profiles for sandstone samples after 2, 4, 6, 7, and 8 dry and wet cycles and the apparent variations in the samples with increasing deterioration cycle times. Initial experiments assumed a maximum of 10 cycles of the sandstone samples to obtain data at each two-cycle interval. However, at the end of the 7th cycle, the S-4 sample appeared to be visibly cracked (Fig. 6d). By the end of the 8th cycle, the S-5 sample exhibited severe surface exfoliation (Fig. 6e). The deterioration experiment was terminated after 8 dry and wet cycles to avoid serious deterioration of the samples, resulting in an irregular surface, which would affect the drilling resistance test.

Fig. 6
figure 6

Sandstone samples after dry and wet cycling experiments: (a) S-1 sample after 2 cycles; (b) S-2 sample after 4 cycles; (c) S-3 sample after 6 cycles; (d) S-4 sample after 7 cycles; and (e) S-5 sample after 8 cycles.

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Fig. 7
figure 7

Drilling resistance-depth profiles of sandstone samples after different numbers of dry and wet cycles: (a) S-1 sample after 2 cycles; (b) S-2 sample after 4 cycles; (c) S-3 sample after 6 cycles; (d) S-4 sample after 7 cycles; (e) S-5 sample after 8 cycles; and (f) comparison of samples after different numbers of deterioration cycles.

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As shown in Figs. 6 and 7, after 2 cycles, the drilling resistance within 0–4.5 mm is slightly lower than that of the undeteriorated sandstone sample, and the drilling resistance in the 4.5–10 mm range is approximately the same as that of the undeteriorated sandstone sample. After 4 cycles, the drilling resistance is significantly lower within the 0–4 mm region than that for the undeteriorated samples. After 6 cycles, the drilling resistance significantly decreased as the range increased to 0–6 mm, and the drilling resistance within 0–0.6 mm was only 0.63 N. After 7 cycles, the depth range of complete deterioration is further extended, with only 1.04 N of drilling resistance in the 0–0.8 mm range. After 8 cycles, the drilling resistance-depth profile clearly changes, and the drilling resistance within 0–1.2 mm is only 1.26 N, indicating that this range has completely deteriorated.

Figures 8 and 9 show the drilling resistance-depth profiles for the clay brick samples after 3, 6, 9, 12, and 15 dry and wet cycles, respectively, and the apparent variations in the samples with increasing deterioration cycle times. At the end of the 3rd cycle, there was no clear variation in the appearance of the B-1 sample. At the end of the 6th and 9th cycles, slight granular exfoliation occurred at the corners of the clay bricks. By the end of the 12th cycle, the B-4 sample exhibited more severe granular exfoliation. After 15 deterioration cycles, the B-5 sample had a large area of missing.

Fig. 8
figure 8

Clay brick samples after dry and wet cycling experiments: (a) B-1 sample after 3 cycles; (b) B-2 sample after 6 cycles; (c) B-3 sample after 9 cycles; (d) B-4 sample after 12 cycles; and (e) B-5 sample after 15 cycles.

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Fig. 9
figure 9

Drilling resistance-depth profiles of clay brick samples after different numbers of dry and wet cycles: (a) B-1 sample after 3 cycles; (b) B-2 sample after 6 cycles; (c) B-3 sample after 9 cycles; (d) B-4 sample after 12 cycles; (e) B-5 sample after 15 cycles; and (f) comparison of samples after different numbers of deterioration cycles.

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As shown in Figs. 8 and 9, after 2 cycles, the drilling resistance-depth profiles did not change significantly, with the drilling resistance slightly decreasing within 0–4 mm, and the drilling resistance in the 4–10 mm range was approximately the same as that of the undeteriorated clay brick sample. Afterward, as the deterioration time increases, the drilling resistance in the depth range of 0–4 mm continues to decrease, but the deterioration depth range does not change significantly.

Quantitative evaluation of deterioration degree

To quantitatively analyse and evaluate the deterioration degree of sandstone and clay brick samples, a deterioration degree index (\(K\)) was proposed according to the results of drilling resistance testing from sandstone and clay brick samples before and after deterioration. \(K\) represents the rate of decrease in the average drilling resistance over the range of deterioration depths. The drilling depth on the drilling resistance-depth profile corresponding to the point at which the drilling resistance begins to stabilize is defined as the deterioration depth, as shown in Fig. 10. The initial data with disturbances at drilling depths of 0–1 mm are removed from the calculation, and the calculation formula for \(K\) is shown in Eq. (1).

$$\begin{array}{c}K=\frac{{DR}_{UD}-{DR}_{D}}{{DR}_{UD}}*100\%\end{array}$$
(1)

where \({DR}_{UD}\) is the average drilling resistance for undeteriorated samples (within the deterioration depth range) and \({DR}_{D}\) is the average drilling resistance for deteriorated samples (within the deterioration depth range).

Fig. 10
figure 10

Schematic of deterioration depth and calculation depth of drilling resistance (\(K\)): \({f}_{UD}(x)\) is the drilling resistance-depth profile of undeteriorated samples; \({f}_{D}(x)\) is the drilling resistance-depth profile of deteriorated samples; i is the drilling depth of the point where the drilling resistance begins to stabilize.

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The drilling resistance data from deteriorated and undeteriorated samples can be obtained as drilling resistance-depth profiles \({f}_{UD}(x)\) and \({f}_{D}(x)\). \({DR}_{UD}\) and \({DR}_{D}\) are the arithmetic mean of the drilling resistance-depth profiles \({f}_{UD}(x)\) and \({f}_{D}(x)\) respectively over a depth range from 1mm to i mm. Table 3 shows the calculation results of \(K\) for the sandstone and clay brick samples at different deterioration cycle times. The average drilling resistance values of the undeteriorated sandstone samples ranged from 26.87 to 28.66 N, and those of the undeteriorated clay brick samples ranged from 15.50 to 19.11 N. The uniformity of the drilling resistance was superior for the fine-grained sandstone samples, with a maximum difference of only 6.7%; while the maximum difference in the drilling resistance for clay brick samples was up to 23.29%, with a high degree of discreteness. Non-homogeneity within the clay brick sample, soft clay minerals approximately 20%, and hard minerals (such as SiO2) may lead to high strength in the localized area of the drill hole. The occurrence of minerals with different hardness could enhance the fluctuations of drilling resistance. In addition, \(K\) gradually increases as the number of deterioration cycles increases, and the deterioration degree of the samples gradually increases. For the sandstone samples, a significant decrease in the drilling resistance occurred at the 4th and 7th cycles. The clay brick samples exhibited a visible decrease in drilling resistance after every three deterioration cycles. The rate of decrease in the drilling resistance with deterioration cycle time for the sandstone sample was significantly greater than that for the clay brick sample.

Table 3 Drilling resistance and drilling resistance index (\(K\)) of the sandstone and clay brick samples before and after deterioration.
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In addition, Table 3 shows that the deterioration depth in the sandstone samples increases with the number of deterioration cycles, and the thickness of the deteriorated layer increases from 3.9 to 7.4 mm, but at the 7th and 8th cycles, the thickness of the deteriorated layer was only approximately 5.5 mm. The thickness of the deteriorated layer fluctuates gradually from 3 to 4 mm in the clay brick samples, and the deterioration degree cannot be accurately determined from the deterioration depth data alone.

Consolidation experiment

Figure 11 shows the experimental process for determining the consolidation effectiveness of the three types of consolidants (PS, B-72, and TEOS) for consolidating sandstone and clay brick samples. There is a clear difference in the penetration consolidation depth of the different types of consolidants. Figure 12 shows the drilling resistance-depth profiles for the sandstone and clay brick samples before and after consolidation for the three types of consolidants. The drilling resistance of the sandstone samples increased within 0–4.1 mm after consolidation with 2 ml of PS solution and further increased within 0–5.4 mm after consolidation with 4 ml of PS solution. Similarly, the drilling resistance of the sandstone samples increased within 0–3.6 mm after consolidation with 2 ml of B-72 solution and further increased within 0–5.4 mm after consolidation with 4 ml of B-72 solution. However, the drilling resistance-depth profiles of the sandstone samples exhibited little change after consolidation with 2 ml and 4 ml of TEOS solution. The drilling resistance of the clay brick samples increased within 0–1.8 mm after consolidation with 2 ml of PS solution and further increased within 0–3.1 mm after consolidation with 4 ml of PS solution. The clay brick samples exhibited a continuous increase in drilling resistance within 0–1.8 mm after consolidation with 2 and 4 ml of B-72 solution, while the increase in the second consolidation was greater. The drilling resistance-depth profiles of the clay brick samples also exhibited little change after consolidation with 2 and 4 ml of TEOS solution.

Fig. 11
figure 11

Sandstone and clay brick samples after consolidation with three types of consolidants.

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Fig. 12
figure 12

Drilling resistance-depth profiles for sandstone and clay brick samples before and after consolidation with three types of consolidants: (a) S-6 sample consolidated with PS; (b) B-6 sample consolidated with PS; (c) S-7 sample consolidated with B-72; (d) B-7 sample consolidated with B-72; (e) S-8 sample consolidated with TEOS; (f) B-8 sample consolidated with TEOS.

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Quantitative evaluation of consolidation effectiveness

To quantitatively analyse and evaluate the consolidation effectiveness of sandstone and clay brick samples, a consolidation effectiveness index (\({R}_{c}\)) was proposed according to the results of drilling resistance testing from sandstone and clay brick samples before and after consolidation. \({R}_{c}\) represents the increase rate of the average drilling resistance over the range of consolidation depths. The drilling depth on the drilling resistance-depth profile corresponding to the point at which the drilling resistance begins to coincide before and after consolidation is defined as the consolidation depth, as shown in Fig. 13. The initial data with disturbances at drilling depths of 0–1 mm are removed from the calculation, and the calculation formula for \({R}_{c}\) is shown in Eq. (2).

$$\begin{array}{c}{R}_{c}=\frac{{DR}_{C}-{DR}_{UC}}{{DR}_{UC}}*100\%\end{array}$$
(2)

where \({DR}_{UC}\) is the average drilling resistance of unconsolidated samples (within the consolidation depth range) and \({DR}_{C}\) is the average drilling resistance of consolidated samples (within the consolidation depth range).

Fig. 13
figure 13

Schematic of the consolidation depth and calculation depth of the consolidation effectiveness index (\({R}_{c}\)): \({f}_{UC}(x)\) is the drilling resistance-depth profile of unconsolidated samples; \({f}_{C}(x)\) is the drilling resistance-depth profile of consolidated samples; j is the drilling depth of the point where the drilling resistance tends to coincide before and after consolidation.

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The drilling resistance data from consolidated and unconsolidated samples can be obtained as drilling resistance-depth profiles \({f}_{C}(x)\) and \({f}_{UC}(x)\). \({DR}_{C}\) and \({DR}_{UC}\) are the arithmetic mean of the drilling resistance-depth profiles \({f}_{C}(x)\) and \({f}_{UC}(x)\) respectively over a depth range from 1 mm to j mm. Table 4 shows the calculation results of \({R}_{c}\) for sandstone and clay brick samples with different reinforcement consolidant types and dosages. After the first and second consolidations with the PS solution, the \({R}_{c}\) values of the sandstone samples were 12.51% and 30.12%, respectively, while the \({R}_{c}\) values of the clay brick samples were 15.66% and 33.33%, respectively. Similarly, after the first and second consolidations with the B-72 solution, the \({R}_{c}\) values of the sandstone samples were 33.42% and 32.54%, respectively, while the \({R}_{c}\) values of the clay brick samples were 14.29% and 45.24%, respectively. Therefore, both the PS and B-72 solutions reinforce the sandstone and clay brick samples; the greater the consolidant dosage used is, the greater the \(R_{c}\) and consolidation effectiveness are. However, after the first and second consolidations with the TEOS solution, the \(R_{c}\) values of the sandstone samples were 9.42% and -8.64%, respectively, while the \(R_{c}\) values of the clay brick samples were 6.18% and -11.17%, respectively. An increase in the consolidant dosage of TEOS instead decreased \(R_{c}\), and the consolidation effectiveness was not satisfactory.

Table 4 Dilling resistance and consolidation effectiveness index (\(R_{c}\)) of the sandstone and clay brick samples before and after consolidation.
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In addition, Table 4 and Fig. 14 shows that the consolidation depth increases with increasing consolidant dosage. However, the consolidation depth does not directly reflect the consolidation effectiveness. The consolidation depth was almost the same for the clay brick samples after the first and second consolidation cycles with the B-72 solution, but the Rc increased from 14.29% to 45.24%.

Fig. 14
figure 14

Consolidation depth and \(R_{c}\) of the sandstone and clay brick samples after the first and second consolidation.

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Discussion

Deterioration degree

The deterioration depth can be determined by the drilling resistance values over a range of drilling depths29,30. This is also confirmed in the drilling resistance-depth profiles for sandstone and clay bricks in Figs. 7 and 9. The drilling resistance-depth profile shows a continuously increasing tendency in the surface deterioration layer and stabilizes when the drill bit enters the fresh layer. However, the deterioration degree cannot be determined accurately from deterioration depth data alone (Table 3). The undeteriorated sandstone and clay brick samples also showed a continuously increasing trend within the 0–1 mm range, even though the surface of the samples had been polished. Similar observations have been reported in other studies, where the drilling resistance-depth profile always involves some initial data interference, and the drilling resistance data are meaningful only within the depth range after the drill bit has completely entered the material51,52. A carbide drill (BOSCH, CYL-2) with a V-shaped cross-section was used in the experiments, as shown in Fig. 5. Before the front end of the drill bit enters the sample completely, the drilling resistance increases as the cross-sectional area of the drill bit increases, resulting in an increase within the 0–1 mm range of the undeteriorated samples. The inclusion of the data from 0 to 1 mm in the calculation will result in a lower calculated average drilling resistance than the true value. Therefore, when calculating \(K\) and \(R_{c}\), the initial data with disturbances at drilling depths of 0–1 mm are removed from the calculation.

Table 5 The testing results of the sandstone samples before and after deterioration.
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Regarding the calculation method of the average drilling resistance value, there is no uniform standard for the depth range chosen. Rodrigues and Costa proposed an average drilling resistance calculation method for low-strength mortars53. Based on a series of processes of segmenting, sorting, selecting, and averaging the data, the smallest 5 or 10 drilling resistance data points in each segment are ultimately selected to calculate the average value. Fernandes and Lourenço excluded the maximum or minimum drilling resistance data and then averaged the drilling resistance values54. Benavente et al. calculated average drilling resistances with data in a depth range of 0.5–25 mm55. Several researchers have directly calculated average drilling resistance values with data from the whole drilling depth range56. In this experiment, the drilling depth corresponding to the point at which the drilling resistance begins to stabilize is defined as the deterioration depth, and the initial data corresponding to disturbances at a drilling depth of 0–1 mm are removed from the calculation. In addition, the effect of deterioration depth and consolidation depth was taken into account when calculating the average drilling resistance value. The data within the deterioration depth (i mm, Fig. 10) or consolidation depth (j mm, Fig. 13) were selected for the calculation of the average drilling resistance value. Based on the variation in drilling resistance values, the deterioration degree index (\(K\)) is defined and calculated. The deterioration degree index (\(K\)) was compared with the weathering index (Fs) proposed by WW/T 0063–201537 (shown in Eq. 3) and the dynamic elastic modulus loss rate (\(\Delta E_{d}\)) proposed by BS EN 12,371:201036 (shown in Eq. 4).

$$\begin{array}{*{20}c} {F_{s} = \frac{{V_{p0} - V_{p} }}{{V_{p} }}} \\ \end{array}$$
(3)
$$E_{d} = \frac{{\rho_{d} V_{s}^{2} \left( {3V_{p}^{2} - 4V_{s}^{2} } \right)}}{{V_{p}^{2} - V_{s}^{2} }}$$
(4)

where \(V_{p0}\) is the primary wave velocity of the undeteriorated samples (m/s), \(\rho_{d}\) is the density of the samples (kg/m3), \(V_{s}\) is the shear wave velocity of the deteriorated samples (m/s), and \(V_{p}\) is the primary wave velocity of the deteriorated samples (m/s).

Tables 5 and 6 show the primary wave velocity (\(V_{p0}\), \(V_{p}\)), shear wave velocity (\(V_{s0}\), \(V_{s}\)), average drilling resistance (\(DR_{UD}\), \(DR_{D}\)) and dynamic elastic modulus (\(E_{d0}\), \(E_{d}\)) of the samples before and after deterioration, as well as the loss rate of the dynamic elastic modulus (\(\Delta E_{d}\)), weathering index (Fs) and weathering degree index (\(K)\) of the samples after deterioration. The primary wave velocity, shear wave velocity, and average drilling resistance gradually decrease with increasing deterioration cycle time. The values of \(\Delta E_{d}\), Fs, and \(K\) gradually increase with the number of deterioration cycles, and the deterioration degree of the sandstone and clay brick samples gradually increases. The \(\Delta E_{d}\) of the sandstone samples reached 38.39% after the 8th deterioration cycle, and the \(\Delta E_{d}\) of the clay brick samples reached 47.07% after the 15th deterioration cycle; both of these values were in a state of extremely serious deterioration according to BS EN 12371:201036 (a sample is considered to experience extremely serious deterioration when \(\Delta E_{d}\) exceeds 30%).

Table 6 The test results of the clay brick samples before and after deterioration.
Full size table

Figure 15 shows the correlation between the deterioration degree index (\(K\)) and the dynamic elastic modulus loss rate (\(\Delta E_{d}\) (%)) as well as the weathering indices (Fs) of the sandstone and brick samples. \(K\) is linearly and positively correlated with both \(\Delta E_{d}\) and Fs, with correlation coefficients for sandstone samples of 0.95 and 0.83, respectively, while the correlation coefficients for clay brick samples are 0.89 and 0.91, respectively, which further verifies the accuracy and reliability of \(K\). Therefore, the deterioration degree of sandstone and clay brick samples can be evaluated by using the deterioration degree index (\(K\)) based on the average drilling resistance.

Fig. 15
figure 15

Correlations between the deterioration degree index (\(K\)) and the dynamic elastic modulus loss rate (\(\Delta E_{d}\)) and between the weathering indices (Fs) of sandstone and clay brick samples: (a) correlation between \(K\) and \(\Delta E_{d}\); (b) correlation between \(K\) and Fs.

Full size image

The above results show that the dynamic elastic modulus loss rate (\(\Delta E_{d}\)) and the weathering index (Fs) are strongly correlated with the deterioration degree index (\(K\)). Especially for the clay brick samples, the variation rates of \(K\) and \(\Delta E_{d}\) are similar. After the 15th deterioration cycle, the \(K\) and \(\Delta E_{d}\) of the clay bricks were 43.75% and 47.07%, respectively, with a difference of only 10%. Compared with obtaining the dynamic elastic modulus loss rate (\(\Delta E_{d}\)) by measuring the ultrasonic wave velocity, the deterioration degree index (\(K\)), which is based on the average drilling resistance and involves controlling factors, including the deterioration depth and the deterioration in the mechanical properties of materials, can reflect the deterioration degree of the samples more directly and accurately.

In addition, the rate of decrease in the drilling resistance with deterioration cycle time for the sandstone sample was significantly greater than that for the clay brick sample (Table 3). The clay brick has a high content of quartz (more than 60%) and exhibits a high level of uniaxial compressive strength. The low content of calcium minerals, like calcite and dolomite, indicates good resistance toward sulfates57. The clay brick has a relatively high level of both free water absorption (15.56%) and forced water absorption (19.05%). Moreover, the saturation coefficient (ratio of free water absorption to forced water absorption) of the clay brick is 0.82, smaller than the critical value of 0.9, suggesting that the clay brick has good water swelling resistance58. The greater vitrification at higher firing temperatures implies the formation of relatively larger pores. The clay bricks used in this paper are fired at high temperatures (1100 ℃). Crystallisation pressure would be much lower in larger pores where no restraint exists for the crystal growth, which indicates its good resistance towards salt crystallisation damage57. In contrast, the sandstone is mainly composed of fine sand (0.06–0.25 mm), which should dissolve faster than a coarse-grained rock due to its higher reactive surface area59. In addition, the sandstone has a high content of calcium minerals such as calcite (> 10%), which will accelerate the sulphate erosion process. These account for the differences in deterioration rates between the sandstone and the clay brick.

The deterioration depth and deterioration degree index (\(K\)), obtained from drilling resistance tests, can be used to determine the optimal consolidation depth and consolidant dosage on-site to achieve accurate conservation and restoration. It is feasible to investigate more accurate consolidation methods for different deteriorated parts in the same material. Further investigation of the optimal consolidation parameters for different materials at varying deterioration depths and degrees is necessary.

Consolidation effectiveness

The drilling resistance-depth profile for the consolidated samples increases in the shallow surface depth range and then converges to coincide with the drilling resistance-depth profile for the unconsolidated samples (as shown in Fig. 12).

The drilling depth on the drilling resistance-depth profile corresponding to the point at which the drilling resistance begins to coincide before and after consolidation is defined as the consolidation depth. The consolidation effectiveness index (\(R_{c}\)) was proposed based on comparing variations in the average drilling resistance over a range of consolidation depths. The consolidation depth does not directly reflect the consolidation effectiveness (Fig. 14). The penetration distribution of the consolidant was not uniform (Fig. 11), even though dropwise infiltration with a dropper was used to maximize the uniformity of penetration. The alteration of material permeability before and after consolidation represents a significant factor influencing the consolidation depth. The mechanism of permeability properties of different materials influenced by different consolidants needs to be further investigated.

In addition, after the first and second consolidations with the TEOS solution, the increase in drilling resistance is concentrated in the 1–2 mm surface layer (as shown in Fig. 12b and d). Similar observations regarding the concentration of consolidants on the surface can also be found in Valentini et al.52, which may be attributed to the insufficient permeability of the consolidant, as well as the evaporation and capillary action of the volatile components in the consolidant60. Furthermore, the porosity of the consolidated materials may prove to be a significant impediment to the consolidation depth achieved by the consolidants. In instances where the first consolidation is unable to fill the majority of surface pores, the second consolidation will preferentially fill the remaining surface pores, which may result in a lack of further increase in the consolidation depth. This phenomenon can be observed in the case of clay bricks consolidated by the B-72 solution, as illustrated in Fig. 14. By comparing the variations in \(R_{c}\), there is only a 2.6% difference between the first and second consolidations when the sandstone samples are consolidated with the B-72 solution. In contrast, the second consolidation showed a 216.6% increase in \(R_{c}\) over the first consolidation when the clay brick samples were consolidated with the B-72 solution. As the total porosity of the sandstone sample is 11.12%, which is much lower than the total porosity of the clay brick sample (32.35%), after the first consolidation with 2 ml of B-72 solution, the solute filled most of the pores; thus, the drilling resistance varied minimally in the second consolidation. The clay brick samples with a higher porosity exhibited a significant increase in \(R_{c}\) after the first and second consolidations, but the consolidation depths varied minimally.

The drilling resistance increased within the shallow surface layer of the samples consolidated with the PS solution and B-72 solution, and the drilling resistance increased within a wider range and magnitude as the dosage of consolidants increased. There was no visible variation in the drilling resistance-depth profiles of either the sandstone or clay brick samples after consolidation with the TEOS solution, and even after the second consolidation, a decrease in the drilling resistance was observed instead.

The dissociation products of PS solutions will result in electrostatic adsorption of metal cations on the clay particles of the sandstone and clay brick, which can alter the structure of the clay particles and form silico-aluminate reticulated colloids. In addition, the potassium ions of PS solutions will exchange and adsorb with particle debris in the sandstone and clay brick, which could make the dispersed particles aggregate into larger agglomerates and form an overall linkage61. These improve the drilling resistance of the material. Moreover, the PS solution has little effect on the permeability of the consolidation material42,43; hence, the consolidation depth of the PS solution exhibited a significant increase after the second consolidation. B-72 solution is a synthetic resin and polymer material with a high strength and fast curing rate, widely used to conserve cultural relics40. Among the three consolidation materials, the sandstone and clay brick consolidated with B-72 solutions exhibited the most significant increase in drilling resistance.

It is widely recognized that the siloxane polymer generated by TEOS solution can strengthen the consolidated material41. Based on the hydrolysis of alkoxyl groups, TEOS solutions could connect dispersed particles with siloxane chains to consolidate and strengthen the deteriorated sandstone and clay brick. However, the TEOS solution in this experiment used anhydrous ethanol as the solvent (Table 2). The volatility of ethanol is pronounced at room temperature, and the rapid volatilisation is not conducive to the homogeneous dispersion and infiltration of the TEOS solution61. This may be a significant factor contributing to the limited increase in drilling resistance observed in the first consolidation by the TEOS solution. Furthermore, at the second consolidation by the TEOS solution, the drilling resistance exhibited a decrease, with \(R_{c}\) demonstrating a negative value. This phenomenon may be attributed to the siloxane polymer generated during the first consolidation, which has obstructed the downward seepage of the pore channels. Consequently, the second consolidation of the TEOS solution is unable to penetrate further (as evidenced by the almost identical consolidation depths of the two consolidation experiments in Table 4). Meanwhile, the siloxane polymers are transported to the material surface by the volatility of ethanol, forming a weaker layer of crust than the sandstone and clay brick. This ultimately results in a decrease in drilling resistance at the drill depth of 0–3 mm after the second consolidation, with a negative value for \(R_{c}\).

These results suggest that the \(R_{c}\) based on the average drilling resistance could directly and accurately reflect the difference in consolidation effectiveness between the sandstone and clay brick samples with different consolidant types and dosages, which can provide an empirical reference for masonry relic reinforcement and restoration work.

Conclusion

Based on the micro-drilling resistance method, drilling resistance was tested and analysed for the sandstone and clay brick samples before and after deterioration, as well as before and after consolidation. Deterioration degree index (\(K\)) and consolidation effectiveness index (\(R_{c}\)), which are based on the drilling resistance, are proposed. The following conclusions can be drawn.

  1. (1)

    In comparison to the undeteriorated samples, a decrease in the drilling resistance was observed in the surface layer of the deteriorated samples, and the range and magnitude of the decrease increased with the number of dry and wet cycles. The deterioration depth can be identified from drilling resistance-depth profiles.

  2. (2)

    The deterioration degree index (\(K\)) based on the average drilling resistance of deterioration depth can accurately evaluate the deterioration degree of sandstone and clay brick samples. The deterioration degree index (\(K\)) was strongly correlated with the dynamic elastic modulus loss rate (\(\Delta E_{d}\)) and the weathering index (Fs).

  3. (3)

    The consolidation effectiveness index (\(R_{c}\)) can directly and accurately evaluate the consolidation effectiveness of sandstone and clay brick samples with different consolidant types and dosages. The greater the amount of consolidant used is, the greater the increase in drilling resistance, but this increase can also be limited by the porosity of the consolidated material.

However, there are some challenges in field applications, for example, for non-homogeneous materials (e.g., mortar; heterogeneous constitution with hard constituents), drilling resistance-depth profiles have a wide range of floating values, which makes it difficult to define the deterioration depth and consolidation depth. The relationship between deterioration depth and deterioration degree, and between consolidation depth and consolidation effectiveness cannot be easily quantified. Further optimization should be explored in the application method of the deterioration degree index (\(K\)) and the consolidation effectiveness index (\(R_{c}\)).