Sustainability analysis of sandstone using smart material by EMI approach

In India, sandstone was broadly used to construct structures like Agra Fort, Red Fort Delhi, and Allahabad Fort. Around the world, many historical structures were collapsed due to the adverse effect of damages. Structural health monitoring (SHM) is very useful to take appropriate action against the failure of the structure. The electro-mechanical impedance (EMI) technique is used to continuously monitor the damage. A piezoelectric ceramic (PZT) is used in the EMI technique. PZT is a smart material used as a sensor or an actuator in a certain specific manner. The EMI technique work in the 30 to 400 kHz frequency range. This technique helped to analyze the hairline crack, location, and severity of damage to structural elements. A 10 cm length and 5 cm diameter sandstone cylinder was used in the experimental work. An electric marble cutter was used to create the artificial damages of 2 mm, 3 mm, 4 mm, and 5 mm respectively along the length, at the same place in specimens. The conductance signature and susceptance signature were measured for each depth of damage. The comparative result of healthy and damaged state with different depth were concluded based on the conductance signature and susceptance signature form the sample. Statistical methods like root mean square deviation (RMSD) is used for the quantification of damage. The sustainability of sandstone has been analyzed with the help of the EMI technique and RMSD values. This paper motivates the application of the EMI technique to the historical building made of sandstone as key material.


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The four sandstone mausoleums within this walled garden, present an exquisite example of Mughal architecture. The structures are respectively tombs of Prince Khusro, tombs of Shah Begum, Khusrau's sister, Nithar, and tombs of Bibi Tamolan Khusrau's tomb was completed in 1622, while that of Nithar Begum's, which lies between Shah Begum's and Khusrau's tombs, was built on her instructions in 1624-25. Most of the structures in the Khusro Bagh are constructed with sandstone masonry (Fig. 1a). Red, brown, yellow, and white color sandstone with lime mortar have been used in Khusro Bagh structures. Primarily large amount of red sandstone is used in these structures.
Sandstone is a natural sedimentary rock. It is available in a vast amount on the earth. Sandstone has been a prevalent construction material from ancient times to till now. The compressive strength and load-carrying capacity were high for sandstone as compared to other ancient construction materials (Qi et al. 2022;Wang et al. 2020). The sandstone has been used in beams, columns, domes, Mihrab arch, flooring, wall, fountains, etc. in structure (Sanday et al. 2001;Prasad et al. 2019). Sandstone is a hard rock that contains sand-size grains of mineral, sand, cementitious material, and organic material (Lazar et al. 2015;Qi et al. 2022). The formation of sandstone is in two phases. In the first phase, sand particle sedimentation occurs through air or water; in the second phase, the compaction occurs with physical pressure and chemical changes. Variable colors are found in sandstone like red, black, brown, cream, dark brown, yellow, pink, white, and light and dark grey. The colors of sandstone depend upon the mineral composition.
These days' sandstone is broadly used in flooring, wall tiles, retaining wall, river training and stone masonry, etc. The use of sandstone in structures does not require any type of coloring. Sandstone is a fabulous construction material due to its shine, color, and softness. Sandstones have been widely used in the construction of new structures such as Ambedkar Park Lucknow and Kashi Vishwanath Corridor Varanasi (Sinha and Kant 2012;Sen et al. 2022).
The preserving of cultural and architectural value of sandstone historical structures is important. India's archaeological structures and heritage sites are at risk of environmental impact (Ural and Dog 2008;Saba et al. 2018). The damage and/or deterioration occurs in the sandstone historic structures due to several reasons (Mahesh et al. 2021;Silva et al. 2022;Fermo et al. 2015). Several types of pollution threaten the existence of these historical structures and pieces of evidence of history (Varotsos et al. 2009). Pollution is a significant cause of historical structure deterioration like white colour marbles of the Taj Mahal is changed to yellowish color due to acid rain (Natarajan et al. 2022). Within two decades, the pollution is gradually rising due to the increasing size of vehicles, factories, workshops, thermal plants, refineries, and residential complexes (Belfiore et al. 2013). CO (carbon monoxide) and SO 2 (sulphur dioxide) are significant causes of deterioration of the sandstone in Gwalior Fort Madhya Pradesh (Pandey and Kumar 2015). The continuous dampness and polluted air affected the quality of the sandstone (Batista et al. 2019). Joerg Ruedrich et al. investigated the deterioration in sandstone under the weather conditions like wet-dry. They found the stability of sandstone is affected Fig. 1 a Historical structure Khusro Bagh. b and c Sample collection of Khusro tombs by the change in the porosity of the sandstone (Ruedrich and Bartelsen 2011). Akoğlu et al. investigated the swelling which plays an important role in the deterioration of sandstone under chloride environment. The deterioration in sandstone is caused by the progression of micro-cracks which increase the separation of layers, especially along the bending plan (Saltik et al. 2010). Rina (Irena) Wasserman investigated that humidity, salt water, and high air pollution are causes of deterioration in sandstone. (Wasserman 2021). Labus and Bochen investigated the deterioration and weight loss of sandstone in different adverse conditions (Labus and Bochen 2012). J.J. Ortega-Calve et al. investigated some bacteria is harmful to sandstone structure like cyanobacteria and chlorophytes that are present near the soil, plants, and surface of the structure (Ortega-calve and Ariiio 1995). The leading causes of the deterioration in the historical building materials are air pollution, temperature, acid rain, salt water, and biological factor (Sayed Hemeda et al. 2018;Vidović et al. 2022).
Structural health monitoring (SHM) is an effective technique for monitoring structures to prevent the catastrophic failure of structures (Ai et al. 2014;Mishra 2021). SHM techniques provide safety, reliability, of structure. SHM is one of the best effective industrialized methods for determining structural integrity. SHM techniques are helpful for recognizing the quality of structure, minor to major structural failure with an early indication of damage. The SHM technique is also used for measurement of early-stage damage to take preventive measure against damage so that the failure of structures can be avoided. It is a process to determine the presence, location, and severity of damages present in the system, and also the remaining service life of the system. In SHM, various techniques have been used to continuously or discontinuously monitor and study the change in the behavior at any stage of the structure (Martínez-garrido and Ergenc 2016). SHM are divided into two groups: first is the global response technique, and the second is the local technique. In global technique, it contains static response and dynamic response technique. The global dynamic response technique is based on low-frequency excitation and their vibration response like displacement, velocity, and acceleration; however, global static response technique is based on only static displacement response. Mostly non-destructive evaluation (NDE) test is conducted to evaluate the change in the structural parameters. The local damage in structural members plays a hostile role in the structure. Many nondestructive tests, such as ultrasonic, X-rays, Eddy current, magnetic particle, dye penetrant, and EMI technique, are used to evaluate structural damage (Maurya et al. 2020). The limitation of these techniques is less sensing range, and most of the method depends on the geometry, material properties, and depth of cracks. The minimum and maximum probability sensitive range of the local technique is provided in Table 1.
The EMI technique is comparatively new in SHM. Last few decades, this technique has been used in buildings, bridges, railways, towers, etc., for monitoring purposes. EMI technology based on the piezoelectric ceramic (PZT) sensor has versatile potential applications in SHM (Dongyu et al. 2014;Bhalla and Soh 2004a). The PZT sensors were used as embedded or surface-bonded inside of the host specimen (Maurya et al. 2022a). The surface-bonded sensor is applied at the surface with the help of high epoxy adhesive (Moharana and Bhalla 2014;Saravanan and Chauhan 2022). The embedded sensor is used inner part of the host structure (Negi et al. 2018). The embedded PZT sensor is called a concrete vibration sensor (CVS). CVS is a packaged sensor designed especially for monitoring RC structures. The CVS is composite in nature, has better compatibility with the surrounding concrete, and can withstand the harsh conditions typically encountered in the RC structures during casting (Kaur and Bhalla 2014). The PZT sensor effectively Ultrasonic pulse velocity 2 mm 5-6 mm Depends upon the properties and geometry of material 2 X-ray 4 mm 10 mm Dependent upon Structural member configuration Better for thickness of member is grater than 12 mm 3 Magnetic particle 2 mm 4 mm surface -4 Dye penetrant 2 mm 10 mm surface -5 Eddy currents (at low frequency) 2 mm 4.5-8 mm Thickness of testing specimen < 12 mm only 6 Eddy currents (at high frequency) 2 mm for surface and 0.5 mm for bore holes 2.5 mm for surface and 1.0 mm for bore holes -monitors and detects incipient damage (Shanker et al. 2011). The incipient damage influences the strength and sustainability of structural members. The PZTs sensor is also used in global dynamic response technique below the frequency range of 100 Hz. The use of the EMI technique in aerospace and automobile sectors indicates the change in function during the operational process and any chance of malfunction. The PZT sensor is also used in parallel and series combination (Priya et al. 2018). Wu et al. investigated the deterioration patterns, physical, and mechanical properties of the West Lake Cultural Landscape of Hangzhou historical site with the help of an ultrasonic pulse wave, Schmidt hammer. The result of the research was the one-time estimation of the damage and evaluation of quality (Wu et al. 2021). The continuous estimation can be possible by PZT sensor in lesser time, with accuracy, and a cost-effective monitoring system. The research work is focused on the analysis of sandstone using the EMI technique for sustainability concern. The conductance and susceptance signatures were evaluated using PZT-based EMI technique at various levels of induced artificial damages in the sandstone. Further, the quantification of damage levels with respect to the healthy state in the considered sandstone specimen was performed using the RMSD index.

Environmental impact on historical structure
The impact of the environment on sandstone structures is significantly less at the initial stages; however, the environmental impact can be observed on these types of structures after a long time. Environmental factors such as acid water, temperature, pressure, wet-dry, humidity, and freeze-thaw are responsible for the deterioration of historic structures (Batista et al. 2019;Manohar et al. 2020). The porosity participates in the deterioration of stone due to environmental effects. The variation of temperature with sudden rainwater is very harmful to the structure. The shrinkage and swelling is a major process of deterioration of the historical stone structure. The porosity of stone rock plays a vital role in durability. The deterioration rate of a stone depends upon the porosity; damage or deterioration rises gradually when porosity increases. Figure 2a-d contain the deterioration of sandstone in the Khusro tombs structure.

Loading impact on historical structure
The loading impact on historical structure is caused by dead load, live load, earthquake load, wind load, and vibration loads. Generally, no new construction occurs on historical structure due to no change in historical originality, so the dead load does not increase. Mostly the vibration load and earthquake loads are very harmful as compared to other loads. Nowadays, development is a need of every city. The growth rate of the infrastructure of the city is very high. The need of human being in the city is the metro, multistory building, water tank, water supply line, electricity and mobile tower, and railway line and station. New construction near the historical site produces vibration due to transport systems, electrification, water supply line, tower, bridges, metro, railway, etc. The heavy vibration equipment in the construction industry produces vibration, which harms the stability of the historic structure.

Properties of sandstone
The sandstone is a sedimentary rock that forms by clastic sedimentation of sand, minerals, organic compounds, and Fig. 2 a, b, c, and d Deterioration of sandstone in Khusro tombs structure 1 3 cementitious matrix material. The size of grains of minerals ranges from 0.06 to 2 mm and the wide range of the strength is from less than 5.0 to over 150 MPa. The strength is depending upon the porosity, cementitious material, matrix material, grain size, and composition. In this experiment, the sandstone sample was made by the collected sandstone from tombs of Prince Khusro in Fig. 1a-c. A lithological characteristic studies of sandstone sample have been conducted (Fig. 3). The sample is characterized by a non-bedded structure; it is compacted in nature. It is containing quartz and orthoclase. The rock has a clastic texture. A subordinate amount of muscovite mineral is present.

Damage in sandstone
The volumetric change in the original objects is called deterioration/damage. There are many causes of damage in sandstone like environmental and loading impact (Korkanç 2013). The environmental deterioration is gradual deterioration. The deterioration of sandstone is due to the erosion process, which causes the spall out of small pieces of sandstone from the structures. The depth of damage increases when the material spalls out from the structure. The depth of damage in sandstone was developed up to 5 mm to conduct the experimentation. An electric marble cutter has been used for artificial damage and which is presented in Fig. 4. The damage depth is 2 mm, 3 mm, 4 mm, and 5 mm. The signature has been measured at every depth of damage (Fig. 5a-d).

Piezoelectric materials
Some unique crystal materials like PZT (Pb (Zr1-xTix) O 3 ), PLZT ((Pb1-xLax) (Zr1-yTiy) O 3 )), lithium niobate (LiNbO 3 ), and quartz (SiO 2 ) have the piezoelectric effect. These types of crystal are also called non-centrosymmetric crystals (Fig. 6). The chemical composition of piezoceramic is (Pb (Zr1 − xTix) O 3 ). Generally, the piezo  ceramic is called PZT. The reason behind calling smart material is its properties; when the voltage is applied to PZT, it generates mechanical force and, when subjected to mechanical force, becomes electrically polarized (Shanker et al. 2011). Various types of PZT patch are commercially available such as specification, sensing frequency range, and shapes. Figure 7 is an experimental PZT patch having a size of 1 cm × 1 cm. (1) where Eq. 1 is the direct effect and Eq. 2 is the converse effect. Equations 1 and 2 can be rewritten in the tensor form as Eq. 3 (Sirohi and Chopra 2000).
where D is the electric displacement vector quantity (3 × 1) and the unit is (C/m 2 ), S is the second order strain tensor (3 × 3), and E is applied external electric field vector (3 × 1) and the unit is (V/m).T is stress tensor (3 × 3) and the unit is (N/m 2 ). is di-electric permittivity. T is the second-order dielectric permittivity tensor under constant stress and the unit is (F/m), d d and d c are the third-order piezoelectric strain coefficient tensors and the unit are respectively (C/N), . 5 a, b, c, and d are 2 mm, 3 mm, 4 mm, and 5 mm depth of damage respectively

Experimental work
The PZT-5H square sensor of 1 × 1 cm is manufactured by Sparkler ceramics Pvt Ltd. The properties of the PZT-5H sensor are given in Table 2. This PZT sensor has contained both nodes on the upper surface. A special type of low resistance single core wire of one meter was attached to PZT nodes. A smooth surface sandstone specimen of 10 cm in length and 5 cm in diameter have been used for testing collected from the Khusro tomb. The center of the specimen diameter is also the center of the PZT patch. The high-strength epoxy has been used to attach the PZT sensor to the sandstone surface. A LCR meter is used to measure the signature conductance and susceptance. One end of the wire is attached to the PZT electrode and another end of this wire is connected to an LCR meter. To analyze the sustainability of sandstone, the manual damage has been created along the length at depths of 2 mm, 3 mm, 4 mm, and 5 mm respectively, at the same place in specimens with the help of a cutter. Figure 3 is the experimental test setup for testing in the laboratory.

EMI technique
EMI technique is NDE method. In the EMI technique, a PZT patch is used to monitor the health of the structure within a   (Maurya et al. 2022b). The incipient and hairline damage evaluation and repair are necessary to prevent the big failure. The EMI technique is helpful to measure the incipient damage, severity, and location of damages. The difference between in global dynamic technique and EMI is the frequency range. EMI technique works on a 30-400 kHz frequency range. An LCR meter or impedance analyzer is used to acquire the signature. The LCR meter measures the admittance which consists of the real part (conductance) and imaginary part (susceptance), when plotted as a function of frequency, gives a unique signature to structures. 1-D and 2-D expressions of electromechanical admittance are written below in Eq. 4a and 4b respectively (Consumption 1994;Bhalla andSoh 2004a, 2004b).
where Y is complex electromechanical admittance, G is conductance, B is susceptance, Z is mechanical impedance, Za is the mechanical impedance of the PZT patch, d is piezoelectric strain coefficient, is the angular velocity, l is halflength of PZT patch, h is the thickness of PZT patch, w is the width of PZT patch, ɛ is electric permittivity, T 33 is complex electric permittivity at constant stress, Y E is complex Young's modulus of elasticity at the constant electric field, k is spring constant, is Poisson's ratio, T is the complex tangent function, Z a,eff is the effective impedance of the PZT patch, Z s,eff is the effective impedance of the structure, and j = √ −1.

Major findings
The variation of conductance has been measured, such as the conductance signature of the health state and at damage depths of 2 mm, 3 mm, 4 mm, and 5 mm, respectively. All conductance signatures are shown compared with healthy state conductance signatures in Fig. 8. The susceptance signature is an imaginary part of the admittance. The variation of the susceptance signature is very slight as compared to the conductance signature. The susceptance signature has been measured at a healthy state and various damage depths such as 2 mm, 3 mm, 4 mm, and 5 mm, respectively, which are in Fig. 9. The percentage RMSD value is used for the evaluation of the damage. The percentage RMSD values were calculated for conductance and susceptance were shown in Fig. 10.
With the help of conductance and susceptance percentage, the curve of RMSD value is plotted in Fig. 11.

Evaluation of conductance and susceptance signature
The purpose of the evaluation is co-related with structural functions like strength gain and losses, damage, and early strength change. There are serval statistical methods like RMSD, mean absolute percentage deviation (MAPD), covariance (Cov), and correlation coefficient (CC)are used to The variation of conductance and susceptance both are calculated individually. A significant change was found in the RMSD value of conductance signatures compared to RMSD values of susceptance signature. The RMSD value of conductance and susceptance at different depths of damage is shown in Fig. 10. The equation of percentage RMSD is shown in Fig. 11.

Results
A comparative result of the conductance signature is in Fig. 8. The continuous shifting has been observed in the conductance signature from baseline up to 5 mm damage. The 2 mm, 3 mm, 4 mm, and 5 mm have observed a continuous increase in damage; then, the signature of conductance also shifts towards the left side. It indicates the severity of the damage. The shifting depends upon the damage level; if the damage is more than 5 mm, then the curve may shift more on the left side. The percentage of RMSD values also depends on damage severity. A linear function generated with the help of the percentage of RMSD values has been correlated in the form of a straight line. Various causes of damage in sandstone are in detail explained in the introduction section. Erosion and crack are the processes of damage in sandstone that can be monitored through the EMI technique. The damage, like erosion and cracks from any sandstone structure, can be measured similarly.

Limitations
The PZT patch is brittle in nature, thus it cannot be removed from the host structure, and the removal can be caused by a broken of PZT patch. The PZT patch sensing zone is limited. The sensing range depends upon material shape, size, and properties of the material. Generally is a 0.4 to 2 m length of sensing zone. Aging, mechanical, electrical, and thermal are some limitations of the PZT patch. The main restrictions in the historical structure are the originality and aesthetic view of the structure. The sandstone masonry structure contains

Conclusions
The historical structure is very important in culture, history, and tourism. Health monitoring is necessary for preventing the adverse effect of structure. The EMI technique is helpful to determine the incipient damage as well as damage severity for the sustainability of historical structure made of sandstone.
There are a lot of restrictions on the historical structure that do not change its originality of structure. Due to this limitation, a non-destructive test is useful for monitoring the health of the historical structure. This technique may be directly used to monitor the health of structures within the restriction of historic structures. The PZT size is tiny, so this technique does not affect the aesthetic or harm the historical structure. This technique is less time consuming.
The baseline of the signature is very useful for the future analysis of damage and rehabilitation of the structure. The conductance signature and susceptance signature are corelated to the strength gain and losses of the specimen.
In this study, the EMI technique was applied for the assessment of the sandstone specimen for the sustainability concern. The surface-bonded PZT patches were considered for the extraction of conductance and susceptance signatures of the considered sandstone. Further, the RMSD index has been applied for the quantification of damage severity.
In future, the durability analysis of sandstone in various aggressive environmental conditions can be performed using the smart material-based EMI technique. The prediction of the residual life of sandstone can be evaluated for the sustainability of the sandstone structures. The strength gain and strength losses of ancient mortar could be measured by the PZT sensor. This research paper is based on the sensor effect of PZT monitoring a structure; moreover, another option for monitoring a structure could be the actuator effect of PZT.