Case studyIndirect estimation of injected mortar volume in historical walls using the electrical resistivity tomography
Introduction
The non-destructive testing (NDT) methods offers today engineers and architects efficient tools to map, indirectly and above all non-invasively, the hidden characteristics of ancient structures to be investigated. These methods are based upon mapping the spatial distribution of one or more selected physical properties (e.g. acoustic velocity, electrical resistivity, dielectric permittivity, etc.). Such distribution, when opportunely calibrated, can provide technicians useful information on heterogeneities eventually present inside the structure that may indicate presence of defects.
NDT can also be advantageously used for quality control of certain aspects of restoration works. The most widely known NDT is perhaps the acoustic method which is used to measure the medium's propagation velocity (Vp) before and after restoration [1], [2], [3], [4], [5], [6]. Relevant increase of velocity, after grouting, indirectly indicates enhancement of the structure's mechanical resistance. Generally, the most significant results are obtained using the tomographic approach, which entails a large number of source-receiver points. However, the availability of all sides of the structure in conjunction with a sufficient high frequency energy source are the two major disadvantages to its use.
Recently, the electrical resistivity tomography technique (ERT) has been tested to map the distribution of injected mortar within a portion of an historical foundation in Venice, Italy [7]. In this special case, the more usual NDT's, such as the acoustic tomography and ground penetrating radar (GPR) reflection/tomography should meet great difficulties, because of both the unavailability of the opposite sides of the wall and the high electrical conductivity of the materials due to being saturated with salt water. The achieved results [7] pushed the authors to extend its use to other test sites, where both sides of the wall to be consolidated were accessible and where the electrical resistivity of the constituting materials was very high.
In this paper, we present the method used for data acquisition and discuss the obtained results highlighting the possibility to attain a quantitative estimate of the injected mortar volumes.
Section snippets
Test site description
The test site belongs to a medieval church, undergoing complete restoration, located at Montepetriolo village, in the neighbourhood of Perugia town, (Central Italy, Fig. 1). Four wall portions were selected for the resistivity test two of which were located on the lateral walls (3, 4 on Fig. 4), while the other two were located on the church's front wall (1, 2 on Fig. 4).
The building materials forming the church's walls are made up of different typologies; the front external walls are composed
Outline of the ERT technique
The resistivity estimation technique is a non-invasive geophysical investigation tool based on the application of Ohm's law. A continuous, or slowly variable, electrical current is injected into the body to be investigated by means of a couple of electrodes, to generate an artificial electric field E. Its distribution and intensity are functions of injected current, position of feeding electrodes and internal resistivity distribution. As a consequence, an apparent resistivity ρa is obtained by
Results
Apparent resistivity data were inverted using the commercial software “RES3DINV” [11], [12], that implements an algorithm based on the Occam's inversion procedure, to get the best 3D estimate of the resistivity distribution. The inverted models were of good quality with an absolute root-mean-squares error (rms) ranging between 10 and 15%. The inversion model of the left front panel is shown as an example in Fig. 6. Similar results were obtained for the remaining panels not shown here. The
Discussion
The images depicted in Fig. 6, Fig. 7 represent suggestive views of the internal resistivity structure of the tested walls. Although providing significant information about the volumes that have been filled with mortar, they lack a more quantitative and meaningful information that can help engineers in selecting points to be verified for mechanical resistance and deformation tests. We stress that there is no deterministic or heuristic relationship between the resistivity and the mechanical
Conclusions
In the present work, we confirmed the usefulness of the employment of the ERT technique in the characterisation of historical walls subject to restoration. Different typologies of construction materials have been studied. Although the obtained results provided suggestive 3D images of the internal structure of the investigated wall portions before and after mortar injection, they lack the quantitative estimate of the volumes really filled voids/cracks with mortar. This latter aspect was treated
Acknowledgements
This work was performed thanks to the financial support of Kimia SpA (Perugia, Italy). Dr Laura Pasetti is warmly acknowledged for her help in field and laboratory data collection.
References (17)
- et al.
Acoustic tomography for evaluation of unreinforced masonry
Constr. Build. Mater.
(1997) - et al.
Application of sonic tests to the diagnosis of damaged and repaired structures
NDT & E International
(2001) - et al.
Non-invasive characterisation of ancient foundations in Venice using the electrical resistivity imaging technique
NDT & E International
(2006) - et al.
Utilisation of sonic tests to evaluate damaged and repaired masonries
- et al.
Use of tomography for diagnosis and control of masonry repairs
- et al.
Integrated geophysical surveys to investigate the scarsella vault of St John's Baptistery in Florence
The Leading Edge
(2002) - et al.
Seismic survey of some pillars of Coliseum
Offset system of electrical resistivity sounding and its use with a multicore cable
Geophysical Prospecting
(1981)
Cited by (23)
Using electrical resistivity tomography and surface nuclear magnetic resonance to investigate cultural relic preservation in Leitai, China
2021, Engineering GeologyCitation Excerpt :ERT has been widely used in engineering geology (Perrone et al., 2014; Van Hoorde et al., 2017; Drahor, 2019), hydrogeology (Chretien et al., 2014; Afshar et al., 2015; Costall et al., 2018), and environmental (Bichet et al., 2016; Hu et al., 2019; Sutra et al., 2020) and soil science (Samouelian et al., 2005; Besson et al., 2010; Carriere et al., 2020). In recent years, many case studies have shown that ERT also exhibits considerable application prospects for archaeological investigations, cultural relic protection investigations and cultural relic restoration quality control (Chen et al., 2018; Deiana et al., 2018; Fischanger et al., 2019; Lange-Athinodorou et al., 2019; Yogeshwar et al., 2019; Abu-Zeid et al., 2006; Abu Zeid et al., 2010). Compared with the small-scale detection of nuclear magnetic resonance (NMR) technology, which is often used to detect the structures of murals and oil paintings, surface nuclear magnetic resonance (SNMR), which is often used for large-scale observations of groundwater, is rarely applied in the field of cultural relic protection.
Using surface nuclear magnetic resonance and spontaneous potential to investigate the source of water seepage in the JinDeng Temple grottoes, China
2020, Journal of Cultural HeritageCitation Excerpt :These methods can also provide new archaeological discoveries and information regarding the mechanisms of cultural relic diseases and the present situation, and thus support multidisciplinary research and the restoration of cultural relics [1–6]. Some studies have discussed the application of geophysical methods for monitoring and detection in the restoration of the foundations and walls of ancient buildings, and have demonstrated that geophysical methods are of great potential value in the quality control of the restoration of cultural relics [7,8]. In this study, surface nuclear magnetic resonance (SNMR) and Spontaneous Potential (SP) were used to detect the occurrence and flow of groundwater in the mountain above the JinDeng Temple grottoes.
Low cost system for measuring the evolution of mechanical properties in cement mortars as a function of mixing water
2020, Construction and Building MaterialsCitation Excerpt :Thanks to this type of resistive techniques, the relationship between hardening at the microstructural level and the variation in electrical conductivity inside the study material [29] can be determined. Also, more advanced techniques such as electrical resistivity tomography (ERT) have been used in the construction sector to determine the volume of mortar in historical constructions for possible rehabilitation [30]. This work consists of the use of Arduino for the development of measuring equipment based on low-cost resistive sensors, which in addition to taking data on the variation of the water content of the mixed inside the mortar specimens, manage that information to relate it to the evolution of its mechanical properties.
Integrated characterization of ancient burial mounds using ERT and limited drillings at the Hepu Han Tombs, in coastal area of Southern China
2019, Journal of Archaeological Science: ReportsCitation Excerpt :Iso-surfaces are acknowledged as an excellent representation of modern 3D geophysical datasets analysis and visualization, as they reduce the size of the data, allowing the identification of each anomaly. Accordingly, iso-resistivity surfaces can accelerate the interpretation by implementing semi-automatic procedures and obviously advance the overall quality and efficiency of ERT data interpretation (Abu-Zeid et al., 2006; Drahor et al., 2008; Zeid et al., 2010; Cozzolino et al., 2014). The Hepu Han Tombs are distributed in the Hepu Basin, a small faulted sedimentary basin developed in the Mesozoic and Cenozoic, located in the south of Guangxi Zhuang Autonomous Region.
Evaluation of consolidation of different porous media with hydraulic lime grout injection
2015, Journal of Cultural HeritageHydrothermal monitoring using embedded sensors of the actual roof system of the Prado Museum
2010, Construction and Building Materials