Elsevier

Journal of Cultural Heritage

Volume 11, Issue 2, April–June 2010, Pages 220-227
Journal of Cultural Heritage

Case study
Indirect estimation of injected mortar volume in historical walls using the electrical resistivity tomography

https://doi.org/10.1016/j.culher.2009.07.001Get rights and content

Abstract

The electrical resistivity tomography (ERT) represents one of the widely used geophysical techniques for the exploration of the subsurface. In the last few years, this method has been demonstrated to be an efficient reconnaissance tool not only for monitoring degradation status of walls and foundations of historical buildings, but also for imaging the spatial distribution of injected mortar, commonly employed for consolidation purposes. A 3D resistivity tomography survey was carried out on four selected wall portions of the historical church of Montepetriolo, Perugia, Central Italy. The obtained 3D resistivity distribution models before and after grouting provided suggestive images of the internal structure of the studied walls. Moreover, the spatial distribution of the zones being filled with mortar was determined quantitatively by scaling the post- to the preinjection resistivity values. Using a well known correlation between resistivity and porosity, established in the geophysical community, a further step towards a more quantitative assessment was attempted to indirectly determine the unit volumes of the injected mortar The obtained results were satisfactory and in some cases almost similar to the yard data. Furthermore, two flat-jack tests confirmed the local increment of the mechanical resistance of the studied left front (2) and right lateral (3) wall portions.

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)

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