MS.D.8: Electrical conductivity investigation of masonry

Abstract

This recommendation specifies a method for investigating electrical conductivity distribution within masonry structures using electro-magnetic conductivity techniques. Electromagnetic fields are propagated into the structure and variations are monitored and recorded. These provide geometrical and electrical information on the materials investigated. Details regarding the principles involved, the apparatus, the method of test, the method of calculation and the contents of the test report are provided. D.8.3 BACKGROUND TO THE TESTWater ingress and moisture movement into structures are important in terms of structural durability. For example, if the road surface of a brick masonry arch bridge permits water entry then the soil fill above the arch barrel may become saturated [1]. This can result in degradation of the mortar between the bricks -giving rise to premature failure. Another example of water inclusion in masonry structures is due to moisture capillary rise from the building foundations. The Architect or Engineer may want to know what is the actual height of water rise in the inside of the wall -this height is generally greater than what shows on the external wall surface [2].In the majority of the cases, salt content is associated with water content in the structure. This phenomenon can also cause great damage to the structure and rapid decay of the masonry wall, and it is therefore a cause of concern.Thus a non-invasive method of determining moisture movement behind or inside the masonry walls would be of great engineering value.Electrical conductivity in porous building materials as a response to electrical fields over the range from DC to 20 kHz AC is influenced to a large extent by the content of moisture and soluble ionic salts and thus offers a relevant ND assessment technique for the following: -moisture content in the masonry salt content in the masonry associated with moisture content -height of moisture capillary rise -thickness of the masonry wall -multi-wythe nature of the masonry wall -composite construction of the masonry structure -presence of voids or inhomogeneities in the wall -presence of metal reinforcements, pipes, drains etc. in the wall. D.B.4 TEST LOCATIONSTest locations are dictated by engineering objectives, however an attempt should be made to measure the variation in material quality or condition throughout the largest possible volume of the structure [3] -typically that with a face area of3m x 3m minimum. From such a large map of the conductivity distribution in the sub surface, it should be possible to identify the area of interest.Since no coupling or contact with the surface of the structure is required, the surface of the structure remains unmarked. As a result of the portability of the instrument, the non-harmful nature of the radiation and the continuous emission and receptivity of electromagnetic fields, the structure can be tested rapidly, safely and without disruption of other activities.As the in-situ calibration is of great importance in the interpretation of the readi...