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Stress screening on repaired mortar surfaces using nondestructive testing methods

: Meinlschmidt, P.; Mehlhorn, L.

Brebbia, C.A.; Jäger, W. ; TU Dresden; Wessex Institute of Technology, Southampton:
Structural studies, repairs and maintenance of historical buildings VI
Southampton: WIT Press, 1999 (Advances in architecture series 6)
ISSN: 1368-1435
ISBN: 1-85312-690-X
International Conference Structural Studies, Repairs and Maintenance of Historical Buildings (STREMAH) <6, 1999, Dresden>
Fraunhofer WKI ()

Two nondestructive testing measurement techniques, namely video holography and thermography are used for the inspection of modern and historical replacement materials (mortars and pigments). Test objects of sandstone or tuff, such as would have been used in a medieval church of Northern Germany, including replicas of historical frescoes, are specially prepared for these measurements. Aim of this test is to observe the reaction of frescoes to temperature changes produced by short heat impacts. Furthermore the quality of the bonding between historical and modern replacement materials and the surrounding historical substance should be measured. Using transient thermography it is possible to separate modern and historical materials as well as slightly different material mixtures of mortars because of the different emission coefficients. Different materials like sandstone and mortar can be measured through thin covering plaster layers, because of the differing heat dissipation into the mate rials. With the video holography technique the poor adhesion of historical replacement mortar to the surrounding structure is detected. In contrast to historical mortar, the modern material shows a much better contact to its vicinity. Moreover it is possible to observe the reaction of different thermal expansion coefficients to anisotropic sandstone material and to measure the reaction of different expansions to thin plaster layers. The result of such temperature changes to thin plaster layers can be observed on many destroyed frescoes in historical churches. Usually the different substructures are clearly visible as badly damaged areas on the surface of a fresco.