Monument Future - Siegfried Siegesmund

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       IN-SITU INVESTIGATION OF STONE CONSOLIDATION EFFECTS WITH IMMERSED ULTRASONIC DOUBLE-PROBE

      IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

       – PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

       VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

      1 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Heritage Science, Prosecká 76, 190 00 Praha 9, Czech Republic, [email protected]

      2 Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, Department of Diagnostics and Conservation of Monuments, Prosecká 76, 190 00 Praha 9, Czech Republic, [email protected]

      3 GEOTRON-ELEKTRONIK, Leite 2, D - 01796 Pirna, Germany, [email protected]

       Abstract

      This paper introduces a portable ultrasonic double-hole probe capable of recording changes in material properties along a depth profile and assessing consolidant penetration depth. Application of the device requires drilling two 20 mm holes of arbitrary distance in the surface layer of the masonry. The holes can be drilled directly into a continuous stone block or advantageously in the mortar joints between blocks, provided that appropriate contacts are achieved between the probe transmitter and receiver and the continuous stone. This moderately destructive method is useful mainly for conducting measurements on stone masonry façades or structures in which drilling holes, with subsequent repair, is acceptable. Consolidation assessment is demonstrated on quartz sandstone blocks in a laboratory as well as on a masonry wall made of the same ashlar blocks. The probe is fully compatible with ultrasonic laboratory equipment, e. g. UKS 12 or UKS 14 Geotron Elektronik.

       Introduction

      Controlled structural impregnation and consolidation of porous materials require a reliable technique that enables the assessment of consolidation effects, especially the penetration depth of consolidating agents.

      Consolidation effects are typically tested on specimens extracted from the treated bodies in the form of drilled cylindrical cores or cuboids cut from the object. The specimens are further cut into thin slices beginning at the treated surface and continuing along the depth. The slices are then tested using destructive methods – typically as discs or short beams loaded in bending (Drdácký & Slížková 2008). The determined strengths indicate changes in mechanical characteristics of the treated material along the depth.

      The individual specimens are also suitable for non-destructive laboratory investigation of the penetration depth of consolidants using ultrasonic measurement in the transmission mode (Sasse & Snethlage 1996).

      Such monitoring should ideally be performed during the consolidation process, i. e. after each impregnation cycle. It is practically impossible, however, to cut samples from an object after each treatment cycle.

      A semi-destructive method that exploits the measurement of resistance to drilling is also used for in-situ testing of stone, but the specific character of the method prevents repeated application at identical points, and the monitored data is thus 234unreliable due to the heterogeneity of the tested material. Ultrasonic measurement, on the other hand, can provide integral information across a larger domain of treated stone objects, and the monitored data is less sensitive to small-scale heterogeneity within the material.

      This paper introduces a new portable ultrasonic double-probe for recording changes in material properties along a depth profile and assessing consolidant penetration depth.

       Double-probe ultrasonic device

      For quality control of stone-masonry conservation, an innovative ultrasonic device has been developed jointly with Rolf Krompholz (GEOTRON-ELEKTRONIK) within the EK FP7 Stonecore project.

      The device consists of two probes: an US transmitter and receiver. They were basically designed to be inserted into holes of 20 mm in diameter drilled into the investigated surface layer at a distance of up to 100 mm from their centres and a depth of up to 60 mm. Certain design features (a flat base and adjustable rods for the transmitter and the receiver) allow for

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