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IN-SITU INVESTIGATION OF STONE CONSOLIDATION EFFECTS WITH IMMERSED ULTRASONIC DOUBLE-PROBE
Miloš Drdácký1, Marek Eisler2, Rolf Krompholz3
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 reproducible insertion of the device into prepared holes in order to acquire a reliable series of measurements for the investigation of changes in material characteristics. The device is portable and fully compatible with ultrasonic laboratory equipment, e. g. UKS 12 or UKS 14 Geotron Elektronik.
The device is robust and well-engineered for problematic outdoor measurement conditions. It is possible, for the first time, to measure the properties of materials in situ with an acceptable impact. The main advantage over the standard drilling technique is that it is possible to monitor the conservation effect during the intervention process because changes can be measured at an identical place and over the same volume of material after individual impregnation steps. This allows restorers to continue or terminate impregnation repetitions in response to the measured impact, i. e. consolidation depth and expected strengthening effects. In such cases, optimum control of conservation and reasonably low material consumption are desirable.
The device can be adapted for measurements across the dimensions of entire stones with holes drilled in joints, as this may be more acceptable on historic stone facades. During such measurements, the probes are fixed in a special rail rig allowing for longer distances between them and providing their firm fixture in the holes.
Figure 1: The double probe scissors set up with the UKS 14 ultrasonic testing system.
