Critical assessment of the “non-destructiveness” of Schmidt hammer test on monumental sandstones: A microscopic and microstructural approach

Abstract

The assumed non-destructive character of Schmidt hammer test is challenged because the practical experience with in situ application of the Schmidt hammer raises a question, whether the response of a tested material is of purely elastic nature. In the recent study, several types of sandstones used as natural building/sculptural stone were tested in dry and/or wet conditions by using N-type Schmidt hammer. In order to preserve phenomena of potential brittle damage, the impacted areas were impregnated with a mixture of a low viscosity epoxy resin and a fluorescent dye. After hardening of the mixture, thin sections have been prepared from cuts perpendicular to the impacted surface. Their microscopic investigation allowed for the detection of characteristic zone of brittle damage (2–5 mm deep and 5–12 mm wide) consisting of (1) a flat, shallow depression – a crater, (2) crushed (pulverized) zone, and (3) microcracked zone. The observed brittle damage is similar to that one caused by similar hardness tests including indentation ones. Based on the current results, it can be concluded that Schmidt hammer test should be considered as microdestructive instead of non-destructive. Its use for the artistic objects with preserved original surface of high art-history value (i.e. natural stone artefacts with original traces of sculptor's tools on the surface) can led to their partial destruction, resulting in an unwanted effect on protected artworks.