Torque Penetrometric Test for the in-situ characterisation of historical mortars: fracture mechanics interpretation and experimental validation

• Published in: Construction & building materials 2017-12, Vol.157, p.509-520
• Main Authors: Marastoni, Diego, Benedetti, Andrea, Pelà, Luca, Pignagnoli, Giacomo
• Format: Article
• Language: English
• Subjects: Analysis; Brickwork; Compressive strength; Enginyeria civil; Estructures de murs; Fracture energy; Fracture mechanics; Historical construction; In-situ testing; Masonry; Materials i estructures; Mechanical properties; Minor Destructive Testing (MDT); Mortar; Nondestructive testing; Penetrometric test; Testing; Àrees temàtiques de la UPC
• ISSN: 0950-0618; 1879-0526
• DOI: 10.1016/j.conbuildmat.2017.09.120

•Novel torque penetrometer for the in-situ mechanical characterisation of mortar.•Toothed nail inserted into the mortar and twisted with a torque-meter until failure.•Proposal of an interpretation theory based on fracture mechanics.•Interpretation theory validated using new and available experimental campaigns.•In-situ applications prove the reliability of the instrument for historical mortars. The assessment of historical structures requires appropriate knowledge of the behaviour of the investigated materials. Concerning masonry, its mechanical characterisation is a challenging task, since its composite nature requires the careful evaluation of the behaviour of its material components. In particular, the experimental assessment of the strength of existing mortar in historical structures still encounters several difficulties. This study investigates a novel Minor Destructive Testing (MDT) technique virtually equivalent to the vane test used for soils. The instrumentation, called herein Torque Penetrometric Test (TPT), is composed of a steel nail with four protruding teeth and a torque wrench. The test consists in inserting the toothed nail into a mortar joint and then applying a torque by means of a dynamometric key, until reaching the failure of the material. This work presents a novel interpretation theory based on basic concepts of fracture mechanics and applied to the micro-mechanical analysis of the stress state induced by the instrument on the investigated mortar. The proposed interpretative theory is validated through the execution of experimental tests in the laboratory and in a historical masonry building. The test proves to be effective for a quick in-situ MDT evaluation of the strength of existing mortars.