The hole-drilling technique for the stresses measurements in brick masonries

Abstract

This paper deals with the application of the hole-drilling technique for the measurement of the stresses in the structural elements of cultural heritage constructions. The method involves glued strain gages at the surface of a piece of masonry. The drilling device is a diamond crown extracting a cylinder of material in a concentric way with the circumference (12 cm diameter) of the strain gages position. At least three strain gages are placed in the area under consideration. The strain gages record the strains originated by stress relaxation after extracting a round of 3.6 cm diameter and 4.8 cm in depth. The stresses before drilling can be deduced with an appropriate mathematical scheme. This experimental procedure has been called Donostia Method by the author in previous works being an ASTM technique. Brick masonry has been built and loaded at the laboratory in order to check the method before actual works on site take place. This constructive element is widely used in cultural heritage constructions. A known stress has been applied to the masonry in order to compare it with the results obtained by means of the hole-drilling technique. The results obtained are very close to what is expected. The technique is harmless for the material and allows the deduction of the principal stresses and their directions for a bi-dimensional state of stresses on the surface. In addition, the method allows for the measurement of tensile stresses. Thus the hole-drilling technique is a minor-destructive method of testing giving more information than the flat jacks technique. The deduction of the stresses is strongly influenced by the constants that relate them with strains measured after drilling. This paper analyzes two different methods to deduce such constants.