Slightly Destructive Tests on masonry

Boreholes provide predominantly qualitative information that is essential to measure the geometry of the structural elements, for example, the foundation level depth and the thicknesses of the masonry layers. In this regard, the drillings may be carried out in foundation elements with variable inclinations, so as to identify the contour of the foundations, especially the presence of any offsets. Boreholes are also useful to inspect the type of materials, the size and the nature of the elements, the presence within a wall of an inner core different from the facings or the presence of parts of masonry which have been restored.

To inspect the structural stratigraphy and to identify the hollows passed through by the drilling, it is necessary to carry out the inner survey of the borehole with the use of a bore-scope, which views images and takes pictures, or with a video-probe, which records video during the inspection.

The taking of cores means samples suitable for the execution of laboratory tests are selected; moreover, boreholes mean additional tests within holes can be performed. For example the dilatometric tests provide information about the mechanical characteristics of the material through the application of hydraulic pressure to the edge of the hole and the measurement of the consequent deformations of the hole diameter. This technique is especially useful for characterizing the masonry with inner core different from its external faces.

A flat jack is made of two thin metal plates welded at the edges, with inlet and outlet ports, which may be pressurized with a hydraulic system. A slot is cut on the surface of the masonry along a mortar joint (or an alignment in the case of chaotic masonry) where measurements can be made. Compressive stress causes the masonry above and below the slot to move together.

Compressive stress in the masonry is measured by inserting a flat jack into the slot and increasing its internal pressure until the original distance between points above and below the slot is restored. The state of compressive stress in the masonry is approximately equal to the flat jack pressure multiplied by factors which account for the physical conformation of the jack and of the slot.

The test can be further implemented by making a second slot parallel to the previous one. Two flat jacks are inserted into the two parallel slots and they are pressurized thus introducing compressive stress in the masonry included between them. By gradually increasing the flat jack pressure and measuring the deformation of the masonry, stress-strain diagrams may be plotted. Maximum compressive strengths may be estimated in certain cases.

Placing the flat jacks within specially made vertical cuts, it is also possible to perform Shear Compression Tests (so called FJ-SCT), with a minor impact on the structure compared to the tests performed with the use of ordinary cylindrical jacks. These tests determine the shear characteristics of masonry which have to be correlated with vertical stress.