The flat jack consists of a welded sheet metal pad with two inlet and/or outlet mouths. This pad, when connected to a pressurisation system, can be inflated with oil, increasing its volume.
The flat jack technique was initially created for testing rocky terrain, and was later appropriated for evaluating the stress state and mechanical characteristics of masonry structures.
This technique, considered slightly destructive, is presented in two forms: simple test and double test.
The simple test allows the stress state at a point in the structure to be determined.
The simple test is performed in two stages:
- Measurement of the change in deformation state caused by a tear performed on the wall to insert the flat jack;
- Determination of the pressure to be inserted in the flat jack to reset the initial deformation state, i.e. to its state prior to performing the tear.
This pressure corresponds to the state of vertical stress to which the tested zone was subjected before the tear, which is the value that is intended to be determined.
The double test allows the behaviour curves of the masonry (vertical stress as a function of the vertical and horizontal deformation of the masonry) to be traced, and thus allows for an estimation of its modulus of elasticity and, through relationships established in the bibliography, the resistant capacity.
This test is usually preceded by the simple test, so initially a flat jack is already inserted into the wall.
A second parallel tear is then made at a higher level than the first, where the second flat jack is inserted.
The two flat jacks are connected to a hydraulic pump that ensures the same level of pressure is introduced into both.
A set of displacement transducers placed between the two flat jacks allows the displacements for the different pressure levels installed in the flat jacks to be monitored.