Flat-jack testing Boviar0 pages

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FLAT JACKS
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> STRUCTURAL DIAGNOSTICS

FLAT-JACK TESTING
> WHOS IT DOES
Control of the tension stress state and of deformability
characteristics on masonry

> DESCRIZIONE
Flat jacks are mainly used in the diagnosis of buildings and
masonry with the purpose of establishing experimentally the
actual static conditions of the structure under investigation.
Other possible applications regard the measurement of stress
in situ (e.g. in tunnel’s lining and in bridge piers).
The single-jack test methodology is based on the variation of
the stress state in a point of the structure under examination
due to a plane cut executed in the direction perpendicular to
the surface. The stress release causes the cut to be closed to
an extent which is assessed through convergency
measurements between one or more points, situated in a
symmetrical position with respect to the cut itself by using a
high-precision strain gauge (see Mayes strain gauge). A
special flat jack is then inserted within the cut and gradually
put under pressure with a special
hydraulic pump (see
M2H16 Glötzl hand pump) until the previously measured
convergency is eliminated. In these conditions the pressure
within the jack is equal to the pre-existing force on the
masonry, minus a constant that takes into account the relation
between the area of the load jack and the area of the cut and
the jack’s stiffness coefficient (Km).
Two parallel jacks installed at about 50 cm from one another
are used and the volume of material delimited by them is
submitted to a mono-axial compression test to identify the
deformability characteristics.
The employment of flat jacks can involve both a recovery
installation and a disposable installation: in this case the jack
is cemented to the cut and will serve for the medium and
long-term measurement of the pressure in the point of
installation.

> CUT
For the realization of cuts where the jacks are housed,
depending on the material that is under test, a saw with a
widia chain can be used for soft materials (bricks, tuff, mortar,
etc.); a particular hydraulic shear (K950-ring) with diamond
disks that allows performing semicircular cuts of 260 mm in depth in very short times, for which special
models of jacks are available (see the card) can be employed for less regular masonry, e.g., limestone
ashlars.