Flat jack Testing for deformability characteristics on masonry

 

WHAT I DOES

Control of the tension stress state and of deformability characteristics on masonry

DESCRIPTION AND APPLICATION

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.
DATA READINGS
Extensometric measurements
The MAYES mechanic precision strain gauge produced in England is used for extensometric measurements with an analogical (or digital, on demand) dial gauge (see the card).
Reference standard length: 200 mm.
Measuring range: 5 mm. Resolution: 2 x 10-3 mm.
Transmitted pressure
Two different oil-pressure pumps produced by the Glötzl Company in Germany with one or two Wika precision manometers and with a recovery circuit for the measurement fluid (see Glötzl M2H16 hand pump card) are available for putting the jacks under load and reading the values.
The version with two manometers (standard range: 0-25 bars and 0-100 bars) is equipped with an switch-over circuit so that it can use the most suitable measurement range for the test to be performed with the maximum reading resolution.

FEATURES

Commented calibration certificates issued on the results of tests performed on samples picked from every production lot are also released.

MODELS
Rectangular
Special model for K3500-3600 shear
Semicircular

SIZE
400 x 200 x 4 mm
240 x 120 x 4 mm
400 x 120 x 4 mm
325 x 120 x 4 mm
350 x 260 x 4 mm

ARE
383 cm2
761,5 cm2

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