Experimental calibration of flat jacks for in-situ testing of masonry

Flat-jack testing method is one of the most commonly used techniques for the structural assessment of existing masonry structures. Single and double flat jacks are usually adopted to evaluate the acting normal stress, or the compressive behaviour of masonry material. Test procedures are codified by international standards (e.g., A.S.T.M D4729-87; C1196-04; C1197-04, R.I.L.E.M TC 177–MDT D.4; R.I.L.E.M. TC 177–MDT D.5), which provide the preliminary calibration of an experimental coefficient (km) and of the effective area (Aeff), which determination influences significantly the reliability of the test. This article presents the result of an experimental study on the calibration of flat jacks for masonry testing. The problem is investigated by several tests carried out on two types of common flat jacks, which differ for geometry and producer. Two calibration methods are adopted in order to relate the pressure values of the flat jack with those of the hydraulic press and load cycles are performed in two different pressure ranges. Finally, a theoretical interpretation of results is made, which gives good predictions of calibration parameters. Results of this investigation highlight the influence of constructive features and service pressure of the jack adopted, for obtaining reliable results from the tests on masonry structures.