PSI - Issue 62

Alessandro Bellini et al. / Procedia Structural Integrity 62 (2024) 315–322

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A. Bellini et al. / Structural Integrity Procedia 00 (2019) 000 – 000

(a) (b) Fig. 2. View of the experimental set-up: (a) detail of strain gauges application; (b) general view of hydraulic machine and instrumentation.

Before starting the loading process, the strain gauges were zeroed. In this way, both the deformation resulting from the application of the compressive force and the strain related to the prestress release phase were measured, with the nominal target being the return to a zero strain value (when the portion of the concrete extracted is isolated from the column and therefore no longer has any residual stress or deformation). After reaching the required axial load, it was maintained constant during the whole prestress release procedure, simulating in this way a concrete element subjected to a known pre-compression. The concrete volume around the strain gauge was then isolated from the rest of the column by means of 4 saw-cuts made with an angle grinder equipped with a 22 cm disc (see Fig. 3a). After some preliminary tests (Romano and Mazzotti, 2022), the geometric configuration was chosen in order to start sawing from a small square cutting perimeter (80×80 mm 2 ) and to minimize the portion of concrete removed. It should be noted that, in general, the choice of the gauge length of the strain gauge is directly related to the geometry of the sample to be extracted, since a certain safety margin is required from the cutting edges. The sequence of tests, for each column, was carried out starting from the specimens at mid-height and then continuing to those closest to the column ends; this procedure was used for minimizing the effect of the previous extractions on the deformation field of the following tests. The most important parameters investigated during the prestress release tests are summarized in Table 1 where, for each test type, the number of tests, the applied compressive stress σ V , the order of making the saw-cuts and the inclination of them with respect to the concrete surface are indicated, according to the description already reported during the presentation of the experimental program. In general, a pause of at least 60 s was needed between two consecutive saw-cuts, in order to guarantee a proper stabilization of the strain release curve. In addition to the stress release tests described, others release tests were carried out on unloaded columns, in order to evaluate the self-equilibrated stresses, produced inside the concrete columns due to differential shrinkage effect, that can develop in relatively young laboratory concrete specimens (McGinnis and Pessiki, 2007; Romano and Mazzotti, 2022). In fact, drying bulky concrete elements produces a faster shrinkage process at the surface with respect to the slower shrinkage developing at the core of the element; the mechanical consequence being the development of tensile stresses in the external layer of the elements and compressive stresses on the inside. When detaching the block from the column, the previously self-equilibrated tensile stresses are released, affecting the final strain value. To measure this contribution, stress release tests without applying any load on the column were carried out (shrinkage release tests in Table 1). A typical example of a prestress release test is presented in Fig. 3b: it should be noted that, after completing the sawing phase, a small portion of concrete remain attached to the column and sometimes it is necessary to gently use a chisel to promote the full detachment of the sample from the surrounding concrete. Fig. 3c shows the typical aspect of an extracted specimen with a truncated pyramidal shape.