PSI - Issue 62

Giulia Rossini et al. / Procedia Structural Integrity 62 (2024) 347–354 Giulia Rossini et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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implemented this stress release method using blunt pyramidal specimens. The authors asserted that the proposed investigation and analysis methodology is easy to perform, has low cost, and the results are reliable. 2. Experimental program The experimental program involved the fabrication of 4 full-scale PRC beams, 10 m long and 80 cm high, with an "I" cross-section, typical of most PRC bridge girders. These beams were used both to analyze the effectiveness of different diagnostic techniques for determining residual prestress and to study the influence of prestress losses on shear bearing capacity, with particular reference to bridge girders. All four beams have the same geometry, but have different reinforcement configurations, both prestressing and mild. Two of the four beams were made with pre-tensioning system with bonded strands at two prestressing levels fixed during the casting phase; while the remaining two beams are identical and have been designed with a hollow corrugated pipe to subsequently allocate unbonded strands with an external post-tensioning system, making it possible to vary the prestressing level during laboratory tests. Several methods purposed in the literature were analyzed to determine residual prestress. In particular, the efficiency (including the invasiveness and ease of use for the operator) and reliability of the results provided were evaluated. A total of 49 release tests based on tension release were carried out: • N° 11 Core Trepanning test (2 on flanges, 9 on web); • N° 24 Saw-Cut (9 on intrados, 15 on web); • N° 14 Blunt Pyramidal specimen (8 on flanges, 6 on web). Core drilling and intrados saw-cuts were conducted in collaboration with a private laboratory. The cores, measuring 10 cm in diameter, were extracted at various depths (3, 10 and 15 cm) to assess the optimal depth ensuring complete isolation of the concrete block and, consequently, thorough stress release. Following the methodology described in the study by Kraľovanec et al. (2021) , intrados saw cuts were implemented with a depth of 3 cm and a spacing of 12 cm. Both these techniques are operator-dependent regarding core drill penetration and cutting speed. In particular during drilling execution, the element is greatly disturbed by instrumentation vibration. Also blunt pyramid specimen tests were carried out by technicians from a private laboratory. Four cuts were made in the concrete at an angle of 45° inclination out of the plane, forming a blunt pyramid with a larger base of approximately 7 cm x 7 cm and a blunt height of about 25 mm, slightly less than the concrete cover. These cuts were executed using a diamond blade mounted on a ∅ 125 mm angle grinder. The grinder is installed on a plate fixed to the beam called "Discovery", which ensures accuracy and regularity for the 4 cutting angles. Therefore this technique has the advantage over the two previously described techniques (core drilling method and saw cut method) of being less operator dependent. Saw cut-web tests were independently conducted without external support. The test is very similar to the test proposed in the literature with saw cuts at the girder intrados, but involves performing the cuts on the web convering a height of around 25 cm. An angle grinder with a ∅ 115 mm diamond blade was used for the cuts, achieving a maximum depth of 30 mm almost coinciding with the cover. The thickness of a single cut was 4 mm. Two cut spacings (6 and 10 cm) and two cut depths (2 and 3 cm) were examined (Fig. 1). Therefore four different geometric configurations (GC) of cuts were considered: • Pair of cuts 10 cm apart and 2 cm deep: SCw_GC_10_2; • Pair of cuts 10 cm apart and 3 cm deep: SCw_GC_10_3; • Pair of cuts 6 cm apart and 2 cm deep: SCw_GC_6_2; • Pair of cuts 6 cm apart and 3 cm deep: SCw_GC_6_3.