PSI - Issue 64

Said Debbakh et al. / Procedia Structural Integrity 64 (2024) 130–136 S. DEBBAKH and al. / Structural Integrity Procedia 00 (2024) 000 – 000

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the concrete. The pull-off equipment is provided with a measurement device that displays the exerted force by digital system and records the maximum force exerted. The test is carried out by direct dolly pull-off using a circular dolly (50 mm of diameter and 20 mm of thickness) bonded to the surface of the concrete with rapid hardening epoxy adhesive, having been cored through the surface. 4. Results and discussions 4.1. Compressive strength derived from Rebound Hammer test Table 2 presents the compressive strength values of both columns resulted from the rebound hammer test after one year of natural environment exposure. The mean compressive strength of RCC-1 and RCC-2 for the three zones (upper, medium and lower) are represented and compared with the mean compressive strength derived from the cores crushing at 28 days (core diameter (Ø) = 103 mm, slenderness ratio (SR) = 2) (Benidir and al., 2023). From table 2, and at first sight, the mean compressive strength values of both columns after one year of exposure, derived from the rebound number measurement are higher than that obtained by crushing the cores at 28 days. For instance, the mean compressive strength of both RCC1 and RCC2 derived from Rebound Hammer are 39.6 MPa and 41.5 MPa respectively. The values from crushing the cores are 20.29 MPa and 23.13 MPa respectively (Benidir and al., 2023). It could be deduced that the results of Rebound Hammer test are different from the results of the crushing test. The strength derived from the rebound Hammer test has increased by 195 % and 180 % for RCC1 and RCC2 respectively when compared to the mean compressive strength obtained by crushing the cores. Accordingly, it is said that the rebound Hammer test has overestimated the compressive strength of both columns. It could be concluded that the rebound Hammer test measures the hardness of concrete surface (Sanchez and al., 2015). Moreover, it is observed that the upper zone of both columns has the weakest compressive strength values compared to the middle and the lower zones, which is consistent with the results from the crushing tests of the cores (Benidir and al., 2023). According to (Akcay, 2004), the upper regions of columns would always be weaker irrespective of the method of construction. On the other hand, it is observed that the rebound hammer test has recorded maximum values of the mean compressive strengths at the lower zone, and this, for both columns, which does not agree with the results previously derived from the cores, where the medium zone was found to exhibit the highest strength. According to this strength distribution after one year of environment exposure, it can be deduced that the rebound hammer test would not be reliable enough for characterizing the compressive strength of concrete of columns, and that the zone effect is steel persisting. The cores would be according to engineers more reliable at representing the quality of concrete. Accordingly, the reliability of the rebound Hammer test depends on several parameters including methodology and rebound index values.

Table 2. Compressive strengths derived from rebound Hammer test for RCC-1 and RCC-2 Reinforced concrete columns RCC-1 Reinforced concrete columns RCC-2 Left side Right side Left side Right side RI D fc (MPa) RI D fc (MPa) Mean D fc (MPa) RI D fc (MPa) RI D fc (MPa) Mean D fc (MPa)

37 37 37 37 42 42 43 44 44

34 34 34 34 44 44 45 47 47

36 37 38 38 39 43 42 41 41

32 34 36 36 38 45 44 42 42

36 33 36 33 38 36 42 44 42 44 42 44 41 42 44 47 44 47

37 37 41 42 42 42 41 43 44

34 34 42 44 44 44 42 46 47

Upper

34

35.3

Medium

40.2

44

Lower

44.5

45.2

Mean

39.6

41.5

Standard deviatio n (std)

5.3

5.4

RI: rebound index Dfc: Derived compressive strength from rebound Hammer number