Assessment of concrete core strength with and without steel bars

The concrete compressive strength is a suitable index to ensure the quality of concrete while the construction is underway. The core samples, which represent the potential strength of concrete, are prepared, cured, and tested according to the relevant standard codes and specifications. On the other hand, determination of the actual strength of concrete in a structure is not easy because it depends on the history of the curing procedure, the adequacy of concrete compaction, and the casting method. Therefore, the question that has always attracted the attention of designers is if the standard test specimens can represent the in-situ strength of concrete. Arriving at the answer to this question becomes even more important when the strengths of standard test specimens are lower than the specified strength. In this case, either the strength of concrete in the structure is lower than the design value or the specimens do not actually represent the concrete strength in the structure. In such cases, the problem would be addressed by drilling and testing some core specimens from the suspected structural member. In addition, there may be no standard specimens at a late age, and it may be necessary to determine the current strength of the structure.Concrete core test is always regarded as an important issue in the area of concrete industry to evaluate the in-situ concrete strength, and sometimes it becomes the unique tool for safety assessment of existing concrete structures. Core test is, therefore, introduced in most building codes. The presence of rebar in the cores affects the results of testing; accordingly, some codes specify that no bars are allowed to be present in the cores, while others account for the bars by introducing a correction factor. In the present experimental research, the parameters that exert significant effects on the strength of the cores containing rebar are examined. To that end, 112 plain and reinforced concrete beams with the bars of 10- and 16-mm diameter (with different arrangements) and water-to-cement ratios of 0.4 and 0.55 have been created. The beams have been kept and cured under air-dried conditions. In order to perform the compression tests, 988 concrete cores of 7.5- and 10-cm diameters with aspect ratios of 1 and 2 have been drilled at 14, 28, and 56 days of age. In the majority of cases, as the water-to-cement content increases from 0.4 to 0.55, there is a larger amount of strength loss in the cores containing the rebar as compared to those without any rebar. The strength of the cores declines by increasing the concrete cover for the bars. For the cores containing a single bar, the reduction which is resulted in the strength in comparison to that of the plain concrete cores is more dramatic in the cores having a bar of larger diameter. On the other hand, the amount of strength drop increases by increasing the number of bars. The largest drop in the strength values, amounting to 23 percent of the plain-concrete core strength, is observed in the concrete cores having two 16-mm bars. Furthermore, the cores containing eccentric rebar show a greater reduction in comparison to the cores with no eccentric rebar.