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TEST SPECIMENS AND EXPERIMENTAL PROGRAM FOR OLYMPIC GROUP BUILDING
The group (L1-L3) is low concrete grade with dimensions 25x35x150 cm (half scale of the common 45x55x300 cm columns). Reinforcement ratio was taken as about one percent of the gross section as common in concrete design and matching the code of practice in buildings. The reference column detailing is shown in Fig. 7. Three 15-cm side length control cubes were cast with each mix. Specimens were demoulded after 24 hours and cured in room temperature until the time of testing. The specimens were tested when the strength of their concrete cube strength reaches 11.77 MPa to simulate real cases of an existing building. The first of them (specimen no. L1) was tested without strengthening while the second, L2, was strengthened then loaded until failure. The third specimen, L3, was first loaded to working load, strengthened then loaded until failure to simulate the actual case of retrofitted column of an existing building.
Arrangement of the measurement devices and reinforcement details of concrete section and retrofitting steel. High-grade deformed steel bars of diameter 12 mm were used as main reinforcement having a yield and ultimate strength 511.6 MPa and 658.5MPa. Mild steel bars of diameter 8mm were used for stirrups of this group, which have yield and ultimate strength 440.5 MPa and 476.9 MPa. Details of concrete reinforcement, external retrofitting steel sections, and arrangement of the measurement devices are shown in Fig. 7. For the used threaded rods used in applying the external pressure and lacing strips used to sustain the attained pressure, tension tests were made to identify the properties of their materials. The stress strain relationship of the 12 mm nominal diameter threaded bar and the sample lacing plate were obtained from tests. Both materials behave as mild steel with different values of yield and ultimate strength. The threaded bars yield at 512 MPa and fail at 545 MPa while lacing plates yield at 440 MPa and fails at 654 MPa. ANALYSIS OF THE RESULTS Low Strength Concrete Tests containing three test specimens (L1 through L3) were performed in the Building Research Institute in Cairo. The machine used is displacement control equipped with loading cell and data logger connected to digital computer. The resulting P-D curve of the three specimens is shown in the following figure at which the applied load is plotted against the measured axial deformation of the column. Strengthening significantly enhances the overall behavior and capacity of the retrofitted specimens. While the ultimate load of the reference column specimen (number L1) was 800 kN, the strengthened specimen (number L2) failed at 1570 kN. The third specimen (number L3) was loaded by its working load and left for one day until no displacement was remarked at a load of 392 kN. then strengthened and the loading continued until failure. These conditions simulate to the real situation of retrofitting of existing buildings while the columns are already loaded. The ultimate load of this sample recorded the uppermost value (1694 kN), which constitutes about 212 percent of the ultimate load of reference sample. Strengthening also enhances the behavior of reinforced concrete columns such that more ductile response is observed for the third specimen (L3). The results indicated also that the applied lateral pressure has no side defect on specimens in spite of the weakness of concrete grade. Many of existing buildings have low concrete grade, the improvements of column behavior with low strength concrete certify the efficiency of the method applied to existing RC buildings. This can be attributed to the fact that the applied external active pressure delays the formation of diagonal shear cracks, limits the width of such cracks and improves aggregate interlock then increase concrete contribution to shear resistance which led to higher load capacity.
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