PSI - Issue 10

M. Petrov et al. / Procedia Structural Integrity 10 (2018) 303–310 M. Petrov et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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The mechanical impact is performed using an electromechanical impact device with standard impact force (100 N) and duration of the excitation pulse (60 μs). The electromagnet was used as the impact device. In order to record the electric signal, a differential electric receiver is used, which significantly reduces the level of electromagnetic interference. The differential electric receiver consists of a measuring electrode, a compensating electrode and a differential amplifier. Each electrode is represented by a 20×20 mm conductive plate and connected to the ground by the input impedance of 10 MOhm.The measuring electrode is placed at the distance of 2 mm from the sample surface. The compensating electrode is placed at the height of 30 mm from the measuring electrode. The operational frequency of the amplifier is 1-100 kHz and the gain ratio is 10. A more detailed description of the differential electric sensor operation is provided in the articles of Fursa et al. (2017a; 2017b). The signals from the differential electrical sensor were registered using multifunction input/output board ‘‘NI USB 6212” connected to the notebook. The signal recording is perfor med with a custom software developed in LabVIEW integrated environment.

Fig. 1. Compression test setup of concrete samples: 1 – press; 2 – measuring probe; 3 – concrete sample; 4 – displacement transducer.

3. Experimental results

Compression tests of laboratory concrete samples reinforced with steel (RC) and fiberglass reinforcement (GFRP-RC) have been carried out. Influence of the reinforcement type, the structural features of reinforced concrete and the concrete strength on the process of their destruction under compression conditions were investigated. Stress strain curves at a constant loading rate have three characteristic stages (Fig.2).

Fig. 2. Stress-strain curves for compression of concrete samples reinforced with a single steel (RC) and fiberglass (GFRP-RC) bar.

The first nonlinear stage (I) is related to compaction of the sample surface. The second stage (II) is the stage of linear quasielastic deformation. Finally, the third stage (III) is the stage of nonlinear deformation and it is associated with formation and growth of cracks. Some distinctive features are observed in the fracture behavior under compression conditions when testing samples with different types of reinforcement and different strength of concrete. Table 1 lists the values of the maximum stress that samples with different reinforcement parameters can withstand.