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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As can be seen from Fig. 3, the distribution of stresses in the area of reinforcing bar location for concrete samples with different type of reinforcement has some differences. In a sample with steel reinforcement, the maximum stresses occur in the steel reinforcement itself and in the layer of concrete adjacent to the reinforcement, and these stresses are concentrated in the regions above and below the reinforcement. In the sample with fiberglass reinforcement, maximum stresses are also concentrated in the area adjacent to the reinforcement, but fiberglass rebar itself is under low mechanical stresses condition. Therefore, the development of cracks in the reinforced sample should begin from the reinforcement in the direction coinciding with the direction of compression. Furthermore, the destructive stresses in the concrete area adjacent to the steel and to the fiberglass reinforcement are achieved at different levels of external loads (Fig.4).

Fig. 4. Dependence of maximum stress in concrete on external pressure for samples with fiberglass and steel reinforcement.

The conducted studies showed that the compressive strength of concrete samples in this set was 34 MPa. As can be seen from Fig.2, the transition of the sample with steel reinforcement into the stage of destruction (III) occurs at approximately 28 MPa, which corresponds to the results of calculating the destructive stresses arising at the boundary of concrete with steel reinforcement (Fig.4). Calculated values of the external pressure at which the destructive stresses occur at the boundary of concrete with fiberglass reinforcement are 33 MPa (Fig.4) and they exceeded the value of external pressure at which the transition into destruction stage (III) for the sample reinforced with fiberglass reinforcement is observed (Fig.2). The beginning of the destruction processes in reinforced concrete under uniaxial compression for concrete reinforced with steel is associated with damages at the boundary of concrete with reinforcement. But for concrete reinforced with fiberglass, the destruction processes are probably associated with damages arising at the boundary of cement matrix with coarse aggregate. Differences in the fracture mechanics of concrete reinforced with steel and fiberglass reinforcement are related to the differ ences in their Young’s modulus. Instances of mechanical stress appearing in reinforced concrete under external load lead to the development of cracks in the radial zone around the reinforcement, at the boundary between the cement matrix and the coarse aggre gate, and in the cement matrix itself. In case of mechanical impact on the surface of a three-dimensional sample, a longitudinal wave propagates into the sample along a hemispherical wavefront and is reflected from defects, in homogeneities and six external boundaries of the sample. As a result, a complex wave pattern is formed in the sample, which is determined by the geometry of the sample, its elastic characteristics, and the duration of the impact and the presence of defects (Lin and Sansalone (1992)). As the load increases, so does the size and quantity of the cracks. The acoustic wave front created by mechanical impact crosses these areas and gets distorted. Since the acoustic wave interacts with the cracks repeatedly, the wavefront distortions accumulate. Parameters of the distortions must be re flected in the spectral characteristics of the electric response since the latter is interdependent with the elastic waves. The interpretation of measured signals in the frequency area was performed by the power spectral density. The normalized energy spectrum provides general information on the change in the spectral composition of electrical responses. Fig.5 shows two-dimensional pictures displaying the behavior of changes in the electric signals spectra during the uniaxial compression of reinforced concrete. Characteristics of electric response registered from concrete samples reinforced with fiberglass bars have similar changes.