Prognosis of fatigue and impact induced damage in concrete using embedded piezo-transducers

Concrete structures are often subjected to a wide array of load conditions, such as fatigue and impact, and the consequent damages. Recently, smart materials, in particular the piezoelectric materials, have received high attention from the point of view of structural health monitoring (SHM). In this connection, the electro-mechanical impedance (EMI) is one of the latest and most effective techniques. The technique harnesses the piezoelectric property of lead-zirconate-titanate (PZT) patches to sense any incipient damage. In the research covered in the paper, embedded PZT patches are employed to monitor the accumulating damage in plain cement concrete subjected to fatigue and impact type loadings. Concrete specimens (150 mm cubes and 100φ × 200 mm cylinders) of grade M-25 are cast with embedded concrete vibration sensors (CVS). The impact loading is simulated by a free falling iron-ball of mass 5 kg dropped from variable heights of 2 m, 2.5 m and 3 m above the top surface of the test cubes. For fatigue tests, a 4000 kN cyclic compression testing machine is used to apply the fatigue load in a sinusoidal form. The admittance signatures from the embedded PZT patches for impact and fatigue tests are acquired using an LCR meter. These signatures are used to determine the PZT-identified equivalent system properties (damping, stiffness and mass) of the specimens at varying damage states. For impact tests, strain history during the impact process is acquired in addition to the admittance signatures. The results for impact tests show an increase in the maximum strain induced in the patch with the number of impacts, while the equivalent damping computed from the admittance signature is found to decrease with increasing damage. For fatigue tests, the equivalent damping is again found to decrease with increasing damage, finally undergoing a phenomenal increase near the failure. Based on these measurements, separate relationships have been developed for impact and fatigue induced damages linking the PZT-identified damping with remaining life.