Effect Of Stress And Material Composition On P-wave Propagation Of Piezoelectric Sensors In Concrete Materials

Nowadays, piezoelectric-based non-destructive health monitoring mostly focuses on the structure damage by analyzing the waveform feature(amplitude and velocity) caused by the damage to evaluate the damage severity and location in concrete structure. However, it’s difficult to apply to the practical engineering for less understanding about the external loading effect on the monitoring result and fewer researches on the mechanism of wave propagation in porous multiphase concrete material. In this paper, a piezoelectric intelligent monitoring platform is established to study the loading effect on the monitoring result and to deeply discuss the propagation rule of longitudinal wave in concrete due to aggregate elastic modulus, area ration and different compactness of cement paste.For the sake of studying loading effect, a couple of piezoelectric smart aggregate sensors embedded in concrete block is used to calculate the wave amplitude and velocity under three different ways of axial loading involved single cycle loading, cyclic loading and monotonic compression fracture loading. Under single cycle loading, the amplitude increases when ?=0.1 due to the compactness effect and decreases when ?=0.3 due to the development of microcracks. As to the cyclic loading, the initial microcracks develop quickly at the beginning of the cyclic loading, the amplitude and velocity have a sharp decline and then stay stable because of the lack of cycle times which cannot produce the next level of crack. Under monotonic compression fracture loading, the cracks in concrete develop gradually with the increase of the stress level, the amplitude and velocity decrease. But the change rate of wave amplitude is significantly greater than that of wave velocity in the same stress level. This means that wave amplitude is more sensitive for damage detection but less stable compared to wave velocity. In addition, the compression stress st ate of concrete is closely related to the wave velocity which can reflect t he stress amplitude to some extent.Based on the finite element software ABAQUS, the polygonal aggregate model and random circular aggregate model are set up to simulate the stress wave propagation effect due to different aggregate elastic modu lus and area rate of aggregate. Results show that considering the geometry effect and random characteristics of the aggregate, the amplitude decreases with the increase of aggregate elastic modulus, while the velocity has a growth trend. The higher frequency, the smaller the amplitude and velocity. Besides, the velocity is proportional to aggregate area rate, but the amplitude fluctuates for the geometry dispersion effect and the non-homogeneity of the model. Furthermore, aggregate shape has great effect on amplitude, while it has a small effect on the velocity, the maximum error is only 2.6%.Homemade piezoelectric sensors are used to monitor the change of velocity in different compactness of cement paste. Results show that wave velocity can monitor the hydration process effectively. With the going of hydration process, wave velocity increases and has a sharp rise in the liquid-solid coupling phase. In addition, the initial setting time and final setting time can be determined around 3 h and 8 h respectively through the velocity curve. The change of wave velocity in the cement paste under three different water cement ratios(0.3, 0.4 and 0.5) during the 28 d curing period has also monitored in the experiment. The velocity is inverse proportional to the water cement ratio and increases as the growth of curing age. It is important to note that within 24 h after casting, the smaller the water cement ratio, the faster wave speed grows, while in the later hydration process, the smaller the water cement ratio, the slower the wave speed grows.