The Numerical Investigation into the Influence of the Scattering Effect from the Aggregates of Concrete on the Transient-elastic-wave-based NDT Technologies

摘要

Recently, the transient-elastic-wave-based nondestructive evaluation technologies are well developed, and then successively applied to monitor the quality of in-situ civil structures. It is worth to note that the principle of present technologies is based on the assumption that concrete is treated as a kind of homogeneous material. It could really simplify the analysis of the inspected results with that assumption when the wavelength of elastic wave is much larger than the dimension of the largest aggregates. However, the scattering of elastic waves by the aggregates does affect the analysis of the received signals while the short-wavelength elastic wave is used to enhance the resolution of inspection. To understand the influence of scattering effect on those methods, a quantitative analysis on concrete material is indispensable. In this paper, the finite difference method was used to simulate the propagation of transient elastic wave inside concrete structure. The force-time function of impact source corresponding to various steel balls was derived from Hertz contact theory. The numerical model made with two materials, the specific size aggregate and the cement paste, was used to simulate the real concrete. And the other model with only one homogeneous material was treated as an ideal concrete. The surface responses of standard impact-and-receive operation were calculated out on these two models. The coherent coefficient of these two received signals was regarded as the quantitative index of signal distortion owing to scattering effect. Relations between that coefficient and possible effective parameters were carefully analyzed and discussed in this paper. The numerical analysis shows that as the ratio of wavelength to aggregate's diameter increases, the scattering effect decreases. When the ratio is larger than 10, the coherent coefficient approaches to 1 and the scattering from aggregate almost vanishes. The concrete could be treated as a kind of homogeneous material and then the analysis assumption of elastic-wave-based NDT principle matches. Besides, the result also shows that the acoustic impedance difference between aggregate and cement paste will seriously affect the coherent coefficient. It would be very useful when NDT is applied on the high-performance concrete structure. In this paper, the result reveals the quantitative analysis of the scattering effect.The achievement of this research could provide very useful information for selecting a proper elastic wave source when performing the NDT inspection on concrete structures. Also, it could be a useful reference for analyzing the received signals regarding the inhomogeneousity of concrete material when developing new elastic-wave-based NDT technologies.