Masayasu Ohtsu

Overview of Visualized NDE for On-Site Measurement

Toward the sustainable society, long service life of infrastructure is going to be an evolutional target in the world. Aging and disastrous damage in concrete structures have updated the urgent demand for continuing maintenance of the structures in service. To this end, a variety of non-destructive testing (NDT) techniques and evaluation methods for diagnosis and prognosis are surveyed. Thus, visualized non-destructive evaluation (NDE) for on-site measurement is overviewed.

3D Visualization of Crack Kinematics by AE-SiGMA

Kinematics of crack nucleation is physically associated with acoustic emission (AE) source mechanisms. Mathematically, kinematical information on cracking is represented by the moment tensor. In quantitative AE waveform analysis, one powerful technique for three-dimensional (3D) visualization of AE source has been developed as SiGMA (Simplified Green’s functions for Moment tensor Analysis), where crack kinematics of locations, types and orientations of AE source is quantitatively analyzed. Starting with theoretical background and principle, the procedure is explained with a sample code.

Cross-Sectional Visualization of Defects by SIBIE

For nondestructive evaluation (NDE) of concrete structures, the impact-echo method has been successfully applied to locate defects and voids in concrete. To compensate the difficulty in selecting the resonant frequency and to visually locate a defect, an imaging procedure named SIBIE (Stack Imaging of spectral amplitude Based on Impact-Echo) has been developed. This is an imaging technique for visualizing defects of which locations are known or identified. Following the basic concept and procedure, applications to concrete structures are discussed. The presence of the ungrouted duct can be visually identified by the SIBIE procedure in both the cases of plastic sheath and metal sheath. In addition, the depth of the surface crack is visually evaluated.

On-Site Damage Evaluation by AE and CT in Concrete

For detailed inspections of concrete structures, non-destructive tests (NDT) are often conducted and then mechanical properties are compared with detected NDT parameters. In this chapter, damage evaluation of concrete by AE and X-ray computerized tomography (CT) tests are presented. The concrete damage is visualized by X-ray CT. Since nucleation of AE events in damaged concrete is affected by crack distribution, the damage parameter estimated is correlated with the dynamic modulus of elasticity E d which is calculated from P-wave velocity. The results confirm that the damage of concrete could be estimated by applying AE measurement, damage parameter in damage mechanics, CT images and modulus E d. It is also shown that the static modulus E 0 is closely correlated with the dynamic modulus E d. Based on a relation between AE rate and the damage parameter, the damage of concrete is quantitatively estimated.

Hybrid NDE for Rebar Corrosion in Concrete

Corrosion of reinforcing steel-bar (rebar) is modelled phenomenologically as corrosion losses of three phases. Corresponding to these phases, high AE activities are characteristically observed twice during the corrosion process. It is shown that the 1st AE activity corresponds to the onset of the corrosion in rebar and at the 2nd period of high activity AE events result from concrete cracking due to the expansion of rebar caused by corrosion products. The expansion caused by corrosion products generates micro-cracks in concrete as corrosion-induced cracks, of which mechanisms can be investigated experimentally by AE. To this end, SiGMA (Simplified Green’s functions for Moment tensor Analysis) is applied to the corrosion process in reinforced concrete. To compare with cracking mechanisms identified by SiGMA, a numerical analysis by the two-dimensional boundary element method (BEM) is performed for stress analysis and applied boundary nodes were determined from a diffusion analysis of chloride contents by the finite element method (FEM). Thus, fracture mechanisms of corrosion-induced cracks are quantitatively evaluated by AE-SiGMA and BEM. For on-site measurement of the corrosion in reinforced concrete, hybrid nondestructive evaluation (NDE) is going to be developed. Here, acoustic emission (AE) measurement is conducted, along with measuring half-cell potentials and polarization resistances at the surface of concrete. In order to identify the corroded area along rebar, the potential inversion by BEM (PiBEM) analysis is applied. Thus, plausible areas of the corrosion in rebar are identified.