Innovative AE and NDT Techniques for On-Site Measurement of Concrete and Masonry Structures

Technical committee TC-239 MCM “On-Site Measurement of Concrete and Masonry Structures by Visualized NDT” has been completed in March, 2016. Thus, historical development and research activities of the committee are summarized as the introduction. The objective of the state-of-the-art report is stated.

Implementation of Non-destructive Evaluation (NDE) procedures in concrete structures seems to be the only way towards a safe and economic infrastructure management. Since concrete structures have reasonably ceased to be regarded as maintenance-free, periodic inspection is deemed necessary. Although destructive techniques can provide information on the strength of the material, this is only based on sampling and cannot be performed in a wide basis. Therefore, NDE methodologies, especially with the capability of visualizing the interior of the structure, have been developed and applied in structures to answer a question that cannot be addressed destructively. This chapter focuses on the current trends of elastic wave and general techniques that are used and new trends with some examples of how NDE assists in the identification of damage and maintenance of concrete structures are highlighted.

Historical masonry buildings often show diffused crack patterns due to different causes in relation to original function, construction technique and loading history. Non-destructive testing methods, and especially the Acoustic Emission (AE) technique, applied as in situ monitoring systems allow reliable evaluation of the state of conservation of these structures and its evolution in time. In this chapter three different case studies are presented to show the AE technique capability on the assessment of damage evolution in ancient brick and stone artworks. All the analyzed structures are located in remarkable Italian sites from the Architectural Heritage point of view.

Newly developed Computerized Acoustic Emission Tomography (AE Tomography) is summarized, which could identify elastic wave velocity distribution and source locations of AE events. Starting with theory of AE Tomography based on ray-trace technique, two- and three-dimensional problems are stated, and capability of AE Tomography is discussed by performing a series of numerical investigations. Furthermore, AE Tomography is applied for evaluation of existing structures as well as laboratory tests to verify an applicability of AE Tomography.

Acoustic emission (AE) is one of the promising techniques for structural health monitoring (SHM) purposes in concrete structures. Due to its passive and non-invasive nature, it can be applied without disturbing the operation while it provides information that no other monitoring technique can. During the last decades great developments have taken place in the technical equipment and the analysis tools. The present chapter intents to summarize the recent trends in AE parameter analysis mainly related to fracture mode identification. This issue carries high importance since fracture of concrete materials and structures includes shifts between different modes. Therefore, its identification supplies information on the damage status. However, the characterization should remain as simple as possible in order to have the potential to be implemented in situ.

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.

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.

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.

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.

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.