Yuichi Tomoda

Phenomenological model of corrosion process in reinforced concrete identified by acoustic emission

The concrete reinforcing steel bar (reinforcing bar) corrosion process is phenomenologically modeled as three phase corrosion loss. Reinforcing bar has corrosion initiated at the first phase. Under aerobic conditions at the second phase, there is a decreased rate of corrosion loss. Due to corrosion products, reinforcing bar expansion nucleates concrete cracking at the third phase of anaerobic corrosion. For such a corrosion process to be identified as reinforced concrete corrosion loss, continuous acoustic emission (AE) monitoring was conducted in a cyclic wet and dry test and an accelerated corrosion test. There was characteristic observation of two high AE activity periods. The slope gradient of AE amplitude distribution and AE waveform parameters were investigated to elucidate these two activities. Cracking mechanisms were classified as the other-type cracks than a tensile crack and AE events were found to be of small amplitudes at the first period of high activity. That ferrous ions vanished at some surface regions was demonstrated by scanning electron microscope examination of the reinforcing bar surface, although no corrosion products were visually observed on the reinforcing bar surface. The corrosions onset in the reinforcing bar results in the first high AE activity is suggested by this, corresponding to the phenomenological models first phase. Corrosion products were visually observed on the surface of the reinforcing bar following the second high AE activity period. It suggested that in concrete around the reinforcing bar, there was nucleation of cracks. Reasonably, AE event amplitudes were found to be fairly large and AE sources were classified as tensile cracks. Concrete cracking is associated with these AE events due to corrosion product expansion that could be observed at the models third phase. Thus, the corrosion loss phenomenological model agrees remarkably with reinforced concrete AE generating behaviors. Reinforced concrete corrosion time could be identified, by AE activity monitoring, at reinforcing bar corrosion initiation and also at the nucleation of cracks in concrete. There is earlier detection of even the latter AE activity than through other nondestructive evaluation techniques.

Acoustic Emission Techniques for Rebar Corrosion in Reinforced Concrete

Corrosion of reinforcing steel is modeled phenomenologically as corrosion losses of three phases. At the 1st phase, corrosion is initiated in reinforcement. The rate of the corrosion loss decreases at the 2nd phase under aerobic conditions. At the 3rd phase of anaerobic corrosion, the expansion of reinforcing steel due to corrosion product nucleates concrete cracking. In order to identify the onset of corrosion and the nucleation of concrete cracking based on this model, continuous AE monitoring in reinforced concrete specimens was conducted in a cyclic wet and dry test. High AE activities were characteristically observed twice during the corrosion process. AE parameter analysis of hits, events and locations demonstrates that AE monitoring is readily applicable to detect the corrosion in rebar and to identify the corrosion process.

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.