No abstract

Continuous acoustic emission (AE) monitoring in reinforced concrete (RC) was conducted to investigate the corrosion process. In experiments of an accelerated corrosion test and a cyclic wet-dry test, two periods of high AE activities were observed. These AE sources are classified by AE indices of the RA value and the average frequency and the b-value of AE amplitude distribution. At the 1st period, generation of small shear-type cracks is identified. From ingress of chloride ions analyzed, chloride concentration at the cover thickness of the reinforcing steel (rebar) was just over the lower-bound threshold for the initiation of corrosion. Although rebars were removed from the specimen, no corrosion products were observed. The surface of rebar was then examined by the scanning electron micrograph (SEM). The results showed that ferrous ions on the rebar surface disappeared, suggesting that the initiation of corrosion is associated with small AE events of the shear type. Approaching the 2nd period, large-scale tensile cracks were identified by AE. Chloride concentration at the cover-thickness was higher than the threshold level prescribed in the codes. Once rebars were removed, corrosion products were visually observed. This implies that the 2nd AE activities correspond to tensile cracks, which obviously result from concrete cracking due to expansion of corrosive products. These results show that the corrosion process of rebars is identified at the onset of rebar corrosion and at the nucleation of concrete cracking by continuous AE monitoring.

Deterioration of reinforced concrete caused by corrosion of reinforcing steel under chloride environment is experimentally studied. Onset of corrosion and nucleation of cracking are estimated by acoustic emission (AE), comparing with the chloride content. Corroded areas are evaluated by the half-cell potential and the polarization resistance. To compensate the potentials, the inverse boundary element method (IBEM) is applied.Results show that AE parameters can effectively evaluate the onset of corrosion and the nucleation of cracking, which are remarkably comparable to chloride concentration. The decrease in the half-cell potential is observed, following high AE activities. It is concluded that AE technique give an earlier warning of corrosion than the half-cell potential measurement. In addition, crack types at onset of corrosion and nucleation of cracking are identified from AE parameters. Corroded areas estimated by IBEM solutions are in good remarkable agreement with those of visual inspection, although results of electrochemical techniques are marginally successful.

Corrosion of reinforcing steel is modeled phenomenologically as corrosion losses of three phases. At the 1 st phase, corrosion is initiated in reinforcement. The rate of the corrosion loss decreases at the 2 nd phase under aerobic conditions. At the 3 rd 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.

Recently， kasadaka-shi ， that is high-bulky paper or low-density paper, has been widely used for book paper and paperboard. Low-density paper is combined with chemicals, fillers or large amount of pulp that makes bulkiness of paper. NICCA CHEMICAL has led and developed a cationic type density