This paper reports an evaluation of the properties of medium-quality concrete incorporating recycled coarse aggregate (RCA). Concrete specimens were prepared with various percentages of the RCA (25%, 50%, 75%, and 100%). The workability, mechanical properties, and durability in terms of abrasion of cured concrete were examined at different ages. The results reveal insignificant differences between the recycled concrete (RC) and reference concrete in terms of the mechanical and durability-related measurements. Meanwhile, the workability of the RC reduced vastly since the replacement of the RCA reached 75% and 100%. The ultrasound pulse velocity (UPV) results greatly depend on the porosity of concrete and the RC exhibited higher porosity than that of the reference concrete, particularly at the transition zone between the RCA and the new paste. Therefore, the sound transmission in the RC required longer times than that in the reference concrete. Moreover, a predictive equation relating the compressive strength to the UPV was developed.

This paper presents two models: one for predicting the electrical resistivity of concrete and the other for the corrosion potential of reinforcing steel. The prediction models were developed based on experimental results, considering various influencing factors. The experiment approach included the concrete mix proportion, chloride content, concrete cover thickness and time of exposure as the parameters. The results revealed that fly ash concrete showed significantly high electrical resistivity even in the presence of chloride ions. The effects of fly ash became more significant when the water to binder ratio was lower. Chloride ions also decreased the corrosion potential of steel in both the OPC and fly ash concrete. The corrosion potential was found less negative for fly ash concrete due to higher electrical resistivity. The prediction results show good agreement with the experimental results of this study and some other researchers.

This study is to incorporate the time-dependent development of microstructures of cement-fly ash concrete into the evaluation of corrosion severity for newly constructed structures and existing structures. The electrical resistivity, corrosion potential, and corrosion rate were determined for twelve series of concrete mixtures prepared with various water-to-binder ratios, fly ash contents, and chloride contents. The time-dependent developments of cement-fly ash paste, including hydration degree, pozzolanic reaction degree, and capillary porosity, are adopted based on previous models. The correlation between the corrosion rate and the time-dependent developments was considered for the proposed equations. The experimental results indicate that low water-to-binder ratio concrete and fly ash concrete show a low corrosion rate of reinforcement. Because the microstructure of concrete became denser, represented by a high degree of hydration and low porosity. This phenomenon increases the corrosion resistance of concrete. Besides, the chloride ions can accelerate the corrosion rate of reinforcement. Its effects of corrosion acceleration are found to be different in fly ash concrete because the fly ash can capture free chloride ions.

The main purpose of this study was to investigate the distribution of potential of steel in patching-repaired concrete. Reinforced concrete specimens were prepared with and without patching repair, made with various concrete mix proportions. Experimental results revealed that patching repair showed good effectiveness for restoring corroded reinforced concrete structures. The highest half-cell potential of steel was observed in a repaired area (patch), surrounded by a lower potential of steel in a non-repaired area (substrate). A comparison between individual concrete and patching-repaired concrete revealed that there was a redistribution of the potential along with the steel in patching-repaired concrete. The variation of the potential depended on the parameters of concrete, which include the water to binder ratio, fly ash content, and chloride content. Finally, a numerical model to simulate the electrochemical corrosion of steel showed good agreement with the measured potential.

Electrochemical chloride extraction – ECE is an effective method to rehabilitate reinforced concrete structure, which has been corroded. This study investigated concentration of chloride remained in concrete and half-cell potential of the steel reinforcement after ECE using interrupting period of electricity current. Efficiency of ECE using Ca (OH)₂ was surveyed with two current density of 0.5 and 1A/m². In this study, ECE treatment was proceeded intermittently in approximately 8 weeks. Results pointed out that chloride concentration decreased to 30 – 60% significantly, especially at in the vicinity of reinforcing steel. Simultaneously, half-cell potential of the steel reinforcement after 4 weeks halted treatment stabilizes in low-corrosion rate.