Ultrasonic Detection of Chloride Ions and Chloride Binding in Portland Cement Pastes

Chloride ions diffuse through the concrete cover and interact with the cement hydration products. As a result, some chloride ions become chemically and/or physically bound. Free chloride ions are the primary cause of steel corrosion in reinforced concrete structures. In this study, ultrasound was used to detect the presence and binding of chloride ions in cement pastes that contained supplementary cementing materials. Four cement pastes with w/c ratio of 0.55 were prepared and cast into cylindrical specimens that were moist cured for 254 days before being oven dried at 105 °C. The dried specimens were vacuum-saturated with NaCl solutions at various concentrations. Through-transmission ultrasonic measurements were performed as a function of time using 500 kHz longitudinal wave transducers. The results indicated exponential relationships between energy/amplitude weighted average frequency and the amount of chloride chemically bound by the cement pastes.