Influence of curing time on chloride penetration and binding capacity in concrete exposed to chloride solutions

This study experimentally analyzed the effects of curing period, fly ash (FA) mixing, and paste amount changes on chloride penetration and bonding capacity to improve the durability of concrete structures exposed to coastal environments. The chloride binding capacity was analyzed by focusing on the immersion time and selecting two representative factors affecting the binding capacity: the use of cementitious binder and the change in paste volume. Concrete specimens were subjected to a curing period of 28 and 91 days, followed by immersion in an aqueous chloride solution. At predetermined intervals of 28, 56, and 91 days after immersion, chloride penetration was measured. To assess the chloride binding capacity, five samples were collected at a depth of 10 mm from the surface and analyzed for acid soluble and water soluble chloride at each depth. The amount of acid soluble chloride on the surface of the concrete was used to determine the chloride penetration of the concrete. The 20 % fly ash concrete (NF20) had 15 %–20 % less chloride penetration than the OPC concrete (C100) with the same paste volume when data from all ages and immersion periods were analyzed. A 6 % increase in paste volume resulted in a 3.9 %–11.8 % decrease in chloride penetration. Additionally, the amount of acid soluble chloride at the concrete surface was found to be 15 %–20 % lower in concrete with 20 % fly ash compared to OPC concrete, with the same paste volume. A 6 % increase in paste volume resulted in a 3.9 %–11.8 % decrease in chloride penetration. The chloride binding capacity with depth from the surface of the concrete was reduced from 0.52 to 0.38 based on the average immersion time of the 28-day-old concrete, as determined by the concrete surface data. A similar trend was observed in 91-day-old concrete, which exhibited a decrease from 0.68 to 0.39. Furthermore, it was observed that the chloride binding capacity at measurement points in close proximity to the concrete surface underwent a gradual decline as the immersion time increased. In support of this, the chloride binding capacity data for the four formulations immersed for 28 and 91 days exhibited a decreasing trend in variance, with the variance decreasing in the order of increasing immersion time and distance from the surface. For a total chloride content of less than 1.0 %, the 91-day cured concrete exhibited a higher chloride binding capacity compared to the 28-day cured concrete. Conversely, for a total chloride content greater than 1.0 %, the 28-day cured concrete demonstrated a comparatively higher chloride binding capacity. • Increased curing age improved chloride penetration resistance and binding capacity. • Binding capacity decreased with longer immersion periods and at greater depths. • Total chloride and bound chloride contents exhibited a linear relationship. • Higher paste volume led to increased chloride penetration, while 20 % fly ash reduced penetration by 15–20 %. • The C100 mix, with higher C 3 S content, showed enhanced chloride binding despite higher chloride penetration.