Relationship between chloride migration coefficient and pore structures of long-term water curing concrete

The chloride migration coefficient is a useful index for quality control and durability design of reinforced concrete. This paper explored the relationship between chloride migration coefficient and pore structures of long-term water curing concrete. Low-field nuclear magnetic resonance (NMR) and water absorption were used to probe the total porosity and pore sizes distribution. An appropriate dosage of supplementary cementitious materials reduced the total porosity and increased the proportion of capillary pores in the concrete. Fly ash, slag and silica fume effectively increased the capillary pores in the concrete. The capillary pores of 0.35 water to cement ratio concrete with 25% slag was 20% higher than that of the 0.35 water to cement ratio plain concrete at 415 days. The capillary pores of 0.45 water to cement ratio concrete with 25% slag was 32% higher than that of the 0.45 water to cement ratio plain concrete at 415 days. The big pores in the concrete were significantly reduced. According to our analysis, pores were divided into four parts: 0 ∼ R12, R12 ∼ R23, R23 ∼ 50 μm, ≥ 50 μm. R12 and R23 were the sequential minimum distribution frequency pore sizes in the pore size distribution figures tested by the Low-field NMR method. The chloride migration coefficient was dependent on the porosity which included the pore size over R23 and the pore radius R3. R3 was the biggest pore size among several peak frequency pore sizes in the figure of pore size distribution. Their relationship was non-linear. This meant the controlling factors of chloride migration in the concrete were larger capillary pores, voids and the porosity made up of these large capillary pores and voids. The persistent evolution of pores determined the permeability of concrete in late age.