Influence of aggregate concentration on gas diffusion coefficient in concrete with various water-cement ratios was obtained under the various relative humidities. The relationships between effective diffusion coefficient and relative humidity in mortar or concrete had construct of three provinces, similar to hardened cement paste. The effective diffusion coefficients were small affected by aggregate concentrations, except for under the absolute dry condition. Using to these results, the gas diffusivity in transition zone was discussed by the theory of composite material with hardened cement paste, aggregate and transition zone.
