Cylindrical concrete specimens were loaded uniaxially. Computer tomography scanning technique was used to examine the evolution of air voids inside the specimens at various loading states up to 85% of the ultimate compressive strength. There was an excellent correlation between the measured nonlinear stress-strain response of the concrete specimen and the total number of air voids generated inside the specimen. All air voids were grouped according to varying sizes. It was detected that each air void size group had its own characteristics in nucleation and growth. The air void initiation and growth were concentrated at air voids of small sizes with high population. The large-size air voids were fairly stable having a high fracture toughness resisting fracture propagation.
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