The measurement for expanding of interfacial crack under various uniaxial compressive load has been done in laboratory. It shows that the interfacial crack will first apear at the bottom of aggregate and then rapidly expand. The stress level, at which a sudden chang of increasing rate of crack width will take place, corresponds to initial stress σ i, a distinguishing characteristic of macro-mechanical behaviour of concrete. And the stress level, at which the increasing rate of crack width on top of coarse aggregate is up to extremely large, corresponds to critical stress σ cr , another distinguishing characteristic of concrete. It is found that the practical size of coarse aggregate has an obvious effect on critical stress. The smaller the partical size, the higher the critical stress.Therefore, in concrete construction the smaller coarse aggregate should be used.
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