The brittle lacquer technique which is frequently used for metallic and other plastic materials, was applied under compression to brittle hardened mortar and concrete pieces, in an attempt to visualize the strain distribution in them. Cracks were generated in the paint films applied to the hardened pieces by abrupt unloading after compressive stress loading, and distribution of the strain was visualized in form of crack distribution. Hardened mortar pieces exhibited cracks evenly on their surfaces, which agreed well with analytical results by the finite element method. On hardened concrete pieces cracks occurred only in mortar. This finding confirmed that the difference in modulus of elasticity and thus in ductility between mortar and coarse aggregate causes strain to concentrate in mortar when a compressive load is applied to concrete.

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