The objective of this research is to establish the deformational compatibility for aggregate phase of concrete in bend pipes used for concrete pumping. The deformational compatibility, which describes the relationship between the strain rate of aggregate phase (particle assembly) developing over a reference section and sectional mean flow speed of the particle assembly, is essential in one-dimensional computation of dense liquid-solid flow in pipe lines. Flow visualization and image processing on model concrete were carried out for development of the mathematical description of the deformational compatibility for aggregate phase in bend pipes, in which the second strain invariant of aggregate phase was adopted as a main parameter associated with stress generation due to particle-to-particle interactions. The authors have proposed a simplified compatibility formulation reduced to one-dimensional condition and applicable to dense liquid-solid flow in bend pipe portions, by relating the sectional averaged invariant of strains to the mean flow speed of solid phase and the curvature of bend pipes.