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Four-point bending tests were conducted up to failure on eleven reinforced concrete (RC) beams and strengthening beams to study the effectiveness of extally pouting ultra high toughness cementitious composites (UHTCC) on improving the fiexural behavior of existing RC beams. The strengthening materials included UHTCC and high strength grade concrete. The parameters,such as thickness and length of strengthening layer and reinforcement in post-poured layer, were analyzed. The flexurai behavior,failure mode and crack propagation of composite beams were investigated. The test results show that the strengthening layer improves the cracking and ultimate load by increasing the cross section area. Introducing UHTCC material into strengthening not only improves the bearing capacity of the original specimens, but also disperses larger cracks in upper concrete into multiple tightly-spaced fine cracks, thus prolonging the appearance of harm surface cracks and increasing the durability of existing structures.Compared with post-poured concrete, UHTCC is more suitable for working together with reinforcement. The load-deflection plots obtained from three-dimensional finite-element model (FEM) analyses are compared with those obtained from the experimental results, and show close correlation.