Based on the excellent mechanical properties of UHPC such as high strength and good toughness, UHPC is used to replace part of NC in the mid-span of beams to form an UHPC-NC composite beam, a design that optimizes both the cost and mechanical properties of the composite beam. The effect of the ratio of UHPC length to calculated span (Lu/L) on the performance of the composite beams in terms of crack load, peak load, mid-span deflection and stiffness was investigated by four-point flexural tests. The study shows that (1) the peak load of full UHPC beam (UB) is 40.2% higher than that of the NB beam, and peak loads of composite beams UN-HB1, UN-HB2 and UN-HB3 can reach 79.8%, 97.6% and 98.7% of the peak load of the UB beam, respectively. (2) Composite beams with larger Lu/L have better deformation capacity, the ductility of UN-HB1 is 22.1% higher than that of NB. (3) The stiffnesses of composite beams UN-HB2 and UN-HB3 increased by 31.7% and 34.2% beam NB, and they were able to reach 83.2% and 84.3% of the stiffness of UB beam. (4) UHPC length ratio of 50% (Lu/L = 0.5) enables the composite beam to achieve satisfactory flexural performance. A calculation model is derived based on variable stiffness non-equal section beams and immediate stiffness, which can accurately predict the mid-span deflection of the composite beams.
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