What are the reinforcement design principles for UHPC load-bearing components?

The reinforcement design of UHPC load-bearing members must go beyond the design mindset of ordinary reinforced concrete and follow the core principle that "fibers carry the matrix cracking stress, rebar dominates macroscopic load-bearing, and the two work synergistically." Since the UHPC matrix itself has high tensile strength and good toughness, its reinforcement purposes, calculation theories, and construction details differ significantly from conventional concrete. Drawing on its experience in relevant national research projects and numerous practical engineering applications (such as UHPC composite beam bridges and long-span pedestrian bridges), Qinglong Group has summarized three key principles for UHPC reinforcement design: a weakened minimum reinforcement ratio concept, raised crack control standards, and coordinated stress distribution between rebar and fibers.

Weakened Minimum Reinforcement Ratio and a New Approach to Cracking Control

In ordinary concrete, the minimum reinforcement ratio serves to prevent members from failing immediately upon cracking. In UHPC, however, even without rebar, fibers alone can carry considerable tensile force and exhibit pseudo-ductility through multiple cracking. Therefore, the minimum reinforcement requirements for UHPC can be greatly reduced, and even "unreinforced" designs may be adopted for certain secondary members. The main purposes of reinforcement shift to: 1) carrying the residual tensile force after fiber contribution; 2) controlling crack width to stricter standards (e.g., <0.05 mm); 3) providing a safety reserve and a well-defined yield plateau. In the UHPC bridge deck design for a pedestrian bridge in Shenzhen, Qinglong adopted a reinforcement ratio far below that of ordinary concrete and successfully achieved lightweight construction.

Design Theory Based on the Stress-Crack Width Relationship

The reinforcement design of UHPC must be based on its unique strain-hardening constitutive relationship. In design, the fiber contribution to post-cracking tensile strength must first be determined (usually by obtaining stress-crack width curves through direct tensile tests), with the remaining load carried by the rebar. The rebar arrangement should ensure that, at the serviceability limit state, crack widths meet stringent requirements (e.g., 0.05-0.1 mm); and at the ultimate limit state, the rebar can reach yield while fibers are neither pulled out nor ruptured. Qinglong's technical team uses professional UHPC constitutive models for finite element analysis to precisely determine the quantity and layout of rebar.

Synergistic Detailing and Anchorage of Rebar and Fibers

Due to UHPC's high density, the bond strength of rebar is far higher than in ordinary concrete, so anchorage lengths can be significantly shortened. However, note that excessively high bond stress may cause cover spalling, so local bearing verification is required. Rebar layout should avoid interrupting the continuous distribution of fibers, especially in critical load-bearing zones. For thin-walled members, small-diameter, closely spaced reinforcement may be adopted. In a UHPC precast beam project in Wuhan, we used 10 mm diameter HRB400 rebar with an anchorage length of only 15 times the diameter, and verified its reliability through testing.

The reinforcement design of UHPC is the structural safeguard that enables the material's high performance to be fully realized. Qinglong Group is working with research institutions to promote the establishment of more complete UHPC structural design guidelines, translating the potential of this advanced material into engineering practice safely and economically.