Why Is the Shear Resistance Advantage of UHPC Difficult to Reflect in Design? Research and Analysis of Qinglong UHPC Shear Characteristics

The theoretical shear strength of UHPC can reach 20-30MPa, 3-5 times that of ordinary concrete, but actual designs often fail to fully exploit it. Main reasons include: structural detailing constraints (minimum dimension restrictions), applicability of shear friction theory, and lagging design codes. Qinglong UHPC tests show that UHPC beams with a 2% reinforcement ratio have 80% higher shear capacity than ordinary concrete, but current codes do not fully reflect this advantage.

In terms of mechanism analysis, the shear advantage of UHPC comes from three aspects: high matrix strength (40% contribution), fiber bridging effect (35% contribution), and crack surface friction (25% contribution). However, the size effect is significant: when the section depth increases from 200mm to 400mm, shear strength is reduced by 30%. In a bridge application project, by optimizing stirrup spacing (densified from 150mm to 80mm) and increasing fiber content (from 2% to 2.5%), shear capacity increased by 25%, but still fell short of the theoretical value. This indicates a need for innovative detailing design.

Mismatch Between Material Properties and Structural Response

UHPC has a compressive strength of 150MPa but a tensile strength of only 8-10MPa, with a low tension-compression ratio (0.05-0.07); shear failure is brittle and must rely on fibers and steel bars for reinforcement; this mismatch limits the realization of its advantages.

Significant Size Effect

When section depth >300mm, shear strength is reduced by 25%-30%; the size effect is even more pronounced in beams without web reinforcement (40% reduction); these effects are not adequately considered in current codes.

Lagging Design Theory

Current codes are based on ordinary concrete tests; calculation theories for fiber action are imperfect; detailing provisions are overly conservative. These lags limit the realization of performance.

Room for Innovation in Detailing Measures

Optimize stirrup spacing (densified to 80-100mm); increase the reinforcement ratio (to 2%-2.5%); install shear keys. These innovations improve shear performance.

Fully exploiting the shear advantage requires theoretical innovation. Qinglong UHPC participates in code revisions to drive technological progress.