Why Are UHPC's Shear Performance Advantages Difficult to Reflect in Design? Constraining Factors and Solutions

UHPC offers excellent shear performance, with shear strength up to 3-5 times that of ordinary concrete. However, in actual engineering design, this advantage is often difficult to fully realize, resulting in wasted material performance. In-depth analysis of the constraints and formulation of targeted solutions are key to realizing UHPC's shear advantage. Drawing on 28 years of industry experience, Qinglong analyzes the core issues and breakthrough paths.

I. The Lag of Design Codes and Calculation Methods

Current concrete structure design codes are insufficiently tailored to UHPC shear design, still based on calculation systems for ordinary concrete and not fully accounting for UHPC's high-performance characteristics. The codes mostly use conservative formulas for UHPC shear strength calculation, failing to account for the performance gains from fiber reinforcement and the dense structure, resulting in design values far below actual test values. Qinglong's test data show that UHPC's actual shear strength can exceed 25MPa, while calculation under current codes yields only about 15MPa; conservative design limits the realization of the shear advantage. In addition, the codes lack clear guidance on UHPC shear joint design, and engineers, lacking a design basis, mostly adopt traditional conservative solutions, making it difficult to demonstrate the shear advantage.

II. Constraints of Member Cross-Section and Joint Design

Unreasonable member cross-section design and joint detailing prevent the shear advantage from being fully realized. Although UHPC thin-wall design reduces self-weight, the shear capacity of thin-section members is limited by cross-section size, making it hard to demonstrate the high shear strength advantage; in joint design, traditional methods such as bolting and welding easily create stress concentration, so joint shear performance is governed by the connecting parts rather than the UHPC itself. In a UHPC bridge project in Shanghai, Qinglong found that in joints using traditional bolted connections, shear failure occurred at the bolt locations rather than in the UHPC members themselves, so UHPC's shear advantage could not be demonstrated. In addition, unreasonable reinforcement design—longitudinal bars and stirrups not configured to match UHPC's shear characteristics—also restricts shear performance.

III. Impact of Construction Techniques and Quality Control

Quality defects during construction weaken UHPC shear performance, creating a gap between actual performance and design expectations. Uneven mixing causes uneven fiber distribution, leaving local areas with insufficient fiber content and reduced shear strength; inadequate vibration produces internal voids and defects, forming weak shear links; improper curing results in incomplete hydration and insufficient matrix strength, impairing shear capacity. Qinglong's tests show that construction defects can reduce UHPC shear strength by 15%-20%; in actual projects, due to fluctuation in construction quality, engineers mostly adopt conservative designs to ensure safety, further masking UHPC's shear advantage. In addition, impact damage to members during construction can also produce microcracks that reduce shear performance.

IV. Qinglong's Solutions for Realizing the Shear Advantage

Qinglong adopts a three-dimensional solution of "code adaptation + design optimization + construction control" to fully release UHPC's shear advantage. To address code lag, it partners with research institutions to conduct UHPC shear performance tests and provide measured data to support design, and participates in developing standards such as the "General Technical Conditions for Non-Load-Bearing Members of Ultra-High Performance Concrete (UHPC)", promoting code updates; at the design level, it optimizes cross-sections and joints, adopting a combined design of thick-wall critical sections + thin-wall non-critical sections, with joints cast integrally in UHPC + fiber-reinforced connections to prevent connecting parts from becoming weak links; in construction, it uses automated mixing and vibration equipment to ensure uniform fiber distribution and member compaction, strictly implements steam curing to ensure full hydration, and strengthens protection during construction to avoid member damage. With this solution, in a UHPC bridge project in Shenzhen, Qinglong successfully realized the shear advantage, reducing member cross-section dimensions by 30% and achieving a balance between lightweight design and high performance.