What Joint Width Should Be Reserved When Splicing UHPC Panels? Dynamic Joint Design Theory and Deformation Control Practice

Determining the reserved joint width for UHPC panel splicing is a systematic design subject that integrates thermal expansion, drying shrinkage deformation, construction tolerances, and visual effects—it is by no means a fixed value. Qinglong Group's design principle is: on the minimal basis of meeting structural deformation requirements, to balance architectural aesthetics and joint material performance, and to carry out “dynamic” and “differentiated” designs. Typically, panel joint width ranges between 4mm and 12mm, determined according to specific conditions.

The calculation basis for joint width is deformation analysis. Three main types of deformation are considered: 1) Thermal deformation: ΔLt = α L ΔT, where α is the coefficient of linear expansion of UHPC (approximately 9-12×10⁻⁶/°C), L is the panel length, and ΔT is the local annual extreme temperature difference. For example, a 3-meter-long panel in an environment with an annual temperature difference of 50°C will undergo a length change of approximately 1.5-1.8mm. 2) Drying shrinkage deformation: high-quality UHPC has a relatively small long-term drying shrinkage value, usually between 0.02%-0.04%, which amounts to about 0.6-1.2mm for a 3-meter panel length. 3) Structural inter-story drift: calculated according to building height and seismic design requirements. The total reserved joint width should be greater than the sum of the above deformations, with an additional safety margin.

In actual projects, we perform refined design based on panel dimensions, installation season, and joint material performance. For example, in a large venue project in Hainan, each UHPC panel measured 1.5m×3.0m; the installation temperature was 25°C, and the maximum summer surface temperature was expected to reach 60°C, with a winter minimum of 5°C. We calculated the maximum thermal deformation to be approximately 1.7mm. The selected silicone sealant has a movement capability of ±25%. The theoretical minimum joint width is therefore: 1.7mm / 25% = 6.8mm. On this basis, allowing for a construction and installation tolerance of about ±2mm, the horizontal joints were ultimately designed at 8mm and the vertical joints at 6mm (since vertical drying shrinkage deformation is dominant). For joints of extra-long or specially shaped panels, the width is appropriately increased to 10-12mm.

Beyond width, the joint detailing is also crucial. We adopt a “pressure-equalized cavity” design, with a cavity and drainage channel set deep within the panel joints, so that even if the exterior sealant partially fails, rainwater will not penetrate into the wall interior. Closed-cell foam backing rods are placed inside the joints to ensure the sealant forms an ideal dumbbell-shaped cross-section and to control the ratio of its thickness to width. Ultimately, through precise joint design, we not only ensure the safety and watertightness of the UHPC curtain wall system under thermal cycling and wind loads, but also make the regular joints part of the rhythmic beauty of the building facade.