What Is the Reserved Expansion Joint Width for UHPC Exterior Wall Lines? Thermal Deformation Calculation and Dynamic Joint Width Design

The reserved width of expansion joints for UHPC exterior wall profiles (such as window trim lines, belt courses, and cornice lines) is not a fixed value, but a “dynamic value” determined through scientific calculation based on the profile length, local climate temperature variations, installation season, and the thermal expansion coefficient of UHPC itself. The design principle of Qinglong Group is: while accommodating thermal deformation and drying shrinkage deformation, to balance aesthetics with the performance of sealing materials, typically setting the expansion joint width between 8-15mm. Blindly unifying joint widths or insufficient allowance are common causes of profile compression deformation or cracking.

The core formula for calculating joint width is: ΔL = α L ΔT. Here, ΔL is the thermal deformation (mm), α is the linear expansion coefficient of UHPC (approximately 9-12×10⁻⁶/℃), L is the segment length of the profile (mm), and ΔT is the maximum temperature difference the profile may experience after installation (℃). ΔT is usually taken as the difference between the extreme temperature the profile surface may reach and the temperature at installation. For example, for a 6m-long UHPC window trim line in a northern region, its surface may reach a summer maximum of 70℃ and a winter minimum of -20℃, giving an annual temperature difference of ΔT=90℃. Its theoretical thermal deformation is then ΔL = 10×10⁻⁶ 6000 90 = 5.4mm. Adding drying shrinkage of about 0.02% (approximately 1.2mm), the total deformation is about 6.6mm.

The movement capability of the selected sealant (usually silicone sealant), such as ±25%, determines the minimum joint width. Minimum theoretical joint width = total deformation / sealant movement capability = 6.6mm / 25% = 26.4mm. However, this value is too large and unsightly, so the practical approach is to divide long profiles into segments to shorten the L value. If a 6m profile is divided into two 3m segments, ΔL is halved, and the calculated minimum joint width is about 13mm. Qinglong Group usually adds a construction tolerance margin of 2-3mm on this basis, setting the joint width at 15mm. Meanwhile, depending on the cross-sectional shape and restraint conditions of the profiles, we set expansion joints at locations such as the middle of straight sections, filling the joints with closed-cell foam backer rods and applying weather-resistant sealant on the outside. Only through such scientific dynamic design can UHPC exterior wall profiles be ensured to expand and contract freely under seasonal temperature variations, remaining safe and durable.