What is the Reserved Width of Expansion Joints for UHPC Exterior Wall Mouldings?

The reserved width of expansion joints in UHPC exterior wall profiles (such as window surrounds, belt courses, and cornices) is a key detailing design that enables them to accommodate thermal deformation and avoid crushing cracks or pull-out failure. The width is not a fixed value; it must be calculated and determined based on “profile length, local extreme temperature difference, UHPC coefficient of linear expansion, and installation fixing method”, while also taking visual aesthetics into account. Qinglong Group’s design principles are “calculation fixes the amount, detailing fixes the width, and sealing fixes the form”, controlling joint width on the premise of ensuring safe deformation. In the construction of UHPC window surround profiles for a high-end residential project in Shenyang, we set 15mm expansion joints for profiles longer than 3 meters based on the local annual temperature difference of 70℃, and filled them with high-elasticity sealant. After withstanding several winters and summers, they remain intact.

Calculation Basis: Determining the Thermal Deformation ΔL

This is the theoretical basis for determining the joint width. The formula is: ΔL = α L ΔT. Here, α is the coefficient of linear expansion of UHPC, taken as 1.0×10⁻⁵/℃; L is the segment length of the profile (m); ΔT is the local annual extreme temperature difference (the difference between the highest and lowest temperatures, which must account for the surface temperature rise of the material caused by solar radiation, typically 20-30℃ higher than the air temperature). For example, for a 4-meter-long profile in a region with an annual temperature difference of ΔT=60℃, the theoretical expansion is ΔL = 1.0e-5 4 60 = 2.4mm. This is the minimum deformation that must be accommodated.

Determining the Design Joint Width W

The design joint width W must be greater than ΔL, with margin left for construction tolerances and the deformation capacity of the sealant. Qinglong’s empirical formula is: W ≥ ΔL / ε + δ. Here, ε is the allowable movement rate of the selected sealant (usually ±25%, i.e. 0.25); δ is the construction and installation tolerance (usually taken as 3-5mm). Substituting the above example, if ε=0.25 and δ=4mm, then W ≥ 2.4/0.25 + 4 = 9.6 + 4 = 13.6mm, rounded up to 15mm. This is the minimum width that meets the functional requirements.

Visual Proportion and Detailing Coordination

While satisfying the calculated width, the joint width must be coordinated with the cross-sectional dimensions of the profile and the facade division proportions of the building. Joints that are too wide may compromise aesthetics. Sometimes it is necessary to reduce ΔL by “rationally dividing the profile length L”, thereby narrowing the joint. For example, by designing a decorative segment in the middle of a 6-meter-long profile, turning it into two 3-meter segments, the ΔL of each segment is halved, and the joint width can be reduced accordingly. Qinglong’s designers take both structure and aesthetics into comprehensive consideration.

Joint Detailing and Sealing

The expansion joint must be filled with a closed-cell foam rod as backing, then injected with weather-resistant silicone sealant. The sealant must bond well with UHPC, and its movement capability must be ≥25%. At special locations such as internal and external corners, the joint treatment requires particular care to prevent cracking. A scientifically designed expansion joint is the “safety valve” that keeps UHPC exterior wall profiles intact and attractive over the long term.