Application of post-casting belt in deformation joint

Application of Post-Casting Belt in Deformation Joints Post-casting belts are an effective solution for managing deformation joints in construction, especially in complex structures like high-rise buildings and their adjacent podiums. These belts help address key challenges such as differential settlement and temperature-induced shrinkage. One of the main purposes of a post-casting belt is to solve the issue of settlement differences between different parts of a building. The high-rise structure and the podium are often designed as a single unit, but during construction, they are temporarily separated. Once the main structure is completed, most of the settlement (typically over 50%) has already occurred. At this point, the two sections are connected by pouring concrete into the post-casting belt, allowing them to function as one integrated system. During the design phase, engineers must account for the different stress conditions in both stages and perform separate strength checks. After integration, additional internal forces caused by late settlement must also be considered. This approach works best when the foundation soil is strong and stable, ensuring that most of the building's settlement occurs before the post-casting belt is filled. Another important function of the post-casting belt is to reduce the impact of temperature-induced shrinkage. Freshly poured concrete undergoes shrinkage during the curing process, while existing structures expand when heated and contract when cooled. Most of the shrinkage happens within the first 1-2 months after construction. Temperature changes can cause significant stress within the structure, especially when movement is restricted, leading to cracks. By installing post-casting belts every 30–40 meters, with widths of 700–1000 mm, the steel reinforcement in the joint can be lapped or continuous, allowing the concrete to shrink freely. This helps minimize shrinkage stress and allows the tensile strength of the concrete to better resist temperature-related stresses. The retention period for the post-casting belt is usually at least one month, during which about 30–40% of the shrinkage is completed. It’s recommended to pour the concrete in the post-casting belt during cooler periods, though temperatures should remain above freezing. A special type of concrete—often with a small amount of aluminum powder—is used to improve workability and reduce early cracking. The strength grade of the new concrete should be higher than the surrounding structure to prevent weak points and potential cracks between the old and new sections. The success of a post-casting belt depends on the quality of the foundation soil. The soil should have high bearing capacity and low compressibility. The location of the post-casting belt should be chosen where the bending moment in beams or slabs is minimal. Beams, in particular, require careful consideration, as they are critical load-bearing elements. The exact placement should also take into account the specific characteristics of each project. Every building is unique, and the decision to use deformation joints—or how to implement them—must be based on the specific construction requirements, structural design, and environmental conditions. A simple and practical approach is always preferable, aiming for optimal performance and long-term durability. Among all types of deformation joints, the settlement joint remains the most crucial. This article shares some of the author’s experience in dealing with settlement joints over the years. By summarizing past practices, it is hoped that future structural designs will incorporate even more refined and effective methods for handling deformation joints.

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