Building Expansion Joint for Masonry Structures: Control and Expansion Joints

Masonry structures require both expansion joints and control joints to prevent cracking caused by thermal movement, moisture expansion, and differential settlement. Understanding the difference between these joint types and their correct design is essential for durable masonry construction. Expansion joints in masonry allow the masonry to expand freely under thermal and moisture loading. They are full-width joints that completely separate adjacent masonry sections, with no masonry ties or reinforcement crossing the joint. The joint is filled with a compressible backer rod and sealed with a flexible sealant. Control joints in masonry are designed to control the location of cracking caused by shrinkage and differential movement. They are partial-width joints that weaken the masonry at a specific location, causing any cracking to occur at the joint rather than at a random location. Control joints are used in concrete masonry units (CMU), which shrink after construction. Brick masonry expands due to moisture absorption after firing. This irreversible expansion is typically 0.5-1.0 mm per meter over the first 20 years. Expansion joints must be provided at intervals of 10-15 meters in brick masonry to accommodate this expansion. The joint must be sealed with a flexible sealant that can accommodate the full expansion movement. Sealant selection for masonry expansion joints must consider the movement range, the substrate material, and the exposure conditions. Polyurethane sealants provide good adhesion to masonry and good movement accommodation. Silicone sealants have better UV resistance and are preferred for exposed joints. The sealant must be compatible with the masonry material and any surface treatment. Joint width design follows the rule that the joint width must be at least three times the expected movement range to keep the sealant strain within the allowable limit. For a movement range of 5 mm, the minimum joint width is 15 mm. The joint depth must be 50-75% of the joint width, maintained by a backer rod.