The Importance of Expansion Joints in Buildings
By: Jimmy Monahan
An expansion joint, also known as a movement joint, is an assembly consisting of a separation in a wall and a flexible material such as a sealant or bond breaker. Bond breaker materials might include: liquids, sprays, rods or tape. They are required to break the bond between building sections to ensure the sections can separate. Since the material is compressible, it can accommodate the movement of adjacent materials. A flexible sealant is applied to close the joint opening and mitigate moisture from entering the joint, along with accommodating movement between wall sections.
Adjacent materials in a wall assembly are affected by temperature differentials, moisture infiltration and stress, which causes movement between wall sections. Temperature movement is the thermal expansion and contraction of building materials and is very common in areas that experience seasonal climate changes. A long masonry wall will expand or contract over its height and length when heated or cooled by ambient temperatures. The individual masonry units will elongate when heated and deform when cooled. The changes in the height and length of the wall will create internal stress within the wall. If the stress is not relieved, cracks will develop.
Elastic deformation is a temporary change in length, volume or shape of a material under stress. Vertical loads, such as dead and live loads, will create stress in building materials. A dead load is the weight of the structure or building on itself. Since they are permanent, the materials that comprise the building are considered dead weight. Live loads are not fixed or permanent, but rather can be variable or moving. Examples of live loads are people, materials, office equipment, and furniture or shelving that is not bolted down. Wind, seismic activity and snow are other loads that cause building materials to deform and deflect in length, volume and shape. A diving board is a good example of this. Imagine a person standing on the edge of a diving board. The board itself is the dead load (self-weight), and the person is the live load. When the person is on the edge of the diving board, you can see the deflection in the board as it dips towards the pool. It is deforming or bending due to the weight of the person. The bigger the person, the greater the deformation, and vice versa. This diving board also experiences variable deformations as the person walks up and down the board, and the deformations become exaggerated when the person jumps up and down on the diving board. These deformations create stresses within the material of the diving board, similar to those affecting building material.
Moisture movement is caused by materials expanding and contracting from increases or decreases in moisture content. Materials such as masonry block, concrete and wood will expand when saturated with water, and return to their original state after drying out. Imagine these building materials as a dry sponge when they are first installed. When the materials are saturated through rain events or leaks, they will expand and grow in size like a dry sponge absorbing water. Similarly, when the material dries out, it will decrease in size. These variations in size create stress within the material, and any abutting material. If these increases and decreases within a wall are not accounted for, saturation and drying cycles will eventually cause the material to fail.
If the properly designed expansion joints are not installed to accommodate the movement of building materials, façade elements will crack and spall. Longer walls with more material will undergo more movement. Additionally, the walls at building corners are extremely susceptible to the deleterious effects of movement. Corner walls are connected at a perpendicular angle and move within two different planes. One wall will move in an east-to-west direction while the other moves in a north-to-south direction. If expansion joints are not designed and installed in these locations, extensive vertical cracks will result from the walls accommodating the movement and alleviating internal stress.
Elastic deformation, thermal expansion/contraction and moisture movement within building walls must be accounted for during design. Therefore, it’s essential that building owners retain exterior restoration or building envelope consultants to inspect their buildings, design repairs, oversee construction, monitor implementation and ensure quality control. Regular maintenance, and periodic scheduled inspections by a building envelope professional, should also be performed to improve the lifespan of a building and ensure public safety.