Expansion Joints are separations provided between adjacent sections to allow movement due to dimensional increases and reductions of the adjacent sections. In pavements, slabs on grade, they are separations filled with a compressible filler material.
In this article we will cover:
- What is an expansion joint
- Expansion joints in concrete
- Control joints VS expansion joints
- Concrete expansion joint material
- Concrete expansion joint filler
- How to install concrete expansion joint
What is an expansion joint?
Expansion joints are vital components of safe and long-lasting construction for buildings, bridges, highways, pipelines, railroad tracks, and more. Similar to joints in the human body, such as the knee or elbow, an expansion joint is a purposefully-designed gap or separation between adjacent sections of a construction. The joint allows materials such as concrete, metal, or asphalt to move, expand, or contract without causing damage or stress to the materials themselves. The natural world causes expansion, contraction, and movement through temperature fluctuations, settling, seismic activity, and other natural forces; thus the expansion joint is designed and built for the purpose of providing the necessary flexibility to withstand those forces. Expansion joints prevent cracks, structural damage, or material failures, creating higher integrity and better longevity for any construction project.
Expansion joints in concrete
Concrete is a composite material created from aggregate, cement, and water. The chemical reactions of the composition allows the aggregate materials to bond together and harden over time. This creates a solid and hard material with high compressive strength, making it a primary choice in construction materials. However, despite the overall durability of concrete, its amalgamated nature means that fluctuations occur naturally. Even if conditions of materials, proportions, air content, mixing speed and technique, moisture content, and air temperature remain absolutely constant and optimal during the concrete-making process, the natural world will impart various forces onto the finished product. Temperature, moisture, and movement variations have the potential to cause cracking, buckling, or other structural damage from weaknesses inherent to even one of the strongest building materials.
Expansion joints are used to prevent damage to concrete from these shifting forces. By planning for and installing expansion joints, builders prevent uncontrolled cracking by creating predetermined locations for movement to occur, and the expansion joint is an assembly that allows for that movement. Expansion joints are also used to ensure an even load distribution across concrete slabs, and they are particularly important for road construction, where constant exposure to the sun creates an abundance of thermal fluctuation. Some of the largest expansion joints in concrete are used in bridge construction for highways, such as in the stretch of Interstate-70 that passes through Glenwood Canyon in Colorado, or the marvel of the Seven Mile Bridge in the Florida Keys.
Control joints versus expansion joints
Although control joints are also building techniques used to manage movement in structures built from concrete, they have some key differences and applications when compared to expansion joints. Control joints are controlled cuts made at regular intervals across a surface of slab poured concrete. These joints are generally cut to the depth of approximately one-fourth of the thickness of the slab of concrete, and the cut is typically performed while the concrete is still wet. As their name suggests, control joints are specifically utilized to control the locations where cracks occur as the concrete shrinks while it dries. Not only does this create greater longevity in the concrete surface, but also it allows for a more uniform, flatter appearance to a poured slab.
The application of these control joints differs from expansion joints in that control joints are used to protect the concrete from changes while the material is drying, while expansion joints prevent damage from environmental factors and forces after the curing process is complete. In other words, expansion joints create space for movement in an otherwise hard surface, so that weaknesses in the material do not cause cracks from movement later.
Concrete expansion joint material
The material used for an expansion joint in concrete varies based on budget and application considerations. There are six main materials used for concrete expansion joints: silicone, rubber, foam, asphalt, fiberboard, and cork.
- Silicone is a specific type of polymer called an elastomer, with a wide variety of uses across many industries. When used in a concrete expansion joint, the silicone creates a highly-flexible and UV-resistant assembly, which makes it perfect for vertical or angled concrete surfaces, or areas exposed to direct sunlight.
- Rubber is an ancient polymer building material, first used by the indigenous cultures of Mesoamerica. It is known for its flexibility, durability, and most important for concrete expansion joints–its weather-resistance. Because rubber has a unique impact-absorbing quality, it is often used for expansion joints in concrete bridges, highways, and industrial floors.
- Polyurethane Foam was formulated in 1937 as a lightweight and thermo-regulating material, Most commonly used for building insulation, manufacturers have also realized polyurethane foam can be formulated to use for concrete expansion joints. The foam is compressive, which gives it cushioning properties perfect for sidewalks, driveways, and building foundations.
- Asphalt joins concrete as one of the most common roadway building materials in the modern world, notably for its durability and ability to withstand thermal expansion and heavy loads. When used as the material for expansion joints, the asphalt creates greater flexibility between concrete slabs, thus improving the longevity of a concrete highway.
- Fiberboard is an engineered wood product made from compressed and chemically altered wood particles. Although it may seem counterintuitive to insert a wood product between concrete slabs, specially-treated fiberboard makes for a lightweight, rigid, weather-resistant, and easy-to-handle expansion joint. Treated fiberboard is best used in flat surfaces between concrete slabs, such as driveways, sidewalks, and patios.
- Cork, like rubber, is another ancient material, having been used by humans for over 5,000 years. Cork is buoyant and impenetrable due to its innate cellular structure. Its water-resistant nature makes it a popular choice for expansion joints in concrete foundations; although technological advancements are causing synthetic cork to outpace the use of natural cork products.
Concrete expansion joint filler
Even the most durable of the above materials degrade over time, which means restoration to expansion joints is an occasional necessity. To fill a compromised concrete expansion joint, a multi-step process is required.
- Clean – Start by removing the existing expansion joint and removing all remaining residue using wire brushes, metal scrapers, and compressed air. It is vital to have a clean canvas to ensure adhesion of the new joint filler material.
- Prep – After the joint has been cleaned, it may be necessary to install a backer rod, depending on the type of expansion joint material chosen.
- Fill – If the expansion joint will be made from a pre-formed material like silicone or rubber, ensure that the joints are measured precisely to fit the existing space. If using a liquid substance like asphalt or foam, the installation must be monitored to ensure a complete and even coverage.
- Seal – After the expansion joint material has been filled, a sealant must be applied to provide additional layers of protection to protect the materials over time.
- Inspect and monitor – A certified professional will use various tools and techniques to measure the success of a concrete expansion joint filler installation. This step is essential to setting a strong foundation (pun intended!) for the life of the material.
How to install concrete expansion joint
Concrete expansion joints are complex construction contraptions; therefore their installation is a process best completed under the guidance of professionals. The process for installing a new concrete expansion joint is the same as the process listed above, with the exception of needing to remove the old joint first. A contractor who specializes in installing concrete will have the multitude of tools and techniques to complete the job properly, including chosen the appropriate material for the expansion joint, depending on what type of concrete slab or brick is being used and how.
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