Why Does a Permanent Movement Joint Require Flexible Polyurethane Joint Sealing?

A permanent movement joint is designed to keep moving throughout the life of a building. As the structure expands, contracts or shifts, the joint opens and closes with it. That’s why it needs a flexible sealant that can handle repeated movement without cracking or losing its bond.

Polyurethane sealant is commonly used for expansion joint sealing and waterproofing because it offers the flexibility, strength and durability needed to keep these joints protected.

Why Do Movement Joints Move?

Buildings are constantly exposed to conditions that cause materials to move. Some of the most common reasons include:

• Temperature changes: Concrete, masonry, metal and other building materials expand in the heat and contract when temperatures drop. Different materials also move at different rates.
• Moisture and shrinkage: Concrete naturally shrinks as it cures and dries, while some materials expand or contract depending on moisture levels.
• Structural movement: Building settlement, wind, vibration and everyday loads can cause adjoining surfaces to shift.

These movements happen repeatedly, so the joint sealant needs to withstand ongoing stretching and compression rather than just a single movement.

Why Is Polyurethane Sealant Suitable for Movement Joints?

Flexible polyurethane sealant is a popular choice for concrete joint sealing, expansion joint repairs and waterproofing applications because of its ability to accommodate movement while maintaining a strong seal.

1. Flexibility to Handle Ongoing Movement: Many polyurethane sealants can accommodate movement of around ±25% of the joint width, with some products offering greater flexibility. This allows the sealant to stretch and compress as the building moves, reducing the risk of cracking or splitting.
2. Ability to Return to Its Original Shape: Polyurethane is an elastic material, meaning it can recover its shape after being stretched or compressed. Unlike rigid fillers that may crack under pressure, a suitable polyurethane sealant can continue performing through repeated movement cycles.
3. Strong Adhesion to Different Surfaces: Polyurethane sealants can bond well to concrete, stone, masonry and certain metals when the surfaces are properly prepared. Good adhesion is important because it prevents the sealant from pulling away from the sides of the joint, which is a common cause of leaks and joint failure.
4. Protection Against Water Leaks: Movement joints are common areas for water to enter a building, especially when the existing sealant has cracked or deteriorated.  Proper expansion joint waterproofing helps prevent water penetration, moisture damage and leaks through concrete joints. This is particularly important for balconies, concrete slabs, car parks and other exposed areas.
5. Durability in Exposed and High-Traffic Areas: Certain polyurethane sealants offer good resistance to abrasion, tearing and everyday wear.

Traffic-grade products are often used for driveway joint sealing, concrete floor joints, car park expansion joints and commercial waterproofing applications where the sealant needs to withstand regular use.

Why Can’t You Use Rigid Fillers for Movement Joints?

Materials such as mortar, grout and hard-setting fillers aren’t designed to stretch with building movement.

When the joint opens, these materials can crack or separate from the surrounding surface. Once that happens, water can enter through the gaps and cause further damage. Rigid fillers can also restrict the natural movement of adjoining surfaces, potentially leading to cracking in nearby concrete or masonry.

Using a suitable flexible joint sealant allows the building to move as intended while keeping the joint sealed.

Getting the Best Results from Polyurethane Joint Sealing

Even a high-quality sealant can fail if the joint isn’t prepared or installed correctly.

For reliable, long-lasting results, a few important steps need to be followed:

• Install a backer rod: This helps control the sealant depth and prevents it from bonding to the bottom of the joint, allowing the sealant to stretch properly.
• Maintain the correct joint dimensions: A width-to-depth ratio of around 2:1 is common for many applications, although the manufacturer’s recommendations should always be followed.
• Prepare the surface properly: Remove dust, dirt, moisture where required, and old sealant before applying the new product. Use a primer when specified.
• Choose the right sealant: Consider the amount of movement, UV exposure, foot or vehicle traffic, chemicals and water exposure.

Proper preparation and application are essential for effective movement joint sealing and long-term waterproofing protection.