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Examining the Three Types of Joints in Concrete

Examining the Three Types of Joints in Concrete

One of the types of joints in concrete.

Isolation, control, and cold joints do different things for your concrete floor.

Key Takeaways:

  • Isolation and control joints are intentional installations
  • These joints reduce damage to a concrete slab as the surface dries
  • Cold joints are accidental weaknesses that appear between two concrete layers
  • A concrete expert can sometimes repair cold joints

It should come as no surprise to learn that cracks sometimes appear on concrete floors. These imperfections can be unsightly, but they also sometimes blend into the floor’s aesthetics and create a unique, distinguished look. 

Your flooring contractor will intentionally create some of these crevices or joints to limit damage to the slab over time. In other situations, random cracks will appear that require repairs to keep the floor looking great. 

Learning about the various joints in concrete floors ensures you understand their purpose and how to handle them as they emerge. Here’s a look at the three types of joints in concrete you could encounter as you move through your project.

What are isolation joints?

Isolation joints have a specific purpose, as they ensure that a concrete slab can separate in particular locations throughout the drying process. They also allow for movement as the base under the floor shifts. Without these isolation joints, the concrete could crack as it dries and shrinks, leaving you with significant damage to repair. 

Concrete shrinks as it dries, so you can’t expect everything to stay in one place. Although concrete reaches full strength in about 28 days, it continues curing indefinitely, so there’s no end to the window when it could crack.

Your contractor will typically install isolation joints where your concrete floor meets another element, like a wall, pipe, or post. Walls have an independent footing that’s deeper than a concrete slab, so they don’t move the same way. Therefore, cracking is likely to occur when isolation joints aren’t present. A floor’s movement could also damage any pipes or drains touching it, so isolation joints are necessary. 

An isolation joint should extend the entire depth of the slab down to its sub-base. Your contractor might then apply a joint filler to give the concrete a more uniform appearance. It might sound counterintuitive to deliberately create separation in a concrete floor, but the result is a more durable surface that will experience fewer issues.

An overview of control joints

Control joints have a purpose similar to isolation joints, as they aim to prevent cracks and other imperfections from forming. The difference is that control joints aren’t placed in areas that border other objects in the room but in strategic locations across the floor.

The role of these control joints is to allow movement caused by shrinkage and temperature changes. The idea is that the concrete won’t crack in other areas because these cuts take the stress off the slab. 

When placing control joints, you’ll want to consider the slab’s thickness as you space them. Put simply, the space between the joints in feet should be no greater than two to three times the slab’s thickness in inches. So if your slab is five inches thick, the control joints should be ten to 15 feet apart. 

You’ll want to cut your control joints soon after pouring the slab. In warm weather, you might only have six to 12 hours to cut these joints after the concrete dries until it begins cracking. These cuts should be 25% of the slab’s depth, so a five-inch-thick slab will require control joints that are 1.25 inches deep. 

Figuring out where to place your control joints is another part of the equation. You can try to put them under walls or carpets as much as possible if you don’t like their appearance, but make sure you lay them out in a manner that will minimize cracking to the surface.

Defining cold joints

Cold joints aren’t installed by a contractor, as they result from a delay or error when pouring the slab. In short, a cold joint is an imperfection in the surface that appears when the first and second layers of concrete cannot form a bond. This situation typically occurs when the first layer begins to set before pouring the second layer due to a delay, stoppage, or low rate of pour placement. 

You want to avoid cold joints whenever possible, but you will have some treatment options if they appear on your surface. The method you’ll utilize depends on the state of the concrete.

If you can get to the cold joint early enough, such as when the surface still has some elasticity, you can remove it manually with vibrating tools. You’ll use these tools to penetrate the surface and then pour fresh concrete over the existing layer to remove the imperfection.

When the cold joint has started to harden, you can use a light handpick to remove loose concrete on the surface without damaging the rest of the slab. You can then place a layer of mortar over the cold joint and cover it with fresh concrete to remove the defect.

Once the concrete completely cures, your only option is to repair the cold joint. You can accomplish this by applying a joint sealant, which will make the floor watertight between the two layers. It’s also possible to strengthen the bond between the two layers of concrete by adding dowel bars or turning the cold joint into a control or other construction joint.

Making the most of your concrete floor

Understanding how an isolation joint, control joint, and cold joint affect your concrete floors is an essential part of the construction process. Using a contractor that understands how to use construction joints to maximize your surface’s durability is also vital. 

Concreate is a concrete flooring expert that does everything but pour the slab. We can ensure you have adequate control and isolation joints before preparing and finishing your floor and can even help repair imperfections like cold joints. Contact us or visit thisisconcrete.com for more information on our concrete flooring services.