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When Are Isolation Joints Used in Commercial Concrete Construction?

  • February 20, 2024

Concrete is a unique material. Due to its properties, concrete slabs are not stationary. 

They expand and shrink in response to the temperature of their surrounding environment. They can also move because of shifting soil below or added loads from above.

If you don’t account for these factors when building your concrete structure, you’re bound to see some damage. Therefore, experienced contractors will use joints in the proper places to mitigate these issues. 

One type of joint is the isolation joint. 

What are isolation joints, what purpose do they serve, and how are they different from other types of joints?

Read on to find the answers to those questions and more.

Table of Contents

1. Where Are Isolation Joints Used?
2. What Are Isolation Joints?
3. Why Are Isolation Joints Necessary?
4. Isolation Joints vs. Contraction Joints
5. Isolation Joints vs. Expansion Joints
6. Isolation Joints vs. Construction Joints
7. Isolation Joints vs. Cold Joints
8. How Isolation Joints are Made
9. Isolation Joint Materials


Where Are Isolation Joints Used?

Where would an isolation joint be placed in a construction project? 

The shortlist includes where slabs meet:

  • Walls
  • Columns
  • Footings
  • Pipes
  • Stairs
  • Machinery

Isolation joints are rarely needed for interior slabs due to the controlled environment inside a structure. 

However, if there are drastic temperature changes inside a building, this could require isolation joints. For example, when a heated office butts up to a walk-in cooler.

Basically, wherever concrete meets another material, and there is a chance of thermal movement, isolation joints will be present.


What Are Isolation Joints?

Isolation joints create separations between concrete slabs and another structure. They are a gap that isolates the slab to allow for the multi-directional movement that concrete is known to do. 

These gaps are created with a filler material that is put in place before the concrete is poured.


Why Are Isolation Joints Necessary?

Isolation joints allow for horizontal and vertical movement from swelling and shrinking with no concrete-to-concrete contact that could cause friction.

Without these joints in place, the poured concrete would bind to the other structure, limiting its movement. If the concrete slab is adjacent to a static structure without an isolation joint, the concrete cannot swell or shrink without causing cracking or buckling. 

Cracks and crumbling can occur due to this differential movement. This cracking can cause moisture to penetrate the slab, resulting in weakness and corrosion to the reinforcements. 


Isolation Joints vs. Contraction Joints

There are many different types of joints in concrete construction. The next few sections will explain how isolation joints differ from the others. 

There are a few key differences between isolation joints and contraction joints:

  • While an isolation joint is installed between the slab and another structural element, contraction joints are placed at intervals within the slab itself.
  • Isolation joints are put in before the concrete is poured, but contraction joints are installed after the concrete is poured.
  • Isolation joints are used to eliminate cracking. Contraction joints are installed to control where the cracking occurs during shrinkage by creating a weakened section in the concrete pavement. 
  • Isolation joints are a full-depth separation between the slab and the other structure, while contraction joints create only a partial separation. 
  • Lastly, isolation joints use preformed joint material to create the joint. Contraction joints are sawed into semi-dry concrete, using no material at all. 

Isolation Joints vs. Expansion Joints

Isolation joints and expansion joints are very similar. In fact, all isolation joints are expansion joints, but not all expansion joints are isolation joints.

Expansion joints can be made between two slabs and at intervals of a large slab, similar to contraction joints. 

Isolation joints completely interrupt contact between two structures, but expansion joints can fully or partially interrupt contact between the concrete sections.

Learn more: Designing and Installing Expansion Joints in Commercial Construction Concrete


Isolation Joints vs. Construction Joints

Isolation joints are premeditated joints placed for the purpose of separating slabs from other elements. Construction joints could come unexpectedly in cases of lack of material, equipment breakdown, or bad weather that stops the construction process.

Isolation joints use a material that will be a permanent part of the structure. However, construction joints use a wood board, steel, or plastic form that will be removed once the concrete is dried. 

Construction joints could become expansion joints if they meet the need. But this would require some forethought or advance notice, since expansion joints must be placed at specific intervals. 

If the construction joint is not in the right place, this could result in the removal of the unfinished section. Alternatively, they can be connected with tie bars or welded wire fabric to continue the section when circumstances allow. 


Isolation Joints vs. Cold Joints

Where most joints we’ve already discussed are necessary, whether preplanned or not, cold joints should be avoided. 

Cold joints are similar to construction joints, except one section has already begun to set. In a sense, cold joints are construction joints gone wrong. 

This could be due to a failure to pour the concrete in time or some stoppage in the machinery. They are not gaps in the concrete, but visible seams. 

Again, isolation joints are placed purposefully to improve the structural integrity of the pour. Cold joints are a mistake. 

Cold joints are also a weakness in the concrete slab, since the concrete doesn’t have a chance to bond correctly. These joints should be rectified to ensure the strength of the slab. 


How Isolation Joints are Made

Before the concrete is poured, isolation joint material is placed adjacent to the elements needing separation. As we’ve discussed, this could be at the junction of the exterior wall, staircases, columns, or footings. 

This material should be measured, cut, and placed at every area the pour needs to be isolated from. These joints should begin at the sub-base and come to the top but not over the thickness of the slab.

However, this can be difficult to exact.

It’s better to leave a little extra material at the top than for the concrete to overflow the joint. Some will pre-score the filler to easily remove the top when ready. Some fillers come with a removable top portion.

Alternatively, a wood rod can be placed at the top of the joint and removed after the concrete hardens to ensure no concrete bonds to the structural element. 

Joint material should be between ½ inch to 1 inch thick. This allows enough padding for the concrete to swell and shrink without causing damage.

Reinforcements cannot pass through the joint. Therefore, placement and calculations of rebar or steel mesh need to consider isolation joint locations.

After the concrete is poured and dried, isolation joints should be examined one more time, and any trimming of joint material should be done. 

Next, the joint is filled with an elastomeric sealant to ensure no moisture or debris can enter the joint. If the sealant can adhere to the filler material, a bond breaker should be installed first. 

Walls Near Footings

Isolation joint construction is done slightly differently for walls near footings. Since isolation joints are typically only needed for exterior walls, most walls will be near footings along the perimeter of the building.

Where a typical isolation joint would be a simple flat piece of material placed between the slab and another structure butting up against the depth of the slab, walls near footings need an isolation joint that creates an L-shape.

This is due to the double protection the concrete slab requires. The joint filler needs to separate the footing from the floor as well as separate the floor from the wall.

To do this, the isolation joint should be under a portion of the slab as well as to the side facing the wall.  

Columns

Columns are another tricky structure to isolate. The joints need to be constructed in the shape of a diamond or circle around the column.  A column is usually the center point of four concrete slabs. This will completely separate the column footing from the adjoining slabs.


Isolation Joint Materials

When making an isolation joint, you have a variety of materials to choose from, including fillers, sealants, waterstops, and bond breakers. 

Fillers

There are many types of joint fillers. Some are reserved mainly for roads or sidewalks. Others are designed to withstand extreme circumstances. 

Each has a unique attribute that works best in a specific environment. 

Asphalt Impregnated

This self-sealing filler is typically used for control joints or expansion joints on sidewalks and roads. It is a permanent, non-absorbing filler that protects the slab from water infiltration. 

Recycled Fiber

Fiber filler is a semi-rigid material that comes precut into easily installed strips. These strips are versatile, tough, and can withstand the heat of summer and the cold of winter. Under compression, it bounces back up to 70% of its original size.

Foam

Foam filler is a unique material that can be shaped around curves and difficult areas. It’s also lightweight and easy to carry and install. Any material protruding from the top of the slab can simply be scored with a box cutter.

Cork

Cork can be semi-rigid or self-expanding. It comes in a roll and is easy to install. Additionally, cork protects against water infiltration and has good compression recovery. 

Rubber

This pliable material makes an excellent isolation joint filler. It is oil-resistant and can recover 95% of its thickness after compression.

Plastic

These are a rigid option for isolation joints, but they are economical and durable. The major times to use plastic isolation joint filler are when there is a strong chance of exposure to acids, chlorine, or other chemicals. 

Sealant

When installing the joint filler material, leaving about ½ inch recess is essential to allow room for sealant

Elastomeric sealants can be made of either silicon, polyurethane, or polysulfide. These sealants are highly elastic, adhesive, and durable. This makes them the most popular choice for contractors.

Another option is to use preformed sealants made from neoprene and bitumen. Instead of being elastic to ensure a constant seal, these joint sealants are in a constant state of compression. They are wider than the gap, lubricated, compressed, and forced into the joint. 

Waterstop

Waterstops are specific joint fillers used for concrete structures underground, such as tunnels, sewage treatment facilities, water reservoirs, and parking garages. 

There are four main types of waterstops:

  • PVC
  • Urethane
  • Bentonite
  • Metallic

Bond Breaker

At times, an isolation joint may need a bond breaker between the joint filler and the joint sealant to ensure that both have free movement and don’t tear. Without this bond breaker, the two materials may form a molecular bond.

Bond breakers can either be either membrane-forming or non-membrane-forming and either water-based or non-water-based. 

In isolation joints, bond breakers are usually in the form of a tape, such as a plastic or metallic strip. 


Conclusion

Isolation joints in concrete construction are necessary to maintain the structural integrity of your slabs. These joints must be properly placed using the correct techniques and materials. 

To ensure your concrete structure has impenetrable joints that will be durable and effective, you should find a professional and experienced concrete contractor.


Contact FMP Construction for your concrete construction needs.

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