Although concrete is known for its strength and durability, it does have some weaknesses. One of them is temperature change.
Rising and falling temperatures cause concrete to expand and shrink. This most often leads to cracks.
Installing rebar can help some, but when concrete covers a wide area, cracking is still a problem.
What can mitigate this issue? Expansion joints.
This article will discuss what an expansion joint is, why they are installed, the various types to choose from, and how to install and maintain them.
An expansion joint is a gap purposely placed in concrete slabs by the construction team that allows room for thermal expansion and contraction.
Expansion joints can also be inserted between a concrete slab and another structure.
The term expansion joint is often grouped with or used interchangeably with control and isolation joints.
The difference between control joints, isolation joints, and expansion joints is very slight but exists nonetheless:
For this article, we will use all three terminologies as one: expansion joints.
Concrete is not elastic but expands and contracts with certain changing variables. This can cause cracking or buckling if there is no room for movement.
As we’ve already pointed out, concrete will fluctuate in size based on external factors, one of them being the temperature of its environment.
Concrete swells up in hot weather and shrinks in cold weather. This constant change in size can cause damage to the concrete slab. Expansion joints are put into place to allow room for this change in size with only minimal, controlled cracking.
Concrete slabs are put under a variety of loads; some come and go, but those that are constant are called dead loads.
Examples of dead loads might include the weight of:
Many of these place significant stress on one area of the slab, which could cause the concrete to weaken in this area. Expansion joints aid in distributing these loads across the entire slab, reducing the total stress on any one spot.
On the other, live load impacts frequently fluctuate.
Examples of live loads include:
These items can bring unexpected forces that dramatically change in an instant. Sudden changes in weight can cause damage to the concrete. Expansion joints provide flexibility to shift with these sudden variations without cracking.
Concrete is a porous material. Any contact with water will weaken its structural stability. On top of that, if moisture reaches the steel reinforcements, these can corrode and weaken as well. It could even cause toxic mold to grow in the concrete.
Expansion joints help prevent moisture issues by providing a trench for the water to accumulate instead of being absorbed into the concrete.
As you will learn in the coming sections, expansion joints are sealed and watertight so no moisture can seep through. This tight seal helps keep any other corrosive substances, such as chemicals and abrasives, out of the concrete cracks as well.
Earthquakes, volcanoes, landslides, and avalanches can be catastrophic to structures of any kind, and concrete structures are no exception.
Expansion joints allow the soil underneath the slab to shift slightly due to vibrations without damaging the concrete. They also help to absorb some of this movement, reducing the stress on the slab.
Cracking, buckling, or crumbling concrete is not a pretty sight. Neither are clunky or abrupt transitions.
Well-designed expansion joints allow room for these necessary movements in designated spaces. If the concrete does crack, it will happen in a straight line below the finished section.
Expansion joints come in all colors and styles to match the architectural design of your concrete structure, ensuring a smoother look.
Expansion joints are almost always used in one way or another when a concrete slab is being made:
Expansion joints can have many characteristics defining and impacting their installation method, including:
When it comes to the installation of expansion joints themselves, you have two options.
This is considered the traditional expansion joint installation, using a barrier to run along the predetermined joint line that will create a break in the concrete slab. This method is often used when expansion joints are between a slab and another structure.
The joint materials, known as backer rods, are usually made from a closed-cell flexible material.
Cork and rubber expansion joints are both flexible, compressible, and resistant to moisture or temperature change. Neoprene is a synthetic rubber that has similar characteristics.
Foam expansion joints are often made with polyethylene. This joint material is lightweight and flexible.
Asphalt-impregnated fiberboard is a rigid option for expansion joint material. It can be cut to fit and is resistant to moisture.
Expansion joints installed after the concrete is poured are traditionally known as concrete control joints.
These joints are sawed or hand-formed into the concrete slab after the concrete pour has semi-dried. The control joints must be installed no later than 12 hours after pouring the concrete, or it can cause premature cracking.
It should be installed as soon as the concrete can be sawed without disrupting the integrity of the slab (roughly 6-18 hours after the concrete pour).
These grooves sawed or formed into the concrete must be at the correct depth, which is at least a quarter depth of the entire thickness of the slab.
You will then need to fill these joints with a sealant after the concrete is fully cured and vacuumed free of debris. You might also require a bond breaker installed before adding the sealant.
Another critical element of precise expansion joint installation is the distance between each placed joint.
Joints need to be placed a certain distance from each other based on the slab’s thickness and the aggregate’s size. Several factors can amend the spacing of expansion joints.
Concrete slabs in residential projects require less spacing between expansion joints, about 8-12 feet. Commercial slabs, however, only require expansion joints every 15-20 feet.
More expansion joints will be needed for concrete slabs in extremely hot or cold climates.
Joints should be placed perpendicular to the anticipated movement and level with the concrete slab. Joints are to be located under walls whenever possible, removing them from sight. This is often the case for residential slabs.
The ACI recommends between ¼ inch to ½ inch for the expansion gap in your joints. The gap must be wide enough to allow for the anticipated expansion but not so wide that it will deform the concrete.
The anticipated expansion is calculated based on the temperature changes in the area and length of the structure:
(Coefficient of Thermal Expansion) × (Temperature Change) × (Length of the Structure)
After expansion joints are installed, they should periodically be maintained to preserve their ability to serve their purpose.
Some of the maintenance recommended on expansion joints is a DIY project, but others need the help of professionals.
Expansion joints need to be sealed with sealant or caulking to prevent water erosion or freeze/thaw damage due to water getting through the concrete cracks.
For joint sealants to be effective, they must possess specific properties like:
Expansion joint filler materials can be broken into two groups: elastomeric or bituminous sealants.
These sealants are durable and provide a good moisture barrier. They also hold up well to temperature changes. Usually made from silicon or polyurethane, elastomeric sealants are self-leveling and more elastic, offering a better option for more movement.
Like elastomeric sealants, bituminous materials are durable, provide a good moisture barrier, and resist temperature change. These include tar or asphalt with aggregate, and can come in preformed sealant strips for easy installation. Bituminous materials are the best choice when minimal shrinkage is expected.
This easy-to-install joint sealer can be made from silicone, rubber, or polyurethane. Preformed sealers are flexible, withstand extreme temperatures, and create a moisture barrier.
Whatever sealant you choose, it must be periodically inspected and possibly replaced if damage is found. These inspections are ideally done during the cold season when the sealer hardens, shows weakness, and can easily be removed.
Weeds can push through anything. If they find a glimmer of sunlight, they will grow towards it, no matter what may be in the way.
This includes expansion joints, a problem that often allows water to come through. For this reason, removing weeds from your expansion joints is integral to maintaining them.
Over time, joints can widen more than what is beneficial. When this happens, joint reforming should be done to reduce the gap. If you find broken or crumbling edges, joint reconstruction must be done. These joints can be resawed and then reinstated.
Expansion joints are critical elements of effective concrete construction. Failing to install them can cause cracks, buckling, and a significant reduction in a concrete slab’s lifespan. Of course, high-quality installation and well-maintained expansion joints help them to fulfill their purpose.
Contact FMP Construction to construct your next concrete structure, including those requiring expansion joints.