On February 7, 2019, falling concrete from an expansion joint hit a car driving on the lower deck of the Richmond-San Rafael Bridge in San Francisco. Fortunately, the driver escaped injury. That wasn’t the case for unlucky drivers who drove on the I-5 in Seattle on April 01, 2022. Ten cars hit the raised expansion joint, and one rolled over.
Failure of expansion joints in concrete is not limited to highways and bridges. According to federal estimates, the top cause (24%) of pedestrian and bicyclist injuries is uneven or cracked sidewalks.
Concrete is an incredibly versatile and durable building material that has created our modern infrastructure and buildings. However, it expands and contracts due to changes in temperature and moisture, known as thermal movement. This movement causes cracking, buckling, and other structural problems. To prevent the problem, workers install expansion joints to allow for movement and reduce internal stress.
Discover the importance of expansion joints in concrete, the different types, FAQs, and best practices for maintaining them.
What Are Expansion Joints, and What Do They Do in Concrete Construction Projects?
Concrete expansion joints are gaps between two concrete slabs or structures that allow for movement when the concrete expands and contracts due to thermal movement and mechanical vibrations. They are used to prevent cracking and damage to the structure. Installers fill the joints with a flexible material to absorb the expansion and contraction forces. Expansion joint filler materials include metal, rubber, or closed-cell foam.
Concrete expansion joints are critical for all concrete buildings, bridges, sidewalks, railway tracks, and concrete slabs to accommodate movement and shrinkage due to temperature variations.
Here are the five critical functions of expansion joints:
- Allows for movement: Expansion joints create a space between concrete elements that permits motion caused by changes in temperature and physical or mechanical vibrations. The ability to expand and contract prevents cracking and damage to the structure.
- Accommodate misalignment: Joints can accommodate misalignment between different parts of the structure, reducing stress on the concrete and improving its durability.
- Reduce noise: Flexible concrete joints can also reduce noise by absorbing vibrations and preventing them from transmitting through the structure.
- Relieve anchor tension: Allowing for movement relieves tension on the anchors holding the concrete.
- Volumetric changes: Concrete expands and contracts laterally due to temperatures. Expansion joints relieve the internal tension, which would otherwise result in cracking.
Wherever separate elements or concrete pours exist, a joint must bisect the entire structure, creating a gap. Most concrete construction projects require multiple days to complete. Each new pour requires a joint to allow for the difference in drying shrinkage to avoid cracks.
Concrete buildings and bridges have various structures and supports, each requiring joints for movement.
Types of Concrete Joints
According to the American Concrete Institute, there are four types of joints in concrete structures: contraction, expansion, construction, and isolation.
1. Contraction Joints
The purpose of contraction joints is to control cracking from shrinking as the concrete cures. Workers create these joints in fresh concrete with a saw or jointing tool. These cuts are between 1/4 to 1/3 of the depth of the slab and are spaced every 3.1 m to 15 m.
Concrete control joints control cracking in concrete slabs such as patios, driveways, sidewalks, parking lots, and industrial floors. The goal of these joints is to allow the concrete to crack in a straight line at the joint and not elsewhere.
2. Expansion Joints
Expansion joints allow for movement in the concrete caused by temperature changes. These joints contain a flexible material, such as rubber or foam, that can compress and expand as needed. The placement of expansion joints should be no more than 30 times the thickness of the concrete slab.
Some common examples include foundations, patios, sidewalks, driveways, basements, bridges, masonry, and where concrete slabs meet. By allowing the concrete to move independently without breaking, expansion joints ensure the longevity and durability of the structure.
3. Construction Joints
Often, there is insufficient time to pour an entire slab on large jobs such as multi-story buildings or highways. Workers place a joint to finish a section as the job ends and have a neat starting place for the next pour. Other reasons for a construction joint are unexpected material shortages, bad weather, or equipment failure.
4. Isolation Joints
Isolation joints separate concrete from other materials, such as walls or columns. Like expansion joints, they use a compressible material, such as foam, to prevent the concrete from bonding to the other material.
To summarize, contraction joints prevent concrete slabs from cracking by creating a weakened line across the slab where it will crack in a controlled manner. Expansion joints are gaps between two concrete slabs or structures that allow for movement when the concrete expands and contracts. Isolation joints separate two different structures, like a building and a bridge. Finally, construction joints are necessary when pouring concrete in stages, such as on a highway or skyscrapers.
Do You Need Expansion Joints in Concrete?
Yes, expansion joints are necessary for concrete to prevent cracks and buckling. Without expansion joints, the concrete may crack and damage over time due to changes in temperature and moisture.
Random cracking from shrinkage often occurs just after pouring and before hardening. Placing grooves in the hardening concrete helps release the stress caused by shrinking. When the crack occurs within the groove, engineers consider it “controlled.” Uncontrolled cracks form across the hardened concrete surface, look bad, and can cause a tripping hazard.
Proper placement of both control and expansion joints is essential to prevent cracks in concrete slabs and sidewalks to ensure the concrete structures’ longevity and durability.
How Can Expansion Joints Help Improve Thermal Performance In A Concrete Structure Or Project?
Concrete thermal performance refers to the ability of concrete structures to withstand movement due to temperature changes without cracking or damage. Expansion joints accommodate this movement, allowing thermal contraction and expansion without inducing stresses into the elements.
Construction engineers specify the joint spacing and gap size to control cracking. High-performance thermal and moisture protection strategies, such as water-resistant and air barriers, are critical in creating better buildings and ensuring proper thermal and moisture control in building enclosures.
In addition, properly installed expansion joints in vertical concrete walls improve thermal performance by preventing air leakage, a common source of energy loss in buildings. Concrete joints help keep the building envelope intact and prevent unwanted cracks.
Finally, expansion joints can also help extend a concrete structure’s lifespan. By allowing for natural movement and preventing stress on the concrete, expansion joints can help prevent damage that can compromise the structure’s integrity over time. This increased durability can lead to lower maintenance costs.
How Do You Properly Install An Expansion Joint In A Concrete Project?
Are you wondering how it’s been done?
The first step in installing expansion joints is determining where to place them. You’ll want to place the joints where there is an abrupt change in the concrete surface, such as at the edge of a pavement section or where a slab meets a wall or a patio meets the pool coping.
For freshly poured concrete, installers use a groover or jointer. Groovers can be hand-held or walking models on long poles for wide slabs. The groover pushes a metal fin into the concrete to form a gap. Alternatively, installers can saw joints in set concrete. We’ll discuss this method shortly.
Next, it is time to fill the actual expansion joint. You can use various materials, including pre-formed joint fillers, foam strips, or rubber strips. Cut to the appropriate size and shape and then insert into the joint. It is vital to ensure enough space is left around the edges of the joint for it to move as needed.
Finally, properly sealing the joint prevents water or debris from entering and causing the joint to fail. Use a specialized sealant or caulk designed for use with expansion joints. The sealant should be applied carefully and evenly along both sides of the joint, ensuring it covers all exposed surfaces.
Sawing concrete joints is a process of cutting joints into concrete slabs to prevent cracking caused by tension.
Sawing Concrete Joints
There are two methods of cutting expansion joints. The first is when the concrete is still curing and relatively soft. The second method is after it’s cured and hard.
A soft-cut concrete saw cuts joints in soft concrete 1/2 inches to 1-1/2 inches deep. It’s best to use it between 6 to 18 hours after pouring. The motor and rotating blade mount on a wheeled support platform. The blade extends through a slot in the skid plate. The skid plate prevents excess cracking and chipping of the concrete during cutting. Before cutting, installers snap a chalk line to follow as a guide for each joint.
Cutting concrete after it has cured requires a circular saw with a diamond blade. Saw cutting makes a thinner line or joint in the concrete surface. However, the aggregate can flick out of the cut leaving jagged, unsightly edges. Saw cutting a joint into a hard concrete surface is not as strong as a hand-grooved joint.
For smaller jobs, use a hand-held circular saw. You’ll need to make a jig from boards to keep the line straight and saw level.
Once you have joints in the concrete, it is time to fill the expansion joint using various materials manufactured for that purpose.
Let’s look at different types of products to fill and seal the joint to maintain its integrity.
Expansion Joint Fillers and Sealers
The choice of filler and sealer can significantly impact the performance of a concrete surface. Using the wrong material for the expansion joint can lead to early joint failure and high repair costs. Materials used for expansion joints must be compatible with the concrete and withstand environmental conditions.
Most concrete joints need a filler and sealer combination. Here are some of the products to consider for your project.
Concrete Joint Fillers
- Polyethylene Filler Board – Polyethylene is a closed-cell product that is semi-rigid, UV-resistant, and bitumen free. It is also non-absorbent and has good thermal insulation properties. Polyethylene is a natural bond breaker. The concrete will not stick to it. It’s appropriate for bridges, highways, runways, and reservoirs.
- Resin Bonded Cork Filler – Ground cork combined with synthetic resin creates a flexible and economical joint filler. Self-expanding cork is ideal for rapidly contracting slabs such as freezer floors or walls. The cork can stretch up to 140% of its original thickness without breaking its bond to adjacent concrete materials.
- Bituminous Fiberboard – This filler combines bitumen (a.k.a. asphalt) and wood fibers into rot-proof, compressible joint filling material. The impregnated fiberboard is flexible and has excellent memory properties to resist continual contraction and expansion. Fiberboard works well in both indoor and outdoor concrete expansion joints.
- Rubber Bearing Strip – Manufactured from a high-quality natural rubber or neoprene rubber compound, it absorbs compressive forces, shock, and vibration. It evenly distributes loads over the entire surface area. It creates a weather-resistant seal that also reduces sound transmission.
- Foam Backer Rods – Backer rods are the construction version of pool noodles. They are dense, closed-cell foam that packs tightly into any joint or crack. Foam backers increase the joint’s elasticity. They reduce the depth of the joint, decreasing the amount of sealant needed to seal the joint.
Choosing the suitable material for the expansion joint ensures its optimal performance and longevity. After filling the joint, it’s best practice to seal the joint with an approved concrete joint sealer.
Concrete Joint Sealers
Sealing the joint with a flexible sealer prevents water and dirt from damaging it. Here are three options:
- Polysulphide Joint Sealant – This sealant adheres to concrete and stone. It has good resistance to most environmental chemicals and UV radiation.
- Polyurethane Joint Sealant – Polyurethane is a high-strength adhesive and all-purpose sealant. It is resistant to abrasion and cold.
- Hot Applied Sealants – They can be polymeric or asphalt and applied hot, around 370°F (188°C).
Permanently seal expansion joints on the exterior of a structure to help prevent water from entering, freezing, and breaking the concrete.
And, speaking of freezing, what precautions should you take when pouring concrete in cold conditions?
Can You Pour Concrete in Freezing Weather?
Pouring concrete in cold weather is inevitable if you live above the 35° latitude north or below it in the southern hemisphere. That covers about two-thirds of the U.S., Canada, Europe, New Zealand, southernmost Australia, and South America.
The challenge of installing concrete in the cold is that it shrinks faster, causing surface and deeper cracks. To combat cold weather, keep the concrete warm as it cures with concrete curing blankets. Inserting the appropriate contraction joints in concrete during cold weather installations is especially critical.
Cutting joints in freezing weather is not uncommon, but it is a problem. An article on ConcreteConstruction.net addressed the issue of Timing Joint Sawing in Cold Weather. A contractor had poured a 46 x 90-foot slab, 5 inches thick. He was supposed to cut control joints every 15 to 18 feet but could not finish the pour until 5 pm. The ambient temperature dipped below freezing, with sleet forecast for the following day. He insulated the concrete to retain heat but did not have a concrete curing blanket, a much better choice. Not sure what to do, he asked the experts.
The experts recommended using a diamond blade to saw a 2-inch deep contraction joint in the middle of the slab. Dividing the section into two equal sections would relieve some stress cracking until they could cut other joints in better weather.
Factors to Consider When Installing Expansion Joints in Concrete
Concrete moves at a measurable rate in relationship to ambient temperature. It has a thermal expansion coefficient or expansion rate of 0.0000055 per linear inch of concrete per 1 degree Fahrenheit of temperature change. The thickness of a concrete slab is not a factor. Concrete moves uniformly on a linear movement regardless of the slab’s thickness.
Concrete expands 0.66 inches for every 100 linear feet of concrete flatwork at 100 degrees Fahrenheit. If we raise the temperature to 160°F, the same slab will expand 1.06 inches. For each mile of concrete highway, engineers must add an expansion factor of 34.85 inches at 100 degrees.
Consider the following factors before laying down chalk lines to install expansion joints in concrete
Slab Thickness
The thickness of the concrete slab will determine the depth of the expansion joints needed. According to the American Concrete Institute (ACI), the minimum thickness for concrete slabs is 4 inches for residential construction and 5 inches for commercial construction. However, it’s essential to note that the actual thickness of a slab can vary depending on the specific project’s requirements.
Expansion Joint Location
The location of the expansion joints will depend on the concrete structure’s design and the concrete’s expected movement. Place expansion joints where the expected tensile stress is the highest, such as changes in direction, openings, or intersections of slabs or walls.
Joint Spacing
The ACI sets guidelines for maximum joint spacing based on various factors, including slab thickness, concrete mix, shrinkage, temperature, reinforcement, and lateral deflection due to thermal loads. The number of expansion joints required for a concrete floor and the distance between each can vary depending on the length and width of the floor slabs.
A general rule of thumb is to place a joint every 2 or 3 feet per one inch of the slab’s thickness. For example, if the concrete slab is 4 inches thick, you can have a slab 8 to 12 feet long before requiring an expansion joint.
Most finishers use the American Concrete Institute’s recommendation for spacing joints at 24 to 36 times the slab’s thickness. Using our previous example, 4 in. thick x 24 = 96 inches or 8 feet. At 36 times, the interval would be one joint every 12 feet.
Proper joint spacing and placement are crucial to avoid cracking and ensure that the joint can accommodate movement and expansion caused by changes in temperature, moisture content, and other factors.
The Best Time to Install Expansion Joints for Optimal Performance
Knowing when to install concrete expansion joints is as important as knowing where to put them. Timing is critical for optimum thermal performance. Why? Installing expansion joints after the concrete hardens may be too late. Cracks can already form.
The best time to cut expansion joints in concrete projects is within 6 to 18 hours after pouring when the concrete has hardened enough to prevent aggregate interlock but before shrinkage cracking occurs.
Installing expansion joints too late means they may not function, and the concrete may have already experienced shrinkage cracking.
Therefore, planning expansion joints before pouring concrete and installing them shortly after provides the best thermal performance.
Additional Benefits of Concrete Construction or Expansion Joints
As mentioned earlier, expansion joints in concrete construction allow the concrete to move independently and prevent cracking, buckling, and other structural damage.
One of the additional benefits of using expansion joints in concrete construction is their ability to achieve thermal performance goals. By incorporating expansion joints into a concrete structure, contractors can ensure that the building maintains its desired level of thermal performance. The joints can help prevent heat transfer through the concrete and improve energy efficiency. Heat transfer can be significant in areas with extreme temperature variations, such as deserts or mountainous areas, where thermal bridging can be a genuine concern.
Another benefit of using expansion joints is that they can help increase the overall durability of a concrete structure and reduce the need for costly maintenance and repairs. Concrete expansion joints can help prolong the life of the building and reduce maintenance costs over time.
Concrete joints can also improve the aesthetic appeal of a building. By minimizing the appearance of cracks or other damage, expansion joints can be part of the design and help maintain a clean and polished look for commercial buildings.
Installing expansion joints in concrete construction improves energy efficiency and structural strength. Joints provide a clean aesthetic appeal.
Best Practices for Maintaining Concrete Joints
After installing concrete joints, maintaining them prevents damage to the structure and extends its lifespan. Here is a list of best practices for maintaining concrete joints:
- Regular inspection and maintenance can keep minor repairs from becoming major issues.
- Regularly clean the joints. Keep them free of debris, dirt, and other contaminants that can cause damage to the joint sealant or filler material.
- Replace damaged or worn-out joint sealant or filler material. Water and dirt infiltration can lead to concrete deterioration and structural damage.
- Use high-quality joint sealant or filler materials compatible with the concrete substrate, weather conditions, and traffic volume for its location.
- Allow for proper curing time for the joint sealant or filler material. Ensure that it adheres properly to the concrete substrate to provide the required level of protection and durability.
- Properly train and educate maintenance personnel to apply best practices and techniques.
- Follow industry standards and guidelines to ensure the joint performs as intended and meets all relevant requirements and regulations.
These best practices can prevent damage and reduce maintenance costs. Regular inspection, cleaning, and replacement of damaged joint sealant or filler material will help ensure the joint performs as intended and meets all relevant requirements and regulations. Proper training and education of maintenance personnel will also help to ensure that best practices and the joint is well-maintained.
Now that we’ve covered concrete joint best practices let’s address frequently asked questions about this topic.
FAQs About Concrete Joints and Placing
- What are the reasons we need concrete expansion joints?
They…
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- Allow concrete pads to expand and contract with temperature and humidity changes.
- Help prevent cracks and buckling in concrete slabs.
- Are essential for preventing cracks in driveways and sidewalks.
- Protect against cracking and strain in concrete flooring.
- Isolate different parts of the slab so that it can move without strain.
Additionally, installers must place a joint between the different sections for large jobs requiring multiple days to finish. Joints allow for differences in shrinkage.
- How far apart should expansion joints be placed in concrete?
Place expansion joints at 24 to 30 times the slab thickness. For a 5-inch thick slab, plan on placing a joint every 10 to 12.5 feet. Another way to estimate is to install joints every two to three feet of the thickness of a concrete slab in inches. Here is an example: 5 inches thick x 2 = 10 feet. 5 in. x 3 = 15 feet. The more joints you have, the more likely the slab will last longer.
- How big can a concrete slab be without expansion joints?
If we use the 2 to 3 times in feet by the slab thickness in inches method, you can have a 4-inch slab between 8 to 12 feet long or wide that doesn’t require a joint.
- What are common mistakes to avoid when installing expansion joints in concrete?
Common mistakes to avoid when installing expansion joints in concrete include:
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- Placing them too far apart
- Not sealing them properly
- Not using an appropriate filling material
- Not installing them at the correct depth
- Filling the joints with concrete or other non-compressible material.
Improper installation can cause water leakage leading to further concrete damage and corrosion of the steel reinforcement.
- How are expansion joints installed in concrete?
There are two primary methods. The first is using a concrete hand groover or jointer that forces a metal fin into the fresh concrete to create a joint. The second uses a soft-cut saw that cuts through the concrete and aggregate to the correct depth. For the soft saw method, the concrete should cure between 6 and 18 hours.
- What is the lifespan of expansion joints in concrete?
The lifespan of expansion joints in concrete varies depending on the material used, traffic, and temperature changes.
- How often should expansion joints in concrete be maintained?
The joint should never fill with dirt, sand, or other non-compressible materials. Inspect the expansion joints in concrete regularly and maintain them as needed, such as by replacing damaged joint fillers or sealants.
- Can expansion joints in concrete be repaired?
Yes, you can repair the expansion joints in concrete by replacing damaged joint fillers or sealants. Never fill the joint with concrete, or it will defeat its purpose.
- What happens if you don’t put expansion joints in concrete?
If you don’t install expansion joints in concrete, it can lead to buckling and cracking, damaging the entire surface. The uneven surface can lead to tripping hazards. For example, in Abington, Pennsylvania, an elderly lady tripped over a three-inch uneven portion of two adjoining sidewalk slabs. She sustained a broken wrist and lacerations to her face. Engineering analysis established that the cause of the sidewalk buckling was inadequate control of the thermal movement of the concrete slabs.
Expansion joints let concrete expand and contract without causing damage.
- What is the recommended distance between expansion joints?
Most concrete contractors control cracks by keeping the maximum distance between joints (in feet) at 2 to 2.5 times the slab’s thickness in inches. The maximum joint spacing for a 6-inch thick slab will be 12 to 15 ft.
Concrete Joints Ensure the Durability and Longevity of The Structure
If you’re wondering how to improve the durability of concrete, always include expansion joints in your plans. Joints allow one concrete slab to move independently of other parts of the floor, building, or structure. Joints allow concrete to shrink as it dries, relieving internal stress. If it does crack, it becomes part of the expansion joint and won’t be noticeable.
To find the best type of expansion joint for your project, consider where and how the structure will move and what type of climate it is in. For safety and best results, consult with a structural engineer or building inspector to design the placement of concrete construction and expansion joints.
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