Pouring concrete for a driveway, sidewalk, patio, basement floor, slab-on-grade, or even a foundation used to be left to the pros. But with rising costs, many DIYers are looking at tackling the task. If you’re looking at picking up a float and trowel, you should know the difference between a control joint vs expansion joint.
A control joint weakens the concrete to draw shrinkage cracks to one location. It is usually cut 1/4 of the way through a slab within 18 hours of the pour. Expansion joints allow the slab or wall material to freely and independently move. Expansion joints are laid out prior to the pour, go all the way through the concrete, and are filled with a waterproof or resistant compressible material.
In this guide, we explain what control and expansion joints are, the types of expansion joints, the differences between expansion and control joints. We discuss placement and size, and when, where, and how to put in different joints. Plus, we look at if and when basement and driveway slabs should have joints, and what the best expansion joint material is. Our goal is to provide you with a better awareness of what control and expansion joints are, and when and where to use them.
- Control Joint vs Expansion Joint: Key Points
- What Is a Control Joint in Concrete?
- What Is an Expansion Joint in Concrete?
- Types of Expansion Joints
- What Is the Difference Between Expansion Joint and Control Joint?
- How Often Do You Need Control Joints in Concrete?
- How Big Can a Concrete Slab Be Without Expansion Joints?
- When to Cut Control Joints in Concrete Slab?
- How to Cut Expansion Joints in Concrete
- Does a Basement Slab Need Expansion Joints?
- Do Concrete Driveways Need Expansion Joints?
- Best Concrete Expansion Joint Material
Control Joint vs Expansion Joint: Key Points
Control joints protect concrete surfaces from shrinkage cracks due to concrete drying. They are not expansion cracks. Expansion cracks are used to control thermal expansion and movement by other forces to prevent the concrete buckling or breaking. In the table below, we compare the key points of control vs expansion joints.
|Key Points||Control Joint||Expansion Joint|
|Purpose||Prevent shrinkage cracks randomly occurring in the screeded surface and potential damage to hard finishes like tile.||Permit free and independent movement and expansion in large surfaces or structures.|
|Characteristics||Grooved or cut lines or joints in the screed or surface of a slab.||A complete or full structural separation between or through two or more components, sections, or parts.|
|When to Make||During the setting stage with a grooving tool or within 6 to 18 hours of the pour with a saw.||Engineered and usually placed prior to the pouring or placement of building components.|
|Joint Size||Typically, the width of the saw blade and 1/4 of the depth of the concrete.||The joint goes all the way through the structural component and is commonly 1/4" to 1” wide, but can be up to 20” wide.|
|Joint Spacing||2 to 3 times in feet the thickness of the slab is in inches, so 8’ to 12’ for a 4” slab.||Commonly range from every 75’ to 150’ depending on design factors. Often closer spacing in unheated or external applications.|
|Placement||Commonly used in floors but can be used in other surfaces too to control radiating shrinkage cracks.||Used and aligned in floors, walls, and ceilings, plus at the perimeters and corners to permit independent movement of structural components.|
|Location||Frequently used to divide large slabs into smaller sections to control where surface shrinkage cracks occur.||Required where two or more building or structural elements join, or to separate large slabs to allow movement and prevent buckling or breakage.|
|Movement Direction||Typically control or absorb horizontal shrinkage or movement.||Control movement in multiple directions and oscillations – horizontally, vertically, diagonally, and rotationally|
What Is a Control Joint in Concrete?
Control joints or contraction joints are typically used in slabs poured on the ground, such as house slabs, basement floors, driveways, garage floors, sidewalks, and patios to control where cracks will happen. Concrete shrinks as it dries and cures and cracks will randomly occur across the slab unless controlled. Concrete will crack. Control joints are intended to control where cracks occur, not to stop them. If there are no expansion joints, concrete will crack wherever it chooses.
Whether cracks occur due to shrinkage when curing, or because of frost, seismic movement, or ground settling, it is best to encourage them to occur where you want them. Planning the location for cracks to happen, if they do, and cutting or placing control joints provides a weakened place for cracks to happen. Control, contraction, or movement control joints are always straight, which means any cracks will be straight, not randomly wandering across the floor.
Control joints are typically cut into the concrete during the setting to finishing stage using a grooving tool. For large pads though, a light-weight dry-cut concrete saw or heavier wet-cut concrete saw is often used within 6 to 18 hours after pouring to cut in control joints. The saw weight determines how soon it can be used, with the lighter one sooner than the heavier one.
The cuts are usually 1” to 3” deep into the concrete, or 1/4 of the pad’s depth – 1” deep per 4” of thickness. Placement should be 2 to 3 times in feet the concrete’s thickness in inches, so every 8’ to 12’ for a 4” thick pad. Contractors often will place cuts to be unobtrusive so they’ll disappear under walls or to create an aesthetically pleasing checkerboard or diamond pattern if remaining visible.
Control joints aren’t always necessary. Pads that have rebar, steel mesh, or are reinforced with additives like fiberglass may have minimal cracking. Having said that, many Engineers will still recommend control joints. Grid patterns are common, with structural points such as support posts being isolated to prevent weakening the concrete around them.
What Is an Expansion Joint in Concrete?
Concrete expands and contracts with temperature changes and can also shift with ground movement or other forces. The larger the surface concreted, the greater the movement potential. Expansion joints, also known as isolation or movement joints, are gaps or spaces typically used to separate different pours, pads, or structural masses so they can freely and independently move from one another without causing structural stresses that lead to buckling or cracking.
The joints provide a gap that is usually filled with a compressible material that forms a bridge between two or more surfaces. They allow movement between components to prevent damage caused by different forces. Foam is typically used to fill expansion joints inside buildings, and an asphalt-impregnated board commonly provides the compressible material used outside.
Expansion joints are often used where one mass of concrete meets another solid mass like a building, bridge, pool, or other structure, or where the length can be affected by expansion. Commercial, industrial, educational, and medical structures will frequently have expansion joints strategically placed to absorb forces experienced within their large concrete surfaces.
Expansion joints allow for movement by soil, ground heaves or settling, temperature changes, seismic action, machinery and vehicle vibration or movement, or even movement by large numbers of people. Expansion joints are recommended every 75 to 150 feet depending on location and climate but may be closer together for some purposes, such as sidewalks. So, most single-family residences won’t require expansion joints in their floors.
Temperature can cause long stretches of concrete, like highway slabs, sidewalks, driveways, or outdoor malls, and patios to expand or contract anywhere from 1/2″ to 1” in all directions. Concrete expanding against soil or gravel isn’t likely to cause damage, but pushing against other concrete masses, will.
The most common use in residential construction occurs where two pours or pads meet. Often between concrete driveways and sidewalks or garage floors, or where a driveway, sidewalk, or patio abuts the house. Expansion joints are also used to separate concrete decks from pools to prevent expansion damage.
Types of Expansion Joints
There are different types of expansion joints used depending on the purpose and location of the concrete. The joints or gaps are used to separate large concrete surfaces or consecutive pours from one another so they can expand, contract, or experience movement without causing damage to surfaces or structures adjacent to them. Expansion joints are usually specified in construction drawings by a Structural Engineer.
Expansion joints may be classified by purpose or material, with four main types of joints by purpose. The two most common types, that many of us may be familiar with, are bridging expansion joints and masonry expansion joints. Our familiarity stems from having seen them in large buildings, sidewalks, malls, or when driving on concrete roads or bridges.
Bridge expansion joints keep different pours, slabs, or structures level where they meet to prevent trip hazards or traffic bumps. They are especially important where movement, vibration, and temperature expansion and contraction could cause damage. The joints are used between two or more surfaces that were poured separately, such as roads and bridges or man-hole access points, or driveways or sidewalks and buildings.
Masonry or brick expansion joints allow for movement within clay bricks, concrete blocks, or poured concrete walls. A vertical expansion joint filled with an elastomeric sealant is used in place of mortar to absorb compressive forces. The masonry expansion joint usually aligns with a horizontal bridge expansion joint in adjacent floors.
Another type of expansion joint is used where steel structures like railways cross concrete structures, which, not surprisingly, are known as railway expansion joints. The fourth type is pipe expansion joints. They are used where concrete’s proximity to pipes carrying high-temperature gasses, steam, or liquid could cause damage from temperature expansion, vibration, or movement.
Expansion joints may also be identified by the material used to form the joint. Some are installed prior to or during the pour, and others are installed after the concrete has set. There are rubber or sponge expansion joints, asphalt or asphalt infused fiber or fabric joints, cork, and metal expansion joints. Some are highly waterproof and self-sealing, others highly flexible, or capable of handling vibrations, and others can take a great deal of pressure and heat.
What Is the Difference Between Expansion Joint and Control Joint?
A control joint is used to weaken the concrete to control where tensile stresses or shrinkage cracks occur in the surface of the slab so they don’t cause unsightly patterns or damage hard finishes like tile. An expansion joint provides space for the concrete to expand or move against another immovable mass due to external forces or temperatures without buckling, breaking, or damaging itself or whatever it is against.
Contraction or control joints are usually formed or cut into the concrete within 2 to 18 hours of pouring. Their depth is typically 1/4 the thickness of the pad, so 1” deep for a 4” thick pad. Expansion joints go all the way through the pour and may be smooth or keyed, with or without smooth or deformed metal dowels to tie them together. The dowels allow longitudinal expansion and contraction, but keep the pieces aligned.
Expansion joints typically range from 1/4″ to 1” wide but can be as wide as 20” depending on purpose and location. Control joints are usually 1/8” to 1/2″ wide or the thickness of the saw blade. An expansion joint is filled with a compressible material to allow free movement, while a control joint may be left clear or filled with mortar, plastic, metal, or even hardboard.
A typical slab will have expansion joints around its perimeter where it abuts other concrete or immovable structures, plus have a grid pattern of control joints cut into its surface. The one prevents cracking or damage due to temperature expansion or other forces, and the other controls where shrinkage cracks occur due to moisture loss or movement within the concrete.
How Often Do You Need Control Joints in Concrete?
Control joint spacing often is based on the thickness and size of the pour. The cuts are often predetermined so they don’t detract from the overall aesthetics of the finish or are incorporated into the finished design. They may also be part of a decorative pattern etched into the concrete surface.
A common rule of thumb is 2 to 3 times in feet apart based on the thickness of the pour in inches. So, a 4” thick slab would have control joints every 8’ to 12’ apart. A 10’ by 10’ or 12’ by 12’ patio may not have any control joints, while a 20’ by 20’ or 24’ by 24’ slab will typically have a joint at the halfway mark both longitudinally and latitudinally.
Another factor affecting control joint spacing is the concrete mix itself. Concrete that contains a high synthetic or steel fiber content, or shrinkage-compensating or zero-shrink cement, or a combination of those, can be 10 times or more larger than those without. However, much depends on location and indoor versus outdoor use, as temperature plays a large role in the occurrence of cracks.
How Big Can a Concrete Slab Be Without Expansion Joints?
Expansion joints go all the way through the concrete pour or structure at strategic locations to allow free movement between components or one pour and another. They range from 1/4″ to 20” wide, and are usually identified by a Structural Engineer in the design drawings.
Pours resting on the ground, like driveways, patios, sidewalks, stairs, and slab-on-grade, typically only require expansion joints where they come in contact with other solid structures, each other, or where length dictates a joint is appropriate.
The physical size of most residential pours isn’t very large and is usually limited to what can be handled and poured by the crew doing the work. However, machinery is available that makes it possible to pour and finish huge tracts of concrete in a single day. Due to the additional expense of using such machines, they are more commonly used for commercial, industrial, or government projects.
The location of expansion joints depends on a huge number of factors, including purpose, location, and climate. It also depends on the type and location of supports, and other structural and ground preparation factors. Warehouse, commercial, and industrial spaces may have support columns spaced 100 to 130 feet apart, with some up to 350 feet apart. The placement of expansion joints typically coincides with the location of those supports.
Expansion joints typically are 75’ to 150’ apart for inside locations and 30’ to 120’ for those outside. However, modern concrete mixes, technology, and machinery make it possible to have larger pours without expansion or control joints. Currently, the ‘world’s largest’ concrete slab without any expansion or control joints, is at the Australian Container Freight Services pier in the Port of Brisbane. It is 50,000 square meters or 538,195.5 square feet in size.
When to Cut Control Joints in Concrete Slab?
Control joints can be placed into the concrete during the screeding stage using a grooving tool, or cut into the concrete after it sets up, usually 6 to 18 hours after it’s poured. The time the concrete takes to set up is temperature and humidity sensitive, which affects when the cuts can be made.
The concrete needs to be hard enough to walk on with the saw without leaving marks, and the saw blade should make a clean cut, not spew out aggregate or create raised edges. However, waiting longer than 24 hours is considered too late as some shrinkage cracks may have already begun to appear.
How to Cut Expansion Joints in Concrete
Control joints run longitudinally, latitudinally, or diagonally across the surface of a slab. They are typically cut into the concrete surface using a grooving tool during the screeding or finishing stage as the concrete sets up. Alternatively, they are cut 1/4 of the way through the set-up slab using a lightweight dry-cut concrete saw or heavier wet-cut concrete saw. Control joints control shrinkage cracks and some movement, but they aren’t the same as expansion joints.
Expansion joints allow the concrete to freely and independently move in multiple directions. They go all the way through the concrete and are usually inserted, or accommodated for, prior to the pour or placement occurring. The joints are used where a pour comes in contact with an existing structure or slab, or where it is structurally required. Expansion joints can run longitudinally, laterally, and diagonally, as well as vertically through walls and successive levels to allow structural sections independent movement.
Expansion joints are typically placed or formed in or at structurally strategic locations and are commonly filled with a compressible material. In rare situations, an expansion joint may be cut through the concrete using a heavy-duty wet-cut concrete saw.
The saw cut may be widened with an additional parallel cut, cleaned out, and then filled with waterproof compressible material to protect steel reinforcing inside the concrete. Cutting in expansion joints requires additional work and expense, especially if it is against a wall or other structural component. An expansion joint can also double as a control joint too.
Location of Construction Joints in Concrete Slabs
Construction joints are expansion joints used in structures to allow independent movement between components and are usually kept at a minimum. They are usually identified and placed in construction drawings by a Structural Engineer who takes all stress factors into account. Typically, joints are aligned with columns or other structural members to allow sections to move independently.
Construction joints may be centered between supports or beam spans, or equitably spaced along long spans to divide the structures into 2, 3, or more parts. The location of the joints depends on design loads and commonly occur where there is a low possibility of shear force and bending momentum. So, if there is a high possibility between supports or beam span, the location is elsewhere.
Does a Basement Slab Need Expansion Joints?
The physical size of a basement floor, support points, temperature, and the loads it is expected to carry commonly determine if expansion joints are required. Most residential basement floors are within a heated shell and often below frost level, and don’t require expansion joints. They also typically rest on gravel within foundation walls and are cooler than the rest of the structure.
For concrete to expand and require expansion joints, the basement would need to be warmed significantly for expansion to occur. However, basement floors are susceptible to shrinkage cracks and would benefit from control or isolation joints at strategic locations. Most basement floors have no need for expansion joints, and many don’t even have control joints.
Do Concrete Driveways Need Expansion Joints?
Most residential concrete driveways will have an expansion joint where they meet the garage floor slab to prevent buckling or cracks. If the concrete drive abuts a structure, there typically is an expansion joint to buffer any thermal expansion or movement. If the road at the end of the drive has a concrete curb there may also be an expansion joint there too.
Few residential concrete driveways are long enough to require an intermediate expansion joint along their length. Temperature is one of the concerns for long drives since the longer the run, the greater the expansion due to heat on hot days. Having said that, though, most concrete driveways will have control joints, which are usually enough to absorb any thermal expansion.
Best Concrete Expansion Joint Material
Expansion joints will expand and contract due to thermal fluctuations, and move with other forces. Horizontal and vertical expansion joints typically range from 1/4″ to 1”, with some reaching up to 20”. The material used to fill the gap needs to be flexible, durable, and resilient. Once compressed, it needs to return to its original thickness and shape as the concrete contracts.
The joints may be filled with asphalt-infused fiberboard, flexible foam, fiberboard, Ceramar expansion foam, sponge rubber, cork, metal, or bridged with interlocking materials to facilitate greater movement while providing support. Some materials also work better inside and others outside.
Ceramar expansion joint material is a foam joint filler that is highly flexible and has a 99% recovery ability, so it will essentially return to its original shape and thickness. Sponge rubber and cork are almost as responsive, with a 95% recovery rate. Asphalt expansion joint material is less expensive, self-sealing, waterproof, flexible, and permanent, but only has about a 70% recovery rate.
As to which is best, much depends on the width of the joint, where it’s located, forces it must withstand, the amount of movement, Code requirements, and even budget. For most residential purposes, 1/2″ thick asphalt infused fiberboard cut into 4” strips will provide 96 feet of joint material. Alternatively, a 50-foot roll of 1/2″ thick 4” wide foam expansion joint material will cost more but may be easier to handle.
Control joints differ greatly from expansion joints. Control joints provide a place for shrinkage cracks to occur so they do not randomly creep across the concrete’s surface. Expansion joints permit the concrete to expand and contract without causing damage and also allow it to move freely and independently of other portions of a structure. Hopefully, you have a better awareness of what control and expansion joints are, and where and when to use them.