Control Joint in Concrete
— Why Are Control Joints Necessary in Concrete?
Everything you need to know about control joints in concrete: types, spacing, depth, installation, advantages, disadvantages, safety, and expert FAQs — all in one comprehensive resource.
📋 Table of Contents
- What Is a Control Joint in Concrete?
- Why Are Control Joints Necessary?
- Types of Control Joints
- How Control Joints Work
- Spacing & Depth Guidelines
- How to Install Control Joints
- Advantages of Control Joints
- Disadvantages & Limitations
- Where Are Control Joints Used?
- Control Joint vs. Other Joints
- Is It Safe? Safety Considerations
- Sealant & Fill Materials
- Cost of Control Joints
- Common Mistakes to Avoid
- Standards & Codes
- FAQs — 14 Expert Questions
- Related Keywords
ANIMATED: HOW A CONTROL JOINT WORKS
Alternative Names for Control Joints
Control joints are referred to by several names in civil engineering literature and codes:
- Contraction joint — the most technically accurate name (concrete contracts/shrinks)
- Dummy joint — because the joint fools the crack into going there
- Weakened plane joint — describes its mechanism
- Saw-cut joint — describes one installation method
- Crack control joint — describes its primary function
Why Are Control Joints Necessary in Concrete?
Concrete, despite being one of the strongest construction materials in the world, has a fundamental weakness: it cracks. Understanding why concrete cracks is essential to understanding why control joints are absolutely necessary.
Primary Causes of Concrete Cracking
WITHOUT Control Joint — Random Cracking
WITH Control Joint — Controlled Cracking
1. Plastic Shrinkage
Fresh concrete loses moisture rapidly through evaporation, especially in hot, windy conditions. This early moisture loss causes plastic shrinkage cracking within the first few hours of placement.
2. Drying Shrinkage
As concrete cures over days, weeks, and months, it continues to shrink as excess mixing water evaporates. A typical concrete mix shrinks by approximately 0.04% to 0.08% during curing — enough to cause significant cracking in unreinforced slabs.
3. Thermal Movement
Concrete expands in heat and contracts in cold. Temperature differentials between the surface and interior, or between seasons, cause repeated cycles of expansion and contraction that eventually crack the concrete.
4. Differential Settlement
When the ground beneath a concrete slab settles unevenly — due to soil compaction, moisture changes, or poor subgrade preparation — the slab cracks along lines of maximum stress.
5. Restrained Shrinkage
When concrete is bonded to another structure (e.g., a wall or existing slab), it cannot move freely, causing restrained shrinkage stress that results in cracking.
Bottom Line: Concrete WILL crack. The only choice engineers have is whether that cracking happens randomly (bad) or in a planned, controlled location (good). Control joints ensure the latter.
Types of Control Joints in Concrete
There are several types of control joints used in concrete construction, each suited to different applications, slab thicknesses, and project requirements.
Saw-Cut Control Joint
The most common type. A diamond-blade saw cuts a groove into hardened or green concrete to a depth of 1/4 the slab thickness. Made within 4–12 hours of placement.
Tooled / Formed Joint
A groover tool is used while concrete is still plastic (fresh) to press a groove into the surface. Faster and cheaper than saw-cutting but requires skilled timing.
Insert / Zip Strip Joint
A pre-formed plastic or hardboard strip (zip strip) is embedded into fresh concrete during finishing to create a weakened plane. Removed or left in place after hardening.
Early-Entry Saw-Cut Joint
A specialized lightweight saw makes shallow cuts within 1–4 hours of placement, before the concrete fully hardens. Reduces risk of raveling and uncontrolled cracking.
Pre-Formed Plastic Inserts
Pre-formed T-shaped or L-shaped plastic inserts are placed in the concrete formwork before casting to create weakened planes at designated locations.
Dowel-Assisted Control Joint
Steel dowel bars are placed across the joint to maintain load transfer between slabs while still allowing controlled cracking and horizontal movement.
Control Joints in Concrete Walls
In concrete masonry unit (CMU) walls and cast-in-place concrete walls, control joints are vertical slots placed at regular intervals to control cracking from thermal movement, settlement, and drying shrinkage. These are typically formed using neoprene or PVC waterstop inserts combined with a backer rod and sealant.
How Does a Control Joint in Concrete Work?
The science behind control joints is elegant in its simplicity. By creating a deliberate weakened plane in the concrete, engineers ensure that when tensile stresses from shrinkage exceed the concrete’s tensile strength, the crack forms at the joint — not randomly across the slab.
CROSS-SECTION: CONTROL JOINT DEPTH
The saw cut weakens the cross-section so that the internal crack forms directly below the joint — invisible from the surface.
Example: 150mm (6-inch) slab → Minimum joint depth = 37.5mm (1.5 inches)
For crack inducers (formed inserts): Effective depth = 1/3 slab thickness is preferred
For early-entry saws: Minimum 25mm or 1 inch, whichever is greater
The Mechanism in 3 Steps
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Weakened Plane Created
The saw cut or formed groove reduces the effective cross-sectional area of the slab at that location by at least 25%, creating a point of minimum resistance.
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Shrinkage Stress Builds
As concrete cures and dries, tensile stresses develop throughout the slab due to restrained shrinkage. These stresses seek the path of least resistance.
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Crack Initiates at Joint
The crack propagates through the remaining concrete beneath the saw cut, forming a clean, vertical crack directly under the joint — hidden from view and easily sealed.
Spacing & Depth of Control Joints — Engineering Guidelines
Proper spacing and depth of concrete control joints are critical to their effectiveness. Joints placed too far apart allow cracks to develop between them. Joints that are too shallow fail to create an effective weakened plane.
Standard Spacing Guidelines
PLAN VIEW — SLAB JOINT SPACING
Example: 4-inch slab → 8 to 12 ft spacing (ACI 360R recommendation)
| Slab Thickness | Min. Joint Depth | Recommended Spacing | Max Panel Size |
|---|---|---|---|
| 100mm (4 in) | 25mm (1 in) | 2.4–3.6m (8–12 ft) | ~9–13 m² |
| 125mm (5 in) | 31mm (1.25 in) | 3.0–4.5m (10–15 ft) | ~14–20 m² |
| 150mm (6 in) | 38mm (1.5 in) | 3.6–5.4m (12–18 ft) | ~20–29 m² |
| 200mm (8 in) | 50mm (2 in) | 4.8–7.2m (16–24 ft) | ~36–52 m² |
| 250mm (10 in) | 63mm (2.5 in) | 6.0–9.0m (20–30 ft) | ~56–81 m² |
Joint spacing (in feet) should not exceed 2 to 3 times the slab thickness (in inches). For a 4-inch slab: 2×4 = 8 ft minimum spacing, 3×4 = 12 ft maximum spacing. Panels should be as square as possible — the length-to-width ratio must not exceed 1.5:1.
Factors That Affect Spacing
- Water-cement ratio: Higher w/c ratios mean more shrinkage → closer joint spacing needed
- Aggregate size: Larger aggregate provides internal restraint and can allow wider spacing
- Reinforcement: Fiber or steel reinforcement reduces cracking potential, allowing slightly wider panels
- Climate: High temperature variation requires closer spacing
- Subgrade conditions: Poorly prepared subgrades require closer spacing
- Curing method: Faster drying = more shrinkage = closer joints
How to Install Control Joints in Concrete — Step by Step
Installing control joints correctly requires careful timing, proper equipment, and adherence to engineering specifications. Here is a complete step-by-step guide:
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Plan the Joint Layout Before Pouring
Mark the joint locations on the formwork using chalk lines or layout strings. Calculate spacing based on slab thickness and applicable standards (ACI 360R, ACI 302.1R). Plan for square panels wherever possible.
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Pour and Finish the Concrete
Place concrete as per the design mix specification. Level, screed, and float the surface to the required finish. If using tooled joints, form the groove during the finishing stage while concrete is still plastic.
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Determine the Correct Cutting Time
For early-entry saws: cut within 1–4 hours after finishing. For conventional wet saws: cut within 4–12 hours. The concrete should be hard enough to walk on but soft enough that the saw does not cause raveling at the joint edges.
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Mark Joint Lines on the Hardened Surface
Use a chalk line to mark the exact joint locations on the cured concrete surface before cutting. Double-check dimensions and ensure the layout matches the design drawings.
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Make the Saw Cut
Use a diamond-blade concrete saw (wet or early-entry) to cut along the chalk line to the specified depth (minimum 1/4 of slab thickness). Keep the saw steady and move at a consistent pace to maintain a uniform cut width and depth.
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Clean and Cure the Concrete
Remove sawing slurry with water. Continue curing the concrete as specified (wet curing, curing compound, etc.) for at least 7 days. Avoid applying sealant until the concrete has fully cured (28 days preferred).
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Fill or Seal the Joint
Once the concrete has cured and any cracking beneath the joint has occurred, seal the joint with a flexible sealant (polyurethane, epoxy, silicone) appropriate for the application. Install a backer rod first to control sealant depth.
Diamond-blade concrete saw (early-entry or wet cut), chalk line, measuring tape, backer rod applicator, caulking gun, polyurethane or silicone sealant, safety glasses, hearing protection, and water supply for wet-cut saws.
Advantages of Control Joints in Concrete
Control joints offer numerous benefits that make them an indispensable element of quality concrete construction. Here are the key advantages:
✅ Advantages
- Prevents random, uncontrolled cracking across slabs and walls
- Improves aesthetics by hiding cracks within planned joints
- Protects structural integrity of the concrete element
- Reduces maintenance costs over the life of the structure
- Extends lifespan of concrete slabs and pavements
- Accommodates thermal movement without damage
- Easy to install with standard equipment
- Cost-effective compared to repair of cracked concrete
- Prevents water infiltration when properly sealed
- Protects reinforcement from corrosion by limiting crack widths
- Complies with building codes and industry standards
- Versatile — works in slabs, walls, pavements, industrial floors
❌ Disadvantages
- Not 100% guaranteed — cracks can still form randomly if joints are misplaced
- Timing is critical — cutting too early or too late reduces effectiveness
- Requires skilled labor for proper design and installation
- Sealants degrade over time and require periodic maintenance
- Reduces load transfer slightly between adjacent panels
- Additional cost of saw cutting, labor, and sealant
- Can accumulate debris in unsealed joints
- Aesthetic issue if sealant color doesn’t match concrete
- Joint edges can chip under heavy forklift or wheeled traffic
- Requires ongoing inspection and resealing every 5–10 years
Limitations & Disadvantages of Concrete Control Joints in Detail
While control joints are highly beneficial, engineers and contractors must be aware of their limitations to ensure effective design and maintenance.
1. Ineffective If Incorrectly Timed
The most common reason a control joint fails is cutting too late. If the concrete has already developed internal cracking before the saw cut is made, the joint serves no purpose. Early-entry saws and constant monitoring of concrete set time are essential.
2. Joint Spacing Errors
If joints are spaced too far apart, cracks will develop between them regardless. Many contractors place joints at 15–20 feet in 4-inch slabs — well beyond the recommended 8–12 foot maximum — leading to mid-panel cracking.
3. Sealant Failure
Control joint sealants are the joint’s most vulnerable component. UV exposure, traffic loading, temperature cycling, and chemical exposure cause sealants to harden, crack, and fail over time. Typical sealant service life is 5–10 years, after which resealing is required.
4. Edge Spalling Under Heavy Traffic
In industrial floors with heavy forklift traffic, the edges of saw-cut control joints are vulnerable to spalling (chipping) as wheeled vehicles repeatedly impact the exposed joint edge. Hard aggregate facings, epoxy nosings, or armored joint edges are required in such applications.
5. Aesthetic Limitations in Decorative Concrete
In decorative concrete applications (stamped, colored, or polished concrete), control joint placement must be carefully planned to align with the decorative pattern — otherwise joints visually disrupt the design.
Where Are Control Joints in Concrete Used?
Control joints are used in virtually every type of concrete construction. Here are the primary applications:
Roads & Highways
Parking Lots
Industrial Floors
Driveways
Sidewalks & Paths
Retaining Walls
CMU Walls
Building Floors
Airport Aprons
Pool Decks
Bridge Decks
Warehouse Slabs
Special Applications
In airport concrete pavement, control joints are used alongside expansion joints and construction joints to manage the complex thermal and loading stresses from aircraft operations. In bridge decks, they are combined with waterproofing membranes to prevent water and chloride ingress. In swimming pool decks, they accommodate the unique combination of water exposure, thermal cycling, and concentrated foot traffic.
Control Joint vs. Expansion Joint vs. Construction Joint — Key Differences
One of the most common points of confusion in concrete construction is the difference between the three main types of concrete joints. Understanding this distinction is essential for proper design.
| Feature | Control Joint | Expansion Joint | Construction Joint |
|---|---|---|---|
| Purpose | Control cracking location | Allow thermal expansion | Mark end of one pour |
| Also Called | Contraction joint, dummy joint | Isolation joint | Day joint, cold joint |
| Depth | 1/4 of slab thickness | Full slab depth | Full slab depth |
| Load Transfer | Via aggregate interlock | Via dowels or none | Via dowels or keys |
| Movement Type | Horizontal (contraction) | Horizontal (expansion) | None intended |
| Fill Material | Flexible sealant | Compressible filler + sealant | Bonded or keyed |
| When Created | After or during pour | Before pour (pre-formed) | At end of each pour |
| Typical Spacing | 8–15 ft (slabs) | 20–100 ft | As required by pour schedule |
Remember: A control joint is NOT a full separation of the concrete — it is a partial-depth groove that guides cracking. An expansion joint IS a full separation that allows physical movement between two concrete sections. These serve completely different purposes and are not interchangeable.
Is a Control Joint in Concrete Safe?
Control joints are a fundamental safety feature in concrete construction — not a compromise. Here is why they enhance safety:
Structural Safety
Random, uncontrolled cracking is a structural risk. Wide, irregular cracks can compromise the load-bearing capacity of slabs and walls, allow water infiltration, corrode reinforcement, and ultimately lead to structural failure. Control joints prevent the development of hazardous random cracks by managing internal stresses.
Trip Hazard Prevention
In pedestrian areas, wide random cracks create trip hazards. Control joints, when properly sealed and maintained, create a smooth, flush surface that eliminates tripping risks associated with uncontrolled cracking.
Durability and Long-Term Safety
Concrete structures without adequate joint design deteriorate faster, particularly in freeze-thaw environments where water infiltrating random cracks causes spalling, delamination, and progressive structural degradation. Control joints protect the long-term integrity of the structure.
Construction Safety During Installation
Saw-cutting operations require proper PPE: safety glasses, hearing protection, dust mask (for dry cutting), and gloves. Early-entry saws should be operated only by trained personnel to avoid accidents on freshly placed concrete.
ACI 302.1R, ACI 360R, and ASTM standards require control joints in concrete construction. Compliance with these codes is a legal safety requirement for commercial and public concrete work.
Control Joint Sealant & Fill Materials
Selecting the right sealant for a concrete control joint is critical to long-term performance. The wrong sealant can fail prematurely, allowing water infiltration, debris entry, and joint edge damage.
| Sealant Type | Best For | Service Life | Notes |
|---|---|---|---|
| Polyurethane (PU) | Exterior slabs, driveways, parking lots | 10–20 years | Best all-around; UV stable, flexible |
| Silicone | Pool decks, wet areas | 15–25 years | Excellent water resistance; paintable versions available |
| Epoxy | Industrial floors, heavy traffic | 5–15 years | Rigid; good load transfer but less flexible |
| Polysulfide | Fuel-exposed areas, airfields | 10–20 years | Resistant to fuel and chemicals |
| Hot-Poured Rubberized Asphalt | Road and highway joints | 5–10 years | Cost-effective for roads; requires heated application |
| Backer Rod + Sealant | All exterior joints | Varies | Backer rod controls sealant depth (2:1 width-to-depth ratio) |
For best performance, the sealant width-to-depth ratio should be 2:1. A backer rod (closed-cell polyethylene foam) should be installed first to control the sealant depth and prevent three-sided adhesion (which causes sealant failure).
Cost of Installing Control Joints in Concrete
Understanding the cost of control joints helps project managers and owners make informed decisions. The cost is minimal compared to the expense of repairing cracked concrete.
| Item | Unit | Typical Cost (USD) | Notes |
|---|---|---|---|
| Saw-cutting (wet saw) | Per linear foot | $0.75 – $2.00 | Varies by slab thickness and location |
| Early-entry saw-cutting | Per linear foot | $1.00 – $2.50 | Premium for speed and reduced raveling |
| Polyurethane joint sealing | Per linear foot | $1.50 – $4.00 | Includes backer rod and material |
| Tooled joint (during pour) | Per linear foot | $0.25 – $0.75 | Lowest cost; included in finishing labor |
| Joint resealing (maintenance) | Per linear foot | $1.00 – $3.00 | Every 5–10 years |
| Crack repair (no joints) | Per linear foot | $5.00 – $30.00+ | Shows the cost of NOT installing joints |
Return on Investment: A properly designed and installed control joint system costs a fraction of the $5 to $30+ per linear foot cost of repairing random concrete cracks. On a 10,000 sq ft industrial slab, the savings can reach tens of thousands of dollars over the slab’s lifespan.
Common Mistakes in Control Joint Installation
Even experienced contractors make avoidable mistakes with control joints. Here are the most common errors and how to prevent them:
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Cutting Too Late
Waiting too long after concrete placement before making saw cuts. Random cracks often develop within 6–18 hours. Always monitor set time and cut as soon as the concrete can support the saw without raveling.
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Insufficient Depth
Cutting less than 1/4 of the slab thickness. The weakened plane is not deep enough to guarantee that the crack will follow the joint. Always verify cut depth with a depth gauge.
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Excessive Joint Spacing
Placing joints too far apart (e.g., every 20 feet in a 4-inch slab). This is the most common cause of mid-panel cracking. Always follow the 24–36× slab thickness rule.
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High Aspect Ratio Panels
Creating elongated panels with length-to-width ratios exceeding 1.5:1. These panels are prone to diagonal cracking at the corners. Always aim for approximately square panels.
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Not Sealing Joints in Wet Areas
Leaving joints unsealed in driveways, parking areas, or any moisture-exposed concrete. Water infiltrating open joints causes subgrade erosion, joint edge spalling, and accelerated deterioration.
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Re-entrant Corners Without Control Joints
L-shaped or irregular slabs create stress concentration at re-entrant corners (inside corners). Without a diagonal control joint at these points, cracks will inevitably radiate from the corner.
Standards & Codes for Concrete Control Joints
Control joint design and installation is governed by internationally recognized standards and codes. Engineers and contractors must comply with the applicable standards for their region and project type.
| Standard / Code | Organization | Topic |
|---|---|---|
| ACI 302.1R | American Concrete Institute | Guide for concrete floor and slab construction |
| ACI 360R | American Concrete Institute | Design of slabs-on-ground |
| ACI 224R | American Concrete Institute | Control of cracking in concrete structures |
| ACI 318 | American Concrete Institute | Building code requirements for structural concrete |
| ASTM C1399 | ASTM International | Average residual strength of fiber-reinforced concrete |
| IS 456:2000 | Bureau of Indian Standards | Plain and reinforced concrete — code of practice |
| BS EN 1992 (Eurocode 2) | BSI / CEN | Design of concrete structures |
| IRC:58 | Indian Roads Congress | Guidelines for rigid pavement design |
Frequently Asked Questions — Control Joint in Concrete
Here are 14 expert answers to the most commonly asked questions about concrete control joints:
A control joint in concrete is a planned, intentional groove or slot cut or formed in a concrete slab or wall to predetermine and control where cracking will occur due to shrinkage, thermal movement, or settlement. It acts as a guided crack path, ensuring that cracking happens in a hidden, manageable location rather than randomly across the slab surface.
Concrete naturally shrinks as it cures and expands or contracts with temperature changes. Without control joints, this movement causes random, uncontrolled cracking. Control joints guide cracks to a specific, pre-planned location beneath the joint — making them invisible, predictable, and easy to seal and maintain.
As a general rule per ACI 360R, control joints should be placed at intervals of 24 to 36 times the slab thickness. For a 4-inch (100mm) slab, joints should be spaced 8 to 12 feet (2.4–3.6m) apart. For a 6-inch (150mm) slab, spacing is 12 to 18 feet (3.6–5.4m). Always ensure panels remain as square as possible.
Control joints should be cut to a minimum depth of one-quarter (1/4) of the slab thickness. For a 4-inch slab, the minimum cut depth is 1 inch. For a 6-inch slab, it is 1.5 inches. Some specifications call for 1/3 depth for maximum effectiveness. Cutting too shallow means the joint may fail to guide cracking to the desired location.
A control joint (or contraction joint) is a partial-depth groove that creates a weakened plane to guide cracking caused by shrinkage. An expansion joint (or isolation joint) is a full-depth gap filled with compressible material that physically separates two concrete sections, allowing them to expand without pushing against each other. Control joints manage shrinkage; expansion joints manage thermal growth.
Yes — control joints are not only safe but are a critical safety requirement. They prevent random uncontrolled cracking that could compromise structural integrity, aesthetics, and water tightness. Building codes and ACI standards require control joints in most concrete slab and wall construction to ensure long-term structural safety.
Early-entry saw cuts should be made within 1 to 4 hours after finishing concrete. Conventional wet saw cuts should be made within 4 to 12 hours. The exact timing depends on the concrete mix, ambient temperature, humidity, and wind speed. The concrete should be hard enough that the saw doesn’t cause raveling, but not so hard that shrinkage cracking has already begun.
Yes — in areas subject to traffic, moisture, or contamination, control joints should be sealed with a flexible joint sealant such as polyurethane or silicone to prevent debris entry and water infiltration. A backer rod should be installed first to control sealant depth. In interior, climate-controlled environments with no traffic, joints may sometimes be left unsealed, but sealing is generally recommended for all applications.
Without control joints, concrete WILL crack randomly due to shrinkage and temperature changes. These random cracks are uncontrolled, irregular, wider, and structurally more harmful than guided cracks at joints. They are also far more expensive to repair — typically $5 to $30+ per linear foot compared to the $0.75 to $2.50 per linear foot cost of installing joints properly from the start.
Yes. Control joints are widely used in concrete masonry unit (CMU) walls, tilt-up panels, retaining walls, and cast-in-place concrete walls to control cracking caused by shrinkage, thermal movement, and settlement. In CMU walls, they are typically placed at 20-foot intervals or at changes in wall height, section, or geometry.
The maximum panel aspect ratio (length to width) between control joints should not exceed 1.5:1 according to ACI 360R. Square panels are preferred to minimize the risk of diagonal cracking at panel corners. Elongated panels with ratios of 2:1 or more are highly prone to cracking regardless of how well the joints are spaced and cut.
Control joints are typically filled with polyurethane sealant (most common), silicone sealant (wet areas), epoxy (rigid industrial floors), polysulfide (fuel-resistant areas), or hot-poured rubberized asphalt (roads). A closed-cell polyethylene backer rod is always installed first in joints deeper than 1/2 inch to control sealant depth and prevent three-sided adhesion failure.
The cost of saw-cutting control joints typically ranges from $0.75 to $2.50 per linear foot for cutting, plus $1.50 to $4.00 per linear foot for sealing with polyurethane. This minimal investment is a fraction of the $5 to $30+ per linear foot cost of repairing random cracks in concrete that lacks adequate joint design.
Control joints are intentional weakened planes formed within a single concrete pour to control cracking. Construction joints are where two successive concrete pours meet — they are the boundary between a previous and new pour. Construction joints are structural interfaces designed with dowels or keyways to transfer load; control joints are crack-guidance features that do not span the full depth of the slab.