Concrete Cracking and Saw Joints

Concrete, like other construction materials, contracts and expands with changes in moisture and temperature. It can also bend, somewhat, depending on how much weight is on it and how it’s supported. If precautions aren’t taken during design and construction to handle these movements, cracks may form.

Common Types of Cracks:

There are several types of common cracks that can occur:

1. Plastic Shrinkage Cracks: These are narrow cracks that form on the surface of freshly poured concrete during the early stages of curing. They occur when the surface of the concrete dries out too quickly before it has had a chance to fully set and harden. These cracks typically appear as random lines or networks on the surface and are often shallow.

2. Cracks Resulting From Improper Jointing: Improper jointing can lead to excessive stress concentrations in concrete, especially at corners or edges where there are no joints. This stress can cause the concrete to crack as it contracts during the curing process. Also, changes in temperature can cause concrete to expand and contract. Without properly spaced joints to accommodate this movement, the concrete may crack due to thermal stresses.
   
   In pavements and other structural applications, traffic loads can induce stresses in the concrete. Without adequate jointing to control these stresses, the concrete may crack under the pressure of heavy loads.



3. Cracks That Result From External Restraint: These cracks occur when the concrete is prevented from expanding, contracting, or deforming freely, leading to the development of stress within the material. These types of cracks can include Curvature Cracking, Corner Cracking, Tensile Cracking, and Warping or Curling Cracks.



4. Cracks Caused By Lack Of An Isolation Joint: Isolation joints are intentional gaps or spaces left between adjacent concrete elements to allow for movement and prevent cracking. When isolation joints are not provided, several types of cracks can develop that compromise the structural integrity and durability of the concrete. Properly incorporating isolation joints into concrete design and construction is essential for controlling cracking and ensuring the long-term performance of concrete structures.



5. Cracks Caused By Freezing & Thawing Cycles: These cracks are primarily caused by the expansion and contraction of water within the concrete due to cyclic freezing and thawing cycles. They occur primarily in regions with cold climates where temperatures fluctuate above and below the freezing point of water.



6. Craze Cracks: Craze cracks in concrete are fine, shallow surface cracks that resemble a spider web or a network of intersecting lines. These cracks are primarily caused by the tensile stresses generated within the concrete surface during the drying and curing process. As the concrete dries and shrinks, especially during the initial stages of curing, tensile stresses develop at the surface due to differential drying rates between the surface layer and the underlying concrete. These stresses can exceed the tensile strength of the concrete, leading to the formation of fine cracks on the surface.



7. Settlement Cracks: These cracks occur when the soil beneath a concrete slab compresses or settles unevenly, causing the concrete to crack as it adjusts to the changing support conditions. They typically occur in structures such as buildings, foundations, and pavements. Settlement cracks in concrete can pose structural and safety concerns, as they may compromise the integrity of the concrete slab and lead to further damage if left unaddressed.




Most concrete cracks typically result from incorrect design and construction methods, including: neglecting isolation and contraction joints and using improper jointing techniques; inadequate preparation of the subgrade; excessive water added to high slump concrete during construction; improper finishing; and insufficient or absent curing.

Methods To Minimize Cracking:

Concrete cracking is common but can be minimized by following basic concreting practices:

• Subgrade and Formwork: Remove topsoil and soft spots, compact the soil or fill, slope for drainage, and avoid placing concrete on frozen subgrades. Ensure formwork is sturdy to withstand concrete pressure.
• Concrete: Use moderate slump concrete, avoid retempering, and specify air-entrained concrete for freezing conditions.
• Finishing: Bull float promptly after screeding, avoid water during finishing, and use broom finish for exterior surfaces. Prevent rapid drying with wind breaks or wet coverings.
• Curing: Start curing immediately, use liquid membrane or damp burlap for at least 3 days, and consider a second application of curing compound.
• Joints: Create contraction joints spaced properly and run tooled or saw-cut joints at the right time. Use isolation joints to prevent bond with adjacent elements.
• Cover Over Reinforcement: Ensure at least 2 inches of concrete cover over reinforcement to prevent contact with salt and moisture, thus preventing cracks caused by rust expansion.

Saw Joints

Saw joints in concrete refer to intentional cuts or grooves made in concrete surfaces after it has hardened but before it fully sets. These joints are created using specialized sawing equipment and are essential for controlling cracking and ensuring the structural integrity of the concrete. Some key aspects of saw joints in concrete include:

• Purpose: Saw joints are primarily installed to control the cracking that naturally occurs in concrete due to shrinkage during the curing process and temperature changes. By creating predetermined lines of weakness, saw joints encourage cracks to occur along these lines, preventing random cracking that can compromise the concrete’s strength and appearance.
• Timing: Saw joints are typically made within the first 24 hours after the concrete is poured and finished. The concrete needs to be firm enough to support the weight of the sawing equipment but still relatively fresh to allow for clean cuts without excessive chipping or spalling.
• Equipment: Specialized concrete saws are used to create saw joints. These saws can be either hand-held or walk-behind machines equipped with diamond blades designed specifically for cutting through concrete. The blades may be wet or dry depending on the specific requirements of the project.
• Spacing and Depth: The spacing and depth of saw joints are critical factors in their effectiveness. The spacing between saw cuts depends on various factors, including the thickness of the concrete slab, its intended use, and environmental conditions. Generally, saw cuts are made at regular intervals to create rectangular or square-shaped sections. The depth of the saw cuts is typically one-fourth to one-third of the slab thickness.
• Pattern: Saw joints can be installed in various patterns depending on the specific requirements of the project. Common patterns include straight lines, diagonal lines, or grids. The choice of pattern may depend on aesthetic preferences, structural considerations, or functional requirements.
• Sealing: After saw joints are installed, they are often filled with a joint sealing compound to prevent water infiltration, reduce the risk of spalling, and prolong the lifespan of the concrete. Joint sealants also help maintain the structural integrity of the concrete by minimizing the intrusion of debris and chemicals.

Saw joints play a crucial role in ensuring the durability and performance of concrete structures by controlling cracking and promoting long-term stability. Properly designed and installed saw joints are essential for maintaining the integrity and appearance of concrete surfaces in various applications, including roads, sidewalks, driveways, and industrial floors.

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Concrete Surface Dusting

Role of Temperature in Cracking

Find helpful information and free educational resources about concrete on our Concrete Resources page at TriangleReadyMix.com/edu. We also have a convenient Glossary of Ready-Mixed Concrete Terms. Triangle Ready Mix is a proud member of L&L Construction Group, along with our sister companies L&L Concrete and East Coast Civil Group.