Concrete is a versatile and durable construction material used in various applications such as buildings, roads, bridges, etc. However, over time, concrete may develop cracks due to environmental conditions, structural damage, or poor construction practices.
In this blog post, we will discuss the causes of concrete cracks and common methods of concrete repairs to restore your concrete surfaces.
Causes of Concrete Cracks
Concrete cracks can occur for various reasons, including environmental and structural issues. Understanding the causes of concrete cracks is essential for determining the most suitable repair method and maintaining the overall integrity of the concrete structure.
Here are some common causes of concrete cracks:
1. Shrinkage
Shrinkage is a natural process that occurs as concrete dries and hardens. There are two types of shrinkage-related cracks, such as:
- Plastic shrinkage cracks. These occur when the concrete is still in its plastic state, and rapid water evaporation causes the concrete to shrink faster than it can handle, resulting in cracks.
- Drying shrinkage cracks. These occur during the curing process when moisture within the concrete evaporates, causing the concrete to contract. If the shrinkage is restrained by reinforcement or adjacent structural elements, it may lead to cracking.
2. Thermal Expansion and Contraction
Temperature changes can cause concrete to expand and contract. When the temperature difference between the concrete’s interior and exterior exceeds the material’s tensile strength, thermal cracks may form. This type of cracking is more common in mass concrete structures, such as dams or large foundations, where temperature variations are more significant.
3. Settlement
Settlement cracks occur when the underlying soil or subgrade settles unevenly, causing the concrete to crack due to differential settlement. These cracks often appear as diagonal or vertical cracks near the corners of slabs, walls, or columns. Factors contributing to settlement include poor soil compaction, erosion, or changes in the moisture content of the soil.
4. Corrosion of Reinforcing Steel
Corrosion-induced cracks result from the corrosion of reinforcing steel bars embedded in the concrete. As the steel corrodes, it expands, creating internal pressure that leads to cracking. These cracks usually run parallel to the reinforcement and are accompanied by rust stains on the surface. Causes of corrosion include exposure to moisture, chloride ions (e.g., from deicing salts), or insufficient concrete cover over the reinforcing steel.
5. Overloading
Overloading cracks occur when the concrete structure is subjected to loads exceeding its design capacity. These cracks may appear diagonal, vertical, or horizontal, depending on the nature and direction of the applied load. Overloading can be caused by improper design, construction errors, or unexpected external forces (e.g., earthquakes or impact loads).
6. Poor Construction Practices
Poor construction practices can also lead to concrete cracking. Some examples include:
- Inadequate concrete mix design. An improper mix of cement, sand, aggregate, and water can result in weak concrete that is more susceptible to cracking.
- Insufficient curing. Inadequate curing can cause the concrete to dry too quickly, leading to shrinkage cracks.
- Improper placement of reinforcement. Incorrectly placed reinforcing steel can cause stress concentrations and lead to cracking.
- Excessive use of water. Adding too much water to the concrete mix can weaken the concrete and increase the risk of cracking.
7. Structural Design Issues
Errors in structural design can lead to concrete cracking. For example, inadequate reinforcement, incorrect sizing of structural elements, or insufficient consideration of loads and environmental factors can all contribute to cracking.
Why Repair Cracks in Concrete?
It’s essential to repair cracks in concrete for several reasons:
- To maintain the structural integrity of the concrete structure
- To prevent water infiltration that can cause further damage
- To reduce the risk of corrosion in reinforcing steel
- To restore the visual appeal of the concrete surface
Types of Concrete Cracks
The appropriate repair method depends on the type of crack and the extent of the damage. Here are the types of concrete cracks:
Based on Causes
- Plastic shrinkage cracks. These cracks form when the concrete is still in its plastic state (before it has completely hardened). Rapid loss of water due to evaporation causes the concrete to shrink, leading to the formation of cracks. Plastic shrinkage cracks are usually shallow and random and can occur on horizontal and vertical surfaces.
- Drying shrinkage cracks. Drying shrinkage cracks occur when the concrete loses moisture during the curing process. As the concrete dries, it contracts, which may cause cracking if the shrinkage is restrained by reinforcement or adjacent structural elements. These cracks may appear as diagonal or random patterns on the surface.
- Thermal cracks. Thermal cracks are caused by temperature changes within the concrete, leading to expansion and contraction. When the temperature difference between the concrete’s interior and exterior exceeds the material’s tensile strength, cracks may form. These cracks are common in mass concrete structures, like dams or large foundations.
- Settlement cracks. Settlement cracks occur when the underlying soil or subgrade settles unevenly, causing the concrete to crack due to differential settlement. These cracks often appear as diagonal or vertical cracks near the corners of slabs, walls, or columns.
- Corrosion-induced cracks. Corrosion-induced cracks result from the corrosion of reinforcing steel bars embedded in the concrete. As the steel corrodes, it expands, creating internal pressure that leads to cracking. These cracks usually run parallel to the reinforcement and are accompanied by rust stains on the surface.
- Overloading cracks. Overloading cracks occur when the concrete structure is subjected to loads exceeding its design capacity. These cracks may appear diagonal, vertical, or horizontal, depending on the nature and direction of the applied load.
Based on Location
- Vertical cracks. Vertical cracks run up and down the concrete surface, typically in walls, columns, or other vertical structural elements. Factors like shrinkage, settlement, or overloading can cause them.
- Horizontal cracks. Horizontal cracks run parallel to the ground, often appearing in a concrete slab, beam, or other horizontal surfaces. Common causes include drying shrinkage, overloading, or reinforcement corrosion.
- Diagonal cracks. Diagonal cracks have a sloping orientation, usually appearing at an angle to the structure’s vertical or horizontal axis. They can result from differential settlement, thermal expansion, or overloading.
Based on Characteristics
- Active cracks. Active cracks continue to widen, lengthen, or change over time due to ongoing movement in the concrete structure. Repairing active cracks requires addressing the underlying cause of the movement and using appropriate repair methods to prevent further damage.
- Dormant cracks. Dormant or passive cracks remain stable and do not change over time. These cracks can generally be repaired using standard crack repair methods without addressing the root cause, as the cracks are no longer progressing.
Common Concrete Repair Methods
1. Routing and Sealing
Routing and sealing is a popular method for repairing concrete cracks in horizontal and vertical surfaces. This technique involves:
- Widening the crack into a V-shape
- Cleaning the crack to remove debris and loose material
- Filling the crack with a suitable sealant material (e.g., polyurethane, silicone, or polymer-based)
This cost-effective technique works well for dormant cracks that do not require additional reinforcement.
2. Epoxy Injection
Epoxy injection is a common method for crack repair in concrete structures where structural strength needs to be restored. The process includes the following:
- Cleaning the crack and sealing its surface with an epoxy material
- Drilling holes (injection ports) along the crack at regular intervals
- Injecting a low-viscosity epoxy resin into the drilled holes, filling the entire crack
- Sealing the injection ports after the epoxy has cured
This method is effective for active and dormant cracks and works well on vertical surfaces.
3. Stitching
Stitching is used to repair cracked concrete slabs or large structural elements. It involves:
- Drilling holes on either side of the crack
- Inserting U-shaped metal bars (staples) into the holes
- Grouting the bars in place with an appropriate repair material (e.g., epoxy or cementitious grout)
This method is suitable for both active and dormant cracks and helps restore the tensile strength of the concrete.
4. Dry Packing
Dry packing is a method for repairing concrete cracks in vertical and overhead surfaces. This technique involves:
- Cleaning the crack and removing any loose material
- Mixing a dry, non-shrink repair material (e.g., Portland cement and sand)
- Packing the material tightly into the crack using a trowel or similar tool
Dry packing is best for small, dormant cracks that do not require additional reinforcement.
5. Polymer Modified Cement Mortar
Polymer-modified cement mortar is another common method for concrete crack repair. This technique involves:
- Preparing the surface by cleaning the crack and removing any loose material
- Mixing a polymer-modified cement mortar (e.g., with added acrylic or latex polymers)
- Applying the mortar to the crack using a trowel or similar tool
This method is suitable for repairing active and dormant cracks in horizontal and vertical surfaces.
Choosing the Right Repair Method
When choosing a repair method, consider the following factors:
- The type and size of the crack (active or dormant, width, and depth). For instance, dry packing is suitable for small, dormant cracks. On the other hand, epoxy injection is best for active cracks that require structural reinforcement.
- The location of the crack (horizontal or vertical surface). Dry packing is better suited to vertical and overhead surfaces, whereas stitching is best for slabs and large structures. It’s because these methods provide additional reinforcement.
- The type of repair material (epoxy, cementitious, etc.). Different types of concrete repair products may be better suited to surfaces or crack characteristics. For example, epoxy is best for active cracks, whereas polymer-modified cement mortar works well on dormant cracks.
- The degree of movement in the concrete structure (shrinkage, settlement, thermal expansion). Active cracks often require additional methods to address the underlying cause of movement.
Final Thoughts
Concrete cracks are a common issue for structural elements. Fortunately, there are many effective repair methods available. But how to fix damaged concrete depends on the specific situation and the desired outcome. Understanding the common concrete repair methods and their applications allows you to select the most appropriate technique for your project. Remember to always consult with a professional if you are unsure about the best approach for repairing cracked concrete.
Photo Credit: MorgueFile