Causes and Treatment of Concrete Cracking

Concrete cracking is a very common and complex problem. Below, I will explain in detail the main causes of concrete cracking, how to identify different types of cracks, and the corresponding treatment and prevention measures.

 

1. Main Causes of Concrete Cracking

Concrete cracking can be categorized into two main types: early cracking and late cracking.

(1) Early cracking (within hours to days after pouring)

Plastic settlement cracking:

Cause: After pouring, concrete is still in a plastic state. Aggregates (gravel, sand) sink, while the cement slurry rises (called "bleeding"). When this settling is hindered by rebar, formwork, or larger aggregate, cracks develop on the concrete surface along the direction of the rebar.
Characteristics: Surface cracks typically occur along the direction of the rebar.

Plastic shrinkage cracks:

Cause: Before the concrete finally sets, surface moisture evaporates faster than the rate at which moisture seeps upward, causing the surface to shrink rapidly while the concrete inside remains plastic, resulting in irregular, network-like cracks and crazing.
Causes: High temperatures, strong winds, low humidity, and inadequate curing.

Temperature stress cracks (caused by hydration heat):

Cause: After pouring large volumes of concrete (such as foundation slabs and beams), the cement hydration reaction releases a large amount of heat, causing the internal temperature to rise sharply (up to 50-70°C). This rapid heat dissipation from the surface creates a significant temperature difference between the inside and outside, generating thermal stress. When the tensile stress exceeds the concrete's early tensile strength, cracking occurs.
Characteristics: Cracks are deep and wide, often occurring at cross-sectional changes or in the middle of the structure.

Construction process issues:

Excessive water addition: Adding water arbitrarily for ease of construction increases the water-cement ratio, severely reducing concrete strength.
Improper vibration: Excessive vibration causes aggregate sinking and water seepage; inadequate vibration results in loose concrete.
Failure to apply a secondary trowel to the surface before initial setting prevents the closure of early plastic cracks.

(2) Late cracking (after hardening)

Drying shrinkage cracks:

Cause: After concrete hardens, excess moisture gradually evaporates, causing volumetric shrinkage. When this shrinkage is constrained by external forces (such as foundations and columns) or internal forces (such as rebar), tensile stresses are generated, leading to cracking. This is the most common type of crack.
Characteristics: Cracks are shallow and fine, often forming an irregular network or parallel lines.

Load-induced cracking:

Cause: The loads borne by the structure (such as deadweight or operational load) exceed its design capacity.
Characteristics: The cracks are relatively wide, and their direction is related to the nature of the load (e.g., vertical cracks in the middle of the beam bottom are bending cracks, while diagonal cracks at the ends of the beam are shear cracks). These cracks require special attention, as they may affect structural safety.

Uneven foundation settlement:

Cause: Uneven foundation soil quality, softening due to waterlogging, or excessive loads lead to uneven foundation settlement, resulting in additional stress within the structure and cracking.
Characteristics: Cracks are often penetrating, with their direction related to settlement.

Alkali-aggregate reaction:

Cause: The alkali in the cement reacts chemically with the active silica in the aggregate, forming an expansive gel. This gel expands in volume after absorbing water, causing concrete cracking.
Characteristics: A map-like or network-like pattern of cracks with silicone gel seeping out of the surface.

Rebar Corrosion Cracks:

Cause: Insufficient concrete cover or carbonization reaching the rebar surface. In the presence of water and oxygen, the rebar rusts, causing the rust to expand several times in volume, cracking the concrete.
Characteristics: Cracks run along the rebar, later accompanied by brown rust.

 

2. Crack Treatment Methods

Before treating cracks, it is necessary to first analyze and determine the crack type, width, depth, stability, and impact on structural safety. Treatment methods are primarily categorized as surface sealing and internal reinforcement.

(1) Non-structural Cracks (Small Width, No Safety Impact)

Surface Sealing Method (Suitable for Micro-Cracks <0.2mm)

Brushing method: Apply a cement-based penetrating crystallizing waterproofing material, epoxy resin, or polymer-modified cement slurry directly to the crack surface to seal the crack and prevent the intrusion of moisture and harmful substances.
Grooving and filling method (suitable for static cracks 0.2-0.5mm wide):

Steps: Chisel a "V" or "U"-shaped groove along the crack → Clean thoroughly → Apply a primer → Fill with epoxy resin mortar, polymer cement mortar, or a specialized sealant.

(2) Structural cracks (larger in width, affecting structural durability or load-bearing capacity)

Low-pressure grouting (injection method) (suitable for cracks 0.1-1.5mm wide)

Steps:

Surface cleaning: Clean the area around the crack.
Inserting grouting nozzles: Attach grouting nozzles at regular intervals along the crack.
Crack sealing: Use sealant to seal the crack surface to prevent grout from leaking.
Pressure grouting: Use a low-pressure syringe to inject epoxy or polyurethane grout into the crack from a grouting nozzle until grout is released from the adjacent grouting nozzle.
Surface finishing: After the grout has solidified, remove the grouting nozzle and smooth the surface.
Structural reinforcement method (suitable for wide cracks that affect bearing capacity)

Bonding fiber composite materials (carbon fiber cloth/plate): High-strength carbon fiber cloth is bonded to the surface of the cracked area, utilizing its high tensile strength to share the load.
Bonding steel plates: Steel plates are bonded to the concrete surface using structural adhesive to increase structural rigidity.
Enlarging the cross-section: A layer of concrete is wrapped around the existing component to increase its cross-sectional dimensions and reinforcement.
Prestressing: Prestressed tendons are used to actively apply pressure to the structure, offsetting some of the tensile stress.
Important: For cracks caused by uneven foundation settlement, alkali-aggregate reaction, etc., the root cause must be addressed first (such as strengthening the foundation) before crack repair.
 

3. Crack Prevention Measures (Key)

Prevention is far better than cure. Strict control should be applied to all aspects of the process, including materials, design, construction, and maintenance.

Materials:

Optimize mix proportions, reduce the water-cement ratio, and use high-efficiency water reducers.
Use well-graded aggregates and reduce cement dosage to reduce hydration heat and shrinkage.
Use low-heat or medium-heat cement for large-volume concrete.

Design aspects:

Ensure proper reinforcement placement and add structural reinforcement (such as crack-resistant steel mesh) in areas prone to cracking (such as around holes and at cross-section changes).
Properly establish expansion joints and post-cast joints (for extra-long structures).

Ensure sufficient concrete cover thickness.

Construction:

The addition of water on site is strictly prohibited.
Strictly control the pouring and vibration processes to ensure uniform compaction and avoid over-vibration and missed vibration.
Implement cooling measures (such as water-cooling aggregates) during hot seasons and insulation measures in winter.
Perform secondary troweling and compaction promptly to eliminate plastic cracks.

Maintenance (critical!):

Early Curing: Immediately cover with plastic sheeting or a curing blanket after pouring to prevent rapid evaporation.
Sufficient Moisturization: After final set, begin regular watering or use a curing agent to keep the concrete surface moist for at least 7-14 days.
Insulation Curing: For large concrete volumes, monitor the temperature difference between inside and outside, implement insulation and moisture curing, and maintain the temperature difference within 25°C.
 

Summary

Crack Types	Main causes:	Treatment Focus	Prevention of core problems
Plastic Shrinkage/Settlement Cracks	Early water loss and impeded settlement	Surface Sealing	Timely screeding and covering to retain moisture
Temperature Cracks	Heat of hydration and large temperature difference between inside and outside	Grouting Reinforcement	Use low-heat cement, cooling, and thermal insulation
Desiccation Shrinkage Cracks	Later water evaporation and shrinkage	Surface Sealing/Grouting	Reduce the water-cement ratio and enhance moisture retention
Load/Settlement Cracks	Overloading and foundation problems	Structural Reinforcement + Root Cause Treatment	Reasonable design to ensure construction quality

When you encounter concrete cracks, don't blindly address them. First, determine their nature and severity. For cracks that are wide, persistent, or potentially impacting structural safety, consult a professional structural engineer or testing company for evaluation. Based on the results, develop a sound treatment plan.
 

Note: The parameters provided in this document are for reference only and are not mandatory. Due to differences in technical characteristics between different brands and models of laser levelers, please consult the manufacturer for a suitable solution before actual operation. This reference document assumes no responsibility for any issues arising from failure to follow the manufacturer's instructions.
 

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