As the most widely used and largest-volume composite material in civil engineering, cement concrete is favored for its superior performance. However, it also exhibits significant weaknesses, such as poor homogeneity, high variability, and susceptibility to cracking—issues that are particularly prominent in mass concrete structures. Cracks not only compromise structural aesthetics and durability but also pose persistent challenges for engineers.
Through years of on-site engineering practice and in-depth observation, we have summarized practical experiences and methodologies to effectively control cracks in mass concrete, thereby enhancing project quality and structural safety.
Causes of Cracks
01Concrete Shrinkage
Shrinkage-induced cracks are the most common type, predominantly occurring in mass concrete structures. Smaller components like beams, slabs, columns, and especially prestressed members rarely develop shrinkage cracks. These cracks form due to volumetric contraction caused by moisture loss during the hardening process.
02Thermal Stress
The heat released during cement hydration raises the internal temperature of concrete, creating a temperature differential between the interior and exterior. This generates thermal stress, damaging the concrete matrix and forming cracks. In mass concrete, due to its large cross-sectional thickness, hydration heat accumulates internally, leading to significant temperature gradients and crack formation.
03Material and Mix Design
Poor-quality raw materials or improper mix proportions directly reduce tensile strength. Excessive cement content, low-grade cement, high water-cement ratios, contaminated aggregates, and inappropriate admixtures can all weaken concrete strength and induce cracks.
04Load Effects
Overloading or uneven load distribution may cause cracks. Load-induced cracks exhibit distinct characteristics depending on the stress type, often appearing in tension zones, shear zones, or areas subjected to severe vibration. Notably, peeling or short cracks along the compression direction in compression zones typically signal that the structure has reached its load-bearing limit, foreshadowing potential failure.
Preventive Measures
Material Optimization
- Low-Heat Cement: Use low-heat or blended cement (e.g., slag Portland cement) to reduce hydration heat.
- Mineral Admixtures: Incorporate fly ash, slag powder, or other supplementary cementitious materials to enhance performance.
- Superplasticizers: Employ high-range water reducers to lower water demand, optimize the water-cement ratio, and improve density and crack resistance.
Construction Techniques
- Layered Pouring: Implement layered placement to reduce thickness per layer, minimize heat buildup, and control thermal gradients.
- Sequencing: Adopt rational pouring sequences and rates to avoid large-scale simultaneous pours and extreme temperature differentials.
- Timely Compaction: Ensure thorough vibration to eliminate air voids and internal porosity.
Temperature Control
- Sensors: Embed temperature sensors in the concrete to monitor real-time thermal changes and trigger cooling or insulation measures.
- Expansion Joints: Install joints at designated locations to mitigate thermal and shrinkage stresses.
Reinforcement Design
- Strategic Reinforcement: Increase steel reinforcement in crack-prone areas to enhance tensile strength and stress distribution.
- Fiber Reinforcement: Add steel or polypropylene fibers to improve crack resistance.
Formwork Removal
- Delay formwork stripping as long as possible. After removal, immediately cover or backfill the concrete to shield it from environmental impacts. The curing period should align with the rapid strength development phase (7–28 days).
Post-Construction Curing
Moisture Retention
- Spray Curing: Use automated spray systems to maintain surface moisture without over-saturation.
- Covering: Apply plastic film, wet burlap, or straw mats to prevent evaporation.
- Curing Compounds: Spray membrane-forming agents to seal in moisture.
- Ponding: Create temporary water pools on the surface for immersion curing.
Thermal Management
- Cooling Pipes: Embed pipes to circulate chilled water, dissipating hydration heat and minimizing thermal stress.
- Chilled Mixing: Add ice or chilled water during mixing to lower initial concrete temperature.
- Insulation: Use insulating blankets or foam boards in cold environments to prevent freezing.
Crack Repair Methods
01
Surface Repair
For non-structural microcracks: Seal cracks with epoxy mortar or cement paste.
Steps:
- Clean the surface to remove debris.
- Apply sealing material to fully fill cracks.
- Finish the surface after curing.
02
Pressure Grouting
For wide/deep structural cracks: Inject epoxy resin, polyurethane, or acrylic into cracks under pressure.
Steps:
- Clean and prepare the crack.
- Install grout ports at intervals along the crack.
- Inject repair material and allow it to cure.
- Remove ports and finish the surface.
03
External Reinforcement
For severe structural cracks: Bond steel mesh or carbon fiber-reinforced polymer (CFRP) sheets to restore integrity.
Steps:
- Clean and prepare the surface.
- Design reinforcement based on crack location and length.
- Apply adhesive and attach reinforcement materials.
We hope that the discussions and insights presented in this article will inspire and provide practical guidance to engineering professionals, fostering collaborative advancements and innovations in the field of civil engineering. For further discussion or communication, you can just feel free to contact us at any time.