Structural and Non-Structural Cracks in Concrete
Concrete is renowned for its durability and widespread use in construction, but like any material, it's susceptible to cracking. Cracks in concrete are generally classified into two types: structural and non-structural. For construction and engineering professionals, understanding the difference between these types of cracks is vital for making informed decisions about repair, maintenance, and overall safety.
Structural Cracks: What You Need to Know
Structural cracks are caused by applied loads, which can affect the stability and safety of a structure. Here are the main types:
1. Flexural Cracks
Flexural cracks occur in reinforced concrete members subjected to bending, typically in the tensile zone at the underside of beams. When beams and slabs bear significant loads, they deflect, causing both the steel reinforcement and surrounding concrete to stretch. If the tension exceeds the concrete's tensile strength, transverse or flexural cracks appear. Initially, these cracks narrow from the surface to the steel, but over time, under sustained loading, they widen and become more uniform across the member.
2. Shear Cracks
Shear cracks, also known as diagonal tension cracks, arise from structural loading or movement after the concrete has hardened. These cracks are caused by the combined effects of bending and shear forces, making beams and columns particularly vulnerable. The presence of diagonal tension cracks is a serious concern, indicating stress from multiple directions within the concrete.
3. Internal Micro-Cracks
Internal micro-cracks develop in areas of severe stress, often due to large differential cooling rates or compressive loading. These microscopic, discontinuous cracks may initially be invisible but can develop into visible signs of structural issues if they become continuous.
Non-Structural Cracks: Causes and Considerations
Non-structural cracks typically result from the inherent properties of concrete, design practices, or in-service conditions. These cracks are categorised into three main areas: pre-hardening (plastic) cracks, cracks in hardened concrete, and cracks due to chemical effects.
1. Pre-Hardening (Plastic) Cracks
Plastic Shrinkage Cracks: These occur within the first six hours after concrete placement, often due to rapid surface drying caused by moisture loss. Factors like strong winds, high temperatures, and low humidity can accelerate this process, leading to surface cracks that may develop into more severe issues if not addressed promptly.
Plastic Settlement Cracks: These cracks occur when concrete settles under its own weight, particularly when excessive bleeding is present and settlement is restricted by reinforcement or formwork. Proper mix design and good compaction practices can help prevent these cracks.
Cracks Caused by Formwork Movement: Formwork movement after the concrete begins to stiffen but before it gains sufficient strength can lead to cracking. Ensuring the formwork remains in place until the concrete is self-supporting is crucial for preventing these cracks.
2. Cracks in Hardened Concrete
Craze Cracking: This type of cracking is characterised by a network of fine cracks on the concrete surface, often due to shrinkage of the cementitious material. While craze cracks affect the appearance of the concrete, they generally do not impact structural integrity.
Drying Shrinkage Cracks: As concrete loses moisture, it undergoes volume reduction, which can lead to shrinkage cracks if the concrete is restrained. Proper mix design and reinforcement placement are key to minimising these cracks.
Early Thermal Contraction Cracks: Occurring within the first two weeks after placement, these cracks result from temperature changes due to the heat of hydration. They are more common in larger and thicker members and can be mitigated by managing temperature differentials.
3. Cracks Due to Chemical Effects
Corrosion of Steel Reinforcement: Expansive forces from corroding steel reinforcement can cause cracks that run parallel to the reinforcement. These cracks often develop over time and can lead to spalling if not addressed.
Alkali-Silica Reaction (ASR) Cracks: ASR occurs when alkali hydroxides in concrete react with reactive aggregates, producing an expansive gel that leads to map or directional cracking. Preventing ASR involves careful selection of aggregates and monitoring the alkalinity of the concrete.
Conclusion: Why Differentiating Cracks Matters
Understanding the distinction between structural and non-structural cracks in concrete is essential for maintaining the integrity and safety of any construction project. Structural cracks can compromise the stability of a building or infrastructure, requiring immediate attention. Non-structural cracks, while generally less critical, still require consideration to prevent long-term deterioration and aesthetic concerns. By identifying and addressing the root causes of these cracks, professionals can ensure the longevity and durability of concrete structures, ultimately contributing to safer and more resilient built environments.
Reference
VicRoads. (2010). Technical Note: Cracks in Concrete (No. 38). December.