Concrete is the most widely used material in construction. Concrete is very resistant to compressive forces to an appreciable extent but has one flaw. It is weak in tension. In order to overcome this flaw, concrete used in construction where tension forces exist, is designed to be embedded with steel reinforcement bars to overcome the tensile forces. When this is done, the concrete specimen is known as a reinforced concrete structure. Another way to overcome the deficient nature of concrete when it comes to tension is to prestress it before it is used in service. This form of concrete construction is known as prestressed concrete.

When sections of the concrete structure experiencing tensile forces exceed the capacity to withstand these forces, cracking manifests. The cracking phenomenon is the splitting of the concrete into distinct parts.

Photo of Cracking in a Concrete Specimen                                                               _{Photo Source: gettyimages – Internet}

Overview of Crack Monitoring:

Crack monitoring in concrete structures is a crucial aspect of structural health monitoring (SHM) that ensures the safety and durability of the structure over time. Cracks in concrete can occur due to various reasons, including thermal expansion, shrinkage, overload, or other mechanical stresses. Monitoring these cracks is essential for identifying potential structural issues before they lead to significant damage or failure.

Objectives of Crack Monitoring:

1. Early Detection: Identify cracks early to prevent further propagation and mitigate potential structural failures.
2. Assessment of Crack Growth: Monitor the rate of crack growth over time to determine whether the crack is stable or expanding.
3. Determination of Cause: Understand the root cause of cracking to implement appropriate repair strategies.
4. Structural Integrity: Ensure that the structure remains safe for use by monitoring critical cracks.
5. Repair Planning: Provide data that can be used to plan maintenance and repair activities.

Common Methods of Crack Monitoring:

1. Visual Inspection:

• Description: Simple and cost-effective method where inspectors visually examine the structure for cracks. Crack widths can be measured using crack width gauges or microscopes.
• Advantages: Quick, inexpensive, and can be performed regularly.
• Disadvantages: Subjective and may miss fine cracks. Not suitable for continuous monitoring.

2. Mechanical Crack Monitors:

• Description: Devices like crack monitoring gauges (e.g., DEMEC gauge) or displacement transducers are placed across cracks to measure their width over time.
• Advantages: Provides more accurate measurements compared to visual inspection.
• Disadvantages: Limited to monitoring specific cracks. Installation can be invasive.

3. Strain Gauges:

• Description: Strain gauges are attached to the surface of the concrete to measure strain, which can indicate the formation or growth of cracks.
• Advantages: Sensitive to small deformations and can provide early warning.
• Disadvantages: Installation requires careful surface preparation, and the gauges are sensitive to environmental conditions.

4. Acoustic Emission (AE) Monitoring:

• Description: This technique listens to the high-frequency stress waves generated by crack growth or other defects in the structure. Sensors are placed on the structure to detect these emissions.
• Advantages: Real-time monitoring and can detect active crack growth. Can cover large areas.
• Disadvantages: Interpretation of data can be complex, and background noise can affect accuracy.

5. Fibre Optic Sensors:

• Description: Fibre optic cables embedded in the concrete can measure changes in strain, temperature, or even crack width. Bragg grating sensors or distributed fiber optic sensing (DFOS) are common methods.
• Advantages: High sensitivity, can monitor over long distances, and provide continuous data.
• Disadvantages: Installation can be costly and complex.

6. Digital Image Correlation (DIC):

• Description: A non-contact optical method where images of the structure are taken over time. The images are then processed to track deformations and crack development.
• Advantages: Provides full-field deformation data and can detect crack initiation.
• Disadvantages: Requires high-quality imaging equipment and advanced software for analysis.

7. Ultrasonic Testing:

• Description: Ultrasonic waves are sent through the concrete, and their reflection and transmission are analysed to detect cracks and other flaws.
• Advantages: Can detect internal cracks and is non-destructive.
• Disadvantages: Requires skilled operators and can be time-consuming.

8. Ground Penetrating Radar (GPR):

• Description: GPR uses radar pulses to image the interior of the concrete. Changes in the reflected signal can indicate cracks or voids.
• Advantages: Non-destructive and can detect subsurface cracks.
• Disadvantages: Interpretation of data can be complex, and results may be affected by moisture content.

9. Electrical Resistance Method:

• Description: Cracks can change the electrical resistance of concrete. Sensors embedded in the concrete can measure these changes to detect crack formation.
• Advantages: Can provide real-time data and is sensitive to crack initiation.
• Disadvantages: Requires embedding sensors during construction, and the method can be affected by environmental factors.

10. Laser Scanning:

• Description: Laser scanners can create detailed 3D models of the structure, allowing for precise measurement of crack widths and deformation.
• Advantages: High accuracy and can cover large areas.
• Disadvantages: Expensive and requires specialized equipment.

Choosing the Right Monitoring Method:

The choice of crack monitoring method depends on several factors:

• Type of Structure: Bridges, buildings, dams, etc., may require different monitoring approaches.
• Location of Cracks: Surface vs. internal cracks.
• Accessibility: Some methods require physical access to the crack, while others can be done remotely.
• Accuracy Requirements: Some methods provide more detailed information than others.
• Cost: More advanced methods tend to be more expensive.
• Environmental Conditions: Methods may need to withstand harsh conditions like moisture, temperature fluctuations, or vibrations.

Case Studies and Applications:

1. Bridges: Monitoring cracks in bridge decks and supports to ensure structural safety. Acoustic emission and fiber optic sensors are often used for continuous monitoring.
2. Dams: Due to the critical nature of dams, fiber optic sensors and ultrasonic testing are commonly used to monitor cracks and potential failures.
3. High-rise Buildings: Strain gauges and mechanical crack monitors can be used to assess cracks in key structural elements like beams and columns.

Building Code Provisions on Crack Widths:

The fact that a structure is noted to experience cracking is not necessarily a cause to be of alarm. Building codes and standards provide provisions and guidelines on allowable crack widths in concrete structures to ensure safety, durability, and aesthetic acceptability. These provisions vary depending on the type of structure, environmental exposure, and the function of the element. Below is an overview of the key building code provisions related to crack widths in concrete structures.

1. ACI 318 (American Concrete Institute):

The ACI 318 code is widely used in the United States for the design of concrete structures. It addresses crack control indirectly through requirements for minimum reinforcement, cover, and spacing.

• Crack Width Control: ACI 318 does not explicitly provide maximum allowable crack widths but focuses on reinforcement detailing to control cracking. Section 24.3 of ACI 318-19 provides guidance on crack control in flexural members, primarily by limiting the spacing of reinforcement.
• Exposure Categories: For aggressive environments, the code suggests more conservative reinforcement designs to limit cracking, particularly in members exposed to moisture, deicing chemicals, or other aggressive agents.

While ACI 318 does not provide direct limits on crack width, typical industry practice suggests:

• 0.4 mm (0.016 in.) for interior exposures (dry environments).
• 0.3 mm (0.012 in.) for exterior exposures (moderate moisture).
• 0.2 mm (0.008 in.) for severe exposure conditions (e.g., exposure to aggressive chemicals).

2. Eurocode 2 (EN 1992-1-1):

Eurocode 2, which is widely used in Europe, provides more specific guidance on acceptable crack widths.

• Crack Width Limits: Eurocode 2 defines allowable crack widths based on the exposure class:
  • 0.4 mm for interior or low-exposure conditions (Class X0).
  • 0.3 mm for moderate exposure conditions (Class XC1 to XC4).
  • 0.2 mm for severe exposure conditions (Class XD, XS).
  • 0.2 mm or less for members in contact with water or other aggressive environments (Class XA, XF).
• Calculation of Crack Width: Eurocode 2 also provides formulas for calculating crack widths based on parameters like concrete cover, bar spacing, and tensile strain.

3. BS 8110 (British Standard):

BS 8110, although partially superseded by Eurocode 2, is still referenced in some regions.

• Crack Width Limits: BS 8110 recommends maximum allowable crack widths based on the exposure condition:
  • 0.3 mm for interior or sheltered conditions.
  • 0.2 mm for exterior or moderate exposure conditions.
  • 0.1 mm for members in aggressive environments (e.g., exposed to chlorides).

4. CSA A23.3 (Canadian Standards Association):

The Canadian standard CSA A23.3 provides guidelines for crack control in reinforced concrete structures.

• Crack Width Limits: CSA A23.3 sets limits based on environmental exposure:
  • 0.33 mm (0.013 in.) for structures with normal exposure.
  • 0.25 mm (0.01 in.) for structures exposed to moisture, deicing chemicals, or other aggressive agents.
• Reinforcement Requirements: Like ACI 318, CSA A23.3 focuses on reinforcement detailing to limit crack widths.

5. IS 456 (Indian Standard):

IS 456 provides guidelines for the design of reinforced concrete structures in India.

• Crack Width Limits: IS 456-2000 suggests maximum permissible crack widths based on the exposure condition:
  • 0.3 mm for moderate exposure conditions.
  • 0.2 mm for severe exposure conditions, such as marine environments.
• Calculation Method: IS 456 also provides methods for calculating the probable crack width in beams and slabs, considering factors like the amount and distribution of reinforcement.

6. AS 3600 (Australian Standard):

The Australian Standard AS 3600 provides provisions for crack control in concrete structures.

• Crack Width Limits: AS 3600 specifies maximum allowable crack widths based on exposure classification:
  • 0.3 mm for structures in non-aggressive environments.
  • 0.2 mm for structures exposed to aggressive environments, such as marine exposure.
• Reinforcement Detailing: AS 3600 emphasizes reinforcement detailing to manage crack widths, particularly for members in severe environments.

7. Concrete Durability and Aesthetic Considerations:

• Durability: Crack width limits are often set to prevent the ingress of harmful substances like chlorides, which can cause corrosion of reinforcement, leading to a reduction in the durability of the structure.
• Aesthetics: In some cases, crack widths are controlled for aesthetic reasons, especially in architectural concrete or exposed surfaces.

8. General Guidelines:

• Interior Structures (Non-Aggressive Environment): Allowable crack widths typically range between 0.3 mm to 0.4 mm.
• Exterior Structures (Moderate Environment): Allowable crack widths typically range between 0.2 mm to 0.3 mm.
• Aggressive Environments (Severe Conditions): Allowable crack widths are generally kept below 0.2 mm to prevent the penetration of harmful agents.

What to do on the domestic setting when one notices cracking in a concrete structure:

Cracking is inevitable in some types of Concrete Structures. This should not be a worry unto anyone once we realize, Concrete Structures are designed with the necessary safeguards in place. The rational is to limit or control the amount of cracking so it does not compromise the structural integrity or the aesthetics of the Concrete Structure.

When one is of a structure which had no cracks manifesting on it in the visual context through appreciation in a start which then manifests cracking, we need to be properly informed of what is taking place in the context of effects through cracking. The cracking could be due to an effect which has brought about a state which is no longer continuous in terms of the cracking after such cracking manifests. It could also be due to the continuous growth in terms of development of the cracking which can compromise the structural integrity of the edifice with time. This is why, as soon as one notices cracking which was not on a concrete structure previously, the most important thing doing is, get a marker, mark the crack profile and if possible, date when the crack was first noticed and get in touch with an expert to assess the crack. If you notice the crack still propagating with time, make sure you mark the cracking in such a way for the distinct crack ends to be indicated for the distinct phases of propagation to be properly identified so the expert called in to monitor the cracking can be properly informed in how the cracking is spreading and what type of cracking is manifesting.

You want to use an arrow to show the direction of spread of the cracking so the expert can look at the situation and be properly informed in assessing the situation through the dynamics of the crack propagation so the necessary interventions can be put in place to address the situation if need be. The good in a properly designed Concrete Structure is this, it gives you ample warning when it is in a state of distress so the necessary remedial actions can be taken to safeguard the structure.

Conclusion:

Crack monitoring in concrete structures is vital for maintaining structural safety and longevity. With advancements in technology, methods like fiber optics, digital image correlation, and acoustic emission provide more accurate and real-time data, enabling better maintenance and repair decisions. However, the choice of method depends on the specific requirements of the structure, the type of cracks, and the resources available.