What is crack monitoring of concrete structures?
Crack monitoring tracks the opening, expansion and closing of cracks in concrete structures to assess structural movement, condition and stability.
The process involves setting up monitoring devices to measure changes in crack dimensions or structural strain over time. This information is crucial for identifying the scale of structural movement, the significance of detected cracks to structural stability, and, the potential causes of this cracking, in order to predict future crack development, ensuring structural safety.
How does crack monitoring work in concrete structures?
Crack monitoring in concrete structures uses a range of techniques and equipment to detect, locate and quantify the formation and future development of cracks. The choice of methods depends on the structure being monitored, environmental conditions and the required accuracy, resolution and time period of measurements.
Visual inspection and manual measurement
This most simple form of crack monitoring involves the regular, manual, measurement of crack widths using simple handheld tools like crack width gauges. This method provides direct localised measurement of cracks but requires frequent ongoing access and may not be suitable for widespread monitoring of large structures, especially those with hard to reach or industrially active areas.
Mechanical and strain gauge installation
Devices like crack meters, displacement gauges or strain gauges can be installed directly to the structure across the crack to be investigated. These devices measure changes to the length and width of cracks or the strain encountered by the concrete and are checked periodically to assess crack severity and development. This method cheaply and simply provides direct, localised monitoring of cracks but gauges still need to be manually checked and as such share many of the same constraints as manual measurement.
Fibre optic sensors
Fibre optic sensors can be used to detect changes in the physical state of a concrete structure based on the effect these changes will have on the transmission and reflection of light within the optical fibres. Changes in strain and temperature alter the lights path and the scale of this can be measured to infer structural movement.
Fibre optic sensors can be embedded within or mounted to the concrete surface and are especially useful for monitoring inaccessible areas and along continuous lengths such as bridges or pipelines.
Light is transmitted through the fibres and recorded using a data logging device installed at the end of the cable allowing instant assessment of structural movement and real time assessment of rapid changes to prevent structural collapse. This technique is however costly and complex, limiting its applicability.
Acoustic emission (AE)
Acoustic emission techniques detect transient elastic waves created by the release of energy that results from the formation of cracks. Sensors are strategically installed on the concrete surface and provide valuable, ongoing information about the propagation of cracks throughout the concrete structure. This method cannot detect existing cracks, only their formation and expansion and is especially useful for real time monitoring of crack propagation in structures such as gas pipelines where any failure could be catastrophic. High levels of ambient noise and vibration can create signal interference and significant expertise is required to interpret results accurately.
Digital Image Correlation (DIC) and photogrammetry
These sophisticated optical investigation techniques involve capturing a series of high-resolution digital images of the surface of a concrete structure over time and comparing them to assess the deformation, displacement and strain exhibited.
DIC applies a random speckle pattern to the images of the concrete surface and movement within this pattern is then used to precisely track structural deformations or the propagation of cracks. Photogrammetry utilises images taken from various angles to produce a 3d model of the structure with calculated depths and dimensions derived from using multiple overlaid images. These techniques both offer high resolution and accuracy without the need to physically touch or scale concrete structures and are therefore are applicable to a huge range of projects.
Seismic Tomography Transmission (STT)
STT employs seismic waves to map the internal structure of large, dense concrete elements such as bridges, dams and building foundations. A point source of seismic compression waves is generated from the concrete surface using mechanical impacts from an accelerated weight drop device and travels through the concrete. The travel time is then captured by an array of sensors (geophones, accelerometers etc) placed strategically on the structures surface. Wave speed variation can then be used to identify anomalies like cracks, voids, or areas of honeycombing and this data is then analysed to create 2D or 3D images pinpointing the exact locations of defects, facilitating targeted maintenance and repair. The complex and expensive test is extremely accurate and well suited to structures with low accessibility and very thick concrete elements.
What is crack monitoring used for?
Deterioration process | Defects | Control of repairs |
How do I carry out crack monitoring?
To carry out effective crack monitoring on a concrete structure, intelligent selection of monitoring techniques is vital. Where possible simple techniques should be used for initial screening and non-critical areas where detailed data is not crucial. Advanced, complex or costly techniques should be used in critical zones, for structures with complex geometries, or where long-term data collection is required without frequent physical inspections.
A simple step-by-step guide that incorporates the best practices and methodologies from various standards and techniques includes the following steps:
Conduct an initial site assessment with a thorough visual inspection to identify visible signs of distress such as cracks, spalling, or discoloration. Document these initial findings with photographs and notes to help in planning the monitoring approach.
Choose your monitoring techniques. For accessible and surface-level monitoring, simple techniques like manual measurement with crack width gauges or mechanical gauges are suitable. For comprehensive, in-depth analysis, especially in critical or hard-to-reach areas, advanced techniques such as Fiber Optic Sensors, Acoustic Emission (AE), Digital Image Correlation (DIC), Photogrammetry, or Seismic Tomography Transmission (STT) can be used.
Manually measure identified crack widths and length or Install mechanical crack meters or strain gauges directly across identified cracks.
If required set up advanced monitoring systems. Embed or attach fibre optic cables, place AE sensors at strategic locations to detect crack propagation, apply a speckle pattern to the concrete surface, strategically image the structure or install geophones and a mechanical impactor for seismic tomography transmission.
Regularly collect and analyse data from sensors or periodically, manually remeasure the crack propagation and reimage the concrete structure.
Analyse collected data to evaluate crack progression, new crack formation, and other structural changes such as deformation or subsidence.
Use software tools for advanced techniques to help interpret the data accurately, identifying critical areas needing attention.
Compile the collected data into a report on the condition of the structure, prediction of future cracking and recommendations for maintenance and repair.
Determine the most suitable interventions. Should the concrete element be replaced? If not what type of ongoing monitoring would be most suitable?
Set up a schedule for continuous maintenance and monitoring. Adjust monitoring techniques and frequency based on the evolving condition of the structure and effectiveness of previous monitoring rounds.
What equipment and expertise are required for crack monitoring?
Visual Inspection and Manual Measurement
Equipment required for this type of monitoring includes crack width gauges, measuring tapes, a magnifying glass and a digital camera to document the findings. Basic training for operators on identifying cracks and measuring them using simple tools would be required along with expertise in concrete structures and crack propagation for interpretation of the results.
Mechanical and Strain Gauge Installation
Specialised equipment such as mechanical crack meters, displacement gauges and strain gauges will be required along with expertise in their installation and calibration, analysing the results from them and assessing the effects this will have on the structure as a whole.
Fibre Optic Sensors
Equipment required includes Fiber optic cables, lasers, optical detectors, and signal processing equipment. Advanced knowledge in optics and photonics is required, as well as skills in handling and installing delicate fibre optic components.
Acoustic emission (AE)
A full review of the equipment and expertise required for acoustic emission monitoring of a concrete structure can be found here.
Digital Image Correlation (DIC) and Photogrammetry
Equipment required includes high-resolution cameras, tripods, speckle application materials (for DIC), image processing and analysis software. Expertise in optical measurement techniques, image processing, and data analysis is also essential.
Seismic Tomography Transmission (STT)
A full review of the equipment and expertise required for acoustic emission monitoring of a concrete structure can be found here.
What are the advantages of crack monitoring?
Crack monitoring allows early detection of cracks, which can signify underlying structural problems, potentially preventing catastrophic failures.
Techniques like fibre optic sensors provide real-time off site data on structural changes, allowing for immediate response to critical conditions.
Crack monitoring allows the prediction of the progression and severity of cracking therefore maintenance can be more efficiently planned reducing downtime, costs and carbon.
Continuous monitoring provides valuable long term data allowing intelligent structural management of the structure's health and understanding of the processes that are causing cracking.
Regular monitoring enhances the safety of the structure by ensuring that damage is addressed before failure.
What are the disadvantages of crack monitoring?
Initial set up costs for advanced monitoring technologies, such as seismic tomography and digital image correlation, can be expensive to install and operate.
Crack monitoring even in its most simple form requires significant technical expertise to operate and interpret the data, necessitating trained personnel, which can be a resource constraint.
Crack monitoring tools often require regular maintenance and calibration to ensure accurate measurements, adding to the operational costs and complicating their operation in hard to reach areas.
Many monitoring techniques are affected by environmental conditions like vibration, temperature, and humidity, potentially compromising data accuracy without bespoke calibration.
How accurate is crack monitoring?
Visual Inspection and Manual Measurement
These methods may be described as moderately accurate, with precision depending on the skill of the inspector and the tools used. Manual measurements using crack width gauges can typically measure widths accurately to within 0.02 mm but human error and the subjective nature of visual assessments can affect accuracy. This method is inaccurate for detecting subtle or internal structural changes.
Mechanical and Strain Gauge Installation
Mechanical gauges and strain gauges offer precise measurements of crack widths and structural deformations, usually accurate to within 0.01 mm contingent on proper installation and calibration of the gauges. Environmental factors and mechanical faults can however still lead to inaccuracies.
Fibre Optic Sensors
These sensors have very high accuracy and are capable of detecting minute changes in strain or temperature that change the light path within the fibre by as little as 1µm. The complex set up, specialised equipment and data interpretation required can however introduce variability if not properly managed
Acoustic Emission (AE)
These techniques are sensitive enough to detect very small increments of crack growth in materials. The smallest amount of crack growth that AE can detect depends on the sensitivity of the equipment and the specific setup, but typically, AE is capable of detecting crack growth as small as 1µm and changes in strain of 1με.
Digital Image Correlation (DIC) and Photogrammetry
DIC offers exceptional accuracy of 0.1 to 1μm depending on the equipment and setup, enabling it to capture extremely small deformations of the surface of structures. Photogrammetry is less accurate (1-5mm). Both methods rely heavily on the quality and resolution of the images captured. Any instability in the camera setup, such as vibrations or shifts, can impact the data accuracy and environmental factors such as lighting conditions and the presence of reflective or absorptive surfaces can affect the image quality. DIC, in particular, requires a controlled environment to achieve the highest accuracy.
Seismic Tomography Transmission (STT)
STT can achieve an accuracy up to 1mm, depending on the specific concrete, the arrangement and quality of sensors used, and the type of seismic source employed. Variations in the material properties of the concrete, such as voids and reinforcement, can affect the propagation of seismic waves, thus influencing the accuracy of the results.
What are the limitations of crack monitoring?
Many monitoring techniques such as acoustic emission techniques can only detect active crack formation and expansion and cannot measure existing cracks.
In large or complex concrete structures installing sensors in all necessary locations may be unfeasible.
Most advanced monitoring systems rely on continuous power supply and data connectivity, which can be disrupted, leading to gaps in data collection or limiting the areas of application.
Ancillary information:
Maturity of test: >10 years
Qualification & interpretation : Inspector and specialist
Service disruption: Dependent on method
Preliminary works: Yes
Time consumption Dependent on method
Cost Dependent on method
Access to element Dependent on method
References and further information: