Crack monitoring describes the continuous observation of cracks in concrete, masonry, and rock in order to make changes in crack width or extent visible. In construction projects involving concrete demolition and special deconstruction, strip-out and cutting, as well as in rock demolition and tunnel construction, this crack monitoring is a central instrument of quality assurance. It helps protect existing structures, control interventions with the lowest possible vibration, and technically substantiate the choice of suitable methods—such as the use of concrete pulverizers or stone and concrete splitters. At the same time, it provides reliable evidence of the effects on surrounding structures and sensitive assets and serves construction-accompanying documentation.
Definition: What is meant by crack monitoring
Crack monitoring refers to the systematic, time-resolved recording and evaluation of crack conditions on structures or in rock. The aim is to quantify crack widths, crack movements (opening/closing), crack extensions, and, where applicable, crack growth rates, analyze influencing factors, and define thresholds for interventions. Crack monitoring is part of structural and building monitoring and is often used together with ground vibration monitoring and settlement monitoring. In the context of deconstruction and rock works, it is used to select and control working methods (e.g., hydraulic splitting, shear and cutter operations as well as cutting works) so that impacts on the surroundings remain controllable.
Fundamentals and measurement methods for crack monitoring
Reliable crack monitoring starts with a baseline measurement, i.e., recording the initial condition including photo documentation and reference values. Building on this, suitable measuring points are selected, measurement intervals and alarm levels are defined, and methods are determined. In practice, mechanical indicators, optical methods, and electronic sensors are used. Temperature and humidity compensation is important, as climatic fluctuations can apparently change crack widths. For interventions with concrete pulverizers or stone and concrete splitters, the monitoring provides the basis for adjusting force application, sequencing, and pacing so that limit values are observed.
Fields of application in demolition, deconstruction, and rock works
Crack monitoring is used wherever existing structures must be preserved or neighboring assets secured. Typical fields of application are:
- Concrete demolition and special demolition: Control of shear and cutter operations on sensitive structural elements; comparison of crack development during sectional deconstruction.
- Strip-out and cutting: Observation of cracks on remaining partition walls and slabs during sawing, drilling, or core drilling.
- Rock excavation and tunnel construction: Monitoring of joint openings, brittle fracture zones, and shotcrete linings; control of crack formation during hydraulic splitting in rock.
- Natural stone extraction: Support of controlled split guidance along natural joints; minimization of unwanted crack branching within the block.
- Special deployments: Protection of listed elements, vibration-sensitive facilities, or dense inner-city environments.
Measurement methods in detail
Mechanical indicators and crack gauges
Mechanical markers, plaster seals with crack indicators, and transparent crack gauges allow simple visual checks. They are suitable for low measurement frequencies, provide robust trend information, and are resistant to construction site conditions. Their accuracy is limited; therefore, they are often combined with periodic photo documentation.
Electronic crack sensors and data loggers
Electronic probes (e.g., inductive or potentiometric displacement sensors, fiber-optic strain measurement, vibrating wire) enable continuous, temperature-compensated measurement series with high resolution. In combination with data loggers, thresholds can be monitored automatically. During work with concrete pulverizers or stone and concrete splitters, this makes it possible to record crack response during individual work cycles and precisely adapt process control.
Optical methods
High-resolution photography, microscopy, and image-based evaluation (e.g., structured light or digital image analysis) support the measurement of very small crack widths and the mapping of complex crack networks. Photogrammetric methods allow area-wide condition assessment, for example on vaults, tunnel walls, or large façades.
Planning the measurement concept and thresholds
An effective concept defines measurement points, time grids, data management, and response plans. Multi-stage limit levels are recommended:
- Information level: Anomaly is documented and verified (e.g., additional measurement, temperature check).
- Pre-warning level: Adjustment of construction methods (e.g., reduction of pacing, switch to lower-vibration tools).
- Intervention level: Short-term work stoppage in the affected area, technical countermeasures, rescheduling of the sequence.
The definition of the levels is based on recognized rules of practice, the conditions of the existing structure, and the project requirements. Binding thresholds are object-dependent and determined on a project-specific basis.
Influencing factors and typical sources of error
- Changes in temperature and humidity: Apparent crack movements due to thermal length changes; compensation required.
- Settlement and creep: Slow deformations can superimpose crack development.
- Installation and adhesion: Inadequately fixed sensors distort measurements.
- Measurement interval: Intervals that are too coarse can miss short-term peaks during individual work steps.
- Documentation: Missing photo data or inconsistent referencing complicate evaluation.
Implementation on site: from monitoring to action control
The strength of crack monitoring lies in the direct derivation of measures. When crack activity increases, working methods are adjusted: reduced individual strokes, changed contact points, modified sequence, or temporary bracing. Hydraulically driven tools can be finely metered; combined with a sophisticated monitoring setup, forces can be targeted and deliberately reduced. Stone and concrete splitters are suitable as a low-vibration alternative in sensitive areas; concrete pulverizers enable controlled removal with locally limited load peaks. Monitoring objectively demonstrates the effectiveness of these adjustments.
Crack monitoring in tunnel and rock works
In rock formations, joint orientation, bedding, and water conduits influence crack propagation. Crack monitoring records openings and shear offsets on joint surfaces as well as the behavior of linings (e.g., shotcrete) under varying loads. In hydraulic splitting in rock, it supports the targeted guidance of the crack front by assessing reactions in the surroundings and accordingly controlling splitting cycles. In tunnels and caverns, measuring points are often arranged in the crown, roof, and sidewalls to detect asymmetrical movements at an early stage.
Documentation, data management, and legal notes
Complete, traceable documentation is an essential component of crack monitoring. This includes measurement logs, photos, location plans of the measuring points, calibration certificates, and records of action adjustments. Data management should be audit-proof; access is regulated on a project basis. Legal requirements may vary depending on the project, contract, and local regulations. The notes in this article are general in nature and do not replace legal advice.
Practical guidance on selecting demolition techniques in the context of monitoring
The choice of equipment significantly influences crack development. In existing structures with sensitive neighbors, methods with low vibration and oscillation excitation are advantageous:
- Stone and concrete splitters: Generate splitting forces inside the element; when applied correctly, they have low far-field effects. Suitable for massive components and rock in crack-critical environments.
- Concrete pulverizers: Enable controlled removal with targeted load application; in combination with crack monitoring, contact points and sequences can be optimized.
- Combination shears, multi cutters, steel shears: For reinforcement, steel beams, and mixed deconstruction; coordinated pacing can reduce crack-critical events.
- Hydraulic power packs: Ensure reproducible, finely metered power supply; parameter settings (pressure, flow) are adapted to monitoring values.
- Tank cutters: In industrial special deployments, separation of shells and internals enables crack-minimizing dismantling of adjacent concrete foundations, provided process control is monitored.
The combination of suitable tools, an appropriate work sequence, and tightly scheduled monitoring reduces the risk of uncontrolled crack propagation and supports quality assurance in the project.
Typical key figures and evaluation
Evaluation considers crack width (mm), change in crack width over time (mm/h), cyclic opening and closing movements, and correlations to work steps. Time series are compared with temperature and humidity data. Event logs (e.g., start/stop of a splitting or pulverizing cycle) facilitate root-cause attribution. Clear visualizations with threshold markers and comments on action adjustments are suitable.
Interfaces with vibration and settlement monitoring
Crack monitoring unfolds its full effect when combined with ground vibration monitoring and settlement measurements. Vibration data provides context (external influences, traffic), and settlement data shows slow deformations. If events align across all three disciplines, process control—such as metering hydraulic pressure and pacing for concrete pulverizers or splitters—can be optimized with particular precision.
Quality assurance, calibration, and competence
Regular functional checks of sensors, documented calibrations, and trained teams are key to reliable results. Uniform naming of measuring points, traceable versioning of measurement plans, and a clear responsibility matrix prevent interpretation errors. In highly sensitive projects, a four-eyes principle for evaluation and approval of measures is recommended.