Crack formation in concrete, masonry, and rock is among the defining phenomena in construction and demolition. It determines load-bearing capacity, durability, and the choice of safe, low-emission methods. In planned deconstruction, crack formation can be deliberately induced and guided, for example using hydraulic wedge splitters or concrete demolition shear from Darda GmbH, to separate components in a controlled manner and decouple load paths. In existing structures, by contrast, it is necessary to assess the causes, progression, and relevance of existing cracks objectively to plan suitable measures.
Definition: What is meant by crack formation
Crack formation refers to the initiation and propagation of separation planes in a material as a result of tensile stresses, shear actions, restraint effects, or fatigue. Cracks may start microscopically small (microcracks) and develop into visible, throughgoing separations. In brittle materials such as concrete and natural stone, cracks preferentially follow weaker zones, fabric boundaries, or pre-existing defects; reinforcement in reinforced concrete influences direction and crack width. In demolition these properties are used to dismantle components with low vibration levels and to deliberately steer crack propagation.
Causes and mechanisms of crack formation
Cracks form when local stresses exceed the tensile strength of a material, or when recurring load cycles lead to fatigue. The most important causes are:
- Load-induced cracks: due to service loads, self-weight, or demolition loads; bending and tension cracks in tension zones, shear cracks in support areas.
- Restraint effects: shrinkage, creep, temperature gradients; early- and late-shrinkage cracks, thermally induced cracks during temperature changes.
- Fabric and manufacturing aspects: nonuniform hydration, aggregate skeleton, pores and inclusions; notch effects at edges and boreholes.
- Fatigue: repeated load cycles produce microcracks that interlink and grow.
- External influences: moisture variations, chemical attack, freeze–thaw cycles that alter stress states.
Crack initiation and crack growth
A crack typically initiates at a notch or inhomogeneity. In concrete and rock, the crack tip opens under tensile stresses; transverse tension reduces crack closure, while compression increases it. With existing reinforcement, crack width is limited by bond forces, while the path is deflected across bars. This mechanics is decisive in concrete demolition when concrete demolition shear introduce crushing forces and guide cracks along weak zones.
Crack formation in concrete demolition and special deconstruction
In concrete demolition and special deconstruction, cracks are deliberately generated to separate components step by step and in a controlled manner. Priority is given to low vibration and noise levels, especially in sensitive areas. Two approaches are established:
- Mechanical splitting: With hydraulic wedge splitters, wedge forces are introduced into boreholes. This creates predictable separation cracks without large-scale vibrations.
- Crushing and pulverizing: concrete demolition shear open cracks in a controlled way, separate cover concrete, and crush components; steel shear cut reinforcement, preventing uncontrolled crack growth in the steel.
Hydraulic power pack supply the tools with defined output, which improves the reproducibility of crack steering. In complex structures, crack formation is combined with cutting and shearing tools (e.g., multi cutters, combination shears) to unload primary members first and only then split.
Controlled crack steering: procedures, parameters, and practice
Steering crack propagation relies on the targeted superposition of stresses, material weaknesses, and geometry. Key factors are:
Drilling patterns and spacing
- Bore diameter and depth define the wedge effect and thus crack initiation.
- Center-to-center spacing steers crack paths: tighter for dense separation joints, wider for coarse breakouts.
- Edge distance prevents spalling and unintended crack deflection at edges.
Wedge forces and hydraulics
- The generated splitting force must exceed local tensile strength but remain below a range that favors uncontrolled secondary cracks.
- Hydraulic power pack with constant delivery improve reaction control; pressure ramp rates influence crack velocity.
Reinforcement and embedded items
- Reinforcement has a crack-closing effect; cracks often pass around bars or are deflected.
- The targeted use of steel shear and multi cutters to cut free the reinforcement reduces unwanted crack branching.
Moisture and temperature state
- Dry concrete fractures more brittlely, moist concrete shows tougher crack tips—this affects crack widths and the required splitting force.
- Temperature gradients create additional stresses; uniform boundary conditions facilitate clean formation of a separation crack.
Crack assessment in existing structures: survey, measurement, classification
Proper assessment of cracks serves to plan demolition steps or repair options. General, nonbinding principles:
- Crack width and path: uniform, fine cracking indicates shrinkage; arching shear cracks point to local overloading.
- Location within the element: cracks in tension zones are expectable; inclined cracks in support areas require special attention.
- Activity: markings and repeated measurements show whether cracks are actively growing. Changes influence the choice of demolition sequence.
- Environment: moisture, frost, chemical influences, and vibrations can intensify crack development.
For precise decisions, the recognized rules of practice are decisive. Depending on the findings, procedures from crack injection to partial relief of the component may be considered; in deconstruction, targeted splitting with subsequent crushing is often the more robust option.
Crack formation in rock excavation and tunnel construction
Rock exhibits natural joints, foliations, and bedding. These discontinuities act as predefined crack paths. In non-explosive rock removal, hydraulic wedge splitters are used to introduce splitting forces into rows of boreholes and activate existing zones of weakness. In this way, blocks can be detached with low vibration levels, which is advantageous in urban areas, near sensitive neighboring structures, or in tunnel heading.
Geological influence
- Bedding and joint spacing determine the required borehole spacing.
- Water in joints reduces friction and can facilitate crack propagation; at the same time, securing measures against uncontrolled detachments must be planned.
Tools in the context of crack formation
- Hydraulic wedge splitter: Generate defined separation cracks with low vibration levels; suitable for massive components, foundations, and rock.
- Concrete demolition shear: Open and enlarge existing cracks, crush concrete members, and enable exposure of the reinforcement.
- Multi cutters and combination shears: Combine cutting and crushing effects to prepare or limit crack growth.
- Steel shear: Cut reinforcing steel in a controlled manner, minimizing unwanted crack formation in steel due to notch effects.
- Tank cutters: For thin-walled metal structures, the focus is on avoiding brittle cracking; controlled cuts prevent unpredictable crack paths in the shell.
Planning: sequence for controlled crack formation
- Existing-conditions analysis: clarify material, reinforcement layout, existing cracks, embedded items, and load paths.
- Crack strategy: define target cracks (separation joint, block sizes), account for boundary conditions.
- Drilling and splitting plan: set hole diameter, depth, grid, and edge distances; determine working pressure and hydraulic pressure.
- Relief: cut reinforcement and connection elements in advance with steel shear or multi cutters to prevent uncontrolled crack branching.
- Split and crush: initiate and extend cracks, and further process components with concrete demolition shear.
- Control and adjustment: observe crack paths, adjust parameters (pressure, sequence), monitor safety and emissions.
Typical errors and how to avoid them
- Edge distances too small: lead to spalling and unwanted crack paths; allow sufficient edge reserves.
- Unsuitable drilling patterns: spacing too large prevents continuous separation joints; too small increases effort without added value.
- Reinforcement not accounted for: deflected cracks, blockages; detect reinforcement layout in advance and cut selectively.
- Unsteady hydraulics: abrupt pressure build-up favors secondary cracking; prefer uniform pressure increase.
- Neglected environmental conditions: strong temperature and moisture gradients change crack mechanics; stabilize boundary conditions.
Safety and environmental aspects
Crack formation alters load-bearing and stability conditions. Therefore, protection zones, shoring, and defined demolition directions must be planned. Tools from Darda GmbH enable methods with low vibration levels and low emissions; nevertheless, dust suppression, noise control, and organized haulage logistics are essential parts of work preparation. Legal requirements and recognized rules of practice must always be observed; project-specific decisions should be underpinned by expert planning.
Documentation and monitoring
Traceable documentation supports both the deconstruction process and quality assurance:
- Crack mapping: record location, orientation, width, and activity.
- Measurement points: use crack markers, measuring wedges, or optical methods to control changes.
- Record-keeping: document drilling patterns, hydraulic pressures, and sequences; justify adjustments.
This allows correlation of crack formation and demolition progress—an essential basis for predictable outcomes in concrete demolition, building gutting, rock excavation, tunnel construction, natural stone extraction, and special operations.