Crushing deformation denotes the local destruction or permanent deformation of materials as a result of high compressive stresses. In demolition and extraction technology, this effect is deliberately used to separate concrete and rock in a controlled manner – for example with concrete pulverizers or rock and concrete splitters – or it occurs unavoidably as a side effect of other cutting and splitting processes. For controlled deconstruction, rock excavation and natural stone extraction, a sound understanding of crushing deformation is essential to open structural elements as planned, expose reinforcement, limit emissions and protect adjacent structures.
Definition: What is meant by crushing deformation
Crushing deformation is understood as the local overloading of a material under compression, in which the compressive strength is exceeded. In brittle materials such as concrete or natural stone, this leads to grain crushing, microcracking and spalling; this is also referred to as crushing failure. In ductile materials (e.g., steel), crushing deformation appears predominantly as plastic deformation with indentations and flow. In practice, crushing zones form at contact surfaces of high surface pressure, for example between the jaws of concrete pulverizers, at wedge and spreading points of rock or concrete splitter cylinders, or at bearing edges. Crushing deformation is to be distinguished from tensile and shear failure, but it often occurs in combination with these fracture modes.
Origin and mechanics of crushing deformation in concrete and rock
The mechanics of crushing deformation are based on concentrated compressive stresses that overload the microstructure. In concrete, the cement matrix first develops microcracks, then aggregates fracture and compact crushing cores form with surrounding crack fans. In natural stone, texture, joints and moisture content determine the extent of grain fragmentation. The decisive factors are:
- Contact geometry and bearing: Small, hard contact areas increase contact stress and favor deep crushing zones; wide bearings distribute the load.
- Hydraulic pressure: Higher operating pressure of the hydraulic power unit increases jaw or wedge force and thus the surface pressure.
- Boundary conditions: Proximity to edges, existing cracks, reinforcement layout and member thickness influence crack propagation and the crushing pattern.
- Loading rate: Slowly increasing loads promote controlled crushing cores; impact-like loads favor spalling.
With rock and concrete splitter devices, a tensile splitting fracture is primarily generated, but high local compressive stresses act on wedge faces and the borehole wall, which can lead to borehole crushing deformation. This is desirable when it facilitates crack initiation, but undesirable when surface quality or edge zone integrity is required.
Crushing deformation with concrete pulverizers: mode of action, application, limits
Concrete pulverizers generate high, directed surface pressure between their jaws. This crushes the concrete in the clamping zone, weakens the cross-section and breaks the structural element in a controlled manner. The method features low vibration levels and is suitable for concrete demolition and deconstruction as well as for building gutting and cutting in existing structures, especially in sensitive environments. For heavily reinforced sections, crushing supports the exposure of reinforcement, which can then be separated, for example, with steel shears or a hydraulic demolition shear.
Work steps in the controlled crushing process
- Select the starting point: Edges, openings and previously weakened areas facilitate defined crack guidance and reduce uncontrolled spalling.
- Observe jaw geometry: Profiling and opening width influence contact stress and the crushing pattern.
- Meter the load build-up: Uniform pressure build-up promotes reproducible crushing cores and minimizes secondary damage.
- Consider reinforcement: Targeted separation of exposed reinforcement to avoid tensile bridges.
Crushing deformation with rock and concrete splitters
Rock and concrete splitter devices operate via wedges inserted into boreholes that split the material in tension. In the contact zone between the wedge and the borehole wall, high contact pressure occurs, creating a narrow crushing zone. This zone helps with split initiation, but in natural stone extraction it must not penetrate too deeply to preserve the visually relevant surface. In rock demolition and tunnel construction, the combination of local crushing and global splitting enables progress with low vibration levels in densely built or sensitive areas.
Influencing factors of borehole crushing deformation
- Borehole diameter and surface quality of the borehole wall
- Wedge geometry, friction and lubrication condition
- Spacing and arrangement of boreholes to control crack lines
- Rock texture, joints, moisture and temperature
Fields of application where crushing deformation is used deliberately
Crushing deformation is not an end in itself, but a means of controlled structural separation in various application areas:
- Concrete demolition and special demolition: Concrete pulverizers crush compressed zones and weaken cross-sections to release components according to position and load.
- Building gutting and cutting: Openings can be produced indoors with low noise and dust; crushing deformation confines the force flow to the tool zone.
- Rock excavation and tunnel construction: Splitters use local crushing cores for crack initiation, while the main fracture occurs in a low-vibration tensile mode.
- Natural stone extraction: The goal is a clean separation pattern with minimal edge-zone influence; crushing deformation should only occur in moderation here.
- Special applications: For complex composite constructions, crushing is combined with cutting and shearing to release material composites in a controlled manner.
Limiting crushing deformation: protection of adjacent components and surfaces
Where crushing deformation should occur only in the tool zone, load- and contact-appropriate procedures help:
- Optimize contact surfaces to avoid unintended point loads.
- Select starting points so that cracks propagate in a defined manner and edge spalling is minimized.
- Increase the load step by step and use intermediate steps to guide the fracture.
- For exposed concrete and natural stone, protect surface zones particularly well and, if necessary, provide alternative pre-weakening cuts.
Material behavior: concrete, natural stone and steel under crushing deformation
Concrete and rock respond in a brittle manner: crushing cores, spalling and crack fans occur. The depth and extent of the crushing zone increase with surface pressure. Steel behaves predominantly ductile; plastic flow dominates, often combined with shear. For composite cross-sections this means: concrete is crushed, reinforcement is then separated with suitable tools – such as steel shears or a hydraulic demolition shear. Cutting torches and thermal methods, by contrast, act not through crushing deformation, but through heat and a separating cut.
Safety: crushing hazards and organizational measures
Crushing deformation is not only a material phenomenon, but also a safety aspect: Hydraulic tools create pinch and shear points. As a general rule, only trained personnel operate hydraulic equipment and work areas are secured. Notes are always non-binding and do not replace operating or safety instructions.
- Mark and secure danger zones, maintain a sufficient safety distance.
- Ensure stable positioning of the tool and the component.
- Wear personal protective equipment, keep lines of sight and communication paths clear.
- Depressurize hydraulic systems and reduce loads in a controlled manner before entering areas.
Quality assurance: assessment of crushing zones
Assessment is performed visually and – if required – with simple test methods. Typical features are edge breakage, grain crushing, pulverization and the course of crack fans. In deconstruction, the predictability of the separation pattern is paramount; in natural stone extraction, the depth of the micro-crushing zone plays a role for surface quality.
Typical failure patterns and causes
- Excessive point loads due to unsuitable contact geometry lead to deep crushing cones.
- Incorrect starting point near uncontrolled weaknesses favors edge spalling.
- Impact-like load build-up causes large-area spalling instead of defined crushing cores.
- Unconsidered reinforcement holds fracture faces together and generates undesirable secondary fractures.
Planning and selection of the method
The choice between concrete pulverizers, rock and concrete splitter devices, shears or thermal cutting methods depends on member thickness, degree of reinforcement, accessibility, permissible emissions and requirements for the separation pattern. Hydraulic power units provide the necessary operating pressure and flow rates to build up the crushing effect reproducibly. A process combination is often expedient: crushing to weaken, splitting to guide cracks, followed by separation of the inserts.