The angle of friction is a key parameter in geotechnical engineering, in the concrete and natural stone domain as well as in force-fit gripping, splitting and cutting of components. In the application fields concrete demolition and special demolition, building gutting and cutting, rock excavation and tunnel construction, natural stone extraction as well as in special demolition, the angle of friction directly affects force transmission, stability and process safety. For the products of Darda GmbH – from rock wedge splitter and concrete splitter via rock wedge splitter to concrete demolition shear, shears and tank cutters – it determines how safely surfaces adhere to each other, how efficiently forces are introduced and how controlled cracks, separation cuts or split lines develop.
Definition: What is meant by angle of friction
The angle of friction (internal angle of friction, φ) describes the relationship between normal stress and shear stress on a contact or shear surface. In practice it states how “grippy” a material or a joint is under load. Mathematically it is linked to the coefficient of friction μ: μ = tan(φ), and φ = arctan(μ). In fracture mechanics and soil/rock mechanics the angle of friction is combined with cohesion c in the Mohr–Coulomb criterion: shear strength τ = c + σ′ · tan(φ). For concrete, natural stone, masonry and also for technical contact pairings (e.g., steel-on-concrete or carbide-on-rock) φ allows an estimate of the maximum transferable shear force for a given normal force.
Influence on rock and concrete splitters and rock wedge splitter
During splitting with Hydraulic Rock and Concrete Splitters, the angle of friction acts at several locations simultaneously: between borehole wall and spreading element, between wedges/cylinders and the bearing surfaces, and in the emerging crack joint. A high angle of friction at the borehole wall promotes holding of the spreading elements and thus the effective introduction of splitting force into the block. At the same time, the internal sliding interface of the wedges should exhibit the lowest possible friction so that the hydraulically generated force is converted into spreading force with minimal loss. Knowledge of φ therefore influences borehole diameter, surface condition (roughness), orientation to bedding/joints, the required contact pressure and the choice of step sequence during splitting – for controlled crack propagation in concrete components or natural stone blocks.
Physical fundamentals and calculation
Friction arises from microscopic interlocking and adhesion components. Two states are practically relevant: static friction with the angle of friction φ_s and kinetic friction with a usually smaller φ_k. For gripping and cutting processes, static friction is decisive because it prevents slipping. From the basic relationship F_max,shear = μ · F_N and μ = tan(φ), it follows that even small changes in φ have noticeable effects on the maximum transferable shear force. In brittle materials such as rock and concrete, roughness, joint infill (dust, fines) and moisture couple the effective angle of friction: dry, roughened surfaces exhibit higher φ values than smooth, dusty or wet surfaces.
Significance in concrete demolition, rock excavation and natural stone extraction
In concrete demolition and special demolition, the angle of friction in crack joints and contact zones decides whether loads are transferred or components slide uncontrollably. For further context, see concrete demolition and special deconstruction. In natural stone extraction, φ determines how borehole rows and splitting direction are aligned to bedding planes or joints so that the split continues along preferred weakness zones. In rock excavation and tunnel construction, the angle of friction of joint surfaces influences the stability of temporary blocks. A moderate increase in contact pressure (e.g., through suitable positioning of splitting elements or through preload via hydraulics) increases usable friction capacity and thus process control.
Angle of friction in concrete demolition shears, combination shears, multi cutters and steel shears
Gripping and cutting tools such as concrete demolition shear, combination shears, multi cutters and steel shears transmit cutting and crushing forces via jaw-side contact surfaces. The minimum gripping force F_G,min results from the required process force F_P and the coefficient of friction μ of the contact pairing (e.g., hardened tooth profiles on concrete/steel): F_G,min ≈ F_P / μ (with safety factor). The larger φ is, the lower the gripping force required for a secure hold. From this arise requirements for:
- Jaw surface profile (toothing, ribbing) for higher effective roughness and thus larger φ
- Clean, dry contact surfaces to avoid impairing adhesion and micro-interlocking
- Sufficient normal force from the hydraulic system to increase the maximum transferable shear force
- Geometry of the component edge (spalled edges reduce the effective bearing area and thus the φ-localized load capacity)
Contact pairs and typical angles of friction
Typical orientation values (variable depending on roughness, moisture, loading rate and temperature):
- Concrete on concrete (rough, dry): φ ≈ 30–40°
- Concrete on concrete (smooth or wet): φ ≈ 20–30°
- Rock (granite/gneiss) on rock, rough: φ ≈ 35–45°
- Rock on rock, smooth/polished: φ ≈ 20–30°
- Steel on concrete (dry, roughened surface): φ ≈ 15–25°
- Rubberized inserts on concrete (dry): φ ≈ 25–35°
These values serve for rough estimation. For project-critical verifications, material-specific tests with representative surfaces and loads should be conducted.
Angle of friction in the borehole: planning splitting processes safely
With rock wedge splitter and concrete splitter as well as rock wedge splitter, friction and contact pressure are decisive in the borehole. Important aspects are:
- Borehole quality: breakouts, slurry, dust and water reduce φ and can promote breakout or slip.
- Alignment: the borehole line relative to joints or reinforcement influences the crack path. A high angle of friction at the borehole wall stabilizes force transmission.
- Internal sliding interfaces: wedges/supports should have low friction with each other so that hydraulic energy is not lost to heat/frictional work.
- Step sequence: preloads, unloads and sequential activation of multiple boreholes exploit differences in φ and c to promote desired fracture lines.
Influencing factors: roughness, moisture, temperature, contamination
The effective angle of friction is not a material constant, but environment-dependent:
- Roughness: rougher surfaces increase φ; polished or worn surfaces decrease it.
- Moisture/water film: generally reduces μ and thus φ; exceptions exist with absorbent, rough surfaces.
- Dust, fines, slurries: act as a separating layer and reduce micro-interlocking.
- Temperature: can change the stiffness of contact materials (e.g., rubber inserts) and thus affect φ.
- Loading rate: dynamic processes can cause transitions from static to kinetic friction (φ_k < φ_s).
Hydraulic power pack, force balance and angle of friction
Hydraulic power units supply the pressure for cylinders, shears and cutters. Via the geometry of the actuators this becomes normal force at the contact zone. Since F_max,shear ∝ tan(φ) · F_N, either a larger angle of friction or a higher normal force improves force transmission. In practice both levers are used: suitable contact surfaces (profiling, cleanliness) increase φ, while a material-appropriate hydraulic load provides the necessary F_N — without exceeding the stability limits of the workpiece.
Angle of friction in concrete and masonry
In concrete, the angle of friction is closely linked to the roughness of the crack joint, the aggregate structure and any joint infill. Torn, interlocked joints show higher φ values than smooth saw cut surfaces. In masonry, φ varies between stone–stone and stone–mortar joints. In special demolition this influences the choice between splitting, shears-based demolition or cutting: where low φ and low cohesion are present, separation cuts can be guided more controllably with lower gripping force; with high φ, crack joints carry more, which can be used for targeted breaking.
Concrete demolition shear in building gutting and cutting operations
During building gutting and cutting of wall and slab areas, concrete demolition shear secure the position of the component against slipping. The necessary gripping pressure depends on the expected process force and the φ of the contact surfaces. In practice, the following has proven effective:
- Keep contacts free of slurries and lubricants
- Select suitable jaw profiles for the respective component surface
- Place gripping points so that the line of action yields favorable lever arms (higher F_N with lower hydraulic load)
- Plan shear direction and potential sliding joints (building edges, crack lines)
This reduces sliding movements and enables controlled execution of cuts or crushing operations.
Tank cutters and metallic contact surfaces
With tank cutters and steel shears, contact pairings with metal occur. Steel-on-steel and steel-on-coated steel usually exhibit lower φ values than rough mineral surfaces. For safe application and guidance, sufficient normal forces and form-fitting elements (e.g., stops, support geometry) are therefore important, so as not to rely on friction alone.
Special demolition: heterogeneous materials and coated surfaces
In special deployments with composite components (concrete–steel composite, coated concrete surfaces, fiber-reinforced matrices), coatings and bonding layers can greatly reduce the angle of friction. Here, a conservative assumption of φ is advisable, planning additional form-fit or support elements and, where possible, a brief on-site friction test for calibration.
Practical determination: field and laboratory approaches
Reliable φ values are obtained via laboratory tests (direct shear, triaxial test). In the field, approximations can be achieved by:
- Tilt-board test: determine the angle at which a test body begins to slide on the target surface (φ ≈ tilt angle).
- Grip test: press on with defined normal force and gradually increase the shear component until sliding; derive μ and φ from this.
- Trial in the borehole: observation of slip/hold under defined load and borehole quality as a qualitative indication of φ.
Such procedures only provide guide values; for safety-relevant decisions, standardized tests are preferable.
Work planning, safety and risk minimization
Underestimating the angle of friction leads to slip, load redistribution and uncontrolled movements. Therefore, safety factors, redundancies (additional shoring/anchor points) and suitable step sequences must be included in the planning. Interventions in load-bearing components require careful assessment; legal and standard requirements are location- and project-dependent and should generally be taken into account without deriving case-by-case assurances from them.
Maintenance and condition of contact surfaces
Worn, polished or contaminated gripping surfaces reduce the angle of friction. Regular cleaning, proper profile maintenance of the jaws of concrete demolition shear and controlled surfaces of splitting elements improve φ and thus process stability. Internal sliding interfaces of wedges/cylinders should be kept functional so that hydraulic energy is efficiently converted into the desired spreading or cutting effect.
Documentation in the project workflow
For repeatable results it is helpful to document assumed angles of friction, contact pairings, surface conditions and achieved gripping/splitting forces. These data simplify fine-tuning in comparable deployments and improve forecasting reliability in concrete demolition and special demolition, building gutting and cutting, rock excavation and tunnel construction, as well as in natural stone extraction.