Surface pressure describes the pressure that arises at a contact surface between tool and material. In concrete demolition, rock excavation, and natural stone extraction, it determines whether material breaks in a targeted manner, deforms plastically, or spalls uncontrollably. For work with concrete demolition shears, stone and concrete splitters, hydraulic power units, as well as shears and cutting tools from Darda GmbH, understanding surface pressure is central: It links the hydraulically generated force with the actual action on the component—and thus with efficiency, precision, and component protection.
Definition: What is meant by surface pressure
Surface pressure p is the acting force F related to the actually load-bearing contact area A: p = F/A. It is often also referred to as contact pressure, support pressure, or bearing pressure. In practice, surface pressure is rarely distributed uniformly; edges, radii, and local roughness produce peaks. Surface pressure is therefore a local loading and differs from the internal stresses in the component that result from load distribution within the material. The decisive point is whether the local pressure exceeds the compressive strength of the material (e.g., concrete, rock, steel) or, via contact mechanisms (friction, wedge effect), generates tensile stresses that initiate cracks.
Formula, units, and typical magnitudes
Surface pressure is calculated as p = F/A. Common units are N/mm² or MPa (1 MPa = 1 N/mm² = 10 bar). For practical orientation:
- Normal to high-strength concrete: compressive strength about 30–150 MPa; tensile strength significantly lower (typically 2–10 MPa).
- Granite and comparable natural stones: compressive strength often 100–250 MPa; tensile strength low.
- Structural steel: yield strength typically 250–500 MPa, higher for high-strength steels.
Example calculation (simplified): If 200 kN act on an effective contact area of 300 mm² (3 cm²), then p ≈ 200,000 N / 300 mm² ≈ 667 N/mm² = 667 MPa. Such local values are possible at cutting edges or teeth, although the real load distribution depends strongly on geometry and contact evolution.
Hydraulic pressure and surface pressure: distinction and relationship
Hydraulic pressure in the power unit (e.g., a few hundred bar) is not identical to the surface pressure at the workpiece. Hydraulic pressure generates, via piston area, transmissions, and geometry, a force at the tool. Only the contact geometry (tooth, wedge, cutting edge, support) converts this force into local surface pressure. A small contact area means higher surface pressure and faster initiation of cracks or plastic deformation—though also higher tool loading.
Importance of surface pressure in concrete demolition and specialized deconstruction
In the deconstruction of concrete components, surface pressure is used to initiate fracture processes in a controlled way. The goal is to maximize pressure where the crack is intended to form and limit it in adjacent areas to avoid spalling and secondary damage.
Concrete demolition shears: teeth, cutting edges, and jaw geometry
With concrete demolition shears, profiled teeth concentrate surface pressure along narrow lines. This locally crushes concrete at the surface while tensile stresses build up in the component, causing fracture. Influencing factors include:
- Jaw shape and tooth geometry (radius, depth, pitch): smaller radii increase surface pressure, promote crack initiation, but also increase tool wear.
- Material condition of the concrete (strength, moisture, reinforcement ratio): dense, dry concretes require higher surface pressures; reinforcement alters the load path.
- Positioning and support: a defined counter-bearing reduces scatter losses and minimizes unwanted spalling.
Stone and concrete splitters: wedge effect and crack steering
Rock splitting cylinders and rock and concrete splitters work with wedge inserts that introduce the force into a narrow gap. The wedge generates high local surface pressure on the flanks of the gap. Critical factors are:
- Borehole selection (diameter, depth, location): carefully placed boreholes enable a defined contact area and thus reproducible surface pressure.
- Wedge geometry and surface condition: clean, well-fitting wedges minimize uneven pressure peaks and steer the crack front.
- Serial application: multiple splitting points distribute the overall loading and keep local peaks controllable.
Factors influencing surface pressure at the contact interface
- Contact area: the smaller the effective bearing area, the higher the surface pressure.
- Shape and edge radii: sharp edges create peaks; defined radii limit extreme values.
- Roughness and fit: rougher surfaces increase friction and local pressures; well-fitting wedges distribute loads more favorably.
- Material properties: compressive strength, modulus of elasticity, and toughness of rock/concrete govern fracture behavior.
- Load path and eccentricity: tools applied at an angle generate non-uniform contact pressures.
- Operating temperature and moisture: affect strengths and frictional behavior, especially for natural stone and young concrete.
Design and rough calculation in practice
In practice, p = F/A is first used as a rough check to assess orders of magnitude. Relevant points:
- Determine the effective force (hydraulic pressure × piston area × transmissions, accounting for losses).
- Estimate the real contact area (tooth line length × contact width; wedge flank × engaged length).
- Allow safety margins, as local peaks exceed the mean surface pressure.
- Observe edge distances to avoid spalling at supports and edges.
Example: concrete separation with a concrete demolition shear: F ≈ 250 kN, initial line contact effectively A ≈ 200 mm × 1.0 mm = 200 mm² → p ≈ 1,250 N/mm² = 1,250 MPa. In reality, the contact spreads as indentation progresses; the peak value drops while cracks propagate.
Measurement and verification methods for surface pressure
Direct measurements are rarely possible, yet there are practical approaches:
- Pressure measurement films for qualitative/semi-quantitative distributions at accessible interfaces.
- Pressure sensors or load-measuring bolts in tool structures for force data.
- Imprints/indentations as indicators of contact peaks.
- Numerical simulation (e.g., finite element) for the design of complex contact geometries.
Fields of application: from gutting works to tunnel construction
In gutting works and cutting, high surface pressures at cutting edges are used to break concrete cover or expose reinforcement. In rock excavation and tunnel construction, surface pressure is deliberately generated in borehole gaps to initiate brittle fracture. In natural stone extraction, controlled pressure enables clean separation faces. In special deployments (e.g., under sensitive boundary conditions), a targeted limitation of surface pressures is important to protect adjacent structures.
Protecting tools and components: controlling surface pressure
The right balance between sufficient and excessive surface pressure protects components and tools:
- Use interlayers/pads to distribute bearing pressure where no fracture is desired.
- Regularly inspect jaws/contours; worn edges alter contact pressures and the working principle.
- Adjust the contact angle; tilted application produces unwanted peaks.
- Load stepwise: approach, check position, then increase—observe crack formation.
Typical failure modes with excessive surface pressure
- Spalling and edge break-offs at supports or columns.
- Uncontrolled crack paths and secondary damage in the component.
- Indentations, notch effects, and premature tool wear on teeth/wedges.
- Local pulverization of the rock (fines formation) that reduces force transmission.
Surface pressure in shearing and cutting operations (steel, tanks, composites)
With steel shears, combination shears, multi cutters, and tank cutters, surface pressure is concentrated along the cutting line on very small contact regions to exceed the yield strength and initiate a controlled cut. Important aspects:
- Condition of the blades (sharpness, angle, radius) determines peak pressures and cut quality.
- Material strength and wall thickness vary the required pressure along the cut.
- Minimal offset of the blades prevents local over-peaks and cold welding.
Practical calculation examples
1) Wedge splitting in a borehole: F ≈ 1,000 kN, assumed initial effective line contact area per flank A ≈ 2 × (120 mm × 0.5 mm) = 120 mm² → p ≈ 8,333 N/mm² = 8,333 MPa initially. As penetration progresses, A increases and p drops locally while the crack advances.
2) Concrete edge bearing: F ≈ 80 kN on a support plate 80 × 60 mm = 4,800 mm² → p ≈ 16.7 N/mm² = 16.7 MPa. If the support lies close to an edge, peaks can be significantly higher; a larger support or interlayer reduces p and prevents spalling.
Note: The examples are idealized. Real contact pressures deviate due to roughness, settlement, misalignment, and material inhomogeneities.
Safety, standards, and documentation
For safe use, follow the manufacturer’s information from Darda GmbH, applicable standards, and recognized rules of engineering practice. Load limits, support conditions, and edge distances should be chosen conservatively. Documented workflows and regular visual and functional inspections help avoid impermissibly high surface pressures. Legal requirements can vary by location; implementation follows the relevant provisions in each case.
Optimization in use: leveraging surface pressure deliberately
- Apply concrete demolition shears so that teeth initiate crack lines while non-critical areas are broadly supported.
- With splitters, plan boreholes (spacing, depth, axis) to guide cracks and minimize the necessary pressure.
- Keep contact surfaces clean; foreign particles increase local peaks and hinder reproducible results.
- Set hydraulic power units to the specified pressure range; excessive pressures increase not only surface pressure but also the risk of damage.