Toothed formwork refers to formwork elements and methods that create a toothed profile at concrete construction joints to ensure mechanical interlock between old and new concrete. The resulting interlocks transmit shear force, limit relative slip, and improve interlock in joints, for example at walls, slabs, foundations, or abutments. In existing structures, the targeted creation or careful release of such interlocks is relevant, especially in concrete demolition and deconstruction, during extensions or structural reinforcement of members. In practice, hydraulic tools such as concrete demolition shear as well as hydraulic wedge splitters by Darda GmbH are frequently used whenever low vibration levels, precision, and controlled work are required.
Definition: What is meant by toothed formwork
Toothed formwork means formwork with profiled form faces or preassembled inserts that produce a regular tooth pattern in the fresh concrete. This toothing—often trapezoidal, triangular, or wave-shaped—increases the contact area, raises the surface roughness, and provides a form-fit load transfer in the construction joint. In combination with connecting reinforcement, shear dowels, or dowel bars, a load-bearing composite is created that takes shear forces, restraint-induced tensile stresses, and local impact loads. Toothed formwork is used both in new construction at planned joints and on existing structures at prepared connection surfaces.
Design and function of toothed formwork
Toothed formwork consists of load-bearing formwork girders and a profiled form face that shapes the tooth pattern. The profiling can be integrated permanently into the face or executed as a replaceable insert. Decisive factors are dimensional accuracy, sufficient stiffness against deformation during concrete compaction, and a surface that can be stripped cleanly while providing suitable roughness for the bond.
Geometry and profile types
- Trapezoidal profile: Good form-fit, robust teeth, universal for wall, slab, and foundation connections.
- Triangular profile: Pronounced interlock with clear load paths, proven for high shares of shear force.
- Wave or sinusoidal: Continuous contact, advantageous under alternating loads and for crack control.
- Step or offset profiles: For precise rework in existing structures, e.g., during partial removal and subsequent concrete infill.
Materials and execution
The form face is usually made of coated wood-based material, metal, or polymer form liners. Important are low absorbency, adequate abrasion resistance, and controlled release-agent management. For elements exposed to water, sealing components (e.g., waterstops, injection hoses) must be integrated in the design; the toothed formwork must provide appropriate accommodations for them.
Fields of application in new build and existing structures
Toothed formwork is used wherever load transfer across construction joints must be ensured: at wall-to-wall and wall-to-slab connections, beams, bearings, foundation haunches, piers, upstands, baffles, and cores. In structural and bridge engineering, it serves to control force redistribution between construction stages. In existing structures, it enables load-bearing connections for strengthening, cross-section enlargements, or extensions. During partial deconstruction, existing joints are exposed or newly created by targeted toothed cuts—preferably with concrete demolition shear for controlled concrete breakout and hydraulic wedge splitters for low-vibration separations, for example in building gutting and cutting as well as in special demolition.
Partial replacement and strengthening of concrete members
When structural members are supplemented or widened with new concrete layers, the quality of the toothed connection surface governs the composite action. During deconstruction, concrete edges are often broken in steps with concrete demolition shears to obtain a defined tooth profile without uncontrolled damage to reinforcement. Split cylinders transmit wedge-shaped pressure into the member and create crack-minimized fracture surfaces, which has proven effective in sensitive environments—for example during ongoing operations.
Concrete demolition and special demolition
When releasing anchored components in toothed joints, a sequential approach is required: expose the joint, relieve loads, release the toothing, cut the reinforcement. For cutting or severing exposed steel, depending on constraints, attachment shear, steel shear, or Multi Cutters by Darda GmbH are used. In this way, components can be removed with low vibration, and new, capable interlocks for reconstruction can be produced.
Design and verification: load path and composite action
The load-carrying behavior is provided by the form-fit of the toothing, the bond between cement paste and aggregates, and the connecting reinforcement. Governing parameters are tooth height, flank angle, spacing, surface roughness, and the share of overlapping reinforcement. For design, the applicable standards and guidelines must be consulted; the notes here are of a general nature. Typical verifications address shear capacity, slip limitation, crack widths, and—where required—the watertightness of the joint.
Joint roughness and surface preparation
Roughness and cleanliness of the contact surfaces are crucial. In existing structures, smooth old-concrete surfaces are roughened by scabbling, milling, blasting, or controlled splitting. Hydraulic wedge splitters enable vibration-reduced roughening, which is advantageous in sensitive areas—for example in rock excavation and tunnel construction near existing structures. Concrete demolition shear allows selective removal of concrete protrusions without weakening the connecting reinforcement. Low-dust methods and thorough cleaning of the joint before re-concreting are important.
Reinforcement layout and connection details
Overlapping reinforcement, shear dowels, or dowel bars must be arranged to act in concert with the tooth profile. Minimum spacings, cover, and anchorage lengths must be observed. For water exposure, sealing elements must be made compatible with the toothing; notch effects at tooth roots must be mitigated with suitable radii or additional reinforcement.
Execution on site
The quality of toothed formwork depends on precise fabrication, correctly positioned reinforcement, and a clean concreting process. The formwork must be anchored so that the profile remains true even under intensive concrete compaction. Concrete mix and consistency must match the profile to avoid voids and to reliably fill the tooth gaps.
Formwork fabrication and installation
- Profile fidelity: Observe face tolerances; provide bracing against deflection.
- Release agent: Apply sparingly and uniformly to avoid gravel pockets and glossy layers.
- Sealing: Fix waterstops and inserts so they do not migrate during compaction.
Concreting and compaction
- Plan pours so the toothing is completely filled; choose suitable compaction tools.
- Match vibration frequency and duration to the profile and reinforcement density to prevent segregation.
- Ensure concrete curing so the tooth profile does not dry out prematurely and the bond surface gains sufficient strength.
Quality assurance and documentation
- Visual inspection of tooth flanks and tips after stripping; documentation with photos and dimensional records.
- Check surface roughness and the position of connecting reinforcement.
- For water-exposed joints: perform leakage test according to project requirements.
Deconstruction, conversion, and refurbishment: creating or releasing the interlock
In existing structures, a toothing is often produced later to rigidly connect new components, or deliberately released to separate elements. The work steps follow a clear sequence and must be adapted to the constraints in concrete demolition and special demolition.
- Expose: Remove coatings and loose layers, mark the joint alignment.
- Pre-separation: Local splitting with hydraulic cylinders to induce cracks along the planned tooth path.
- Profiling: Break out tooth by tooth with concrete demolition shear, rework edges, protect reinforcement.
- Cut steel: Cut reinforcement in a targeted manner with attachment shear, Multi Cutters, or steel shear from the Darda system.
- Cleaning: Remove dust and slurry; if required, apply a bonding bridge according to project specifications.
- New concreting: Add connecting reinforcement, set formwork, concrete, and cure as planned.
Typical sources of error and how to avoid them
Errors in profile, roughness, or concrete technology reduce composite action and lead to increased slip and crack widths. Careful planning, accurately fitting formwork, and controlled execution are therefore essential.
- Insufficient tooth geometry: Select tooth height/angle in line with the project; provide a mock-up area.
- Smooth joint surfaces: Roughen surfaces consistently; remove dust and release-agent residues.
- Gravel pockets at tooth roots: Adapt compaction to the geometry; optimize placement sequence.
- Missing reinforcement overlap: Coordinate connection details early; perform clash checks.
- Incompatible sealing: Match sealing elements to the profile; secure installation against distortion.
- Uncontrolled deconstruction: Remove components sequentially; use hydraulic wedge splitters and concrete demolition shear for careful separation.
Occupational safety, environment, and boundary conditions
Work on toothed joints requires precise coordination of load rerouting, shoring, and sequence. The relevant regulations on occupational safety apply; the following notes are general in nature: minimize vibration, noise, and dust—especially in sensitive areas such as hospitals, laboratories, or heritage buildings. Depressurize hydraulic systems and inspect them regularly, use hose-rupture safeguards, and avoid leaks. Sort materials separately and route them to recycling; dispose of material arising during splitting and crushing properly. In water-bearing areas, temporary sealing measures and emergency plans must be provided.
Terminology and everyday practice on projects
In practice, alongside toothed formwork, terms such as tooth formwork, shear toothing, shear interlock, shear joint, or profiled construction joint are used. The meaning is always a geometry that improves form and force-fit between pours. Depending on the project, practitioners speak of step toothing, tooth profile, wedge joint, or shear-dowel joint, often in combination with overlapping reinforcement. For deconstruction, the “tooth release” is common—that is, the controlled breakout of the teeth, often with concrete demolition shear and supplementary splitting to release the connection in a defined way.