Formwork anchors are central components in concrete construction: They hold opposing formwork elements in position during concreting, secure the spacing, and transfer fresh concrete pressure. Over a structure’s lifecycle they concern designers, contractors, and deconstruction teams alike—from selection and design to later remediation or selective deconstruction. In many existing structures, remaining anchor rods, sleeves, and cones influence strategies for concrete demolition and special demolition. Tools such as concrete pulverizers or hydraulic rock and concrete splitters from Darda GmbH enable controlled, low-vibration methods in these situations and facilitate separation of concrete and steel.
Definition: What is meant by formwork anchor
A formwork anchor is a detachable or permanent connection between two opposing formwork faces that ensures the required spacing of the formwork parts and takes up the resulting forces from fresh concrete pressure. Typically, formwork anchors consist of an anchor rod with a robust thread, anchor plates or anchor points with cones on both sides, spacers or sleeves, and corresponding closures. After the concrete has hardened, anchors—depending on the system—are partially removed, cut off flush, or remain as built-in components in the member and are closed off at the surface.
Structure, components and operating principle
In practice, a formwork anchor system is composed of several coordinated components. The core function is to secure the spacing of the formwork and to transfer the transverse forces that arise during concreting in a controlled manner into the formwork facing or into the formwork girders.
Typical components
- Anchor rod with a robust, site-ready thread (e.g., trapezoidal thread in common diameters); material selection ranges from unalloyed steel to corrosion-resistant variants.
- Anchor sleeve or spacer to ensure the defined wall cross-section and to control the ingress of cement paste.
- Anchor cones and finishing cones for clean anchor points, defined breakout after stripping, and visual requirements (exposed concrete) where applicable.
- Counterplates with nut or tensioning mechanism to clamp the anchor with frictional engagement.
- Closure systems for the anchor holes (cementitious mortars, resins, cone plugs, sealing washers where increased watertightness is required).
Operating principle
During concreting, the hydrostatic fresh concrete pressure acts on the formwork faces. This load is short-circuited via the anchors as tensile force between the opposing sides of the formwork. After hardening, the external anchor components are released; depending on the system, the inner part (e.g., sleeve, cone) remains in the member. Anchor points that appear at the surface are filled, pressure-grouted, or closed with cones, particularly where increased watertightness or exposed concrete requirements apply.
System variants and selection criteria
The choice of system is based on load level, wall thickness, surface requirements, watertightness, and the intended approach during deconstruction.
Common variants
- Continuous anchor rods with clamping on both sides: high flexibility, simple installation, partial removal or flush cutting depending on specification.
- Sleeve systems with removable rods: reduce steel remnants in the member and simplify later treatment of anchor points.
- Cone systems for exposed concrete: defined, recurring anchor patterns; cleanly removable and closable after stripping.
- Special solutions with increased watertightness requirements: additional sealing elements, sealing washers, or multi-stage closure concepts for waterproof structures.
Selection criteria from practice
- Fresh concrete pressure and formwork heights determine the required anchor capacity and quantity.
- Corrosion protection and durability, especially if anchor parts remain in the concrete.
- Deconstruction concept: The less metal in the cross-section, the easier the separation in selective demolition using concrete pulverizers or rock and concrete splitters.
- Surface requirements (exposed concrete, grid pattern, reprofiling depth).
- Watertightness and the sealing concept for anchor points in water-exposed members.
Design, load path and boundary conditions
Formwork anchors are designed for tension; governing parameters include fresh concrete pressure, formwork height, concreting rate, temperature, and concrete viscosity. Additionally, edge distances, wall thicknesses, and formwork stiffness influence the required number and arrangement of anchor points.
Planning aspects
- Determination of fresh concrete pressure according to recognized engineering practice.
- Specification of anchor spacing in line with wall thickness, girder grid, and desired surface pattern.
- Verification of anchor components including cones and bearing faces.
- Consideration of exposed concrete classes and tolerances for recurring anchor patterns.
Design information should always follow current standards and guidelines as well as manufacturer specifications. Concrete designs are project-specific and performed by qualified parties.
Installation, sealing and surface quality
Correct installation of formwork anchors determines not only the formwork’s structural safety but also the subsequent watertightness and appearance of the concrete surface.
Installation notes
- Accurate alignment of the anchors in the formwork plane; avoid skewing.
- Tight cone and sleeve interfaces to prevent washout and honeycombing.
- Controlled release after hardening; defined removal of cones.
- Closure of anchor points with suitable mortars or plugs; coordinated sealing concept for water-exposed members.
Surface appearance
Anchor points shape the appearance of exposed concrete surfaces. A uniform grid and cleanly formed cones and closures are decisive. For representative members, anchor positions are therefore coordinated early and aligned with the formwork planning.
Formwork anchors in existing structures: impact on deconstruction and remediation
Anchor components often remain in existing walls, slabs, or shafts. These steel parts influence the choice of deconstruction methods. For selective concrete demolition and special demolition, methods that specifically separate steel and concrete or release components with low vibration are suitable.
Low-vibration separation and deconstruction methods
- Concrete pulverizers: Grabbing, crushing, and downsizing concrete while simultaneously exposing anchor rods, which are then cut with steel shears or Multi Cutters.
- Rock and concrete splitters: Create controlled cracks along anchor axes or around anchor cones; particularly useful in sensitive environments (gutting, monument preservation, vibration-sensitive plants).
- Steel shears and Multi Cutters: Cold-cutting of anchor rods, stirrups, and embedded parts; suitable for confined workspaces.
- Combination shears: Combine cutting and pressing functions for mixed cross-sections.
- Hydraulic power packs: Supply the tools efficiently and mobile, including special operations with limited power supply.
Remediation of typical anchor points
- Closure and reprofiling of anchor holes with mineral systems; in multiple layers if required.
- Subsequent sealing under moisture exposure with coordinated sealing elements or injections; approach to be defined project-specifically.
- Removal of corroded anchor remnants, exposure with pulverizers or split wedges, followed by passivation and reprofiling.
Areas of application: practical relevance and typical scenarios
Formwork anchors are encountered in numerous construction tasks. Their handling during deconstruction requires different tool and process decisions depending on the environment and objectives.
Concrete demolition and special demolition
When removing walls with anchor remnants, concrete pulverizers enable selective downsizing and exposure of steel components. Rock and concrete splitters are used to release components with low noise and vibration, for example in inner-city settings or on sensitive existing structures.
Strip-out and cutting
During interior strip-out, flush-cut anchor rods often interfere with subsequent sawing or milling. Steel shears or Multi Cutters cold-cut these, and Hydraulic Power Units provide the necessary power. This allows openings and breakthroughs to be produced efficiently without excessive secondary damage.
Rock demolition and tunnel construction
For cast-in-place inner linings or tunnels with high water ingress, watertight closure of anchor points is especially important. In deconstruction, concrete pulverizers and splitters serve to open anchor areas in a controlled manner to remediate leaks or create connections without unnecessarily weakening the load-bearing structure.
Natural stone extraction
Formwork anchors appear only marginally here, for example in auxiliary constructions for concrete additions or plinths. Knowledge of steel remnants in the stone–concrete composite nevertheless helps align separation strategies: splitters act on natural joints, while steel shears safely cut remaining metallic embedded parts.
Special operations
In areas with limited emission budgets (vibration, noise, dust) or in sensitive facilities, low-vibration methods are required. Targeted splitting around anchor points and cold cutting with shears reduces emissions and accelerates subsequent steps.
Equipment selection by component and objective
The choice of tool depends on concrete strength, reinforcement and anchor density, accessibility, and the required emission level.
- Thick, heavily reinforced walls with many anchor remnants: concrete pulverizers for the concrete, then steel shears/Multi Cutters for the anchor rods.
- Exposed concrete with dense grid patterns: splitters to release cone areas in a controlled manner; minimal edge spalling.
- Confined shafts or interior spaces: compact pulverizers and Multi Cutters with suitable hydraulic power packs.
- Assets with high protection requirements: prioritize low-vibration splitting technology, supplemented by cold-cutting methods.
Occupational safety, quality and environment
Handling formwork anchors requires careful work—both during concreting and during deconstruction.
Safety aspects
- Controlled pre-tensioning/release of anchors; avoid hazards from spring-back.
- When cutting or splitting, pay attention to stress states; establish protection zones.
- Personal protective equipment, especially cut and splash protection.
Quality assurance
- Documentation of anchor arrangement for exposed concrete surfaces.
- Inspection and rework of anchor points (watertightness, pull-off strength of reprofiling).
- For water-exposed members, coordinated sealing concepts for anchor penetrations.
Environmental aspects
- Low-vibration methods reduce impacts on the surroundings.
- Dust and noise reduction through suitable separation methods and hydraulic power packs with demand-based output.
- Clean separation of concrete and steel facilitates recycling streams.
Typical failure patterns and appropriate measures
- Leaking anchor points: Causes range from inadequate closure to material incompatibilities; remedy with adapted reprofiling or injections.
- Corrosion of remaining anchor parts: Local exposure with concrete pulverizer/split wedge, removal or passivation, durable surface protection.
- Breakouts at cones: Reprofiling with suitable mortars; for exposed concrete, coordinated color and texture matching.
Practical guide: procedure for removing and closing anchor points
- Investigation: Record drawings, grid, and materials of the anchor components; trial exposure.
- Exposure: Locally open the concrete with concrete pulverizers or create controlled cracks with rock and concrete splitters.
- Cutting: Cold-cut anchor rods with steel shears or Multi Cutters; flush or recessed, depending on the repair concept.
- Preparation: Bondable, clean edges; corrosion protection for remaining metal parts.
- Closure: Suitable mortar/plug; multi-stage sealing build-up if required.
- Control: Visual inspection; where applicable, document functional or watertightness testing.
This schematic procedure must be adapted to the respective project, the concrete structure, and the requirements. Binding specifications are defined on a project-specific basis by qualified professionals.