Prestressed concrete

Prestressed concrete stands for high load-bearing capacity with slender cross-sections. Tensioned strands or wires place the concrete in favorable compressive stresses, so that tensile actions are largely compensated. This construction method characterizes bridges, halls, slab fields and precast elements. For planning, maintenance and especially for deconstruction, understanding the stored prestressing forces is crucial. In demolition and special deconstruction, depending on the situation, hydraulic concrete crushers, rock and concrete splitters, steel shears, multi cutters or combination shears as well as suitable power units from Darda GmbH are used to work in a controlled, low-vibration and material-friendly manner. Comprehensive risk assessments, defined method statements and, where appropriate, trial cuts support safe and economical sequences in practice.

Definition: What is meant by prestressed concrete?

Prestressed concrete is concrete whose load-bearing behavior is improved by intentionally introduced prestressing forces. Through tendons (wires, strands or bars), tensile forces are introduced into the concrete before or after service loads act. A distinction is made between pre-tensioning in a pre-tensioning bed (prestressing before casting) and post-tensioning (prestressing after hardening). Prestressing reduces cracking, limits deflection and enables long spans with low structural depth. The tendons are arranged as bonded systems (grouted duct) or as unbonded systems and are anchored at anchorages or deviation points. Bonded systems develop composite action along the duct, while unbonded systems allow limited redistribution at the anchorages, which affects inspection, repair and deconstruction strategies.

Configuration, working principle and component types

Prestressed concrete components consist of concrete, reinforcement and tendons. The tendons usually run in the lower tension zone (under bending), are tensioned, and keep the concrete-in spite of prestress losses-in a favorable stress state. Common components include slab beams, hollow-core slabs, bridge girders, prestressed slab systems, or prestressed precast elements. Internal prestressing is arranged within the cross-section (ducts with grout), while external prestressing runs outside the cross-section and is introduced via deviations and anchor plates. Critical details include deviators, local bearing and bursting reinforcement, and anchorage zones with concentrated force introduction.

Types of prestressing and methods

• Pre-tensioning in a pre-tensioning bed (pre-tensioning method): Tendons are tensioned along a stressing bed against rigid abutments and then cast in; after hardening they are released, transferring the forces into the concrete.
• Post-tensioning (cast-in-place or precast composite): Ducts are cast in, tendons are installed after hardening, tensioned and grouted (bonded) or left ungrouted (unbonded).
• Internal vs. external prestressing: Tendons guided within the cross-section (internal) or outside (external) with deviations and anchor heads.
• Combined or staged stressing: Multiple stressing operations, flat ducts in slabs or deviated multi-strand tendons for optimized bending lines and construction phasing.

Tendons and materials

Common are 7-wire strands, wires or bars made of high-strength prestressing steel. Ducts (metal or plastic) and grout ensure bond and corrosion protection. Anchor plates and wedge anchors transfer forces into the concrete. Key influencing factors include relaxation of prestressing steel, creep and shrinkage of concrete, and friction in the ducts. Quality-assured grouting, robust sheathing and verified corrosion barriers are decisive for service life and safe handling during later interventions.

Structural behavior, design and durability

Prestressing generates compressive stresses that superimpose the tensile stresses from service loads. This limits crack widths and controls deflections. Design generally follows the applicable concrete standards (e.g., Eurocode frameworks), including ultimate and serviceability limit states as well as durability and fatigue verifications. Time-dependent effects (creep, shrinkage, relaxation), second-order influences, shear at deviators and anchorage bursting forces must be considered; fire behavior and robustness concepts are verified where required.

Crack widths, deflection, prestress losses

Prestress losses arise from friction, anchor seating, relaxation, creep and shrinkage. They are accounted for when defining the target prestress. Cracking may still occur under exceptional actions and requires a component-specific assessment. Predictive models and, where necessary, on-site monitoring (deflection checks, strain measurements) support verifying serviceability over the life cycle.

Corrosion protection, grouting and ducts

Bonded systems with grout protect the strands and ensure force transfer. Unbonded systems rely on grease or HDPE sheathing and are particularly sensitive during deconstruction with regard to possible residual prestress. In existing structures, grout quality remains a durability concern. Non-destructive testing and local inspections help detect voids, water ingress or damage to sheathing and anchorage zones.

Prestressed concrete in existing structures: identify, test, document

Before interventions, the prestressing system must be identified: position of tendons, anchorage zones, deviation points and year of construction. Project documentation, reinforcement scanning, low-damage investigations and trial openings provide clues. Documentation of prestressing records, grouting reports and re-analyses supports decisions for repair or deconstruction. Consistency checks between plans and findings reduce uncertainty and inform method selection.

Typical indicators and evidence

• Visible anchor plates or end caps at end diaphragms and abutments.
• Deviation zones with thickened cross-sections or concrete buildups.
• Precast girders with characteristic pre-tensioning bed marks and bearing plates.
• Construction periods and uses in which prestressed system structures were common.

Investigation and measurement methods

• Georadar and electromagnetic scanning to locate ducts, strands and reinforcement.
• Endoscopy through small openings to assess duct condition, grout presence and anchor heads.
• Lift-off tests, controlled detensioning at isolated tendons or vibration-based back-calculation to estimate effective prestress.
• Core extraction and material testing to determine compressive strength, density and carbonation depth in anchorage regions.
• Targeted trial openings with documented re-closure, coordinated under a project-specific risk and method statement.

Deconstruction of prestressed concrete: particular challenges

Prestressed concrete stores potential energy in its tendons. Uncontrolled cutting can result in sudden release. Therefore, safe work zones, gradual stress release, load-free conditions and suitable cutting and crushing methods are essential. Hydraulic tools from Darda GmbH enable low-vibration, precise interventions in concrete and reinforcement-the sequence of work steps is crucial and must be defined on a project-specific basis. Where feasible, stresses are redirected or reduced in advance through detensioning, propping and segmentation.

Safety aspects and general principles

• Careful assessment of the existing structure and definition of a deconstruction-specific load-bearing concept (temporary shoring, segmentation, load redistribution).
• Protection zones against snap-back or component drop, shielding and controlled working direction.
• Section-by-section exposure of anchorage and deviation zones prior to separation cuts.
• Minimization of dust, noise and vibration as well as water and waste management.
• Defined emergency procedures, exclusion zones and stop-work criteria for unexpected tendon behavior.
• Work supervised by qualified personnel with competence in prestressing systems and hydraulic cutting methods.

Methods and sequence in controlled demolition

Common is segment-by-segment removal with prestressing forces reduced in advance, local exposure of tendons, and controlled cutting in defined, secured areas. For unbonded systems, particular attention is paid to residual strand tension. For bonded systems, removal of surrounding concrete and targeted cutting-free of strands is typical, always with adequate restraint or unloading of adjacent components. Where required, detensioning using jacks at anchor heads precedes core cuts or crushing to minimize stored energy.

Tools and low-vibration methods

Concrete crushers from Darda GmbH break concrete locally, reduce cross-sections and expose tendons in a defined manner. Stone and concrete splitters work in drill holes with controlled hydraulic pressure to split concrete quietly and with low vibration-useful in sensitive environments such as hospitals, laboratories or densely built inner cities. Steel shears and multi cutters are used to cut reinforcement and-with appropriate securing-strands or wires. Combination shears unite cutting and crushing, which offers advantages when exposing and cutting in confined zones. hydraulic power units supply the tools with the required output and facilitate mobile on-site work. In particular configurations and special operations, tank cutters are also used, for example when pipelines, tank connections or inserts must be cut in combined steel-concrete systems. Remote placement of power units and consistent hose management improve safety and ergonomics in tight workspaces.

Typical application areas of prestressed concrete and related deconstruction requirements

Prestressed concrete appears in new and existing construction across various sectors, resulting in specific methods for repair, strip-out and deconstruction.

• Concrete demolition and special deconstruction: Segmental removal of a prestressed structure with concrete crushers; targeted exposure and securing of anchor heads; steel shears for cutting exposed reinforcement.
• Strip-out and cutting: In buildings with prestressed slabs, stone and concrete splitters support openings without widespread cracking; multi cutters assist with the separating removal of metallic installations.
• Rock removal and tunneling: For combined structures (e.g., anchor blocks, abutments), stone splitting cylinders enable controlled alternatives to blasting with low vibration.
• Natural stone extraction: Indirect connection via technology transfer-hydraulic splitting for precise fractures; the know-how feeds into safe splitting of concrete components.
• Special operations: Work in sensitive areas with strict emission limits, where hydraulic, low-spark methods with concrete crushers, combination shears and suitable power units offer advantages.
• Marine and port structures: Controlled exposure and segmentation of anchor blocks, crossheads or slab fields with low-vibration methods under constrained access and environmental requirements.

Cutting and separation techniques in prestressed concrete

The choice of technique depends on the prestressing system, cross-section, accessibility and environmental requirements. The aim is controlled redirection or elimination of critical stress states before actual cutting.

• Wire saws and wall saws: Precise separation cuts, often combined with prior exposure of tendons.
• Core drilling: Relief drillings, access to anchorage zones and preparation for stone and concrete splitters.
• Hydraulic pressing/splitting: Hole-based splitting locally reduces cross-section and stress levels.
• Hydraulic concrete crushers: Selective crushing, controlled removal without excessive vibration.
• Relief slot cutting: Pre-cuts to steer crack propagation and reduce edge spalling ahead of main separation cuts.

Targeted use of concrete crushers in prestressed concrete

Concrete crushers allow layer-by-layer biting of concrete to reveal tendons and assess tensile forces. Strands can then be cut under restraint in a controlled manner. The method is particularly suitable at edges, ribs and webs of girders as well as in anchorage and deviation areas where precise material removal is required. Progressive reduction of the cross-section limits sudden energy release and supports predictable behavior.

Stone and concrete splitters in prestressed concrete

Splitters act in drilled holes with high, localized pressure. In prestressed components, this enables opening cross-sections with negligible edge damage, for example to create openings in slabs or reduce anchor blocks. The low vibration input protects adjacent components and infrastructure-an advantage in existing buildings and when working near sensitive facilities. Accurate drilling patterns and staged pressurization further enhance control.

Quality, environment and sustainability in prestressed concrete deconstruction

Selective deconstruction of prestressed concrete promotes recycling rates and reduces emissions. Hydraulic methods lower noise and vibration; dust is minimized by targeted wetting and extraction. Source-separated sorting of concrete, reinforcement and prestressing steel facilitates reuse. Clean cut-edge quality supports subsequent reconstruction. Early planning of transport routes, temporary storage and on-site processing improves material recovery.

Low-emission working methods

Hydraulic concrete crushers and stone and concrete splitters operate with low sparking, which is advantageous in sensitive areas. Water demand is lower than with wet sawing methods, simplifying logistics and disposal. Electrically driven power units can reduce local exhaust emissions and contribute to compliance with indoor or tunnel work requirements.

Documentation and aftercare

Accompanying measurements, photo documentation and records of prestress handling increase traceability. Edge sealing, temporary shoring and protective measures are continuously checked and adapted to construction progress. Digital logs for tool use, pressure levels and sequence steps, combined with sign-off by responsible specialists, support quality assurance.

Standards, planning and responsibility

Planning, execution and deconstruction of prestressed concrete follow the relevant technical codes for concrete construction. Interventions in prestressed structures require qualified specialist personnel. Information on standards and methods is to be understood generally; specific measures must be planned and verified on a project-specific basis. Hydraulic tools such as concrete crushers, stone and concrete splitters, combination shears, multi cutters, steel shears and tank cutters from Darda GmbH cover different tasks within professional, controlled operations. Typical project documentation includes risk and method statements, permits and clear role allocation with site monitoring.

• Defined planning documents: deconstruction concept, load paths, temporary works and verification of stability.
• Qualification and briefing: competence in prestressing, tool-specific training and coordinated communication channels.
• Monitoring and acceptance: hold points for tendon handling, pre-cut inspections and final checks of residual risks.