Prestressing methods describe the targeted introduction, holding, and controlled release of tensile forces in components and rock. In planning, construction, and deconstruction they play a central role: from prestressing concrete girders to activating anchors to the deliberate generation of tensile stresses to trigger cracks and splitting. In practice, prestressing methods apply to use cases such as concrete demolition and special deconstruction, interior demolition and cutting, rock excavation and tunnel construction, natural stone extraction, as well as special operations. Tools from Darda GmbH—such as concrete pulverizers or stone and concrete splitters—are used wherever stress states are exposed, safely reduced, or used for crack guidance.
Definition: What Is Meant by Prestressing Methods
Prestressing methods are methodological procedures in which tensile stresses in materials or structures are built up, transferred, secured, or reduced again. In concrete construction, the term often stands for the prestressing method (with or without bond, pre- or post-tensioning). In deconstruction, it includes the safe exposure, stress relief, and separation of prestressed concrete, prestressing steel, and anchors. In rock and natural stone, targeted tensile loading is used to form fracture planes in a controlled manner—for example through hydraulic splitting with stone and concrete splitters.
Fundamentals and Principles of Prestressing and Prestress Technology
Prestressing methods are based on the principle of deliberately generating tensile forces and introducing them into a component via suitable load paths. In prestressing, the introduced prestress compensates for future service loads; in deconstruction, existing prestresses are identified, reduced in a controlled way, and components are separated with minimal residual stress. In rock, introducing local tensile stresses serves mechanically favorable, low-vibration removal.
Prestressing Systems in Concrete Construction: Pre- and Post-Tensioning
Various prestressing systems have become established in construction practice. Knowing them is essential for planning, site supervision, and deconstruction, because tendon paths, anchorage zones, and deviators determine the load path.
System Overview
- Prestressing with bond (grouted post-tensioning): Tendons are grouted after stressing. Force transfer occurs via bond into the concrete; deviator points and anchorage zones are essential.
- Prestressing without bond: Tendons run in ducts, with force transfer only at the anchors. Accessibility of tendons is advantageous in deconstruction but requires specific stress-relief steps.
- Pre-tensioning (immediate bond): Stressing before concreting and transfer by direct bond after hardening; typical for precast elements.
Materials and Components
- Prestressing steel (wires, strands, bars), anchor heads, stressing jacks, ducts, grout, deviators.
- Influencing factors: friction losses, wedge seating, relaxation, temperatures, creep and shrinkage of concrete.
Prestressing Methods in Deconstruction: Safe Stress Relief, Separation, and Load Redistribution
For prestressed members, safety comes first. The goal is controlled release of stored elastic energy before components are separated or reduced in size. Darda concrete pulverizers are often used to expose tendons; steel shears or Multi Cutters perform defined cutting of prestressing steel.
Process Steps (Simplified Sequence)
- Investigation: Review drawings, detection and probes; determine the location of anchor heads, deviators, tendon paths, and bond conditions.
- Structural concept: Load redistribution, shoring, temporary support beams; determination of the permissible sequence for stress relief.
- Exposure: Open concrete locally with low vibration levels, e.g., using concrete pulverizers; make anchorage zones accessible.
- Stress relief: Stepwise reduction of existing prestress. For unbonded systems: defined unloading at anchors. For bonded systems: controlled notching and cutting concept.
- Separation: Cut prestressing steel with suitable cutting tools (e.g., steel shears, Multi Cutters); avoid spring-back or whiplash effects (protective measures, coverings, minimize free length).
- Removal of concrete sections: After stress relief, mechanical downsizing and removal; followed by material sorting.
Induced Tensile Stresses for Controlled Fracture: Hydraulic Splitting
Hydraulic splitting exploits the material behavior of concrete and natural stone: both are strong in compression but sensitive in tension. Stone and concrete splitters and stone splitting cylinders from Darda GmbH generate locally high spreading forces that initiate cracks along weak planes. The method features low emissions and low vibration levels and is suitable for precise removals, e.g., in interior demolition, rock demolition and tunnel construction, or in natural stone extraction.
Advantages of the Splitting Method
- Targeted crack guidance and dimensional accuracy.
- Low noise and vibration loads; protection of sensitive adjacent structures.
- Reduced secondary risk compared with percussive methods.
Typical Application Areas
Concrete Demolition and Special Demolition
Prestressed girders, hollow-core slabs, or bridge components require a stress-relief concept. After exposure with concrete pulverizers, tendons are cut in a controlled manner. Splitters assist with low vibration levels opening of massive sections or with targeted separation of foundations in the context of concrete demolition and special deconstruction.
Interior Demolition and Cutting
During strengthening and partial deconstruction, prestress zones are opened locally. Steel shears and Multi Cutters cut reinforcement and prestressing steel, while stone and concrete splitters create openings without large-scale damage.
Rock Excavation and Tunnel Construction
During heading and in niches, the stress state of the rock mass is considered. Hydraulic splitting enables controlled removal and reduces vibrations. Tension anchors or temporary grouted anchors can hold loads until removal is complete.
Natural Stone Extraction
Splitting cylinders introduce targeted tensile stresses to free blocks along natural joints. This increases the yield and quality of the extracted raw blocks and aligns with natural stone quarrying applications.
Special Operations
Complex steel installations, tanks, or composite members require special planning for stressing and cutting. Tank cutters or specialized shears are used where material thicknesses and cross-sections are beyond usual dimensions.
Tools and Equipment in the Context of Prestressing Methods
The selection of equipment follows the stress state of the component, accessibility, and the objective (stress relief, separation, splitting). Hydraulic power packs from Darda GmbH supply the attachments as required.
- Concrete pulverizers: Precise exposure of anchor heads, strand and bar paths; opening concrete cover with minimal edge damage.
- Stone and Concrete Splitters / Stone Splitting Cylinders: Generate local tensile stresses for crack initiation and controlled detachment.
- Steel Shears / Multi Cutters: Separate prestressing steel, reinforcement, and steel sections after stress relief or in a secured environment.
- Combination Shears: Flexible use in composite sections (concrete–steel composite) when cutting tasks vary.
- Tank Cutters: Cutting thick plates and vessel components in special operations, matched to stress and residual forces.
- Hydraulic Power Packs: Supply and sensitive control of the connected tools; important for reproducible process steps.
Planning Parameters: Design, Sequence, and Control
Prestressing and deconstruction concepts account for mechanical and organizational influencing factors. A clear sequence and measurability increase safety and quality.
- Prestress levels, friction and deviator losses, relaxation; documented prestress values.
- Bond condition (with/without bond), duct systems, grout quality.
- Member geometry, support conditions, temporary shoring and redistributions.
- Interfaces to neighboring structures, allowable deformations, vibration limits.
- Measuring and control devices (displacement, strain, force), release points for subsequent steps.
Occupational Safety, Environmental, and Emission Aspects
Handling stored prestress energy requires heightened attention. Protective measures must be adapted to the specific hazards; applicable standards and regulations must be observed.
- Personal protection: Cordoning off the hazard zone, retention systems during cuts, coverings against whiplash effects.
- Emissions: Noise and vibration minimization; hydraulic splitting and pulverizer work are often advantageous.
- Dust and water: Dust suppression, safe handling of process water; material separation for recycling.
- Tool safety: Regular safety inspection of shears, pulverizers, splitting cylinders, and hydraulic hose lines; depressurized condition before coupling/uncoupling.
Best Practices and Common Pitfalls
Experience and a clear method avoid disruptions and increase efficiency.
- Early investigation and documentation of prestressing systems; no assumptions without evidence.
- Step-by-step stress relief instead of large-scale, uncontrolled cuts.
- Appropriate sequence: relieve first, then cut, and finally downsize.
- Select tools suited to the material and cross-section; assess cutting capacity realistically.
- Use crack guidance through splitting tools deliberately instead of unguided breakage.
- Continuous monitoring, clear communication paths, defined stop criteria.
Documentation and Monitoring
Transparent procedures and traceable measurements are key quality characteristics of prestressing methods.
- Component and tendon IDs, exposure locations, photos before/after each step.
- Measurement logs (forces, displacements, strains), releases, acceptance points.
- Evidence of shoring, load redistribution, and boundary conditions.
- Feedback of experience for future projects and standardization of procedures.
Material- and Component-Specific Particularities
In bridges, precast slabs, or composite girders, tendon paths and anchorage zones differ significantly. Corroded or insufficiently grouted systems can show unexpected behavior. Therefore, probes and careful exposure with Darda concrete pulverizers are advisable before cutting or splitting begins. In rock, joints and bedding favor crack propagation—here, stone and concrete splitters can be used to deliberately exploit the natural structure.