Fibre addition

Fibre addition describes the targeted mixing of short, discontinuous fibres into mortar and concrete mix designs. The aim is to limit crack formation, increase toughness and residual load-bearing capacity, and improve element behaviour under tension and flexural tension. In planning and execution, but also in deconstruction, fibre addition plays an important role: fibre-reinforced concrete (FRC) behaves differently from conventional concrete during demolition and thus influences the selection and use of concrete pulverizers or hydraulic splitters (wedge) from Darda GmbH in areas such as concrete demolition and special deconstruction, interior demolition, rock excavation and tunnel construction, or special operations. Beyond performance while in service, the altered fracture pattern and fragment cohesion under load also affect dust, noise and vibration control concepts during deconstruction.

Definition: What is meant by fibre addition?

Fibre addition means mixing metallic or non-metallic fibres into cementitious construction materials. The fibres act as micromechanical crack bridges: they limit crack openings, increase energy absorption capacity, and can improve the ductility of the composite. Depending on fibre material, geometry and dosage, different effects are achieved, such as early-stage crack width limitation (microfibres) or residual load-bearing capacity after cracking (macrofibres). These properties are important for the serviceability of industrial floors, shotcrete in tunnel construction, or components with increased robustness – and they shape fracture and size-reduction behaviour in deconstruction using equipment from Darda GmbH. Fibre addition does not replace code-required bar or mesh reinforcement where structural design demands classic reinforcement; it complements it where appropriate.

Mechanism of action, fibre types and application limits

In cementitious matrices, fibres do not provide a classic reinforcement function like bars; instead, they act in a distributed manner. Decisive is the pull-out or rupture behaviour of the fibre in the cracked composite. This results in increased toughness, post-cracking load contribution, and a limitation of crack widths. The load transfer depends on bond properties, fibre orientation and slenderness ratio; strain-hardening effects can be achieved in narrow windows, while most FRCs show controlled softening with useful residual strength.

Steel fibres

Steel fibres (with hooks, anchorage or deformed surface) provide high residual tensile and flexural tensile load contributions. They improve impact toughness and reduce brittleness. Typical applications include industrial floors, precast elements or shotcrete for tunnel construction. In deconstruction, steel fibres often keep fragments connected for longer; concrete pulverizers must cut or tear fibres as well. Protruding fibre bundles may require rework with steel shear or hydraulic demolition shear. Surface rusting of exposed ends is common but typically superficial in alkaline matrices; metallic fibres can be magnetically separated during processing and recycling.

Synthetic fibres

Polypropylene (PP) and polyethylene (PE) fibres are used as micro- or macrofibres. Microfibres address early shrinkage cracking and reduce the risk of explosive spalling under fire exposure by melting and forming micro-channels for steam pressure relief. Macrofibres provide – within limits – crack-bridging functions. During demolition, synthetic fibres can generally be torn effectively with concrete pulverizers; the splitting behaviour approaches that of conventional concrete, yet remains tougher. Chemical resistance and low density support uniform distribution, but fibre length and dosage must be matched to pumping and placing methods.

Glass, basalt and carbon fibres

Alkali-resistant glass fibres (AR glass) and basalt fibres improve surface and edge stability, while carbon fibres increase strength in thin-walled elements. They are more common in special components. In selective deconstruction, finer fracture surfaces and increased edge integrity can be expected; adjusting jaw forces and cutting geometries can improve process stability. With brittle high-modulus fibres, attention to edge chipping and dust management is advisable; adequate PPE and extraction reduce irritation from fine fibre fragments.

Application limits

Very high fibre contents increase mixing and pumping demands, can reduce workability, and raise tool wear during deconstruction. The suitability of the fibre type depends on the exposure and application area; adherence to recognized rules of practice is essential. Aggregate grading, maximum grain size and fibre geometry must be coordinated to avoid blocking in pump lines and to prevent segregation; approvals and performance-based verifications may be required for structural uses.

Dosage, distribution and mix design

The effect of fibre addition depends on dosage (by volume or mass fraction), fibre length and diameter (slenderness ratio), surface characteristics, and uniformity of distribution. A balanced mix design ensures workability and performance. Target values should be linked to measurable performance indicators such as residual flexural strength classes or crack width limits, not solely to kilogram per cubic metre specifications.

• Dosage: Microfibres are usually in the range of a few kilograms per cubic metre; macro- and steel fibres higher – always sized project-specifically.
• Workability: Adjusted aggregate grading and concrete admixture reduce balling and maintain pumpability. For self-compacting concretes with fibres, viscosity-modifying admixtures support stability.
• Fibre distribution: Even, clump-free mixing prevents local weaknesses and ensures reproducible properties. Visual checks during trial batches help calibrate the mixing regime.
• Water content: The effective water-cement ratio must not be inadmissibly altered by fibre addition; fresh concrete tests help with control.
• Performance verification: If required, flexural tests on prisms or round panels document residual load-bearing capacity and inform deconstruction planning.

Recommended mixing sequence

1. Homogenize dry components.
2. Add part of the mixing water and admixtures, set the workability level.
3. Slowly and evenly sprinkle in fibres, increase mixing time, add remaining water.
4. Test fresh concrete (consistency, homogeneity) and fine-tune as needed. Where necessary, adjust admixtures and mixing energy to prevent fibre balling.

Placement and processing: Cast-in-place concrete, shotcrete, precast elements

The placement of fibre-containing concretes follows the same basic principles as conventional concrete, but requires more careful control of homogeneity and deaeration. Fibre orientation can be influenced by flow paths and vibration; execution should aim for uniform dispersion and avoid segregation at edges and openings.

Cast-in-place concrete

During placement, sufficient compaction and controlled curing are important, as fibres can influence early-age shrinkage cracking. Edges and anchorage zones benefit from uniform fibre distribution. Internal vibration should be adapted to avoid excessive settlement or fibre clustering; finishing tools must be matched to prevent fibre pull-out at the surface where appearance or abrasion resistance is critical.

Shotcrete (tunnel construction)

Fibre-reinforced shotcrete is widespread in tunnel face support. Macrofibres or steel fibres provide residual load contribution between anchor points. During later deconstruction, for example in cross-section enlargement, fibre addition changes removal behaviour: concrete pulverizers work efficiently when size reduction is performed in layers and fibre bridges are deliberately torn. Nozzle setup, accelerator dosage and rebound management require coordination to limit losses and ensure target fibre content in place.

Precast elements and industrial floors

In slabs, chambers, edge beams or industrial floors, fibre addition increases robustness against impact and fatigue loading. In selective deconstruction, the combination of pre-splitting and subsequent trimming supports a clean separation. For joint-reduced floor systems, residual flexural strength and crack width control are key parameters to document during production and later refurbishment planning.

Effect of fibre addition on demolition, deconstruction and size reduction

Fibre-reinforced concrete exhibits tougher behaviour in fracture. Crack faces remain interconnected for longer; fragments are held together by fibre bridges. This results in practical consequences for equipment selection and operation. Tool engagement, stroke control and sequencing determine how reliably fibre bridges are severed while keeping vibration and dust low; wear parts may require closer monitoring with high steel fibre contents.

Concrete pulverizers

Concrete pulverizers by Darda GmbH are designed for controlled size reduction. For fibre-containing elements, a sequential approach is recommended: pre-crack along planes of weakness, then follow up to separate remaining fibre bridges. Steel fibres may protrude along the cut edge; a short trimming pass or snipping with steel shear or hydraulic demolition shear prevents hook formation. An adjusted stroke and closing speed supports material-appropriate crack propagation. Penetrating tooth profiles and alternating bite positions improve the chance of rupturing residual bridges efficiently.

Hydraulic splitters (wedge)

Splitters, such as hydraulic rock and concrete splitters, develop high wedge forces for brittle separation. Fibre addition reduces the tendency to pure brittle fracture; borehole spacing, splitting direction and number of setting points must be planned accordingly. In elements with steel fibres, combining pre-splitting with subsequent processing using concrete pulverizers can increase efficiency. With low fibre content, the split pattern usually remains predictable; at high contents, tougher crack paths are to be expected. Borehole diameter and embedment depth should be harmonized with the element thickness and support conditions to control crack initiation.

Combination shears, multi cutters and steel shears

If fibre whiskers remain after crushing, they can be removed flush with steel shear or hydraulic demolition shear. Multi cutters support separating local inserts or trimming fibre-reinforced edges. This reduces trip edges and injury hazards. Short, controlled cuts minimize spalling and improve the recyclability of separated fractions.

Hydraulic power packs

Hydraulic power packs from Darda GmbH provide the working pressure for pulverizers, shears and splitters. In fibre-containing concrete structures, a slightly longer working cycle may be required; stable oil flow and precise controllability facilitate material-adapted processing. Matching flow rate and pressure to tool requirements avoids unnecessary load peaks and supports consistent cutting performance.

Application areas relevant to fibre addition

Concrete demolition and special deconstruction

Fibre-reinforced concrete is common in deconstruction-friendly designs, industrial floors or machine foundation blocks. Concrete pulverizers enable controlled size reduction with low dust and low vibration levels; with steel fibres, increased fibre bridge formation is to be expected, which influences the cutting sequence. Where access is limited, combining pre-splitting and targeted biting passes helps to maintain process stability.

Interior demolition and cutting

In interior demolition of fibre-reinforced walls and slabs, pre-cracking with concrete pulverizers is often combined with saw cuts. Fibre bundles along cut faces can be trimmed afterwards to achieve a clean joint. Dust extraction and careful staging of operations support compliance with indoor emission limits and shorten rework.

Rock excavation and tunnel construction

In tunnel construction, fibre-reinforced shotcrete is used as temporary or permanent lining. When widening cross-sections or removing temporary shells, a layer-by-layer pulverizer method ensures controlled detachment without impairing the load-bearing action of adjacent areas. Precondition surveys of lining thickness and residual strength guide the choice of bite depth and spacing.

Natural stone extraction

In natural stone extraction itself, fibre addition plays no role. It becomes relevant where fibre-reinforced shotcrete shells were used for stabilization and later need to be removed. Splitters and concrete pulverizers can be coordinated to release stabilization layers without unnecessarily damaging the natural stone. Clean interfaces facilitate subsequent restoration or new anchorage systems.

Special operations

In areas with fire exposure, microfibres are used to limit spalling. During later deconstruction, edge zones often remain more intact and tougher; an adapted pulverizer guidance with short gripping strokes improves control. In safety-sensitive environments, combining splitting and pulverizing minimizes the introduction of vibrations. Sequenced bites and limited penetration reduce the risk of uncontrolled fragment release where adjacent operations continue.

Quality assurance, testing and documentation

To assess fibre-reinforced concrete, tests of fresh concrete (consistency, homogeneity) and hardened concrete (flexural tensile behaviour, residual load-bearing capacity, crack width limitation) are used, among others. Traceable documentation of fibre type, content and mixing procedure facilitates later deconstruction concepts. For demolition, as-built documents are helpful, since fibre content and type influence the equipment setup and work steps. Performance-related specifications and retained test reports simplify planning and permit risk-based method statements for size reduction.

Occupational safety and environmental aspects

During processing, fibre dust and mechanical irritation can occur. Personal protective equipment, low-dust methods and orderly handling of fibres must be considered. In deconstruction of fibre-containing components, watch for protruding fibre ends; clean cutting with shears reduces injury risk. Metallic fibres can often be magnetically separated in the recycling process; source-separated sorting improves recovery routes. Legal requirements must be checked in a project-specific and general manner. Extraction at the tool, eye and hand protection, and safe handling of cut whiskers in collection containers support occupational safety and environmental hygiene.

Planning and tendering in the context of deconstruction

Even at the planning stage, it is advisable to consider fibre addition in specifications and as-built documentation. For deconstruction, trial areas and test demolitions are useful to assess the composite’s toughness and optimally coordinate the interaction of concrete pulverizers, hydraulic splitters (wedge) and shears. A clear sequence – pre-splitting, layer-by-layer crushing, trimming of fibre whiskers – increases process reliability and reduces rework. Where appropriate, performance-based tender items describing target residual strengths lead to more robust execution than sole mass-per-volume declarations.

Typical practical mistakes and how to avoid them

• Fibre balling due to improper mixing – choose slow feeding and sufficient mixing time.
• Underestimated toughness in deconstruction – adapt pulverizer sequencing and gripping strokes, place splitting points closer.
• Loss of workability – match aggregate grading and concrete admixture to fibre content.
• Incomplete documentation – record fibre type and dosage to define equipment selection and work sequence in a targeted manner.
• Safety risks from fibre whiskers – consistently snip them flush with steel shear or hydraulic demolition shear.
• Neglecting performance verification – define and check residual strength parameters to align execution and deconstruction methods.