Concrete Rebound Losses in Shotcrete Explained

Rebound losses occur primarily during the application of shotcrete: part of the material applied to the surface bounces off, falls to the ground, and can usually no longer be incorporated into the structure. This affects material consumption, quality, and occupational safety and later influences deconstruction. In application areas such as concrete demolition and special demolition as well as rock excavation and tunnel construction, understanding rebound losses is essential for properly planning methods, work sequences, and tools—such as concrete pulverizer or rock wedge splitter and concrete splitter.

Definition: What is meant by concrete rebound losses

“Concrete rebound losses” refers to the portion of shotcrete (cement paste, sand, aggregates) that, upon impact on the substrate, rebounds and falls to the ground as loose rebound material. Causes include the high impact velocity of the particles, impact angle, aggregate gradation, moisture content, and the geometry and roughness of the substrate. Rebound affects the composition of the applied layer (e.g., binder enrichment at the surface) and thus the adhesion, density, durability, and the subsequent removal behavior during deconstruction.

Causes, key figures, and measurement of rebound losses

Rebound losses occur to varying degrees depending on the process (dry- or wet-mix shotcrete), nozzle pressure, nozzle distance, application angle, aggregate (size, shape, gradation), admixtures (e.g., accelerators), layer thickness, position (overhead, wall, floor), and workforce qualification. Typical rebound rates range—depending on boundary conditions—from single-digit to mid double-digit percentages and can be determined by weighing (input quantity vs. net built-in amount) or by accompanying tests (layer thickness, density, sampling and sieving of rebound material). Lower rebound rates mean more efficient material use, less rework, and more homogeneous layers—advantages that are noticeable later in special demolition.

Formation and influencing factors in shotcrete

The rebound mechanism is physically governed by the momentum of the aggregate. If the particle strikes without sufficient embedding in plastic cement paste, it leaves the surface again. In the dry-mix process, higher air content, rough particle characteristics, and unfavorable nozzle handling promote rebound; in the wet-mix process, appropriately adjusted consistency often achieves lower rebound fractions.

Typical influencing factors in practice

Substrate: High roughness and a load-bearing, clean substrate improve embedding; contaminated or smooth surfaces increase rebound.
Nozzle handling: A small nozzle stand-off and an application angle close to 90° reduce rebound; large distance and shallow angle increase it.
Mix design: A well-graded particle-size distribution, rounded aggregates, and suitable consistency lower rebound; overly coarse aggregate increases it.
Element position: Overhead work generally shows higher rebound losses than vertical surfaces.
Accelerators: Excessive dosages can reduce adhesion and promote rebound; coordinated dosage has a stabilizing effect.

Relevance for concrete demolition and special demolition

Rebound material influences the quality and uniformity of shotcrete layers. Areas with elevated rebound shares often show lower density or reduced pull-off strength. During deconstruction, this affects dust formation, fracture patterns, and separation cuts. In tunnels and bench areas, where shotcrete is regularly used for support, layers with deviating structure can often be removed in a controlled manner using a concrete pulverizer. For thicker or inhomogeneous layers, the combined use of a rock wedge splitter and concrete splitter can define the separation joint before continuing with pulverizers or combination shears.

Influence on the choice of concrete pulverizer and rock wedge splitter and concrete splitter

Thin, locally debonded shotcrete zones can be removed with low vibration levels using pulverizers; the reduced adhesion favors clean breaks.
For thicker, multi-layer support shells, splitters create controlled cracks that facilitate the subsequent grip of the pulverizer.
In areas with strong aggregate enrichment in the rebound (e.g., accumulations of coarse particles), irregular fracture must be expected; tool selection should provide sufficient jaw travel and cutting force.

Cutting and separation work in the vicinity of rebound material

Rebound material forms loose piles that soil work areas, cover rails and travel paths, and hinder cutting and separation work. Proactive clearing before deploying combination shears, multi-cutters, or steel shear makes it easier to safely expose reinforcement and built-in components. Where shotcrete has been applied as temporary support on rock or existing structures, the combination of a pulverizer followed by steel cutting supports selective deconstruction.

Material management: collecting, separating, and utilizing rebound material

Rebound is generally not standard-compliant concrete. It often contains excessive proportions of coarse aggregate and too little binder. Reuse for the original purpose is therefore mostly ruled out. Depending on the site concept and legal framework, possible options include use as backfill material or separate collection for proper disposal. Cleanliness and separation by material type reduce disposal costs and facilitate recycling. Organized construction logistics keeps routes clear for the use of hydraulic demolition tools and reduces accident risks.

Dust and noise reduction in deconstruction

Loose rebound piles increase the tendency to generate dust. A combination of regular clearing, localized wetting, and the choice of low vibration levels methods—such as removal with a concrete pulverizer or creating separation cut with a rock wedge splitter and concrete splitter—supports low-emission work, particularly in tunnels, existing buildings, and during building gutting and cutting.

Quality assurance: assessing layers with a high rebound share

Areas with elevated rebound losses can often be identified visually (rough surface, aggregate blooming) and acoustically (hollow sound when tapped). For a reliable assessment, non-destructive testing, pull-off tests, or core extraction can be considered. The findings influence deconstruction planning: removal sequence, position of splitting boreholes, gripping directions of the pulverizer, and the separation of reinforcement (e.g., with steel shear) can be specified more precisely.

Practical guide: reducing rebound losses and simplifying deconstruction

Prepare the substrate: clean, roughen, moisten—improves embedding and reduces rebound.
Optimize the mix design: well-graded particle-size distribution, suitable aggregate shape, and consistent workability reduce rebound and improve layer quality.
Train nozzle handling: constant distance, favorable application angle, and position-appropriate handling (especially overhead) are crucial.
Use accelerators in a coordinated way: only as much as needed; avoid overdosing.
Control layer thicknesses: work in several thin layers to limit rebound and sloughing.
Clear rebound early: promptly remove loose piles to ensure safe standing areas for pulverizer and splitting work.
Plan deconstruction: first check areas with a presumed high rebound share; remove with low vibration levels there, define gripping routes, and expose reinforcement cleanly.
Coordinate tool combinations: concrete pulverizer for controlled removal, rock wedge splitter and concrete splitter for defined crack guidance; if necessary, follow with cutting of inserts using hydraulic demolition shears.
Prioritize occupational safety: dust suppression, adequate lighting, safe traffic routes; compliance with applicable regulations.

Occupational safety and legal notes

Rebound material can create slippery surfaces, slide on slopes, and block access routes. Appropriate safeguards, regular clearing cycles, and coordinated material logistics are integral components of a safe construction site. Information on disposal and potential material reuse must always be verified for the specific project and region. Legal requirements and technical rules must be observed in each case; the guidance provided here is general in nature.

Relation to application areas of Darda GmbH

In rock excavation and tunnel construction, shotcrete is used for support; rebound losses govern material flows and the subsequent deconstruction strategy. In concrete demolition and special demolition, insights into rebound zones enable targeted selection of methods—such as selective removal with a concrete pulverizer—to expose reinforcement and protect adjacent components. In building gutting and cutting, early clearing of rebound material eases access for the hydraulic power pack and facilitates clean separation of built-in components. In special operations scenarios, for example under confined conditions or in sensitive areas, understanding the formation and distribution of rebound supports the planning of low-vibration, controlled work steps. In natural stone extraction, shotcrete is less common, but when stabilizing slopes or benches the same principles apply: minimize rebound, secure layer quality, and make later removal predictable.