Sprayed formwork

Sprayed formwork refers to a shell made with shotcrete or spray mortar that serves as lost or temporary formwork. It is applied directly to rock, existing concrete, or reinforcement and forms a stable, geometry-following surface for subsequent construction processes. In practice, sprayed formwork is encountered primarily in tunnel construction, in rock and slope stabilization, in excavation pits, as well as in repair works and cross-section enlargements. Equally important is the controlled processing and selective deconstruction of sprayed formwork: typically, a concrete demolition shear or a hydraulic wedge splitter (e.g., hydraulic rock and concrete splitters) is used, supported by hydraulic power pack units (hydraulic power units), to remove or adjust layers with low vibration levels, precisely, and with minimal material damage. In specialist practice, sprayed formwork is also referred to as sprayed concrete formwork or shotcrete form shell; typical layer thicknesses range from approximately 30 to 120 mm depending on geometry and loading, with tolerances defined by the subsequent lining or backfill.

Definition: What is meant by sprayed formwork?

Sprayed formwork is understood as a thin-walled shell produced by shotcrete or spray mortar that primarily assumes the function of formwork. It creates a dimensionally stable surface that either remains permanently on the structure (lost sprayed formwork) or is partially or completely removed after fulfilling its task (temporary sprayed formwork). In contrast to load-bearing shotcrete constructions, sprayed formwork focuses on shaping, profile retention, and support for subsequent cast-in-place concrete or backfill. Depending on requirements, sprayed formwork can be executed with or without reinforcement and supported by anchors, lattice girder beams, or mesh reinforcement (fabric). For lost sprayed formwork, durability, compatibility with waterproofing, and defined cover to reinforcement are additionally relevant, while for temporary shells the emphasis is on fast application and controlled removal.

Build-up, methods, and material properties

Sprayed formwork is produced using the wet- or dry-spray method. The choice depends on geometry, construction stage, conveying distances, and environmental conditions. The goal is a compact, well-adhering layer with defined thickness and sufficient early strength to take loads from self-weight, backfill pressure, and vibrations. Wet-spray enables low dust and good compaction at higher outputs, while dry-spray offers flexibility for long conveying distances and small set-ups. Accelerators are selected to achieve early strengths in the lower MPa range within hours, without compromising long-term performance.

Layer build-up

A multi-layer application is typical: a bond coat for adhesion, a main layer for shell action, and, if applicable, a leveling and protective layer. In reinforced execution, mats, lattice girders, or single layers are fixed with spacers; tie-back anchoring ensures load transfer into the substrate or existing structure. Edge zones and transitions are built up in shorter passes to avoid sagging and to keep rebound low.

• Typical bond coat thickness: approx. 10 to 20 mm, applied onto a saturated surface-dry substrate.
• Main layer thickness: usually 40 to 100 mm per pass, adapted to curvature and support spacing.
• Finishing and protection: approx. 5 to 15 mm for smoothing, protection, or to meet waterproofing tolerances.

Materials

Standardized shotcretes/spray mortars are used for uniform quality, often with accelerators for shorter stand times. Fibers are common in slope stabilization. The consistency must suit the nozzle technique to minimize rebound and increase bond tensile strength. Alkali-free accelerators and optimized grain curves reduce dust and rebound, while steel or synthetic fibers can improve edge stability and crack control. Exposure class requirements, maximum aggregate size, and water binder ratio are defined in the project specification.

Equipment and tools

In addition to spraying equipment, profile gauges, layer thickness measurements, and pull-off tests are used. For preparation and finishing processes, hydraulic tools are employed: a concrete demolition shear for edges and openings, a hydraulic wedge splitter for low-vibration removal zones, steel shear for reinforcement, and hydraulic power pack units as the energy source. Laser-based profile checks and simple control templates help meet tight tolerances for the subsequent lining or waterproofing.

Fields of application for sprayed formwork in construction

Tunnel construction and underground works

In tunnel construction, sprayed formwork often serves as a profile and form shell for subsequent inner linings. It follows the tunnel heading and creates a smooth, receptive surface for waterproofing and concreting. For profile corrections or replacements, sprayed formwork is locally trimmed back; here, a concrete demolition shear and a hydraulic wedge splitter enable precise work under limited headroom and sensitive boundary conditions. Interfaces to invert construction, block joints, and drainage outlets are coordinated to avoid water pockets and differential settlements.

Rock breakout, excavation pits, and slope stabilization

On rock and loose ground, sprayed formwork stabilizes the terrain, binds loose sections, and acts as a shell for backfills or injections. Anchoring and mesh reinforcement distribute loads. To expose anchor heads, create penetrations, or remove damaged areas, hydraulic tools are used that operate with low vibration levels. Where necessary, staged application and curing reduce the risk of sloughing at steep slopes or highly fractured rock faces.

Existing structures, building gutting, and cross-section enlargement

In refurbishment, sprayed formwork serves as lost formwork for overlays and cross-section additions. It enables clean geometries on soffits, slab edges, and vaults without elaborate conventional formwork. Selective removal for revealing reinforcement or for service penetrations is achieved in a controlled manner with a concrete demolition shear; for thick layers, stresses are reduced with a hydraulic wedge splitter before load-bearing areas are finished. Particular attention is paid to bond to the existing structure and to corrosion protection of exposed reinforcement ends.

Execution: substrate, bond, and reinforcement

The performance of sprayed formwork stands and falls with substrate preparation. Loose components must be removed, surfaces roughened, and cleaned. Moisture and temperature must be within the permissible range. Substrates are preferably brought to a saturated surface-dry condition to limit suction and improve bond, and any separating substances are removed before spraying.

Substrate preparation

• Removal of shells, layers, and damaged areas using low-vibration methods; in confined conditions, a concrete demolition shear has proven effective.
• Breaking up thick, inhomogeneous zones with a hydraulic wedge splitter to avoid crack formation in adjacent components.
• Exposing starter reinforcement, cutting mats with steel shear; energy supply via hydraulic power pack units.
• Cleaning by vacuuming or oil-free compressed air and pre-wetting to saturated surface-dry to promote adhesion.

Bond and layer thickness

Bond coats are applied with appropriate nozzle technique. Spray angle, nozzle distance, and feed rate influence rebound and pores. Layer thickness is governed by geometry, backfill load, and construction stage. Locally increased thicknesses require additional tie-back anchoring or reinforcement. As a guide, a near-perpendicular spray angle with a constant stand-off of roughly 0.5 to 1.0 m and short application cycles reduces rebound and ensures dense layers; drumming areas are corrected immediately.

Thickness, load action, and temporary loads

Even though sprayed formwork primarily performs a formwork function, it must safely take self-weight, construction-stage, and backfill actions. For temporary sprayed formwork, the focus is on early load-bearing capacity; for lost sprayed formwork, durability and interaction with subsequent concreting are additionally relevant. Early strengths in the range of approximately 1 to 3 MPa are typically targeted before exposing the shell to vibration or backfilling, while deflection limits and permissible tolerances are aligned with the follow-on trade.

Design aspects

• Shell action: curvature and support conditions improve load-bearing capacity.
• Tie-back anchoring: anchors transfer loads into rock or existing concrete.
• Reinforced shells: mats and lattice girders limit crack widths and stabilize edges.
• Joints and transitions: working joints, drip edges, and terminations are detailed to control crack initiation and water flow.
• Serviceability: surface tolerances and permissible deformations are set to meet requirements of waterproofing and inner lining.

Quality assurance and control

• Pull-off tests to evaluate bond quality.
• Layer thickness measurements, especially at critical nodes and edges.
• Visual inspection for voids, honeycomb structure in concrete, cracks, and edge stability.
• Documentation of mix design, spraying parameters, temperatures, and curing.
• Monitoring of rebound quantities and recording of accelerator dosage to ensure reproducible quality.
• Early strength checks where required for construction-stage releases.

Deconstruction, openings, and adjustments on sprayed formwork

During the construction process, subsequent openings, profile corrections, or partial deconstruction of sprayed formwork are often required. The goal is a selective, low-vibration intervention that protects the surroundings and does not unnecessarily damage the bond zone. Sequenced cutting, dust suppression, and protection of adjacent waterproofing reduce secondary effects during trimming or removal.

• Concrete demolition shear: precise nibbling of edges, window openings, pipe penetrations; well suited near sensitive installations.
• Hydraulic wedge splitter: controlled widening of existing cracks/joints to relieve stresses; minimizes secondary damage.
• Steel shear and combination shears: cutting mats, stirrups, and anchors; useful in confined spaces.
• Multi Cutters: for composite materials and mixed layers at detail points.
• Hydraulic power pack: energy-efficient supply for the tools on site.

Occupational safety, environment, and health protection

Sprayed formwork requires careful dust, noise, and rebound management. Protective clothing, low-dust methods, and dust extraction increase safety. For deconstruction, separation joints must be planned to avoid uncontrolled break-offs. Removed shotcrete can be sent to recycling, provided the boundary conditions are appropriate. Suitable respiratory protection, hearing protection, and eye protection are selected according to measured exposure, and in underground works ventilation and water spray systems reduce airborne particulates.

Advantages and limitations of sprayed formwork

Advantages

• Form freedom and adaptability to complex geometries.
• Rapid execution, short downtimes thanks to high early strengths.
• Low formwork effort, especially on soffits and in tight spaces.
• Can be combined with anchoring and reinforcement for local strengthening.
• Selective, low-vibration adjustments and deconstruction with hydraulic tools are feasible even near sensitive structures.

Limitations

• Surface quality depends on method and workmanship; post-treatment may be necessary.
• Rebound and dust generation require experience and protective measures.
• Layer thicknesses are limited; additional measures are necessary for high backfill loads.
• Interfaces with waterproofing and embedded components require tight tolerances and coordinated detailing.

Planning and interface management

Careful planning considers construction stages, backfilling sequences, anchor layouts, layer thicknesses, and subsequent concreting. Interfaces with waterproofing, drainage, and embedded components must be clarified early. For future penetrations, integrating placeholders is advisable; if that is not possible, openings are subsequently cut out cleanly with a concrete demolition shear and reinforcement adjusted with steel shear. Mock-ups and coordinated control sections help to fix acceptable surface classes and tolerances, while digital profile checks facilitate interface control in constrained geometries.

Practical tips for the construction site

1. Prepare a load-bearing, clean substrate; mark critical areas.
2. Fix reinforcement and anchor layouts before spraying, protect edges.
3. Match nozzle technique to geometry; keep spray angle and distance constant.
4. Work in layers, remove rebound, avoid voids.
5. Continuously check layer thicknesses, use profile gauges.
6. Ensure curing, avoid vibrations in the early phase.
7. Define separation joints early for corrections and openings; keep hydraulic tools ready with a suitable hydraulic power pack.
8. Plan drainage outlets, terminations, and transitions before spraying to prevent water traps.
9. Use trial areas to calibrate accelerator dosage, rebound control, and achievable surface quality.

Relevance for the application areas of Darda GmbH

Sprayed formwork appears in many fields where Darda GmbH tools are used: in concrete demolition and special demolition for selective corrections and dismantling, in building gutting and cutting for subsequent openings, in rock breakout and tunnel construction as temporary shell during tunnel heading and for profiling, in natural stone extraction for securing and targeted removal of sprayed layers, and in special demolition wherever confined spaces and sensitive environments require low vibration levels methods. A concrete demolition shear delivers clean cut edges on shells and overlays; a hydraulic wedge splitter reduces stresses and facilitates the controlled detachment of thick layers. Hydraulic power pack units supply the required tools, steel shear and Multi Cutters take care of cutting the reinforcement. In combination, these low-vibration methods enable precise adjustments with minimal collateral damage to the substrate and adjacent installations.