The spraying method is a central construction and repair technique in which mortar or concrete is applied to a substrate under pressure and compacted in place. In concrete demolition, special demolition, as well as in rock excavation and tunnel construction, spraying is closely linked to preparatory and accompanying steps: damaged or loose concrete layers are removed, reinforcement is exposed, rock surfaces are secured, and are then upgraded with spray mortar or shotcrete. Tools such as concrete pulverizers or hydraulic rock and concrete splitters enable low-vibration surface preparation before the spray material is applied.
Definition: What is meant by the spraying method
The spraying method refers to the pneumatic or hydraulic conveying and layer-by-layer application of spray mortar or shotcrete to structural or rock surfaces. The material is transported through hoses to the nozzle either as a dry mix with subsequent water addition (dry spraying) or as a pumpable wet mix (wet spraying) and is accelerated there with compressed air. Upon impact, high compaction energy is generated; the layers interlock geometrically and mechanically with the substrate, even in overhead or wall positions. Typical objectives include securing the rock surface, provisional or permanent tunnel face support, concrete replacement in repairs, and fire protection of structural elements.
Relevance in concrete demolition, special demolition and tunnel construction
The spraying method is essential wherever surfaces must be quickly stabilized or upgraded after deconstruction and separation works. In concrete demolition and special demolition, damaged concrete is first removed in a controlled manner—e.g., with concrete pulverizers, combination shears, or Multi Cutters. This is followed by substrate preparation (reprofiling, cleaning, roughening) before spray mortar is applied as a concrete replacement and corrosion protection for exposed reinforcement. In rock excavation and tunnel construction, shotcrete layers serve as an immediate support step for the stability of the tunnel face and the excavation surface; fiber reinforcement and accelerators support early load-bearing behavior. Also, during building gutting and cutting of structures, there is often a need to secure openings, edges, and bearing areas with spray mortar to safely enable subsequent steps. In natural stone extraction, applying local protective shotcrete layers to unstable areas can support extraction, while in special operations (for example, in sensitive areas or where access is restricted) low-vibration preparatory work with rock and concrete splitters creates the basis for a high-quality spray pattern.
Overview of method types
Dry spraying method
In dry spraying, a dry mortar or concrete mix is continuously conveyed by a rotor machine with compressed air; water is added only at the nozzle. Advantages include a flexibly controllable water content, simple start/stop maneuvers, and suitability for small to medium material quantities. Typical aspects: increased dust generation, higher rebound (bounce-back of aggregate), and more demanding nozzle technique.
Wet spraying method
In the wet spraying method, a plant- or site-mixed, pumpable suspension is conveyed and accelerated at the nozzle with compressed air. Advantages include lower dust, reduced rebound quantities, defined mix designs, and high placement rates, which makes it particularly suitable for large-area applications in tunnel construction. Process stability depends on pumpability, temperature, accelerator dosage, and hose routing.
Manual, robotic, and climbing spraying
Depending on accessibility, manual spraying, remotely controlled robotic spraying, or climbing spray systems are used. Robotics increases repeatability and occupational safety in confined or hazardous areas, while manual spraying offers advantages in detailing for smaller repairs.
Materials and mix designs
Spray mortar and shotcrete consist of cements, hydraulic binders, well-graded aggregates, admixtures (e.g., set accelerators, plasticizers), and, where applicable, additives. Depending on the task, steel or polymer fibers are used to improve crack distribution and energy absorption. Important parameters include grading curve, fines content, water-binder ratio, air content, and early strength development. For fire protection, special lightweight aggregates and formulations with increased temperature resistance are used.
Substrate preparation and deconstruction prior to spraying
The quality of the spray result stands and falls with substrate preparation. Loose concrete parts, delaminations, and damaged material must be completely removed; the aim is to achieve load-bearing, rough, and clean surfaces. Concrete pulverizers enable controlled removal of edge zones, corners, and wall sections without undesirable vibrations. In massive structural elements or rock, rock and concrete splitters create defined separation joints or relief cuts to deliberately release spalling. After deconstruction, reinforcement is exposed, prepared to a corrosion-compliant standard, and, if necessary, supplemented or re-anchored. Only then does spraying follow in suitable positions and layer thicknesses.
Equipment, components, and process control
Depending on the method, the conveying unit (pump or rotor machine), material hopper, hoses, nozzle, compressed air supply, and water/additive lines must be coordinated. Process control includes consistent feed rates, appropriate air flow rates, and stable nozzle geometries. A uniform stand-off distance, a favorable impact angle, and guiding the nozzle in overlapping passes prevent voids and spray shadow behind the reinforcement. Hydraulically driven tools on site—such as those used in preceding deconstruction—are supplied by suitable hydraulic power units; work areas must be organized logistically so that material and personnel flows do not interfere with each other.
Execution: Layer build-up and detail points
- Fill recesses and anchor pockets first, then build up the surface in multiple layers.
- Adjust layer thickness per pass to the mix design, position (wall/ceiling), and reinforcement density.
- Consistently remove rebound material; do not rework it into fresh layers.
- Arrange construction and field joints in a planned manner; keep edges moist and rework in time.
- In overhead areas, dose accelerators and fibers in a coordinated way to avoid dripping.
Quality assurance and typical key values
A coordinated quality assurance program includes testing of consistency, fresh concrete temperature, early and final compressive strength, pull-off bond strength, layer thickness control, and surface homogeneity. Sampling is carried out depending on the method using cut-outs, core drills, or test specimens. Key success factors include consistent mix control, clean hose runs, trained nozzle technique, and documented ambient conditions (temperature, humidity, wind).
Occupational safety and environmental aspects
Spraying generates dust, rebound, and noise. Suitable dust reduction measures (e.g., coordinated nozzle water addition in dry spraying), personal protective equipment, shielding, and organized construction logistics are essential. Rebound material must be collected separately and properly disposed of. Water from equipment cleaning and residual quantities must be handled in accordance with applicable environmental and disposal rules. Requirements from relevant standards and regulatory approvals must be followed; project-specific protection concepts must be prepared before work begins.
Interfaces with tools and equipment from Darda GmbH
The sequence of deconstruction, substrate preparation, and spray application requires precise interfaces. When exposing damaged zones and creating defined edges, concrete pulverizers make an important contribution, especially in structurally relevant areas. Rock and concrete splitters as well as hydraulic wedge splitters are used when low-vibration separations in massive components or rock are required—such as prior to shotcrete support in tunnel heading. Combination shears, versatile Multi Cutters, and steel shears cut reinforcement or built-in parts so that spray application can be carried out in accordance with reinforcement requirements. Hydraulic power packs provide the power supply for these tools. In special deconstruction tasks where tanks or pipelines must be opened, suitable cutting tools such as tank cutters enable safe access before surfaces are cleaned and, if necessary, upgraded with spray mortar to prevent spalling or edge break-offs.
Practical application examples
- Concrete demolition and special demolition: Selective removal of chloride-damaged edge zones with concrete pulverizers, followed by spray mortar as concrete replacement and reinforcement protection.
- Rock excavation and tunnel construction: Advance driving with immediate shotcrete support; localized relief using rock and concrete splitters at critical sections prior to spray application.
- Building gutting and cutting: Creating new openings and separation cuts, subsequently securing cut edges and bearing areas with spray mortar.
- Natural stone extraction: Temporary securing of disturbed zones by thin shotcrete layers to stabilize the extraction face.
- Special operations: In areas with restricted access, a combination of low-vibration deconstruction and targeted wet spraying for rapid stabilization.
Limits, risks, and alternatives
The spraying method reaches its limits in very densely reinforced areas, extremely smooth or contaminated substrates, and under adverse climatic conditions. Alternatives include conventionally shuttered cast-in-place concrete, grouting and injection methods, or prefabricated elements. Risks such as insufficient adhesion, voids, shrinkage cracking, or excessive rebound are minimized through careful substrate preparation, appropriate mix designs, trained nozzle operation, and documented execution.