The term “Overlay” is used in construction in two ways: as a surface overlay (coating, protective layer, or mortar application) on concrete, steel, or natural stone, and as a structural overlay in the sense of an upstand concrete beam for load transfer. Both meanings are relevant for planning, maintenance, and deconstruction. In practice, an overlay influences the choice of methods and tools—especially for selective deconstruction with concrete pulverizers or for controlled splitting with hydraulic rock and concrete splitters from Darda GmbH in application areas such as concrete demolition and deconstruction, building gutting and cutting, as well as tunnel construction and special operations.
Definition: What is meant by Überzug
An overlay is either an applied layer serving a function (e.g., corrosion protection, waterproofing, chemical resistance, visual enhancement, or leveling) or a structural, upstanding beam made of concrete that takes loads and transfers them into walls, columns, or bracing systems. While surface overlays are executed with layer thicknesses from a few tenths of a millimeter up to several millimeters (for mortar and spray-mortar overlays, even centimeters), the structural overlay is part of the load-bearing system. It differs from a downstand beam (located below the slab) and from a lintel (local beam over openings) by its position and integration into the overall structure.
Fields of application and relevance over a structure’s life cycle
Overlays shape planning, execution, repair, and deconstruction. In new construction, material selection and processing determine durability (e.g., protection against concrete carbonation and chloride contamination). During service life, overlays influence inspection, cleaning, and maintenance. In deconstruction, they govern methods, emissions control, and the safe separation of construction waste fractions. Especially for selective tasks—such as in building gutting, special demolition, or tunnels—controllable tools like concrete pulverizers and hydraulic wedge splitters from Darda GmbH help remove overlays in sections or make structural overlays load-free and crush them.
Structure and types of overlays
The spectrum ranges from thin coatings to load-bearing elements. Decisive factors are function, layer thickness, adhesion to the substrate, chemical and mechanical resistance, and structural behavior.
Surface-protection overlays (coatings and mortar overlays)
Surface overlays serve the protection and function of components. Typical examples are polymer-bound coatings (e.g., EP, PU, acrylate systems), mineral overlays (e.g., PCC or SPCC mortars), silicate or silane impregnations, as well as sprayed mortar/shotcrete as a covering overlay. They address concrete carbonation, chloride contamination, chemical attack, abrasion, or crack bridging. For durability, substrate preparation, pull-off strength, pore structure, and suitable environmental conditions are key.
Structural overlay (upstand concrete beam)
The structural overlay is a reinforced concrete beam standing on the slab or deck. It improves continuous and support regions, compensates height offsets, or reroutes loads. Relevant for design are cross-section, reinforcement, shear checks, and connections. In deconstruction or conversion, exposing the reinforcement and controlled load redistribution play a central role.
Overlays in concrete demolition and special demolition
In deconstruction, overlays often appear as the first layer. They can conceal reinforcement, embedded parts, or defects and influence the demolition strategy. Clean separation of layers facilitates construction waste sorting and single-grade recycling. Tools from Darda GmbH enable controlled interventions: concrete pulverizers for precise nibbling and crushing of concrete overlays, hydraulic wedge splitters for crack-guided detachment of massive layers, Multi Cutters and steel shears for metallic overlays or casings, and tank cutters for coated tank walls in special operations.
Selective removal of surface overlays
Depending on the layer type, different methods are suitable: mechanical removal (milling, blasting, chiseling, removal by shears), hydraulic splitting, water jet cutting, or thermal processes for special coverings. Concrete pulverizers permit segmental removal down to the sound substrate—particularly at edges, upstands, and in areas with concealed reinforcement. Hydraulic wedge splitters create controlled crack patterns and help release thick mortar or shotcrete overlays over large areas with low vibration levels. Metallic overlays, sheets, or encapsulated inserts are separated with Multi Cutters or steel shears; for tanks with coatings, tank cutters are used, with attention to low-emission methods.
Deconstruction of structural overlays
Before removal, loads are redistributed and the overlay is decoupled section by section. Subsequently, hydraulic wedge splitters produce crack lines along the planned separation cuts; concrete pulverizers take over the crushing. Hydraulic power packs supply the tools with the required working pressure and flow, even under confined conditions such as in tunnel construction. In special operations—e.g., above sensitive installations—low-vibration methods are advantageous.
Planning, substrate assessment, and quality assurance
Solid planning starts with recording layer build-up, thickness, adhesion, and any potential hazardous substance content. Visual checks are supplemented by low-destructive methods, such as pull-off tests, rebound hammer, assessment of drill dust, rebar location, and chloride/carbonation tests. Results govern the choice of tools, the cut lines, and the sequence of steps. For quality assurance, release tests of the substrate (e.g., pull-off, roughness) and seamless documentation are advisable. For new coatings, the technical rules and manufacturer-specific system requirements apply; binding decisions are project-specific and are made based on applicable standards and approvals.
Occupational safety, emissions control, and environment
Overlays may contain substances that release dust, vapors, or fibers during processing. Depending on the age and use of the structure, these include, for example, PAH, PCB, or asbestos-containing legacy coatings. Protective measures include low-dust working, dust extraction, wetting, protective enclosure, appropriate personal protective equipment, emissions measurements, and compliant disposal logistics. Hazard analysis, operating instructions, and training must be prepared before starting; legal requirements must be observed project-specifically.
Practical guide: step sequence for de-coating and deconstruction
The following sequence has proven effective in many situations and is adapted to the project.
- Survey: layer build-up, material data, structural function, accessibility, sensitive adjacent areas.
- Material assessment: sampling, laboratory tests, classification of wastes and cleaning procedures.
- Separation concept: sequence, cut and split lines, load redistribution, intermediate conditions.
- Setup: barriers, enclosure, media, hydraulic power packs, tool logistics.
- Preparatory cuts: expose connections, release embedded parts with Multi Cutters or steel shears.
- De-coating: mechanical removal, segmental nibbling with concrete pulverizers, crack-controlled release with hydraulic wedge splitters.
- Size reduction and handling: gripping, separating, single-grade placement for transport and recycling.
- Finishing: substrate testing (e.g., pull-off), leveling, preparation for new build-up or continued deconstruction.
Typical damage patterns and repair with overlays
Surface overlays often exhibit dewetting, blistering, cracks, debonding, chalking, or chemical degradation. Causes include moisture, inadequate substrate preparation, unsuitable systems, or climatic conditions. Remedy comes from a combination of de-coating, substrate repair (e.g., reprofiling with PCC), and a new function-appropriate overlay. For preparation, controlled material removal is crucial; precise tools—such as concrete pulverizers—enable removal down to the load-bearing zone without unnecessarily destroying base material. For structural overlays, crack widths, deflections, and connections are key; depending on the findings, local structural reinforcement, load rerouting, or partial deconstruction may be carried out.
Particularities in tunnel construction, natural stone extraction, and special operations
In tunnel construction, shotcrete often acts as an overlay for stabilization and as a carrier for waterproofing. Its removal requires controlled methods with low vibration levels, minimal block detachment, and good dust control. Hydraulic wedge splitters and concrete pulverizers allow segmental work under tight space conditions. In natural stone extraction, a thin mortar or protective overlay can temporarily secure edges; crack-guided splitting helps during removal. In special operations—such as on coated tanks or installations—steel shears, Multi Cutters, and tank cutters are used to separate metallic and coated layers, accompanied by low-emission working methods.
Terms and distinctions in context
In everyday use, overlay, coating, topping, leveling layer, or skim coat are often equated but differ in function and layer thickness. An overlay can act hydrophobically, as a seal, as waterproofing, or as a load-bearing element. From a structural perspective, overlay, downstand beam, lintel, and beam are distinguished by geometry and integration. These distinctions are relevant for defining deconstruction methods, tests, and the selection of tools—such as concrete pulverizers or hydraulic wedge splitters from Darda GmbH—in a targeted manner.
Site checklist
- Documents: as-built records, test reports, structural boundary conditions, approvals.
- Layer analysis: type, thickness, adhesion, rebar position, potential hazardous substances.
- Method selection: de-coating mechanically/hydraulically, splitting, crushing, cutting.
- Equipment deployment: hydraulic power packs, concrete pulverizers, hydraulic wedge splitters, Multi Cutters, steel shears, tank cutters as required.
- Emissions protection: dust, noise, low vibration levels, media management, extraction.
- Safety at work: PPE, access routes, emergency plan, training, monitoring.
- Disposal/recycling: separation of fractions, logistics, documentation of compliance.