Sintered concrete refers to concrete whose surface zone or matrix has been densified, vitrified, or sintered by high temperatures or chemo-thermal processes. In practice, sintered concrete occurs mainly where concrete has been exposed over time to strong heat, hot exhaust gases, or intense radiation sources—such as in tunnels after fires, in industrial shafts, furnace areas, or flue gas ducts. For deconstruction this matters because its hardness, brittleness, and abrasiveness differ markedly from ordinary concrete. As a result, separation techniques, tool selection, and work sequence change, for example when using concrete demolition shear or concrete splitter from Darda GmbH in the application areas of concrete demolition and special demolition, building gutting and concrete cutting, as well as rock demolition and tunnel construction.
Definition: What is meant by sintered concrete
Sintered concrete is concrete whose cement paste and fine aggregate have been partially sintered by thermal action—that is, bonded cohesively below the melting temperature. The result is a dense, partly glassy sinter skin or a deeper densified zone. Typical characteristics include reduced porosity, increased surface hardness, color changes up to dark, glass-like layers, and pronounced brittleness. Sintered concrete can occur either locally (millimeters to centimeters) or at greater depth, depending on the temperature–time profile, moisture, aggregate type, and exposure to chemically aggressive gases. In industrial settings, refractory concretes with a thermally densified matrix are also colloquially described as sintered concrete.
Formation and microstructure
Its formation is tied to thermal limit states. From about 200–300 °C the cement paste dehydrates; at higher temperatures (≥ 500–600 °C) strength-reducing processes occur; in temperature windows above approx. 700–900 °C, fines can vitrify locally and form a sinter skin. Hot gases, soot, and salt loads from exhausts promote reaction layers that further densify the surface. Depending on the aggregate (e.g., limestone, silicate rocks) and fines content, glassy, sometimes color-changed zones form with high surface hardness and low water uptake.
Typical triggers
- Tunnel fires or vehicle fires in enclosed structures
- Thermally loaded furnace and shaft areas, combustion chambers, flue ducts
- Hot-gas plumes along industrial runs and local exit points
- Prolonged radiant heat at equipment foundations
- Fire or explosion events in existing structures (special operation)
Properties and on-site diagnostic features
Sintered concrete often shows a hard, dense, and sometimes glassy-looking surface zone. This layer can appear glossy, be darkly discolored, and exhibit sharp-edged spalling. At the same time, the substrate is often thermally pre-damaged: microcracking, strength loss, and delaminations may lie beneath the hardened surface. This leads to a complex combination of high surface abrasiveness and brittle behavior in depth.
Test and diagnostic methods
- Surface inspection: glassy skin, discoloration, spalling, fire marks
- Rebound hammer: elevated values on the sinter skin, lower in deeper zones
- Ultrasonic pulse velocity: inhomogeneous travel times due to microcracking
- Core drilling/polished section: evidence of sinter skin, vitrification, delaminations
- Water drop test: reduced absorbency of the surface zone
Relevance for concrete demolition and special demolition
For deconstruction planning, sintered concrete means surfaces are extremely wear-intensive for cutting and drilling equipment, while the overall matrix can respond in a brittle manner. This combination favors controlled mechanical separation methods with minimal additional thermal loading. In many cases, concrete splitter as well as concrete demolition shear from Darda GmbH are particularly suitable because they induce targeted stress states and operate without sparking or additional heat generation—an advantage in contaminated or fire-affected environments.
Mechanical behavior under load
The sinter skin acts as a hard, thin shell with high local strength and brittleness. Beneath it, the load-bearing capacity can be reduced. Mechanical splitting methods exploit this contrast: induced tensile stresses open existing weaknesses below the sinter skin so that the hard surface breaks and the cross-section can subsequently be dismantled in a controlled manner.
Suitable separation and deconstruction techniques
In practice, a stepwise approach has proven effective: first create stress cracks, then fragment in a controlled way and separate by material type. This reduces wear, sparks, heat, and undesired emissions.
Splitting with concrete splitter and rock wedge splitter
Hydraulic splitting with hydraulic rock and concrete splitters relies on defined drilling patterns and controlled tensile stresses. This is advantageous with sintered concrete, as the hard surface zone is broken up and deeper pre-damage is utilized.
- Plan the drilling pattern: adapt edge distances, drilling depth, and spacing to the member cross-section; with a sinter skin, use slightly smaller spacing.
- Produce the boreholes: diamond wet-drilled to minimize heat input and dust; a pilot borehole makes insertion of splitting wedges easier.
- Set the split cylinders: relieve load, align the split direction with existing cracks and weaknesses.
- Splitting process: increase pressure stepwise, score the sinter skin, propagate the crack, and free the cross-section.
- Secondary breakage: downsize released pieces and prepare them for transport or further processing.
Secondary breakage with concrete demolition shear
Concrete demolition shear from Darda GmbH act with strong shear and generate highly localized compressive and tensile stresses. With sintered concrete, a progressive biting along the split zones is recommended: first break the sinter skin in a targeted manner, then shear the core. This controls edge breakouts and reduces splintering.
Alternative tools: combination shears, multi cutters, steel shear, tank cutter
In composite areas with reinforcement, built-ins, or plant components, combination shears and multi cutters can be used to separate mixed matrices. Steel shear helps expose and separate reinforcing steel. Tank cutters are useful in special operations when sinter-affected concrete must be dismantled at tank, pipeline, or equipment foundations. The goal is always to separate concrete and metal cleanly and minimize fire load or spark generation.
Hydraulic power packs as the energy source
Robust hydraulic power units are key to maintaining constant pressure and reproducible splitting and cutting forces. Important are stable flow rates, finely metered pressure stages, and good heat dissipation so that performance remains reliable even under high abrasiveness and in tunnels or shafts.
Drilling and cutting techniques for sintered concrete
The sinter skin increases tool wear and cutting forces. Diamond-wet methods reduce heat and dust generation. For drilling and sawing cuts, a coordinated strategy is recommended to avoid glazing of the bond and gumming of the diamond segments.
- Moderate feed; adapt speed and infeed to the glaze fraction
- Ensure sufficient cooling and flushing; filter the flushing water
- Select segments for high abrasiveness and hard surface layers
- Pre-milling/scoring the sinter skin reduces edge chipping
- Lay out drilling patterns more densely to better introduce splitting forces
Occupational safety, emissions, and environmental protection
Sintered concrete can create sharp-edged, glassy splinters and release high levels of fine dust. In fire-affected areas, soot, PAHs, and other residues are possible. Protective measures should therefore be chosen proactively. Legal and regulatory requirements must always be observed; the following points are general in nature.
- Personal protective equipment with cut and puncture protection, eye and face protection
- Effective dust binding, negative pressure containment, and point extraction (dust extraction)
- Prefer wet methods; collect and treat contaminated flushing water
- Use low-spark techniques; avoid ignition sources (dust suppression)
- Collect materials separately; disposal according to classification by a certified disposal company
Special application cases in the deployment areas
In concrete demolition and special demolition, sinter-damaged zones occur after fires or near furnaces; splitting followed by shear processing minimizes emissions. For building gutting and concrete cutting within industrial plants, near-surface sinter skins are often found over short lengths, requiring precise, low-vibration separation. In rock breakout and tunnel construction, sintering primarily affects tunnel shells, fire protection layers, and shotcrete; segmental splitting followed by crushing is proven. In natural stone extraction it is rare but can occur on fire-exposed structures of processing plants. Special demolition includes situations with unknown contamination or residual fire loads, where low-spark, hydraulic methods are advantageous.
Planning, structural analysis, and boundary conditions
Before starting, the degree of damage should be determined: visual inspection, simple surface tests, and—if needed—supplementary investigations. Load-bearing components should be assessed by specialists in case of doubt. The work sequence (drilling, splitting, secondary breakage, separation) must be planned so that load redistributions remain controlled and demolition pieces are safely guided. Temporary shoring, barriers, and a regulated material flow are key boundary conditions.
Terminology and related phenomena
To be distinguished from sintered concrete are pure sinter skins without deeper matrix changes, carbonated surface zones without glaze content, and thermally weakened concrete without significant surface vitrification. Likewise, refractory concrete designed for high-temperature operation differs from normal concrete subsequently damaged by sintering. For deconstruction, the practical consequence matters: a hard, thin surface layer plus a brittle substrate require a zone-adapted combination of splitting and crushing.
Practical workflow: procedure for a sinter-damaged tunnel shell
Example scenario from deconstruction practice: A tunnel shell exhibits a glassy surface zone and cracks. The work sequence could be as follows:
- Cordon off the area, set up ventilation and dust management, and arrange sampling of the soot layer (general, not case-specific).
- Moisten the surface, map the sinter skin visually, define the drilling pattern.
- Set diamond-wet pilot boreholes, then complete the drilling pattern.
- Insert concrete splitter from Darda GmbH into the boreholes; continue crack formation stepwise until the member separates.
- Perform secondary breakage with concrete demolition shear from Darda GmbH; expose reinforcing steel and separate it with steel shear.
- Remove materials in sorted streams; collect and properly treat flushing water.
Checklist: preparation and execution
- Record the damage pattern (sinter skin, depth, crack distribution)
- Define the separation concept (drilling, splitting, shear sequence)
- Select tools to match the hard surface zone and abrasiveness
- Size the hydraulic power pack and plan pressure stages
- Prepare dust, water, and waste management
- Implement protective measures and barriers
- Document the work sequence; continuously check outcomes