Oxidation of reinforcing steel—often also referred to as reinforcement corrosion—is one of the primary causes of damage to reinforced concrete components. It occurs when steel reinforcement in concrete rusts under the influence of oxygen and moisture. The result is cracking, spalling, and a loss of load-bearing capacity. For planning, repair, concrete demolition and special demolition, a precise understanding of the mechanisms and effects is essential. In practice, the corrosion condition influences the selection and operation of hydraulic tools such as concrete demolition shears or hydraulic splitters from Darda GmbH—for example, for selective exposure of reinforcement, controlled breaking of components, or low-vibration deconstruction in sensitive environments.
Definition: What is meant by oxidation of reinforcing steel
Oxidation of reinforcing steel refers to the electrochemical reaction of iron in the steel with oxygen and water to form iron oxides (rust). In fresh, dense concrete, the steel surface is passivated by the high alkalinity. If this passivation layer is lost—due to concrete carbonation or chloride contamination—corrosion begins. Rust has a larger volume than the original steel and thus generates internal stresses in the concrete cover. Typical damage patterns are longitudinal cracks over the reinforcement, spalling of the cover layer, cross-sectional losses in bars, and associated reductions in load-bearing capacity and serviceability. In practice, this is also described as reinforcement corrosion, chloride-induced corrosion, or carbonation-induced corrosion.
Causes and electrochemical mechanisms
The main triggering mechanisms are concrete carbonation and chloride contamination. In carbonation, carbon dioxide from the air reacts with alkali hydroxides in the cement paste; the pH value drops, the passive layer dissolves, and corrosion can begin if oxygen and moisture are present. Chlorides—from de-icing salts, seawater, or industrial influences—penetrate the concrete cover and locally destroy the passive layer; pitting often occurs, which insidiously weakens cross-sectional load capacity. Cracks, insufficient cover, high porosity, repeated wetting, and temperature fluctuations also accelerate the process. Electrochemically, local anode and cathode areas form; the steel dissolves at the anodes, and oxygen reduction occurs at the cathodes. The rust that forms expands and causes expansive stresses in the cover layer, promoting cracking and spalling.
Symptoms and diagnosis in existing structures
Early indications include fine, longitudinal cracks over reinforcement layers, rust staining, and hollow-sounding cover zones. In advanced stages, spalling with exposed, rusted reinforcement and locally reduced cross-sections appear. A systematic condition assessment combines visual findings with minimally destructive and destructive testing to plan repair, deconstruction, or partial dismantling in a targeted manner.
Testing and measurement methods
- Half-cell potential mapping and concrete resistivity measurement to estimate corrosion probability.
- Determination of concrete carbonation depth and chloride contents at reinforcement depth.
- Determination of cover depth and reinforcement layout using ground-penetrating radar/ferroscan.
- Core samples, thin sections, and metallographic examinations to verify mechanisms.
- Crack mapping, hammer sounding, and endoscopy to locate voids and delaminations.
Implications for load-bearing capacity, durability, and usability
Cracking and spalling reduce the protective function of the concrete and accelerate corrosion. Cross-sectional losses in the reinforcement weaken bending and tensile capacity, while bond between steel and concrete decreases. This can lead to increased deformations, lower ductility, and a more brittle failure mode. Statements on load-bearing capacity and remaining service life of structure are project-specific and require expert evaluation; the following notes are general and non-binding.
Relevance for deconstruction, concrete demolition and special demolition
The corrosion condition influences how components are selectively opened, separated, and recovered during concrete demolition and special demolition. Corrosion cracks guide fracture lines and can promote controlled breaking. At the same time, unpredictable break edges can occur, making adapted tool handling and safeguarding necessary. Hydraulic concrete demolition shears are suitable for precise removal of carbonated or chloride-contaminated cover layers to expose reinforcement. Hydraulic rock and concrete splitters—including splitting cylinders—use predrilled holes to split components with low vibration, which has proven effective in sensitive environments. Exposed bars are cut with steel shears or Multi Cutters. Hydraulic power packs provide the required pressure and flow rate for the tools. In complex special demolition scenarios—such as combined dismantling of steel and concrete components—additional combination shears or, if steel tanks/piping are involved, cutting torches may be required.
Typical work steps in selective deconstruction
- Define deconstruction sections based on the corrosion map, including safety measures and shoring.
- Remove the cover layer with concrete demolition shears to open reinforcement and relieve corroded zones.
- Targeted splitting of thick sections with hydraulic splitters to guide cracks without blasting.
- Cut, recover, and bundle reinforcement with steel shears or Multi Cutters.
- Source-separate fractions for construction waste sorting and recycling.
Equipment selection, hydraulics, and handling technique
Tool selection depends on component thickness, degree of reinforcement, accessibility, and environmental constraints (noise, vibration, sparks). Concrete demolition shears enable controlled nibbling of the cover; their jaw opening and crushing force should match the component geometry. Hydraulic splitters develop high splitting forces in the borehole, which is advantageous for massive foundations or near sensitive infrastructure. Hydraulic power packs must be matched to the required operating pressure and flow rate of the connected tools; adequately sized lines and couplings secure performance and reduce heat build-up. Directional load management—for example, preferentially opening existing corrosion cracks—improves the predictability of fracture edges.
Specific scenarios
Chloride-contaminated components (bridges, parking structures, coastal areas)
Chlorides often cause localized, deep attack fields. For deconstruction, a sequential approach is recommended: first remove the cover with concrete demolition shears, then split or break the affected areas section by section. Low-spark, hydraulic methods are advantageous in chloride-rich and dust-sensitive environments, for instance during building gutting and concrete cutting in ongoing operations.
Carbonated façades, balconies, slab edges
Edge spalling and disturbed bond require precise edge treatment. Concrete demolition shears allow controlled “nibbling” in small bites. For thick bearings or brackets, splitting cylinders can guide the crack line. This keeps the load on adjacent components low—a plus for special demolition in occupied buildings.
Tunnel construction and rock bond
In tunnels and underground structures, vibrations and sparks are critical. Corroded anchor heads and reinforced linings are preferably opened hydraulically. Hydraulic splitters and concrete demolition shears enable controlled release of the bond, benefiting rock breakout and tunnel construction. Exposed reinforcement is cut with steel shears or Multi Cutters.
Occupational safety, health, and environment
- Risk of spalling and falling due to the volume increase of rust: provide protective enclosure, safety nets, and secured work platforms.
- Residual stresses in reinforcement: pre-tension/relieve cut lines and plan cutting sequence.
- Dust and rust: minimize dust by wetting, dust extraction, and appropriate protective measures (dust suppression and dust extraction).
- Prefer low-spark methods, especially in explosion-prone or fire-sensitive areas.
- Noise and vibration management: hydraulic methods are generally low vibration and well controllable, supporting noise control and low vibration levels.
- Environment: collect chloride- and fine-dust–laden material separately and dispose of or process it in accordance with local requirements.
Planning, documentation, and quality assurance
A structured process includes the survey of the existing structure, definition of deconstruction stages, selection of suitable hydraulic tools, trial areas to optimize crack guidance, and continuous documentation of removal, separation cuts, and material flows. Measurements of carbonation and chlorides support the decision for concrete repair as well as for the partial deconstruction or deconstruction route. Seamless separation into concrete, reinforcing steel, and any contaminated fractions forms the basis for recycling and verification.
Prevention and repair in existing structures
Where deconstruction is not planned, preventive and repair measures are considered: increasing cover in additions, dense surface protection systems, hydrophobic treatments, electrochemical re-alkalization, chloride extraction, or cathodic protection. For localized damage, the cover layer is selectively removed, the reinforcement is derusted and/or supplemented, and the concrete is properly replaced. Here too, concrete demolition shears can help with gentle opening. Decisions are project-specific and follow the applicable rules of the art.
Application areas and relation to Darda GmbH tools
Oxidized reinforcement influences working methods across several application areas: In concrete demolition and special demolition, existing cracks accelerate controlled splitting with hydraulic splitters. In building gutting and concrete cutting, concrete demolition shears allow selective exposure of bars for subsequent cutting with steel shears or Multi Cutters. In rock breakout and tunnel construction, low vibrations are essential; splitting cylinders and precise shearing protect the remaining structure. In natural stone extraction, experience with guiding cracks by splitting provides valuable analogies for working along corrosion-induced weak zones in concrete. Special demolition covers situations with elevated requirements for low-spark operation, noise control, or limited access—here hydraulic tools and coordinated hydraulic power packs excel in controllability, without relying on promotional statements.