Carbonation

Carbonation is a central topic in the planning, structural repair, and deconstruction of reinforced concrete. It changes the chemical composition of the concrete, influences the corrosion risk of the reinforcement, and thus the service life of a structure. For practical work — for example in concrete demolition and deconstruction, in building gutting and concrete cutting, or in refurbishment works — a deep understanding of carbonation helps to select demolition methods and tools safely, efficiently, and in a material-appropriate way. This is especially relevant for force-based methods with concrete pulverizers as well as rock wedge splitters and concrete splitters from Darda GmbH, because carbonation can significantly influence crack formation, fracture surfaces, and the handling of reinforcement.

Definition: What is meant by carbonation

Carbonation is the reaction of carbon dioxide (CO₂) from the air with the alkaline components of the cement paste, especially calcium hydroxide. Calcium carbonate is formed and the pH value of the concrete structure decreases. This pH drop is decisive: the reinforcement is passivated in a strongly alkaline environment; once the carbonation front reaches the reinforcement layer, passivation can be lost, which promotes corrosion. In addition to durability, carbonation also affects mechanical properties, such as density, stiffness, and local brittleness of the edge zone.

Chemical basics and influencing factors

In simplified terms, carbonation proceeds via the diffusion of CO₂ into the pores of the concrete. In the presence of moisture, CO₂ reacts with calcium hydroxide and other hydrated phases. Initially, the transformation leads to a densification of the pore structure in the edge zone; in the long term, however, the pH drop predominates. The rate essentially depends on moisture content (optimal at medium relative humidity), porosity and water–cement ratio, concrete cover, crack degree, temperatures, CO₂ concentration, and exposure (interior/exterior). Dense, well-cured concretes carbonate more slowly; cracked or dried-out surfaces carbonate faster.

Effects on load-bearing behavior and durability

Carbonation usually changes the concrete at the surface toward higher hardness and brittleness, which can seemingly increase local compressive strengths. Critical, however, is the depassivation of the reinforcement and the possible corrosion that follows: rust formation leads to volume increase, crack formation, spalling, and loss of cross-section. In practice, this appears as hollow-sounding areas, cracks along reinforcement, and spalling. For planners and executors in deconstruction, this means that load reserves and failure modes cannot be derived solely from the original structural analysis but must take the carbonation state into account, including potential residual load-bearing capacity reductions.

Detection, testing, and documentation

To assess carbonation depth, the color change with phenolphthalein on fresh fracture surfaces or core drilling surfaces is often used. In addition, visual findings (spalling, cracks), measurements of concrete cover, moisture determinations, and, if necessary, laboratory analyses are common. Systematic documentation supports the choice of methods in concrete demolition and special demolition or in building gutting and concrete cutting.

Procedure for an indicative test

1. Expose a fresh concrete surface by core drilling or a fracture sample.
2. Apply the test solution and read the carbonation depth as the colorless zone.
3. Compare with the existing concrete cover to assess reinforcement risk.
4. Supplementary findings: crack mapping, moisture, visible corrosion traces.
5. Documentation as a basis for the repair or deconstruction concept.

Consequences for deconstruction methods and tool selection

The carbonated edge zone is often denser and more brittle, while backing areas — especially with corroding reinforcement — may be weakened by cracks. This leads to practical consequences for selecting and guiding tools. Concrete pulverizers bite the component edge, generate controlled cracks, and release concrete from reinforcement in a targeted manner. Rock wedge splitters and concrete splitters act from the core and split massive cross-sections from the inside out. Both methods benefit from a correct assessment of carbonation depth to anticipate fracture lines and safely expose reinforcement layers.

Use of concrete pulverizers

• Advantageous on carbonated, brittle edge zones: the pulverizer can bite off defined edge pieces and progress along existing microcracks.
• With corroded reinforcement, irregular spalling is to be expected; an adapted bite sequence reduces uncontrolled fractures.
• In building gutting and concrete cutting, the pulverizer enables work with low vibration levels, which is important in existing buildings with sensitive neighboring elements.
• After biting: exposed bars can be cut with a steel shear or multi cutters once the bars are accessible.

Use of rock wedge splitters and concrete splitters

• Suitable for thick-walled structural elements where the dense carbonated edge zone makes biting difficult: splitting from the inside induces controlled tensile stresses.
• Pre-drilled holes should be positioned to respect carbonation depths and reinforcement layers in order to steer the split line.
• In concrete demolition and special demolition as well as in tunnel environments (removal of old, carbonated shotcrete linings), low emissions and controlled fracture patterns are an advantage.
• In combination with Darda GmbH hydraulic power units, finely metered forces can be applied to avoid uncontrolled pulverization of brittle zones.

Practical guide: procedure according to the structural condition

The combination of carbonation state, crack pattern, and reinforcement layout determines the approach. The goal is to release material in a targeted way, minimize dust and vibrations, and increase safety.

Edges, slabs, and floor slab edges

• Carbonated edges are hard and brittle: apply concrete pulverizers with sequential bites along the edge, exploit cracks, and expose reinforcement in a controlled manner.
• With strong carbonation and high edge tensile strength: pre-score or pre-drill, then continue with a pulverizer or split cylinders.

Massive structural elements and foundations

• Plan drilling patterns that bypass or deliberately cut reinforcement; use splitters for internally controlled fracture formation.
• Corroded layers cause unplanned spalling: consider accessibility and fall protection at an early stage.

Reinforcement after the concrete removal

• Cut exposed bars with a steel shear or combination shears; for dense reinforcement, the use of multi cutters helps.
• For tanks and metallic inserts in special operations, a cutting torch is an option, provided the environment is prepared accordingly.

Prevention and repair in existing structures

To slow down carbonation, use dense concretes with low water–cement ratios, adequate concrete cover, careful curing, and surface protection systems. In existing structures, carbonated edge zones are often removed and reprofiled; in addition, coatings, hydrophobization, or — where suitable — electrochemical methods are considered. Decisions should be based on an object-specific condition analysis and take use, exposure, and maintenance strategy into account.

Typical measures

• Removal of damaged zones back to non-carbonated, load-bearing concrete; work in a targeted way with concrete pulverizers to control cracks.
• Expose, assess, and, if necessary, supplement corroded reinforcement; select concrete repair materials and surface protection according to the applicable rules.
• Documentation of carbonation depth to define maintenance intervals.

Application areas at a glance

Carbonation plays a role in several application areas and influences the choice of methods and equipment from Darda GmbH:

• Concrete demolition and special demolition: Edge-zone brittleness favors pulverizer-based removal; corroded reinforcement requires flexible cutting sequences.
• Building gutting and concrete cutting: In interiors with carbonated surfaces, dust- and vibration-reduced approaches are required; concrete pulverizers and splitters help to release components selectively.
• Rock excavation and tunnel construction: With tunnel linings made of shotcrete, carbonation can harden the surface; splitting technology or targeted pulverizer bites produce controlled fracture patterns.
• Natural stone extraction: In natural stone extraction, carbonation of concrete is not decisive, but splitters are relevant for brittle-elastic rocks; differences from concrete must be considered when metering forces.
• Special operations: In structures with mixed materials (concrete, steel, tanks), carbonation influences the removal strategy; combining a pulverizer, splitter, steel shear, and cutting torch enables coordinated workflows.

Occupational safety, emissions, and sustainability

Carbonated edge zones can produce fine mineral dust; appropriate extraction, wetting, and personal protective equipment are important. Low-vibration approaches with concrete pulverizers and splitters help protect adjacent structural elements and reduce noise. In the interest of resource conservation, it is worthwhile to separate components so that reinforcing steel and aggregates can be captured for construction waste sorting and recycling.

Planning and documentation

Forward-looking planning combines findings on carbonation with reinforcement drawings, material properties, and access conditions. Pilot areas or preliminary tests are advisable to define fracture behavior and tool sequence, for example via a field trial/test or a test cut. The results should be documented in a traceable manner to safeguard workflows, safety, and quality.