Thermal crack

Thermal cracks occur when concrete or natural stone members deform unevenly due to heating and cooling, leading to tensile stresses that exceed the material’s tensile strength. They are significant for planning, repair, and for controlled demolition: they influence structural behavior, cut alignment, equipment use, and safety distances. In concrete demolition and special deconstruction, during building gutting and concrete cutting, as well as in rock breakout and tunnel construction, existing thermal cracks can be deliberately utilized or must be reliably mitigated. Tools such as concrete pulverizers or rock and concrete splitters from Darda GmbH allow precise, low-vibration work along defined crack lines or across them, without unintentionally activating the structure. When assessed early and documented consistently, thermal cracking insights support predictable sequencing, reduced rework, and safer interfaces with adjacent structures.

Definition: What is meant by a thermal crack?

A thermal crack is a crack in concrete, masonry, or natural stone caused by temperature differences or rates of temperature change. It forms when thermally induced strains cannot occur freely due to restraint. Typical cases include cracking in massive members (hydration heat in young concrete), in members subjected to strong solar radiation followed by cooling, or in members with unequal boundary conditions (e.g., a strongly cooled surface and a warm core). Unlike pure drying shrinkage, thermal strain and the resulting internal stress are central to a thermal crack. Cracks often run transverse to the principal strain direction, are often straight to moderately branched, and may concentrate at edges, supports, anchor points, or at transitions between members. In practice, mixed mechanisms occur frequently, so that thermal effects act together with shrinkage or moisture movements.

Causes, mechanisms, and typical crack patterns

Thermal cracks can be traced to the interaction of thermal strain, member geometry, and restraint. Relevant mechanisms include:

• Hydration heat in young concrete (early stage): heat development in the core, faster cooling at the surface, resulting in tensile stresses at the surface or in the transition zone.
• Daily and seasonal cycles: heating from solar radiation, night-time cooling, wind cooling, precipitation, freeze-thaw cycles.
• Uneven boundary conditions: some areas shaded, others exposed; varying member thicknesses; rigid supports or fixity.
• Material parameters: coefficient of thermal expansion, modulus of elasticity, tensile strength, thermal diffusivity, moisture content, and pore structure.
• Extreme events: fire exposure with subsequent quenching or cold shock; temperature gradients from hot process exhausts, district heating lines, or adjacent thermal masses.
• Composite interfaces: restraints from connected slabs, claddings, or inserts that impede free thermal movement.

Overview of crack patterns

• Near-surface, relatively shallow cracks under rapid temperature changes (thermal shock, wind cooling, cold exposure).
• Deeper crack zones in massive members due to internal temperature gradients.
• Cracks along geometric transitions (cantilevers, supports, notches), often oriented transverse to the temperature gradient.
• In rock: temperature- and moisture-induced joints, intensified in weathering-susceptible rocks and at free edges.
• Map-like grid cracking on large exposed surfaces where expansion is restrained in multiple directions.

Identification and differentiation in existing structures

Before interventions in deconstruction or during building gutting, distinguishing thermal cracks from shrinkage or settlement cracks is important. A structured approach combines observation, measurement, and comparison with boundary conditions.

Indicators of thermal cracks

• Correlation with exposure: cracks on highly weathered or sun-exposed faces, at edges or boundary zones.
• Temporal evolution: formation after weather changes or in the early hardening stage of massive concrete.
• Morphology: rather straight, grid-like patterns; orientation transverse to the expected strain direction.
• Measurements: temperature and strain readings; crack width development depending on time of day/season.
• Reversibility: measurable cyclic opening and closing with temperature, without permanent growth under constant mechanical load.

Differentiation

• Versus shrinkage cracks: less dependent on moisture loss, more governed by temperature gradients.
• Versus load-induced cracks: no direct correlation to load combinations; activity often reversible (crack opening varies with temperature).
• Versus chemical or durability-related cracking: absence of reaction products and typical discolorations; correlation with thermal boundary conditions rather than exposure to aggressive agents.

Measurement and monitoring methods

• Thermography and point sensors: identify temperature gradients driving restraint.
• Mechanical crack gauges or displacement transducers: track daily and seasonal width variations.
• Strain gauges or fiber optics: capture strain fields near suspected restraint locations.
• Documentation protocol: photo log with scale, orientation, timestamp, and environmental data for later comparison.

Relevance for concrete demolition and special demolition

Thermal cracks influence the choice of methods, the cutting sequence, and the placement of supports. For controlled demolition, the following applies:

• Crack lines can be used as natural weaknesses to guide break lines.
• Unwanted crack propagation should be avoided by suitable sequencing, pre-separations, and local unloading.
• Low-vibration methods reduce the risk of uncontrolled activation of existing cracks.
• Active monitoring at critical interfaces allows timely adjustment of forces and tool positioning.

Practical guidance on equipment use

• Concrete pulverizers: approach cracks, dose the opening motion, break in small segments, respect load paths.
• Rock and concrete splitters or stone splitting cylinders: adapt drill-hole spacing to the crack path; start with low splitting pressure and increase after checking crack propagation.
• Hydraulic power pack: keep pressure and flow rate stable; monitor the unit’s temperature to ensure consistent response.
• Additionally: combine Multi cutters, hydraulic shears, and concrete pulverizers to create access and selectively relieve edges.
• Peripheral protection: use local shielding and spacers to prevent spalling at visible edges and finishes.

Building gutting and cutting: cut alignment along thermal cracks

During strip-out works in existing buildings, existing thermal cracks can support cut planning or require additional safeguards.

Approach

1. Survey and crack mapping: document crack width, depth, path, and activity; mark critical zones.
2. Pre-cutting/relieving: make relief cuts transverse to the crack path to prevent spalling.
3. Segmented dismantling: divide members into manageable units; plan gripping and cutting edges.
4. Tie-back anchoring/shoring: temporarily secure before opening load-bearing crack regions.
5. Interfaces: decouple sensitive finishes and building services along expected break lines to avoid collateral damage.

Rock breakout and tunnel construction: handling thermally activated joints

In rock masses, seasonal temperature changes can open or close joints. This affects drilling patterns, splitting sequences, and stabilizations.

Procedure

• Adapt the drilling pattern to joint systems; apply wedge splitters or stone splitters along existing weakness planes.
• Consider cyclic temperature fluctuations: during phases of larger crack openings, lower splitting forces can be used, while requirements for rockfall protection increase.
• Prefer low-wear, precise splitting techniques to avoid vibration-induced joint activation.
• Stabilization: where joints daylight or intersect excavations, install temporary support before applying splitting forces.

Natural stone extraction: using temperature-induced weakness zones

In natural stone extraction, joints created by temperature cycles facilitate detaching larger blocks. Rock and concrete splitters and stone splitting cylinders can respect natural bedding and jointing to produce clean separation faces, while hydraulic power pack systems ensure finely metered energy input. Coordinated sequencing improves block yield, edge integrity, and the quality of exposed faces.

Measures to limit thermal cracks in new construction and their significance for later deconstruction

Preventive measures in new construction improve durability and make later deconstruction more predictable.

Prevention

• Temperature control during concreting: appropriate concrete mix, low hydration heat, and where necessary cooling or insulation measures.
• Expansion joints: defined separation joints and control joints to avoid uncontrolled cracking.
• Member thicknesses and staging: construct massive cross-sections in stages; carry out concrete curing consistently.
• Curing and protection: maintain moisture, apply curing compounds, and use thermal blankets to limit gradients and early-age cracking.

Benefit in deconstruction

• Planned joints and defined weakness zones reduce uncontrolled fractures and facilitate the use of concrete pulverizers.
• Documented temperature control during construction supports forecasting of crack locations and the selection of splitting and cutting methods.
• As-built records of joints, embeds, and restraint points enable targeted sequencing and fewer unplanned interventions.

Special application: metallic components after temperature exposure

The term thermal crack is used primarily in the context of concrete and natural stone. In metallic components, thermal effects can cause embrittlement or other types of cracking. In special cases with steel shear or cutting torch, the material condition after severe heating or cooling must therefore be checked before cutting. In case of doubt, lower cutting forces, smaller cutting segments, and additional fixings are advisable. Attention should be paid to potential heat-affected zones, loss of toughness, and microcrack initiation near welds or prior flame cuts.

Planning, documentation, and quality assurance

Systematic recording of the crack situation increases safety and efficiency.

Recommended steps

1. Crack mapping with classification of width and depth; record exposure conditions.
2. Temperature monitoring (surface/core) before and during the intervention, where relevant.
3. Trial steps with a concrete pulverizer or hydraulic splitter while observing crack activity; fine-tune parameters.
4. Continuous control of the removal sequence and the shoring; adapt measures if crack opening changes.
5. Acceptance criteria: define stop-work thresholds for unexpected crack growth, excessive vibration, or temperature excursions.

Safety and general legal notes

Work on crack-prone members requires careful hazard analysis, appropriate safety measures, and qualified personnel. The information in this article is of a general nature and does not replace a project-specific assessment or binding requirements. For interventions in load-bearing members, the relevant standards, guidelines, and regulatory requirements must be observed. Equipment use must be planned to protect people, adjacent members, and infrastructure.

• Establish exclusion zones, lifting plans, and emergency procedures for each intervention stage.
• Verify tool compatibility with the substrate and crack activity; maintain equipment to ensure predictable output.
• Document safety checks and sign-offs before shifting to higher forces or new working faces.