The concrete structure determines how concrete forms, ages, develops cracks, and behaves during controlled deconstruction. Anyone who intends to selectively separate, split, or crush concrete must understand the microstructure consisting of cement paste, aggregate, pores, and any reinforcement. This is precisely where the proper selection and application of tools such as concrete demolition shears and stone and concrete splitters comes in: the microstructure controls crack propagation, fracture energy, and thus the appropriate approach in the fields of concrete demolition and special deconstruction, building gutting and cutting, rock breakout and tunnel construction, natural stone extraction, and special applications. Darda GmbH combines compact tools and hydraulic solutions that work in a structure-appropriate manner and help minimize vibration, noise, and dust.
Definition: What is meant by concrete structure
Concrete structure refers to the internal makeup of concrete from the micro to the member scale. It comprises the cement matrix (cement paste), the distributed aggregates, the pore system (gel and capillary pores, air voids), the interfacial transition zone between aggregate and matrix (ITZ), and embedded elements such as reinforcing steel or prestressing. This structure arises from cement hydration, the arrangement and quality of the aggregates, the water–cement ratio, and curing. Mechanical properties such as compressive and tensile strength, modulus of elasticity, toughness, and fracture energy are derived from the concrete structure—key quantities for design, repair, and deconstruction.
Structure and microstructure of concrete
Concrete is a multiphase composite. Its microstructure governs macroscopic behavior during removal, cutting, crushing, and splitting.
Cement paste and hydration phases
The cement matrix consists predominantly of C-S-H phases, ettringite, and other hydrates. The density, distribution, and orientation of these phases determine stiffness and crack initiation under tensile and shear loading.
Interfacial transition zone (ITZ)
The boundary layer between aggregate and matrix is often more porous and crack-prone. Cracks tend to propagate along this ITZ. Tools that generate locally high compressive stresses—such as wedges of stone and concrete splitters—exploit these weak zones to steer crack fronts in a controlled manner.
Pore system and moisture state
Capillary and gel pores control stiffness as well as creep and shrinkage tendencies. Moisture content influences fracture energy: dry concrete behaves more brittle; moist concrete can respond locally with greater toughness. This matters for low-vibration methods because it makes crack propagation more predictable.
Reinforcement and bond
Steel in concrete increases ductility and alters crack paths. In the deconstruction of reinforced concrete members, concrete demolition shears attack the bond locally and separate concrete from reinforcement, while steel shears and combination shears precisely cut the exposed bars. Hydraulic power units provide the required energy, mobile or stationary.
Influencing factors on the concrete structure
The scatter of in-situ properties is large. The following factors shape the microstructure and thus the demolition strategy:
- Water–cement ratio (w/c): Low w/c yields a dense matrix, higher strength, lower porosity; high w/c promotes a porous ITZ and facilitates splitting.
- Aggregate type and grading: Hard rock increases wear and fracture energy; soft aggregates favor shear failure in the cement paste. Coarse grading influences crack paths.
- Admixtures/additions: Fly ash, silica fume, and air-entrainers change the microstructure and frost–de-icing salt resistance.
- Maturity and curing: Early drying leads to microcracks; prolonged curing densifies the microstructure.
- Aging and environmental actions: Carbonation, chloride ingress, and alkali–silica reaction (ASR) produce structural weakening or internal expansive pressures.
- Stress state: Prestressing affects crack formation; increased requirements for diagnostics and procedure apply to prestressed concrete.
Mechanical properties and fracture behavior
For choosing tools and sequencing in deconstruction, parameters such as compressive strength, tensile and splitting tensile strength, modulus of elasticity, and fracture energy are decisive. Concrete fails in tension in a brittle manner; cracks initiate at defects and within the ITZ.
Failure modes and crack steering
- Splitting failure: Can be deliberately triggered by local wedge forces. Stone and concrete splitters generate controlled crack wedges in massive members.
- Crushing and flexural failure: Concrete demolition shears exploit high contact pressures; reinforcement often remains partially integrated and can subsequently be cut with steel shears.
- Shear failure: Relevant at slab edges, beam bearing regions, and composite joints.
Concrete structure in deconstruction: investigation and assessment
Before deploying tools, the member’s structure should be recorded. Non-destructive and selectively destructive testing provides the basis for decisions on procedure in concrete demolition and special deconstruction.
Typical steps of the condition survey
- Review drawings, construction age, concrete mix (if available), and past repairs.
- Site visits: visual inspection, mapping of cracks, moisture zones, spalling, and potentially prestressed areas.
- Localization of reinforcement and services; determination of cover and cross-sectional geometry.
- Material sampling or comparative testing to estimate strength and density.
Tool selection depending on the concrete structure
The concrete structure determines the appropriate separation and size-reduction method, the sequence of work steps, and the drive power of the hydraulic power packs.
Concrete demolition shears in reinforced concrete
Concrete demolition shears work with concentrated compressive force. They are suitable for members with significant reinforcement where concrete and steel are to be separated successively. High density and strong aggregates require adapted gripping and cycle strategies. In building gutting and when cutting openings, concrete demolition shears are particularly advantageous where vibrations and noise must be limited.
Stone and concrete splitters for massive cross-sections
Stone and concrete splitters deploy hydraulic wedges. They induce cracks along the ITZ and existing weak zones. In unreinforced or lightly reinforced concrete as well as in massive foundations, machine blocks, and parapets, members can be broken down quietly, with low dust, and in a controlled manner into transportable segments—an advantage in special deconstruction and special applications in sensitive environments.
Combination shears, Multi Cutters, steel shears and tank cutters
Combination shears and Multi Cutters combine gripping, crushing, and cutting for mixed structures of concrete, masonry, and metal. Steel shears cut exposed reinforcement, sections, and plates, while tank cutters are used on large-area steel components. These tools complement concrete demolition shears and splitters in sequential deconstruction.
Areas of application: structure-appropriate approach
Concrete demolition and special deconstruction
For high-strength members with a dense matrix, a combination of pre-splitting to initiate cracks followed by shear-based removal is recommended. This reduces fracture energy and controls crack paths.
Building gutting and cutting
In existing buildings with limited vibration, concrete demolition shears perform precise partial demolition. Splitters help create openings in thick walls or foundations without affecting adjacent areas.
Rock breakout and tunnel construction
The interfaces between concrete and rock—such as at anchor heads or shotcrete shells—require tools that treat both composite material and rock in a structure-appropriate manner. Splitters transfer well from natural stone extraction to massive concrete foundations.
Natural stone extraction
The mechanics of splitting in anisotropic rocks is closely related to crack steering in concrete: crack tips seek weak zones. This analogy supports the choice of wedge positions and splitting sequences.
Special applications
In vibration-sensitive zones—such as at historic structures or above sensitive facilities—splitting and shear-based methods favor low-vibration operations.
Special concretes and their structural implications
- High-strength concrete: Much denser matrix, higher fracture energy; often a combination of pre-splitting and shear-based removal is beneficial.
- Fiber-reinforced concrete: Microfibers bridge cracks; size reduction requires adapted cutting and crushing strategies, possibly with additional steel shears for fiber bundles.
- Lightweight concrete: Porous aggregates, lower density; splitters are efficient, but edge stability during fixation must be considered.
- Shotcrete: Heterogeneous layer thicknesses and bond to the substrate; shear-based, segmental removal increases control.
Typical damage mechanisms and relevance for deconstruction
Carbonation
pH reduction and reinforcement corrosion lead to cracks parallel to the surface. Concrete demolition shears can remove loose zones; corroded reinforcement is then cut.
Chloride-induced corrosion
Localized cross-sectional losses and undercutting alter crack routes. Segmentation with splitters reduces uncontrolled spalling.
Alkali–silica reaction (ASR)
Expansive gel formations produce network cracking. The existing crack network facilitates splitting but requires careful stabilization during removal.
Sustainability, resource conservation, and material separation
Structure-appropriate methods improve recycling quality and efficiency. Splitting produces larger, clean fragments; shears separate concrete from reinforcement, facilitating sorting. The goal is high fraction purity, reduced energy input in crushing, and lower emissions.
Recommendations
- Select segment sizes to optimize transport and reprocessing.
- Expose reinforcement as early as possible to obtain clean steel and concrete fractions.
- Size hydraulic power packs according to load to use energy efficiently.
Occupational safety, emissions, and surrounding requirements
Vibrations, noise, and dust must be minimized. Splitting and shear-based methods are considered low-vibration and enable controlled work in sensitive areas. Dust and debris protection, secure fixation of members, and well-coordinated cutting and splitting sequences are integral to planning. Legal requirements and local regulations must be observed in general.
Practical guide: structure-appropriate approach step by step
- Condition analysis of the concrete structure, reinforcement, and load paths.
- Define demolition targets: openings, segmentation, complete deconstruction.
- Select the method: concrete demolition shears for reinforced concrete and precise removal; stone and concrete splitters for massive or unreinforced areas; complement with steel shears, combination shears, Multi Cutters, or tank cutters depending on the material mix.
- Dimension and provide the hydraulic power packs.
- Set up a trial area, observe crack behavior, optimize the sequence.
- Work in segments with safeguards against tipping and uncontrolled fractures.
- Continuous monitoring of vibration, dust, and noise.
- Material separation, removal, and documentation of results for subsequent steps.
Interfaces: concrete–steel and embedded elements
Inserts such as reinforcement, embedded components, and connected steel constructions steer crack paths. Shear-based removal exposes the steel parts, splitters divide the concrete along the ITZ, and subsequently steel shears and tank cutters perform metal separation. This sequence protects adjacent members and reduces rework.