Test specimens

Test specimens are the basis for reliable statements about material properties in construction, deconstruction, rock and tunnel construction, as well as natural stone extraction. They form the link between laboratory values and the practical mode of operation on the construction site. From the characteristic values of concrete or rock samples, safe and efficient procedures for concrete demolition and special demolition, building gutting and cutting, rock excavation and tunnel construction, natural stone extraction and special demolition can be derived. In this way, they directly influence the selection of hydraulic methods, such as the use of concrete pulverizers or hydraulic wedge splitters, and the parameterization of the associated hydraulic power units.

Definition: What is meant by test specimens

A test specimen is a material piece that is deliberately manufactured or taken from a structure or a geological formation, on which tests are performed under defined conditions. For concrete, these are often cubes, cylinders or prisms made from fresh concrete, or concrete cores (specimens) from existing components. In rock and natural stone testing, cores and cuboid blocks predominate. The goal is to determine characteristic values (e.g., compressive strength, tensile strength, modulus of elasticity) and derive the material response under load, impact, or hydraulic splitting.

Types of test specimens and typical applications

Depending on the material, standards, and question at hand, different specimen shapes and sizes are used. Their proper selection is crucial so that laboratory values can be reliably transferred to real components or rock masses—such as when deciding whether a structure should be reduced with concrete pulverizers or hydraulically split with hydraulic wedge splitters.

• Concrete cubes (typically 150 mm): standardized determination of compressive strength for normal and high-performance concrete (HPC).
• Concrete cylinders (e.g., 150/300 mm): compressive strength, modulus of elasticity, creep, flexural tensile strength; widely used internationally.
• Concrete prisms: flexural tensile strength, fracture energy, shrinkage and creep tests.
• Cores from existing concrete (varying diameters/lengths): condition assessment in existing structures (e.g., ahead of deconstruction, building gutting, or concrete separation/cutting).
• Rock cores (UCS, Brazilian, point-load testing): characterization of strength and anisotropy for rock excavation and tunnel construction.
• Natural stone blocks/prisms: compressive and flexural tensile strength, frost–de-icing salt resistance, water absorption—relevant for natural stone extraction.
• Mortar and screed prisms: pull-off, flexural tensile strength, compressive strength for assessments in interior deconstruction.

Test methods and parameters obtained from test specimens

Specimens yield parameters that describe fracture behavior and crushability. Combined with structure geometry (wall thickness, spans), reinforcement, and rock fabric, they provide a realistic picture for planning deconstruction or extraction works.

Concrete: mechanics, durability, microstructure

• Compressive strength (e.g., to DIN EN 12390): central parameter for resistance to crushing; important for sizing jaw forces in concrete pulverizers.
• Splitting tensile and flexural tensile strength: decisive for crack initiation under hydraulic spreading; relevant for the effectiveness of hydraulic wedge splitters.
• Modulus of elasticity and Poisson’s ratio: deformation behavior; influence split widths and crack propagation patterns.
• Density, water absorption, porosity: indicators of microstructure and aging (e.g., concrete carbonation) that influence the fracture pattern.
• Ultrasonic pulse velocity, rebound hammer: low-damage screening methods for orientation prior to sampling.

Rock and natural stone: structure, anisotropy, abrasivity

• Uniaxial compressive strength (UCS): key for choosing between mechanical crushing and hydraulic splitting in rock.
• Brazilian test (indirect tensile strength): prognosis of crack formation under spreading loads.
• Point-load index: quick comparative value for changing rock layers.
• Bedding, joint spacing, anisotropy: determine preferred splitting directions—important for positioning rock wedge splitters.
• Abrasivity (e.g., Cerchar): influence on tool wear and maintenance intervals.

From lab values to method: derivation for deconstruction, extraction, and tunnel construction

The decision between crushing, splitting, cutting, or shearing follows the material parameters and the structure. Well-documented specimen results reduce risks, optimization loops, and downtime.

• High compressive strength but low tensile strength (typical of concrete): favors hydraulic splitting; with dense reinforcement, concrete pulverizers with high crushing performance are suitable to first break the concrete and expose reinforcement.
• Older concrete with carbonation: higher surface hardness but often brittle; a combination of concrete pulverizers followed by splitting accelerates removal.
• Rock with pronounced joints: splitting devices exploit natural weaknesses; in massive, anisotropic rock, the splitting direction is oriented by the core.
• Natural stone extraction: targeted split lines along the fabric enable large-format, low-defect blocks; core analysis is used to align the split cylinders.
• Building gutting and cutting in existing structures: specimens from components with composite systems (e.g., bonded screeds) facilitate separation; crushing with concrete pulverizers and cutting off metals with steel shears or tank cutters can be coordinated effectively.

Manufacture, sampling, and storage of test specimens

Only correctly obtained samples deliver reliable results. Manufacture and sampling follow standards and good practice.

1. Manufacture from fresh concrete: careful compaction, plane-parallel faces, defined curing and storage (moisture/temperature) until testing.
2. Sampling from existing components: proper core drilling, cooling, and marking; avoid microcracks through gentle cutting. Core axes are documented to interpret fabric or reinforcement effects.
3. Rock specimens: record orientation relative to bedding/joints; edge finishing for specimens intended for bending tests.
4. Storage and transport: protect from drying, frost, and shocks; complete chain of custody from extraction to the laboratory.

Influencing factors and typical sources of error

Scatter can be minimized when disturbances are recognized and controlled. This increases the significance of results and thus planning reliability when deploying hydraulic wedge splitters and concrete pulverizers.

• Specimen geometry and size effect: small specimens tend to show higher strengths; transfer to massive components must be calibrated.
• Moisture content and temperature: significantly influence strength and stiffness.
• Loading direction: anisotropy in rocks and oriented microstructures in concrete (e.g., due to shrinkage cracks) change the fracture pattern.
• Reinforcement influence in the core: local strengthening distorts results; prefer cores without reinforcement or evaluate separately.
• Edge-zone and carbonation effects: surfaces can be atypically hard; consider deeper core segments separately.
• Core end preparation (end grinding): uneven end faces cause transverse tensile stresses and false breaks.

On-site quality assurance: quick tests and decisions

Alongside laboratory tests, field methods support timely fine-planning of work steps, such as selecting borehole spacing for split cylinders or the bite sequence with concrete pulverizers.

• Rebound hammer and ultrasonics: quick classification of zones with differing quality.
• Pull-off and bond tests: assessment of composite systems prior to building gutting.
• Point-load on rock cores: rapid comparison between layers in tunnel heading.
• Trial field/test demolition: validation of assumed forces and spacings in representative areas before large-scale work.

Documentation, safety, and sustainability

Clean documentation of specimens and test data increases transparency and facilitates methodological adjustments as the project progresses. Occupational safety during drilling, sawing, and crushing includes dust and noise reduction measures, safe handling of heavy specimens, and protection against uncontrolled crack propagation. The parameters also enable strategies for selective separation that improve the recycling rate, by using concrete pulverizers for controlled crushing and clean separation of reinforcing steel.

Practice-oriented parameterization for splitting and crushing

Transferring specimen parameters into concrete settings is iterative and project-specific. The following guidelines have proven effective:

• Concrete with high density and low w/c ratio: plan smaller borehole spacing and higher spreading pressures; with dense reinforcement, first crush with concrete pulverizers, then split.
• Rock with pronounced foliation: place split wedges along weakness planes; reduced forces are often sufficient to achieve clean separation faces.
• Heterogeneous existing components: take specimens from all zones (core/edge/areas with additional compaction) and align the sequence of operations (crushing, splitting, cutting) accordingly.
• Hydraulic power packs: select flow rate and working pressure so that the force–time curve matches the fracture mechanics; low-shock load ramps favor controlled crack propagation.

Application examples

Interior demolition of a reinforced concrete structure

Cores indicate a concrete compressive strength class of C30/37 with low splitting tensile strength and moderate reinforcement density. Procedure: selective crushing with concrete pulverizers to expose bars, followed by hydraulic splitting of wall panels. Result: controlled crack propagation, reduced vibration levels.

Advance in massive gneiss

According to rock cores, UCS is high but anisotropy is distinct. Procedure: align rock wedge splitters along foliation; lower spreading pressures are sufficient; crushing of offsets with multi cutters in secondary breakage.

Natural stone extraction in sandstone

Test values show homogeneous tensile strength; defined borehole rows and split initiation deliver large-format blocks with low reject rates. Specimen data are used to fine-tune hole spacing.