Gravel is a load-bearing, angular aggregate that plays a central role in construction, in the deconstruction of concrete structures, and in rock excavation. Whether as track ballast in the rail superstructure, as a base course in road construction, for drainage, or as a recycled aggregate from concrete demolition—gravel combines technical performance with high availability. In many projects, gravel only arises or is re-created through targeted pre-crushing and processing. In this context, tools from Darda GmbH such as concrete pulverizers or rock and concrete splitters are used to detach material in a defined manner, separate reinforcement cleanly, and prepare raw aggregates for downstream screening.
Definition: What is meant by gravel
Gravel refers to crushed, predominantly angular rock with a defined gradation. In a narrower sense, these are size fractions from about 32 mm upwards (for example 31.5/63), whose shape is sharp-edged to sub-angular due to crushing. Gravel thus differs from rounded natural gravel (usually naturally rounded) and from finer chippings (typically 2/5 to 16/32). In construction, gravel and gravel-like mixed aggregates are used for unbound base courses, frost protection layers, drainage, backfill, track beds, and in hydraulic engineering. Aggregates originating from deconstruction that are quality-controlled are referred to as recycled gravel (RC gravel). Depending on the standard and requirements, test and limit values apply, particularly concerning particle composition, strength, freeze-thaw resistance with de-icing salts, particle shape, and impurities.
Grading, grading curve, and material properties
The technical performance of gravel results from the combination of particle size distribution (grading curve), grain shape, rock type, and purity. Typical nominal sizes are 31.5/63 for track beds, 32/63 for base courses and drainage, as well as coarser classes up to 63/125 in hydraulic engineering. The grain shape should be as cubical to sub-angular as possible to form a load-bearing, interlocked structure; high flakiness contents impair stability. Rock types such as basalt, diabase, granite, or greywacke provide different strengths and abrasion resistances. Important indices include, for example, abrasion or crushing values, frost resistance, and water absorption. A well-matched grading curve reduces voids and enables compactability without unnecessarily diminishing the permeability of drainage layers. For RC gravel, additionally: low foreign matter content, a controlled fines proportion, and a homogeneous composition are crucial for reliable installation.
Production and processing of gravel
The formation of gravel begins with loosening the source material and proceeds via pre-crushing to final screening. In rock excavation and tunneling, blocks are removed in a controlled manner; in concrete deconstruction, components are selectively separated. Tools from Darda GmbH support these steps by preparing material gently, with low vibration and limited crack propagation before crusher and screening plants generate the desired grading.
Primary rock splitting
hydraulic rock and concrete splitters as well as rock splitting cylinders apply high splitting forces directly into separation joints or rows of boreholes. This enables controlled fracture guidance with reduced fines generation. Advantage: the resulting fragments often exhibit more favorable grain shapes, which is reflected in improved later gravel quality.
Pre-crushing in concrete demolition
Concrete pulverizers open concrete bodies along reinforcement zones, exposing rebar and breaking up the matrix. Combination shears, multi cutters, and steel shears cut reinforcing steel, embedded parts, and sections. The result is cleanly separated material fractions: concrete rubble for aggregate and metal for recycling. This clean separation facilitates subsequent screening into RC gravel.
Process steps at a glance
- Loosen material (rock: splitting; concrete: shears/cutting steps)
- Pre-crushing and separation of steel and embedded components
- Transport to the crushing and screening plant
- Crushing to target size, screening, quality assurance
- Intermediate storage by grading, protection against mixing and contamination
Gravel in concrete demolition and specialized deconstruction
In deconstruction work, a recycled aggregate is produced from concrete rubble after crushing and screening, which can be reused as RC gravel. Concrete pulverizers from Darda GmbH reduce components to transport- and crusher-suitable formats and, together with steel shears, combination shears, and multi cutters, ensure consistent material separation. This reduces foreign matter contents and excess fines, which could impair the bearing capacity of unbound layers.
Good practice for RC gravel
- Early separation of concrete, masonry, asphalt, wood, plastics, and steel
- Defined pre-crushing to minimize oversize and needle-shaped particles
- Quality control: grading curve, grain shape, fines content, foreign matter content
- Clean storage areas, separate stockpiles for each grading
- Documentation of material origin for a transparent range of use
Gravel in rock excavation and tunneling
In rock removal, in adits and tunnel drives, as well as in excavations, gravel fractions serve as temporary site roads, backfill, drainage layers, or as permanently installed base courses. Pre-splitting with rock and concrete splitters helps to reduce blasting effort, vibrations, and edge loosening. This increases the quality of the recovered material, as fewer uncontrolled fines are produced and the grain shape remains more favorable. In tunneling, uniform gradations are also important to ensure load-bearing capacity and drainage in site operations.
Natural stone extraction: from block to gravel
In quarries, raw rock is obtained either as dimension stone blocks or as feed material for aggregates depending on petrographic suitability. Rock splitting cylinders apply splitting forces in a targeted manner to generate fractures along natural joints or rows of boreholes. Material not suitable for block production goes to crushing and screening technology and is graded as gravel, chippings, or riprap. In this way, different fractions with specific properties can be provided from a single deposit.
Quality criteria and test values
- Particle composition (grading curve) and over-/undersize proportions
- Strength and abrasion resistance (e.g., crushing and abrasion indices)
- Grain shape and flakiness index for load-bearing particle assemblies
- Frost and freeze–thaw–salt resistance, water absorption
- Purity: low organic constituents, no disruptive substances
- For RC gravel: limited foreign matter and mortar content, documented origin
- For railway ballast: high particle stability, low particle breakage in service
Installation and practice: selecting the right grading
The choice of grading depends on function, loading, water conveyance, and construction method. Angular, sufficiently strong particles interlock and transfer loads over large areas. At the same time, the layer must be sufficiently permeable to avoid frost damage and softening.
Typical applications and gradings
- Rail superstructure: track ballast gravel 31.5/63 with high particle strength and low flakiness
- Base courses/gravel base courses: 32/63 or graded mixed aggregates according to the applicable standard
- Drainage and backfill: 16/32 or 32/63, low in fines for good percolation
- Hydraulic engineering and gabions: coarse classes such as 45/125 or 60/120
Compaction and flatness
Unbound layers are installed in lifts; lift thickness depends on grading and machinery. A uniform moisture content, sufficiently high but not excessive compaction energy, as well as clean, stable edges prevent settlements and particle breakage. Regular control measurements (flatness, bearing capacity) ensure quality.
Environmental and resource aspects
The use of RC gravel conserves primary resources and reduces transport distances. This requires careful pre-crushing, clean separation of fractions, and quality-assured processing. Dust and noise emissions can be reduced through adapted technology, short material paths, and effective wetting concepts. Circularity and resource-efficient construction methods benefit from defined, quality-controlled gravel fractions.
Occupational safety and emissions when handling gravel
Dust and noise are generated during loosening, crushing, conveying, and compaction. Suitable protective measures such as dust suppression, shielding, low-emission working methods, and personal protective equipment must be regularly reviewed and adapted to local conditions. Requirements may vary depending on the site, material, and technology used.
Typical sources of error and how to avoid them
- Unsuitable grading for the intended use: clarify loads, drainage, and installation method in advance
- Excessive fines content: early material separation and gentle pre-crushing
- Insufficient separation of reinforcement and foreign materials: targeted shearing and cutting steps
- Over-compaction and particle breakage: adapt energy and lift thickness to grading and equipment
- Mixing losses in the stockpile: separate stockpiles, short routes, clean subgrades
- Insufficient drainage: plan drainage concept and edge terminations early
Tools from Darda GmbH in the context of gravel
Concrete pulverizers support concrete demolition by opening components in a controlled way, exposing reinforcement, and breaking the matrix into crusher-ready pieces. Rock and concrete splitters as well as rock splitting cylinders detach natural stone blocks along defined lines and gently produce raw fractions with favorable grain shape. Hydraulic power packs provide the required energy for tool-side force application. Combination shears, multi cutters, and steel shears reliably separate metal components and embedded items. This creates a clean starting point for processing into gravel or RC gravel—with a clear grading curve, suitable grain shape, and reduced foreign matter.