Understanding Gravel Sizes
Gravel and aggregate sizes are classified by the diameter of individual particles, measured in millimetres. The size of aggregate used in a construction project has a direct impact on the strength, workability and durability of the finished product. In Tanzania, aggregates are typically categorised into crusher dust, 6mm, 10mm, 14mm, 20mm, 40mm and larger sizes. Each size serves specific purposes in construction, from the finest crusher dust used in paving and concrete to large riprap stone used for erosion control. Understanding gravel sizes and their appropriate applications helps contractors, engineers and homeowners make informed decisions about material selection for their projects.
Crusher Dust (0-4mm)
Crusher dust, also known as quarry dust or stone dust, is the finest fraction produced during the crushing of rock. Particles range from dust-sized up to 4mm. Crusher dust is commonly used as a bedding material for paving blocks and interlocking stones, providing a stable, compactable base that locks pavers in place. It is also used in concrete production as a partial or complete replacement for river sand, though it requires careful mix design due to its higher fines content. In Tanzania, crusher dust is widely available from quarries and is more economical than river sand in many areas. Tanzibaba supplies high-quality crusher dust suitable for paving, concrete and general construction applications.
6mm To 10mm Aggregates
6mm and 10mm aggregates are small crushed stone sizes commonly used in concrete production, particularly for precast elements, concrete blocks and thin concrete sections where larger aggregates cannot fit. They are also used in asphalt production for surface dressing and in drainage layers where their relatively uniform size allows water to pass through. In decorative applications, 10mm aggregate is popular for garden paths, decorative concrete and exposed aggregate finishes. These smaller sizes are also used in filter media for water treatment and as pea gravel for playgrounds and landscaping. In Tanzania, 10mm aggregate is a standard size stocked by most aggregate suppliers including Tanzibaba.
14mm To 20mm Aggregates
14mm and 20mm aggregates are the most commonly used sizes for general concrete construction in Tanzania. They are the standard coarse aggregates for reinforced concrete in foundations, slabs, columns and beams. The 20mm size is particularly popular because it provides good concrete strength while being small enough to work well with typical reinforcement spacing. 14mm aggregate is often used when reinforcement is closely spaced or for higher-strength concrete mixes where better workability is required. These sizes are also used in asphalt concrete for road surfaces and in granular base courses for pavement construction. Tanzibaba supplies both 14mm and 20mm crushed stone aggregates from selected quarries, tested to ensure consistent quality.
20mm To 40mm Aggregates
20mm to 40mm aggregates are larger crushed stones used in heavy construction applications. They are commonly specified for mass concrete works such as large foundations, retaining walls, culverts and bridge abutments where reinforcement spacing permits larger aggregates. These sizes are also used in road construction as base course material and in drainage applications such as French drains, septic tank soakaways and filter media. The larger particles provide excellent drainage characteristics while maintaining good load-bearing capacity. In Tanzania, 40mm aggregate is frequently used in infrastructure projects and industrial foundations. The cost per tonne of larger aggregates is typically lower than smaller sizes because less crushing is required.
40mm+ Aggregates
Aggregates larger than 40mm are classified as riprap, boulders or gabion stone. These are used for specialised applications requiring large, heavy stones. Riprap is used for erosion protection on riverbanks, shorelines and embankments, where its size and weight resist water flow. Gabion stone is used in wire mesh baskets to build retaining walls, bridge abutments and erosion control structures. Larger aggregates are also used in railway ballast to support and stabilise railway tracks. In Tanzania, these large aggregates are sourced from quarries with suitable rock formations and are essential for infrastructure projects, particularly in hilly areas and along the coast where erosion control is needed.
How To Choose The Right Size
Choosing the right aggregate size depends on your specific application and project requirements. For concrete work, the maximum aggregate size should not exceed one-quarter of the minimum member thickness or three-quarters of the clear spacing between reinforcement bars. For road base, a well-graded material from crusher dust up to 40mm provides the best compaction and load distribution. For drainage, use clean, single-sized aggregate that allows water to flow freely through the voids. For decorative purposes, choose a size and colour that matches your aesthetic vision. Tanzibaba offers free technical advice to help customers select the correct aggregate size for any project anywhere in Tanzania.
Aggregate Size Chart
The following chart summarises the standard aggregate sizes available in Tanzania and their typical applications. Crusher dust (0-4mm) is used for paving block bedding, concrete fine aggregate, floor screeds, backfill and base for synthetic grass. 6mm aggregate is used for precast elements, concrete blocks, thin concrete sections, drainage filters and decorative finishes. 10mm aggregate is used for concrete blocks, thin structural concrete, asphalt surface courses, exposed aggregate finishes, garden paths and landscaping. 14mm aggregate is used for reinforced concrete with closely spaced reinforcement, columns, beams, asphalt binder courses and road base components. 20mm aggregate is the standard size for reinforced concrete in foundations, slabs, columns, beams, retaining walls and most structural applications — it is the most commonly used coarse aggregate size in Tanzania. 25mm aggregate is used for mass concrete, larger structural elements, precast products and road base. 40mm aggregate is used for mass concrete fill, strip foundations, road sub-base, French drains, septic tank soakaways and filter media. Larger sizes exceeding 40mm including riprap, gabion stone and armour stone are used for erosion control, riverbank protection, coastal defences, railway ballast and gabion basket filling. Understanding this size chart helps contractors, engineers and homeowners quickly identify the appropriate aggregate for their specific construction need, avoiding the common mistake of using the wrong size which can compromise structural performance or waste money on unnecessarily large or small material.
Applications By Size Grade
Each aggregate size grade has specific applications where it performs optimally. Crusher dust excels in applications requiring tight compaction and a smooth level surface — its angular particles interlock firmly when compacted, making it ideal for paving block bedding where pavers must be held securely in place. In concrete, crusher dust works as fine aggregate filling the voids between coarse particles, though its higher fines content requires careful mix design. 6mm and 10mm aggregates are the go-to sizes for concrete blocks and precast elements where the smaller particles fit comfortably within mould dimensions and around dense reinforcement. The 10mm size is particularly popular for exposed aggregate concrete finishes where the small stones create an attractive textured surface. 14mm aggregate bridges the gap between the smaller precast sizes and the standard 20mm structural size, making it ideal for columns and beams where reinforcement is closely spaced. 20mm aggregate is the workhorse of the construction industry — it provides excellent concrete strength while remaining small enough for standard reinforcement spacing of 150mm or greater. For road construction, 20mm and 40mm aggregates form the structural layers that distribute traffic loads. 40mm aggregate is the preferred size for drainage applications because its larger particles create substantial interconnected voids that allow water to flow freely while maintaining structural stability. Selecting the right size for each application ensures the best combination of strength, workability, economy and durability in the finished construction.
Quality Grades By Size
Aggregate quality requirements vary by size grade and intended application, with different specifications governing each category. For fine aggregates (0-4.75mm), the key quality parameters include gradation within specified limits, silt content below 3 percent, organic impurities absent, and water absorption below 5 percent. Crusher dust must be well-graded without excessive ultrafine particles that increase water demand in concrete. For medium aggregates (6-14mm), quality focuses on particle shape with flakiness index below 35 percent, gradation consistency, and strength meeting the aggregate crushing value requirement below 30 percent. These sizes are critical for precast and block production where particle shape directly affects mould filling and surface finish. For standard structural aggregates (20mm), the full range of quality tests applies — strength, shape, gradation, durability, cleanliness and chemical stability. The 20mm size must have consistent gradation with minimal variation between batches to ensure predictable concrete performance. For coarse aggregates (25-40mm), strength and durability become even more important because these larger particles carry higher individual loads in mass concrete and road base applications. The Los Angeles abrasion test value should be below 30 percent to ensure the aggregates resist wear under traffic loading. Larger sizes used for erosion control (40mm+) must be hard, durable and resistant to weathering over decades of exposure. Tanzibaba grades all aggregates to specific quality levels and provides test certificates confirming compliance with the relevant standards for each size grade supplied.
Gravel Size Selection For Drainage
Selecting the correct gravel size for drainage applications is critical for ensuring long-term performance and preventing system failure. Drainage systems rely on the voids between gravel particles to allow water to flow freely, so the particle size directly determines the drainage capacity. For French drains and subsurface drainage, clean 20-40mm single-sized gravel provides excellent flow rates because the uniform particle size creates large, interconnected voids that allow water to pass through without restriction. Using a well-graded material with mixed sizes in drainage applications is a common mistake — the smaller particles fill the voids between larger ones, dramatically reducing permeability and potentially blocking the drainage system over time. For septic tank soakaway fields, 40mm clean gravel provides the largest voids and highest flow capacity, which is essential for dispersing treated effluent into the surrounding soil. Filter drains that protect perforated drainage pipes use a layered system with progressively coarser gravel — fine sand immediately around the pipe, followed by 10-20mm gravel, then 20-40mm gravel, and finally the backfill material. This graded filter prevents fine soil particles from migrating into the drainage pipe and blocking it. For surface water drainage around building perimeters, 20mm clean gravel in a French drain channel provides effective water collection and conveyance. In all drainage applications, the gravel must be washed or screened to remove fines that could clog the system. Tanzibaba supplies clean, single-sized gravel specifically screened for drainage applications, ensuring reliable performance in drainage systems across Tanzania.
Frequently Asked Questions
What is the most common gravel size for concrete?
The most common gravel size for concrete in Tanzania is 20mm crushed stone, often combined with 10mm aggregate to create a well-graded mix. For standard residential slabs and foundations, a blend of 10mm and 20mm aggregates produces the best concrete density and strength.
What size gravel is used for road base?
Road base typically uses a well-graded crushed stone ranging from crusher dust up to 40mm. This material, sometimes called graded crushed stone or crusher run, compacts tightly to form a stable, load-bearing layer. Tanzibaba supplies road base material meeting TANROADS specifications.
Can I use crusher dust for concrete?
Yes, crusher dust can be used in concrete as a partial or complete replacement for river sand. However, the mix design must be adjusted to account for the higher fines content and different particle shape. Crusher dust concrete typically has higher strength but may require more water. Tanzibaba can advise on the right mix design.
What is the difference between 10mm and 20mm aggregate?
The main difference is particle size — 10mm aggregate has particles up to 10mm in diameter, while 20mm has particles up to 20mm. 10mm aggregate is used for thinner sections, closely spaced reinforcement and precast elements. 20mm aggregate is the standard size for general reinforced concrete construction.
What size gravel is best for drainage?
For drainage applications, clean single-sized gravel between 20mm and 40mm is best. The uniform size creates open voids that allow water to flow freely. Washed gravel or crushed stone is preferred because it contains no fines that could clog the drainage system over time.
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