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.
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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