Stone chips and sand: choosing aggregates for concrete
, 2 minute read, by Mashrur Corporation
Aggregates make up most of the volume of concrete. Cement and steel get the attention, but a mix made with dirty sand or flaky stone will not reach its strength whatever the cement brand.
Coarse aggregate: stone chips or brick chips
Crushed stone is the standard coarse aggregate for structural concrete. Bangladesh has little hard rock of its own, so a large share of stone is imported, with domestic supply from the north and north-east of the country.
Brick chips, known as khoa, have long been used here as a cheaper alternative. They are lighter and more porous than stone and give lower strength, so they are generally kept to lightly loaded work. For columns, beams and slabs of multi-storey buildings, follow the engineer's specification, which will normally call for stone.
Size and shape
- 20 mm down (three-quarter inch) graded stone suits most reinforced concrete.
- A smaller maximum size, around 12 mm, is used for thin sections and congested reinforcement.
- Angular, roughly cubical pieces bond well. Flat and elongated pieces weaken the mix.
- A graded mix of sizes packs tighter than single-size stone and needs less cement paste.
Fine aggregate: two kinds of sand
Coarse sand, usually sold as Sylhet sand, is the sand for structural concrete. Fine local river sand is used for plaster, brickwork mortar and filling. The two are often blended for particular mixes. Using fine sand alone in structural concrete raises the water demand and lowers strength.
Simple checks on site
- Rub sand between wet palms. If it stains them, it carries silt or clay.
- Shake sand with water in a clear bottle and let it settle. A thick layer of fine material on top means too much silt.
- Look for shells, roots, coal and clay lumps in the heap.
- Check that stone is not coated in dust. Dust stops cement paste gripping the surface.
- Near the coast, be cautious of salt in sand, which corrodes reinforcement.
Buying and measuring
Aggregates are sold by volume, usually per cubic foot, measured in the truck. Loose material settles in transit, so agree in advance how and where the load will be measured. Store sand and stone on a hard, clean base so the bottom layer is not lost into mud.
This is general guidance. Standards, duties and procurement rules change, so confirm current requirements for your project before acting on it.