Published by Build Hub Company Limited · Updated
Why size is the first question
Aggregate is most of the volume of any concrete, and the largest stone in the mix has to fit through the gaps it is being poured into. If the stone is too big for the space between reinforcing bars, or for the cover between the bar and the formwork, it bridges, leaves voids and produces honeycombed concrete that no amount of vibration fixes. That is why section thickness and bar spacing — not the price per load — decide the size you order.
As a working principle, the maximum aggregate size should comfortably pass between the closest bars and sit within the concrete cover. Thin, congested elements want a smaller stone; thick, lightly reinforced sections can take a larger one and benefit from the reduced cement demand.
Matching aggregate to the element
- Slabs, beams and columns with normal reinforcement — a medium graded concreting aggregate that passes between the bars and within cover
- Congested pours, thin sections and heavily reinforced columns — a smaller graded aggregate so the mix flows around the steel
- Mass concrete, footings and blinding — a larger stone is acceptable and economical where there is room for it
- Sub-base and hardcore under slabs, paving and roads — a crushed, well-graded material that locks together when compacted
- Drainage, soakaways and French drains — a clean, single-sized stone with the fines washed out so water can move through it
Graded, single-sized and crushed — what the words mean
Graded aggregate contains a spread of particle sizes so the smaller stones fill the gaps between the larger ones; it packs densely and is the normal choice for concrete and sub-base. Single-sized aggregate is mostly one size, leaves voids between the stones on purpose, and is what you want for drainage. Crushed aggregate has angular faces that interlock, which is why it compacts into a stable sub-base and bonds well in concrete; rounded material rolls and does neither as well.
Consistency across the pour programme
On a project pouring concrete for weeks, the aggregate needs to stay the same from the first load to the last. A change in source changes the water demand and the workability of the mix, and the site notices it as a change in finish or a mix the crew starts adding water to. Plan the aggregate against the pour dates rather than buying opportunistically load by load.
Checks on delivery
- Dust and fines coating the stone — a heavily coated aggregate weakens the bond and needs washing or rejecting
- Clay lumps, soil or organic matter mixed through the load
- A visible spread of sizes for graded material; a uniform size for drainage stone
- Stockpile on clean, firm ground away from the sand so the two do not mix at the edges
Ordering aggregate for a Zanzibar site
Tell us what the aggregate is for — slab, columns, footings, sub-base or drainage — and, for concrete, the section thickness and reinforcement spacing. Add the pour dates so loads can be sequenced, and confirm where the truck can tip. Aggregate is supplied by the load across Unguja and is usually ordered with sand and cement on one quotation; crusher dust and pavers are added when the order includes hardstanding or paving.