Concrete blocks: hollow, solid and rock-cut

What concrete blocks are

Concrete blocks, also called cement blocks or concrete masonry units (CMU), are cast from a stiff mix of cement, sand and aggregate in a block machine, then cured. They are laid in courses with cement mortar to build walls, fences and foundations. Blocks for walls are made in several widths, so one wall can carry load while a thinner one only divides rooms.

The names vary by market. Cinder block and breeze block are other names buyers use for the same kind of unit.

Hollow and solid blocks

Hollow blocks have voids running through them, which reduces weight and leaves space for steel bars and grout where a design calls for reinforcement. Solid blocks have no voids. Which one a wall needs, and which hollow cores are filled, is a decision for the designer.

Block sizes: nominal and actual

Blocks are sold by their nominal size, which is the block plus one mortar joint. Angelus Block, a US manufacturer, explains this on its page on CMU dimensions and sizes: the common 8 × 8 × 16 in block has a specified size of 7.625 × 7.625 × 15.625 in (194 × 194 × 397 mm, our conversion), and the actual block may vary by up to 1/8 in (3 mm) either way under ASTM C90, the US standard for loadbearing units.

Block sizes: nominal and actual

Use	Nominal size (W × H × L)	Approximate metric
Loadbearing wall	8 × 8 × 16 in	19 × 19 × 39 cm
Partition wall	3.5 × 8 × 16 in	9 × 19 × 39 cm

Sources: Angelus Block; Dimensions.com for the partition size and metric values.

Angelus lists nominal widths of 4, 6, 8, 10, 12 and 16 in. Tanzanian suppliers also list blocks in inches, as below, so check that a “6-inch block” from two suppliers means the same thing.

Rock-cut building blocks in Tanzania

Not every walling block is cast concrete. Mosha Integrated Investment Ltd says it machine-cuts building blocks from natural rock at its quarries in the Kilimanjaro and Arusha regions, and lists them in four sizes: 4 × 8 × 16, 5 × 8 × 16, 6 × 8 × 16 and 7 × 8 × 16 in (Mosha building blocks). Cut stone blocks are solid, so the hollow-block advice above does not apply to them.

Mortar, plaster and damp walls

The mortar joint is part of the block’s nominal size, so the joint thickness must match what the block size assumes. Plaster and mortar can crack as they dry; for those mixes Kitovu holds Polypropylene Fibre for Concrete and Mortar in Zanzibar, 12 mm monofilament fibre meant to control plastic shrinkage cracks, at 0.9 kg per m³.

Concrete blockwork below ground or around wet rooms also needs a waterproof layer. K11 Polymer Cement Waterproof Coating is a brush-on cement coating whose data sheet lists walls and basements among its uses.

What to check when you buy

1. Hollow or solid, and the width in inches or millimetres.
2. The actual size of a sample block against the nominal size.
3. That blocks are square and even, with no broken corners.
4. How long they have cured before delivery.
5. For loadbearing walls, the strength the maker declares and the standard behind it.
6. The quantity per square metre of wall for the chosen size.

For pressed soil blocks that need little mortar, see interlocking bricks; for blocks with open patterns for ventilation, see louver blocks.