Concrete block

A concrete block is a hollow concrete masonry unit (widely called a besser block, after the manufacturer) laid in mortar to build a wall. Because the units are hollow, the cores can be left empty, or filled with grout and steel to make reinforced masonry. It is the standard unit for retaining walls, fences, garages, sheds, and load-bearing walls, designed under AS 3700 (or AS 4773 for small buildings). “Concrete block” and CMU (concrete masonry unit) mean the same thing.

The common forms:

• Standard hollow block (the besser): the workhorse, cores up, for grouted/reinforced walls.
• Bond-beam / knock-out block: a channel block for horizontal reinforcement courses and lintels.
• Solid / paving / split-face / besaflor: solid units, and finished-face blocks where the block is left exposed.

Blocks come in modular sizes (commonly 390 mm long x 190 mm high, in 90/140/190 mm widths) so they course out neatly, and they are laid in a mortar of the specified class.

For a builder the practical points are: match the block strength and the mortar class to the engineer’s spec (a retaining or load-bearing wall is not a job for whatever mortar is handy); plan coursing off the 190/390 module so you minimise cutting; keep grout cores clean and aligned if the wall is reinforced, and follow the reo detail for bar size and which cores fill; allow for control joints to manage shrinkage cracking; and decide the finish early (face block left bare, rendered, or bagged), because that changes the block you order and the care taken laying it. Exposed besser needs clean, consistent joints; a wall to be rendered is more forgiving.

Also known as: Besser block, concrete masonry unit, CMU, block (blockwork).

Category: Materials / Masonry.

Related

• CMU
• Mortar mixes
• Reo reinforcement

See also

• Cement render
• AS 3700 masonry structures
• Reinforced masonry

References

• AS 3700 (Chalkline) (verified 2026-06-09)

---

Last updated: 2026-06-09. Verified: 2026-06-09. Quarterly review for currency.