Concrete masonry units (CMU) have hollow cores. In retaining walls, foundation walls, bond beams and any wall with vertical rebar, some or all of those cores are filled with grout or concrete. Ordering the right amount matters: too little stops the pour halfway up a wall, and too much is wasted concrete you pay for.
The calculation
Grout volume = wall area × wall thickness × core void fraction × share of cores filled × (1 + waste)
The void fraction is the hollow share of a block. Standard two-core hollow blocks are roughly 45 to 50% void; the actual figure depends on the manufacturer, so check their data sheet.
Typical grout volumes, fully grouted
| Block | Grout per sq ft of wall | Grout per 100 sq ft |
|---|---|---|
| 6 in | about 0.21 cu ft | about 0.8 cu yd |
| 8 in | about 0.29 cu ft | about 1.1 cu yd |
| 10 in | about 0.37 cu ft | about 1.4 cu yd |
| 12 in | about 0.45 cu ft | about 1.7 cu yd |
These figures assume 46% void and no waste.
Partial grouting
Many walls only need grout in the cells with vertical rebar, for example every 32 or 48 in. Choose the share of cells filled; one cell in four of an 8 in block wall needs about a quarter of the solid-grouted volume. Bond beams along the top course are extra: add them as a short wall.
Example
An 8 in block wall 25 ft long and 4 ft high (100 sq ft), fully grouted, needs about 29 cu ft of grout. With 10% waste, that is 1.19 cubic yards, or 54 bags of 80 lb premix yielding 0.6 cu ft each.
Grout or concrete?
Masonry grout is a fluid mix of cement, sand and small aggregate (pea gravel) poured at a slump of 8 to 11 in so it flows around rebar and fills every void. Ordinary concrete with large aggregate can bridge in narrow cores and leave voids. For more than about a cubic yard, ready-mix grout delivered by truck and pumped is far easier than mixing bags.
Pouring tips
Fill in lifts of no more than 5 ft at a time unless the wall is designed for high-lift grouting, and consolidate each lift with a rod or a small vibrator. Clean out mortar droppings at the base of the cores first. Follow the engineer’s drawings and local building code for rebar size and spacing.