Road base grades explained: 2.1, 2.3 and 2.5

What road base is

Our road base is crushed blue rock (blue metal) and fines in a 40mm blend. The fines fill the gaps between the stone so it compacts into a firm, consistent layer.

What CBR means

CBR stands for California Bearing Ratio — a test of how much load a compacted material can carry. In Queensland, unbound pavement materials are specified under Transport and Main Roads technical standards. The higher the number, the stronger the base.

The three grades

Compaction matters

Road base needs moisture and machine compaction for best results. Lay it in layers, dampen each one (see preparing a base for pavers) and compact with a plate compactor or roller before adding the next.

A cheaper alternative

For access roads, driveways and paving sub-base, recycled concrete is a cheaper option (compare it with blue metal) that still makes a firm base. Where an engineer has specified a grade, use that grade.

What CBR means

CBR stands for California Bearing Ratio. It is a standard test of how well a compacted material resists load, so a higher CBR means a stronger base. That is why 2.1 (CBR80) suits heavily loaded areas, 2.3 (CBR45) suits most home driveways and paths, and 2.5 (CBR15) is a lower grade for light-duty fill and general levelling.

Laying and compacting road base

• Strip topsoil and soft ground until you reach firm subgrade.
• Spread the road base in layers of about 100–150mm.
• Dampen each layer. Road base compacts best when it is moist, not dry and not soaking.
• Compact each layer with a plate compactor or roller before adding the next.
• Shape the surface with a slight crossfall so water runs off.

Drainage and edges

Water is what breaks down most driveways and pads. Make sure the finished surface sheds water to the sides, and keep the edges firm with kerbing, timber edging or a compacted shoulder. Where water pools against a slab or path, a strip of drainage aggregate helps carry it away.

How much to order

Work out the area and the finished depth, then add around 15–20% for compaction. Our volume calculator gives the cubic metres, and we can confirm the grade and quantity when you request a quote. For engineered slabs or commercial pavements, always follow the specification.