Sub-base aggregate is a granular material in which different particle sizes come together in a defined distribution and interlock under compaction to carry load. A single-sized pile of crushed stone will not compact; the job of sub-base material is to fill the voids with smaller particles and build a load-bearing skeleton.
The layers of a road: sub-base, base, surfacing
A road cross-section is built up in stages from the bottom. At the base is the prepared subgrade. On top of it comes the sub-base: a more economical, coarse and thick layer. Above that sits the base course, whose grading is controlled more tightly, and on top the asphalt or concrete surfacing.
Each layer spreads the load coming from above and passes it to the layer below. Savings made in the lower layers are stolen from the life of the asphalt above: surfacing laid on a base with insufficient bearing capacity settles and cracks in its first winter.
Grading: the real name of the mix
The performance of sub-base aggregate lies in its particle size distribution. Coarse particles form the skeleton, medium particles the interlock, and fines fill the voids. When the distribution is off, two typical problems appear:
- Too many fines. The material turns to mud when wet, bearing capacity drops and frost susceptibility rises.
- Too few fines. The voids are not filled, compaction is not achieved and the layer stays loose.
That is why sub-base aggregate is not simply what comes out of the quarry; it is a mix built under control after crushing, screening and, where needed, washing. Choosing fractions and aggregate quality are the other links in this production chain.
Compaction and moisture content
Sub-base material does not compact properly either dry or wet. Every material has an optimum moisture content at which it reaches maximum density; field compaction is done close to that value. Layer thickness matters too: in a thick layer placed in one lift, the bottom does not compact while the top looks as if it has.
- Material is placed in lifts, and each lift is compacted separately.
- Compaction is checked with field tests; judging by eye is not accepted.
- Material placed on a summer site dries quickly; watering is part of compaction.
Which material for which job?
A rough guide:
- Fill and levelling: more economical, coarse mixes.
- Sub-base: well-graded sub-base aggregate.
- Base course: more tightly controlled, screened material; under heavy traffic, plant-mixed blends are preferred.
- Drainage layer: washed coarse fraction with a low fines content.
The final decision depends on the project’s traffic loading and ground investigation. Before material leaves for site, it should be known which layer it is going into; the same pile does not suit every job.
