Road sub-base and base course aggregate specification

What a road sub-base and base course aggregate specification really separates

The sentence heard on every site is that the material comes from the same quarry, so the layer should not matter. In fact the two layers carry different jobs. Sub-base is the thick transition between the formation and the structure above it: it spreads load and it drains. Base course sits immediately under the bound layers, takes the highest concentration of stress, and is expected to interlock once compacted. Different jobs mean different acceptance criteria.

The practical consequence for a crushing and screening operation is that one feed has to yield two distinct products, stockpiled separately. A plant selling both layers off a single pile eventually sends a load back after a test result.

The measured properties and their test methods

Both layers are judged on the same properties; what changes is how tight the expectation is. When the method is written into a contract, the current edition of the standard governs.

The numeric limits are deliberately absent from the table. They are read from the employer's specification, the road class and the design traffic. What the plant must do is not memorise limits but convert the contractual limit set into a production control plan.

Why base course has to be cleaner

Base course receives wheel load through a thin bound layer. As fines and clay increase, the material loses bearing capacity once saturated, and that loss shows up at the surface as rutting or settlement. This is why washed production, or at least effective dedusting, is preferred for base. Sub-base is not held to the same standard because it lies deeper and its drainage function dominates.

Building the grading inside the plant

The target band is hit with a screening combination, not with a crusher setting alone. Output straight from a tertiary crusher rarely falls inside a specification band by itself; two or three fractions are usually blended in controlled proportions to bring the curve inside the envelope. The blend is not a fixed recipe: it is re-tuned as the feed changes and as screen media fatigues, which is why sampling during production is not optional.

Segregation during handling is a separate risk. Widely graded material dropped from height collects coarse at the toe of the pile and fine at the crown. Loading from one face of that pile can produce a delivery that fails, while cutting vertically through the pile does not.

A specification without compaction control means nothing

Correct grading compacted at the wrong moisture will not reach target density. Optimum moisture is established in the laboratory, and layer thickness, roller type and pass count are planned around it on site. Checking the compaction record before blaming the material settles most disputes. How this connects to mixing and laying equipment is developed on the road base plant mix facility page.

The production control plan

Meeting a specification is a matter of continuity, not a single certificate. A typical plan runs at least one grading per shift, a daily fines check, weekly plasticity and periodic fragmentation testing. Results are filed with date and stockpile number, because in a rejection argument that record is all that remains. Site logistics and plant setup are collected under jobsite and mobile plant.

Common questions

Can recycled material be used in these layers

It can. What decides is not origin but whether the material passes the same tests. With crushed concrete, fines generation and water absorption behaviour deserve extra monitoring.

Is screened pit-run enough for sub-base

Sometimes, but the decision is made by test. Screened pit-run frequently sits at the margin on plasticity and particle shape, and a crushing stage has to be added.

Can one stockpile serve both layers

It should not. Declaring two specifications from one pile destroys both the record keeping and the traceability of the test results.