How to Interpret Sieve Analysis Data for Road Base Course Design

Sieve analysis turns a bulk sample of roadbase into a particle size distribution, showing how much material passes through each selected sieve. Read correctly, the results reveal whether an aggregate is well graded, overly uniform or prone to segregation. They also provide the evidence needed to compare a material with an Australian road authority specification or a project-specific design envelope.

No single sieve result determines suitability. A reliable interpretation combines grading, fines content, particle shape, durability, strength, compaction and field performance. The following process connects laboratory numbers with practical decisions on base course design.

Preparing a representative sample

Representative sampling matters because a laboratory cannot correct for an unrepresentative sample. For stockpiled material, the sample should capture particles across the full stockpile rather than only surface material or one loader bucket. Sampling after crushing and screening may be appropriate for process control, while acceptance sampling should follow the method and frequency stated in the Australian project specification.

After collection, the sample is air-dried or oven-dried according to the applicable test method, then broken down without crushing individual particles. The dry mass is recorded before the material is passed through a cleaned stack of sieves. Typical Australian sieve openings for coarse aggregate are 19.0, 9.50, 4.75, 2.36, 1.18, 0.600, 0.300, 0.150 and 0.075 mm.

The selected stack must suit the nominal aggregate size and expected grading. For example, a 20 mm crushed rock base generally needs a larger opening than a 10 mm screening aggregate. The bottom pan collects material passing the smallest sieve, while material retained above the top sieve is recorded separately. Strict mass accounting is essential, especially when a washed sieve analysis is used.

Building the grading curve

For each sieve, calculate the percentage retained by dividing the dry mass retained on that sieve by the total dry sample mass, then multiply by 100. Add the individual percentages to obtain cumulative retained mass. Cumulative passing is the difference between 100 percent and cumulative retained at the corresponding sieve.

The percentages passing are plotted against sieve opening on a semi-log grading curve, with particle size on the horizontal logarithmic axis and percent passing on the vertical linear axis. Each point represents the proportion of material finer than a stated size. Australian engineers commonly call this the PSD curve or simply “the grading”.

A typical calculation illustrates the distinction clearly. If 22 percent of a sample is retained on the 4.75 mm sieve, 78 percent passes it. That 22 percent is not the percentage passing, and confusing the two can produce the wrong design decision. Likewise, 8 percent passing the 0.075 mm sieve indicates 8 percent of the sample is finer than 75 micrometres.

Comparing results with Australian practice

Australian specifications commonly control a base course through upper and lower limits for percent passing at selected sieve sizes. Relevant references may include the applicable Austroads guidance, AS 1724.1, or requirements issued by state authorities such as Transport for NSW, Main Roads Victoria or the Queensland Department of Transport and Main Roads. Engineers often refer to the product as “roadbase”, “base course” or “graded crushed rock”, but those everyday labels should not be treated as interchangeable technical specifications.

Aggregate geology and supply conditions vary considerably across Australia. A quarry near Melbourne may produce basaltic crushed rock, while materials around Sydney and Perth may be derived from sandstone or limestone, with imported basalt also used in some markets. Regional supplies can differ in weathering, abrasion resistance and particle shape. A passing result in one district is therefore not a universal approval for a substitute aggregate elsewhere.

Climate also affects the grading target. Heavy wet-season rain in northern Queensland creates a stronger need for stability and drainage, while prolonged dry conditions can make excessive fines generate dust. Coastal salt exposure, heat and restrained water further complicate Australian pavement work. For engineers comparing Indonesian and Australian documents, local climate, quarry source and project specification must be considered alongside the laboratory result.

Standards and approvals should always be read within their governing jurisdiction. A test performed under one national accreditation framework is not automatically accepted under another without contract and regulator confirmation. The importance of checking scope rather than assuming status from a similar name is illustrated by this award scheme comparison.

Assessing fines and grading shape

The percentage passing 0.075 mm is one of the most important grading checks because it represents the fine fraction. A small amount of fines may help bind the base, but excess fines can increase sensitivity to water, reduce permeability and create pumping under repeated wheel loads. Low fines content may improve drainage while leaving a material vulnerable to segregation if the remaining particles are poorly graded.

Fines content alone does not identify the minerals. Two samples can have the same percentage passing 75 micrometres but behave differently because one contains inert rock dust and the other contains reactive clay. Atterberg limits, clay content and chemical or durability tests may therefore be required. Base course design should use the complete test schedule, not rely on PSD data in isolation.

Uniformity and curvature coefficients can provide an additional grading check. Cu equals D60 divided by D10, while Cc equals D30 squared, divided by D10 multiplied by D60. Here, D10, D30 and D60 are the particle sizes corresponding to 10, 30 and 60 percent passing. Broadly, Cu and Cc are often associated with a suitably graded granular material, but road authority envelopes remain the deciding criteria because a very broad grading may contain too many fines.

Connecting gradation to construction

A well-graded base packs efficiently and can achieve high strength at lower compaction. A uniform aggregate may form voids, while a gap-graded material can experience internal erosion. Poorly controlled grading also makes segregation more likely during tipping, spreading and traffic. Contractors in Australia therefore inspect crusher settings and manage stockpiles as part of producing a stable “20 mm roadbase”, rather than relying on the crusher’s nominal product size.

Moisture and compaction must develop alongside the grading. Water helps particles rearrange during rolling, but the amount required depends on the fines fraction and aggregate absorptivity. Contract documents commonly express compaction as a percentage of maximum dry density, although the target, moisture condition and test method must come from the specification. A trial section should establish whether the proposed grading and plant combination can achieve the required density without breaking down particles.

Construction records should link each test result to a production lot, stockpile or placement area. Changes in quarry, crusher configuration or weather can alter the effective grading even when the nominal material remains the same. Where results move toward an envelope limit, additional testing and process adjustment are more reliable than accepting an isolated result without investigation.

Verifying laboratory and field quality

The laboratory report should identify the sampling method, test standard, sieve sizes, sample description, drying treatment, individual masses retained and cumulative percentages passing. Results should be checked for balance, sensible rounding and agreement with the declared nominal size. A digital grading curve is useful only when its underlying calculations and units have been reviewed.

In Australia, clients often require testing by a laboratory accredited by the National Association of Testing Authorities, or NATA, for the relevant field of testing. The accreditation scope must cover the exact test involved. Field density, moisture and sieve results require calibrated equipment, documented procedures and competent personnel. Sampling during live traffic also demands a work-zone risk assessment and controls consistent with occupational health and safety management systems.

UP. PPP applies SNI ISO/IEC 17025 and ISO 45001 while providing laboratory and field investigation services within Jakarta’s public works environment. Its stated direction supports infrastructure assessment for government and private-sector road and bridge projects. For Australian and international partners, the applicable accreditation scope and project specification should still be confirmed before results are accepted.

Converting results into design decisions

A defensible assessment follows a fixed sequence: confirm representative sampling, verify calculations, plot the grading curve and compare each required point with the specification. Fines, uniformity and curvature are then considered alongside strength and durability tests such as CBR, triaxial compression, Los Angeles abrasion or soundness testing. Trial-section results and construction moisture complete the chain between laboratory approval and field acceptance.

A compact comparison helps separate raw measurements from design interpretation:

Measurement	What it indicates	Design question
Individual percentage retained	Material captured on one sieve	Is one fraction unexpectedly dominant?
Cumulative percentage passing	Proportion finer than a stated size	Does the aggregate satisfy the grading envelope?
Percentage passing 0.075 mm	Total fine fraction	Could excess fines affect stability or drainage?
Cu and Cc	Breadth and shape of grading	Does it behave as a densely graded granular material?
Deviation from the envelope	Distance below or above a limit	Is the result acceptable, or does production need adjustment?
CBR, density and moisture	Strength and field compaction response	Can the material support the intended pavement loading?

Engineering judgment sits at the centre of this process. Passing a grading envelope confirms only one property, while a borderline result may require evaluation of production consistency, drainage, loading and maintenance exposure.

Road and bridge projects in Jakarta can engage UP. PPP for accredited sieve analysis, laboratory testing, elevation and infrastructure assessments supported by experienced technical personnel. Early representative testing and clear grading records give designers, contractors and quality teams a stronger basis for road base course approval and construction control.