Step 01
Test the constituents
Each aggregate stockpile is graded and tested for abrasion, shape, soundness, specific gravity and absorption; the binder is tested for penetration, softening point and ductility.
Job Mix Formula
The recipe your asphalt plant runs to, established by testing your actual aggregates and your actual binder rather than by copying a formula from another job.
Job Mix Formula: what it actually tells you
A job mix formula sets the proportion of each aggregate stockpile, the binder content and the tolerance around each, so the plant can reproduce the same mix load after load. It has to be developed from the materials that will actually be used, because two sources of the same nominal aggregate behave differently once they are graded and tested.
The aggregate work comes first. Each stockpile is graded, and the proportions are blended mathematically until the combined grading sits inside the specified envelope with room to move. Aggregate quality is checked at the same time: Los Angeles abrasion for toughness, flakiness and elongation for shape, soundness for durability, and water absorption because absorptive aggregate drinks binder that should be coating the surface.
Optimum binder content is then found by the Marshall method. Specimens are made at a range of binder contents and tested for stability, flow, air voids, voids in mineral aggregate and voids filled with bitumen. The optimum is the content that satisfies all of the criteria at once, which is usually a narrow window and often not the value that maximises stability.
At a glance
- Binder contents trialledTypically 5, in 0.5 percent steps
- Compaction50 or 75 blows per face by traffic category
- Design criteriaStability, flow, air voids, VMA, VFB
- TurnaroundTypically 7 to 10 working days
How we run it
The sequence we follow on site and in the laboratory, and the point of each stage.
Step 02
Blend the gradation
Stockpile proportions are calculated so the combined grading falls within the specification envelope, avoiding the restricted zone and leaving tolerance for plant variation.
Step 03
Prepare Marshall specimens
Specimens are compacted at a series of binder contents, typically five, at the blow count specified for the traffic category.
Step 04
Test and plot
Stability, flow, bulk and maximum specific gravity are measured, and air voids, VMA and VFB are calculated and plotted against binder content.
Step 05
Select and issue
The binder content meeting every criterion is selected, verified by a confirmation batch, and issued as a job mix formula with tolerances for each sieve and for binder content.
What you receive
- Combined aggregate gradation against the specification envelope
- Stockpile blend proportions for the plant
- Optimum binder content with the design curves
- Marshall stability, flow, air voids, VMA and VFB at optimum
- Aggregate quality results
- A job mix formula with working tolerances
When this test is the right one
- Before any asphalt production begins on a project
- When an aggregate source or binder supplier changes
- When the specification or traffic category changes
- Where laid asphalt has failed and the mix design is under review
- For approval submissions to NHA, a consultant or a client
Methods and standards
Every result is reported with its method reference and the calibration status of the instrument used.
ASTM D6926 preparation of asphalt mixture specimens using Marshall apparatusASTM D6927 Marshall stability and flowAASHTO R35 Superpave volumetric designASTM C131 Los Angeles abrasionASTM D5 penetration of bituminous materialsNHA general specifications for asphaltic concrete
Common questions
What engineers most often ask us about job mix formula.
Only if the aggregate sources, the stockpile gradings and the binder are genuinely the same, which is rarer than people assume. A different quarry, or even a different crushing run from the same quarry, shifts the grading and moves the optimum binder content.
VMA is the space between the aggregate particles that binder and air have to share. If it is too low there is physically no room for enough binder to make a durable film, so the mix looks acceptable on stability while being under bound and prone to ravelling. It is the criterion most often violated by an over dense gradation.
Tolerances on each control sieve and on binder content, set to the governing specification. They define the band the plant must stay within, and they are what site acceptance testing is judged against.
Often ordered alongside
Most investigations combine several of these. We scope them together so you are not paying to mobilise twice.
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Soil & Rock Testing
The laboratory is where samples become design parameters.
Read moreNeed job mix formula on your project?
Send us the location, the structure and your programme. You get a scoped quotation with the standard, the number of tests and the turnaround written into it.