Every asphalt road starts life as a recipe. Before a premix plant produces a single tonne for a project, the contractor has to show that a particular blend of aggregates and a particular bitumen content will produce a pavement that is strong enough to carry traffic, flexible enough not to crack, and dense enough to keep water out. In Malaysia, that recipe is almost always developed and checked using the Marshall method.
Where the Requirement Comes From
For public road works, asphaltic concrete is specified under the JKR Standard Specification for Road Works, Section 4: Flexible Pavement (JKR/SPJ/2008-S4). It defines the asphaltic concrete mix types by aggregate gradation, including:
- AC 10 and AC 14: wearing course mixes, the top layer that traffic runs on.
- AC 28: binder course, the layer beneath the wearing course.
The specification requires the contractor to propose a job mix formula for each mix type. Each trial combination of aggregate gradation and bitumen content is put through the Marshall test procedure and volumetric analysis, with specimens compacted at 75 blows per face.
The specification refers to ASTM D 1559, the original Marshall method. ASTM withdrew D 1559 in 1998. Specimen preparation and the stability and flow test are now published separately as ASTM D6926 and ASTM D6927.
Step 1: Preparing the Specimens
Aggregates are blended to the target gradation and mixed with bitumen at a range of trial bitumen contents. For each bitumen content, a set of cylindrical specimens is compacted in a standard mould using the Marshall compaction hammer, which drops a fixed weight a fixed height onto each face of the specimen. The specification calls for four specimens at each bitumen content.
Step 2: Density and Volumetric Analysis
Before any load is applied, each specimen is weighed dry, submerged in water, and saturated surface-dry to find its bulk specific gravity. Combined with the specific gravities of the aggregate and bitumen, this gives the mix's volumetric properties:
- Air voids (VTM): the percentage of the compacted mix that is air. Too high and the pavement lets in water and oxidises quickly; too low and it can bleed and rut in hot weather.
- Voids in mineral aggregate (VMA): the space between aggregate particles available for bitumen and air. It needs to be large enough to hold a durable bitumen film.
- Voids filled with bitumen (VFB): how much of that space is actually filled with bitumen.
Step 3: Marshall Stability and Flow
Each specimen is brought to a standard test temperature in a water bath, then loaded across its diameter (perpendicular to the cylinder's axis) in the Marshall apparatus until it fails.
- Stability: the maximum load the specimen carries, an indicator of its resistance to deformation under traffic.
- Flow: how much the specimen deforms up to that maximum load, an indicator of its flexibility.
A mix with high stability but very low flow may be too stiff and prone to cracking. A mix with high flow may deform and rut under heavy vehicles.
Step 4: Choosing the Design Bitumen Content
Stability, flow, density, and each volumetric property are plotted against bitumen content. The design bitumen content is chosen as the value that satisfies all the specification limits at the same time, balancing strength, durability, and workability. The result becomes the job mix formula that the premix plant must then produce within the specified tolerances.
After Approval: Checking What Gets Laid
Mix design is only the first half. During construction, the laid asphalt is checked against the approved job mix formula. Typical checks include bitumen extraction and grading to confirm the plant is producing the approved blend, and cores cut from the finished pavement to check thickness and compacted density.
Note: The limits for stability, flow, and each volumetric property depend on the mix type and the edition of the specification in the contract. Always design and assess against the project's own specification.
