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D242 – Standard Test Method Technical Guide
📐 General Description and Scope
This specification covers mineral filler added as a separate ingredient for use in asphalt mixtures. As defined in the standard, mineral filler shall consist of finely divided mineral matter such as rock dust, slag dust, hydrated lime, hydraulic cement, fly ash, loess, or other suitable mineral matter. The material must be sufficiently dry to flow freely and essentially free from agglomerations at the time of use.
🟦 Physical Gradation and Purity Requirements
The physical requirements of the standard specify strict gradation limits to ensure consistent performance in the asphalt mixture. The filler must meet the following sieve analysis criteria. Additionally, mineral fillers prepared from rock dust, slag dust, loess, and similar materials must have a plasticity index not greater than 4. They must also be essentially free from organic impurities.
| 🟦 Sieve Size | 📐 Percent Passing (by Mass) |
|---|---|
| 1.18 mm (No. 16) | 100 |
| 600 µm (No. 30) | 97 to 100 |
| 300 µm (No. 50) | 95 to 100 |
| 75 µm (No. 200) | 70 to 100 |
⚙️ Sampling and Testing Methodology
Proper sampling and testing are critical for verifying compliance. Samples must be obtained according to Practices C50/C50M, C183/C183M, or Test Methods C311/C311M, whichever is most appropriate for the material being sampled. Random samples must be taken at intervals not exceeding each 300 tons of material. The minimum field sample size is 5.0 kg [11 lb], which is then reduced to a minimum of 2.5 kg [5 lb] for testing.
Grading is determined in accordance with Test Method D546, while the plasticity index is determined according to Test Methods D4318.
| ⚡ Parameter | 📏 Specification / Requirement |
|---|---|
| Minimum Field Sample Size | 5.0 kg [11 lb] |
| Minimum Laboratory Test Sample | 2.5 kg [5 lb] |
| Sampling Interval | Not exceeding each 300 tons of material |
| Grading Test Method | ASTM D546 |
| Plasticity Index Test Method | ASTM D4318 |
💡 Technical Tip: While the standard specifies a maximum plasticity index of 4 for most mineral fillers, Note 1 explicitly states that this limit is not appropriate for hydrated lime and hydraulic cement. These materials have inherent chemical properties that make the plasticity index requirement unsuitable.
📌 Industry Application Note: The filler must be essentially free from organic impurities. The ability to flow freely without agglomeration is a crucial property at the time of mixing, ensuring uniform dispersion within the asphalt binder.
❓ Frequently Asked Questions
🔍 What types of materials can be used as mineral filler?
The standard allows for rock dust, slag dust, hydrated lime, hydraulic cement, fly ash, loess, or other suitable finely divided mineral matter.
💡 What is the plasticity index requirement and is it universal?
The requirement is a plasticity index not greater than 4. However, it is not universal; it applies to materials like rock dust and slag dust but not to hydrated lime or hydraulic cement.
⚡ How much of the mineral filler must pass the No. 200 (75 µm) sieve?
The gradation requirement states that 70 to 100 percent must pass the No. 200 sieve.
📌 What is the required minimum size for the field sample and the laboratory test sample?
The minimum field