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D2728-98 – Standard Test Method Technical Guide
📐 Scope, Terminology, and Classification
ASTM D2728 – 98 (Reapproved 2019) standardizes the practice for selecting and applying road tars in pavement construction and maintenance. The standard critically defines mixture types, binder viscosities, and application methods to ensure proper material performance. All values in this practice are stated in SI units.
The standard defines several key terms. A dust palliative is a light application of a low-viscosity bituminous material for the express purpose of bonding dust or preventing dust nuisance. Four types of plant mix are specified, prepared in a central plant and compacted at the job site: cold-lay tar concrete (medium-viscosity, less dense aggregate), hot-lay tar concrete (high-viscosity, dense aggregate), sand mix (high-viscosity, sand aggregate), and soil mix (low-to-medium viscosity, soil aggregate, often stockpiled). A patch mix is designed for small areas and applied at ambient temperature using hand-laying and hand-compaction techniques. Sand aggregate is specifically defined as non-plastic mineral aggregate essentially all passing the 4.75-mm sieve with a minor percentage passing the 75-µm sieve.
⚙️ Recommended Uses and Temperature Selection
This practice provides a comprehensive guide for matching road tar grades to specific construction procedures. Selection of a particular grade depends on local practice, equipment availability, traffic loads, and environmental conditions. The table below summarizes the recommended uses based on viscosity.
| 🟦 Tar Grade Viscosity | 📐 Recommended Construction Use | 🎯 Specific Method |
|---|---|---|
| Low | Dust Palliative / Prime Coat | Light spray or surface application |
| Medium | Cold-Lay Concrete / Soil Mix | Plant mix, ambient or near-ambient laydown |
| High | Hot-Lay Concrete / Sand Mix | Plant mix, immediate hot laydown |
Application temperatures are critical for achieving proper viscosity. The standard specifies minimum and maximum temperatures for each grade. In general, the lower end of the temperature range is selected when aggregate or pavement surface temperatures are elevated.
| 🌡️ Road Tar Grade | 📏 Viscosity Level | ⚡ Application Temp Range (°C) |
|---|---|---|
| RT-1, RT-2 | Low | 20 – 60 |
| RT-3, RT-4, RT-5 | Medium | 40 – 80 |
| RT-6, RT-7, RT-8 | Medium / High | 70 – 110 |
| RT-9, RT-10, RT-11, RT-12 | High | 100 – 150 |
✅ Standard Compliance Note: All temperature values and specifications in this standard are provided in SI units, which are to be regarded as the standard.
📊 Significance in Pavement Construction
The practice emphasizes that the recommendations serve as a guide for construction and maintenance. By standardizing the definitions of mixture types (cold-lay, hot-lay, sand mix, soil mix) and their corresponding tar viscosities, this practice ensures the binder provides adequate workability, aggregate coating, and pavement durability. Proper application temperature is crucial for achieving the necessary compaction and long-term performance. The specificity in defining sand aggregate and patch mix provides a consistent framework for mix design and field repairs.
⚠️ Project Specification Alert: While this standard provides the framework, the selection of a particular grade will depend on local practice, equipment availability, traffic conditions, and environmental factors applicable to the specific project