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D3387-11 – Standard Test Method Technical Guide
🧪 Scope and Principles of the GTM Test
ASTM D3387 – 11 defines a testing methodology using the U.S. Corps of Engineers Gyratory Testing Machine (GTM) to evaluate the compaction and shear properties of bituminous paving mixtures. The objective is to compact the specimen to the ultimate in-place density under the anticipated vertical stress while monitoring the process in terms of unit mass and shearing resistance, including the development of plastic properties. This method is intended for use in mixture design, plant control testing, and accelerated traffic simulation for mixtures containing asphalt cement, asphalt binder, cutback asphalt, and asphalt emulsion.
Particular attention is given to the plastic properties associated with the compaction phenomenon. Users are cautioned that this standard involves hazardous materials and operations, and it is the responsibility of the user to establish appropriate safety practices and determine the applicability of regulatory limitations prior to use.
🟢 Key Insight: The air-filled roller mode is considered a better mechanical analog of the interaction between a pneumatic tire and the pavement structure, as the gyratory angle varies according to the resistance encountered during the gyratory kneading process.
⚙️ Apparatus, Mold Specifications, and Modes of Operation
This test method covers two separate modes of operation of the Gyratory Testing Machine (GTM): (1) the oil-filled roller mode, and (2) the air-filled roller mode. The machine accommodates a variety of mold sizes to cater to different maximum aggregate particle sizes, as specified in the table below. The values stated in inch-pound units are regarded as the standard.
| 🟦 Mold Diameter | 📏 Max. Aggregate Size | ⚡ Max. Specimen Height |
|---|---|---|
| 4 in. (101.6 mm) | 1 in. (25.4 mm) | 8 in. (203.2 mm) |
| 6 in. (152.4 mm) | 1.5 in. (38.1 mm) | 10 in. (254.0 mm) |
| 8 in. (203.2 mm) | 2.0 in. (50.8 mm) | 12 in. (304.8 mm) |
The selection between oil-filled and air-filled rollers depends on the specific testing objectives. The air-filled mode is often preferred for simulating the dynamic load of traffic because the machine angle varies according to the resistance offered by the specimen.
🟡 Critical Procedure: The maximum permissible bitumen content is indicated directly by the first evidence of a progressive increase in shear strain (as indicated by a progressive increase in the gyratory angle) accompanied by a progressive reduction in shear strength (as indicated by a progressive reduction in roller pressure).
📊 Key Terminology and Measured Properties
Understanding the specific terminology in ASTM D3387 is essential for accurate testing and interpretation of results. These parameters directly characterize the shear resistance and plastic flow of the bituminous mixture during the compaction process.
| 📌 Term | 🎯 Definition |
|---|---|
| Gyratory Angle | A measure of the magnitude of the gyratory kneading action applied to the specimen during compaction. |
| Equilibrium Density | The density when a rate of densification of 1 lb/ft³ (16 kg/m³) per 100 revolutions of the GTM roller carriage is reached. This duplicates the ultimate in-place density. |
| Critical Roller Pressure (p’) | The roller pressure required for a Gyratory Shear Factor (GSF) equal to unity. That is, the roller pressure when shear stress (SG) equals maximum shear strength (τmax). |
By monitoring the gyratory angle and roller pressure during compaction, the operator can directly assess the development of plastic properties and identify the maximum permissible binder content for the mixture.
❓ Frequently Asked Questions
🔍 What is the primary difference between the oil-filled and air-filled roller modes in the GTM?
In the oil-filled roller mode, the gyratory angle is typically fixed. In the air-filled roller mode, the machine angle varies dynamically according to the resistance encountered during the