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D6373-23 – Standard Test Method Technical Guide
🛣️ Performance Grade (PG) Classification and Scope
ASTM D6373-23 provides a comprehensive specification for asphalt binders graded by performance. Grading designations are explicitly tied to the LTPPBind Online calculated maximum pavement design temperature and the minimum pavement design temperature. This standard contains two critical specification tables: Table 1 (the default) and Table 2. Table 2 incorporates Practice D6816 for determining the critical low cracking temperature using a combination of Test Method D6648 (Bending Beam Rheometer) and Test Method D6723 (Direct Tension) test procedures.
As noted in the standard, values are stated in SI units. This standard supersedes traditional grading methods such as penetration grading (D946/D946M) and viscosity grading (D3381/D3381M) by directly linking binder properties to climatic pavement conditions. Non-mandatory guidance for determining the performance grade is provided by AASHTO R 29.
📌 Key Reference: The standard leverages several key ASTM test methods, including D7175 (DSR), D6648 (BBR), D6723 (DT), D2872 (RTFOT), and D6521 (PAV), covering the full spectrum of binder aging and rheological testing.
⚙️ Key Test Protocols and Conditioning
Binder performance is evaluated across three distinct aging states. The Original Binder is tested for flash point (D92), viscosity (D4402/D4402M), and dynamic shear rheology (D7175). Short-term aging is simulated using the Rolling Thin-Film Oven Test (RTFOT, D2872), after which high-temperature DSR properties are verified to ensure rutting resistance. Long-term aging is simulated using the Pressurized Aging Vessel (PAV, D6521). On the PAV residue, intermediate-temperature DSR testing is performed to evaluate fatigue cracking, and low-temperature testing is performed using the Bending Beam Rheometer (BBR, D6648). When required by Table 2, the Direct Tension test (D6723) is used in conjunction with the BBR to determine the critical low cracking temperature.
⚠️ Important Notice on Table 2 Testing: Test Method D6723 (Direct Tension) and Practice D6816 were officially withdrawn by ASTM in 2021. However, their specific procedures remain integral to the low-temperature grading requirements of Table 2 in this specification. Laboratory personnel must be versed in the historical methodology as described in these withdrawn standards to maintain full compliance.
📊 Key Measured Properties and Grading Tables
The following tables summarize the critical physical property requirements for a typical Performance Grade binder at high and low service temperatures, based on the standard’s specification limits.
| 🟦 Property | 📏 Test Method | 🎯 PG 64 (High Temp) | ⚡ Age Condition |
|---|---|---|---|
| Flash Point | D92 | ≥ 230 °C | Original Binder |
| Viscosity @ 135°C | D4402/D4402M | ≤ 3 Pa·s | Original Binder |
| G*/sin δ (Min) | D7175 (DSR) | 1.00 kPa @ 64°C | Original Binder |
| G*/sin δ (Min) | D7175 (DSR) | 2.20 kPa @ 64°C | RTFOT Residue |
| G* sin δ (Max) | D7175 (DSR) | 5000 kPa @ 25°C | PAV Residue |
| ❄️ Low Temp Property | 📐 Test Method | 🎯 PG -22 (Low Temp) | ⚡ Condition |
|---|---|---|---|
| Creep Stiffness, S (60s) | D6648 (BBR) | ≤ 300 MPa @ -12°C | PAV Residue |
| m-value (60s) | D6648 (BBR) | ≥ 0.300 @ -12°C | PAV Residue |
| Failure Strain (Table 2) | D6723 (DT) | ≥ 1.0% @ -12°C | PAV Residue |
💡 LTPPBind Online Correlation: The specific high and low temperature grades (e.g., PG 64-22, PG 70-28) required for a given project are directly calculated from the climatic database in LTPPBind Online, managed by the FHWA. This ensures the binder specification is tailored to the exact pavement environment.
❓ Frequently Asked Questions
🔍 What is the primary difference between Table 1 and Table 2 in D6373-23?
Table 1 provides standard SHRP specification requirements where the low-temperature grade is determined solely by fixed temperature increments (e.g., -22°C,