CoreLok Gmb Calculator

For the calculation of Gmb (Bulk Specific Gravity) of compacted asphalt using the CoreLok method (AASHTO T 331).

Bulk Specific Gravity (Gmb)

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Intermediate Values

Chart visualizing mass and volume components.

What is the Calculation of Gmb using CoreLok?

The **calculation of Gmb using CoreLok** is a precise method for determining the Bulk Specific Gravity (Gmb) of a compacted asphalt mixture. This property is a critical indicator of pavement quality, as it’s used to calculate density and air voids, which directly influence the durability, strength, and resistance to water damage of an asphalt pavement. The CoreLok procedure, standardized as AASHTO T 331, is an alternative to traditional methods like AASHTO T 166 (Saturated Surface-Dry). It is particularly effective for specimens with high air voids or absorptive aggregates where water can infiltrate the sample during conventional testing, leading to inaccurate results.

The method involves placing a dry, weighed asphalt core into a special polymer bag, sealing it under vacuum, and then weighing it in both air and water. The vacuum sealing ensures the bag conforms tightly to the specimen’s surface, providing an accurate measurement of its volume without water absorption. This makes the **calculation of Gmb using CoreLok** a more reliable test for porous or coarse-graded mixtures.

Gmb using CoreLok Formula and Explanation

The formula for the **calculation of Gmb using CoreLok** is derived from Archimedes’ principle and corrects for the volume and mass of the vacuum bag. The accepted formula is:

Gmb = A / [ (E – F) – ( (E – A) / D_bag ) ]

This formula precisely determines the specimen’s volume by subtracting the bag’s volume from the total volume of the sealed package.

Variables Table

Variables used in the Gmb calculation.

Variable	Meaning	Unit	Typical Range
A	Mass of the dry sample in air	grams (g)	1500 – 5000 g
E	Mass of the vacuum-sealed sample in air	grams (g)	Slightly > A
F	Mass of the vacuum-sealed sample in water	grams (g)	~50-60% of E
D_bag	Specific gravity of the plastic bag film	Unitless	~0.922

Practical Examples

Example 1: Standard Hot Mix Asphalt (HMA) Core

Consider a standard laboratory-compacted HMA sample.

• Inputs:
  • Mass of Dry Sample (A): 2150.5 g
  • Mass of Sealed Sample in Air (E): 2162.0 g
  • Mass of Sealed Sample in Water (F): 1235.0 g
  • Specific Gravity of Bag (D_bag): 0.922
• Calculation:
  • Bag Mass = 2162.0 – 2150.5 = 11.5 g
  • Bag Volume = 11.5 / 0.922 = 12.47 cm³
  • Sealed Volume = 2162.0 – 1235.0 = 927.0 cm³
  • Sample Volume = 927.0 – 12.47 = 914.53 cm³
  • Gmb = 2150.5 / 914.53 = 2.352
• Result: The Bulk Specific Gravity (Gmb) is 2.352.

Example 2: Open-Graded Friction Course (OGFC)

OGFC mixtures are highly porous, making the CoreLok method ideal.

• Inputs:
  • Mass of Dry Sample (A): 1850.0 g
  • Mass of Sealed Sample in Air (E): 1863.5 g
  • Mass of Sealed Sample in Water (F): 980.0 g
  • Specific Gravity of Bag (D_bag): 0.922
• Calculation:
  • Bag Mass = 1863.5 – 1850.0 = 13.5 g
  • Bag Volume = 13.5 / 0.922 = 14.64 cm³
  • Sealed Volume = 1863.5 – 980.0 = 883.5 cm³
  • Sample Volume = 883.5 – 14.64 = 868.86 cm³
  • Gmb = 1850.0 / 868.86 = 2.129
• Result: The Bulk Specific Gravity (Gmb) is 2.129. For more information, see our guide on {related_keywords}.

How to Use This CoreLok Gmb Calculator

Follow these steps for an accurate **calculation of Gmb using CoreLok**:

1. Prepare the Specimen: Ensure your asphalt sample is clean, dry, and at room temperature. It should be dried to a constant mass.
2. Enter Mass of Dry Sample (A): Weigh the dry specimen on a calibrated scale and enter the value in the first input field.
3. Seal and Weigh: Place the specimen in a CoreLok bag, seal it using the vacuum device, and weigh the sealed package in air. Enter this value as Mass of Sealed Sample in Air (E).
4. Submerged Weight: Weigh the sealed package while fully submerged in a water bath. Enter this as Mass of Sealed Sample in Water (F).
5. Check Bag SG: Confirm the specific gravity of the bag film (D_bag). The default of 0.922 is standard, but check your bag’s specifications.
6. Interpret Results: The calculator instantly provides the Gmb. The intermediate values help verify the calculation steps, such as sample and bag volume.

Key Factors That Affect Gmb Calculation

1. Aggregate Properties: The specific gravity, absorption, and texture of the aggregates are the primary drivers of the final Gmb.
2. Asphalt Binder Content: The amount of asphalt binder fills voids between aggregates, increasing the overall density and Gmb.
3. Air Voids: The level of compaction determines the percentage of air voids in the mixture. Higher compaction leads to lower air voids and a higher Gmb.
4. Measurement Accuracy: Precise and calibrated scales are essential. Small errors in mass measurements can significantly affect the final **calculation of Gmb using CoreLok**.
5. Bag Sealing Integrity: A perfect vacuum seal is critical. Any air leaks will cause the bag to not conform to the sample, leading to an inaccurate volume measurement.
6. Water Temperature: The density of water changes with temperature. While the formula assumes a standard density, tests should be conducted at a consistent temperature (e.g., 25°C). You can learn about advanced mixture designs in our article about {related_keywords}.

Frequently Asked Questions (FAQ)

1. Why is the Gmb important for asphalt pavements?

Gmb is a fundamental property used to calculate percent compaction and air voids. These values are critical for ensuring a pavement will resist deformation, fatigue, and moisture damage over its design life. A correct **calculation of Gmb using CoreLok** is step one.

2. When should I use the CoreLok method instead of SSD (AASHTO T 166)?

The CoreLok method is required for specimens that absorb more than 2.0% water by volume. It is highly recommended for coarse-graded mixtures like SMA (Stone Matrix Asphalt) or OGFC, as the SSD method can be inaccurate due to water draining from large surface voids during weighing.

3. What is a typical Gmb value?

Typical Gmb values for dense-graded hot mix asphalt range from 2.200 to 2.500. Lower values are expected for more porous mixtures.

4. What happens if the bag is not sealed properly?

If the bag has a leak or does not fully conform to the specimen, the measured volume will be incorrect (usually larger than the true volume), resulting in a calculated Gmb that is artificially low.

5. Can I reuse the CoreLok bags?

No, CoreLok bags are single-use only. They are cut open to remove the specimen after the submerged weight is taken.

6. Where does the Specific Gravity of the Bag (0.922) come from?

This value is determined by the bag manufacturer and is a physical property of the polymer film used. It is essential for correcting the volume calculation.

7. How does this relate to Maximum Specific Gravity (Gmm)?

Gmb is the bulk specific gravity of the *compacted* mixture, including air voids. Gmm (Theoretical Maximum Specific Gravity) is the specific gravity of the *loose* mixture with no air voids. The difference between Gmm and Gmb is used to calculate the air void percentage: % Voids = 100 * (1 – Gmb / Gmm). Understanding {related_keywords} is key to mastering pavement materials.

8. Does the water bath temperature matter?

Yes, specific gravity is a ratio of densities, and water density changes with temperature. AASHTO T 331 specifies a water bath temperature of 25 ± 1°C (77 ± 2°F) for consistency.