Apparent Specific Gravity
Coarse Aggregate Specific Gravity: An Essential Guide and Test Procedure
Overview
Coarse aggregate specific gravity, a pivotal test in the construction industry, measures the density of an aggregate sample relative to water. This test provides insights into the aggregate’s weight in various states and helps determine other crucial parameters. Moreover, its applications span across Superpave mix design and the identification of material properties.
Understanding Coarse Aggregate Specific Gravity (CASG)
What is it?
The CASG test calculates the specific gravity of a coarse aggregate sample by comparing its weight to the weight of an equal volume of water, thus illuminating the aggregate’s density and quality.
The test evaluates the aggregate’s weight under three distinct conditions:
Oven-dry (devoid of water).
Saturated surface-dry (pores filled with water).
Fully submerged underwater.
From these, one can determine the aggregate’s apparent, bulk, and bulk SSD specific gravities, as well as its absorption capability.
Why is it important?
The specific gravity of aggregates is vital for:
Understanding weight-to-volume relationships.
Calculating volume-related metrics like voids in mineral aggregate (VMA) and voids filled by asphalt (VFA).
Evaluating aggregate durability and predicting its asphalt binder absorption rate.
The standard test for CASG is dictated by AASHTO T 85 and ASTM C 127.
The Science Behind Specific Gravity and Absorption
Specific Gravity: Represents a material’s density compared to water's density at 73.4°F (23°C). By default, water at this temperature has a specific gravity of 1.
Absorption: Quantifies the water that an aggregate can retain within its pore structure. High absorption can indicate a non-durable aggregate and forecast the asphalt binder amount it might absorb.
Applications of Specific Gravity
Superpave Mix Design: Specific gravity helps in converting aggregate and asphalt binder weights into volumes.
Material Contamination Identification: Deviations in specific gravity can indicate impurities or contaminations.
Material Change Detection: Monitoring specific gravity can help detect shifts in material properties, hinting at changes in the source material.
Types of Aggregate Specific Gravity
Specific gravity types differ based on how the volume of the aggregate's water-permeable voids is addressed:
Apparent Specific Gravity (Gsa): Considers only the solid volume of the aggregate.
Bulk Specific Gravity (Gsb): Incorporates the void volume within the aggregate.
Bulk Saturated Surface Dry (SSD) Specific Gravity: Factors in the weight of water within the voids.
Effective Specific Gravity (Gse): Excludes voids permeable to asphalt.
Test Procedure
Note: This is a summary. For a full procedure, refer to AASHTO T 85 and ASTM C 127.
Preparation:
Acquire a sample of coarse aggregate retained on the No. 4 (4.75 mm) sieve, based on the nominal maximum aggregate size (NMAS).
Wash and oven-dry the sample at 230°F (110°C).
Soak the aggregate in room-temperature water for 15-19 hours.
Testing:
Dry the sample to a saturated surface dry (SSD) state.
Weigh the SSD sample underwater using a pre-conditioned basket.
Dry the sample again in an oven at 230°F (110°C) until a constant mass is achieved.
Let the aggregate cool at room temperature for 1-3 hours, then determine its mass.
Results:
Determine the aggregate's bulk, bulk SSD, apparent specific gravities, and absorption. Interpret the results in line with industry standards, keeping in mind that specific gravities can vary based on the aggregate type.
Conclusion
The Coarse Aggregate Specific Gravity test is vital for ensuring the quality and properties of aggregate used in the construction industry. Properly conducted, this test can ensure the durability, efficiency, and cost-effectiveness of construction materials and projects.