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Coarse Aggregate: Definition & Density

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  • 0:04 What Are Coarse Aggregates?
  • 0:52 Locating Material Sources
  • 1:39 Calculating Aggregate Density
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor: Nicole Dufalla
Coarse aggregates are components found in many areas of the construction industry. They have structural uses such as a base layer or drainage layer below pavements and in mixtures like asphalt and concrete. This lesson explores the various types of coarse aggregates.

What are Coarse Aggregates?

Do you know what makes up the concrete that we walk and drive on everyday? While the finished product is uniform and strong, concrete is made of many components, but is mostly made up of materials known as coarse aggregates. Coarse aggregates have a wide variety of construction applications because they resemble standard rock particles, as opposed to fine aggregate, which more closely resembles sand.

Coarse aggregates are an integral part of many construction applications, sometimes used on their own, such as a granular base placed under a slab or pavement, or as a component in a mixture, such as asphalt or concrete mixtures. Coarse aggregates are generally categorized as rock larger than a standard No. 4 sieve (3/16 inches) and less than 2 inches.

Locating Material Sources

Coarse aggregate is mined from rock quarries or dredged from river beds, therefore the size, shape, hardness, texture and many other properties can vary greatly based on location. Even materials coming from the same quarry or pit and type of stone can vary greatly.

Most generally, coarse aggregate can be characterized as either smooth or rounded (such as river gravel) or angular (such as crushed stone). Because of this variability, test methods exist to characterize the most relevant characteristics, since exact identification would be impossible. Several key characteristics that are frequently used to describe the behavior of coarse aggregates include relative density (or specific gravity), bulk density, and absorption.

Calculating Aggregate Density

The density of coarse aggregate is used in many applications, such as for calculating required amounts of aggregate for concrete or asphalt mix designs. Most frequently, the density of coarse aggregate is described using the specific gravity, otherwise known as the relative density, of the coarse aggregate. The relative density is a ratio that describes the density of the aggregate relative to the density of water.

A typical range for natural coarse aggregate is 2.4 to 2.9, meaning that the coarse aggregate is 2.4 to 2.9 times the density of water. Therefore, multiplying the specific gravity and the density of water will result in the density of the aggregate itself, in units of mass per unit volume. The specific gravity is measured by the standard testing procedure from the American Society for Testing and Materials (ASTM) C127: Standard Test Method for Relative Density (Specific Gravity) and Absorption of Coarse Aggregate.

In this test method, the absorption is also tested. Coarse aggregate absorption is the moisture content, or amount of water, in the coarse aggregate when the aggregate is fully saturated with a dry surface. This indicates the potential amount of water aggregate particles can absorb when used in construction or incorporated into mixtures and density measurements, and mix designs are adjusted based on this absorption capacity.

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