Measuring Density in Aggregate and Why it Matters
The density of aggregate refers to the measure of mass per unit volume of granular material such as crushed stone, sand, or gravel. Density is a crucial consideration in the construction industry when calculating the amount needed for particular projects and determining the strength and durability of the mix. Yet there are many measures of density.
Two density calculations are particularly helpful to construction professionals as they consider the properties of certain types of aggregates and how these will influence their specific project: bulk density and relative density. Each calculates a distinct though related density measurement and both are useful in specific applications.
Density Measurements for Aggregate
There are two main measurements for calculating the density of aggregates for building professionals: bulk density and relative density.
1. Bulk Density
Bulk density, also known as apparent density, calculates the mass of the aggregate material per unit volume. The bulk density measurement includes the particles and also the empty spaces between them. This measurement is expressed in kilograms per cubic meter (kg/m³) or pounds per cubic foot (lb/ft³).
Builders use the bulk density measurement to calculate how much aggregate material they need for a given job such as mixing concrete or asphalt or filling void spaces. The bulk density of a particular aggregate gives construction professionals insight into its workability, strength, and durability.
2. Relative Density
The relative density, or specific gravity of aggregate material measures the ratio of the density of the material compared with the density of water of a certain temperature and pressure. The measurement calculates how much volume the material will displace when placed into water. The calculation of relative density is expressed as a number without units.
Water has a relative density of 1. Aggregates have a higher relative density than water. The range of aggregate relative densities is typically between 2.0-2.9. Industry professionals test aggregates for relative density using either an oven dry basis or a saturated surface dry basis depending on the application for which the aggregate is being used.
Relative density measurements are helpful to consider both the density of a substance and its porosity. More porous aggregates have more void spaces that the water will fill. These aggregates take up less space in the water than less porous aggregates. Absorption is a crucial consideration in aggregates when they are used in concrete mixes. Aggregates with high absorption rates can impact the workability, strength, and durability of the mix because of their increased need for water.
Projects that Require Density Measurements
Building professionals use both relative and bulk density measurements in various circumstances and projects, depending on the specific applications.
- Concrete mixes – Bulk density and relative density calculations are crucial for mixing concrete. Builders use the bulk density of aggregate types to determine the quantity of aggregate, sand, and cement they need for desired strength and workability of concrete mixes. Relative density measurements help builders to assess the porosity and compactness of the aggregates, which impacts the water ratio in the mix.
- Asphalt mixes – Bulk density is the most important measurement for aggregates in asphalt to predict its strength and longevity. Asphalt mixes combine aggregates with bitumen. Water is not a component of the asphalt mix, so a relative density measurement is not as useful as a bulk density measurement.
- Load-bearing capacity – The bulk density of particular aggregates impacts the load-bearing capacity of soil layers in geotechnical engineering. The bulk density calculation helps to assess the compaction and stability of the soil for various projects.
- Aggregate quality assessments – Relative density is useful in evaluating the quality of a particular aggregate material for certain jobs. Aggregates that are less porous and have a higher density compared to water are better for heavy construction projects like load-bearing or structural designs. Lightweight aggregates are typically more porous but work well for designs where there are strict weight requirements.
Relative and Bulk Density Ranges for Types of Aggregates
| Aggregate Type | Particle Size | Relative Density | Bulk Density |
| Fine Aggregate | ≤ 4.75mm | 2.4-2.9 | 1,400 to 1,800 kg/m³ (87 to 112 lb/ft³) |
| Medium Aggregate | 4.75mm-19mm | 2.4-2.9 | 1,400 to 1,800 kg/m³ (87 to 112 lb/ft³) |
| Coarse Aggregate | 4.75mm-37.5mm | 2.4-2.9 | 1,350 to 1,650 kg/m³ (84 to 103 lb/ft³) |
| Lightweight Aggregate | 4.75mm-37.5mm | 0.9-2.2 | 800 to 1,400 kg/m³ (50 to 87 lb/ft³) |
| Heavyweight Aggregate | 4.75mm-37.5mm | 2.5-4.0 | 2,500 to 4,000 kg/m³ (156 to 250 lb/ft³) |
Optimizing the Density of Aggregate Mixes
Optimizing the mix of a group of aggregates is a way to achieve the highest bulk density while still maintaining the desired performance characteristics. When manufacturers do this, they allow builders to improve material efficiency, enhance strength and durability, and decrease the permeability of the mix to outside elements like water and other substances.
- Well-graded aggregate mix – Selecting a well-graded aggregate mix that contains a variety of particle sizes is vital in reducing void spaces to increase the density of the mix.
- Proper moisture content – Controlling the moisture in the aggregate mix allows builders to increase the density of the aggregates as excessive moisture leads to less density by unit volume.
- Aggregate shape and texture – Angular or rough-textured granules adhere to one another and improve overall compactness better than round and smooth-textured particles.
- Aggregate material type – Certain aggregate types are more dense than others and are desirable for strength and durability, though lightweight aggregates are useful in specific applications.
- Vibration and compacting – Applying vibration or mechanical compaction helps reduce the air voids in an aggregate mix and increases the bulk density.