Relative density of aggregate, also known as specific gravity of aggregate, is a key property used in concrete mix design and quality control. It represents the ratio of the density of aggregate to the density of water. This value helps engineers understand the strength, quality, and suitability of aggregates for concrete and road construction works. Relative density directly affects concrete strength, durability, and mix proportions.
What Is Relative Density of Aggregate
Relative density of aggregate is defined as the ratio of the mass of aggregate to the mass of an equal volume of water. Since it is a ratio, it has no unit. Relative density indicates how heavy or light an aggregate is compared to water and is used to assess aggregate quality and performance in concrete.
Why Relative Density of Aggregate Is Important
Relative density plays an important role in concrete mix design calculations. It helps in determining volume proportions of aggregates and influences the strength and durability of concrete. Aggregates with higher relative density are generally stronger and more durable, while very low values may indicate porous or weak aggregates.
Importance of Relative Density
- Used in concrete mix design
- Indicates aggregate strength and quality
- Helps calculate yield and voids in concrete
- Affects workability and density of concrete
- Essential for quality control
Types of Relative Density of Aggregate
Relative density of aggregate is classified into different types based on moisture condition of aggregates.
Bulk Relative Density (Oven-Dry)
This is the ratio of oven-dry mass of aggregate to the volume of aggregate including permeable pores.
Bulk Relative Density (SSD)
SSD relative density is calculated when aggregate pores are fully saturated but surface is dry. This condition is commonly used in concrete mix design.
Apparent Relative Density
Apparent relative density considers only the solid material volume and excludes permeable pores. It gives a higher value compared to bulk relative density.
Relative Density of Aggregate Formula
The formula for calculating relative density depends on the test method and condition of aggregate.
General Formula
Relative Density = Mass of aggregate / Mass of equal volume of water
Bulk Relative Density (SSD)
Relative Density = Mass of SSD aggregate / (Mass of SSD aggregate − Mass in water)
This formula is widely used in laboratory testing.
Test for Relative Density of Aggregate
The relative density of aggregate is determined in the laboratory using standard test methods. For coarse aggregate, a wire basket or pycnometer is used. For fine aggregate, a pycnometer is commonly used.
Basic Test Steps
- Aggregate is washed and oven-dried
- Aggregate is soaked and brought to SSD condition
- Weights are measured in air and water
- Relative density is calculated using standard formula
Typical Values of Relative Density of Aggregate
The relative density of aggregate varies depending on type and source.
Common Relative Density Values
- Natural sand: 2.5 – 2.7
- Crushed stone: 2.6 – 2.8
- Lightweight aggregate: 1.2 – 2.0
Aggregates with relative density less than 2.4 are usually considered lightweight.
Factors Affecting Relative Density of Aggregate
Several factors influence relative density. These include mineral composition, porosity, moisture content, and degree of compaction. Aggregates with higher porosity tend to have lower relative density.
Relative Density of Aggregate in Concrete Mix Design
In concrete mix design, relative density is used to calculate the absolute volume of aggregates. Accurate relative density values ensure correct proportioning of materials, which directly affects strength, durability, and economy of concrete.
Difference Between Relative Density and Bulk Density
Relative density should not be confused with bulk density. Relative density is a ratio without units, while bulk density is mass per unit volume and is expressed in kg/m³. Relative density relates to material quality, whereas bulk density relates to packing and handling.
FAQs on Relative Density of Aggregate
What is the relative density of aggregate
Relative density of aggregate is the ratio of aggregate density to the density of water.
What is the normal value of relative density of aggregate
For most natural aggregates, it ranges between 2.5 and 2.8.
Why SSD condition is used
SSD condition represents the actual condition of aggregates in concrete mix design, making calculations more accurate.
Is higher relative density always better
Higher relative density generally indicates stronger aggregate, but other properties like shape and durability must also be considered.
Conclusion
Relative density of aggregate is a fundamental property that influences concrete mix design, strength, and durability. It helps engineers select suitable aggregates and calculate accurate mix proportions. By understanding and testing relative density properly, better quality control and long-lasting concrete structures can be achieved.
