Specific Gravity/Relative density of Coarse Aggregates- ASTM Standards

Neenu أغسطس 30, 2021

The specific gravity of an aggregate is defined as the mass of the aggregate in air divided by the mass of an equal volume of water. Its value ranges between 2.5 to 3.0.  For example, an aggregate with a specific gravity of 2.50 implies it is two and one-half times heavier than water.


The specific gravity of coarse aggregates is an important parameter that affects the weight and volume of the concrete mixtures, along with its workability, strength, and durability. Coarse aggregates can exist in various moisture content levels based on which their specific gravity values alter.

Moisture Content of Coarse Aggregates

Each construction aggregate is made out of some solid matter and voids, that may or may not contain water. As aggregate mass varies with the moisture content, we determine the specific gravity of aggregate at a fixed moisture content.

There are four moisture contents defined for aggregates, which is dependent on the amount of water that is held within the pores on the surface of the particles.

  1. Damp/Wet
  2. Saturated surface-dry
  3. Air-dry
  4. Oven-dry
Aggregates with pores connected to the surface are filled with water and free water on the surface comes under damp/wet conditions. Saturated surface dry aggregates have pores that are connected to the surface filled with water and no free water on the surface. Air-dry aggregates have a dry surface with some water in the pores. Oven-dry aggregates have no water on the pores.

The volume of the aggregate particle is considered the volume of the solid matter and internal pores. Based on whether choose an oven-dry or saturated surface dry mass, the specific gravity values vary. 

Most normal-weight aggregates have a bulk specific gravity SSD between 2.4 and 2.9.


Determination of Specific Gravity of Coarse Aggregates

The test method to determine the specific gravity of aggregates is explained in ASTM C 127, "Specific Gravity and Absorption of Coarse Aggregate".

Test Apparatus

✅Balance
The balance shall be equipped with suitable apparatus for suspending the sample container in water from the center of the platform or pan of the balance.

✅Sample Container
A wire basket of 3.35mm [No:6] fine mesh, or a bucket with a capacity of 4 to 7 l [ length equal to breadth]. It must take aggregates with a size of 37.5 mm (1.5 inches) or smaller.

Wire Basket for Testing Specific Gravity of Coarse Aggregate
Wire Basket for Testing Specific Gravity of Coarse Aggregate

Larger aggregates may require larger containers. The container must prevent air trapping when submerged in water.

✅Water Tank
A watertight tank into which the sample container is placed while suspended below the balance.

✅Sieves
Sieves with a size 4.75 mm or larger  (No:4) are required.

✅Oven
An oven of sufficient size, capable of maintaining a uniform temperature of 110 6 5 °C (230 6 9 °F).

Sampling and Resizing Coarse Aggregates

  • The aggregates are sampled as per Standard Practice for sampling Aggregates, D 75
  • The sampling and resizing of coarse aggregates are performed as per ASTM C702 by employing Method A or Method B as recommended by the standards. Read More on this Here: How to Sample Coarse According to ASTM Standards?.
  • After this, remove all the particles passing through the 4.75 mm sieve by performing dry sieving, and thorough washing.
  • If the sample contains a substantial amount of particles smaller than 4.75 mm, use a 2.36 mm sieve (No.8) in place of 4.75 mm (No.4).
  • If aggregates smaller than 4.75 mm (No. 4) are used in the sample, check to ensure that the size of the openings in the sample container is smaller than the minimum size aggregate.

Procedure

  1. The test sample is dried in an oven to a constant mass at a temperature of 110 ± 5 °C and cool in air. Cool for 1 to 3 hours for test samples of 37.5 mm nominal size and for a longer period for larger sizes until the aggregate has cooled to a temperature that is comfortable to handle.
  2. Immerse the aggregate in water at room temperature for a period of 24 ± 4h.
  3. The sample is then removed from the water and dried with a large absorption cloth until visible films of water are removed. Large particles are wiped separately. This is a surface drying operation and avoids evaporation of air from the aggregates during the drying operation. 

    Drying Coarse Aggregates Using Cloth - SSD Determination

  4. Determination of Mass of Aggregate in Air (B): Determine the mass of the test sample in the saturated surface-dry condition. Record this and all subsequent masses to the nearest 0.5 g or 0.05 % of the sample mass, whichever is greater.
  5. Determination of Mass of Aggregate in Water(C): Now determine the saturated-surface-dry test sample in the sample container and determine its apparent mass in water at 23± 2.0 °C. Always take care to remove all entrapped air before determining its mass by skating the contained while immersing in the water. The container should be immersed to a depth sufficient to cover it and the test sample while determining the apparent mass in water. Wire suspending the container should be of the smallest practical size to minimize any possible effects of a variable immersed length. Before adding aggregates into a wire bucket immersed in water, the whole arrangement with the wire basket inside the water weight must be made to zero reading.
    6. Specific Gravity Frame For Coarse Aggregates

  6. Determination of Mass of Water Displaced: The difference between the mass in air and the mass when the sample is submerged in water equals the mass of water displaced by the sample.
  7. Determination of Mass of Oven-dry Sample (A): Dry the test sample in the oven to constant mass at a temperature of 110 6 5 °C, cool in the air at room temperature for 1 to 3 h, or until the aggregate has cooled to a temperature that is comfortable to handle (approximately 50 °C), and determine the mass.
💡NOTE
The apparent mass of the sample in water is less than that in air. The loss in mass is equal to the mass of the water displaced. Hence, the loss in mass is the mass of a volume of water equal to the aggregate volume lost.


Calculations

1. Bulk Specific Gravity or Specific Gravity ( Oven Dried Sample) Sd

Calculate the relative density (specific gravity) on the basis of oven-dry aggregate as follows: 

Bulk Specific gravity or relative density = A / (B-C)

2. Bulk specific gravity or relative density (SSD)

Ss = B/(B-C)

3. Apparent Specific Gravity or Apparent Relative Density (Sa)

Apparent specific gravity (Sa) = A/(A-C)

3. Percentage Absorption (A)

Absorption % = [(B-A)/A] x 100;

4. Relation between Sa, Ss. Sd and A



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