Theory:
For design of concrete mix, information about the specific gravity and water
absorption of the coarse aggregates are required. Specific gravity of aggregate provides valuable information on its quality and properties. If the specific gravity is above or below than the value normally assigned to a particular type of aggregate; it may indicate that shape
and grading of aggregate has altered. It is also important in determination of moisture contact and in many concrete mix design calculations. It is also required for the calculation of volume yield of concrete.
Objective:
To determine specific gravity and water absorption of coarse aggregate.
Reference: IS: 2386 (Part-3)-1963.
Apparatus: Balance of capacity 5kg weight, box wire basket 200mm in diameter, water container for immersing the wire basket, absorbent cloth for surface drying of the sample and thermos static drying oven.
Material:
coarse aggregates (5 Kg),
Procedure:
1. A sample of not less than 2 Kg of the aggregate shall be thoroughly washed to
remove finer particles and dust, drained and then placed in the wire basket and
immersed in distilled water at a temperature between 22°C to 32°C with a cover of at least 5 cm of water above the top of the basket.
2. Immediately after immersion the entrapped air shall be removed from the sample by lifting the basket containing it 25 mm above the base of the tank and allowing it to drop 25 times at the rate of about one drop per second.
3. The basket and aggregate shall remain completely immersed during the operation and for a period of 24 ± l/2 hours afterwards.
4. The basket and the sample shall then be jolted and weighed in water at a temperature of 22°C to 32°C (weight A).
5. The basket and the aggregate shall then be removed from the water and allowed
to drain for a few minutes, after which the, aggregate shall be gently emptied from the basket on to one of the dry clothes, and the empty basket shall be returned to the water and weighed in water ( weight B).
6. The aggregate placed on the dry cloth shall be gently surface dried with the cloth, transferring it to the second dry cloth when the first will remove no further
moisture. The aggregate shall then be weighed (weight C).
7. The aggregate shall then be placed in the oven in the shallow tray, at a temperature of 100 to 110°C and maintained at this temperature for 24 ± l/2 hours. It shall then be removed from the oven, cooled in the airtight container
and weighed (weight D).
8. The specific gravity, apparent specific gravity and water &sorption shall be
calculated as.
Specific gravity = {D⁄[A − (A − B)]}...(1)
Apparent Specific gravity = [D/(D − (A − A))]...(2)
Water absorption (in %) = 100 × [(C − D)/D...(3)
Where,
A = Weight in g of aggregate and basket in water
B = Weight in g of empty basket in water
C = Weight in g of the saturated surface - dry aggregate in air
D = Weight in g of oven dried aggregate in air.
Observation:
Results and discussions:
Following results are obtained for the provided coarse aggregate specimen:
a) Specific gravity: __________.
b) Apparent specific gravity: __________.
c) Water absorption : __________ %.
Precautions:
(Discuss about the precautions to be taken while conducting this experiment)
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Questions:
1. What is the relationship between specific gravity of aggregate and the strength?
2. What are the other methods available for determination of specific gravity and water absorption of coarse aggregate?
