Theory:
Specific gravity of an aggregate is defined as the ratio of the mass of solid in a given volume of sample to the mass of equal volume of water at 4ππ πΆπΆ. However, all rocks contain some small amount of void and the apparent specific gravity includes this voids. The specific gravity of aggregates is an indirect measure of material’s density and its quality. A low specific gravity may indicate high porosity and therefore poor durability and low strength.
Some of the pores contained by aggregates are permeable while others are impermeable.
Accordingly, two types of specific gravities are defined absolute specific gravity and
apparent specific gravity. If both the permeable and impermeable voids are excluded to determine the true volume of solids, the specific gravity is called true or absolute specific gravity of the aggregate. But true specific gravity has not much of practical use as volume of impermeable internal pores is too difficult to determine.
In contrast, for the determination of apparent specific gravity the impermeable internal pore is added to the effective volume of the aggregates (does not include the permeable
pores).
Mathematically:
Apparent Specific Gravity = mass of aggregate mass of water occupying the volume equal to that of solids of aggragate excluding permeable pores
The apparent specific gravity is realistic one to use for concrete mix proportioning. The apparent specific gravity of most rocks lie between 2.6 to 2.7. Apparent specific gravity can be determined on the basis of surface dry condition (SSD) or oven dry condition (OD), according to the moisture condition of the aggregate.
In saturated surface dry (SSD) situation, the pores of the aggregate are fully filled with water and the surface is dry. This condition can be obtained by immersing coarse aggregate in water for 24 h followed by drying of the surface with a wet cloth. When the aggregate is under the SSD condition, it will neither absorb water nor give out water during the mixing process. Hence, it is a balanced condition and is used as the standard index for concrete mix design.
In contrast, the oven dry condition is obtained by keeping the aggregate in an oven at a temperature of 110o πΆπΆ for 24 hrs. Due to heating, all the water from internal permeable pores gets evaporated and hence it reaches a constant weight. When the aggregate is under OD condition, it will absorb water during the concrete mixing process until the internal pores are fully filled with water.
Similarly, water absorption is also provides an idea about strength of aggregate.
Aggregates having more water absorption are more porous in nature and generally considered unsuitable. Usually, water absorption of coarse aggregate is about 0.5% by weight whereas water absorption of fine aggregate is about 2.0% by weight. Moreover, water absorption values are used to calculate the change in the weight of the aggregate while proportioning and mixing of concrete. Extra water is added to cater the need of water absorption.
Objective:
To determine specific gravity and water absorption of fine aggregate.
Reference:
IS: 2386 (Part-3)-1963.
Apparatus:
Pycnometer, 1000-ml measuring cylinder, thermostatically controlled oven,
taping rod, filter papers and funnel.
Material:
Fine aggregates (500 g)
Procedure:
1. Place 500 g of fine aggregate in a tray and cover it with distilled water at a
temperature of 22 to 32°C. Remove air entrapped in or bubbles on the surface of
the aggregate by gentle agitation with a rod. Keep the sample immersed under
water for 24 Hrs.
2. Carefully drain the water from the sample, by decantation through a filter paper. Air dry the aggregate and solid matter retained on the filter paper, to remove the surface moisture. When the material just attains a “free-running” condition, weight the saturated and surface-dry sample (A).
3. Place the aggregate in the pycnometer and fill the remaining space by distilled
water. Eliminate entrapped air by rotating the pycnometer on its side, covering
the hole in the apex of the cone with a finger. Weight the pycnometer with this
condition (B).
4. Empty the contents of the pycnometer into a tray. Ensure that all the aggregate is
transferred. Refill the pycnometer with distilled water to the same level as before
and measure the weight at this condition (C).
5. Carefully drain the water from the sample, by decantation through a filter paper. Oven-dry the aggregate in the tray at a temperature of 100 to 110o
C for 24 hrs. During this period, stir the specimen occasionally to facilitate proper drying. Cool the aggregates calculate its weight (D).
6. Calculate the specific gravity, apparent specific gravity and the water absorption
as follows:
Specific gravity = {D/[A − (B− )]}.....(1) Apparent Specific gravity = {D/[D − (B − C)]}.....(2)
Water absorption (in %) = 100 × [(A − D)/D]......(3)
Where, A = Weight in g of saturated surface-dry sample
B = Weight in g of pycnometer containing sample and filled with distilled water
C = Weight in g of pycnometer filled with distilled water only
D = Weight in g of oven dried sample only.
Observation:
Results and discussions:
Following results are obtained for the provided fine 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 purpose of conducting water absorption test?
2. What are the limitations in determination of specific gravity using pycnometer?
3. Define unit weight, bulk density?
4. Provide proofs of equations 1, 2 and 3.
