WATER ABSORPTION



SPECIFIC GRAVITY AND WATER ABSORPTION OF AGGEGATES 

IS: 2386 - Part 3

1. Introduction The specific gravity of a solid substance is the ratio of the weight of a given volume of material to the weight of an equal volume of water (at 20°C). In effect, it tells how much heavier (or lighter) the material is than water. A change in aggregate mineral or physical properties can result in a change in specific gravity.

Aggregate specific gravity is needed to determine weight-to-volume relationships in the design of concrete and bituminous mixes.  In bituminous mix design specific gravity is needed to calculate various volume-related quantities such as voids in mineral aggregate (VMA), and voids filled by bitumen (VFB). Absorption is an indicator of aggregate durability as well as the volume of the bituminous binder it is likely to absorb. It shall be noted that water absorption test results will not be consistent in the case of highly porous aggregates.

Based on the size of aggregates, three main methods are specified for determining specific gravity and water absorption are included.

(i)  Method I - The size of the aggregate is large than 10 mm.

(ii) Method I or II - The size of aggregates is between 40 mm and 10 mm.

(iii) Method III - The size of the aggregate is smaller than 10 mm. 

2. Method I – Aggregate larger than 10 mm

2.1 Apparatus 

(i) A balance or scale of capacity not less than 3 kg, readable and accurate to 0.5 g suitable to suspend the basket containing the aggregate sample to be suspended for weighing in water.

(ii) A well ventilated oven, thermostatically controlled, to maintain a temperature of 100 to 110°C.

(iii) A wire basket of not more than 6.3 mm mesh or a perforated container of convenient size, preferably chromium plated and polished, with wire hangers not thicker than 1 mm for suspending it from the balance.

(iv) A stout watertight container in which the basket may be freely suspended

(v) Two dry soft absorbent cloths each not less than 75 cm x 45 cm.

(vi) A shallow tray of area not less than 650 cm2.\

(vii) An airtight container of capacity similar to that of the basket.

2.2 Procedure

a) A sample of aggregates not less than 2000 gm is taken for the test. Two tests shall be made and it is recommended that the two samples should not be tested concurrently. Aggregates, artificially heated, are not normally tested.

b) The test sample is washed thoroughly, drained and then placed in the wire basket.

c) The basket with sample is immersed in distilled water in the container at a temperature between 22°C and 32°C with a cover of at least 5 cm of water above the top of the basket

d) Remove the entrapped air from the sample by lifting the basket 25 mm from the base of the container and allowing it to drop 25 times at the rate of about one drop per second.

e) The basket and aggregate is kept completely immersed during the operation and for a period of 24±½ hours afterwards.

f) The basket and the sample are then jolted and weighed (Weight A1) in water at a temperature of 22 to 32°C.

g) If it is necessary to transfer the basket with aggregates to a different tank for weighing, it is jolted 25 times in the new tank before weighing (Weight A1).

h) The basket with the aggregates is removed from the water and allowed to drain for a few minutes.

i) Thereafter, empty the aggregates from the basket on to one of the dry clothes, and the empty basket returned to the water, jolted 25 times and weighed in water (Weight A2).

j) The aggregate placed on the dry cloth is gently surface dried with the cloth. Transfer it to the second dry cloth when the first will remove no further moisture. It shall then be spread out not more than one stone deep on the second cloth, and air dried keeping away from direct sunlight or any other source of heat for not less than 10 minutes.

k)  After the aggregates are completely surface dry (which with some aggregates may take an hour or more) it shall then be weighed (Weight B)

 l) During the period of air drying, the aggregates must be turned over at least once and a gentle current of unheated air may be used after the first ten minutes to accelerate the drying of difficult aggregates.

m) The aggregate is then oven dried in the shallow tray, at a temperature of 100 to 110°C for 24±½ hours.

n) After 24 hours, the sample is removed from the oven, cooled in the airtight container and weighed (Weight C). 

2.3 Calculation

Specific gravity (Bulk), apparent specific gravity and water absorption are calculated as follows:

Specific gravity (Bulk) = C/ (B-A)

Apparent specific gravity = C/ (C-A)

Water absorption (% of dry weight) = 100(B-C)/ C where

A= Weight of the saturated aggregate in water (A1-A2), gm.

B = Weight of the saturated surface-dry aggregate in air, gm.

C = Weight of oven-dried aggregate in air, gm

2.4  Reporting of results

The individual and mean of the two results are reported to three decimal places. The size of material tested, and if artificial heating has been done shall also be stated.

3. Method II – Aggregate between 40 mm and 10 mm

3.1 Apparatus

(i) A balance or scale of capacity not less than 3 kg, readable and accurate to 0.5 g suitable to suspend the basket containing the aggregate sample to be suspended for weighing in water.

(ii) A well ventilated oven, thermostatically controlled, to maintain a temperature of 100 to 110°C.  

(iii) A wide mouthed glass jar of about 1.5 litres capacity with flat round lip and a plane disc of ground glass plate to close the jar water tight.

(iv) Two dry soft absorbent cloths each not less than 75 cm x 45 cm. (v) A shallow tray of area not less than 650 cm2. (vi) An airtight container of capacity large enough to hold the sample. 

3.2 Procedure

a) A sample of aggregates about 1000 gm is taken for the test. Two tests shall be made and it is recommended that the two samples should not be tested concurrently. Aggregates, artificially heated, are not normally tested.

b) The test sample screened on a 10 mm sieve is washed thoroughly, drained and then immersed in distilled water in the glass jar

c) The sample is kept immersed in distilled water in the jar at a temperature between 22°C and 32°C.

d) Immediately after immersion, remove the entrapped air from the sample by hand rotation of the jar in clockwise and anti-clockwise directions. Repeat this after the end of soaking period.

e) The sample in the jar is kept completely immersed during the operation and for a period of 24±1/2 hours afterwards.

f) The jar is overfilled with distilled water and the plane ground glass is placed over the lip of the jar, ensuring that no air is entrapped in the jar. The jar is dried outside and the weight taken (Weight A).

g) Thereafter, drain the jar and transfer the aggregates on to one of the dry clothes. Fill the jar with water and weigh (Weight B).

h) The aggregate placed on the dry cloth is gently surface dried with the cloth. Transfer it to the second dry cloth when the first will remove no further moisture. It shall then be spread out not more than one stone deep on the second cloth, and air dried keeping away from direct sunlight or any other source of heat for not less than 10 minutes

i) After the aggregates are completely surface dry (which with some aggregates may take an hour or more) it shall then be weighed (Weight C).

j) During the period of air drying, the aggregates must be turned over at least once and a gentle current of unheated air may be used after the first ten minutes to accelerate the drying of difficult aggregates.

k) The aggregate is then oven dried in the shallow tray, at a temperature of 100 to 110°C for 24±½ hours.

l) After 24 hours, the sample is removed from the oven, cooled in the airtight container and weighed (Weight D).

3.3 Calculation

Specific gravity (Bulk), apparent specific gravity and water absorption are calculated as follows:

Specific gravity (Bulk) = D/ [C - (A-B)]

Apparent specific gravity = D/ [D - (A-B)]

Water absorption (% of dry weight) = 100(C-D)/ D where

A= Weight of the jar and aggregate filled with water, gm.

B= Weight of jar filled with distilled water, gm.

C = Weight of the saturated surface-dry aggregate in air, gm.

D = Weight of oven-dried aggregate in air, gm. 

3.4  Reporting of results

The individual and mean of the two results are reported to three decimal places. The size of material tested, and if artificial heating has been done shall also be stated.

4. Method II – Aggregate smaller than 10 mm

4.1 Apparatus

(i) A balance or scale of capacity not less than 3 kg, readable and accurate to 0.5 g suitable to suspend the basket containing the aggregate sample to be suspended for weighing in water.

(ii) A well ventilated oven, thermostatically controlled, to maintain a temperature of 100 to 110°C.

(iii) A glass pycnometer of about 1litre capacity with a metal conical screw top with a 6 mm diameter hole at the apex or a wide mouthed glass jar of about 1.5 litres capacity with flat round lip and a plane disc of ground glass plate to close the jar water tight.

(iv) A hair drier or so for supplying a current of warm air.

(v) A shallow tray of area not less than 325 cm2. (vi) An airtight container of capacity large enough to take the sample.

4.2 Procedure

a) A sample of aggregates about 1000 gm for 4.75 mm to 10 mm size or 500 gm if finer that 4.75 mm is taken for the test. Two tests shall be made and it is recommended that the two samples should not be tested concurrently. Aggregates, artificially heated, are not normally tested.

b) The test sample passing a 10 mm sieve is placed in a tray and kept immersed in distilled water at a temperature between 22°C and 32°C.

c) Immediately after immersion, remove the entrapped air from the sample by gentle agitation with a rod.

d) The sample is kept completely immersed for a period of 24±1/2 hours.

e) The water is drained from the sample by decantation through a filter paper. The aggregate retained on filter paper is exposed to a gentle current of warm air.  Ensure uniform drying by stirring till the material just attains a free running condition. Avoid over drying of the material. The weight of the saturated surface dry (SSD) aggregate is taken (Weight A).

f) Transfer the aggregates to the pycnometer and fill it with water. Remove the entrapped air from the sample by rotating the pycnometer on its side while the hole in the apex of the cone is covered with a finger. Top up the pycnometer up to the hole with distilled water ensuring that any froth on the surface is removed.

g) The pycnometer is dried outside and the weight taken (Weight B). Thereafter, drain the pycnometer and transfer all the aggregates into the tray.

h) The pycnometer is refilled with distilled water to the same level as before, dried out side and weight taken (Weight C). The difference in temperature of water in the pycnometer shall not vary more than 2°C.

i) The saturated surface dry aggregate is then oven dried in the shallow tray, at a temperature of 100 to 110°C for 24±½ hours. During this period, the aggregates are stirred to facilitate drying.

j) After 24 hours, the sample is removed from the oven, cooled in the airtight container and weighed (Weight D). 

NOTE: Instead of pycnometer if glass jar is used, the procedure is the same except that the jar is filled to overflowing and the glass plate slid over to exclude any air bubbles.

4.3 Calculation

Specific gravity (Bulk), apparent specific gravity and water absorption are calculated as follows:

Specific gravity (Bulk) = D/ [A - (B-C)]

Apparent specific gravity = D/ [D - (B-C)]

Water absorption (% of dry weight) = 100 (A-D)/ D where

A= Weight of the saturated surface dry sample in gm.

B= Weight of the pycnometer or glass jar and sample filled with distilled water in gm.

C = Weight of the pycnometer or glass jar filled with distilled water in gm.

D = Weight of oven-dried aggregate in air in gm.

4.4  Reporting of results

The individual and mean of the two results are reported to three decimal places. The grading of material shall be stated.

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