Specific gravity and water absorption of aggregates

Constroquick

Specific gravity and water absorption of aggregates are most important properties of aggregates which needs to be considered while designing concrete and roads.

Specific gravity is important to find out quantity, quality and toughness of aggregates

Water absorption is important to know percentage of water absorption, porousness of aggregates.

specific gravity and water absorption of aggregates

Specific gravity and water absorption of aggregates

There are two types of specific gravities,

  1. Apparent specific gravity and
  2. Bulk specific gravity.

Apparent Specific Gravity, Gapp,

It is computed on the basis of the net volume of aggregates i.e the volume excluding water-permeable voids.

Thus, Gapp = [(MD/VN)]/WWhere, MD is the dry mass of the aggregate, VN is the net volume of the aggregates excluding the volume of the absorbed matter, W is the density of water.  

Bulk Specific Gravity, Gbulk,

It is computed on the basis of the total volume of aggregates including water permeable voids.

Thus, Gbulk = [(MD/VB)]/W  Where, VB is the total volume of the aggregates including the volume of absorbed water.  

Water Absorption: 

Water absoption is measured by  weighing the aggregates dry and in a saturated surface dry condition, with all permeable voids filled with water.

The difference of the above two is MW.

MW is the weight of dry aggregates minus weight of aggregates saturated surface dry condition.

Thus, Water Absorption = (MW/MD)*100

The specific gravity of aggregates normally used in road construction ranges from about 2.5 to 2.9.

Water absorption values ranges from 0.1 to about 2.0 percent for aggregates normally used in road surfacing.

Objective:

  1. To measure the strength or quality of the material.
  2. To determine the water absorption of aggregates.

Water Absorption and Specific Gravity of Aggregates

For any kind of aggregates, method of test is different according to their size.

For Coarse aggregate (10mm – 40mm), given method is used.

For Fine Aggregates (less than 10mm), Pycnometer test is used.

Apparatus Required for test:

  1. A balance of capacity about 3kg,
  2. Oven to maintain temperature at 100-110° C.
  3. A wire basket of not more than 6.3 mm mesh ( at least 5 liter capacity)
  4. A container for filling water and suspending the basket
  5. An air tight container ( at least 5 liter capacity)
  6. A shallow tray and two absorbent clothes, each not less than 75x45cm.

Procedure

  1. Take 2kg of sample (coarse aggregates) which is dry and clean.
  2. Take sample in basket and immersed it into distilled water and jolt is 25 times to remove the entrapped air and remain immersed completely at least for 24 hours.
  3. After 24 hours, take the weight of basket and sample suspended in water and note is as W1 gm.
  4. Remove the basket and allow sample to dry on absorbent sheet and drain it completely. While aggregates drying, take the empty weight basket in water tank after jolting it 25 times. And note down it as W2 gm.
  5. Allow aggregates to surface dry, change absorbent sheet and dry sample for 10 more minutes. And take the sample aggregates (surface dried) as W3 gm.
  6. Then put aggregate in tray at dried it in oven for 24 hours at temperature of 110° C. Take the weight of aggregate as W4 gm.

Observations of Test

Weight of Sample in tank with basket (Completely saturated) = W1 gm

Weight of empty basket (Suspended in water) = W2 gm

 Weight of aggregate (surface dried) = W3 gm

Weight of oven dried aggregate = W4 gm

Weight of saturated aggregate in water = W1 – W2 hm

Weight of water equal to the volume of the aggregate = W3–(W1–W2) gm

Formulas:

Specific gravity = W3/ (W3– (W1– W2))

Apparent specific gravity = W4/ (W4– (W11– W2))

Water Absorption = ((W3– W4) / W4) X 100 

  • Specific gravity: 2.4 to 3 (for road construction), 2.68 (average)
  • Water Absorption: should not more than 0.6 per unit by weight

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