Specific Gravity and Water Absorption Test on Aggregates
To determine the specific gravity and water absorption of given sample of aggregates.
a) Density basket
b) Weighing balance
c) Water tank
e) IS sieves- 10mm and 20mm.
The specific gravity of an aggregate is considered to be a measure of strength or quality of the material. Stones having low specific gravity are generally weaker than those with higher specific gravity values. The specific gravity test helps in the identification of stone.
Water absorption gives an idea of strength of rock stones having more water absorption are more porous in nature and are generally considered unsuitable unless they are found to be acceptable based on strength, impact and hardness.
1) Take about 2kg of given aggregates passing IS 20mm sieve and retained on 10mm sieve.
2) Keep the aggregate in density basket and then keep the basket in water.
3) Allow the aggregate and basket to be in water for 24 hours.
4) After 24 hours find the suspended weight of basket with aggregate.
5) Remove the basket out of water and remove the aggregate.
6) Keep the empty basket back in water and find the suspended weight.
7) Wipe the surface of aggregate using a cotton cloth to make them surface dry.
8) Find the weight of surface dry aggregate in air.
9) Keep the aggregate in oven at 110° C for 24 hours.
10) Now find the weight of dried aggregate in air.
11) Then specific gravity and Water absorption is calculated from the relation:
Weight of aggregate taken = 2000gm
I.S. sieve used 20mm and 10mm
The specific gravity of given aggregate sample = ………………….
The water absorption of given aggregate sample = ………………...%
The specific gravity of aggregates normally used in road construction ranges from about 2.5 to 3.0 with an average value of about 2.68. though high specific gravity of an aggregate is considered as an indication of high strength, it is not possible to judge the suitability of a sample of road aggregate without finding the mechanical properties such as aggregate crushing, impact and abrasion values.
Water absorption of an aggregate is accepted as measure of its porosity. Sometimes this value is even considered as a measure of its resistance to frost action. Water absorption value ranges from 0.1 to about 2.0 percent for aggregate normally used in road surfacing. Stones with water absorption upto 4.0 percent have been used in base courses. Generally a value of less than 0.6 percent is considered desirable for surface course, though slightly higher values are allowed in bituminous constructions. IRC has specified the maximum water absorption value as 1.0 percent for aggregates used in bituminous surface dressing and built-up spray grout.
To study the resistance of aggregates to weathering action, by conducting accelerated weathering test cycle
a) Sodium sulphate or magnesium sulphate
c) Weighing balance
d) IS sieves
1) In order, to quicken the effects of weathering due to alternate wet-dry or freeze-thaw cycles in the laboratory, the resistance to disintegration of aggregate is determined by using saturated solution of sodium sulphate or magnesium sulphate.
2) Clean, dry aggregates of specified size is weighed and counted. Then immersed in the saturated solution of sodium sulphate or magnesium sulphate for 16 to 18 hours.
3) Then the aggregates are dried in an oven at 105-110°C to a constant weight, thus making one cycle of immersion and drying.
4) The number of such cycles is decided by prior agreement and then the specimens are tested. After completing the final cycle, the sample is dried and each fraction of aggregate is examined visually to see if there is any evidence of excessive splitting, crumbling or disintegration of the grains.
5) Sieve analysis is carried out to note the variation in gradation from original. The coarse aggregate fraction of each size range is sieved on specified sieve sizes.
IRC has specified 12percent as the maximum permissible loss in soundness test after 5 cycles with sodium sulphate, for the aggregate to be used in bituminous surface dressing, penetration macadam and bituminous macadam constructions.
Bitumen Adhesion/Stripping Test
Several laboratory tests have been developed to determine the adhesion of bituminous binder to an aggregate in presence of water. These tests may be classified into six types:
a) Static immersion test
b) Dynamic immersion test
c) Chemical immersion test
d) Immersion mechanical test
e) Immersion trafficking test and
f) Coating test
The static immersion test is very commonly used as it is quite easy and simple. The principle of this type of test is by immersing aggregate fully coated with the binder in water maintained at specified temperature and by estimating the degree of stripping. The result is reported as the percentage of stone surface that is stripped off after the specified time periods.
IRC has specified the maximum stripping value as 25 percent for aggregate to be used in bituminous construction like surface dressing, penetration macadam, bituminous macadam and carpet.
IRC has specified that stripping value of aggregates should not exceed 25percent for use in bituminous surface dressing, penetration macadam, bituminous macadam and carpet constructions, when aggregate coated with bitumen is immersed in water bath at 40°C for 24 hours.
Aggregate Impact Test
To determine the impact value of the given aggregate
Aggregate impact apparatus
IS sieves (12.5 mm, 10.0 mm and 2.36 mm)
Cylindrical measure and cylindrical cup
Toughness is the property of a material to resist impact. Due to traffic loads the road stones are subjected to the pounding action or impact and there is possibility of stones breaking into smaller pieces. The road stones should therefore be tough enough to resist fracture under impact. A test designed to evaluate the toughness of stones i.e. the resistance of the stones to fracture under repeated impacts may be an impact test for road aggregate.
The aggregate impact value indicates a relative measure of the resistance of an aggregate to a sudden shock or an impact, which in some aggregate differs from its resistance to a slow compressive load. The method of tests specifies the procedure for determining the aggregate impact value of coarse aggregate.
1. Take clean and dry aggregate and sieve on IS 12.5 mm and 10.00 mm sieve.
2. Collect the aggregate passing IS 12.5mm sieve and retained on IS 10.0 mm Sieve.
3. Find the weight of empty cylindrical measure. Let the weight be „a‟ g.
4. Fill the aggregate in the cylindrical measure in three layers, tamping each layer 25 times with the rounded end of the tamping rod.
5. Roll the tamping rod over aggregate surface and remove excess aggregate, if any
6. Find the weight of the cylindrical measure with aggregate. Let the weight be „b‟ g. Thus the weight of aggregate = W1 = ( b-a )
7. Transfer all the aggregate from the cylindrical measure to the test cylinder in one layer and tamp the layer 25 times with the rounded end of the tamping rod.
8. Fix the test cylinder firmly to the base of the impact tester.
9. Adjust the height of fall of the plunger to 380+ 5mm and set the blow counter to zero.
10. Lift the plunger gently and allow it to drop. This is one blow. Give 15 such blows.
11. Take out the test cylinder and sieve the crushed material on IS 2.36 mm sieve. Find the weight of material passing the sieve. Let weight be W2 g.
12. Find the weight of aggregate retained on this sieve. Let the weight be W3 g.
Then, Aggregate impact value = W2 / W1 * 100 %
And percentage of dust = W3 / W1* 100 %
The Impact value of given aggregate sample is = ………….. %
value Various agencies have specified the maximum permissible aggregate impact values for the different types of pavements, those recommended by the Indian Roads Congress are given below:
For deciding the suitability of soft aggregates in base course construction, this test has been commonly used. A modified impact test is also often carried out in the case of soft aggregates to find the wet impact value after soaking the test samples. The recommendations is given in the below table.
Aggregate impact value is used to classify the stones in respect of their toughness property as indicated below:
<10% Exceptionally strong
Strong 20-30% Satisfactory for road surfacing
>35% Weak for road surfacing
The aggregate impact value should not normally exceed 30% for aggregate to be used in wearing course of pavements. The maximum permissible value is 35% for bituminous macadam and 40% for water bound macadam base courses.