EXPERIMENT NO. 3 & 4
SPECIFIC GRAVITY (RELATIVE DENSITY) AND WATER ABSORPTION TEST FOR AGGREGATES
RELATED THEORY Water Absorption
Specific Gravity is defined as ratio of weight of solid to the weight of an equal volume of gas free distilled water (no dissolved air/impurities) at a stated temperature.
It is the ratio of weight of water absorbed to the weight of dry sample expressed as a percentage. It will not include the amount of water adhering to the surface of the particles.
Any material which is retained on BS sieve #4 (ASTM sieve 4.75mm) is known as coarse aggregate.
Any material which is passing BS sieve #4 (ASTM sieve 4.75mm) is known as fine aggregate.
SATURATED SURFACE DRY (S.S.D.) CONDITION
It is the condition related with the aggregate particles in which the permeable pores of the aggregate particles are filled with water but without free water on the surface of the particles.
OVEN DRIED SPECIFIC GRAVITY
It is the ratio of the oven dried density of the aggregate to the density of the gas free distilled water at a standard temperature (i.e. 4 oC).
SATURATED SURFACE DRY SPECIFIC GRAVITY
It is the ratio of the saturated surface dry density of the aggregate to the density of the gas free distilled water at a standard temperature (i.e. 4 oC).
APPARENT SPECIFIC GRAVITY
It is the ratio of the apparent density of the aggregate to the density of the gas free distilled water at a standard temperature (i.e. 4 oC).
TYPES OF CRUSH AVAILABLE IN PAKISTAN
1- SARGODHA CRUSH
Sargodha crush possess the following properties;
- Greenish/Dark Gray in color
- High strength
- Usually elongated particles
2- MARGALLA CRUSH
Margalla crush possess the following properties;
- Light Gray in color
- Low in strength
3- SAKHI SARWAR CRUSH
Sakhi Sarwar crush possess the following properties;
- Whitish in color
In this test method we will determine the relative density (i.e. specific gravity) and the water absorption of the coarse aggregates.
- The knowledge of the specific gravity is important for the concrete technologist to determine the properties of concrete made from such aggregates.
- It is used for the calculation of the volume occupied by the aggregates in various mixes and generally it ranges from 2.5 to 3.
- The pores at the surface of the particles affect the bond between the aggregate and the cement paste thus influences the concrete strength. Smaller the number of pores, higher will be the specific gravity hence more will be the bond strength and more concrete strength.
- Though higher specific gravity of aggregate is considered as an indication of its high strength; it is not possible to judge the suitability on this basis alone without finding other mechanical properties like aggregate abrasion value etc.
- Water absorption is a measure of porosity of aggregates and its resistance to frost action.
- Higher water absorption means more pores hence aggregate will be the considered as weak.
- Water absorption value ranges from 0.1 – 2.0% for aggregate normally used in roads surfaces.
- Aggregates with water absorption up to 4.0% are acceptable in base coarse.
Balance 5 Kg capacity readable to 0.5g
- Sample container in the form of a wire mesh bucket of capacity 4000 – 7000 cm3 and not more than 6.3mm mesh.
- Suitable arrangement for suspending the container in water from center of the balance.
- A container for filling water and suspending the wire mesh bucket.
- Shallow tray and absorbent cloth.
- Thermostatically controlled oven.
Take representative sample. Reject all material passing #4, weight of sample to be used for the test would depend upon the nominal maximum size as given in table below.
|Nominal Maximum Size||(mm)||13||19||25||38||50||63||75||90|
|(in.)||½||¾||1||1 ½||2||2 ½||3||3 ½|
|Maximum Sample Wt.||2||3||4||5||8||12||18||25|
- Thoroughly wash the aggregates to remove any dust. Oven dry and cool the aggregates for 1 to 3 hours and then immerse in water for 24 hours.
- Remove the specimen from water and roll it in a large absorbent cloth until all visible films of water are removed.
- Weight the specimen in saturated surface dry conditions.
- Place the saturated surface dry specimen in wire mesh bucket and weight it in water. Shake the bucket to remove all entrapped air before weighing.
- Dry the sample to constant weight in oven, cool and weigh.
Weight of oven dried aggregate in air (gm) = A = 2493
Weight of saturated surface dry aggregate in air (gm) = B = 2502
Weight of saturated aggregate and bucket in water (gm) = W1 = 3197
Weight of bucket in water (gm) = W2 = 1572
Weight of saturated aggregate in water = C (gm) = (W1 – W2) = 1625
Oven Dried Bulk Specific Gravity = Sd = A / (B – C) = 2493/(2502-1625) = 2.842
Saturated Surface Dry Bulk Specific Gravity = Ss = B / (B – C)
= 2502 / ( 2502 – ( 3197-1572 )) = 2.853
Apparent Specific Gravity = Sa = A / (A – C) = 2493/(2943-1625) = 2.872
Water Absorption, = WA = [(B – A) / A] x 100 = [(2502-2493)/2493] x 100 = 0.361
If the aggregate is not oven-dried before soaking, specific gravity values may be significantly higher. This is because in the normal procedure the water may not be able to penetrate the pores to the center of the aggregate particle during the soaking time. If the aggregate is not oven-dry to start, the existing water in the aggregate pore structrure may be able to penetrate further into the pores (AASHTO, 2000c).
Make sure to use cloth and not paper towels. Paper towels may absorb water in the aggregate pores.
INTER RELATIONSHIP BETWEEN SPECIFIC GRAVITIES AND WATER ABSORPTION
- Ss = (1 + A/100)Sd
- Sa = 1/(Sd – A/100) = Sd/(1 – Asd/100)
- A = (Ss/Sd – 1)*100 = [Sa – Ss/Sa(Ss -1)] x 100
Specific gravities of our samples came out to be 2.8 which lies in the general range of specific gravity values. Water absorption value of our sample comes out to be 0.361, Value ranges from 0.1 – 2.0% for aggregate normally used in roads surfaces. So our sample cannot be used in road surfaces whereas aggregates with water absorption up to 4.0% are acceptable in base coarse. So our sample can be used as base coarse.
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