Exp 6 Relative Density & Water Absorption

Experiment No. 6:

Standard test method for the determination of relative density (i.e. specific gravity) and water absorption of the coarse aggregates. (ASTM C-127-04)

Scope & Significance:

In this test method we determine the relative density (i.e. specific gravity) and the water absorption of the coarse aggregates.
The information obtained from specific gravity is helpful in the following ways;
1-         The knowledge of the specific gravity is important for the concrete technologist to determine the properties of concrete made from such aggregates.
2-         It is used for the calculation of the volume occupied by the aggregates in various mixtures.
3-         The pores at the surface of the particles affect the bond between the aggregate and the cement paste and thus influence the concrete strength.
4-         Normally it is assumed that at the time of setting of concrete, the aggregate is in the saturated and surface dry condition. If the aggregate is to be batched in the dry condition, then it is assumed that sufficient amount of water will be absorbed from the mix to bring the aggregate in the saturated condition. If an additional amount of water is not added as a cover for the absorbed water, the loss of work-ability is resulted.

Limitation: The limitation of the test is that, it can not be used for the light weight aggregates.

Related Theory:11

Aggregates

Aggregates may be classified as;
i-                 Coarse Aggregates
ii-               Fine Aggregates

Coarse Aggregates

Any material which is retained on BS sieve #4 (ASTM sieve 4.75mm) is known as coarse aggregate.

Fine Aggregates

Any material which is passing BS sieve #4 (ASTM sieve 4.75mm) is known as fine aggregate.

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Types Of Crush Available In Pakistan

1-   Sargodha Crush

Sargodha crush possess the following properties;

  • Greener in color
  • High strength
  • Usually elongated particles

2-   MargHalla Crush

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Marghalla crush possess the following properties;

3–   Sakhi Sarwar CrushGrayish in color

  • Low in strength

Sakhi Sarwar crush possess the following properties;

  • Whitish in color

Absorption

It is the increase in the mass of the aggregate due to the penetration of water into the pores of the particles during a prescribed period of time. The term absorption does not include the amount of water adhering to the surface of the particles. Water absorption is expressed as percentage of the dry mass.

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 Dry Density

It is the mass of the oven dried aggregate per unit volume of the aggregate particles. The term volume includes the volume of the permeable and the impermeable pores and does not include the volume of the voids between the particles.

Saturated Surface Dry (S.S.D) Density

It is the mass of the saturated surface dry aggregate per unit volume of the aggregate particles. The term volume includes the volume of the permeable and the impermeable pores which are filled with water and does not include the volume of the voids between the particles.

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Apparent Density

It is the mass per unit volume of the impermeable portion of the aggregate particles.
OR
It is the mass per unit volume of the solid portion of the particles excluding the voids. 

Specific Gravity/Relative Density

It is the ratio of the density of the aggregate material to the density of the gas free distilled water at a standard temperature (i.e. 4 oC). The relative density is a dimensionless quantity and is expressed as oven dried, saturated surface dry and apparent

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).

Apparatus:

  • Balance
  • Sample container
  • Water tank
  • Sieves
  • Oven

PROCEDURE:

The sample of the aggregate is immersed in water for 24hrs to essentially fill all the pores. Remove the test sample from the water and roll it in a large absorbent cloth until all visible films of water are removed. Wipe the larger particles individually. A moving stream of air is permitted to assist in the drying operation. Take care to avoid evaporation of water from aggregate pores during the surface-drying operation. Determine the mass of the test sample in the saturated surface-dry condition. Record this and all subsequent masses to the nearest 0.5 g or 0.05 % of the sample mass, whichever is greater. In order to calculate the volume of the aggregate, immediately place the saturated-surface-dry test sample in the sample container and determine its apparent mass in water at 23±2.0 °C. Take care to remove all entrapped air before determining its mass by shaking the container while immersed. The difference between the mass in air and the mass when the sample is immersed in water equals the mass of water displaced by the sample. This mass of water equals the volume of water displaced because
mwater = Vwater            (ρwater=1 gm/cm3)15
ρwater  = mwater x Vwater
Vwater = Vaggregate

Dry the test sample in the oven to constant mass at a temperature of 110±5 °C, cool in air at room temperature 1 to 3 h, or until the aggregate has cooled to a temperature that is comfortable to handle (approximately 50 °C), and determine the mass in order to calculate the oven specific gravity of the specimen.

Observations & Calculations:

Table no. 1: Calculation of Specific Gravity

NAME OF

SAMPLE

Weight of Agg.

Wagg

Volume of

Water

Volume of Water + Agg.

Volume of

Aggregate

Vagg

Specific Gravity

=Wagg/Vagg

(gm)

(mL)

(mL)

(mL)

 

Chenab Sand

500

500

690

190

2.63

Laurence pure Sand

500

500

690

190

2.63

Sargodha Crush Saturated

500

500

675

175

2.86

Margala Crush Saturated

500

500

685

185

2.70

Margala Crush

500

500

695

195

2.564

Sargodha Crush

500

500

675

175

2.86

Relative Density

Comments:

Specific gravity is used for calculating yield of concrete or the quantity of aggregate required for a given volume of concrete

•  Rough textured, angular, elongated particles require more water to produce  workable concrete than do smooth, rounded, compact aggregates.
• Aggregates should be relatively free of flat and elongated particles (limit to 15% by weight of total aggregate).
• Important for coarse and crushed fine aggregate  – these require an increase in mixing water and may affect the strength of the concrete, if cement water ratio is not maintained.
• If water content of the concrete mixture is not kept constant, the compressive strength, workability, and other properties will vary from batch to batch.
• If aggregates or concrete absorbs so much water that when the water freezes and expands the concrete cannot accommodate the build up of internal pressure, pop–outs may occur.16
Specific Gravity is the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. Water, at a temperature of 73.4°F (23°C) has a specific gravity of 1.Specific Gravity is important for several reasons. Some deleterious particles are lighter than the good aggregates. Tracking specific gravity can sometimes indicate a change of material or possible contamination. Differences in specific gravity can be used during production to separate the bad particles from the good using a heavy media liquid.
Specific gravity is critical information for the Hot Mix Asphalt Design Engineer. It is used in calculating air voids, voids in mineral aggregate (VMA), and voids filled by asphalt (VFA). All are critical to a well performing and durable asphalt mix. Water absorption can also be an indicator of asphalt absorption. A highly absorptive aggregate could lead to a low durability asphalt mix.
In Portland Cement Concrete the specific gravity of the aggregate is employed in calculating the percentage of voids and the solid volume of aggregates in computations of yield. The absorption is important in determining the net water-cement ratio in the concrete mix. Knowing the specific gravity of aggregates is also critical to the construction of water filtration systems, slope stabilization projects, railway bedding and many other applications.

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