Properties of aggregates plays very important role in selection of aggregates for different kinds of construction. Different activities requires the different aggregates depending on their properties.
For example, for road construction high density aggregates are required.
For compound wall low density or recycled aggregates are required. For compound wall load carrying capacity is very low.
Followings are some of the properties of aggregates,
1. Composition:
Generally Aggregates are derived from rocks, so that aggregates has same composition of parent rock.
Igneous rock is good for aggregates.
Aggregates should be free from impurities like mica, shell, coal, bones, clay etc. Impurities reacts with alkalis in cement and leads to cracking, expansion and deterioration of concrete. There are many test to detect the impurities in concrete.
2. Size & Shape:
Size & shape of aggregates directly has impact on strength of concrete, bond strength, workability of concrete, cement requirement and cost required for concrete.
Rounded, elongated and flaky aggregates offer good workability but less bond strength and mechanical strength. Angular & irregular aggregates offers lesser workability but good bond strength and mechanical strength.
According to IS-456; the maximum size of aggregates that can be used in PCC and RCC are:
one fourth of the minimum dimension of the concrete member.
one fifth of the minimum dimension of the reinforced concrete member.
The minimum clear spacing between reinforced bars or 5 mm less than the minimum cover between the reinforced bars and form, whichever is smaller for heavily reinforced concrete members such as the ribs of the main bars.
3. Surface Texture:
Surface texture of aggregates impacts on strength and bond strength of concrete.
Surface roughness, smoothness, porosity affects the bonding of aggregates with concrete.
Smoothness and roundness of aggregates offer good workability but less bond strength and mechanical strength.
Roughness and porosity of aggregates offers lesser workability but good bond strength and mechanical strength. Pores in aggregates are filled with cement in concrete which offers bonding and enhances the strength of concrete.
4. Specific Gravity
To Find out the specific gravity need to take test for specific gravity. Its measures strength of aggregates, water absorption and porosity of aggregates.
Specific gravity is nothing but the ratio of the weight of given volume of aggregates to the weight of an equal volume of water.
The specific gravity of aggregates generally ranges between 2.5-3. For recycled aggregates the specific gravity may be less than 2.4.
Lesser specific gravity indicates the low strength (weaker aggregates) and more porosity. Higher specific gravity indicates more strength of aggregates and less porosity (which increases the durability of concrete.)
For the important, huge construction the high specific gravity material should be used.
Stone
Trap
2.9
Granite
2.8
Gravel
2.66
Sand
2.65
Table: Specific gravity of materials
5. FINENESS MODULUS
Fineness modulus is a ratio of the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 mm to 150 micron to this sum by 100.
Fineness modulus indicates the grading of aggregates.
More fineness of aggregates means the aggregates is coarse. Lesser fineness aggregates mean aggregates is finer.
6. POROSITY & ABSORPTION
Porosity is defined as the voids or pores present on aggregates due to the formation processes of parent rock.
Absorption is defined as the percentage of water absorbed by the dry aggregates when it is completely immersed in water for 24 hrs.
Depending upon the Moisture content in aggregates, it can exist in any of the 4 conditions.
Moist or wet aggregates (Very high moisture content)
7. BULKING OF SAND
It can be defined as in increase in volume of sand due to increase of moisture content. The ratio of the volume of moist sand to the volume of sand when dry, is called bulking factor.
Why fine sand bulk more than coarse sand:
Surface volume of fine sand is more than the coarse sand, if we consider same quantity of both. When water is added to dry sand, thin film of water formed on sand particle. This film of water tends to push particles apart from each other due to surface tension and thus increases the volume and bulks up. This happens only when the sand is fine and water added is moderate.
In case of fully saturated sand, the thin film broke down and sand doesn’t bulk up.
8. Bulk Density
Bulk density is the weight or mass of the aggregate required to fill a container of specified unit volume. Its unit is kg/m3
Bulk density of coarse aggregate depends upon:
The shape of aggregate
Grading of the aggregate
Degree of the compaction
Bulk Density (kg/m3
River sand
Fine
1440
Medium
1520
Coarse
1600
Beach or river shingle
1600
Broken stone
1600
Stone screenings
1440
Broken Granite
1680
Table: Value of bulk density
9. Voids:
Tiny spaces in particles of aggregates is called void. More void in aggregates makes the concrete porous.
Average Void Percentage
River sand
Fine
43
Coarse
35
Mixed and moist
38
Mixed and dry
30
Broken stone, graded
25 mm maximum size
46
50 mm maximum size
45
63 mm maximum size
41
Stone screenings
48
Table: Average Void Percentage
10. Impact Value :
Impact values the factor to measure the resistant of aggregates against shocks and impact. A higher impact value indicates the toughness of aggregates. This test is very important incase of road construction.
Crushing values the factor to measure the resistant of aggregates against hardness. The crushing value gives the resistance of a coarse aggregate to crushing under the application of compressive load.
Properties
Limiting Values (Percentage)
For Wearing Surfaces
Other Than for Wearing Surfaces
Crushing value
30
45
Impact value
30
45
Abrasion Value (Los Angeles)
30
50
Table: Values for mechanical properties
13. Deleterious Materials
Aggregates should be free from impurities like mica, shell, coal, bones, clay etc. Impurities reacts with alkalis in cement and leads to cracking, expansion and deterioration of concrete. There are many tests to detect the impurities in concrete. This leads to weaken the bond strength.