IMPORTANCE OF ENGINEERING PROPERTIES OF AGGREGATES
Properties or characteristics of aggregate influence the quality of the concrete mix. Properties of Aggregates aggregate affects on the strength and durability of the concrete. Aggregate is a broad encompassing boulder, cobbles, crushed stone, gravel, air-cooled, blast furnace slag, native and manufactured sands, and manufactured and natural lightweight aggregates. These aggregate properties given below are very important for good concrete :
PROPERTIES OF AGGREGATES
- Particle size
- Particle Shape
- Specific Gravity
- Bulk Density
- Water Absorption
- Deleterious Material
- Alkali Reactivity
The size and shape of the aggregate influence the quantity of water and cement in concrete mix. Aggregates may be further described by their respective sizes.
|Boulders||More than 256mm|
|Coarse gravel||4.75 mm – 64mm|
|Fine aggregate||75 micron to 4.75mm|
Boulders and cobbles are generally not used in their as-mined size but are crushed to make various sizes of coarse aggregate and manufactured sand and mineral filler. Gravels and naturally occurring sand are produced by the action of water and weathering on glacial and river deposits. These materials have round, smooth surfaces and particle-size distributions that require minimal processing.
These materials can be supplied in either coarse or fine-aggregate sizes. Fine aggregates have 100% of their material passing the 10 mm sieve. Coarse aggregates have the bulk of the material retained on 4.75 mm sieve.
Aggregates comprise the greatest volume percentage in portland-cement concrete, mortar, or asphaltic concrete. In a portland cement concrete mix, the coarse and fine aggregates occupy about 60 to 75% of the total mix volume. For asphaltic concrete, the aggregates represent 75 to 85% of the mix volume. Consequentially, the aggregates are not inert filler materials. The individual aggregate properties have demonstrable effects on the service life and durability of the material system in which the aggregate is used, such as portland-cement concrete, asphaltic concrete, mortar, or aggregate base.
The maximum size of aggregate should be less than One-fourth of the minimum dimension of the concrete member and One-fifth of the minimum dimension of the reinforced concrete member. The size of aggregate may be dependent upon some conditions. In general, 40 mm size aggregate used for normal concrete and 20 mm size is used for high strength concrete.
Read more : Sieve analysis of aggregate
Depending on the size, we can divide it into 6 types Rounded, Irregular, Angular, Flaky, Elongated, Flaky and elongated.
Rounded are aggregates smoothed by weathering, erosion, and attrition. Rocks, stone, sand, and gravel found in riverbeds are common rounded aggregates.
Irregular Aggregate shaped Irregular by attrition, but they are not fully rounded. Irregular Aggregate reduced workability to rounded aggregates. Angular aggregates Used for higher strength concrete, angular aggregates come form of crushed rock and stone. Angular aggregates can be offset by filling voids with rounded or smaller aggregates.
Flaky aggregates Defined as that are thin in comparison to length and width. Flaky aggregates increase surface area in a concrete mix. Elongated aggregates also add more surface area to a mix it means more cement paste is needed in concrete. Elongated aggregates are longer than they are thick or wide. Flaky and elongated aggregate is a mix of the previous two and the least efficient form of aggregate with regards to workability.
The bond between angular particles is greater than that between other shape particles. Properly graded angular particles can take advantage of this property and offset the increase in water required to produce concrete with cement content and strength
equal to that of a smooth-stone mix.
Read Also : Flakiness & Elongation Index test of Aggregate
The specific gravity of an aggregate is considered to be a measure of strength or quality of the material. Low specific gravity Aggregates are generally weaker than those with high specific gravity. Low specific gravity generally indicates porous, weak and absorptive aggregate. This property helps to the identification of good and suitable aggregates. Specific gravities of aggregate vary between 2.0 and 3.0
Specific gravities are primarily of two types :
- Apparent specific gravity
- Bulk specific gravity
The bulk density or unit weight of aggregate is the mass or weight of the aggregate that required to fill a container of a specified unit volume.It is generally expressed by kg/liter. The bulk density of aggregates depends upon the size and shape of the aggregate.
Read more : Bulk density test of aggregate
Water absorption is the difference between the weight of very dry aggregates and the weight of the saturated aggregates with surface dry conditions.
water absorption of aggregate may affect the properties of concrete in various ways, where both the fresh and
hardened concrete properties are affected. it effects on workability loss of the fresh concrete as well as compressive strength and density of the concrete.
The volume of dry sand increases due to absorption of moisture. These volume increase of dry sand is known as bulking of sand. When dry sand comes in contact with moisture, a thin film is formed around the particles, which causes them to get apart from each other.
Deleterious materials mean aggregate properties (Organic impurities, Clay, silt & dust Salt contamination) that can cause of negative effect on concrete. Harmful aggregate affects the strength and durability of the concrete.
Impurities in Aggregates Erratic setting times and rates of hardening may be caused by organic impurities in the aggregates, primarily the sand. Deleterious materials reduced durability can be caused by soft particles, chert, clay lumps and coal, lignite, or other lightweight materials in the aggregates. Coal and lignite may also cause staining of exposed concrete surfaces.
Deleterious materials may cause of following effects on concrete:
- Reaction with cement properties
- Reducing strength and durability of concrete
- Change in setting times of concete
This properties is measure by the crushing value and impact value of aggregates. crushing value gives a relative measure of the resistance of an aggregate to under compressive load. Aggregate impact value gives a relative measure of the resistance of an aggregate to sudden impact.
Coarse-aggregate hardness is measured by the Los Angeles Abrasion Test. These tests break the aggregate down by impacting it with steel balls in a steel tumbler.
Read more: Los Angeles Abrasion Test
Aggregate soundness is measured testing with “Sodium Sulfate or Magneisum Sulfate.’’ This test measure
the amount of aggregate degradation when exposed to alternating cycles of wetting and drying in a sulfate solution.
Aggregates that contain certain forms of silica or carbonates may react with the alkalies present in portland cement (sodium oxide and potassium oxide). The reaction product cracks the concrete or may create pop-outs at the concrete surface. The reaction is more pronounced when the concrete is in a warm, damp environment.
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