AGGREGATES

Aggregate are the inert materials, such as sand, stone, surkhi, used for mortar, cement concrete, Asphalt concrete, and other civil engineering applications.
Aggregate, in building and construction, material used for mixing with cement, bitumen, lime, gypsum, or other adhesive to form concrete or mortar. The aggregate gives volume, stability, resistance to wear or erosion, and other desired physical properties to the finished product. Commonly used aggregates include sand, crushed or broken stone, gravel (pebbles), broken blast-furnace slag, boiler ashes (clinkers), burned shale, and burned clay.
Depending upon size aggregate may be classified as:
• Fine Aggregate
• Coarse Aggregate

Fine Aggregate

Those particles passing the 9.5 mm (3/8 in.) sieve, almost entirely passing the 4.75 mm (No. 4) sieve, and predominantly retained on the 75 µm (No. 200) sieve are called fine aggregate. e.g
 Sand

• Chemically sand = silica (SiO2)
• Fine minerals
• The order of 0.1 to 1.0 mm diameter
• Contain at least some absorbed water
• Fill voids between the coarser aggregate
• Giving high packing factor
• Reducing amount of open (or interconnected) porosity in the finished concrete
• Reducing disintegration of the concrete due to repeated freezing and thawing during service
• It may be 1) pit sand or quarry sand, 2) river sand 3) sea sand

Bulking of Sand
When the sand is wet then each particles gets a coating of water which due to surface tension, keeps them apart thereby causing an increase in the volume of sand. This increase in volume due to the sand being wet is known as “bulking of sand”.

Bulking is more in finer sand than in coarser sand. In measuring sand by volume, its bulking should be considered. At about 4%, its value increases maximum to 25%.

 Surkhi
Powdered broken brick locally called surkhi is used as fine aggregate in lime mortar.
Surkhi is prepared by finely grinding well burnt good quality bricks free from un-burnt bricks.

Used in mortars.

Coarse Aggregate

Those particles that are predominantly retained on the 4.75 mm (No. 4) sieve, are called coarse aggregate. e.g

(i) Stone Ballast;
(ii) Gravel or Shingle;
(iii) Brick Ballast; and

(iv) Clinker.

Shape of Coarse Aggregate

Shape of three dimensional irregular bodies as coarse aggregate is difficult to describe; but is the vital property affecting the workability of fresh concrete and also its strength and durability. The characteristic of parent rock from which coarse aggregates have been produced and also the type of crusher used for crushing influence the shape of coarse aggregates. Rounded, Angular, Flaky, Elongated and Irregular are some types on shapes of coarse aggregates. Rounded particles are fully water-worn or completely shaped by attrition. Angular particles possess well-defined edges formed at the intersection of roughly planar faces. Flaky particles have thickness small relative to the other two dimensions. Elongated particles are usually angular of which the length is considerably larger than the other two dimensions. Irregular particles are naturally irregular, partly shaped by attrition and having rounded edges.
                                               

Aggregate Gradation

• Gradation, meaning the distribution of particles sizes within the total range of sizes, can be identified on a graphs as “well graded”, “uniform”, or “gap graded”.
• The gradation of a particular aggregate is most often determined by a sieve analysis and is an important feature of aggregates as specified by ASTM C-136.

Aggregate Gradation

• In a sieve analysis, a sample of dry aggregate of known weight is separated through a series of sieves with progressively smaller openings.
• Once separated, the weight of particles retained on each sieve is measured and compared to the total sample weight.
• Particle size distribution is then expressed as a percent retained by weight on each sieve size.

Sieving 

POWDER can be separated into various size fractions by vibrating sieve loaded with sample to enable the particles of size less than that of the mesh openings to pass through and the over size to remain in the sieve.

Sieve Analysis 

Sieve analysis helps to determine the particle size distribution of the coarse and fine aggregates.This is done by sieving the aggregates as per IS: 2386 (Part I) – 1963. In this we use different sieves as standardized by the IS code and then pass aggregates through them and thus collect different sized particles left over different sieves.

The apparatus used are –

i) A set of IS Sieves of sizes – 80mm, 63mm, 50mm, 40mm,31.5mm, 25mm, 20mm, 16mm,   12.5mm, 10mm, 6.3mm,4.75mm, 3.35mm, 2.36mm, 1.18mm, 600µm, 300µm, 150µm         and 75µm.

ii) Balance or scale with an accuracy to measure 0.1 percent of the weight of the test sample.

Grading Of Aggregates for Concrete

•  Aggregate comprises about 85 % volume of mass concrete
• Concrete contains aggregate upto a maximum size of 150 mm
• Way particles of aggregate fit together in the mix, as influenced by the gradation, shape, and surface texture
• Grading effects workability and finishing characteristic of fresh concrete, consequently  the properties of hardened concrete
• Good grading implies, sample of aggregates containing all standard fractions of aggregate in required proportion such that the sample contains minimum voids
• Well graded aggregate containing minimum voids will require minimum paste to fill up the voids in the aggregate
• Minimum paste means less quantity of cement and less quantity of water, hence increased economy, higher strength, lower-shrinkage and greater durability

Gap Grading

•  Voids created by higher size filled up by immediate next lower size
•  Lower size may not be accommodated in the available gap due to small voids left out which can reduce density
•  Voids created by a particular size can accommodate second or third lower size only
•  For example voids created by 40mm can accommodate 10mm & 4.75mm but not 20mm, this concept is called Gap Grading

Advantages of Gap-Graded Mix

• Increase strength and reduce creep and shrinkage
• Requirement of sand reduced by 26 to 40%
• Specific area of total aggregates will be reduced due to less sand
• Requires less cement as net volume of voids is reduced