AGGREGATES

DEFINITION: Inert or chemically inactive materials which when bonded together with cement forms concrete

70% to 80% of concrete is composed of aggregates Properties of aggregates affect properties of concrete. Avoids cracking and gives strength to concrete. Eg: natural sand, gravel, crushed stone etc.

CLASSIFICATION According to nature of formation According to size Natural aggregates Artificial aggregates According to size Coarse aggregates Fine aggregates All-in aggregates According to shape Rounded aggregates Irregular or partially rounded aggregates Flaky aggregates Angular aggregates Elongated aggregates

According to nature of formation Natural aggregates: obtained from natural sources

Artificial aggregates: Artificial aggregates: manufactured from raw materials by fusing in rotary kiln

According to size: Coarse aggregates: retained on 4.75mm sieve. max. size for plain concrete – 150mm max. size for reinforced concrete – 60mm or less

Single sized aggregates Coarse aggregates: Graded aggregates Single sized aggregates

Fine aggregates:. passing through 4. 75mm seive and retained on 0 Fine aggregates: passing through 4.75mm seive and retained on 0.15mm sieve.

Fine aggregates: Natural sand Crushed stone sand Broken brick fine aggregate

All – in aggregates Comes from pit or river bed Used without seperating into different sizes. Contains fine and coarse aggregates Used for unimportant works

According to shape Rounded aggregates: Surface area minimum Concrete requires lesser cement – low strength

Irregular aggregates: natural irregularity due to attrition with rounded edges

Flaky aggregates: usually angular – thickness is small relative to other dimensions

Angular aggregates: - well defined edges - concrete – more cement – high strength and durability

Elongated aggregates: Elongated aggregates: length is considerably greater than other two dimensions concrete – poor quality and less durable.

Sources of aggregates: Pit Sand River Sand Sea Sand

Pit Sand: pits dug at a depth of 1 Pit Sand: pits dug at a depth of 1.5m to 2m in soil sharp, angular porous and free from harmful chemicals for making mortar

River Sand: obtained from river beds and banks bright and clear – sharp or rounded for mortar and plastering

Sea Sand: seashores brown and rounded contains salts – not used for construction

Manufactured Sand: manufactured in stone crushers less impurities better control over size and quality

Requirements: Hard, strong, durable Should not react with cement or steel Angular and rough surface Free from organic substances Contains stone gravels and sand or in various combinations

Grading of aggregates Representation of particle size in a sample of aggregates Good grading – all standard sizes in required proportions Sieve analysis – determination of proportions of particles in an aggregate by seperation on sieves

Sieve analysis 80 mm 40 mm 20 mm 10 mm 4.75 mm 2.36 mm 1.18 mm 600 microns 300 microns 150 microns pan

Sieve analysis

Weight of residue on each sieve % weight retained Cummulative % weight retained Fineness modulus = WR – sum of cummulative % retained

type Fineness modulus Fine sand 2.2 to 2.6 Medium sand 2.6 to 2.9 Sands are classified as: type Fineness modulus Fine sand 2.2 to 2.6 Medium sand 2.6 to 2.9 Coarse sand 2.9 to 3.2

Based on gradation well graded – good representation of all sizes

Poorly graded: excess of of certain sizes

Gap graded: one or intermediate sizes are missing