Aggregates

Aggregates are inert filler materials, chemically unreactive, which

are mixed with cement to make concrete.

Added up to 70-80% by volume in ordinary concrete to provide bulk.

Their Characteristics influence the properties of concrete.

Sand is known as fine aggregates whereas crushed basalt is referred to as coarse aggregates.

Manufacture of Aggregates in Mauritius

In Mauritius, aggregates are produced in accordance to BS 882.

Huge rocks from excavation sites or sugar cane fields are transported to stone crushers.

The production of the aggregates involves:

- Crushing reduce in size

- Screening separate according to size

- Product handling and storage

1- Vibrating feeder ll Jaw crusher lll Impact crusher lV Vibrating Screen (separator)

0-5mm 5 -20mm 20-40 mm

0-5mm

The boulders are placed in a vibrating feeder, where large rocks, smaller sized rocks and soils are separated.

The smaller sized rocks are sent to the first stockpile.

The remaining boulders are then sent to crushers.

Crushers are classified according to the stage of crushing which they accomplish:

Primary

Secondary

Tertiary

A Primary crusher receives the stone directly from the vibrating feeder and produces the first reduction in size.

Larger boulders are fed into a jaw crusher (primary crusher) which can crush up to 250 tons of rock per hour. It reduces the boulders size by 1/3 of its original size.

Jaw crusher operates by allowing the stone to flow in between the

space of 2 jaws, one of which is stationary and the other is movable. The distance between the jaws decreases as we go downwards.

If additional reduction in size is required, before feeding to secondary crusher, the output from the primary is sent to a roller crusher.

Output of the primary crusher(s) is fed to a secondary crusher (impact crusher) which further reduces the stone size.

The crushed stones are then sent to a vibrating separator where they are divided according to their sizes.

Stones of size < 40mm are further subgraded (20 - 40mm, 5-20mm, 0-5mm,) and sent to 3 different stockpiles.

Stones > 40mm are sent back to the impactor.

2 TYPES OF AGGREGATES- Fine aggregates and coarse aggregates

Tertiary crushers are capable of producing large quantities of uniformly fine crushed stones.

Classification of Aggregates

ORIGIN : Igneous, Sedimentary, Metamorphic

SIZE : Crushed basalt (macadam), Coral sand or Rock sand

SHAPE : Rounded, Elongated, Flaky, Irregular, Angular, F;aky and elongated

TEXTURE : Rough, smooth

WEIGHT : Heavy, Normal, Lightweight- Density

Origin

Aggregates are obtained by crushing large rocks.

If parent rock is of:

- Igneous origin, aggregates produced are of good quality and high strength.

Igneous Rocks

Intrusive (plutonic): coarse-grained; granite

Shallow Intrusive: fine-grained; riolite,andesite, basalt

Extrusive: fine-grained; tuff, pumice, basalt

hard, tough, strong : excellent aggregate.

Sedimentary Rocks (cost effective - near the surface)

about 80% of aggregates

Natural sand and gravel

Sandstone, limestone (dolomite), chert, flint, graywacke

Metamorphic Rocks: slate, gneiss : excellent to poor

- Sedimentary origin, produce weaker aggregates

Size of Aggregates

According to size, aggregates are classified as: coarse, fine and all-in aggregates.

Coarse aggregates

Aggregates retained on a 4.75 mm sieve are classified as coarse.

Fine aggregates

Aggregates passing through a 4.75 mm sieve are called fine. The smallest size of fine aggregate (sand) is 0.06 mm.

All- in aggregates

It is a mixture of different fractions of fine and coarse

aggregates.

The deficiency of any particular fraction of aggregate can be corrected by using all-in aggregates, but they are not recommended for quality concrete.

In ordinary concrete work, a maximum size of 20 mm is used for coarse aggregates.

In Mauritius, the common sizes are 6, 10, 14 and 20mm.

There are certain factors that limit the maximum size of aggregates to be used:

(a)The size of the section of concrete to be casted, which must not be < 4 times the size of the aggregate.

(b) Concrete cover for embedded steel/ PVC pipes which must not

be < than the maximum size of aggregates.

(c) The size of the bar and bar spacing (to allow the concrete to

flow through easily).

Shape & Texture

The shape of aggregates determine the volume of void space that

will be left within a concrete mix.

The texture determines the effectiveness of bonding with

cementing material.

Shape

Rounded aggregate

-is generally obtained from rivers or seashore and produce minimum void (about 32 %) in the concrete. It accepts lesser cement paste. It provide poor interlocking bond which make it unsuitable for high strength concrete.

The shapes of aggregates are classified as rounded, irregular, angular, elongated, flaky and flaky & elongated.

Irregular aggregates

- have voids of about 36 % and require more cement paste than that of rounded aggregates. Because of its irregularity in shape (partly by attrition), they develop good bond and are suitable for making ordinary concrete.

Angular aggregates

- have sharp, angular and rough particles having maximum voids of about 40%. They provide very good bonding and are most suitable for high strength concrete.

Flaky aggregates

-They generally orient in one position with water and air vids underneath. Thus they adversely affect durability of concrete and are restricted to a maximum of 15 % only.

Elongated aggregates

- Usually angular in which length is larger than the other 2 dimensions.

Flaky & Elongated aggregates

- Material having length considerably larger than width and the width larger than thickness

Weight of Aggregates

The heavier the parent rock source, the heavier the aggregates produced.

Heavyweight Aggregates (4000-8500 kg/m3)

-are made from iron shots or lead shots. They provide an effective and economical use of concrete for radiation shielding, by giving necessary protection against X-rays, gamma rays and neutrons. It is also used for weight coating of submerged pipelines.

Normal Aggregate (2300-2500 kg/m3)

Compact bluish basalt produces Normal aggregates and are suitable for most structural buildings. Normal aggregates can be divided into natural and artificial.

Natural aggregates include crushed rock, sand and gravel.

Artificial aggregates include air cooled blast furnace slag and broken bricks.

Lightweight Aggregate(350 500 kg/m3)

Highly porous basalt produces Lightweight aggregates. They are mostly used in insulating screeds and in the manufacture of precast concrete blocks.

Concretes made with lightweight aggregates have good fire resistant properties.

They are highly porous and absorb considerably greater quantities of water than do normal aggregates.

Properties of Aggregates

Properties need to be considered when selecting aggregates for making concrete.

Their properties are directly related to that of their parent rock source.

Bonding- Texture

- A rough surfaced aggregates enables a firm grip between aggregates and cement paste.

Size of aggregate- Grading of aggregates

Influence packing density and void content, amount of aggregates to be used, cement and water requirements of the mix

Workability, stability, strength, stiffness, durability, segregation and permeability of the concrete mix (fresh and hardened).

Grading- Determination of particle size distribution- Gap v/s continuously graded aggregates

Fineness Modulus

Bulk Density

Bulk density (dry-rodded unit weight) mass of the aggregate that would fill a unit volume of bulk aggregate: affects the following concrete behaviour: mix design, workability, and unit weight- inclusive of the individual particle and the volume of voids.

Properties of Aggregates

Properties of Aggregates

Flakiness index and elongation index

Porosity and moisture content

Specific gravity of aggregates- Absolute s.g and Apparent s.g

Soundness and Cleanliness of aggregates

Must be clean & durable & Free from organic impurities & dust (WHY?)

It may prevent the cement paste from coating the aggregate properly thus:

- preventing bonding

- reduce strength of the concrete

Properties of Aggregates

Strength

Ability of an aggregate particle to stand up to pulling or crushing forces- Aggregate Crushing Value (ACV).

- The Aggregate Impact Value (AIV) is a test carried out to determine its resistance to impact (Toughness of aggregate).

- Los Angeles Abrasion Test- resistance of an aggregate to wear

- well shaped cubical stones provide higher resistance to impact as compared to flaky and elongated stones.

- The lower the ACV, the stronger the aggregate

- Aggregate with his Los Angeles Value will tend to dust during production and handling- environmental and mix problems

Others

Alkali- Aggregate

Reactions- Alkali silica reaction (ASR) and Alkali carbonate reaction (ACR)

Segregation- 2 Types

In most concrete, aggregates are more or less chemically inert. However, some aggregates react with the alkali hydroxides in concrete, causing expansion and cracking over a period of many years. This alkali-aggregate reaction has two forms: alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).

Alkali-silica reaction (ASR) is of more concern because aggregates containing reactive silica materials are more common. In ASR, aggregates containing certain forms of silica will react with alkali hydroxide in concrete to form a gel that swells as it adsorbs water from the surrounding cement paste or the environment. These gels can induce enough expansive pressure to damage concrete.

Typical indicators of ASR are random map cracking and, in advanced cases, closed joints and attendant spalled concrete. Cracking usually appears in areas with a frequent supply of moisture, such as close to the waterline in piers, near the ground behind retaining walls, near joints and free edges in pavements, or in piers or columns subject to wicking action. Petrographic examination can conclusively identify ASR.

Alkali-silica reaction can be controlled using certain supplementary cementitious materials. In-proper proportions, silica fume, fly ash, and ground granulated blast-furnace slag have significantly reduced or eliminated expansion due to alkali-silica reactivity. In addition, lithium compounds have been used to reduce ASR. Although potentially reactive aggregates exist throughout North America, alkali-silica reaction distress in concrete is not that common because of the measures taken to control it. It is also important to note that not all ASR gel reactions produce destructive swelling.

Alkali-carbonate reaction (ACR) is observed with certain dolomitic rocks. Dedolomitization, the breaking down of dolomite, is normally associated with expansion. This reaction and subsequent crystallization of brucite may cause considerable expansion. The deterioration caused by alkali-carbonate reactions is similar to that caused by ASR; however, ACR is relatively rare because aggregates susceptible to this phenomenon are less common and are usually unsuitable for use in concrete for other reasons. Aggregates susceptible to ACR tend to have a characteristic texture that can be identified by petrographers. Unlike alkali carbonate reaction, the use of supplementary cementing materials does not prevent deleterious expansion due to ACR. It is recommended that ACR susceptible aggregates not be used in concrete.