Aggregates

AGGREGATESAGGREGATES

DEFINITION

A combination of different sizes and shapes normally of stones. Maximum size is 75 mm.

USES OF AGGREGATES

As an underlying material for foundations and pavements

As an ingredients in Portland cement concrete and asphalt concrete.

Selection of aggregates

• aggregates shall be hard, durable and clean and free from adherent coatings and organic matter and shall not contain appreciable amount of clay.

• Aggregates shall not contain harmful impurities such as iron pyrites, alkalis, salts, coal, mica, shale or other materials which will affect hardening and attack reinforcement.

Classification of aggregates

Based on size: classified into 2 categories:

• fine aggregates - those aggregates which pass through 4.75 mm sieve or aggregates with size less than 5 mm.

• Coarse aggregates – those aggregates Passing through 75 mm sieve and entirely retained on 4.75 mm sieve OR those

aggregates with size greater than 5 mm.

Classification of aggregates

Based on source or method of manufacture: classified into 2 categories:

• Natural aggregate/uncrushed aggregate- Those from the river beds, river sand and ex-mines. Normally rounded in shape and have smooth surface texture.

• Manufactured aggregate / crushed aggregate –those obtained by mechanically crushing rocks, boulders, or cobbles. Normally angular in shape and have rough surface texture

Classification of aggregates

Based on Density: Based on specific gravity or density

measured in bulk, aggregate is divided into 3 types:

• Lightweight aggregate

• Normal-weight aggregate

• Heavyweight aggregate

Normal-weight aggregate Crushed stone, gravel and ordinary sand are

examples of normal weight aggregate. They are commonly used in manufacture of

normal weight concrete, asphalt concrete and roadway sub-base.

The average values of sp.gr. For sand and gravel are 2.6 and 2.65 respectively. Bulk density of normal weight aggregate is around 1520 to 1680kg/m3.

Lightweight aggregate Lightweight fine aggregate is any aggregate with

bulk density less than 1120kg/m3 and lightweight coarse aggregate is any aggregate with bulk density less than 880kg/m3.

They are commonly used as ingredients in the manufacture of lightweight concrete, for making lightweight masonry blocks (to improved their thermal and insulating properties and nailing characteristic), and lightweight floor and roof slabs.

2 types of lightweight aggregate:• Natural lightweight aggregates (eg: palm oil shell, rice husk,

etc) • Manufactured (also called synthetic) lightweight aggregates.

Heavyweight aggregate Those aggregate with high density and is

used primarily in the manufacture of heavyweight concrete, employed for protection against nuclear radiation and as bomb shelter.

The unit weight of heavyweight concrete varies from 2400kg/m3 with sp.gr range from 4.0 to 4.6.(eg: mineral ores and barite)

Physical properties of aggregates

Strength Hardness Toughness Durability Porosity Water absorption

Strength of aggregates In practice, majority of normal

aggregates are considerably stronger than concrete

A good average value of crushing strength of aggregates is 200N/mm2.

Hardness of aggregates Hardness of aggregates is the ability of the

aggregate to withstand wear or load or applied pressure.

This hardness is depending on the type of parent rock

The test that can obtain the hardness is the abrassion test.

A satisfactory aggregate should have an abrassion value of not more than 30% for aggregates used for wearing surfaces and 50% for aggregates used for non wearing surfaces.

Toughness Is the resistance of aggregate to failure by

impact. This can be determined by aggregate impact

test. The aggregate impact value shall not exceed

45% by weight for aggregate used for concrete other than those used for wearing surfaces and 30% for concrete for wearing surfaces.

Durability of aggregates Durability is ability of the aggregate to

withstand external or internal damaging attack or in other words the soundness of aggregate.

This can be obtained by carrying out the Soundness test.

Porosity Aggregate normally have pores of various

sizes. Aggregates will absorb water when it is dry but

normally release water in the concrete mix when it is wet.

The amount of water and its rate of permeation depends on the size and volume of aggregate

Since the aggregate comprises 75% of the concrete volume, it is essential to note that porosity of an aggregate contribute to the overall porosity of concrete.

Tests on aggregates

a.Texture and shapes test i.e. test for shapes of aggregates:1. round shape –usually natural aggregates2. irregular shape- a combination of different shapes3. angular shape- usually of crushed stone4. flaky shape- where the thickness is less than its length and

width5. elongated- usually angular where its length is larger than its

width and thickness6. flaky and elongated- its length is larger than its width and its

width is larger than its thickness. In terms of surface texture, the aggregates may have a smooth

texture, or coarse/rough texture or fissures or porous. For production of concrete the aggregates which have angular

shape and coarse texture are recommended to have high bond strength.

Strength tests• Aggregates crushing strength

• Los Angeles abrasion test

• Aggregates Impact value test

• Ten percent fine test

Tests on aggregates (Cont’d)

Physical properties:

• Specific gravity: usually 2.6 or 2.7

• Water absorption test: aggregates absorb water because of their porosity.

If all the pores are filled with water the aggregates are said to be saturated and surface dry.

If all the water inside the pores are removed by drying, the aggregates are said to have maximum dry weight.

Tests on aggregates (Cont’d)

Coarse and fine aggregates to be used for making concrete should be well graded. Gradation means the particle size distribution of aggregates. Test for grading of aggregates is carried out using the sieve analysis method.

Grading of aggregates:

Sieve analysis test equipment

EXAMPLE OF SIEVE ANALYSIS OF SAND

Sieve Mass Cumulative Mass %Size retained mass retained passing passing

10 mm 0 0 287 100 5 mm 6 6 281 982.36 mm 17 23 264 92 1.18 mm 32 55 232 81 600µmm 48 103 184 64 300µmm 81 184 103 36150µmm 86 270 17 6 Pan 17 287 - - 287

A sieve analysis of 250g of sand gives the

following results

Sieve size Mass retained 10 mm 0 5 mm 5 2.36 mm 311.18 mm 38600µmm 38300µmm 79 150µm 51Pan 8

M.S. 30- Typical grading of fine aggregates % passing by weight B.S. 410 Zone 2 Zone 3 Test Sieve ------------------------------------------------------ 10.0 mm 100 100 5.0 90-100 90-100 2.36 75-100 85-100 1.18 55-90 75-100 600µm 35-59 60-79 300µm 8-30 12-40 150µm 0-10 0-10

To calculate moisture content of AggregatesExample:A sample of damp aggregate weighing 2.35 kg is dried by hair dryer until it just reaches the free-running(saturated surface dry) condition. It is then found to weigh 2.24 kg. After drying in the oven at 110°C to constant mass, it is found to weigh 2.15 kg. Based on dry mass, calculate:•free-water content•the total water content

Solution: a) Free-water content in aggregate is defined as the water content

when the aggregate is in saturated surface dry conditions. i.e. initial weight = 2.35 kg weight at SSD = 2.24 kg free water content = 0.11 kg % of free-water content based on dry mass = (0.11/2.15) x 100 = 5.1% b) Total water content = free-water content + water absorbed by aggregates = 0.11 + (2.24 – 2.15) = 0.2 kg OR 2.35 – 2.15 = 0.2 kg (i.e overall weight - wt of max. dry aggregates) % of total water content based on dry mass, = 0.2/2.15 x 100 = 9.3%