AGGREGATES

AGGREGATES

  Aggregates are the granular materials which added to cement with water to form concrete.

The reason for adding aggregates in concrete:

·        Greatly reduce the cost

·        Reduce heat release and also reduce thermal stress

·        Reduce the occurrence of shrinkage of  concrete

·        Produce a concrete with satisfactory plastic properties when the concrete is in fresh condition

Desirable properties of aggregates:

1.     Have low thermal conductivity

2.     Have graded well

3.     Have small or no moisture movement

4.     Suitable shape and texture which no adversely affect the properties of fresh and hardened concrete

5.     Sufficiently strength and strong

6.     Free from constituents which can reach harmfully with cement

Strength

Natural aggregates have high strength than normal concrete. Most aggregates are stronger than the concrete strength specified. Aggregates with moderate and low strength reduce the stress in the cement paste beside increase the durability of the concrete. Igneous rocks are stronger than sedimentary and metamorphic rocks.

Cleanliness

Aggregates should be free from substances which:

o   Reduced bond with aggregate such as clay and oil

o   Expandable  such as bituminous coal

o   Decomposable such as organic mater

o   Attract of moisture such as salt

o   Causes staining such as pyrites

o   Chemically incompatible with cement such as sulfates and organic matter

Porosity and absorption of water

 Surface of aggregates which have contained pores can absorb and hold water. This is acceptable condition as long as there is no or little moisture movement.

Before concrete mixing, aggregates can be in the either 4 moisture condition as shown below:

Ø Completely dry

Ø  Air dry which pores partially filled with water

Ø  Saturated with water and surface dry

Ø  Wet with excess water on surface

The below figure refer to the possible moisture conditions of aggregates

 

TOUGHNESS OF AGGREGATES

Toughness of aggregates is defines as resistance to failure by impact which can determine by testing of aggregates impact.

HARDNESS

Hardness of aggregates is defined as the resistance of an aggregate to wear which can determine by an abrasion test.

Soundness

The soundness is the ability of aggregate to withstand excessive change in volume to prevent shrinkage occurs.

SURFACE AND TEXTURE OF AGGREGATES

It can affect the properties of the concrete.

BONDINIG OF AGGREGATES

 The bonding is to interlocking the aggregate and hydrated cement paste. A clean and rougher surface can improve the bonding.

TYPES OF AGGREGATES

1.     Heavy aggregates

2.     Normal aggregates

3.     Light weight aggregates

The figure refer to the types of aggregates

HEAVY AGGREGATES

It is effectively and economically used for radiation shielding and for weight coating of submerged pipeline.

                  the above figure refer to the heavy aggregates concrete         

NORMAL AGGREGATES

It is suitable use for most purpose and generally consists of various combinations of sand, gravels, stones and different sizes of crushed rocks.

                  the above figure refer to the normal aggregates concrete                            

LIGHTWEIGHT AGGEGATES

It is used in the purpose of producing lightweight concrete. It can be processed from natural materials and By-products. Natural materials such as expanded clay or expended shale while by-products such as foamed slag or sintered pulverized fuel ash.

                             the above figure refer to the lightweight aggregates
                     

Grading requirement

Grading of aggregates indirectly affects the strength of the concrete which have to considerate the surface area of aggregates, workability and segregation.