Aggregates

Different sizes and types of aggregates.

Aggregates are granular material such as sand, gravel, crushed stone, blast-furnace slag, and lightweight aggregates. These are added to cement with water to form concrete. The objective is to use as much aggregate as possible, binding the particles together with the hardened cement paste. Normally. the aggregates occupies approximately 65% to 80% of the volume of concrete.
The advantages to use aggregates in concrete are :
# Reduce the cost.
# Reduce heat output and therefore reduce thermal stress.
# Reduce shrinkage of concrete.
# Help to produce a fresh concrete to flow more easily.Provide dimensional stability.


Types of Aggregates
1. Lightweight Aggregates
These types of aggregates are used to produce lower density concretes, which are very good in reducing the weight of structures and also have better thermal insulation than normal weight concrete. Nowadays, artificial lightweight aggregates are commonly used in concrete because they are widely available . They can be divided into three main types:
# Sintered pulverised fuel ash
# Expanded clay or shale
# Foamed slag 
Lightweight aggregates are not as rigid as normal density aggregates, and therefore produce a lower elastic modulus and higher creep and shrinkage concrete. For the strength of concrete, the properties is depend on the lightweight aggregate type and source.

2. Normal Density Aggregates

The natural materials including gravels, igneous rocks (basalt & granite) and sedimentary rocks (limestone & sandstone)  are used to make concrete. All of the rocks have densities between the range of 2.55 to 2.75 and the produced concretes have densities between the range of 2250 to 2450 kg per meter cube. As a conclusion, the densities are depending on the mix proportions.

3. Heavyweight Aggregates

For the structures such as radiation shielding that required the concretes of high density, heavyweight aggregates can be used. The concretes that produce by barytes have densities between the range of 3500 to 4500 kg per meter cube while the concretes that produce by using steel shot have densities about 7000kg per meter cube.

Properties of Aggregates

1. Strength

Normal weight aggregates are generally considerably stronger than those of the hardened cement paste, and therefore do not have a major influence on the strength of most concretes. 

2. Porosity and Absorption of Water

Lightweight aggregates contain high porosity and normal aggregates also contain pores which can absorb and hold water. 

Before concrete mix, aggregates can be in one of the 4 moisture conditions such as:-

• Completely dry
• Air dry, pores partially filled with water
• Saturated with water and surface dry
• Wet with excess water on surface

3. Surface Characteristics

Surface texture developed the bond between an aggregate particle and a cementing material. A rough texture is producing a stronger bond and create a stronger portland cement concrete. A stronger bond also affected by the cleanliness of the surface which must not be contaminated by mud, clay or other materials.