Concrete and Concrete Materials
Concrete is a mixture of four basic materials: Portland cement, fine aggregates, coarse aggregates, and water.
When first mixed, concrete is in a semi-liquid state and is referred to as plastic concrete. When the concrete hardens, but has not yet gained structural strength, it is called green concrete.
After concrete has hardened and gained its structural strength, it is called cured concrete.
Various types of concrete can be obtained by varying the basic materials and/or by adding other materials to the mix. These added materials are called admixtures.
A) Portland Cement:
Portland cement is a finely ground powder consisting of varying amounts of lime, silica, alumina, iron, and other trace components. A) Portland Cement (continued):
While dry, it may be moved in bulk or can be bagged in moisture-resistant sacks and stored for relatively long periods of time.
Portland cement is hydraulic cement because it will set and harden by reacting with water, with or without the presence of air.
B) Aggregates for Concrete:
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are used mainly for this purpose.
Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.
Natural sand, gravel, crushed stone, blast furnace slag, and manufactured sand (from crushed stone, gravel, or slag) are the most commonly used aggregates in concrete.
In some cases, recycled crushed concrete is used for low-strength concrete applications. There are other types of special aggregates for concrete.
C) Water for Concrete:
Normal drinkable water (unless it is extremely hard or contains too many sulfates) may be used to make concrete.
High concentrations of chlorides, sulfates, alkalis, salts, and other contaminants have corrosive effects on concrete and/or metal reinforcing rods, mesh, and cables.
Click on each item in the list to know their effect on concrete:
Sulfates can cause disintegration of concrete.
2. Alkalis, Sodium carbonate, and Bicarbonates
Alkalis, sodium carbonate, and bicarbonates may affect the hardening times as well as the strength of concrete.
D) Admixtures for Concrete:
Admixtures are materials added to a concrete mix before or during mixing to modify the characteristics of the final concrete.
They may be used to improve the workability during placement, increase strength, retard or accelerate strength development, and increase frost resistance.