Components of Modern Concrete
In this article “Components of Modern Concrete“, it is useful to define concrete and the principal concrete-making components. The concepts that follow are derived from ASTM C 125 and ACI Committee 116.
Concrete is a composite substance composed primarily of a binding agent in which aggregate particles are embedded. The binder of hydraulic cement concrete is a combination of hydraulic cement and water.
The below image shows that large structures of varying shapes may be constructed using cast-in-place and precast concrete elements.
Aggregate is granular material, such as sand, gravel, crushed stone, crushed blast furnace slag, or building and demolition scrap, that is combined with a cementing medium to form concrete or mortar.
The term coarse aggregate refers to aggregate particles bigger than 4.75 mm (No. 4 sieve), while the term fine aggregate refers to aggregate particles less than 4.75 mm but greater than 75 m. (No. 200 sieve). Gravel is the coarse aggregate formed as rock disintegrates naturally due to weathering.
The name sand is sometimes used to refer to fine aggregate formed by natural weathering or stone grinding. Crushed stone is a byproduct of industrial crushing of big blocks, boulders, or cobblestones. Iron blast-furnace slag is a by-product of the iron industry. It is obtained by crushing blast-furnace slag that has solidified due to gradual cooling under ambient conditions.
Aggregate from building and demolition waste is a byproduct of concrete, brick, or stone rubble disposal.
Mortar is a composite material composed of sand, cement, and water. It is comparable to concrete but without the coarse aggregate. Grout is a composite substance composed of cementitious material and aggregate, usually fine aggregate, to which ample water is applied to achieve a pouring quality without segregation of the constituents. Shotcrete is a term that refers to mortar or concrete that is pneumatically transferred through a hose and projected at high velocity onto a surface.
Cement is a finely pulverized, dried substance that is not a binder in and of itself but acquires the binding property upon hydration (i.e., from chemical reactions between cement minerals and water). When the hydration products of a mortar are stable in an aqueous environment, it is referred to as hydraulic cement.
The most frequently used hydraulic cement is portland cement, which is mainly composed of reactive calcium silicates; the calcium silicate hydrates produced during the hydration of portland cement are primarily responsible for its adhesive properties and are stable in an aqueous setting.
The preceding description of concrete as a mixture of hydraulic cement, aggregates, and water omits a fourth component: admixtures, which are commonly used in modern concrete mixtures.
Admixtures are components other than aggregates, cement, and water that are applied to the concrete batch prior to or after mixing. Admixtures are still widely used in concrete due to the many advantages associated with their use. Chemical admixtures, for example, can affect the rate of cement hydration, thus altering the setting and hardening characteristics of the cement paste.
Water-reducing admixtures can plasticize freshly mixed concrete by lowering the surface tension of the water; air-entraining admixtures can increase the durability of concrete subjected to cold weather; and mineral admixtures such as pozzolans (materials containing reactive silica) can mitigate thermal cracking of mass concrete.
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