Concrete is a composite construction material made primarily with aggregate, cement,
and water. There are many formulations of concrete, which provide varied properties,
and concrete is the most-used man-made product in the world.
Concrete is widely used for making architectural structures, foundations, brick/block
walls, pavements, bridges/overpasses, motorways/roads, runways, parking structures,
dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats.
Concrete technology was known by the Ancient Romans and was widely used within the
Roman Empire—the Colosseum is largely built of concrete and the concrete dome of
the Pantheon is the world's largest. After the Empire passed, use of concrete became
scarce until the technology was re-pioneered in the mid-18th century.
In modern times, researchers have experimented with the addition of other materials
to create concrete with improved properties, such as higher strength or electrical
conductivity.
There are many types of concrete available, created by varying the proportions of
the main ingredients below. In this way or by substitution for the cementitious
and aggregate phases, the finished product can be tailored to its application with
varying strength, density, or chemical and thermal resistance properties.
"Aggregate" consists of large chunks of material in a concrete mix, generally
a coarse gravel or crushed rocks such as limestone, or granite, along with finer
materials such as sand.
"Cement", commonly Portland cement, and other cementitious materials such
as fly ash and slag cement, serve as a binder for the aggregate.
Water is then mixed with this dry composite, which produces a semi-liquid that workers
can shape (typically by pouring it into a form). The concrete solidifies and hardens
to rock-hard strength through a chemical process called hydration. The water reacts
with the cement, which bonds the other components together, creating a robust stone-like
material.
"Chemical admixtures" are added to achieve varied properties. These ingredients
may speed or slow down the rate at which the concrete hardens, and impart many other
useful properties.
"Reinforcements" are often added to concrete. Concrete can be formulated
with high compressive strength, but always has lower tensile strength. For this
reason it is usually reinforced with materials that are strong in tension (often
steel). Concrete can be damaged by many processes, such as the freezing of trapped
water.
"Mineral admixtures" are becoming more popular in recent decades. The
use of recycled materials as concrete ingredients has been gaining popularity because
of increasingly stringent environmental legislation, and the discovery that such
materials often have complimentary and valuable properties. The most conspicuous
of these are fly ash, a by-product of coal-fired power plants, and silica fume,
a byproduct of industrial electric arc furnaces. The use of these materials in concrete
reduces the amount of resources required as the ash and fume acts as a cement replacement.
This displaces some cement production, an energetically expensive and environmentally
problematic process, while reducing the amount of industrial waste that must be
disposed of.
The mix design depends on the type of structure being built, how the concrete is
mixed and delivered, and how it is placed to form the structure.
Concrete is strong in compression, as the aggregate efficiently carries the compression
load. However, it is weak in tension as the cement holding the aggregate in place
can crack, allowing the structure to fail. Reinforced concrete adds either steel
reinforcing bars, steel fibers, glass fiber, or plastic fiber to carry tensile loads.
Chemical admixtures are materials in the form of powder or fluids that are added
to the concrete to give it certain characteristics not obtainable with plain concrete
mixes. In normal use, admixture dosages are less than 5% by mass of cement and are
added to the concrete at the time of batching/mixing. The common types of admixtures
are as follows.
Accelerators speed up the hydration (hardening) of the concrete.
Retarders slow the hydration of concrete and are used in large or difficult pours
where partial setting before the pour is complete is undesirable.
Air entrainments add and entrain tiny air bubbles in the concrete, which reduces
damage during freeze-thaw cycles, increasing durability.
Plasticizers increase the workability of plastic or "fresh" concrete,
allowing it be placed more easily, with less consolidating effort.
Pigments can be used to change the color of concrete, for aesthetics.
Corrosion inhibitors are used to minimize the corrosion of steel and steel bars
in concrete.
Bonding agents are used to create a bond between old and new concrete.
Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.
Concrete can be damaged by many processes, such as the expansion of corrosion products
of the steel reinforcement bars, freezing of trapped water, fire or radiant heat,
aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by
fast-flowing water, physical damage and chemical damage (from carbonatation, chlorides,
sulfates and distillate water)
