In its simplest form, concrete is a mixture of paste and aggregates. The paste, composed
of portland cement and water, coats the surface of the fine and coarse aggregates.
Through a chemical reaction called hydration, the paste hardens and gains strength to
form the rock-like mass known as concrete.

Within this process lies the key to a remarkable trait of concrete: it's plastic and malleable
when newly mixed, strong and durable when hardened. These qualities explain why one
material, concrete, can build skyscrapers, bridges, sidewalks and superhighways, houses
and dams.

                                           Other Ingredients

Although most drinking water is suitable for use in concrete, aggregates are chosen
carefully. Aggregates comprise 60 to 75 percent of the total volume of concrete. The type
and size of the aggregate mixture depends on the thickness and purpose of the final
concrete product. Almost any natural water that is drinkable and has no pronounced taste
or odor may be used as mixing water for concrete. However, some waters that are not fit
for drinking may be suitable for concrete.

Excessive impurities in mixing water not only may affect setting time and concrete
strength, but also may cause efflorescence, staining, corrosion of reinforcement, volume
instability, and reduced durability. Specifications usually set limits on chlorides, sulfates,
 alkalis, and solids in mixing water unless tests can be performed to determine the effect
the impurity has on various properties. Relatively thin building sections call for small
coarse aggregate, though aggregates up to six inches (150 mm) in diameter have been
 used in large dams. A continuous gradation of particle sizes is desirable for efficient use
of the paste. In addition, aggregates should be clean and free from any matter that might
affect the quality of the concrete.

                                            Hydration Begins

Soon after the aggregates, water, and the cement are combined, the mixture starts to
harden. All portland cements are hydraulic cements that set and harden through a
 chemical reaction with water. During this reaction, called hydration, a node forms on the
surface of each cement particle. The node grows and expands until it links up with nodes
 from other cement particles or adheres to adjacent aggregates.

The building up process results in progressive stiffening, hardening, and strength
development. Once the concrete is thoroughly mixed and workable it should be placed in
forms before the mixture becomes too stiff.

During placement, the concrete is consolidated to
compact it within the forms and to eliminate potential
 flaws, such as honeycombs and air pockets. For
slabs, concrete is left to stand until the surface
 moisture film disappears. After the film disappears
from the surface, a wood or metal handfloat is used to
smooth off the concrete. Floating produces a relatively
even, but slightly rough, texture that has good slip
resistance and is frequently used as a final finish for
 exterior slabs. If a smooth, hard, dense surface is
required, floating is followed by steel troweling.

             
                                              Curing:
                  Curing ensures the continued hydration of the cement and the strength
 gain of the concrete. Concrete surfaces are cured by sprinkling with water fog, or by
using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods
prevent evaporation of the water by sealing the surface with plastic or special sprays
 (curing compounds).

Special techniques are used for curing concrete during extremely cold or hot weather to
 protect the concrete. The longer the concrete is kept moist, the stronger and more
durable it will become. The rate of hardening depends upon the composition and fineness
 of the cement, the mix proportions, and the moisture and temperature conditions. Most
of the hydration and strength gain take place within the first month of concrete's life
cycle, but hydration continues at a slower rate for many years.
 Concrete continues to get stronger as it gets older.

                                     The Forms of Concrete

Concrete is produced in four basic forms, each with unique applications and properties.
Ready-mix concrete , by far the most common form, accounts for nearly three-fourths
 of all concrete. It's batched at local plants for delivery in the familiar trucks with revolving
drums. Precast concrete products are cast in a factory setting. These products benefit
from tight quality control achievable at a production plant. Precast products range from
concrete bricks and paving stones to bridge girders, structural components, and panels
for cladding.

Placing & Finishing Concrete Mixing, transporting, and handling of concrete should be carefully coordinated with placing and finishing operations. Concrete should not be deposited more rapidly than it can be spread, struck off, consolidated, and bullfloated. Concrete should be deposited continuously as near as possible to its final position. In slab construction, placing should be started along the perimeter at one end of the work with each batch placed against previously dispatched concrete. Concrete should not be dumped in separate piles and then leveled and worked together; nor should the concrete be deposited in large piles and moved horizontally into final position. Consolidation In some types of construction, the concrete is placed in forms, then consolidated. Consolidation compacts fresh concrete to mold it within the forms and around embedded items and reinforcement and to eliminate stone pockets, honeycomb, and entrapped air. It should not remove significant amounts of intentionally entrained air. Vibration, either internal or external, is the most widely used method for consolidating concrete. When concrete is vibrated, the internal friction between the aggregate particles is temporarily destroyed and the concrete behaves like a liquid; it settles in the forms under the action of gravity and the large entrapped air voids rise more easily to the surface. Internal friction is reestablished as soon as vibration stops. Finishing Concrete that will be visible, such as slabs like driveways, highways, or patios, often needs finishing. Concrete slabs can be finished in many ways, depending on the intended service use. Options include various colors and textures, such as exposed aggregate or a patterned-stamped surface. Some surfaces may require only strikeoff and screeding to proper contour and elevation, while for other surfaces a broomed, floated, or troweled finish may be specified. In slab construction, screeding or strikeoff is the process of cutting off excess concrete to bring the top surface of the slab to proper grade. A straight edge is moved across the concrete with a sawing motion and advanced forward a short distance with each movement. Bullfloating eliminates high and low spots and embeds large aggregate particles immediately after strikeoff. This looks like a long-handled straight edge pulled across the concrete. Jointing is required to eliminate unsightly random cracks. Contraction joints are made with a hand groover or by inserting strips of plastic, wood, metal, or preformed joint material into the unhardened concrete. Sawcut joints can be made after the concrete is sufficiently hard or strong enough to prevent raveling. After the concrete has been jointed, it should be floated with a wood or metal hand float or with a finishing machine using float blades. This embeds aggregate particles just beneath the surface; removes slight imperfections, humps, and voids; and compacts the mortar at the surface in preparation for additional finishing operations. Where a smooth, hard, dense surface is desired, floating should be followed by steel troweling. Troweling should not be done on a surface that has not been floated; troweling after only bullfloating is not an adequate finish procedure. A slip-resistant surface can be produced by brooming before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression.  FOR MORE INFORMATION SEE THE PORTLAND CEMENT WEB SITE AT: http://www.cement.org/basics/concretebasics_concretebasics.asp