Concrete is a hard and chemically inert structural material consisting of a mixture of aggregate (usually sand and gravel), cement, and water. This mixture, known as Portland cement, is the dominant cementing agent used in the production of concrete. When the aggregate is mixed with dry Portland cement and water, it forms a fluid slurry that can be easily poured and shaped into any form. The hydration process of the cement reacts with the water to form a hard matrix that binds the materials together into a durable stone-like material with many uses. The time it takes for concrete to set allows it to be molded into shapes, as well as to perform a variety of mechanized processes.
The hydration process is exothermic, meaning that ambient temperature plays an important role in the setting time. Additives such as pozzolans or superplasticizers are often included in the blend to improve the physical properties of the wet mix, delay or accelerate curing time, or change the finished material. Most concrete is poured with reinforcing materials such as reinforcing bars embedded to provide tensile strength, resulting in reinforced concrete. In the past, lime-based cement binders such as lime putty were often used, but sometimes with other hydraulic (water resistant) cements such as calcium aluminate cement or Portland cement to form Portland cement concrete (named for its visual resemblance to Portland stone). There are many other types of non-cementitious concrete with other methods for joining aggregates, including asphalt concrete with a bituminous binder, which is often used for road surfaces, and polymer concretes that use polymers as a binder. While concrete is itself a building material, mortar is a bonding agent that normally holds bricks, tiles and other masonry units together.
The Mayans used concrete in the ruins of Uxmal and it was mentioned by John L. The roof was flat and had been covered with cement. The floors were concrete, in some places hard but due to prolonged exposure they were broken and now crumbled underfoot. But throughout the wall it was solid and consisted of large stones embedded in mortar, almost as hard as rock. Structures that use Portland cement concrete usually include steel reinforcement because this type of concrete can be formulated with a high compressive strength but it always has a lower tensile strength.
Therefore, it is generally reinforced with materials that are strong in tension, typically steel reinforcing bars. Fine and coarse aggregates make up most of a concrete mix. Sand, natural gravel and crushed stone are mainly used for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements for natural aggregates, while various manufactured aggregates are also allowed, including air-cooled blast furnace slag and bottom ash. Concrete production is the process of mixing the various ingredients: water, aggregate, cement and any additives to produce concrete. Once the ingredients are mixed, workers must put the concrete in place before it hardens.
In modern use, most of the production of concrete is carried out in a large type of industrial facility called a concrete plant or batch plant. In general use, concrete plants come in two main types: pre-mix plants and central mix plants. A pre-mixed plant mixes all ingredients except water while a central mixing plant mixes all ingredients including water. A central mixing plant offers more precise control of concrete quality through better measurements of the amount of water added but should be placed closer to the job site where the concrete will be used as hydration begins at the plant. A concrete plant consists of large storage hoppers for various reactive ingredients such as cement, storage of bulk ingredients such as aggregates and water, mechanisms for the addition of various additives and amendments, machinery for accurately weighing, moving and mixing some or all of those ingredients, and facilities for dispensing mixed concrete often to a concrete mixer truck. Modern concrete is usually prepared as a viscous fluid so that it can be poured into forms which are containers erected in the field to give concrete the desired shape. Concrete formwork can be prepared in several ways such as sliding forming and steel plate construction.
Alternatively, concrete can be mixed into dry non-fluid forms and used in factories to manufacture precast concrete products. An engineer decides on design mixing ratios after analyzing the properties of the specific ingredients being used instead of using a 'nominal mixture' of 1 part cement 2 parts sand and 4 parts aggregate (the second example above). A civil engineer will custom design a concrete mix to exactly meet site requirements and conditions fixing the proportions of the materials and often designing a package of additives to adjust the properties or increase the performance envelope of the mixture. Design mix concrete can have very wide specifications that cannot be met with more basic nominal mixtures but the involvement of an engineer often increases the cost of concrete mix. Proper curing of concrete leads to higher strength and lower permeability and prevents cracking when the surface dries prematurely. Care should also be taken to avoid freezing or overheating due to exothermic setting of cement.
Improper curing can cause fouling reduced strength poor abrasion resistance and cracking. The terms asphalt (or asphalt) concrete bituminous asphalt concrete and bituminous mixture are typically used only in engineering and construction documents which define concrete as any composite.