Concrete is a composite material composed of fine and coarse aggregate bonded together with a cement past that hardens, or cures, over time. It is the most used man-made material on earth. Concrete’s usage worldwide, ton for ton, is twice that of steel, wood, plastics, and aluminum combined.
Concrete’s strength and durability are based on proportioning and mixing of its raw ingredients, including cement, sand, aggregates, and water, as well as the type of aggregate used. Fine and coarse aggregates, such as sand, natural gravel, or crushed rocks, make up the bulk of a concrete mixture. The size distribution of the aggregate determines how much binder is required. Portland cement is the most common cement used as a concrete binder. Different types of binding materials other than cement, such as lime and bitumen, can be used to create many types of concrete.
The portland cement is mixed with water to create a paste that coats the surface of the fine and coarse aggregates. Soon after the aggregates, water, and the cement are combined, the mixture starts to harden through a chemical reaction called hydration. Once the concrete is thoroughly mixed, it is poured in formworks before it 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.
After the exposed surfaces of the concrete have hardened sufficiently to resist marring, curing begins. Curing makes up for the moisture loss due to evaporation and ensures the consistent hydration of the cement so that the concrete continues to gain strength. 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. Concrete continues to get stronger as it gets older. While most of the hydration and strength gain takes place within the first month of concrete’s life cycle, hydration continues at a slower rate for many years.
Because concrete is the world’s most used man-made resource, it has an extensive range of standards and specifications to ensure a level of consistency between manufacturers of the material. In the U.S., concrete conforms to the many standards developed by the American Society for Testing and Materials (ASTM). The wide range of concrete and cement standards ranges from abrasion testing and physical tests to strength, sulfate content, and much more. These standards allow laboratories around the world to test and evaluate concrete mixtures to ensure their strength and safety, and help to identify the various properties of concrete including strength, elasticity, hardness, and workability.
When selecting concrete for a project, it is necessary to choose the proper grade, which is the strength of a concrete mixture after 28 days, when concrete has nearly reached full strength. Grade of concrete is denoted by “M”, which stands for mix. For example, for M5 grade, the mixed proportion can be 1:5:10, where 1 is the ratio of cement, 5 is the ratio of sand, and 10 is the ratio of the aggregate based on the volume or weight of materials.
The grade of concrete construction is selected based on structural design requirements. There are two types of concrete mixes, nominal mix and design mix. Nominal mix concretes are those generally used for small scale construction, while design mix concretes are those for which mix proportions are obtained from various lab tests. Once suitable mix proportions are known, then its ingredients are mixed in the ratio as selected
Concrete is generally used in two types of construction, plain concrete construction and reinforced concrete construction. In plain concrete construction, the concrete is poured and casted without use of any reinforcement. For larger structures, concrete can be reinforced with metal structures like rebar, fiber or other agitates in order to increase the structure’s strength or performance capabilities. Without reinforcement, constructing modern structures with concrete material would not be possible.