Modern concrete is vastly different from its old-world counterparts, and uses mix designs that can be quite complex at times. The mix is decided based on what weather condition the structure is going to be exposed to, the maximum load that the structure will bear, and last but not the least, the cost of various components in it. After taking all these factors into consideration, a mix is prepared using cement (portland or other variety), coarse and fine aggregates, water, and other chemicals as required. The method of mixing and the conditions in which it can be used depends on the specific type of concrete. Various types of concrete have been developed to be used for different purposes. Following are some types of concrete:
- Regular Concrete:
This is the layman's term for describing concrete that is mixed by following the instructions given by the manufacturer of cement on the packet. This type of concrete is often mixed in improvised containers and the aggregate used is sand or other commonly found material. Its compressive strength ranges from 1,450 psi (pounds per square inch) for binding concrete to 5,800 psi for structural concrete. It can also be prepared by just adding water to a pre-mixed mixture of cement and aggregates. The typical ratio by weight of portland cement, dry sand, dry stone, and water in regular concrete is 1:2:3:0.5.
- High Strength Concrete:
This is prepared by reducing the water-to-cement ratio to 0.35 or lower. This renders a high compressive strength to this concrete, generally greater than 6,000 psi. To prevent the formation of free calcium hydroxide crystals due to lower water content in high strength concrete, silica fume is added to it while mixing. Lower water content also results in reduced workability of high strength concrete, and superplasticizers are added in order to compensate for this. The aggregates are carefully selected so that they can resist the high loads imposed on concrete to prevent failure.
- High Performance Concrete:
This variety is specifically designed to conform to a set of standards that are not limited to strength alone, e.g., long life in severe environment, density, permeability, long-term mechanical properties, etc. While all high strength concretes are high performance, not all high performance concretes are high strength. In other words, high strength concretes are a subset of high performance concretes, but not the other way round.
This variety is also known by the trade name of Gunite. It uses compressed air to shoot concrete onto a frame/structure, and is commonly used against vertical soil/rock surfaces to eliminate formwork. It is commonly used for rock support in tunneling or on construction sites where groundwater seepage is a problem. It is also used to fix loose soil or rocks at construction sites.
- Pervious Concrete:
As the name suggests, this variety is pervious or porous and has a network of cavities to allow air and water to pass through. This is prepared by altogether eliminating fine aggregate from the concrete mix and using a relatively low quantity of cement paste to bind the large aggregates. The water seeps through pervious concrete at around 5 gal/ft2/min (gallons per square feet per minute). This concrete is generally used for construction of roads to eliminate the need for an artificial drainage system. It also ensures that the groundwater tables get replenished evn though the earth is covered with concrete.
- Autoclaved Aerated Concrete:
This variety was invented by the Swedish inventor and architect Johan Axel Eriksson in the mid-1920s. It is produced by adding an air entraining agent as aggregate to the concrete, e.g., expanded clay pellets, cork granules, and vermiculite. Due to its porous structure, this concrete is lightweight, fire resistant, and has good insulating properties. When used in the construction of buildings, it reduces the load on the air conditioning system. It is generally used in the form of tiles/blocks that have a high workability due to their porous structure.
- Cork-Cement Composite:
Cork granules have a density of about 18.7 lb/ft3 (pounds per cubic feet), which is lower than most aggregates used for making lightweight concretes. For this reason, cement-cork composites are widely used. These composites have a density ranging from 25 to 94 lb/ft3, compressive strength from 145 to 3770 psi, and flexural strength from 72.5 to 580 psi. They have lower thermal conductivities, lower densities, and good absorption characteristics.
- Roller Compacted Concrete:
This type of concrete is placed on the surface of the earth and compacted using large hevay rollers typically used for earthwork. This technique gives the concrete mix a high density which cures overtime into a strong monolithic block. It is typically used for concrete pavements and for constructing dams. It generates less heat while curing due to the lower cement content than the conventional concrete. This variety is also known as rollcrete.
- Glass Concrete:
This variety of concrete uses recycled glass fragments as aggregate material. Apart from enhancing the aesthetic appeal, glass also gives better long-term strength and better thermal insulation to this type of concrete according to recent research being carried out at the Columbia University in New York.
- Asphalt Concrete:
This is a composite material using asphalt as a binding agent instead of cement. The aggregate used in this is sand, gravel, crushed stone, slag, recycled concrete, and geosynthetic aggregate. This type of concrete was refined and brought to its current state by the Belgian inventor Edward de Smedt. It is widely used in the construction of road surfaces, parking lots, and airport runways. It is also finding usage as the core of embankment dams.
- Rapid Strength Concrete:
This variety develops high strength within hours of being manufactured. This allows the early removal of formwork, which allows the construction work to be completed in record time. It is used in building road surfaces that become fully operational in a very short period of time.
- Rubberized Asphalt Concrete:
This a variety commonly used for building pavements that contains regular asphalt concrete mixed with recycled rubber tires. This technique of constructing pavements was pioneered by the city of Phoenix, Arizona in the 1960s.
- Polymer Concrete:
This is a group of concretes that use polymers to bind the aggregates. Its various types are polymer-impregnated concrete, polymer concrete, and polymer-portland-cement concrete.
- Geopolymer Concrete:
This is the green alternative to the ordinary portland cement, and is made from inorganic aluminosilicate (Al-Si) polymer compounds made from 100% recycled industrial waste, like fly ash and slag. Geopolymer concrete is said to have greater chemical and thermal resistance, and better mechanical properties, which are highly useful at both normal and extreme conditions.
- Refractory Cement:
This concrete is commonly used in high-temperature applications, e.g., masonry ovens and refractories. If normal portland cement-based concretes are used in such places, it generally results into damage to the refractory at elevated temperatures, but refractory concrete has a high thermal resistance. This class includes calcium aluminum cements, fire clay, ganisters, etc.
- Concrete Canvas:
It is a flexible concrete-impregnated fabric that hardens on hydration to form a thin, durable, fire-resistant, and waterproof concrete layer. It contains a three-dimensional fiber matrix, containing a specifically formulated concrete mix. It finds a variety of uses, e.g., in water management, erosion control, ditch lining, slope stabilization, pipeline protection, mining, ground resurfacing, etc.