Types of concrete
Concrete is produced in a variety of compositions, finishes and performance characteristics to meet a wide range of needs.
Mix design
Modern concrete mix designs can be complex. The choice of a concrete mix depends on the need of the project both in terms of strength and appearance and in relation to local legislation and building codes.The design begins by determining the requirements of the concrete. These requirements take into consideration the weather conditions that the concrete will be exposed to in service, and the required design strength. The compressive strength of a concrete is determined by taking standard molded, standard-cured cylinder samples.
Many factors need to be taken into account, from the cost of the various additives and aggregates, to the trade offs between the "slump" for easy mixing and placement and ultimate performance.
A mix is then designed using cement, coarse and fine aggregates, water and chemical admixtures. The method of mixing will also be specified, as well as conditions that it may be used in.
This allows a user of the concrete to be confident that the structure will perform properly.
Various types of concrete have been developed for specialist application and have become known by these names.
Concrete mixes can also be designed using software programs. Such software provides the user an opportunity to select their preferred method of mix design and enter the material data to arrive at proper mix designs.
Historic concrete composition
Concrete has been used since ancient times. Regular Roman concrete for example was made from volcanic ash, and hydrated lime. Roman concrete was superior to other concrete recipes used by other cultures. Besides volcanic ash for making regular Roman concrete, brick dust can also be used. Besides regular Roman concrete, the Romans also invented hydraulic concrete, which they made from volcanic ash and clay.Some types of concrete used to make garden sculptures and planters have been called composition stone or composite stone. There is no single precise formula that differentiates composition stone from other lime-cemented concretes, which is unsurprising because the term predates modern chemical science, being attested since at latest the 1790s. In the 19th and later centuries, the term artificial stone has encompassed various human-made stones including numerous cemented concretes.
Modern concrete
Regular concrete is the lay term for concrete that is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. The ingredients in any particular mix depends on the nature of the application. Regular concrete can typically withstand a pressure from about 10 MPa to 40 MPa, with lighter duty uses such as blinding concrete having a much lower MPa rating than structural concrete. Many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water.Typically, a batch of concrete can be made by using 1 part Portland cement, 2 parts dry sand, 3 parts dry stone, 1/2 part water. The parts are in terms of weight – not volume. For example, of concrete would be made using cement, water, dry sand, dry stone. This would make of concrete and would weigh about. The sand should be mortar or brick sand and the stone should be washed if possible. Organic materials should be removed from the sand and stone to ensure the highest strength.
High-strength concrete
High-strength concrete has a compressive strength greater than 40 MPa. In the UK, BS EN 206-1 defines High strength concrete as concrete with a compressive strength class higher than C50/60. High-strength concrete is made by lowering the water-cement ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond.Low W/C ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.
In some applications of high-strength concrete the design criterion is the elastic modulus rather than the ultimate compressive strength.
Stamped concrete
Stamped concrete is an architectural concrete that has a superior surface finish. After a concrete floor has been laid, floor hardeners are impregnated on the surface and a mold that may be textured to replicate a stone / brick or even wood is stamped on to give an attractive textured surface finish. After sufficient hardening, the surface is cleaned and generally sealed to provide protection. The wear resistance of stamped concrete is generally excellent and hence found in applications like parking lots, pavements, walkways etc.High-performance concrete
High-performance concrete is a relatively new term for concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is also high-performance, not all high-performance concrete is high-strength. Some examples of such standards currently used in relation to HPC are:- Ease of placement – HPC can be consolidated adequately by gravity and fills gaps between bars without vibration.
- Compaction without segregation
- Early age strength
- Long-term mechanical properties
- Permeability
- Density
- Heat of hydration
- Toughness
- Volume stability
- Long life in severe environments
- Depending on its implementation, environmental
Ultra-high-performance concrete
Ultra-high-performance concrete is a new type of concrete that is being developed by agencies concerned with infrastructure protection. UHPC is characterized by being a steel fibre-reinforced cement composite material with compressive strengths in excess of 150 MPa, up to and possibly exceeding 250 MPa.UHPC is also characterized by its constituent material make-up: typically fine-grained sand, fumed silica, small steel fibers, and special blends of high-strength Portland cement. Note that there is no large aggregate. The current types in production differ from normal concrete in compression by their strain hardening, followed by sudden brittle failure. Ongoing research into UHPC failure via tensile and shear failure is being conducted by multiple government agencies and universities around the world.
Micro-reinforced ultra-high-performance concrete
Micro-reinforced ultra-high-performance concrete is the next generation of UHPC. In addition to high compressive strength, durability and abrasion resistance of UHPC, micro-reinforced UHPC is characterized by extreme ductility, energy absorption and resistance to chemicals, water and temperature. The continuous, multi-layered, three dimensional micro-steel mesh exceeds UHPC in durability, ductility and strength. The performance of the discontinuous and scattered fibers in UHPC is relatively unpredictable. Micro-reinforced UHPC is used in blast, ballistic and earthquake resistant construction, structural and architectural overlays, and complex facades.Ducon was the early developer of micro-reinforced UHPC, which has been used in the construction of new World Trade Center in New York.
Low-density structural concrete
Ceramic aggregates with a density below that of water are used for low density structural concrete. These aggregates may include expanded clays and shales, preferably with water absorption below 10%. For structural concrete only coarse low density aggregates are used, with natural sand as the fine aggregates. However, lower percentages are used for moderate density concretes.The concrete can develop high compressive and tensile strengths, while shrinkage and creep remain acceptable, but will generally be less rigid than conventional mixes. The most obvious advantage is the low density, but these concretes also have low permeability to water and greater thermal insulation. Resistance to abrasion by ice is similar to normal concrete. Disadvantages are that the water absorption by the aggregates may be relatively high, and vibrational consolidation can cause the low density aggregate to float. This can be avoided by minimising vibration and using fluid mixes. Low density has advantages for floating structures.
Self-consolidating concrete
The defects in concrete in Japan were found to be mainly due to high water-cement ratio to increase workability. Poor compaction occurred mostly because of the need for speedy construction in the 1960s and 1970s. Hajime Okamura envisioned the need for concrete which is highly workable and does not rely on the mechanical force for compaction. During the 1980s, Okamura and his Ph.D. student Kazamasa Ozawa at the University of Tokyo developed self-compacting concrete which was cohesive, but flowable and took the shape of the formwork without use of any mechanical compaction. SCC is known as self-consolidating concrete in the United States.SCC is characterized by the following:
- extreme fluidity as measured by flow, typically between 650–750 mm on a flow table, rather than slump
- no need for vibrators to compact the concrete
- easier placement
- no bleeding, or aggregate segregation
- increased liquid head pressure, which can be detrimental to safety and workmanship
In 2005, self-consolidating concretes accounted for 10–15% of concrete sales in some European countries. In the precast concrete industry in the U.S., SCC represents over 75% of concrete production. 38 departments of transportation in the US accept the use of SCC for road and bridge projects.
This emerging technology is made possible by the use of polycarboxylates plasticizer instead of older naphthalene-based polymers, and viscosity modifiers to address aggregate segregation.