Along with advances in the ways that concrete is brought to the site, the types of formwork in which it is cured, and how it is placed at high elevations, its mechanical and chemical properties have made great advances in the past century. Again, many of these were developed and used in the first half of the twentieth century, but are valid for today's applications and further improvements.
Lightweight Concrete: It can be made from a variety of aggregate types. A few of these are scoria, pumice, vermiculite, perlite, herculite and polystyrene beads. Extra-lightweight aggregates, such as: polystyrene beads, perlite and vermiculite are considered low density and can produce concrete weights as low as 50 pounds per cubic foot (800 kg/m3). Their compressive strength is between 100 and 1,000 psi (0.69-6.90 MPa) - not one that will hold a structural load; however, they have high insulative qualities. Intermediate range structural lightweights include pumice, scoria and herculite. With compressive strengths from 1,000 to 2,500 psi (6.9 to 17.2 MPa), they are considered topping. Structural lightweight concrete has the highest compressive strength of the three categories with a minimum of 2,500 psi (17.2 MPa) up to and sometimes exceeding 6,000 psi (41.4 MPa). Its weight can range from 90 to 120 pounds per cubic foot (1441 to 1922 kg/m3). Absorption, particle size, shape, and the surface texture of the aggregate affect the properties of lightweight concrete.
Lightweight concrete as a structural material has many uses. This includes multistory building frames, curtain walls, shell roofs, folded plates, precast elements, pipes and tubes, and oceangoing ships, to name only a few. Lightweight concrete is especially appropriate for tall buildings having many floors. The additional cost of this material is often recovered from the reduction in the building's weight and consequent saving in the cost of columns and foundations. The 52-story One Shell Plaza of 1971 in Houston, Texas, is a lightweight structure from top to bottom and is still the tallest lightweight building in the world.
High Strength Concrete (HSC): HSC is also known as micro silica concrete or condensed silica fume concrete (CSF). Silica fume is a byproduct of the electric arc furnace production of silicon and ferro-silicon alloys. Its first structural use was in Norway in 1971. In addition to CSF, this material also contains Portland cement, water, crushed gravel of granite or limestone, fine sand and superplasticizers. All of these are combined to create a product that has strength greater than 5,000 psi (34.5 MPa) and even up to 20,000 psi (138 MPa). When CSF is mixed with water, a chemical reaction occurs creating crystals which physically fill any voids in the concrete containing pore water; thus, creating both a water-resistant and a high-strength material. HSC is much more brittle than the ordinary concrete and must be mixed and placed with care.
Even with the precautions for handling, this type of material is finding more and more uses in the construction industry. The current trend for HSC is to pump it to soaring heights. New equipment has been developed to keep the materials from segregating thus retaining its strength. HSC is used for high-rise apartments and skyscrapers like those already built or being built in Malaysia, China and the United States. Its appeal for this type of building is that with higher strengths, members can be made smaller. The sizes of columns using HSC and reinforcing steel in tall buildings are greatly reduced and consequently the usable floor space is increased yielding higher revenues.
High Performance Concrete (HPC): Use of HPC truly began in 1927 when engineers building a tunnel through the Rocky Mountains near Denver needed a quick way of supporting the loads on the tunnel. At that time HPC was in the research stages and was not yet ready to enter the market. The engineers prevailed upon scientists to allow its use. Eventually, the tunnel was built using this material. Why were the builders so interested in HPC? The answer lies in its ability to reach an adequate maturity in 24 hours rather than 7 days for regular concrete. In this way and others to be described in the following, HPC is very different from conventional reinforced concrete, even that which contains admixtures.
The definition of HPC encompasses more than just high-early performance. It is a mixture whose properties include increased strength and better performances in the areas of durability, ductility, density, mixture stability and chemical resistance, to name only a few. These will change depending on the type of admixture combined with cement, aggregates and water for the final product. Building industry professionals are interested in increasing productivity by decreasing the amount of time for concrete to reach its strength and the amount of material required to carry the loads of a structure as well as have improved stability and toughness.
From the evidence presented in the literature, HPC is very flexible with applications to many classifications of construction. It is well known that time, money and labor costs together is a matter of great concern in the building industry. HPC, with its low water/cement ratio, can develop strength of 3,500 to 6,000 psi (24.1 to 41.4 MPa) within 24 hours of placing. This performance speeds the time for project completion and may reduce cost with the reduction of waiting time and more reuse periods for formwork. Higher strengths that can be achieved by HPC also add a few other beneficial effects to the structure. These features of HPC make it appropriate for applications to high-rise buildings.