Concrete is one of the most commonly used construction materials in the world. Wherever there is any inkling of infrastructure, one can almost always find concrete there as well. Not all concrete is the same, however. There are various types of concrete that exist in the world for numerous uses. One of the oldest concrete recipes from the Romans was a mix of volcanic ash and hydrated lime. But it has been a few centuries since then; in that time, concrete has developed and has become both advanced and increasingly diverse.
Most commonly, regular concrete is created by mixing Portland cement with both an aggregate and water-chemical mixtures. Often times, cement and concrete are incorrectly interchanged: concrete is the hard, rock-like substance that is so frequently seen in urbanized areas. Cement is an ingredient, the powder, used in the creation of concrete. It is the most-produced material on Earth and will continue to be so long as there is a need to create, rebuild, or improve infrastructure.
High-strength concrete is different from normal-strength concrete in the amount of force it can resist without breaking. The American Concrete Institute differentiates high-strength from normal-strength at a compressive strength of over 6,000 psi (pounds square inch). In addition to varying the proportions of the materials used in normal-strength concrete, silica fume is added to the mixture in order to strengthen the bond between the cement and the aggregate. However, this admixture causes the cement to hydrate much faster, meaning that it dries quicker than usual. In order to keep consistent the balance between workability and strength, a superplasticizer is added to high-strength concrete. This slows down the chemical reaction between the cement and water, allowing for workers to place the concrete at a more effective pace.
High-Performance Concrete (HPC)
High-performance concrete, in contrast to high-strength concrete, is not necessarily known for its compressive resistance. While high-performance concrete can include a high compressive strength, other characteristics used to define “high performance” are the ease of placement without affecting strength, long-term mechanical properties, toughness, and longevity in various weather conditions among others.
Ultra High-Performance Concrete
This type of concrete is more often than not pre-mixed in bags because of the numerous ingredients needed to make it. It includes Portland cement, silica fume, quartz flour, and fine silica sand. However, high-range water reducers, water, and other steel or organic fibers are used to increase the strength of the mixture. Ultra-high performance concrete is particularly durable because of the combination of fine powders. Other types of concrete normally need a steel rebar or reinforcing to retain the intended structure, but UHPC is generally self-placing in addition to its incredible compressive strength of up to 29,000 psi. Its post-cracking longevity is one of UHPC’s strong points because even after this concrete cracks, it still is able to maintain structural integrity with an impressive tensile strength of 725 psi.
Stamped concrete is another type of concrete that is very commonly used. Often seen in parking lots, pavements, or other like high-traffic areas, stamped concrete has more of an architectural application. Once concrete has been laid, a kind of mold can be placed on top of, or stamped, onto the hardening concrete to create the appearance of natural stone. Once the floor has been hardened, it will likely be sealed to increase the longevity of the dried mixture.
Normally, concrete requires a mechanical vibration while being set in order to release excess air that may be in the mixture. Self-consolidating concrete eliminates the need for mechanical consolidation (the vibrations) mainly through its malleable viscosity. Being able to control the flowability and stability, as achieved by using high-range-water-reducing admixtures, allows concrete to be placed quicker. Not only does this save time, but because there is no need for the mechanical consolidation, self-consolidating concrete saves labor, saves money, and makes it easier for workers to fill restricted or hard-to-reach areas.
Invented by taxidermist Carl Akeley in 1907, the initial dry method for placing shotcrete was by using a compressed air nozzle to shoot dry mix and injecting water through a separate hose at the head of the nozzle while the dry material is hurled toward the wall. The wet-mix shotcrete was developed later in the 1950’s and is only slightly different than the dry-mix shotcrete wherein dry-mix shotcrete involves the continuous feeding of a hopper through which dry mix would shoot through a nozzle and mix at the point of exit. Wet-mix shotcrete, however, involves the use of pre-mixed concrete. The concrete has already been prepared and therefore only involves one pump. The upside to using wet-mix shotcrete is that dry-mix shotcrete creates more waste (excess powder that falls to the floor), more rebound off the wall, and wet-mix shotcrete can place a larger quantity in a smaller amount of time.
Also known as lime concrete, limecrete is a type of concrete where instead of using cement in the mix, lime is replaced. Doing so has certain benefits environmentally and health-wise. Environmentally, lime absorbs carbon dioxide as it sets and allows natural products like wood, straw, and hemp to be used as fibers without fear of composting or deterioration since limecrete controls moisture. In terms of health, lime plaster draws moisture out from inside which means that humidity control is more regulated, resulting in mold growth prevention. Furthermore, limewash and lime plasters are non-toxic so they do not contribute to air pollution inside like other paints would.