Types of concrete
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.
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.
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.
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.
The wear resistance of stamped concrete is generally excellent and hence found in applications like parking lots, pavements, walkways etc.
These strengths generally require well-graded hard rock aggregates, a fairly high proportion of cement plus fly ash, water-reducing admixtures, and the silica fume, with relatively low water content.
The rich mixes may cause high heat of hydration in thick placements, which can be moderated by using a higher proportion of fly-ash, up to 30% of the cement content.
[6][7][8] 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.
Micro-reinforced UHPC is used in blast, ballistic and earthquake resistant construction, structural and architectural overlays, and complex facades.
[4]: Ch 4, 3, 2, 2 The concrete can develop high compressive and tensile strengths, while shrinkage and creep remain acceptable, but will generally be less rigid than conventional mixes.
[4]: Ch 4, 3, 2, 2 The defects in concrete in Japan were found to be mainly due to high water-cement ratio to increase workability.
During the 1980s, Okamura and his Ph.D. student Kazamasa Ozawa at the University of Tokyo developed self-compacting concrete (SCC) which was cohesive, but flowable and took the shape of the formwork without use of any mechanical compaction.
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.
It is often used for concrete repairs or placement on bridges, dams, pools, and on other applications where forming is costly or material handling and installation is difficult.
Pervious concrete is installed by being poured into forms, then screeded off, to level (not smooth) the surface, then packed or tamped into place.
Due to the low water content and air permeability, within 5–15 minutes of tamping, the concrete must be covered with a 6-mil poly plastic, or it will dry out prematurely and not properly hydrate and cure.
Pervious concrete can significantly reduce noise, by allowing air to be squeezed between vehicle tyres and the roadway to escape.
A University of California professor of engineering sciences, P. Kumar Mehta, has even just recently found a way of converting abandoned rice husks into Portland cement.[...]
The variable density reduces strength[18] to increase thermal[18] and acoustical insulation by replacing the dense heavy concrete with air or a light material such as clay, cork granules and vermiculite.
[citation needed] The use of recycled glass as aggregate in concrete has become popular in modern times, with large scale research being carried out at Columbia University in New York.
This feature has advantages such as removing the formwork early and to move forward in the building process very quickly, repaired road surfaces that become fully operational in just a few hours.
It is made from inorganic aluminosilicate (Al-Si) polymer compounds that can utilise recycled industrial waste (e.g. fly ash, blast furnace slag) as the manufacturing inputs resulting in up to 80% lower carbon dioxide emissions.
Greater chemical and thermal resistance, and better mechanical properties, are said to be achieved for geopolymer concrete at both atmospheric and extreme conditions.
[33][38][39][37] CarbonCure Technologies uses waste CO2 from oil refineries to make its bricks and wet cement mix, offsetting up to 5% of its carbon footprint.
[33][37] Solidia Technologies fires its brick and precast concrete at lower temperatures and cures them with CO2 gas, claiming to reduce its carbon emissions by 30%.
[33][37] Carbonaide uses carbon dioxide in the curing phase of precast concrete production and has demonstrated up to 40% savings in cement consumption with their first client.
Ginger Krieg Dosier of bioMASON has developed a method for producing bricks without firing kilns or significant carbon release.
[33][41] One research team found a way to use a form of microalgae called coccolithophores to mass produce calcium carbonate via photosynthesis at a faster rate than corals.
Researchers at the Bartlett School of Architecture are developing materials aimed to support the growth of poikilohydric plants such as algae, mosses and lichens (organisms having no mechanism to prevent desiccation).
These air bubbles enhance the workability of the concrete during placement and improve its durability when hardened, particularly in regions prone to freeze-thaw cycles.
