Types of concrete

There are many types of concrete, variations of installation, composition, finish and performance characteristics.

A highway paved with concrete.

Regular concrete paving blocks

Concrete in sidewalk stamped with contractor name and date it was laid

Mix design

Modern concrete mix designs can be complex. The design of a concrete, or the way the weights of the components of a concrete is determined, is specified by the requirements of the project and the various local building codes and regulations.

The design begins by determining the "durability" 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 (Portland or other cementitious material), 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 softwares provide the user an opportunity to select their preferred method of mix design and enter the material data to arrive at proper mix designs.

Regular concrete

Regular concrete is the lay term describing 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. This concrete can be produced to yield a varying strength from about 10 MPa (1450 psi) to about 40 MPa (5800 psi), depending on the purpose, ranging from blinding to structural concrete respectively. 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, 1-cubic-foot (0.028 m³) of concrete would be made using 22 lb (10.0 kg) cement, 10 lb (4.5 kg) water, 41 lb (19 kg) dry sand, 70 lb (32 kg) dry stone (1/2" to 3/4" stone). This would make 1-cubic-foot (0.028 m³) of concrete and would weigh about 143 lb (65 kg). The sand should be mortar or brick sand (washed and filtered if possible) and the stone should be washed if possible. Organic materials (leaves, twigs, etc) should be removed from the sand and stone to ensure the highest strength.

High-strength concrete

High-strength concrete has a compressive strength generally greater than 6,000 pounds per square inch (40 MPa = 5800 psi). High-strength concrete is made by lowering the water-cement (W/C) 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 which has a superior surface finish. After a concrete floor has been laid, floor hardeners (can be pigmented) are impregnated on the surface and a mold which may be textured to replicate a stone / brick or even wood is stamped on to give a attractive textured surface finish. After sufficient hardening the surface is cleaned and generally sealed to give a 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 (HPC) is a relatively new term used to describe 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
• 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

Self-consolidating concretes

Further information: Self Leveling Concrete

During the 1980s a number of countries including Japan, Sweden and France developed concretes that are self-compacting, known as self-consolidating concrete in the United States. This self-consolidating concrete (SCCs) is characterized by:

• extreme fluidity as measured by flow, typically between 650-750 mm on a flow table, rather than slump(height)
• no need for vibrators to compact the concrete
• placement being easier.
• no bleed water, or aggregate segregation
• Increased Liquid Head Pressure, Can be detrimental to Safety and workmanship

SCC can save up to 50% in labor costs due to 80% faster pouring and reduced wear and tear on formwork.

As of 2005, self-consolidating concretes account for 10-15% of concrete sales in some European countries. In the US precast concrete industry, 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.

Vacuum concretes

The use of steam to produce a vacuum inside of concrete mixing truck to release air bubbles inside the concrete is being researched. The idea is the steam will remove the air that is trapped inside the concrete. The steam will condense into water and will create low pressure, pulling out air from the concrete. This will make the concrete stronger due to there being less air in the mixture.

Shotcrete

Main article: Shotcrete

Shotcrete (also known by the trade name Gunite) uses compressed air to shoot concrete onto (or into) a frame or structure. The greatest advantage of the process is that shotcrete can be applied overhead or on vertical surfaces without forming. 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. Shotcrete is frequently used against vertical soil or rock surfaces, as it eliminates the need for formwork. It is sometimes used for rock support, especially in tunneling. Shotcrete is also used for applications where seepage is an issue to limit the amount of water entering a construction site due to a high water table or other subterranean sources. This type of concrete is often used as a quick fix for weathering for loose soil types in construction zones.

There are two application methods for shotcrete.

• dry-mix – the dry mixture of cement and aggregates is filled into the machine and conveyed with compressed air through the hoses. The water needed for the hydration is added at the nozzle.
• wet-mix – the mixes are prepared with all necessary water for hydration. The mixes are pumped through the hoses. At the nozzle compressed air is added for spraying.

For both methods additives such as accelerators and fiber reinforcement may be used.

Pervious concrete

Pervious concrete contains a network of holes or voids, to allow air or water to move through the concrete.

This allows water to drain naturally through it, and can both remove the normal surface-water drainage infrastructure, and allow replenishment of groundwater when conventional concrete does not.

It is formed by leaving out some or all of the fine aggregate (fines). The remaining large aggregate then is bound by a relatively small amount of Portland cement. When set, typically between 15 % and 25 % of the concrete volume is voids, allowing water to drain at around 5 gal/ft²/ min (70 L/m²/min) through the concrete.

Installation

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.

Characteristics

Pervious concrete can significantly reduce noise, by allowing air to be squeezed between vehicle tires and the roadway to escape. This product cannot be used on major U.S. state highways currently due to the high psi ratings required by most states. Pervious concrete has been tested up to 4500  psi so far.

Cellular concrete

Aerated concrete produced by the addition of an air entraining agent to the concrete (or a lightweight aggregate like expanded clay pellets or cork granules and vermiculite) is sometimes called Cellular concrete, lightweight aerated concrete, variable density concrete, foamed concrete and lightweight or ultra-lightweight concrete.

Not to be confused with Aerated autoclaved concrete which is manufactured off site using an entirely different method.

In the 1977 seminal work on A Pattern Language Towns, Buildings and Construction, architect Christopher Alexander wrote in pattern 209 on Good Materials:

"Regular concrete is too dense. It is heavy and hard to work. After it sets one cannot cut into it, or nail into it. And it's surface is ugly, cold, and hard in feeling unless covered by expensive finishes not integral to the structure.

And yet concrete, in some form, is a fascinating material. It is fluid, strong, and relatively cheap. It is available in almost every part of the world. 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.

Is there any way of combining all these good qualities of concrete and also having a material which is light in weight, easy to work, with a pleasant finish? There is. It is possible to use a whole range of ultra-lightweight concretes which have a density and compressive strength very similar to that of wood. They are easy to work with, can be nailed with ordinary nails, cut with a saw, drilled with wood-working tools, easily repaired.

We believe that ultra-lightweight concrete is one of the most fundamental bulk materials of the future.

The variable density is normally described in kg per m³, where regular concrete is 2400 kg/m³. Variable density can be as low as 300 kg/m³ although at this density it would have no structural integrity at all and would function as a filler or insulation use only. The variable density reduces strength to increase thermal and acoustical insulation by replacing the dense heavy concrete with air or a light material such as clay, cork granules and vermiculite. There are many competing products that use a foaming agent that resembles shaving cream to mix air bubbles in with the concrete. All accomplish the same outcome: to displace concrete with air.

Cork-cement composites

Waste Cork granules are obtained during production of bottle stoppers from the treated bark of Cork oak. These granules have a density of about 300 kg/m³, lower than most lightweight aggregates used for making lightweight concrete. Cork granules do not significantly influence cement hydration, but cork dust may. Cork cement composites have several advantages over standard concrete, such as lower thermal conductivities, lower densities and good energy absorption characteristics. These composites can be made of density from 400 to 1500 kg/m³, compressive strength from 1 to 26 MPa, and flexural strength from 0.5 to 4.0 MPa.

Roller-compacted concrete

Main article: Roller-compacted concrete

Roller-compacted concrete, sometimes called rollcrete, is a low-cement-content stiff concrete placed using techniques borrowed from earthmoving and paving work. The concrete is placed on the surface to be covered, and is compacted in place using large heavy rollers typically used in earthwork. The concrete mix achieves a high density and cures over time into a strong monolithic block. Roller-compacted concrete is typically used for concrete pavement, but has also been used to build concrete dams, as the low cement content causes less heat to be generated while curing than typical for conventionally placed massive concrete pours.

Glass concrete

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 greatly enhances the aesthetic appeal of the concrete. Recent research findings have shown that concrete made with recycled glass aggregates have shown better long term strength and better thermal insulation due to its better thermal properties of the glass aggregates.

Asphalt concrete

Strictly speaking, asphalt is a form of concrete as well, with bituminous materials replacing cement as the binder.

Rapid strength concrete

This type of concrete is able to develop high resistance within few hours after being manufactured. This feature has advantages such as removing the formwork early and to move forward in the building process at record time, repair road surfaces that become fully operational in just a few hours.

Rubberized concrete

While "rubberized asphalt concrete" is common, rubberized Portland cement concrete ("rubberized PCC") is still undergoing experimental tests, as of 2009 .

Polymer concrete

Polymer concrete is concrete which uses polymers to bind the aggregate. Polymer concrete can gain a lot of strength in a short amount of time. For example, a polymer mix may reach 5000 psi in only four hours. Polymer concrete is generally more expensive than conventional concretes.

Geopolymer or green concrete

Geopolymer concrete is a greener alternative to ordinary Portland cement made from inorganic aluminosilicate (Al-Si) polymer compounds that can utilise 100% recycled industrial waste (e.g. fly ash and 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 by the manufacturer at both atmospheric and extreme conditions.

Similar concretes have not only been used in Ancient Rome (see Roman concrete) as mentioned but also in the former Soviet Union in the 1950s and 1960s. Buildings in Ukraine are still standing after 45 years so that this kind of formulation has a sound track record.

Limecrete

Limecrete or lime concrete is concrete where cement is replaced by lime.

Refractory Cement

High-temperature applications, such as masonry ovens and the like, generally require the use of a refractory cement; concretes based on Portland cement can be damaged or destroyed by elevated temperatures, but refractory concretes are better able to withstand such conditions.

Concrete cloth

A recent innovation is the concrete cloth. It consists of a three-dimensional fiber matrix, containing a specially formulated dry concrete mix. A PVC backing on one surface of the cloth ensures the material is completely waterproof, while hydrophilic fibers on the opposite surface aid hydration by drawing water into the cement. Concrete cloth can be used to rapidly create waterproof, fireproof, fiber-reinforced thin concrete forms across a wide range of applications: rapid trackway or landing surfaces, structural reinforcement, ground stabilization, ballistic protection and sterile concrete shelters.

Innovative mixtures

On-going research into alternative mixtures and constituents has identified potential mixtures that promise radically different properties and characteristics.

One university has identified a mixture with much smaller crack propagation that does not suffer the usual cracking and subsequent loss of strength at high levels of tensile strain. Researchers have been able to take mixtures beyond 3 percent strain, past the more typical 0.1% point at which failure occurs.

Other institutions have identified magnesium silicate (talc) as an alternative ingredient to replace Portland cement in the mix. This avoids the usual high-temperature production process that is very energy and greenhouse gas intensive and actually absorbs carbon dioxide while it cures.

Gypsum concrete

Main article: Gypsum concrete

Gypsum concrete is a building material used as a floor underlayment used in wood-frame and concrete construction for fire ratings, sound reduction, radiant heating, and floor leveling. It is a mixture of gypsum, Portland cement, and sand.