Concrete is one of the widest used construction material whether it is a dam or a house or a bridge or flyover. Concrete is basically of three components water, aggregate and cement. Aggregate acts as a filling agent while cement mostly Portland cement act as a binding agent.
Concrete is basically the most popular artificial material on earth. It has many advantages beside easy availability of the constituents it is powerful in compression however weak in tension which can be compensated by using reinforcing bars of steel as in Reinforced Cement concrete.
Concrete is easily and readily prepared and fabricated in all sorts with advantage being turned into any shape desired at site. Concrete is leveled by screeding and smoothed out with trowel or a float. Normally it would be poured into a wood formwork and then finished.
There are different types of concrete out there depending upon type of constituents used in there
keeping in view of the demand at site.
Some common types are as follows :-
1. Normal concrete
2. High performance concrete
3. High strength concrete
4. Air entrained Concrete
5. Light weight concrete
6. Self compacting Concrete
8. Pervious concrete
9. Roller compacted concrete
Normally concrete also known as normal weight concrete is relatively strong in compression and weak in tension. It has low coefficient of thermal expansion and it shrinks while setting. Density of concrete is around 2240 – 2400 Kg/meter cube. (150 lb/cu.ft.). Compressive strength is usually around 3000 to 6000 psi. Normal concrete has 1-2 % air content but is not durable for severe conditions like freezing and thawing. A freshly poured concrete usually sets within 30 – 90 minutes depending upon local temperature and moisture conditions. Just after coming in contact with water the normal hydration reaction starts and concrete sets. It mostly develops its strength after 7 days and usually attain more than 80% strength after 28 days.
High Strength Concrete
Whenever we are talking about strength of non-reinforced concrete we are talking about compression strength or compressive strength denoted by fc’. As the name suggests this type of concrete has extraordinary strength than normal strength or normal concrete. By one definition HSC is mostly the one having cube strength between 60 to 100 N/mm2, although higher strengths can also be achieved. Mix design of high strength concrete is influenced by properties of cement, sand aggregates and water cement ratio. The strength according to one standard is said to be greater than 40 MPa. Mostly high strength is achieved by lowering water cement ratio upto 0.35 or even lower. Often Silica fume is also added to prevent the formation of free calcium hydroxide crystal in the cement matrix. It is usually less workable and thus if needed super-plasticizers can be used.
High Performance concrete
As the name suggests high performance concrete refers to all those sorts of concrete which are optimum in all the standards usually adopted in common application of concrete, thus not only strength it has ease of placement, optimum heat of hydration, compaction without segregation, have early age strength, permeability is sufficient, density is more, life is more even in severe environments, toughness is more.
High workability is attained by super plasticizers, they lower the water cement ratio to 0.25 which is the amount required only for hydration process.
High durability is attributed to fly ash and silica fume which modify the e mineralogy of the cement; it enhances the compatibility of ingredients in concrete mass and reduces the CH amount. Fly ash also causes ball bearing effect increasing workability.
The admixtures are 20-25% fly ash of partial replacement of cement and rest 70% is Ordinary Portland Cement.
As it is not usually durable against freezing and thawing so air entrained agents can also be utilized.
Properties of high performance concrete mix
Strength of high performance concrete ranges from 10000 psi – 15000 psi
Water cement ratio can be reduced to 0.25
Air entrained Concrete
In this type of concrete tiny air bubbles are intentionally created. Mostly an air entraining agent called surfactant (a type of chemical) is used. Air bubbles are created during mixing of the plastic concrete which mostly survive upto end as hardened concrete. The basic purpose of air entrainment is to make it durable for concrete in extreme climate where freeze-thaw effect is there while second purpose is to increase workability of concrete while in plastic state.
Normal concrete will have small capillaries produced by the evaporation of water in concrete. These capillaries are invaded by water from the environment and the freezing of this water can cause a lot of stress in concrete because of expansion in volume that accompanies the freezing. An Air entrained concrete allows the expansion to take place without causing any further stresses as air bubbles are capable of being compressed.
The amount of entrained air is maintained between 4 and 7 percent for best freezing thawing resistance though variations can be made depending on specific conditions.
Normally air entrained concrete has a drawback it has low strength as compared to normal concrete.
Light weight concrete
Structural light weight concrete has an in-place density or unit weight on the order of 90 to 115 lb/ft3 or 1440 to 1840 kg/m3. Lightweight concrete is defined as having an air dry density not exceeding 2000 Kg/m3, but can be as low as 400 kg/m3.
One of the most common uses for light weight concrete is with floor, roof or bridge decks; other include pavement system, masonry blocks etc.
Lightweight aggregates used in structural lightweight concrete are typically expanded shale, clay or slate materials that have been fired in a rotary kiln to develop a porous structure. Other products such as air-cooled blast furnace slag are also used. There are other classes of non-structural LWC with lower density made with other aggregate materials and higher air voids in the cement paste matrix, such as in cellular concrete.
The required properties of the lightweight concrete will have a bearing on the best type of lightweight aggregate to use. If little structural requirement, but high thermal insulation properties, are needed then a light, weak aggregate can be used. This will result in relatively low strength concrete.
Self Compacting Concrete
Self compacting concrete or SCC is a flowing concrete mixture that is able to consolidate under its own weight. The high fluid nature of SCC makes it suitable for placing in difficult conditions and in sections with congested reinforcement. Use of SCC can also help minimize the bearing related damages on the formwork that are induced by vibration of concrete. Another advantage of SCC is that the time required to place large sections is considerably reduced.
Self compacting concrete has more finer content and less coarser aggregate. Self compacting concrete also uses superplasticisers in large amount and a viscosity modifying agent (VMA) in small doses.
As a high-performance concrete, SCC delivers these attractive benefits while maintaining all of concrete\’s customary mechanical and durability characteristics. Adjustments to traditional mix designs and the use of superplasticizers creates flowing concrete that meets tough performance requirements. If needed, low dosages of viscosity modifier can eliminate unwanted bleeding and segregation.
Shortcrete is basically not a product or something you can pick or touch, it is basically a process of placing concrete to achieve high strengths and low permeability.
Shortcrete is a concrete conveyed through a hose and pneumatically projected at high velocity onto a receiving surface.
They are strong, durable, resistant to disasters, fires, mold, insects and vermin, and have low permeability, good thermal mass, and create tight envelopes. Although the hardened properties of Shotcrete are similar to conventional cast-in-place concrete, the nature of the placement process provides additional benefits, such as very fast erection, particularly on complex forms or shapes, including curved walls and arches.
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.
Pervious concrete (also called porous concrete, permeable concrete, no fines concrete and porous pavement) is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other sources to pass directly through, thereby reducing the runoff from a site and allowing groundwater recharge. When set, typically between 15% and 25% of the concrete volumes are voids, allowing water to drain.
The majority of pervious concrete pavements function well with little or no maintenance. Maintenance of pervious concrete pavement consists primarily of prevention of clogging of the void structure.
In preparing the site prior to construction, drainage of surrounding landscaping should be designed to prevent flow of materials onto pavement surfaces. Soil, rock, leaves, and other debris may infiltrate the voids and hinder the flow of water, decreasing the utility of the pervious concrete pavement.
Roller Compacted Concrete
Mostly abbreviated as R.C.C but it must not be confused with Reinforced Cement Concrete. The word is enough to define itself; roller compacted concrete is a relatively stiff concrete mix that is spread with a paving machine and is than compacted with the help of a roller which is usually steel drum vibratory roller.
Method of construction used for roller compacted concrete is simple and conventional.
Roller-compacted concrete has the same basic ingredient as conventional concrete: cement, water, and aggregates, such as gravel or crushed stone.
But unlike conventional concrete, it\’s a drier mix—stiff enough to be compacted by vibratory rollers. Typically, RCC is constructed without joints. It needs neither forms nor finishing, nor does it contain dowels or steel reinforcing.
These characteristics make roller-compacted concrete simple, fast, and economical.
These qualities have taken roller-compacted concrete from specialized applications to mainstream pavement. Today, RCC is used for any type of industrial or heavy-duty pavement. The reason is simple. RCC has the strength and performance of conventional concrete with the economy and simplicity of asphalt. Coupled with long service life and minimal maintenance, RCC\’s low initial cost adds up to economy and value.
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