various type of Cement Concrete
|Normal Cement Concrete:|
The cement concrete in which basic ingredients or components i.e. coarse / fine aggregate, water, cement are commonly used is known as normal cement concrete. It is also may be called normal weight cement concrete or normal strength cement concrete. The setting time of cement concrete is 30 – 90 minutes depending upon the moisture availability in atmosphere, fineness properties of cement etc.
The rate development of the concrete strength starts just after 7 days the general strength values is 10 MPa to 30 MPa . After almost 28 days 70 – 80% of the total strength is achieved. Almost after 90 days 95% of the strength is achieved.
Properties of Normal Cement Concrete:
The slump value varies from 25 mm – 125 mm.The value of strength of concrete is strong in compression side and weak in tension tension side.Air content is basically 1 – 2 %.Normal cement concrete is not durable type of concrete against severe environmental conditions e.g. freezing and thawing action.
High strength concrete:
Compressive strength value of high strength cement concrete mixes is usually higher than normal concrete .High strength cement concrete is basically made by lowering the water cement (W/C) or water cement ratio to 0.35 or lower than normal requirement.Most of the time mineral admixture silica fume or micro silica is added to prevent the formation of free calcium hydroxide crystals in the cement mixture, which might reduce the concrete strength with the cement paste and aggregate bond. With very low w/c ratios and the use of silica fume make concrete mixes significantly less workable condition, which is particularly likely to be create a problem in case of high-strength concrete applications where denser reinforcement bar cages are mostly to be used. For adjusting control of the reduced workability in the high strength cement concrete mix, superplasticizers are commonly added to high-strength mixtures as a chemical admixture.The coarse and fine aggregate must be selected carefully for high strength mixes, as poor quality aggregates may not be strong enough to resist or carry the loads imposed on the casted concrete and may cause failure to start in the aggregate for that reason.
High Performance Concrete:
This concrete mix exist with the following main properties as described below:
High compressive strength.
High workability value.
High durability performance.
Ease of placement at any site condition.
Compaction without any segregation and bleeding.
Early age strength gain.
Long-term mechanical properties:
Density will be high.
Heat of hydration of cement.
Toughness of concrete.
Volume stability of concrete.
Long life in severe environmental condition.
Preparation of high strength concrete:
High strength concrete mixes can be prepared with proper selection of ingredients and optimization of concrete mix design.High workability in mixes is obtained by using super plasticizers, it’s addition lower the water cement ratio to 0.25 which is the almost minimum amount required only for hydration process of cement.High durability of concrete is attributed by fly ash and silica fume which modify the mineralogy of the cement component; it actually enhances the compatibility between ingredients in concrete mass and reduces the CSH gel amount . Fly ash also may causes ball bearing effect increasing of workability of concrete.The external additive mineral admixtures are 20-30% fly ash of partial replacement of cement content and rest 70% is Ordinary Portland Cement content.As in this case it is not usually durable against environmental freezing and thawing action ,so air entrained agents can also be utilized in such cases.
Properties of high performance concrete mixture:
Concrete mixture strength of high performance grade concrete ranges from 40 mpa to 100 mpa the water cement ratio can be reduced to 0.25.
Air Entrained Concrete:
The greatest achievements in field of concrete technology is development of air entrained cement concrete. It is basically used where the cement concrete is vulnerable to freezing and thawing action. It is mostly prepared by adding the air entraining agent or admixture.
The air entrainment in cement concrete with following functions:
It is mostly minimize the surface tension of water and then bubbles are formed.Next the air entraining agents prevents the coalescing i.e. the combining of several bubbles. The diameter of these air bubbles ranges form 10 micrometer to 1000 micrometer size and in entrapped air condition the diameter of air bubbles is much higher than 1mm. The air entraining agents or air entrained admixtures are likely used for the purpose of making of entrained air in concrete.
FREEZING AND THAWING action:
There are commonly two phenomena regarding the freezing and thawing action on cement concrete.whenever the existing water inside concrete mass freezes it expands 8-12% due to this actual increase in the size it exerts pressure on its surrounding portion and thus creating a tensile force due to which micro cracks appear in the outer face of cement concrete. Due to freezing action these micro cracks are develop into fissures which actually results in disruption of cement concrete.Whenever the air entrained agents are present in concrete, the extra amount of air is water expands these air bubble which provide them thin or narrow space and the exertion of pressure is also prevented.
Secondarily is osmotic type pressure: In case of cement concrete structure there are two parts, frozen and unfrozen condition. As the water content is normally higher in the frozen part now, the osmotic pressure is normally developed and water also tends to flow towards the low water concentration portion. If capillary water are not available, then the water develops hair cracks.Now normal concrete can not sustain 3-4 cycles of freezing and thawing action where as the AEA type of concrete can sustain 100 cycles of same condition.
The draw backs of Air Entrained Concrete:
It has low strength as compare to normal type cement concrete.
Light Weight Cement Concrete:
The special type i.e light weight concrete, which has substantially lower mass per unit volume of concrete then the concrete made of ordinary ingredients is called lightweight concrete. The aggregates which is used are lighter in weight.
Density of light weight concrete is 230 kg/m³ -1840 kg/m³ .
Strength of light weight concrete blocks varies from 8 MPa – 40 MPa .Some times air entrained admixtures are also added to it which giving resistance to freezing and thawing action along with adequate strength.
The uses of Light weight concrete:
wherever the extra load is not applied commonly e.g. parapet wall, road lining etc. or to reduce dead load then light weight concrete used.
SELF COMPACTING CONCRETE:
It is the uncommon type of concrete where no mechanical vibration is required. The concrete is basically compacted due to its own weight . It is also may be called self consolidated concrete or flowing type of concrete. It can be alsos specifically categorized as high performance cement concrete as the ingredients are the typically same, but in this type of concrete workability is too much increased.
This self-consolidating type cement concrete is characterized by the following properties:
Extreme fluidity properties as measured by flow condition, typically between 650-750 mm on a flow table, rather than slump value (height).There is no need for vibrators to compact this concrete.
Placement is being much easier.
No bleed water, or aggregate segregation occured.
Uses and Applications of Self Compacting Concrete:
It is used in such location where unreachable for mechanical vibrations. e.g. underground concrete structure, deep wells or at bottom of deep sea area. SCC can save up to 40% in labor costs due to 90% faster pouring of concrete and reduced wear and tear on concrete formwork.
Shotcrete concrete also uses compressed air to shoot concrete onto (or into) a frame or concrete structure.
Shotcrete is some kind of mortar or (usually) cement concrete which conveyed through a hose pipe and pneumatically projected at through a shortcrete nozzle with high velocity or speed onto the surface of structure. Shotcrete also undergoes placement and compaction at the same time of application due to the heavy force with which it is projected from the nozzle.It can also be impacted onto any type or any shape of surface, including vertical or overhead areas of structure .Shotcrete is more or less frequently used against vertical soil or rock surfaces areas, as it eliminates the need for formwork criteria.It is sometimes used for rock sliding support, especially in horizontal tunneling under work. Shotcrete is also used for applications where water seepage is an vital issue to limit the excess amount of water entering a construction fild site due to its high water table or other subterranean water sources.
This type of concrete is often used as a form of quick fixing for weathering of loose soil types in all construction zones.
Pervious type concrete:
Pervious type of cement concrete actually contains a network of holes or voids, to allow air or water to move through the concrete mass. This situation allows water to drain naturally through it, and can both condition remove the normal surface water for drainage infrastructure, and allow the replenishment of groundwater when conventional concrete does not perform the same features.It is basically formed by leaving out some or the entire fine aggregate (sand), the remaining large size aggregate then is bound by a relatively small amount of Portland cement material.When the concrete set, typically between 14% and 28% of the concrete volumes are found voids, allowing water to drain on that situations.The majority part of pervious concrete pavements actually function very well with little or no maintenance cost. Maintenance work of pervious type cement concrete pavement consists primarily of prevention of any clogging of the void in structure.In preparing the concrete site prior to before construction, drainage of surrounding landscaping should be designed in such a way to prevent the flow of materials onto concrete pavement surfaces. Any type soil, rock, leaves, and other debris may infiltrate the voids or air and hinder the flow of water through it, and decreasing the utility of the pervious cement concrete pavement.
Roller compacted concrete:
Roller compacted type concrete, sometimes called commercially rollcrete, it is a low-cement-content stiff concrete which placed using different techniques borrowed from earth moving and paving work as road job.The concrete is actually placed on the surface which need to be covered, and it is compacted in such place using large heavy type rollers typically used in earthwork job.The concrete mixes achieves a very high density and cures over a long time into a strong monolithic section of block.The roller compacted concrete is typically used for concrete pavement purpose. Roller compacted concrete dams can also be self built, as the low cement content causes less exothermic heat which generated while curing than typical form of conventionally placed massive concrete pours such areas.