12-10-2016, 10:21 AM
1458731106-CONCRETE.docx (Size: 28.56 KB / Downloads: 5)
CONCRETE
Concrete is a mixture of cement, sand, stone aggregate and water. Cement usually in powder form, acts as a binding agent when mixed with water and aggregates. This combination, or concrete mix will be poured and harden into the durable material with which we are all familiar. There are three basic ingredients in the concrete mix
1. Portland Cement
2. Water
3. Aggregate`
Portland cement: - the cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together.
Water: - Water is needed to chemically react with the cement (hydration) and too provide workability with the concrete. The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ration. The lower the w/c ration, the stronger the concrete (higher strength, less permeability)
Aggregate:- Sand is the fine aggregate. Gravel or crushed stone is the coarse aggregate in most mixes.
PLAIN CEMENT CONCRETE
Plain Cement Concrete (PCC) is used to provide rigid impervious bed to in foundation where the earth is soft and yielding. PCC can use over brick flat or without brick flat soiling or brick flat soiling. Plain cement concrete can also called only “cement concrete (CC)” or “binding Concrete”.
General Specification of plain Cement Concrete
• In plain cement concrete aggregate should be hard durable and free from impurities
• Fine aggregate should contain sharp, angular, grain.
• Cement should be fresh Portland cement
• Mixing should be done by hand mixing or by machine mixing.
• Laying and compaction should be done before setting of concrete , .i.e. within 30minutes
REINFORCED CONCRETE
Reinforced concrete is a composite material in which concrete’s relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength and or ductility. The reinforcement is usually, though not necessarily, steel reinforcing bars (rebar) and is usually designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking and/ or structural failure. Modern reinforcement concrete can contain varied reinforcement material made of steel, polymer or alternate composite material in conjunction with rebar or not. Reinforced concrete may also be permanently stresses (in compression), so as to improve the behavior of the final structure under working loads. In the United states, the most common methods of doing this are known as pre-tensioning and post tensioning
For a strong ductile and durable construction the reinforcement needs to have the following properties at least
• High relative Strength
• High toleration of tensile strain
• Good bond to the concrete, irrespective of pH, moisture, and similar factors
• Thermal compatibility, not causing unacceptable stresses in response to changing temperatures.
• Durability in the concrete environment, irrespective of corrosion or sustaine stress for example.
TYPES OF CONCRETE
1. Normal Concrete
2. High Strength Concrete
3. High Performance Concrete
4. Pervious Concrete
5. Self Compacting Concrete
6. Roller Compacted Concrete
7. Fiber Reinforced Concrete
8. High Volume Fly Ash Concrete
9. Foam Concrete
10. Geo-Polymer Concrete
11. Polymer Impregnated Concrete
Normal Concrete
The concrete in which common ingredient i.e. aggregate, water, cement are used is known as normal concrete. It is also called weight concrete or normal strength concrete. It has a setting time of 30-90minutes depending upon moisture in atmosphere, fineness of cement etc. The development of the strength starts after 7 days the common strength values is 10Mpa (1450psi) to 40Mpa (5800psi). At about 28 days 75-80% of the total strength is attained. Almost at 90 days 95% of the strength is achieved.
High Strength Concrete
Compressive strength of high strength concrete mix is usually greater than 6000 pounds per square inch. High strength concrete is made by lowering the water cement (w/c) ratio 0.35 or lower. Often Silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement, which might reduce the strength at the cement aggregate bond. Low w/c ration 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 in the high strength concrete mix, super plasticizers are commonly added to high strength mixtures, aggregate must be selected carefully for the high strength mixes, as weaker aggregate may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate.
High Performance Concrete HPC is used for concrete mixture which possesses high workability, high strength , high modulus of elasticity , high density, high dimensional stability, low permeability and resistance to chemical attack. HPC started off as high strength concrete in 1970’s and made possible by advent of super plasticizer. HPC a better version of high strength Concrete, low w/c ration (0.3-0.35), Finer cement, more C3S in cement
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 remove the normal surface water drainage infrastructure and allow replenishment of groundwater when conventional concrete does not. It is formed by the leaving out some or the entire fine aggregate, the measuring the large aggregate then is bound by a relatively small amount of Portland cement. When set, typically between 15% and 25% of the concrete volumes are void, allowing water to drain. The majority of pervious concrete pavements functions well with little or no maintenance. Maintenance of Pervious Concrete pavement consists primarily of prevention of clogging of the wood structure. In preparing the site prior to construction drainage of surrounding landscaping should be designed to prevent flow of materials onto the pavement surfaces, soil, rocks, leaves, and other debris may infiltrate the voids and hinder the flow of water, decreasing the utility of the pervious concrete pavement
Self Compacting Concrete
The concrete where no vibration is required. The concrete is compacted due to its own weight. It is also called self consolidated concrete or flowing concrete. It can be also categorized as high performance concrete as the ingredients are the same, but in this type of concrete workability is increased. This self consolidating is characterized by:
• Extreme fluidity as measured by the flow, typically between 650-750mm on a flow table, rather than slump (height)
• No need for vibration to compact the concrete
• Placement being easier
• No bleed water, or aggregate segregation.
Roller Compacted concrete
Roller compacted concrete, sometimes called roll concrete, 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 the concrete pavement, roller compacted concrete dams can also be built, as the low cement content causes less heat to be generated while curing than typical for conventionally placed massive concrete pours.
Fiber Reinforced Concrete (FRC)
The process of micro cracks at the mortar – aggregates interface is responsible for inherent weakness of plain concrete. The weakness can be removed by inclusion of fiber in mix. FRC can be defined as a composite material consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed fibers. It may also contain pozzolans and other admixtures commonly used with conventional concrete.
High Volume fly ash concrete
Fly ash (pulverized fuel ash) is a by- product of an electricity generating plant using coal as fuel. It used to be disposed of as industrial waste. Because a large amount of fly ash can be produced by power plants ever year, fly ash disposal became a major environment issue in many countries. To solve the disposal problem, since the early1960’s many countries have started to incorporate fly ash into concrete.
In recent years, attempts have been made in many countries to utilize a large amount of fly ash in concrete without losing its early age compressive strength. The HVFA concrete that has developed recently uses fly ash of 60% or more by weight of cement. Desirable mechanical and durability properties of HVFA concrete have been achieved by careful selection of mix preparations and the use of chemical super plasticizers.
Foam Concrete
Foamed concrete also known as cellular concrete is a specialized product that contains more than 20% by volume of air. This is achieved by the incorporation of pre-formed foam or foaming surfactant in to a cementinious base mix.
The greatest advantages of foamed concrete are its flow ability, self-compacting and self-leveling nature, light weight and low dimensional change. In addition, the material exhibits controlled low strength, excellent thermal insulation properties, good load-bearing capacity and can be easily re-excavated, If necessary.
Geo Polymer Concrete (GPC)
The production of one ton of cement emits approximately one ton of carbon dioxide to the atmosphere which leads to global warming conditions. A need of present status is, should be built additional cement manufacturing plants or finds alternative binder systems to make concrete. On the other scenario huge quantity of fly ash are generated around the globe from thermal power plants and generally used as a filter material in low level areas. Alternative binder system with fly ash to produce concrete eliminating cement is called “GEO-POLYMER CONCRETE”. Alkali activation of alumina and silica containing blast furnace slag powder, known as GGBS (Ground granulated blast furnace slag) and fly ash typically used in Geo-polymer concrete.
Polymer Impregnated (PIC)
PIC is the widely used polymer concrete. The precast conventional concrete is cured and dried in oven, and then a low viscosity monometer is diffused through the open cell and polymerized by using radiation, application of heat or by chemical initiation. The monomer used is Methylmethacrylate, Styrene, Acrylonitrile, etc….
Application of PIC: Prefabricated structural elements, prestressed concrete, marine works, Nuclear power plants, Sewage works, for water proofing structures, Industrial applications.
STAGES OF CONCRETE
Concrete has two main stages
1. Fresh Concrete
2. Hardened Concrete
FRESH CONCRETE
Fresh concrete should be stable and should not segregate or bleed during transportation and placing when it is subjected to forces during handling operations of limited nature. The mix should be cohesive and mobile enough to be placed in the form around the reinforcement and should be able to cast in required shape without losing continuity or homogeneity under the available techniques of placing the concrete at a particular job. The mix minimum voids under the existing facilities of compaction at the site. A best mix from the point of view of compatibility should achieve a 99% elimination of the original voids present
HARDENED CONCRETE
One of the most important properties of the hardened concrete is its strength which represents the ability if concrete to resist forces. If the nature of the force is to produce compression, the strength is termed compressive strength. The compressive strength of hardened concrete is generally considered to be the most important property and is often taken as the index of the overall quality of concrete. The strength can indirectly give an idea of the most of the other properties of the concrete which are directly related to the structure of hardened cement paste. A stronger concrete is dense, compact, impermeable and resistant to weathering and to some chemicals. However, a stronger concrete may exhibit higher shrinkage with consequent cracking, due to the presence of higher cement content.
Some of the other desirable properties like shear and tensile strength, modulus of elasticity, bond, impact and durability etc…are generally related to compressive strength. As the compressive strength can be measured easily on standard sized cube or cylindrical specimens, it can be specified as a criterion for studying the effect of any variable on the quality of concrete. However, the concrete gives different values of any property under different testing conditions. Hence method of testing, size of specimen and the rate of loading etc… are stipulated while testing the concrete to minimize the variations in test results. The statistical methods are commonly used for specifying the quantities value of any particular property of hardened concrete.