concrete

concrete:

    concrete is a mixture of cement water and aggregate which takes the shape and texture of its mould or formwork on site.when it cured at a suitable temperature and humidity,it will become harden. Concrete is a strong hard building material composed of sand and gravel and cement and water. It is used for making buildings, roads, bridges, vessels pipes etc etc etc.   As the concrete formulations develop concrete is increasing its range of applications such that it is making inroads into those presently monopolised by metals.To enable concrete to withstand tensile loads it is often reinforced with steel rebars or with natural or artificial fibres...
    there are some different type of concrete,such as dense concrete and less dense concrete.dense concrete has a density over 2000kg/m3.less dense concrete is made by aerating the mix,by using lightweight aggregrate or by omitting the fine aggregate are classified as 'lightweight concrete'.tensile strength is given to concrete by reinforcement steel bars. 

    The Properties of concrete are its characteristics or basic qualities.The four main properties of concrete are workability,cohesiveness,strength and durability.Concrete has three different states,that is plastic,setting and hardening.In each state it has different properties.

At plastic state, the concrete is first mixed it is like 'bread dough'.It is soft and can be worked or moulded into different shapes. In this state concrete is called plastic. Concrete is plastic during placing and compaction.The most important properties of plastic concrete are workability and cohesiveness.A worker will sink into plastic concrete.

At setting state, concrete then begins to stiffen. The stiffening of concrete, when it is no longer soft, is called setting.Setting takes place after compaction and during finishing.Concrete that is sloppy or wet may be easy to place but will be more difficult to finish.A worker leaves footprints in setting concrete.

At hardening state, after concrete has set it begins to gain strength and harden. The properties of hardened concrete are strength and durability.Hardened concrete will have no footprints on it if walked on.

     Workability means how easy it is to place,handle,compact and fi nish a concrete mix.
Concrete that is stiff or dry may be difficult to handle, place, compact, and finish and, if concrete is
not constructed properly, concrete will not be as strong or durable when finally hardened. A slump
test can be used to measure the workability of concrete.Workability is affected by the amount of cement paste.The cement paste is the soft or liquid part of the concrete mix. The more paste
mixed with the coarse and fine aggregates, the more workable a mix.beside,it also can affected by the aggregate grading.Well-graded, smooth, rounded aggregates improve the workability of a mix.To make a more workable mix,we can add more cement paste,use well graded aggregates and use an admixture.Never try to make a mixture more workable by just adding more water because this lowers the strength and durability of concrete.
     Well made concrete is a naturally strong and durable material.It is dense, reasonably watertight, and able to resist changes in temperature, as well as wear and tear from weathering.Strength and Durability are affected by the density of the concrete. Denser concrete is more watertight (or less permeable).Concrete durability increase with strength.Well made concrete is very important to protect the steel in reinforced concrete.Strength of concrete in the hardened state is usually measured by the compressive strength by using the compression test.Strength and Durability of concrete are affected by fews factors.First,compaction. Compaction is removing the air from concrete. Proper compaction results in concrete with an increased density which is stronger and more durable. second,curing.Curing is keeping concrete damp for a period, to allow it to reach maximum strength. Longer curing will give more durable concrete.third,weather. Warmer weather will cause concrete to have a higher early strength.forth,type of cement.Different types of cement will affect concrete properties.for example, how
quickly or slowly concrete gains strength.fifth,water to cement ratio.Too much water and not enough cement means concrete will be weaker and less durable.
The water to cement ratio (W/C) is the weight of the water divided by the weight of cement.The lower the ratio, the stronger
the concrete.

      Cohesiveness is how well concrete holds together when plastic.Cohesiveness is affected by the aggregrate grading.Graded Aggregate means that there is a range of size of aggregates, from large
rocks to small sands.Well-graded aggregates give a more cohesive mix, too much
coarse aggregate gives a boney mix.A mix that has too much water will not be cohesive and may separate and bleed.

      There are two main tests to be done on concrete,first,the slumping test.
The slump test shows the workability of concrete.Workability measures how easy the concrete is to place, handleand compact.

Second,the compression test.The compression test shows the best possible strength concrete can reach in perfect conditions.The compression test measures concrete strength in the hardened state.Testing should always be done carefully. Wrong test results can be costly.sampling is  the first step is to take a test sample from the large batch of concrete. This should be done as soon as discharge of the concrete commences. The sample should be representative of the concrete supplied.The sample is taken in one of two ways.first,for purposes of accepting or rejecting the load.Sampling after 0.2 m3 of the load has been poured.For routine quality checks: Sampling from three places in the load.

     A concrete mix is designed to produce concrete that can be easily placed at the lowest cost.
The concrete must be workable and cohesive when plastic, then set and harden to give strong and
durable concrete.The mix design must consider the environment that the concrete will be in; ie exposure to sea water,trucks, cars, forklifts, foot traffic or extremes of hot
and cold.Concrete is a mixture of cement, water, coarse and fine aggregates and admixtures.
The proportions of each material in the mixture affects the properties of the final hardened
concrete. These proportions are best measured by weight. Measurement by volume is not
as accurate, but is suitable for minor projects.As the cement content increases, so does strength and durability.Therefore to increase the strength, increase the cement content of a mix. Adding more water to a mix gives a weaker hardened concrete.Always use as little water as possible, only
enough to make the mix workable.As the Water to Cement ratio increase, the strength and
durability of hardened concrete decrease.To increase the strength and durability of
concrete, decrease the Water-Cement ratio.

     In modern times, researchers have experimented with the addition of other materials to create concrete with improved properties, such as higher strength or electrical conductivity.

There are many types of concrete available, created by varying the proportions of the main ingredients below. In this way or by substitution for the cemetitious and aggregate phases, the finished product can be tailored to its application with varying strength, density, or chemical and thermal resistance properties.Recently the use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by-product of coal-fired power plants. This use reduces the amount of quarrying and landfill space required as the ash acts as a cement replacement thus reducing the amount of cement required.

      The mix design depends on the type of structure being built, how the concrete will be mixed and delivered and how it will be placed to form this structure.first,cement.Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster.It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay and grinding this product (called clinker) with a source of sulfate (most commonly gypsum).

      second,water. Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it and allows it to flow more freely.Less water in the cement paste will yield a stronger, more durable concrete; more water will give a freer-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete, to form a solid mass.

      Third,aggregate.Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.

The presence of aggregate greatly increases the robustness of concrete above that of cement, which otherwise is a brittle material and thus concrete is a true composite material.

Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients.

        Forth,reinforcement concrete .Installing rebar in a floor slab during a concrete pour.

Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding either steel reinforcing bars, steel fibers, glass fiber, or plastic fiber to carry tensile loads. Thereafter the concrete is reinforced to withstand the tensile loads upon it.

 

     Fifth, chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing^. The common types of admixtures are as follows.

• Accelerators speed up the hydration (hardening) of the concrete. Typical materials used are CaCl₂, Ca(NO₃)₂ and NaNO₃. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored.
• Retarders slow the hydration of concrete and are used in large or difficult pours where partial setting before the pour is complete is undesirable. Typical polyol retarders are sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.
• Air entrainments add and entrain tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles, thereby increasing the concrete's durability. However, entrained air entails a trade off with strength, as each 1% of air may result in 5% decrease in compressive strength.
• Plasticizers increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics. Superplasticizers (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Compounds used as superplasticizers include sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers.
• Pigments can be used to change the color of concrete, for aesthetics.
• Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.
• Bonding agents are used to create a bond between old and new concrete (typically a type of polymer) .
• Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.

     Sixth,mineral admixtures and blended cements.There are inorganic materials that also have pozzolanic or latent hydraulic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures), or as a replacement for Portland cement (blended cements).

• Fly ash: A by-product of coal-fired electric generating plants, it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.
• Ground granulated blast furnace slag (GGBFS or GGBS): A by-product of steel production is used to partially replace Portland cement (by up to 80% by mass). It has latent hydraulic properties.
• Silica fume: A by-product of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface to volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability.
• High reactivity Metakaolin (HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high-reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.