It facilitates the spreading of cement over the fine aggregate. Steel reinforcement bars The steel reinforced bars used in reinforced concrete are mild steel round bars, medium tensile steel, high tensile steel. Functions:- 1. Bond between concrete and steel can be improved by use of deformed bars.
The deformed bars are provided with lungs, or deformations on surface of bars to minimize the slippage of bars in concrete.
Ordinary Portland Cement OPC is the most common cement used in general concrete construction when there is no exposure to sulphates in the soil or groundwater. Lafarge Malaysia's OPC is sold in bulk. But it has the property when it combined with lime to produce a stable pozzolana compound it has definite cementations properties.
By adding the additional pozzolana such as fly ash calcium hydroxide coverts into calcium silica hydrated gel. It gains strength more quickly than OPC, though the final strength is only slightly higher. The one-day strength of this cement is equal to the three-day strength of OPC with the same water-cement ratio. Pile foundation 2. In Coastal area Works 3. Mass Construction Dams, Marine constructions 2. Hydraulic Engineering Concrete 3.
Retaining wall construction Quick Setting Cement This type of cement is manufactured by reducing the amount of gypsum and adding small amount of aluminiumsulphate to accelerate setting time of cement As the name suggests, it is used where the works needs to be done quickly 1. In Underwater Constructions 2. Artificial Marble 2. Used as a base coat before painting 2. Used to avoid the shrinkage of concrete 1. Used in repair works to create a bond with old concrete surface 2. Used in Hydraulic Structures Fineness test 2.
Setting time test 3. Strength test 4. Soundness test 5. Heat of hydration test 6. Chemical composition test Ratio of percentage of lime to percentage of silica, alumina and iron oxide when calculated by Cao Fineness of cement is tested in two ways. So that it is avoided from least vulnerable to damages from external activities. Then the mortar is filled into a cube mould of 7. Compact the mortar. Aggregate is the general term applied to those inert or chemically inactive materials which, when bonded together by cement, form concrete.
Most of the aggregates used are naturally occurring aggregates such as crushed rock, gravel and sand. Artificial and processed aggregates may be broken brick or crushed air-cooled blast furnace slag. Natural sands are generally used as fine aggregate. Sand may be obtained from pits, river, lake or sea-shore. When obtained from pits, it should be washed to free it from clean and silt.
Sea shore sand may contain chlorides which may cause efflorescence, and may cause corrosion of reinforcement. Hence it should be thoroughly washed before use. Angular grained sand produces good and strong concrete, because it has good interlocking property, while round grained particles of sand do not afford such interlocking.
Natural gravels can be quarried from pits where they have been deposited by alluvial or glacial action and are normally composed of flint, quartz, schist and igneous rocks.
Coarse aggregates are obtained by crushing various types of granites, hard lime stone and good quality sand stones. Hard and closed-grained crystalline lime stones are very suitable for aggregate, is cheap, but should be used only in plain concrete. Grading limits and maximum aggregate size are specified because grading and size affect the amount of aggregate used as well as cement and water requirements, workability, durability of concrete.
In general, if the water-cement ratio is chosen correctly, a wide range in grading can be used without a major effect on strength. Sieve Size mm 40 mm 20 mm 16 mm As per IS the grading limit of coarse aggregate, both for single size as well as graded should be as per the table given below. Sieve Size mm 63 mm 40 mm 20 mm 16 mm It has to be graded from its minimum to maximum size.
IS recommends the following grading limit for fine aggregates. WATER Water acts as lubricant for the fine and coarse aggregates and acts chemically with the cement to form the binding paste for the aggregate and reinforcement. Water is also used for curing the concrete after it has been cast into the forms.
Water used for both mixing and curing should be free from injurious amount of deleterious materials. Portable waters are generally considered satisfactory for mixing and curing of concrete. Less water in the cement paste will yield a stronger, more durable concrete; more water will give an 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. Limit of Solids water Organic: Mg per liter Inorganic: Mg per liter Sulphate: mg per liter Chloride: mg per liter for RCC work and mg per liter for concrete not containing steel.
Suspended matter: mg per liter The strength and workability of concrete depend to a great extent on the amount of water used. For a given proportion of the materials, there is an amount of water which gives the greatest strength.
Amount of water less than this optimum water decreases the strength and about 10 percent less may be insufficient to ensure complete setting of cement. More, than optimum water increases the workability but decrease the strength. The use of an excessive amount of water not only produces low strength but increases shrinking, and decreases density and durability.
According to Powers, cement does not combine chemically with more than half the quantity of water in the mix. Water- cement ratio needs to be about 0. Concrete is known by its grade which is designated as M15, M20 etc. Thus, concrete is known by its compressive strength. M20 and M25 are the most common grades of concrete, and higher grades of concrete should be used for severe, very severe and extreme environments.
Grades of concrete Strength: It is one of the most important properties of concrete and influences many other describable properties of the hardened concrete.
The mean compressive strength required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix. The other factor affecting the strength of concrete at a given age and cured at a prescribed temperature is the degree of compaction. Durability The durability of concrete is its resistance to the aggressive environmental conditions.
High strength concrete is generally more durable than low strength concrete. In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the water-cement ratio to be used. The degree of workability required depends on three factors. These are the size of the section to be concreted, the amount of reinforcement, and the method of compaction to be used.
For the narrow and complicated section with numerous corners or inaccessible parts, the concrete must have a high workability so that full compaction can be achieved with a reasonable amount of effort. This also applies to the embedded steel sections. The desired workability depends on the compacting equipment available at the site.
The workability of concrete can be measured by various tests, such as:- 1. Slump test 2. Compaction factor test 3.
Vee-bee Test Slump Test Degree of workability Slump mm Use for which concrete is suitable Very low 0 - 25 Very dry mixes; used in road making. Roads vibrated by power operated machines Low 25 - 50 Low workability mixes; used for foundations with light reinforcement.
Roads vibrated by hand operated Machines Medium 50 - Medium workability mixes; manually compacted flat slabs using crushed aggregates. Normal reinforced concrete manually compacted and heavily reinforced sections with vibrations High - High workability concrete; for sections with congested reinforcement. Compacting Factor Test 7 Wipe clean the outside of cylinder of concrete and weigh to nearest 10gm.
F Uses Very Low 0 - 25 0. VeBe Time Test 7 The time required for the shape of concrete to change from slump cone shape to cylindrical shape in second is known as Vibe Degree. The test fails if VeBe Time is less than 5 seconds.. And the test must be created when no collapse or shears slump in concrete Precast Cast-in-situ Elements are manufactured in a controlled casting environment and have it is easier to control mix, placement and curing.
Elements are manufactured on site and hence it is difficult to control mix, placement and curing. Quality can be controlled and maintained easily. Quality control and maintenance is difficult.
Less labours are required. It does, however, vary approximately in proportion to the square root of compressive strength. The tensile strength of concrete in flexure is quite important when considering beam cracks and deflections. For these considerations, the tensile strengths obtained with the modulus of rupture test have long been used. The load is increased until failure occurs by cracking on the tensile face of the beam.
The modulus of rupture is then determined from the flexure formula. The stress determined in this manner is not very accurate because, in using the flexure formula, it is assumed that the concrete stresses vary in direct proportion to the distances from the neutral axis.
The tensile strength of concrete may also be measured with the split-cylinder test. The cylinder will split in half from end to end when its tensile strength is reached. The tensile strength at which splitting occurs is referred to as the split-cylinder strength Ft and can be calculated with the following expression, in which P is the maximum compressive force, L is the length, and D is the diameter of the cylinder:.
Even though pads are used under the loads, some local stress concentrations occur during the tests. In addition, some stresses develop at right angles to the tension stresses. As a result, the tensile strengths obtained are not very accurate. It is extremely difficult in laboratory testing to obtain pure shear failures unaffected by other stresses.
As a result, the tests of concrete shearing strengths through the years have yielded values all the way from one-third to four-fifths of the ultimate compressive strengths. The aggregates used in concrete occupy about three-fourths of the concrete volume. Since they are less expensive than the cement, it is desirable to use as much of them as possible. Both fine aggregates usually sand and coarse aggregates usually gravel or crushed stone are used. Any aggregate that passes a sieve in which has number of wires spaced mm on centres in each direction is said to be fine aggregate.
Material of a larger size is coarse aggregate. Aggregates are to be strong, durable, and clean. If there are dust or other particles present, they can interfere with the bond between the cement paste and the aggregate. These concretes are also sometimes called high-performance concretes because they have other excellent characteristics besides just high strengths. For example, the low permeability of such concretes causes them to be quite durable as regards the various physical and chemical agents acting on them that may cause the material to deteriorate.
High-strength concretes are sometimes used for both precast and pre-stressed members. They are particularly useful in the precast industry where their strength enables the production of smaller and lighter members, with consequent savings in storage, handling, shipping, and erection costs.
In addition, they have sometimes been used for offshore structures, but their common use has been for columns of tall reinforced concrete buildings, where the column loads are very large. In recent years, a great deal of interest has been shown in fibre reinforced concrete, and today there is much ongoing research on the subject.
The fibres used are made from steel, plastics, glass, and other materials. The compressive strengths of fibre reinforced concretes are not significantly greater than they would be if the same mixes were used without the fibres.
The resulting concretes, however, are substantially tougher and have greater resistance to cracking and higher impact resistance. The use of fibres has increased the versatility of concrete by reducing its brittleness. It is to be noted that a reinforcement bar provides reinforcing only in the direction of the bar, while randomly distributed fibres provide additional strength in all directions.
Steel is the most commonly used material for the fibres. The resulting concretes seem to be quite durable, at least as long as the fibres are covered and protected by the cement mortar. Concretes reinforced with steel fibres are most often used in pavements, thin shells, and precast products as well as in various patches and overlays. Glass fibres are more often used for spray-on applications as in shotcrete. The fibres used vary in length from about 6 mm up to about 75 mm while their diameters run from approximately 0.
For improving the bond with the cement paste, the fibres can be hooked or crimped. In addition, the surface characteristics of the fibres can be chemically modified in order to increase bonding. Typically, the aspect ratios used vary from about 25 up to as much as , with being about an average value.
Other factors affecting toughness are the shape and texture of the fibres. The compressive strength of concrete is dictated by exposure to freeze-thaw conditions or chemicals such as sulphates. These conditions may require a greater compressive strength or lower water—cement ratio than those required to carry the calculated loads. For concrete exposed to chemicals, the amount of fly ash or other pozzolans is limited. The steel reinforcement used for concrete structures may be in the form of bars or welded wire fabric.
Reinforcement bars are referred to as plain or deformed. The deformed bars, which have ribbed projections rolled onto their surfaces patterns differing with different manufacturers to provide better bonding between the concrete and the steel, are used for almost all applications.
Plain bars are not used very often except for wrapping around longitudinal bars, primarily in columns. Plain round bars are indicated by their diameters in mm.
Reinforcement bars were formerly manufactured in both round and square cross sections, but today all bars are round. Welded wire fabric is also frequently used for reinforcing slabs, pavements, and shells, and places where there is normally not sufficient room for providing the necessary concrete cover required for regular reinforcement bars. The mesh is made of cold-drawn wires running in both directions and welded together at the points of intersection.
The sizes and spacings of the wire may be the same in both directions or may be different, depending on design requirements. Wire mesh is easily placed and has excellent bond with the concrete, and the spacing of the wires is well controlled.
Concrete and Reinforced Concrete satyendra November 14, 5 Comments admixtures , aggregate , compressive strength , concrete , reinforced concrete , reinforcement , reinforcement bars , steel , tensile strength , Concrete and Reinforced Concrete Concrete is a composite building material made from a mixture of sand, gravel, crushed rock, or other aggregates coarse and fine held together in a stone like mass with a binder such as cement and water. Advantages and disadvantages of reinforced concrete as a structural material Reinforced concrete is an important material available for construction.
It has considerable compressive strength per unit cost compared with most other materials. It has high resistance to the actions of fire and water and, in fact, is the best structural material available for situations where water is present. During fires of average intensity, members with a satisfactory cover of concrete over the reinforcement bars suffer only surface damage without failure.
Reinforced concrete structures are very rigid. It has a low-maintenance cost. As compared with other materials, it has a very long service life. Under proper conditions, reinforced concrete structures can be used indefinitely without reduction of their load carrying capabilities. This can be explained by the fact that the strength of concrete does not decrease with time but actually increases over a very long period, measured in years, because of the lengthy process of the solidification of the cement paste.
It is usually the only economical material available for footings, floor slabs, basement walls, piers, and similar applications. A special feature of concrete is its ability to be cast into an extraordinary variety of shapes from simple slabs, beams, and columns to great arches and shells, making it widely used in precast structural components.
In most areas, concrete takes advantage of inexpensive local materials sand, gravel, and water and requires relatively small amounts of cement and reinforcing steel. A lower grade of skilled labour is needed for erection as compared with other materials such as structural steel. Concrete has a very low tensile strength, requiring the use of tensile reinforcement zero strength after cracks develop. It needs mixing, casting, and curing, all of which affect the final strength of concrete.
Forms are required to hold the concrete in place until it hardens sufficiently. In addition, false work or shoring may be necessary to keep the forms in place for roofs, walls, floors, and similar structures until the concrete members gain sufficient strength to support themselves. The cost of the forms used to cast concrete is relatively high. The cost of form material and artisanry may be equal to the cost of concrete placed in the forms. The low strength per unit of weight of concrete leads to heavy members.
Lightweight aggregates can be used to reduce concrete weight, but the cost of the concrete is increased. Similarly, the low strength per unit of volume of concrete means members are relatively large, an important consideration for tall buildings and long span structures. The properties of concrete vary widely because of variations in its proportioning and mixing. Furthermore, the placing and curing of concrete is not as carefully controlled as is the production of other materials, such as structural steel and laminated wood.
Reinforced concrete competes against more durable building technologies, like steel frame or traditional bricks and mortar. Around the world, it has replaced environmentally sensitive, low-carbon options like mud brick and rammed earth — historical practices that may also be more durable.
Early 20th-century engineers thought reinforced concrete structures would last a very long time — perhaps 1, years.
In reality, their life span is more like years, and sometimes less. Building codes and policies generally require buildings to survive for several decades, but deterioration can begin in as little as 10 years.
But there is still a lack of knowledge about their composite qualities — for example, in regard to sun-exposure-related changes in temperature. The many alternative materials for concrete reinforcement — such as stainless steel, aluminium bronze and fibre-polymer composites — are not yet widely used. The affordability of plain steel reinforcement is attractive to developers. But many planners and developers fail to consider the extended costs of maintenance, repair or replacement.
There are technologies that can address the problem of steel corrosion, such as cathodic protection , in which the entire structure is connected to a rust-inhibiting electric current. There are also interesting new methods to monitor corrosion, by electrical or acoustic means.
Another option is to treat the concrete with a rust-inhibiting compound, although these can be toxic and inappropriate for buildings.
Fundamentally, however, none of these developments can resolve the inherent problem that putting steel inside concrete ruins its potentially great durability. This has serious repercussions for the planet.
Concrete is the third-largest contributor to carbon dioxide emissions, after automobiles and coal-fuelled power plants. Concrete also makes up the largest proportion of construction and demolition waste, and represents about a third of all landfill waste.
Recycling concrete is difficult and expensive , reduces its strength and may catalyse chemical reactions that speed up decay.
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