Introduction

• Concrete is a mixture of sand, gravel, crushed rock, or other aggregates held together in a rocklike mass with a paste of cement and water.
• Vertical spacing between the layers of bars should be 25mm minimum.
• Sometimes one or more admixtures are added to change certain characteristics of the concrete such as its workability, durability, and time of hardening.
• Concrete has a high compressive strength and a very low tensile strength.
• Reinforced concrete is a combination of concrete and steel wherein the steel reinforcement provides the tensile strength lacking in the concrete.
• Steel reinforcing is also capable of resisting compression forces and is used in columns as well as in other situations.
• The aggregates used in concrete occupy about three-fourths of the concrete volume and are less expensive than the cement, it is desirable to use as much of them as possible.
• Both the fine aggregates (usually sand) and coarse aggregates (usually gravel or crushed stone) are used.
• Any aggregate that passes a No.4 sieve (about 6mm in size) is said to be fine aggregates.
• Material of a larger size is coarse aggregates.
• Water used in mixing concrete shall be clean and free from injurious amounts of oils, acids, alkalis, salts, organic materials, or other substances that may be deleterious to concrete or reinforcement.
• Cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together.
• Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together.
• Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete.
• Ordinary Portland Cement 33 Grade – IS 269:1989
• Ordinary Portland Cement 43 Grade – IS 8112: 1989
• Ordinary Portland Cement 53 Grade – IS 12269: 1987
• Forms are required to hold the concrete in place until it hardens sufficiently.
• Concrete and steel reinforcing work together beautifully in reinforced concrete structures.
• Reinforcing bars have tensile strengths equal to approximately 100 times that of the usual concretes used.
• Reinforced concrete 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.
• Concrete has a very low tensile strength, requiring the use of tensile reinforcing.
• The great shortcoming of concrete is its lack of tensile strength, but tensile strength is one of the great advantages of steel.
• The low strength per unit of volume of concrete means members will be relatively large, an important consideration for tall buildings and long-span structures.
• Deformed wire has indentations pressed into bars instead of rolled-on deformations
• There are several types of reinforcing bars, designated by the ASTM, available in different grades as Grade 40, 50, Grade 60, and so on, where Grade 50 means the steel has a specified yield point of 50,000 psi, Grade 60 means 60,000 psi and so on.
• Air-entraining cement
• The two materials bond together very well so there is little chance of slippage between the two; thus, they will act together as a unit in resisting forces.
• The properties of concrete vary widely because of variations in its proportioning and mixing.
• Reinforced concrete has great resistance to the actions of fire and water and, in fact, is the best structural material available for situations where water is present.
• Reinforced concrete is a low-maintenance material.
• Reinforcing bars are subject to corrosion, but the concrete surrounding them provides them with excellent protection.
• As compared with other materials, reinforced concrete has a very long service life.
• Expansive cement
• Plain bars are not used very often except for wrapping around longitudinal bars, primarily in columns.
• The excellent bond obtained is the result of the chemical adhesion between the two materials, the natural roughness of the bars, and the closely spaced rib-shaped deformations rolled onto the bars’ surfaces.
• In addition, falsework 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.
• Deformed bars, which have ribbed projections rolled onto their surfaces, are used for almost all applications.
• The low strength per unit of weight of concrete leads to heavy members.