Concrete is strong in compression but weak in tension.?Therefore, reinforcement is needed to resist the tensile stresses resulting from the loads. Additional reinforcement is occasionally used to reinforce the compression zone of concrete beam sections. Such steel is necessary for heavy loads in order to reduce long-term deflections.

Steel reinforcement for concrete consists of bars, wires, and welded wire fabric, all of which are manufactured in accordance with ASTM standards. The most important properties of reinforcing steel are:

1. Elastic modulus,

2. Yield strength,

3. Ultimate strength,

4. Steel grade designation

5. Size or diameter of the bar or wire

To increase the bond between concrete and steel, projections called deformations are rolled on the bar surface as shown in Fig.26.1. The deformed wire has indentations pressed into the wire to serve as deformations. Except for wire used in spiral reinforcement in columns, only deformed bars, deformed wires, or wire fabric made from smooth or deformed wire may be used in reinforced concrete.

It is necessary to ensure that the wet concrete mix passes through the reinforcing steel without separation. Since the graded aggregate size in structural concrete often contains 3/4 in. (19 mm diameter) coarse aggregate, minimum allowable bar spacing and minimum?required cover are needed. Additionally, to protect the reinforcement from corrosion and loss of strength in case of fire, codes specify a minimum required concrete cover. Some of the major requirements of ACI Code 318 are:

(1) Clear distance between parallel bars in a layer must not be less than the bar diameter ?or 1 in. (25.4 mm).

(2) Clear distance between longitudinal bars in columns must not be less than 1.5, or 1.5 in. (38. 1 mm).

(3) Minimum clear cover in cast-in-place concrete beams and columns should not be less than 1.5 in. (38.1 mm), this same cover requirement also applies to stirrups, ties, and spirals.

Every structure is proportioned as to both architecture and engineering to serve a particular function. Form and function go hand in hand and the best structural system is the one that fulfills most of the needs of the user while being serviceable, attractive, and, hopefully, economically cost-efficient. Although most structures are designed for a life span of 50 years, the durability performance record indicates that properly proportioned concrete structures generally have longer useful lives.

Reinforced concrete systems are composed of a variety of concrete?structural elements that, when synthesized, produce a total system.The components can be broadly classified into: floor slabs, beams, columns, walls, and foundations.

Floor Slabs ?Floor slabs are the main horizontal elements that transmit the moving live loads as well as the stationary dead loads to the vertical framing supports of a structure. They can be proportioned such that they act in one direction (one-way slabs) or proportioned so that they act in two perpendicular directions (two-way slabs).

Beams??Beams are the structural elements that transmit the?loads from floor slabs to vertical supporting columns. They are normally cast monolithically with the slabs and are structurally reinforced on one face, the lower tension side, or both the top and bottom faces. As they are cast monolithically with the slab, they form a T-beam section for interior beams or an L beam at the building exterior, as seen in Fig. 26. 2.

Columns??The vertical elements support the structural floor?system. They are compression members subjected in most cases to both bending and axial load, and are of major importance in the safety considerations of any structure. If a structural system is also composed of horizontal compression members, such members would be considered as beam-columns.

Walls??Walls are the vertical enclosures for building frames.?They are not usually or necessarily made of concrete but of any material that aesthetically fulfills the form and functional needs of the structural system. Additionally, structural concrete walls are often necessary as foundation walls, stairwell walls, and shear walls that resist horizontal wind loads and earthquake-induced loads.

Foundations??Foundations are the structural concrete elements?that transmit the weight of the superstructure to the supporting soil.