Concrete is artificial stone made from two main components: cement paste and aggregates.
Aggregates usually consist of natural sand and gravel or crushed stone. The paste hardens as a result of the chemical reaction between cement and water and glues the aggregates into a rock-like mass. Reinforced concrete structures utilize the best qualities of concrete and steel - concrete's high compressive strength and steel's high tensile strength. The main idea behind reinforced concrete is to provide steel reinforcement at locations where tensile stresses exist that the concrete cannot resist. Due to its strength, only a relatively small amount of steel is needed to reinforce concrete. Steel's ability to resist tension is around 10 times greater than concrete's ability to resist compression. It is very important to note that reinforcement in concrete structures is effective only if it is appropriately used, strategically placed, and in proper quantity.
Prestressed concrete is a special type of reinforced concrete in which internal compression stresses are introduced to reduce potential tensile stresses in the concrete resulting from external loads. High-strength steel tendons are embedded within the concrete and subjected to a tensile stress imposed by special equipment (jacks). The two main methods of prestressed concrete construction are
- pretensioning: when the tendons are tensioned before the concrete has hardened
- posttensioning: when the tendons are tensioned after the concrete has hardened
Reinforced concrete buildings consist of several structural components (or members). The basic components of a reinforced concrete building are (see Figure 1)
- floor and roof systems
These structural components can be classified into horizontal components (Floors, roofs, and beams) and vertical components (columns and walls). According to another classification, the part of the building above ground is called the superstmcture, while the part below ground (including foundations, basemen!, and other underground structures) is called the substructure. The role of each structural component is briefly explained below.
|Figure :1 Components of a reinforced concrete building.|
The floor and roof systems are the main horizontal structural components in a building. They carry gravity loads and transfer them to the vertical components (columns and/or walls), and also act as horizontal diaphragms by transferring the lateral load to the vertical components of a structure. The most common floor and roof systems are listed below (see Figure 2):
• Slab-beam-and-girder: The slabs are supported by beams, which are in turn supported by girders (see Figure 2a). A girder is a large beam that carries loads from the beams framing into it. Beams around the outside edges of the floor are called spandrel beams.
• Slab band: This is a uniform slab with a thickened slab portion along the column lines parallel to the longer spans (see Figure 2b).
• Flat slab: This is a system without beams, where a slab is supported by round or square columns (see Figure 2c). In this system, the design may also require a flared cone shaped cap on the top of the column, called the capital, and a thickened slab above it, called the drop panel.
• Flat plate: This is similar to the flat slab, except that there are no drop panels or capitals, as shown in Figure 2d. Columns are typically of circular or square shape.
|Figure :2 Floor systems in reinforced concrete buildings: a) slab-beam-and-girder floor; b) slab bands; c) flat slab; d) flat plate; e) slab with beams; f) joist floor.|
• Slab with beams: The beams frame into columns and support floor or roof slabs, as illustrated in Figure 2e. They provide moment interaction with the columns (this interaction is essential for the frame to resist lateral loads).
• Joist floor (pan joist): This system consists of a series of closely spaced joists (similar to small beams), spanning in one or two directions, topped by a reinforced concrete slab cast integrally with the joists, and beams spanning between the columns perpendicular to the joists (see Figure 2f).
• Waffle slab: This is a two-way reinforced concrete joist floor. Waffles are hollow spaces between the joists.
Slab on grade is a very common form of slab construction that is placed directly on the ground. It is also called "floor on ground." It is possible to confuse this term with the term "floor system." The basic difference is that a slab on grade is supported by the earth beneath it, whereas a floor system is supported only by columns at a few distinct locations.
Beams transmit the loads from the floors to the vertical supports (columns). Beams are usually cast monolithically with the slab and are subjected to bending and shear.
Columns are vertical components that support a structural floor system. Columns are usually subjected to combined axial load and bending.
Walls provide the vertical enclosure for a building. Bearing walls carry gravity loads only, whereas shear walls have a major role in carrying lateral loads due to wind and earthquakes. Concrete walls built in the basements of buildings are subjected to lateral soil pressure in addition to gravity loads - such walls are called basement walls.
Foundations transmit the weight of the superstructure to the supporting soil. There are several types of foundations. Spread footings transfer the load from the columns to the soil. Walls are supported by strip footings. Other types of foundations include combined footings, which support more than one column; piles which may be driven into dense soil strata beneath; and raft foundations, where several columns rest upon a raft or a mat distributing the column or wall loads over a uniform soil bearing area.