Unit 2 concrete structures

LEAD-IN

1. Match the following words to make the essential terms of the unit.

concrete	concrete
reinforced	site
building	plant
In-situ	constructions
central	concrete
Portland	cement

2. Answer the questions.

1. What man-made construction materials do you know?

2. Which man-made construction material is one of the most ancient ones?

3. What well-known structures are presented in the photos?

4. Which man-made material was used in their construction?

Text 1 concrete structures

C oncrete is one of the major structural material. The components of concrete are Portland cement, coarse aggregates such as ground stone, fine aggregates such as sand, and water. In the mix, water combines chemically with the cement to form a gel structure that bonds the stone aggregates together. The upper limit of concrete strength is set by that of the stone used in the aggregate. The binding gel structure forms slowly, and the design strength is usually taken as that occurring 28 days after the initial setting of the mix.

The base of modern industrial construction are reinforced concrete structures. Reinforced concrete is applied for one and multi-storey buildings of industrial and dwelling purposes. This kind of concrete often serves as a shell for spans, tanks, subways, transport tunnels, bridges and as a dome for warehouses and circuses.

The component parts of the reinforced concrete are readily available throughout the world at fairly low cost. Portland cement and reinforcing bars are easily manufactured; aggregates such as sand and crushed limestone can be easily obtained. In 100 years, reinforced concrete has risen from an experimental material to the most widespread form of building construction.

There are two methods of fabricating reinforced concrete. The first is to pour the liquid material into forms at the building site; this is so-called in-situ concrete. The other method is called precast concrete, in which building components are manufactured in a central plant and later brought to the building site for assembly.

I n-situ concrete is used for foundations and for structural frames. In low-rise buildings, where vertical loads are the main concern, a number of framing systems are used to transmit the loads. When these systems are applied, a technique called posttensioning is often used. The steel reinforcement in the form of wire cables casts into the concrete. After the concrete has set and gained its full strength, the wires are permanently stretched taut using small hydraulic jacks, bending the entire floor slab into a slight upward arch. This reduces deflection and cracking of the concrete when the load is applied. Concrete columns are usually of rectangular or circular profile and are cast in plywood or metal forms.

Precast concrete structural members are fabricated under controlled conditions in a factory. These elements are usually pretensioned, which is similar in principle to posttensioning. The reinforcement is again steel wire, but the wires are put into tension (stretched) on a fixed frame, formwork is erected around the taut wires, and concrete is poured into it. After the concrete has set and gained its full strength, the wires are cut loose from the frame. As in posttensioning, this gives the precast floor members a slight upward arch, which reduces deflection.

P recast prestressed floor elements are made in a number of configurations. These include beams of rectangular cross section, hollow floor slabs. Precast concrete columns are usually not prestressed and have projecting shelves to receive floor slabs. At the building site, precast members are joined together by a number of methods, including welding together metal connectors cast into them or pouring a layer of in-situ concrete on top of floor slabs, binding them together. Precast prestressed construction is widely used, and it is the dominant form of construction in Russia and Eastern Europe.

TASKS