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Concrete, being an artificial stone possesses the same good qualities as natural stones. It is strong in compression and weak in tension, therefore it has been substituted for natural stone in arch bridges because the arch is strong in compression. The greater strength of concrete permits slender sections with elegant lines and at the same time provides strength equal to that of masonry structures of far greater mass. Concrete is fire resistant and therefore it has become one of the most common building materials all over the world. The strength of concrete is measured in kg/cm² of force, needed to crush a sample of a given age or hardness, but concrete’s strength is affected by environmental factors, especially temperature and moisture. It would be reasonable to add that the cost of natural stone is at least double that of precast concrete.

Plain concrete does not easily resist wind, earthquakes, vibrations and other forces and is therefore unsuitable in many structures. At the beginning of the 19^th century concrete was reinforced with metal bars (fig. 7.1). The idea was to transfer the tensile stress from concrete to the reinforcement (fig. 7.1b). Concrete that has been hardened onto imbedded metal (usually steel) is called reinforced concrete. The reinforcing steel, which may take the form of rods, bars or mesh, contributes to tensile strength. At present reinforced concrete is suited to short and medium bridge spans (up to 40-60 m). Nevertheless, having sufficient strength and permanency it is also rather competitive with metal for longer spans which remain in service for hundred of years. Reinforced concrete spans are of a great variety because of their ability to work in compression and tension as well as in flexure. The durability and appearance of a bridge depend on the concrete strength.

Concrete is used for the large-scale manufacturing of simple beams, continuous girders, cantilever-beam systems, arches, frames and combined systems (arch and beam or arch and truss), etc. The builders use monolithic or cast in situ reinforced concrete, precast concrete, prestressed concrete and other concrete grades. Prestressed concrete is an important variation of reinforced concrete. Prefabricated reinforced concrete is manufactured at ready-mix-plants and the bridge members are mounted in situ from ready-made segments and blocks. High-strength concrete is made by reinforcement (fig. 7.1c, 8) and prestressed with jacks (fig. 7.1c, 9).

When reducing the structure’s weight, coarse aggregate such as crushed rock, pebbles and gravel is substituted with slag and claydite. This results in light concrete. All aggregate materials must be clean and free from soft particles or vegetable matter, because even small quantities of organic soil compounds result in chemical reactions that seriously affect the strength of the concrete. The mixture must have just enough water to ensure that spaces between the aggregate are filled, and that the concrete is liquid enough to be poured and spread effectively.

Bridges made of reinforced concrete offer the following advantages:

1. Substantial saving of steel, which is a scarce building material.

2. Substantial reduction of maintenance costs as compared with metal bridges.

3. Greater rigidity as compared with metal bridges.

4. Long-term service life (80-100 years).

5. Various high-level architectural forms.

The disadvantages of reinforced concrete bridges may be enumerated as:

1. Great dead weight.

2. Great labour and Production time increases for bridge elements.

3. The development of hairline cracks inducing reinforcement deterioration.

4. Hidden hollow spaces, which are difficult to remedy.

5. Problems with concrete placement under low temperature.