Pre-reading Discussion. 1. Which construction materials can you name?

1. Which construction materials can you name?

2. How can you describe concrete in your own words?

3. Is it possible to consider concrete to be the main construction material?

4. For what purpose do usually people use concrete?

5. In designing a building the choice of construction materials is of vital importance, isn’t it?

 

Concrete technology has undergone a constant process of development over the past 50 years. Today, it

provides planning engineers and architects with a broad range of possibilities in terms of both structural and

formal design. In office construction, for example, high-strength concrete offers scope for saving space by reducing

the dimensions of the load-bearing structure and thereby increasing the rentable floor area. This type of

concrete also allows the construction of building elements that have a great resistance to weathering, that are

durable and that, in certain circumstances, can protect the environment against harmful liquids.

High-strength concrete has a long tradition. With extremely low water/cement ratios (below 0.4) and with

the addition of pozzolanic, highly-reactive aggregates such as powdered silica or metakaolin, a compression

strength of up to 150 N/mm2 can be achieved (compared with a strength of 20-50 N/mm2 for normal concrete).

Assuming the same amount of reinforcement and the use of concrete C70/85, the cross-section could be reduced

by 30 per cent. In many situations, a reduction in the amount of reinforcement may be required instead,

in order to facilitate the execution of the work. In most cases, however, a solution will be sought between these

two extremes.

Only by using this type of concrete was it possible to keep the thickness of the walls on the lower floors

within reasonable limits. With concrete of standard strength, a wall thickness of about 2 m with a very large

amount of reinforcement would have been necessary. It was possible to reduce the thickness to roughly 1.40 m

and to use only a moderate amount of reinforcement. Nevertheless, for walls of this thickness, it is necessary to

reduce the cement content to well below normal levels and to add pulverized fly ash to the mix. Preliminary

trials allowed the appropriate combination of cement and fly ash to be determined in order to avoid deleterious

cracking through the discharge of setting heat. In Germany, solutions of this kind require a special certification

of the relevant state planning authorities. Even with the use of an optimum mix of concrete, however, newly

constructed concrete elements still need appropriate curing subsequently to prevent heat escaping too quickly.

In view of the very low water/cement ratio and the addition of pozzolanic additives, high-strength concrete

is not only strong, but also dense. Both properties can be exploited in areas like bridge building, where highly

durable concrete is required to ensure long life as well as slender cross-sectional dimensions. Where harmful

liquids are used in buildings such as clinics, hospitals or chemical laboratories, additional measures will be necessary

to protect the ground and groundwater from contamination. Dense concrete mixes have proved particularly

suitable in such cases.

The advantages of high-density concrete slabs include their greatly reduced permeability in comparison

with normal concrete, and their higher tensile strength. In slabs of smaller area (up to a maximum dimension of

15 m), their greater strength means that they are not subject to cracking. Cracks in construction elements are

especially critical in buildings where water-polluting organic liquids are used. With the use of high-density

concrete, it is often possible to do without an additional protective coating.

As far as cementicious building materials are concerned, new developments have taken place in recent

years that will considerably extend the use of concrete in this area. This applies in particular to high-strength

concretes with compressive strengths of up to 800 N/mm2 (in comparison with standard applications of about

300 N/mm2). Considerable reductions in the cross-sections of reinforced and prestressed concrete members can

be achieved with this type of concrete. Slender elements also mean a lower overall weight, as well as a more

sustainable form of construction through the conservation of resources.

New methods of construction are also emerging through the use of so-called "reactive-powder concrete", as can

be seen in the bridge constructed in Quebec, Canada, in 1997. In the meantime, this technology has

been applied to other structures, such as the footbridge in Seoul, South Korea.