High – Strength Concrete

Today, new types of concrete are available that mark a considerable advance on normal concrete in terms

of their strength and ductile behavior. It has been possible, for example, to increase the compressive strength.

In addition to purely technological developments, there has also been a great increase in the use of cementicious

elements in composite forms of construction. Of special interest in this respect is the creation of fiberreinforced

and textile-reinforced concrete.

Fibre-reinforced concrete, for example, has helped to improve the ductile properties of the material. Indeed, in

certain situations, such as load-bearing walls in housing construction, fibres can replace conventional steel rod reinforcement.

Glass-fibre-reinforced concrete is mostly used for slender constructional elements, e.g. roof coverings,

shell structures, and facade slabs.

Textile-reinforced concrete is a logical development of glass-fibre-reinforced concrete, since it allows the

direction of the load-bearing reinforcement to be controlled, in contrast to the random arrangement of reinforcing

fibres. With textile-reinforced concrete, it is possible to create extremely thin and lightweight elements,

which have a great potential in architectural design. The development of self-compacting concrete marks a

quantum leap in processing techniques. The properties of this type of concrete afford virtually unlimited scope

for design in terms of unit geometry and surface treatment. The materials used in high-performance concrete

usually mean that it is considerably more expensive than normal concrete. Depending on the application, an increase

in costs ranging from 50 to as much as 200 per cent or more may be expected. These figures are related

to a cubic meter of concrete, however, so that the additional costs may be offset in part by reductions in the

cross-sectional dimensions of elements and the resulting increase in rentable space.

The fibres used may be of plastic, glass or steel. Plastic fibres are mostly used to reduce cracking as a result

of early shrinkage in concrete, but they also serve to increase fire resistance; for example, in high-strength concrete.

Polypropylene fibres, which are most commonly used for this purpose, vaporize at high temperatures, but

the precise mechanism involved has not been finally established. Glass fibres are used to reduce cracking in setting

concrete, but they also have a structural function in smaller building elements. In addition, they provide an

alternative to asbestos, which was widely used as a means of reinforcing cement-bonded elements in the past.

In view of its ductile properties, its high strength and durability, glass-fibre concrete has a wide range of applications,

including semi-finished products and other elements. The use of steel-fibre-reinforced concrete is also

possible in the field of engineering construction; for example, in precast reinforced concrete floor elements or

for load-bearing walls without additional steel reinforcement. It is also used in industrial floor finishes or for

securing excavations in tunnel construction.

Textile-reinforced concrete is a logical development of fibre-reinforced concrete. Textile-like structures allow

the alignment of the load-bearing reinforcement to be controlled and facilitate an economical exploitation

of the material. In conventional reinforced concrete construction, the concrete has the additional function of

protecting the reinforcement against corrosion. The use of technical textiles made of glass or carbon means that

the concrete cover can be reduced, thereby allowing the construction of thin-walled and three-dimensionally

shaped elements. The use of textile reinforced concrete is conceivable in many areas, even for complex loadbearing

shell structures. Used in precast elements as a kind of "integrated formwork" in combination with insitu

concrete, it offers a number of advantages.

Various applications of textile-reinforced concrete are possible today, including the construction of facade

slabs with simple geometric forms, and the creation of shuttering components integrated in compound wall and

floor systems.

Production processes need to be developed that will allow the economical manufacture of various building

components with these materials. One possible application for textile-reinforced concrete lies in the creation of

finely dimensioned forms that would allow the actual load-bearing behavior to be visualized. As far as the surface

design is concerned, similar scope exists with this type of concrete as with self-compacting concrete.

Textile-reinforced concrete has a high load-bearing capacity, even with comparatively small cross-sectional

dimensions. It is, therefore, a sustainable form of construction, since the use of raw materials is relatively low,

thus helping to conserve resources. The high performance of textile-reinforced concrete is shown by the construction

of carving skis, which have been tested and function well.

The greatest step forward in concrete technology in recent years is certainly the development of selfcompacting

types of concrete. Self-compacting concrete differs from vibrated concrete in that it contains a

greater proportion of fine-grain cement and aggregate. The concrete acquires its self-compacting properties in

conjunction with high-performance agents.

Roughly, 30 per cent of the volume of normal concrete consists of the cement-paste matrix. The maximum

size of the aggregate may be 8, 16 or 32 mm. The largest ingredient is the aggregate, which makes up more than

70 per cent of the volume. Depending on the water/ cement ratio, the volume of water will probably be slightly

less than 20 per cent. Self-compacting concrete differs from standard forms of concrete in that it generally contains

aggregate with a maximum size of 16 mm.

In high-performance concrete (i.e. high-strength and high-density concrete) there is a considerably greater

proportion of cement. The fine-grain material is generally formed by cement and micro silica. In most cases, the

coarse-grain aggregate will also have a maximum size of 16 mm (as in self-compacting concrete), but the water

content will be somewhat lower. In view of the much greater proportion of the bonding agent, the water/ cement

ratio will also be somewhat lower.

Self-compacting concretes usually have a slump value of between 600 and 800 mm, which provides some

indication of the yield point of this type of concrete. The run-out time from the conical discharge funnel - which

as a rule lies between 10 and 20 seconds - is an indirect measure of the viscosity of the concrete. The combination

of these two values determines the consistency of the self-compacting concrete.

Tests also have to be carried out to determine whether self-compacting concrete can flow between the reinforcement.

So-called "block ring tests" have been developed for this purpose. The spacing of the bars should be

coordinated with the maximum size of the aggregate.

The choice of the appropriate mix proportions and the production of self-compacting concrete presuppose

specialist knowledge in this field. Although it is not necessary to use vibrators with self-compacting concrete, it

is possible to achieve sharply delineated forms and surface textures.

The Science Centre provides a further example of the architectural design scope offered by

self-compacting concrete today. This material is particularly relevant in the field of prefabrication, where there

is also a social dimension to this form of construction.