Introduction - Design of Reinforced Concrete - page 21

compound. Quite a few pozzolans are available that can be used satisfactorily in con-
crete. Two of the most common ones are fly ash and silica fume. Here only silica fume is
discussed.
When silica fume is used, it causes increases in the density and strength of the con-
crete. These improvements are due to the fact that the ultrafine silica fume particles are
dispersed between the cement particles. Unfortunately, this causes a reduction in the
workability of the concrete, and it is necessary to add superplasticizers to the mix. Super-
plasticizers, also called high-range water reducers , are added to concretes to increase
their workability. They are made by treating formaldehyde or napthaline with sulphuric
acid. Such materials used as admixtures lower the viscosity or resistance to flow of the
concrete. As a result, less water can be used, thus yielding lower water-cement ratios and
higher strengths.
The addition of organic polymers can be used to replace a part of the cement as the
binder. An organic polymer is composed of molecules that have been formed by the
union of thousands of molecules. The most commonly used polymers in concrete are
latexes. Such additives improve concrete's strength, durability, and adhesion. In addi-
tion, the resulting concretes have excellent resistance to abrasion, freezing, thawing
and impact.
Another procedure that can increase the strength of concrete is consolidation . When
precast concrete products are consolidated, excess water and air are squeezed out, thus
producing concretes with optimum air contents. In a similar manner, the centrifugal
forces caused by the spinning of concrete pipes during their manufacture consolidate the
concrete and reduce the water and air contents. Not much work has been done in the con-
solidation area for cast-in-place concrete due to the difficulty of applying the squeezing
forces. To squeeze such concretes it is necessary to apply pressure to the forms. You can
see that one major difficulty in doing this is that very special care must be used to prevent
distortion of the wet concrete members.

1.14 FIBER-REINFORCED CONCRETES

In recent years a great deal of interest has been shown in fiber-reinforced concrete, and
today there is much ongoing research on the subject. The fibers used are made from steel,
plastics, glass, and other materials. Various experiments have shown that the addition of
such fibers in convenient quantities (normally up to about 1 or 2% by volume) to conven-
tional concretes can appreciably improve their characteristics.
The strengths of fiber-reinforced concretes are not significantly greater than they
would be if the same mixes were used without the fibers. The resulting concretes, how-
ever, are substantially tougher and have greater resistance to cracking and higher impact
resistance. The use of fibers has increased the versatility of concrete by reducing its brit-
tleness. The reader should note that a reinforcing bar provides reinforcing only in the di-
rection of the bar, while randomly distributed fibers provide additional strength in all
directions.
Steel is the most commonly used material for the fibers. The resulting concretes seem
to be quite durable, at least as long as the fibers are covered and protected by the cement
mortar. Concretes reinforced with steel fibers are most often used in pavements, thin
shells, and precast products as well as in various patches and overlays. Glass fibers are