RHEOLOGY OF LOW WATER/BINDER RATIO MIXTURES
While from a strength point of view it is essential to use the lowest possible
water/binder ratio, we must remember that high-performance concrete
has to be transported and placed with relative ease using conventional
construction procedures (reviewed in Chapter 11).
In field applications, concrete usually must retain an adequate
workability for about an hour and a half. In precast plants, where placing
is more rapid, it is generally sufficient to ensure a high workability for up
to half an hour.
Concrete rheology is essentially governed by physical and chemical
factors. Among the physical factors are the grain size distribution and
shape of the aggregates. In very low water/binder ratio systems, the grain
size distribution and shape of the cement particles can also play an
important role in determining the rheology of fresh concrete. Among the
chemical factors affecting the rheology of fresh concrete are the initial
reactivity of the cement and supplementary cementitious materials when
in contact with water, and the duration of the so-called ‘dormant period’.
These two fundamental aspects of concrete rheology will be examined
separately in sections 5.6.2 and 5.6.3. Among other factors affecting the
rheology of concrete are the general condition of the mixer and its mixing
efficiency, particularly its shearing action, the temperature of highperformance
concrete after it has been mixed and the ambient
temperature.
5.6.1 Optimization of grain size distribution of aggregates
A considerable amount of work has been undertaken on the optimization
of the grain size distribution of powders or aggregate skeletons. However,
the objective in these cases was usually the reduction of the porosity of the
final product rather than improvement in concrete workability. Most of
the present state of the art is the result of early work by Fuller and
Thompson (1907) in North America and by Bolomey (1935), Faury (1953)
and Caquot (1937) in France. Most of these authors arrived at different
recommendations which are still in use. It is well established, for example,
The water/binder ratio law 97
that flat and elongated aggregate particles are detrimental to workability,
and that cubic or spherical aggregates produce better workability.
However, little research has been carried out on the influence of the grain
size distribution of aggregates on high-performance concrete workability
(de Larrard, 1987; de Larrard and Buil, 1987). Therefore, the rules that
were developed and successfully used in usual concrete have often been
extended to high-performance concrete.
5.6.2 Optimization of grain size distribution of cementitious particles
Fundamental research has been carried out to highlight the need for
optimizing the grain size distribution of cementitious materials for
improving workability and strength. Bache was one of the first to point
out the advantages of adding silica fume to very low water/binder ratio
concrete mixtures to improve workability (Bache, 1981). He explained the
beneficial effect of adding such a fine powder in such a concrete mixture
by the fact that when minute spherical silica fume particles are well
dispersed in the cement-water system, they can displace water molecules
from the vicinity of cement grains, so that entrapped water molecules
between flocculated cement particles can be freed and thus contribute to
fluidizing the mixture. Detwiler and Mehta (1989) later showed by using
an inert carbon black having similar grain size to silica fume that the
chemical nature of the very fine particles was not critical.
5.6.3 The use of supplementary cementitious materials
By replacing some of the cement by a supplementary cementitious
material containing no C3S, C3A or C4AF, the rheology of any given
concrete mixture becomes easier to control, provided that the grain size
distribution and shape of the particles of the supplementary cementitious
material are almost the same as those of the replaced cement. Such
replacement is attractive from an economic viewpoint since the price of a
supplementary cementitious material is usually lower than that of cement
(except for silica fume, in most cases). Moreover, it will be seen later that
savings can also be obtained from the reduction in the dosage of
superplasticizer necessary to achieve the desired workability. However,
the optimum dosage of cementitious material has to be found, taking into
account strength requirements at early age.