Building Materials, Third Edition

ƒ™—2g



2—2g

2g™

 RVQ

fc = w/cA

B

where w/c represents the water-cement ratio of the concrete mix and A and B are empirical
constants. When the workability is adequate, it appears that the water-cement ratio holds the
key to the porosity of both the hydrated cement paste (HCP) and transition zone. Furthermore,
with a low water cement ratio it is generally observed that considerable high strength gains are
achieved for very small decreases in the w/c ratio. The main problem with this is that with the
decreasing water content, fresh concrete becomes more and more difficult to mix, place, and
consolidate. So for the production of HSC, the opposing effects of w/c ratio on consistency and
strength of concrete cannot be harmonized without the use of water reducing admixtures like
superplasticizers. As discussed earlier, since it is the transition zone that is the weakest
component of the HSC, so at a given w/c ratio, the strength of concrete mix can be increased
significantly by simply reducing the maximum size of coarse aggregate. This has a beneficial
effect on the strength of transition zone due to increased surface area.
The requirements of low water cement ratio and small aggregate size mean the cement
content of concrete mix will be high, generally above 385 kg/m^3. Cement contents of
approximately 600 kg/m^3 and even higher have been investigated, but found undesirable.
With the increasing proportion of cement in concrete a strength plateau is reached, that is, there
will by no more increase in strength with further increase in cement content. This is probably
due to the inherent inhomogeneity of the HCP, in which the presence of large crystals of
calcium hydroxide represent weak area of cleavage under stress. Such inhomogeneous and
weak area in transition zone are vulnerable to micro cracking even before the application of
external load which can happen as a result of development of stresses due to thermal-shrinkage
or drying-shrinkage. The increase in cement content results in increased heat of hydration, and
drying shrinkage of concrete. As a result of this micro-cracking, considerable differences arise
between the elastic response of the cement paste and that of the aggregate.
When this inhomogeneity becomes strength limiting in concrete, the solution is to modify
the microstructure so that components causing inhomogeneity are eliminated or reduced. In
case of plain cement concrete (PCC), an inexpensive and effective way to achieve this is by
incorporation of puzzolanic materials into concrete mixture such as fly ash. Fly ash reacts with
calcium hydroxide to form a reaction product that is similar in composition and properties to
the principal distributions of OPC; also the puzzolanic reaction is accompanied by a reduction
in large pores—an effect that is equally important for the enhancement of strength of the
system. At a given w/c ratio, when low calcium fly ash with calcium oxide content less than
(6%) is used as a partial replacement of OPC, the early strength (1-and 7-day) of concrete at
normal temperatures may be reduced almost in direct proportion to the amount of fly ash
present in the total cementing material (cement + fly ash). However, high calcium fly ash with
calcium oxide content more than 15% or ground blast furnace slag shows a significant strength
within the first 7 days of hydration. Highly reactive puzzolanas such as condense silica fume
and rice husk can make a strength contribution even at 3 days.
Although normal water reducing admixtures can be used for making high-strength concrete
mixes, concretes with very high consistency (200 to 250 mm slump) and more than 70 MPa