Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 26

the aggregate. It gives a physical uniformity of cement paste structure
through avoiding bleeding effects and creating a smaller scale gel structure.
Thermal stresses are reduced compared to attempting to improve durability
by increased cement content.
Any tendency of the aggregate to alkali-silicate reaction will be limited
since the alkalies will be consumed in a nondeleterious dispersed reaction
with the silica fume.
The combined effect of these factors is to provide resistance to sulfates,
chlorides, and general aggressive chemicals. Two aspects that are not
necessarily greatly improved by silica fume addition are (1) carbon-
ation and (2) resistance to freezing and thawing deterioration. In the
case of carbonation, the consumption of the free calcium hydroxide in
the pozzolanic reaction counteracts the beneficial effect of the reduced
permeability to some extent. However, silica fume concrete has lower
electrical conductivity (Vennesland, 1981), which will assist in providing
greater resistance to steel corrosion.
Resistance to deterioration by freezing and thawing poses an interesting
question for high-strength concrete in general. There has been debate about
the presence of freezable water in the small pores in silica fume concrete.
There is no question either that entrained air still provides greater resis-
tance to freezing and thawing of saturated concrete or that it makes high
strength much more difficult and expensive to attain. The question, espe-
cially with silica fume concrete, is whether laboratory tests using saturated
concrete are realistic. 1 If the concrete is not saturated, there may be no
water to freeze and cause damage. A different answer to this question may
be appropriate in an exposed high strength column and in a bridge deck.
2.5.3 Cohesion and resistance to bleeding
These properties certainly make silica fume a most desirable ingredient
of pumped concrete (and also of self-compacting concrete). A particularly
severe test of pumpability occurs in stop-start situations. Many mixes
pump satisfactorily on a continuous basis but fail to restart after a delay.
The usual cause of this effect is internal bleeding. There is no better cure
for this problem than silica fume. Using fly ash, silica fume, and a high-
performance superplasticiser enabled single-stage pumping of concrete to
over 600 m height on the Burj Khalifa, the world's tallest building.
Resistance to bleeding also means resistance to bleeding settlement. An
important technique for very high-strength columns is to fill steel tubes from
the base with fluid, self-compacting concrete. The authors have experienced
this technique in four-story lifts, but there may be almost no limit to the
height attainable from the viewpoint of the concrete. Such columns often
1 See Section 6.3.