Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 59

placing (reduced fine aggregate content and reduced slump [= higher yield
stress] because the rounded aggregate will have a lower plastic viscosity
and so can have a higher yield stress for equivalent workability) rounded
gravel may give as good or better results, depending on the particular use.
Forty years ago, Day made concrete of 85 to 90 MPa from London area
gravel (which is one of the gravels that has been claimed to give inferior
results for high-strength concrete). Gravels tend to have been adequately
tested by the formation process as regards weaker particles and moisture
movement susceptibility. However, this provides no security against alkali-
aggregate reactivity and any coatings on pit gravels in particular should be
regarded with suspicion.
The subject of coatings on coarse aggregate is worth consideration.
Generally if the coating is removed during the mixing process (and assum-
ing it to be chemically inactive) it is not likely to cause a severe problem.
Very fine material will merely add to the water requirement in the same way
as fine aggregate silt. This will increase water requirement but, unless exces-
sive, should cause only a small strength depression. However, if a coating
remains intact after the concrete is in place, a substantial effect on strength
and durability can occur through loss of bond. The amount of fine material
adhering to coarse aggregate is often substantially affected by the weather,
with more material adhering during wet periods. This effect should be con-
sidered when looking for causes of strength variations in concrete.
The ideal maximum size for a coarse aggregate has usually been assumed
to be 40 mm or 20 mm (1½ inch or ¾ inch) according to the size of sec-
tion and the reinforcement spacing. However, there has been a worldwide
trend to higher concrete strengths and work done many years ago in the
United States (Blick, 1974) is gradually being rediscovered the hard way in
many other places. This work showed that the optimum size of aggregates
depended on the required strength level, being smaller for higher strengths.
That is provided optimum is defined as that which gives the minimum
cement requirement for a given strength.
If optimum is defined in terms of water/cement ratio or shrinkage or (less
certainly) wear resistance, larger sizes may be best. Although the optimum
size may vary from 40 mm at 20 MPa to 14 or even 10 mm at strengths
over 50 MPa, the margin is not usually large and little harm is done by
standardising on 20 mm. One exception to this is where difficulty is experi-
enced in obtaining a high strength, in which case a smaller aggregate should
certainly be tried. It is interesting to note that this effect has now been seen
to extend further than most would have believed possible. In reactive pow-
der “concretes” with strengths of several hundred megapascal, the coarsest
aggregate used is a fine sand.
A smaller maximum aggregate may also be required for pumpability. The
maximum aggregate has the greatest effect on friction factor as discussed
in Chapter 8 on mix design.