Unchanging concepts - Concrete Mix Design, Quality Control and Specification - page 256

cement content reductions (say, 100 kg/m 3 [170 lb/yd 3 ]) were clearly pos-
sible but no reduction was in fact made on the following grounds:
1. The possible saving of $60,000 was trivial compared to the total proj-
ect cost of several hundred million dollars.
2. The huge strength margin virtually ensured that there would be no
delays due to strength problems.
3. The very high early-age strength permitted early stripping with no
concern for damage, weather conditions, need for intensive in situ or
early-age testing, and so on.
4. The additional safety margin against any unexpected factors was also
of some value.
As another example, Australia's billion-dollar Parliament House is a
major concrete structure, containing about one quarter million cubic
metres of concrete. At around $25 million, the cost of the concrete supply
represented about 2.5% of the total cost. It really would not matter very
much if this cost increased by 5% to 2.63% of total cost.
Of course, the extra cost in the case of the Rialto would be a little less
trivial if the same argument were applied to the whole of the concrete in
the project, but the real point is that this attitude could never be taken by
an independent concrete supplier because the cost would probably exceed
the entire profit margin. The strength margin (but more likely 5 MPa [700
psi] than 15 MPa [2000 psi]) could therefore only come about by either
the owner specifying a higher grade or the builder ordering a higher grade
than specified. Either party might take this action on the basis of expedit-
ing construction or at least of avoiding any risk of delay. In fact the best
way of organising this is for the owner to specify a higher strength but
to impose a cash penalty rather than rejection or further investigation
for strength shortfalls of up to 5 MPa (700 psi) (or whatever margin has
been allowed). The same effect could be obtained by offering a bonus for
excess strength (of course within a strict limit) and not raising the speci-
fied strength.
The benefits accruing from the proposed technique (of specifying a higher
strength than strictly necessary and providing a cash penalty for strength
deficiencies within the margin) would be as follows:
1. A relaxed attitude to minor strength deficiencies by the owner
2. A keener attitude to minor strength deficiencies by the concrete
supplier
3. A smoother running project
4. The provision of better concrete, probably at only a very marginal
overall cost increase