Unchanging concepts - Concrete Mix Design, Quality Control and Specification - page 264

negate the change. This is providing one is sufficiently knowledgeable (and
has done the necessary prior investigations) to correctly interpret results at
a range of early ages.
The control process should be considered as a whole, ensuring value for
money in several different types of expenditures, for example:
1. Batching equipment
2. Quality of testing
3. Frequency of testing
4. Computer equipment
5. Computer software
The ability to work to a 1 MPa (150 psi) lower mean strength for a given
specified strength is worth about 5 kg of cement per cubic metre (8.4 lb/
cu yd). This is a sufficient saving (on high volume production) to pay for a
very elaborate control system. The ability to detect a downturn in strength
a day earlier may avoid a major penalty. It may also justify a lower safety
margin.
It should be noted that all criteria relate to the standard deviation of
results. Lower variability concrete is easier to control more precisely.
As already noted, this is not tautology but a recognition of a multiplier
effect of control improvement. A reduction of 1 MPa in standard devia-
tion makes a direct difference of 1.28 or 1.65 MPa to the required target
strength (depending on whether the specification is based on 90% or 95%
above). It will make at least a further 1 MPa reduction in the strength
margin required for the detection of a change. Improved quality control
may also be a major sales point. The standard deviation of the concrete
strength is obviously affected by the quality and effectiveness of both the
batching system and the testing process, as well as by the variability of
input materials.
The frequency of testing is an important cost factor to be weighed against
the quality of testing; the securing of additional data, such as slump, con-
crete temperature, and density; and the cost of result analysis. The cost of
elaborate analysis is rapidly reducing compared to that of physical testing
and an increase in one can justify a reduction in the other.
The ability of a control system to combine results from many different
grades of concrete into a single analysis can be equivalent to a several-fold
increase in testing frequency.
The time between a downturn and its detection and rectification is also
affected by the age at test. The days in which mix revisions were based
on 28-day test results are hopefully gone, but the choice of test age in
the interval of 1 to 7 days is open to consideration. In temperature-stable
tropical conditions, 3 days is a good choice. Depending on the protection
provided to the specimens, and on the time of collection, a 3-day strength