Testing - Concrete Mix Design, Quality Control and Specification - page 150

Figure 7.3 Automatic updating of K value (slope of strength versus log equivalent age graph).
the proportions of different chemical compounds present in a cement by
their different rates of heat generation.
It is immaterial to our purposes what happens after 7 days, since this is
covered by the addition of the average 7- to 28-day strength gain for the par-
ticular mix. However a change of slope prior to 7 days would mean that the
slope of the line joining an early result and that at 7 days on the strength ver-
sus log (EA) graph would be affected by the particular early age. It was seen
that the requirement for satisfactory operation is that the graph should be a
straight line up to a predicted “control age” and that the amount to be subse-
quently added should be the average strength gain from the control age to 28
days. Therefore it is recommended that, for any new mix, a number of speci-
mens are taken from a single batch of concrete, tested at a range of ages, and
plotted on the strength versus log(EA) graph to determine the age at which
the slope changes for that particular mix. This of course is quite different to
the practice of using a predetermined strength versus maturity graph. There is
no suggestion that subsequent mixes will have the same strength at the same
EA, only that the rate of strength development will change at the same EA.
If the change in slope occurs at later than 7 days, then it will be conve-
nient to continue to use 7 days as the control age. It is only when the change
occurs earlier that the control age must be changed to get accurate results.
Having selected or established the control age, one specimen is always
tested at the control age and at least one at some earlier age. The ConAd