Testing - Concrete Mix Design, Quality Control and Specification - page 146

temperature at which concrete does not gain strength. This is often taken to
be +10°F or -12°C. It is also often taken as 0°C for convenience, although
concrete does gain strength at 0°C (but see Figure 11.2 and associated
explanation for selecting a different value).
The equivalent age (EA) concept is older and more accurate, but also
more complicated. The concept was not originated specifically for concrete
but as a general concept for all chemical reactions. The general formula is
attributed to Arrhenius. The concept was applied to concrete in the 1930s
in USSR in the form of coefficients by which the length of time at each
temperature should be multiplied to give equivalence.
The relationship is exponential and is given by the formula:

∑

− QT (1/ − /) T

EA = ( te )

a s

where
EA = Equivalent age (hours)
Q = activation energy divided by the gas constant
T a = temperature (°K) for time interval t
T = Time (hours) spent at temperature
T s = reference temperature (°K = °C + 273)

The reference temperature ( T s ) is the standard curing temperature at
which test specimens are kept. In many parts of the world it is 20°C (293K)
in Australia it is 23°C in temperate zones and 27°C in tropical zones; it
may be that 30°C would be appropriate in some tropical countries (if this
is the average temperature of unheated curing tanks). The Q value can
range from below 4000 to over 5000 depending on the characteristics of
the particular cement. It is often taken as 4200.
A discussion of the relative merits of these two approaches follows, but it
is important for the general reader not to get lost in the detail and worried
about minor pitfalls, but to realise that the basic concept is very simple and
enables powerful solutions to two problems:

1. Prediction of 28-day strength from an early-age test

2. Establishment of the strength of in situ concrete

Previously the first problem was approached by setting down a fixed
accelerating (heated curing) regime and experimentally determining a
correlation curve. The second problem used to be handled by setting a time,
such as 7 or 14 days before some activity such as stripping, depropping,
stressing, or lifting was permitted. Alternatively “field cured” specimens
were used, assuming that cylinders cured alongside in situ concrete would
have a similar maturity. This of course is very far from the truth. Almost
any sort of rough application of any maturity approach is vastly superior to
these “old-fashioned” solutions.