Mix design - Concrete Mix Design, Quality Control and Specification - page 211

8.6 INTRODUCTION OF SUPPLEMENTARY
CEMENTITIOUS MATERIALS

Using this empirical mix design methodology, there are two paths that can
be taken when adding supplementary cementitious materials. With current
batch records, it is possible to ascertain how the materials will behave in
most situations. So, when the production and quality control data are avail-
able for a set of materials that will provide a good guide as to how the
components will perform in different dosages and blend percentages.
The European standard on the concrete (EN 206) sets conditions for
consideration of mineral additions in partial substitution of cement in con-
crete in the formulation. It defines two concepts: the concept of the coeffi-
cient k and the concept of equivalent performance. Regarding the first, this
standard defines a coefficient, k , taking into account the activity of mineral
additives. To quantify the activity within the meaning of resistance, by
transposition, we associate the resistance of mortar to those of concrete.
The idea is to compare, in a given period, the compressive strength of two
mortars with the same proportions of sand and water, first with cement
alone, and then when in proportion of cement is replaced by a mass of min-
eral addition is determined experimentally by an activity index, i :

f

A

i =

f

wA −

where f w − A and f A represent the strength of the mortar, either with a ratio
“ x ” of additions substituted for cement ( f A ) or without addition ( f w-A ).
With regard to the concept of equivalent performance, the European
standard states that concrete with added mineral must have similar
performance to that of a reference concrete without addition, especially
toward the aggression of the environment and its durability. It also provides
recommendations for limits on the composition and properties of concrete
according to the exposure class. The method applied involves the concept of
equivalent binder. We define the equivalent amount of binder as the sum of a
quantity of cement and an additional amount of the weighted coeficient k .

L eq = C + kA

This k coefficient is determined for fly ash and silica fume with all types of
cement. In case the cement used was CEM I type, the coefficient values are
determined taking into account the specific standards.
The calculation of the coefficient k is as follows:

• It is recognised that the relationship between k and i is linear.