K-Value concept for cement additions
K-value comes
from EN 206 and defines it as a factor which considers the activity of a type
II addition, but the values used are not practical considering the need to use
cement additions for durability and environmental aspects.
No one envisaged that the UK would ever use the
k-value concept for ggbs and hence the Note at the end of BS 8500-2 clause 4.2.3.4. The
UK has always used the 4.2.3.2 equivalent performance of combinations concept
(EPCC) whereby 100% of the fly-ash, ggbs or limestone is included as cement for
purposes of calculating w/c ratio.
It could be argued that the EN 206 K-value
concept could be used & implemented in the UK, however they are considered as
being very conservative, and they also anticipate the use of additions with
either CEM I or CEM II/A. For example:
- The value for fly-ash is 0,4 where the mass of
fly-ash is no more than one third the mass of CEM I used. Therefore, if the maximum
100 kg of fly-ash is used with 300 kg of CEM I then only 40 kg of the fly-ash
is regarded as cement to calculate the w/c ratio. For 300kg of CEM II/A the
maximum amount of fly-ash that can be used is 75kg. Any other fly-ash added is
thus regarded as aggregate.
- For ggbs EN 206 was going to contain a similar
clause where the k factor is 0,6 where the ggbs added is not greater than the
amount of CEM I or CEM II/A, but it could not be agreed therefore as it stands
as a ´Note´ containing the requirements in Annex L (GGBS Note is line 7). If
this were limited to an active content of no more than the CEM I or CEM II/A,
and where no more than 60% of the ggbs is regarded as cement then the excess
ggbs would be leave it as an uncategorised cement type
In all, by using this concept for both of these
cement additions it would excessively increase the binder content impacting the
rheology of the concrete supplied, and make it economically and environmentally
unviable to use in a market which is focused on reducing it carbon footprint.
Although not stated, the EN 206 k factor for
fly-ash and ggbs are default values where there is no test data required.
Theoretically a user may want to try interpreting CEN TR 16639 to see if it is
worth establishing their own k-values from the information they have available.