Civil Engineering Reference
In-Depth Information
with current technology is around 5 kg m
-3
mPa
-1
for concretes above 50 mPa.
Below that threshold, minimum
bi
seems to follow the 250 kg m
-
³ binder
content line, which corresponds roughly to the minimum cement content in
many national standards (aBnT, 2006; Grube and Kerkhoff, 2004). Since
most of the concrete is below 50 mPa class, the actual binder intensity is
much higher than the possible minimum.
as an example, we estimated a
ci
based on very simple information. BFS,
Fa and SF were considered to be Co
2
free. The clinker was considered an
average value of 1 kg Co
2
by 1 kg of clinker. aggregate emissions, mostly
related to transportation distances, were also disregarded. minimum
ci
values
are around 2 kg m
-3
mPa
-1
, and correspond to high fractions of typical Co
2
intensity which is between 3 and 9 kg m
-3
mPa
-1
. and, unlike
bi
, Co
2
intensity
seems not to be as sensitive to compressive strength.
The combination of
bi
and
ci
allows demonstrating the risks of judging
environmental impact based only on one indicator. The data shows that it is
possible to produce concrete with very low
ci
values - which could imply
an efficient concrete in response to global warming concerns - both using
very little pure Portland cement (
ci
= 4 kg m
-3
mPa
-1
) or with a very high
bi
due to the heavy use of 'zero Co
2
emissions' supplementary materials.
This strategy is even cited in the LEED certification as a way for 'green'
concretes (US Green Building Council, 2009). however, the foreseen
change in Co
2
allocation procedures for the supplementary cementitious
materials will obviously change this equation and certainly will raise the
Co
2
intensity level. it will make Co
2
intensity more dependent on binder
intensity. For more detailed discussions about
bi
and
ci
, see Damineli et al.
(2010).
as can be seen in Fig. 2.3, average data from two ready-mix concrete
(RMC) producers were obtained. Data from the first one (triangles) are
representative of a 3 month production interval, and from the second one
(squares) represents 6 months. Since these ready-mix concrete producers are
among the best ones in the Brazilian market, it is assured that the analysis
can be considered as a good parameter for assessing the current state of
technology in the market. RmC1 works with concretes in an interval of
20-40 mPa, with a
bi
varying from 8 to 12 kg m
-3
mPa
-1
and average near
10 kg m
-3
mPa
-1
- almost double the best
bi
found in literature. on the other
hand, RmC2 reached
bi
near 6.5 to 7 kg m
-3
mPa
-1
. But maximum values
are also higher than RmC1. market averages seems to be something near
10 kg m
-3
mPa
-1
.
2.3.2 Strategies to increase binder use efficiency
Binder use efficiency is strictly dependent on two concepts: (1) maximization
of particle mobility with lowest water content possible; and (2) an increase of
