Civil Engineering Reference
In-Depth Information
(hunger and Brouers 2009); and (3) water. Due to a problem of component
mass and density, water is the fluid that can move the fines, and the union
of water plus fines creates a paste; and the paste becomes the fluid for the
aggregates. it can be inferred that two different pack models are necessary for
controlling concrete rheological behavior, one for the paste and another for
the aggregates. Because of this, the distance between particles for concrete
suspensions is usually calculated by two indexes: the mPT (maximum paste
thickness, which measures the distance between aggregates and, consequently,
the mobility of them and is determined by the paste); and the iPS (inter-
particle spacing, measuring distance between fines, is determined by the
water) (Pandolfelli et al. 2000):
È
Í
Ê
Á
1
1 -
P
of
ˆ
˜
˘
˚
˙
2
1
IPS =
VSA
¥
V
s
-
where VSA = volumetric surface area, calculated by the product of specific
surface area and solid density;
V
s
is the volumetric solid fraction on suspension;
and
P
of
is the pore fraction on system when all particles are in touch in the
maximum packing condition:
˘
˚
˙
where VSa
c
= VSa of coarse fraction;
V
sc
is the volumetric coarse solid
fraction on suspension; and
P
ofc
is the porosity of coarse fraction on system
when all particles are in touch in the maximum packing condition.
A concrete design with high binder efficiency is one that uses the lowest
water content while allowing the highest iPS possible, since it is the
condition when the flowability of the paste is increased at the same time
as the compressive strength is the highest possible due to the lowest water
content. Conversely, a high-binder efficient concrete is also one that uses the
lowest paste content while allowing the highest possible mPT, because in
this condition the rheology requirements are the same, even though there is
a low paste content - which implies the lowest binder content for the same
compressive strength.
adequate concrete rheology is currently reached by increasing paste content
to increase the mPT, where, for current concretes, pastes are composed
of water and cement. This implies the use of a binder for guaranteeing
rheological parameters which, consequently, decreases binder use efficiency
since the w/c ratio is normally kept constant, which, however, does not change
compressive strength, but the increase in paste content increases binder
content.
The use of inert fines for providing rheology for the concrete is an effective
way of decreasing binder content by replacing it with a very low impact
È
Í
1
1 -
P
ofc
1
V
sc
-
Ê
Á
ˆ
˜
2
MPT =
VSA
c
¥
