main parameters affecting the aggregate packing
density are the shape and combined particle size distribution of the
used in the mix. Once the aggregate
supply has been selected, the only
parameter that the mix designer can manipulate is the fine/coarse
concrete technology, the term “particle”
is used for fine aggregate, the coarse
aggregate(s), and the fibers (if any) and the term “paste” is
used for Water,
cement, other cementitious materials, and air. It is the paste phase that
lubricates the particles.
make concrete or shotcrete, not only
must all the
space (voids) between the particles be filled with cement paste, but
paste must also be added to provide the desired level of workability.
there is an optimum fine/coarse aggregate ratio (and aggregate
minimizes the amount of voids in the aggregate skeleton, thus
amount of paste required to fill the voids. Extra paste is required,
beyond that required to fill the voids, and its presence affects the
workability or, in this case, the pumpability of the wet-mix shotcrete (Chapdelaine
and Beaupré 2000). Normally provision of extra paste to
the extent of 6% is
considered a good practice although it depends on hose dia & hose
content of the aggregate phase can be easily determined using
methods/procedures described in basic soil mechanics manuals. The
have been built by testing different combinations of fine aggregate,
aggregates, and fibers (if any).
curves for mixtures with different fiber contents
Figure above shows the void contents for various proportions of coarse
aggregates (as a fraction of the total combined aggregate mass) for
mixtures containing 0, 1, and 2% (by volume) of synthetic macrofibers. Each
mixture has a minimum void content that indicates optimum particle
optimum ratio depends upon the shape and gradation of both the fine and
aggregates and the fiber type and content.
fibers are present, the aggregate packing is modified, which leads to a
optimum coarse/total aggregate ratio and void content. These curves
obtained with standard concrete sand and a 10 mm (0.394 in.) maximum
crushed granite coarse aggregate. In this figure, the mixture without
having a coarse/total aggregates content of 47% has a void content of
practice, this mixture should contain 29% of cement paste plus
of extra paste* in order to have good pumpability (Chapdelaine and
2002). For the same ratio, the paste content should be increased when
are present; 1% of fibers not only moves the optimum toward a mixture
sand, but increases the void content to 35% (35% + 6% = 41% paste
for good pumping). This is only an example; one should remember that
numbers are affected not only by the aggregates used, but also by fiber
geometry and content. However, it illustrates the importance of this
a minimum paste content to fill the voids between aggregates and fiber
particles is an essential factor with respect to concrete pumpability.
implied in Figure above, there are two
ways of achieving this: either
the paste content (increase cement, water, or air content) and/or reduce the
aggregate void content by optimizing the fine/coarse aggregate ratio.
options have the same effect: increasing the workability (and
the concrete mixture.
there are limits related to cost, mechanical, and durability
to how much the paste content can be increased. Because of the
effects on material costs, shrinkage, durability, and mechanical
of voids in the aggregate skeleton and hence the paste required in the
should always be implemented first.
significance of Coarse Aggregate to Fine aggregate ratio.
“Understanding Wet-Mix Shotcrete: Mix Design, Specifications, and
Marc Jolin and Denis Beaupré