Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 57

to the SS of the fines used, or more strictly, to the MSF of the mortar
(since the coarse aggregate makes a minor contribution to MSF). So, if the
water content (Wm) of a mortar per cubic metre (of mortar) were deter-
mined, either experimentally or by a yet to be discovered calculation, that
of the concrete (Wc) would be readily calculable as
Wc = Wm × (Required MSF of concrete - SS of coarse aggregate)/
MSF of mortar
Interesting workability research is being done at ICAR (University of
Texas at Austin) on the development and use of a highly portable rheometer.
The ICAR rheometer was beta tested on the concrete for the Burj Khalifa
during pumping. Sinan Erdogan investigated the effects of particle shape in
both coarse and fine aggregates. The investigation is at too great a depth
to present here and includes substantial work using x-ray tomography and
microtomography to actually measure the shape of individual particles, in
addition to using the rheometer. Very briefly he found that the particle shape
of coarse aggregate does not greatly affect yield stress (which is essentially
what the slump test measures) but does greatly affect the plastic viscosity
(which is the part of workability the slump test does not reveal). Equally
clear conclusions are not reached in respect of fine aggregates and those
interested should consult the thesis (Erdogan and Fowler 2005).
3.2 COARSE AGGREGATE
The properties of a coarse aggregate depend on the properties of the basic
rock, the crushing process (if crushed), and the subsequent treatment
of the aggregate in terms of separation into fractions, segregation, and
contamination.
Most rock has an adequate basic strength for use in most grades of con-
crete. Even manufactured and naturally occurring lightweight aggregates,
which can be readily crushed under a shoe heel, are used to make concrete
with an average strength up to 40 MPa (although they do require a higher
cementitious content than dense aggregates). Exceptions to this are some
sandstones, shales, and weak limestones. A different type of exception is
that use involving wear and impact resistance can require a more stringent
selection of rock type.
Generally, however, the stability of a coarse aggregate is more impor-
tant than its strength. Rock, which exhibits moisture movement (swelling
and shrinking), will add to concrete shrinkage. Again sandstone tends to be
among the worst offenders, but some basalts will also display moisture move-
ment, and some breccias or conglomerates may be quite strong mechanically
and yet literally fall part after a few cycles of wetting and drying.