Road Science: The Chemistry of Aggregates.
by Tom Kuennen, Contributing Editor
of Road Building
Face of It
depends on its surface
he higher-performing bituminous and portland
cement concrete mixes of today are becoming more
complex, and to ensure performance, more scrutiny
is being given to the chemistry and composition of the aggregates that go into those mixes.
And a growing amount of that scrutiny is aimed at the
surface of a piece of aggregate, because it’s at that interface
– where rock meets liquid asphalt or cement paste – that
pavements can succeed or fail.
Aggregates are the component of a composite material,
such as bituminous asphalt or portland cement concrete,
which resists compressive stress. Aggregates in asphalt or
concrete have a wide variety of sizes, from coarse material
to sand, bound in a matrix by a cementing medium.
The degree of porosity of the surface of a particle of aggregate can make or break a mix. The ﬁlm of liquid asphalt
that enrobes a piece of aggregate bonds better if it can be
absorbed into the surface of the rock.
If asphalt binder loses its grip on aggregate or “strips”,
16 July 2012 Better Roads
and the pavement begins to fall apart, it may be because too
much moisture was present on the surface of the aggregate
when mixed. If too much dust was present on the surface
of coarse or ﬁne aggregate particles, the liquid asphalt will
mix with the dust and not bond very well with the aggregate, and stripping also will occur.
“Physical and chemical properties of aggregates at the
micro scale strongly impact the adhesive bond (strength
and durability) between bitumen and aggregate,” write
Amit Bhasin and Dallas Little with the Texas Transportation
Institute (TTI), in their paper, Characterizing Surface Properties of Aggregates Used in Hot Mix Asphalt, published by ICAR,
the International Center for Aggregates Research. “These
properties include surface free energy, chemical interaction
potential, and speciﬁc surface area.”
The surface free energy of aggregates – a manifestation of
material surface physical chemistry characteristics – is one
way aggregates can be classiﬁed for future performance.
Surface free energy of aggregates can impact the interface
between the asphalt and the aggregate (adhesive fracture),
or fracture within the asphalt binder or mastic itself (cohesive facture), researchers at Texas A&M University’s TTI say.
“The intrinsic surface forces that take part in fundamental
adhesion can be attributed to the fact that atoms and molecules in that region usually possess reactivity signiﬁcantly
different from units in the bulk,” say Arno Hefer and Dallas
Little in their ICAR report, Adhesion in Bitumen-Aggregate Systems
and Quantiﬁcation of the Effects of Water on the Adhesive Bond.
“In the bulk phase, a unit experiences a uniform force
ﬁeld due to interaction with neighboring units,” Hefer and
Little write. “However, if a surface is created by dividing the
bulk phase, the forces acting on the unit at the new surface
are no longer uniform. Due to the missing interactions, the
units are in an energetically unfavorable condition, i.e. the
total free energy of the system increased. This increase in
energy is termed the ‘surface free energy’ or more accurately the ‘excess surface free energy’ ... [s]imple, efﬁcient
and reliable measurement of surface energy is an important
consideration for implementation of this technology.”
For asphalt pavements, the goal is to analyze the physiochemistry of aggregates and binder to select combinations
of liquid asphalt and aggregates that are more resistant to
moisture damage, will perform best with modiﬁers and
other additives, and whose performance can be predicted.
In portland cement concrete, alkali-silica reaction (ASR)
begins at the cement paste/aggregate interface. ASR is a
chemical reaction that occurs between alkalis contributed
primarily by cement, and a reactive form of silica from