On the Face of It

Aggregate performance depends on its surface

The 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”, 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 reactive aggregate, which form an alkali/silica gel. Under the right conditions – particularly enough available moisture – the gel will expand and produce stresses and damage in the concrete.