Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85926
RoadScience by Tom Kuennen, Contributing Editor SPECIAL SERIES A 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 fi ne aggregate particles, the liquid asphalt will mix with the dust and not bond very well with the aggre- gate, and stripping also will occur. "Physical and chemical properties of aggregates at the The Chemistry of Road Building Materials On the T 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 Proper- ties 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 specifi c surface area." The surface free energy of aggregates – a manifestation of Face of It Aggregate performance 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 ag- gregates 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 ag- gregate can make or break a mix. The fi lm 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 material surface physical chemistry characteristics – is one way aggregates can be classifi ed 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 (cohe- sive 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 mol- ecules in that region usually possess reactivity signifi cantly different from units in the bulk," say Arno Hefer and Dallas Little in their ICAR report, Adhesion in Bitumen-Aggregate Systems and Quantifi cation of the Effects of Water on the Adhesive Bond. "In the bulk phase, a unit experiences a uniform force fi eld 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 accu- rately the 'excess surface free energy' ... [s]imple, effi cient and reliable measurement of surface energy is an important consideration for implementation of this technology." For asphalt pavements, the goal is to analyze the physio- chemistry of aggregates and binder to select combinations of liquid asphalt and aggregates that are more resistant to moisture damage, will perform best with modifi ers 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