What is 'Steric Effect' and what does it have to do with making a road last longer?

January 28, 2009

Atoms have volume. Molecules are composed of many atoms. Whenever molecules are squeezed together repulsing, but stabilizing energy is redistributed between the atoms to counteract destabilization due to overlapping electron clouds. This repulsion phenomena is called the Pauli or Born effect; and such a condition frequently leads to a distortion of the preferred molecular shape. Loss of the preferred molecular shape is called a "steric effect" or "steric hindrance".

Steric hindrance occurs when the size-density of molecular groups within any proximate region causes sufficient distortion to inhibit chemical reactions which might otherwise occur. Steric hindrance may be engineered to create 'steric protection' of molecular structures; thereby stabilizing the targeted molecular communities against being damaged from unwanted outside forces.

It is noteworthy that crumb rubber used in most hot asphalt ARB pavement to date substantially functions as a flexibilizing stabilizer providing essential anti-oxidant protection to the asphalt, i.e. steric protection. This is the primary reason ARB constructed roads have become the longest wearing, flexible pavement structures currently in use; often outlasting SBS modified asphalt 'super pave' surfaces by a factor of 3:1. However, because of the larger crumb size typically used (20-40 mesh), the crumb rubber behaves more as an aggregate rather than a reinforcing component of the adhesive system; thereby limiting the ultimate utility of the crumb rubber component.

The AROS™ resin technology has been engineered to optimize steric protection against water and air induced, oxidative embrittlement of the hydrocarbon molecules in asphalt by the use of high loadings of very finely divided crumb rubber (80-600 mesh). Additionally the AROS™ resin technology also has been compounded in a manner which embeds the finely divided crumb rubber into the adhesive where it provides a dual function as a reinforcement; thereby achieving optimal, superior transmission of load dynamics throughout the pavement crossection such that unequaled fatigue resistance to dimensional displacement such as cracking, rutting and shoving is achieved.

Accelerated fatigue testing performed to date reveals that the AROS™ resin technology may significantly extend the useful life of ARB pavement beyond that exhibited by current, hot mix ARB systems.