Hydrophobic chemical treatment of aggregate surfaces to re-engineer the mineral/bitumen interface and improve bitumen adhesion

Abstract

Strong, water-resistant adhesion between aggregate chip and the bitumen binder is vital for achieving effective chipsealing and pavement waterproofing. However, mineral aggregates are hydrophilic, while bitumen is hydrophobic, so the two systems are fundamentally poorly compatible. Chemically this means that the aggregate surface/water (S/W) interface is energetically favoured over the aggregate surface/bitumen (S/B) interface. The presence of water on the aggregate surface or infiltration of water into the S/B interface can therefore destroy the adhesion of bitumen to aggregate surfaces. In this study the surface of chipsealing aggregates has been rendered hydrophobic by treatment with alkyl phosphonate (A-Phon) or alkyl phosphate (A-Phos) surface modifiers. Following this treatment, the interfacial free energy of the S/B interface becomes favoured over that of the S/W interface. Using this method, the adhesion mechanism changes from one where discrete polarisation forces dominate to one where more general dispersive forces dominate. Vibrational spectroscopy was used to probe the mode of binding of the alkylating species to the mineral surface, identifying a mixture of binding modes and tenacity of the bound molecules towards hydrolysis and physical attrition. The surface spreading factor of water and hence the surface free energies for water sorption on the aggregate surface was probed via contact angle analysis. The hydrolytic stability of the treatment was further tested via accelerated weathering in boiling water followed by contact angle analysis. Various physical and rheological bitumen tests and adhesion tests were performed and compared with both non-modified chips and adhesion agent-modified bitumen systems and indicated very strong aggregate/bitumen adhesion that resisted de-bonding by water. The method would be equally applicable to asphalt concrete aggregates.

Keywords:

• chipseal
• bitumen
• adhesion
• surface free energy
• aggregate interface
• surface modification
• hydrophobic

Disclosure statement

The presence of Dr Fertel as an author and materials supplier to the project is noted as a potential conflict of interest.

Additional information

Funding

The authors acknowledge the Ministry of Business, Innovation and Employment (MBIE), New Zealand for research funding through project OPSX1501 – Waterproof Roads.