Weathering behaviour of road aggregate of andesitic composition in a New Zealand quarry

Abstract

The process of weathering progressively affects the characteristics of geomaterials. Despite prior assertions, this process may happen in a relatively short time and can cause ageing and deterioration of the structure of aggregates. This study investigates the effect of natural weathering processes on characteristics of road materials, exposed within a quarry that produces road materials compliant with New Zealand specifications for premium base course material. While not an exact replica of in-use weathering conditions, this case study serves as an observable analogue, recording the chemical and mineralogical degradation of andesite aggregate materials. The variations in chemical and physical properties of roading materials of andesitic composition were investigated using image processing and a variety of analytical methods, including thin section petrography, X-Ray Fluorescence (XRF), and X-ray Diffraction (XRD). Whilst initially there was almost no obvious sign of weathering in fresh materials, within a short time-frame (less than 15 years) intense weathering was evident. The immediate loss of bases (notably alkalies) is the primary chemical trend in the natural weathering, although the rate of loss appears to decrease with time. It is consistent with the results of chemical indices of weathering, including Chemical Index of Weathering and Alteration (CIW and CIA), Weathering Index of Parker (WIP), Plagioclase Index of alteration (PIA), and A-CN-K diagram. Furthermore, XRD analysis revealed the presence of clay minerals in the weathered materials, with kaolinite and smectites the product of relatively short-term of weathering. Thin-section petrography indicates that the original textural features of the andesites were significantly changed during weathering. The results suggested that the grain size and zonation of primary minerals are critical controls in the weathering process and glass and plagioclase phenocryst are the least stable mineral phases in the examined andesitic aggregate materials.