Despite the fact that the in-service weathering process can cause undesirable changes to unbound granular materials (UGMs), there is no reliable preconstruction laboratory test method to evaluate their weatherability. Most existing durability tests do not take into account either the properties of the parent rock or in-service environmental conditions. This study proposes a series of modifications to the New Zealand standard weathering quality index (WQI) method to improve its effectiveness in evaluating the weatherability of two sources of andesitic UGM aggregates. The modifications proposed include adjusting the gradation of aggregates, the wetting and drying process, and the chemical composition and temperature of the weathering solution, so that they can be more representative of the conditions of the UGMs in the pavement structure. The quality and quantity of produced fines (passing the 75 μm sieve), the physical degradation and the retained abrasion resistance of aggregates were also adopted to assess the weatherability of materials. It can be concluded that the standard WQI fails to evaluate the potential weatherability of andesite road aggregates. The findings also reveal the influence of the modified solution on both physical and chemical weathering of selected aggregates, and, as expected, marginal materials are more significantly affected.