Climate and the durability of South African road aggregates: Fid-Bau Portal

Climate and the durability of South African road aggregates

Weinert, H. H.

Abstract The absence of noteworthy unconsolidated fluviatile deposits, and the complete absence of unconsolidated glacial deposits can be explained from the South African geological history and topography. Apart from the crushing of fresh rock, South African road “gravel” and “sand” therefore have to be obtained from weathered rock. The use of such aggregates involves the risk that they may deteriorate below the design standards because weathering does not cease when the aggregate is removed from its source; indeed, the rate of weathering mostly increases after the aggregate has been placed in the pavement. Assessing the durability of the aggregate is therefore a primary requirement during selection. The durability of an aggregate, which differs according to the pavement layer in which it is used, depends on the petrological type of the material, of which quartz is an important indicator. Durability also depends on environment. Of the environmental conditions that control weathering, climate is the most important. South Africa's climate varies from extremely arid to subtropical-humid with either summer or winter rainfall. These widely different conditions have been accommodated in an expression known as the Weinert N-value which is a ratio of the computed evaporation during the warmest month ($ E_{J} $) to the annual precipitation ($ P_{a} $). It reads:$$N = \frac{{12 E_J }}{{P_a }}$$ A classification based on the absence or presence of quartz has been developed for South African road aggregates. The durability of the resulting nine groups is assessed in relation to the climatic environment at the material's source. The wide variation of petrological and environmental conditions is incorporated in the South African standards for road aggregates, which some may consider rather strict.