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Climate and the durability of South African road aggregates

  • Weinert H. H. 
International Symposium on Aggregates Theme 3. Tests on Aggregates

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 (EJ) to the annual precipitation (Pa). 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.

Keywords

Kaolinite Road Construction Crystalline Rock Weathered Rock Diamictites 

Le climat et l'altérabilité des granulats routiers Sud-Africains

Résumé

L'absence de gisements importants d'alluvions et l'absence totale de moraines en Afrique du Sud peuvent être expliquées par l'histoire géologique et par la topographie du pays. En Afrique du Sud on doit donc obtenir des graviers et des sables routiers soit à partir de roches saines concassées soit à partir de roches altérées par les intempéries. L'emploi de tels granulats altérés entraîne le risque de leur altération ultérieur au dessous des limites des spécifications puisque l'altération ne se termine pas au moment de l'éloignement du granulat de son gisement. En effet la vitesse de l'altération ordinairement s'accroît après la mise en œuvre du granulat dans la chaussée. L'estimation de l'altérabilité d'un granulat est donc essentielle lorsqu'on le sélectionne. L'altérabilité d'un granulat, qui peut varier selon les exigences des couches de chaussée, dépend du type pétrologique du matériau—dont le quartz constitue un indicateur important—et de son milieu. Le climat est l'élément le plus important du milieu régissant l'altération. Les climats sud-africains s'échelonnent entre un climat extrêmement sec et un climat semi-tropicalhumide où la précipitation annuelle se produit en hiver ou en été. La valeur N de Weinert qui est le rapport de l'évaporation estimée pendant le mois le plus chaud (EJ) à la précipitation annuelle (Pa) rend compte de ces conditions très différentes. La formule est ainsi conçue:
$$N = \frac{{12 E_J }}{{P_a }}$$

Les granulats routiers sud-africains ont été classés en neuf groupes selon la présence ou l'absence du quartz. L'altérabilité de ces groupes est estimée par rapport au milieu climatique du gisement du matériau. Les normes sud-africaines des granulats routiers—que certains considéreraient comme assez sévères—tiennent compte de cette variation importante des conditions de milieu et des conditions pétrologiques.

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References

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Copyright information

© International Association of Engineering Geology 1984

Authors and Affiliations

  • Weinert H. H. 
    • 1
  1. 1.Council for Scientific and Industrial ResearchNational Institute for Transport and Road ResearchPretoriaSouth Africa

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