Applied Geology of Wellington Rocks for Aggregate and Concrete

Abstract

This study was initiated to examine geological aspects of Wellington greywacke-suite rocks in relation to their end use as an engineering material - aggregate, particularly for concrete. An attempt has been made to map (at least in part), identify and categorise rocks for quarrying in the Wellington region, to evaluate and quantify their properties as aggregates and to appraise their qualities in concrete – in short to equate rock geology to aggregate and concrete performance as a tool for resource management. Study of bedding 1ed to a classification into three lithofacies and some 70 representative samples were examined petrographically. For engineering purposes, Wellington rocks may be divided into two categories, greywacke and argillite, each having separate and distinct mineralogies and chemistries which do not alter significantly between lithofacies. Greywacke is coarser and may be distinguished from argillite texturally at a mean grain size of 5 phi (0.031 mm). Rock properties, in particular strength, modulus, density, hardness and degradation tendencies, are linked directly or indirectly with mean grain size. Argillites, though more dense, are generally weaker, softer, less elastic and degrade more readily than greywackes, the latter property being readily assessed from a newly devised test based on the destruction of chlorite by hydrochloric acid. As aggregates, greywackes produce similar particle shapes irrespective of grading. Argillites, which are generally more angular, produce concretes which are more difficult to work. Physical properties of aggregate, inherently those of its parent rock, are reflected in concrete made from it. The possibility of laumontite promoting cement alkali-silicate reaction is obviated by the mode of occurrence of minerals within the rock. Although argillite aggregates are unsuitable in certain environments and return lower strength in concrete than do greywacke aggregates, they still have a place in low strength concrete applications.