FAQ >> Classifying aggregates in an asphalt

Classifying aggregates in an asphalt

Geology is the study of rocks. It covers a wide spectrum, ranging from the classification of the many rock types. the study of their origins and variations in mineralogical composition to their structure and alteration over many millions of years.

Basically, the level of in-service performance of a particular rock depends on its geological properties. For example, consider the high levels of skid resistance exhibited by certain sandstones and gritstones. These materials consist, essentially, of sand grains which are held together by a cementing matrix. It is this sandpaper-like surface texture which gives the aggregate its high frictional properties.

It is advisable to ask certain questions when considering the potential suitability of a particular aggregate. These should, for example. include details of the type of rock, its composition. its chemistry. its grain size. whether it has been altered, its tendency to degrade. abrade or break under trafficking and whether bitumen will adhere to the surface. These and other questions need to be answered in order to understand fully how an aggregate will perform under the variety of conditions which may exist in a pavement.

The possible range of aggregates available for use in asphalts is potentially vast. However, if an aggregate is to be used in a particular mixture, a certain quality is required and this rules out many potential sources. Rounded gravels, for example, are rightly excluded for use in particular asphalts. Materials for use as wearing courses almost always require high values of skid resistance and this severely limits the number of potential sources. Considerable research and experience has resulted in the majority of main roads in the UK being surfaced with aggregates belonging to a single type or group, i.e. the gritstone group.

In terms of their use in an asphalt, the types of potentially available aggregate may be broken down into three main groups.

  • Natural一his includes all those sources where the rock is naturally occurring in the ground and obtained by conventional blasting and quarrying. excavating or dredging from land, river, estuarine or marine deposits.
  • Artificial/synthetic一these may be formed as a result of industrial processes (such as those carried out by the steel industry) or specifically made for high performance purposes. e.g. calcined bauxite used in high friction surfaces.
  • Recycled一the recycling of existing asphalts is growing significantly in importance and in many countries this is now a major source of aggregate.

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