FAQ >> Why test aggregates?

Why test aggregates?

It is important to consider why aggregates need assessment. It must be recognized that aggregates are a fundamental component of asphalts. The chosen aggregate must have certain properties to withstand the stresses imposed both at and within the road surface and sub-surface layers. The resulting performance of the constituent particles is dependent upon the manner in which they are bound together and the conditions under which they operate. Their assessment is carried out in a laboratory where a range of test methods is used to predict subsequent in-service performance.

Testing is therefore a process of prediction that provides a quantifiable indication of quality which may then be implemented in the form of specification requirements to rank different aggregates in relation to preconceived levels of performance. However, there is nothing to guarantee that a laboratory assessment of an aggregate to meet specification requirements will fully predict its potential for successful in-service performance given the changing in-service expectations of a modern road surfacing.

This is particularly true since aggregates are now being used in ways and exposed to levels of stress not experienced previously nor even anticipated in the original development of certain testing methods and specification limits. This increase in performance expectation from the finished structure prompts basic questions such as the following.

  • What aggregate properties should be measured?
  • Can these be quantified?
  • What do current test method measure?
  • What factors influence the test method?
  • Does the test method relate to actual in-service conditions?
  • Does the method predict performance?
  • Is the test method cheap and does it use simple equipment?
Any improvement in the prediction process of assessing aggregate performance will ultimately result in better value for money and the improved use of limited reserves of aggregate. Although performance is now common to many aspects of engineering, very little is understood about the factor that influence and control how aggregates perform in use. There have been many instances of materials failing prematurely despite having met specified requirements.

Traditionally, the immediate response has been to blame the bitumen, the surfacing contractor or the weather. Aggregates have rarely been blamed initially as the reason for failure. If premature failure occurs then it may he argued that the specification requirements and/or the testing methods used were unable to predict the suitability of the aggregate for the given set of in-service conditions, that is the prediction system which had been used had failed to perform. One of the main aims of this post is to make practising engineers aware of the ability to predict aggregate performance and the limitations of existing standards and specifications.

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