DETERMINATION OF AIR VOIDS CONTENT OF ASPHALT MIXTURES

The process of developing harmonised test methods in Europe has highlighted that many different test procedures can exist to measure a single property of asphalt mixtures. In some cases, the use of different equipment and procedures can produce the same test results provided that each method is adequately defined, such as for binder content. In other cases, it is not possible to achieve this equivalence, which has led to much debate and deliberation within the CEN specialist groups charged with developing common European test methods. A third category of properties require to be measured in different ways under different circumstances in order to obtain meaningful results. One such property is air voids content, which is discussed here. There are numerous ways of determining the densities of asphalt mixtures that are used in the calculation of air voids content, each of which will invariably give different results. Differences in the corresponding air voids contents of 2 to 3 per cent would not be unusual without making any allowance for the precision of the tests. For the densier mixtures, in particular, such differences could lead to quite different evaluation of likely performance. Unfortunately, many researchers have largely overlooked the differences that can occur in the calculation of air voids contents of asphalt mixtures. The methods used have rarely been described in published research and other technical papers where air voids content is a critical variable and so, comparison between different researchers' findings is fraught with difficulties. A review of the various means of determining density and the effect on calculated air voids content is carried out to establish what voids in the mixture are actually being assessed by the different procedures. (A) For the covering abstract see IRRD E100657.