Aggregates & Roadbuilding Magazine - March/April 2004... -
[Cached Version]
Published on: 5/4/2006
Last Visited: 6/14/2008
Paul Lum, director of new product commercialisation and quality assurance at Lafarge Canada Inc., explained in his paper "What mix designers have learned" that recent work included an evaluation of current asphalt aggregates to assess their fit with Superpave requirements.It was found, among other things, that the percentage of crushed faces required was dependent on mix design based on the number of gyrations and location of a mix in the pavement elevation.On the issue of using local aggregate deposits for Superpave mixes, Lum explained that local sources have been successfully used in the 50 and 75 (N design) gyration Superpave mixes that are generally required for lower traffic volumes.In locations subject to heavier loading however, 100 and 125 gyration Superpave mixes typically require washed screenings or manufactured (crushed) sand to retain the necessary air voids in the mixture.Lum added that this requirement does not represent a major change, as these high gyration Superpave mixes are used in applications similar to the MTO's existing Heavy Duty Binder Course (HDBC) and Dense Friction Course (DFC) mixes which require 100 per cent manufactured fine aggregates. According to Lum, there has been no difficulty in introducing RAP into Superpave mixes. Superpave mixes have been classified according to the nominal maximum size (NMS) of the aggregate, where NMS is one size larger than the first sieve to retain more than 10 per cent of the sample.The asphalt cement (AC) content of Superpave mixes may be determined from the Asphalt Institute manual SP2 which sets out AC content for a particular mix classification.Turning to equipment and testing, Lum stressed the importance of gyratory compactor maintenance and calibration, noting that sampling and sample handling, equipment calibration, equipment maintenance, internal angle and mould wear can all cause variability in test results between different gyratory compactors.A Dynamic Angle Validator (DAV) can be utilised to calibrate the internal angle of gyratory compactors and proper angle calibration will minimise variances in test results caused by differences in internal mould angles between different compactors.See sidebar, "The gyratory compactor angle issue". Lum concluded by describing a situation in which all calibrations and procedures, including internal angle, had been confirmed for a gyratory compactor, but where a study still obtained 1.5 per cent higher air voids.
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