TMR currently requires CBR test specimens to be compacted to 95% Standard Proctor maximum dry density (MDD). This density ratio is the minimum characteristic density ratio TMR specifies for some subgrade materials more than 300 mm below the subgrade level. As such, it provides a conservative estimate of the CBR at the subgrade level as such layers are compacted to higher standards. Other Australian road agencies compact their CBR specimens to higher density ratios and, as a consequence, their CBR values are higher. This report proposes TMR changes its test method to test specimens compacted to 97% Standard Proctor MDD, a value appropriate at subgrade level and in closer agreement with CBR values used by other road agencies.
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