The proposal to grade asphalt cements by viscosity at 140 F, and to eliminate grading by penetration at 77 F, has been made to facilitate high temperature pavement construction operations, and particularly to eliminate delayed rolling.
This paper is critical of this proposed change to grading asphalt cements by viscosity at 140 F for the following reasons:
1. The effect this proposed change would have on long term pavement performance has been overlooked or ignored.
2. Elimination of delayed rolling can be achieved much more effectively by discarding steel-wheel rollers, and by employing properly operated pneumatic-tire rollers equipped for rapid adjustment of tire inflation pressure.
3. One of the proposed new viscosity grades, AC 12, would include all of the current paving grades, 40 to 50, 60 to 70, 85 to 100, 120 to 150, and 150 to 200 penetration, and this proposal implies, therefore, that pavement performance will be the same if asphalt cements ranging from 40 to 50 to 150 to 200 penetration are used.
4. Photographic evidence is presented to show the drastic difference in pavement performance that can occur when 85 to 100 versus 150 to 200 penetration asphalt cements are used under the severe climatic conditions that occur in the interior of Canada, and probably in the neighboring United States.
5. Differences of 50 and 70 per cent in Marshall stabilities of paving mixtures determined at 140 and 39.2 F, respectively, occur when they are made with asphalt cements of the same viscosity at 140 F, but with widely different penetrations at 77 F. These large differences in Marshall stability would influence both warm weather and cold weather pavement performance.
6. If the proposed grading by viscosity at 140 F were narrowed or circumscribed in any way, so as to eliminate asphalt cements with low penetrations at 77 F, this would exclude asphalt cements that satisfy current ASTM, AASHO, and Asphalt Institute specifications, which are based on grading by penetration at 77 F, and that have excellent records of service performance.
7. Grading asphalt cements by penetration at 77 F makes low viscosity asphalt cements available which, due to their lower resistance to compaction, facilitate: (a) attaining compaction to 100 percent of laboratory compacted density by rolling during construction, (b) achieving faster compaction by traffic to 100 per cent of laboratory compacted density, (c) achieving compaction to much higher density by rolling during cold weather construction; all of which will result in longer pavement life, in higher load carrying capacity per inch thickness of pavement, and in better pavement performance.