Technical Note

 INFLUENCE OF COMPACTION TEMPERATURE
ON SEAM MIX AIR VOID CONTENT

 Background

 It has been observed in the field that “rigid” 50%SEAM/50%Bitumen wt. binder mixes exhibit higher air void contents after compaction than lower SEAM/Bitumen weight ratio mixes. While studies by the U.S. Bureau of Mines^* have indicated that such “rigid”, high air void content mixes still exhibit higher Marshall stabilities, even when densified at temperatures as low as 85 °C, than highly compacted conventional asphalt mixes, low mix air voids are generally favored by most agencies to minimize mix access to air, in order to reduce bitumen oxidation, and to reduce water intrusion in mix, in order to minimize moisture damage potential.  Actual field performance of higher void SEAM mixes has been shown to be good. <Ref: SEA pavement evaluation [FHWA-Liddle &U 0f W- Mahoney] Sulphur-84 International Conf. 1984>

 Laboratory Mix Compaction Studies

Laboratory Marshall compaction studies in Figure 1 confirm that while mix air void content increases when mixes are compacted at a lower temperature, this increase is not significant for the “flexible” 30%Sulphur/70%Bitumen mix but starts to become significant at lower temperatures for the “semi-rigid” 40%Sulphur/60%Bitumen mix and is very significant for the “rigid” 50%Sulphur/50%Bitumen mix.

<  30%Sulphur/70% Bitumen l  40%Sulphur/60% Bitumen p  50%Sulphur/50% Bitumen * Reference: US Department of Transportation FHWA-IP-80-14

Recommendations

In order to reduce mix air void content, it is recommended that SEAM mixes be compacted at acceptably high temperature levels. The options for doing this are the following:

1.)   “Tightening up” breakdown and intermediate rolling to compact mixes immediately after placement.

2.)   Use of a mass transfer vehicle (with a re-mixer) to improve mix temperature uniformity, and thereby improve mix compaction potential.

3.)   Design “semi-rigid” and “rigid” mixes to a lower air void content, e.g. 2% target, anticipating the higher air void content in the field. This may require slight changes in aggregate gradation.