Making the Grade – How Do Municipal Marshall Mixes Fare Under Superpave Specifications?

Over the past few years, the Ontario Ministry of Transportation (MTO) has gradually increased their usage of Superpave asphalt mixes in provincial paving contracts. It is anticipated that a complete transition to Superpave will occur at the provincial level within the next few years through the efforts of the multi-stakeholder Ontario Superpave Implementation Committee. In early 2002, a study was conducted by the MTO to investigate how traditional Marshall mixes used by the province fared under the Superpave specifications. Results of that investigation indicated that the mixes tested generally fell within Superpave specifications, but that some adjustments would be required. However, the asphalt mixes tested under the MTO study (HL-1, HL-4, DFC and HDBC) are generally not used by municipalities in large quantities. In the City of Ottawa, the primary mixes used for residential streets and arterials are HL-3 and HL-8. Mixes such as HL-1, SMA and HDBC are used, but in smaller quantities for heavier trafficked routes, bridge decks and the Transitway. In order to assess municipal mixes under Superpave criteria, the City of Ottawa retained numerous asphalt mix samples during normal quality assurance operations during the 2002 paving season. These mixes were compacted to 100-gyrations in the gyratory compactor to observe the gyration level at which the 4% air void criterion (Ndes) was met. Significant adjustments to mixes currently used in the City of Ottawa will be required to meet Superpave specifications including (but not limited to) a reduction in the amount of natural sand, a slight reduction in asphalt cement content and gradation adjustment to fall below the restricted zone. Both internally and through the Ontario Superpave Implementation Committee, the City of Ottawa will continue to work toward implementation of Superpave and provide other municipalities with technical expertise during the transition.