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Tender mixes are defined as asphalt concrete (AC) mixes that shove excessively under steel wheel rollers, do not densify, and remain soft for extended periods of time after construction. Improper aggregate gradation, very little aggregate angularity, low filler content, high aggregate moisture content, and improper compaction procedures are the principal causes of tender mixes. However, under critical mix conditions the type of asphalt used can also determine the tenderness, or conversely the toughness, of an AC mix. Laboratory tests are examined to establish those physical properties of the asphalt that influence the toughness of a mix. These properties are related to the pavement toughness as measured in a full-scale paving laboratory in which actual paving construction practices are simulated. Pavement toughness is established at 100 F and at a uniform mix density. The penetration at 77 F and viscosities at 100, 140, and 275 F (or any combination of these properties) of the original asphalt show very poor to fair correlations with pavement toughness. An excellent correlation is obtained between pavement toughness and the viscosity at 100 F of asphalts recovered from test pavements. The viscosity at 140 F of aspalts exposed in the rolling thin film oven (RTFO) also relates well to pavement toughness. An excellent correlation for all asphalts tested results when the rolling thin fim (RTF) viscosity of the asphalt and toughness of the mix are compared at the same test temperature of 100 F. The viscosity at 140 F of an RTF residue is proposed as a practical method for grading setting qualities of paving asphalts. This viscosity can be determined rapidly and routinely in the laboratory.
grading (classifying), pavements, asphalts, bituminous cements, viscosity, setting time, toughness, penetration, rolling thin film test
Santucci, L. E.
Research engineer, Chevron Research Co., Richmond, Calif.
Schmidt, R. J.
Senior research associate, Chevron Research Co., Richmond, Calif.