Determination of Moisture in Hot-Mix Asphalt and Relationship with Tender Mixture Behavior in the Laboratory

Abstract

In recent years, a significant percentage of coarse-graded Superpave® asphalt mixtures have exhibited tender mix behavior. Although the underlying causes of the phenomenon are not well understood, many experts believe the main cause of the tender zone problem is related to the amount of fluids, particularly excess moisture, in the asphalt mixture. Unfortunately, some asphalt technologists consider the accurate determination of moisture in an asphalt mixture to be a difficult prospect. The purpose of the present research was to determine if the tender mix behavior exhibited by some coarse-graded Superpave mixtures is related to moisture content and to develop procedures to accurately measure the moisture content in an asphalt mixture. The research indicated that conventional oven drying (110°C) was an acceptable alternative to the standard distillation procedure (ASTM D1461). The use of the microwave oven procedure for determination of moisture content also appeared to be acceptable, although in the present research there was a significant problem with breakage of the glass containers. The best sampling container for determination of the moisture content after a storage period was a 4-L aluminum paint can. When the paint can was used and sealed, the moisture content after 72 h of storage at room temperature was statistically equal to the moisture content determined immediately after sampling of the asphalt mixture. Other containers studied (paper grocery bag and plastic oven bag) indicated lower moisture content values after storage and a consistent loss of moisture the longer the sample was stored.

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