The difference in volume is the volume of air in the compacted HMA mixture. The difference between Gmm and Gmb is volume.Relationship with Other Specific Gravities However, increased asphalt binder content can also potentially make a HMA mixture more likely to rut or shove. For instance, if adequate compaction is not being achieved, increasing asphalt binder content will fill more voids with asphalt binder and thus lower the air void content for the same amount of compaction. If percent air voids is used as a primary quality assurance characteristic, there can be a tendency to control this characteristic at the expense of others. During HMA production and pavement construction, theoretical maximum specific gravity should be determined at regular intervals because it may change over time as the asphalt binder content and properties as well as aggregate properties vary over time. This bulk specific gravity is then compared to the most current theoretical maximum specific gravity to determine air voids. Percent air voids is typically calculated using Gmm and Gmb in the following equation:Įach time density is to be determined a measure of bulk specific gravity is made by either coring the pavement and determining bulk specific gravity on the sample or using a non-destructive testing method. Although this is not wrong, since density is used to calculate percent air voids, the fundamental parameter of concern is always percent air voids. The terms “percent air voids” and “density” are often used interchangeably.
#DENSITY AND SPECIFIC GRAVITY LAB REPORT CONCLUSION PORTABLE#
Once Gmm is known, portable non-destructive devices can be used to measure HMA density in-place. Percent air voids is calculated by comparing a test specimen’s bulk specific gravity (Gmb) with its theoretical maximum specific gravity (Gmm) and assuming the difference is due to air. This volume is typically quantified as a percentage of air voids by volume and expressed as “percent air voids”. Therefore, the characteristic of concern in compaction is the volume of air within the compacted HMA. This is because compaction reduces the volume of air in HMA. In-place air void measurements are used as a measure of compaction (Figure 2). In-place Density MeasurementĪs previously discussed, theoretical maximum specific gravity is needed to calculate air void content therefore, it is involved in in-place air void determination during HMA pavement construction. The volume is calculated by weighing the mass of the water displaced by the sample and dividing by the unit weight of water.
The mass is determined by measuring the dry mass of the sample either at the beginning of the test or after it has been dried at the end of the test. The basic premise of the maximum specific gravity is to divide the mass of the sample by the volume of the sample excluding the air voids.
It is also used to calculate the amount of asphalt absorbed in a HMA mixture (Vba), which is then used in determining the effective asphalt content (Pbe). Theoretical maximum specific gravity is used along with bulk specific gravity values from field cores and laboratory compacted specimens to calculate air voids and the in-place air voids of a HMA pavement. The theoretical maximum specific gravity test is integral to Superpave mix design as well as field quality assurance. Maximum theoretical specific gravity sample.
The standard theoretical maximum specific gravity test is: Theoretical maximum specific gravity is then the sample weight divided by its volume. Theoretical maximum specific gravity is determined by taking a sample of loose HMA (i.e., not compacted), weighing it and then determining its volume by calculating the volume of water it displaces (Figure 1). This calculation is used both in Superpave mix design and determination of in-place air voids in the field. Theoretical maximum specific gravity is a critical HMA characteristic because it is used to calculate percent air voids in compacted HMA. Theoretical maximum specific gravity can be multiplied by the density of water (62.4 lb/ft3 or 1000 g/L) to obtain a theoretical maximum density (TMD) or “Rice” density (named after James Rice, who developed the test procedure). Thus, theoretically, if all the air voids were eliminated from an HMA sample, the combined specific gravity of the remaining aggregate and asphalt binder would be the theoretical maximum specific gravity. The theoretical maximum specific gravity (Gmm) of a HMA mixture is the specific gravity excluding air voids.
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