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1、OverviewConstruction quality is crucial to the long-term pavement performance. Construction factors such as surface preparation, placement, joint construction and compaction/consolidation have an overwhelming effect on pavement performance, which cannot be ignored or compensated for in mix or struct
2、ural design.CompactionCompaction is the process by which the volume of air in an HMA mixture is reduced by using external forces to reorient the constituent aggregate particles into a more closely spaced arrangement. This reduction of air volume produces a corresponding increase in HMA density (Robe
3、rts et al., 19961).Figure 1: A Steel Wheel and a Pneumatic Tire Roller Working Side-by-Side.Compaction is the greatest determining factor in dense graded pavement performance (Scherocman and Martenson, 19842; Scherocman, 19843; Geller, 19844; Brown, 19845; Bell et. al., 19846; Hughes, 19847; Hughes,
4、 19898). Inadequate compaction results in a pavement with decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture susceptibility (Hughes, 19847; Hughes, 19898).Compaction Measurement and ReportingCompaction reduces the volume of air in HMA.
5、Therefore, the characteristic of concern is the volume of air within the compacted pavement, which is typically quantified as a percentage of air voids in relation to total volume and expressed as “percent air voids”. Percent air voids is calculated by comparing a test specimens density with the den
6、sity it would theoretically have if all the air voids were removed, known as “theoretical maximum density” (TMD) or “Rice density” after the test procedure inventor.Although percent air voids is the HMA characteristic of interest, measurements are usually reported as a measured density in relation t
7、o a reference density. This is done by reporting density as: Percentage of TMD (or “percent Rice”). This expression of density is easy to convert to air voids because any volume that is not asphalt binder or aggregate is assumed to be air. For example, a density reported as 93 percent Rice means tha
8、t there are 7 percent air voids (100% 93% = 7%). Percentage of a laboratory-determined density. The laboratory density is usually a density obtained during mix design. Percentage of a control strip density. A control strip is a short pavement section that is compacted to the desired value under clos
9、e scrutiny then used as the compaction standard for a particular job.Pavement air voids are measured in the field by one of two principal methods: Cores (Figures 2 and 3). A small pavement core is extracted from the compacted HMA and sent to a laboratory to determine its density. Usually, core densi
10、ty results are available the next day at the earliest. This type of air voids testing is generally considered the most accurate but is also the most time consuming and expensive. Nuclear gauges (Figures 4 and 5). A nuclear density gauge measures in-place HMA density using gamma radiation. Gauges usu
11、ally contain a small gamma source (about 10 mCi) such as Cesium-137 located in the tip of a small probe, which is either placed on the surface of the pavement or inserted into the pavement. Readings are obtained in about 2 3 minutes. Nuclear gauges require calibration to the specific mixture being t
12、ested. Usually nuclear gauges are calibrated to core densities at the beginning of a project and at regular intervals during the project to ensure accuracy.Each contracting agency or owner usually specifies the compaction measurement methods and equipment to be used on contracts under their jurisdic
13、tion.Figure 2: Core ExtractionFigure 3: Pavement CoreFigure 4: Thin Lift Nuclear Density GaugeFigure 5: Taking a Nuclear Density ReadingFactors Affecting CompactionHMA compaction is influenced by a myriad of factors; some related to the environment, some determined by mix and structural design and s
14、ome under contractor and agency control during construction (see Table 1).Table 1: Factors Affecting CompactionEnvironmental FactorsMix Property FactorsConstruction FactorsTemperatureAggregateRollers*Ground temperature*Gradation*Type*Air temperature*Size*Number*Wind speed*Shape*Speed and timing*Sola
15、r flux*Fractured faces*Number of passes*Volume*Lift thicknessAsphalt BinderOther*Chemical properties*HMA production temperature*Physical properties*Haul distance*Amount*Haul timeFoundation supportA Note on the Time Available for CompactionHMA temperature directly affects asphalt binder viscosity and
16、 thus compaction. As HMA temperature decreases, the constituent asphalt binder becomes more viscous and resistant to deformation resulting in a smaller reduction in air voids for a given compactive effort. As the mix cools, the asphalt binder eventually becomes stiff enough to effectively prevent any further reduction in air voids rega
