Overview
Construction 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 structural design。
Compaction
Compaction 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 cloly spaced arrangement. This reduction of air volume produces a corresponding increa in HMA density （Roberts et al.， 1996［1]）.
Figure 1: A Steel Wheel and a Pneumatic Tire Roller Working Side-by-Side.
Compaction is the greatest determining factor in den graded pavement performance (Scherocman and Martenson, 1984[2]; Scherocman， 1984［3]； Geller, 1984[4］; Brown， 1984［5］; Bell et. al., 1984［6]; Hughes, 1984[7］; Hughes, 1989［8］). Inadequate compaction results in a pavement with decread stiffness， reduced fatigue life, accelerated aging/decread durability, rutting， raveling， and moisture susceptibility （Hughes, 1984［7]candy是什么意思; Hughes， 1989[8］).
Compaction Measurement and Reporting
Compaction reduces the volume of air in HMA。 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 expresd as “percent air voids"。 Percent air voids is calculated by comparing a test specimen’s density with the density 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 to 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 becau any volume that is not asphalt binder or aggregate is assumed to be air。 For example， a density reported as 93 percent Rice means that 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 ction that is compacted to the desired value under clo scrutiny then ud as the compaction standard for a particular job.
Pavement air voids are measured in the field by one of two principal methods:
∙conspiracy theoryCores (Figures 2 and 3). A small pavement core is extracted from the compacted HMA and nt to a laboratory to determine its density. Usually, core density 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 usually 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 inrted into the pavement。 Readings are obtained in about 2 – 3 minutes. Nuclear gauges require calibration to the specific mixture being tested. 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 ud on contracts under their jurisdiction.
Figure 2: Core Extraction | wcg什么意思 Figure 3： Pavement Core |
anbFigure 4： Thin Lift Nuclear Density Gauge | vals Figure 5： Taking a Nuclear Density Reading |
| |
Factors Affecting Compaction
HMA compaction is influenced by a myriad of factors； some related to the environment， some determined by mix and structural design and some under contractor and agency control during construction (e Table 1）。
recentlyTable 1: Factors Affecting Compaction |
Environmental Factors | Mix Property Factors | Construction Factors |
Temperature | Aggregate | Rollers |
*Ground temperature | *Gradation | ＊Type |
*Air temperature | *Size | *Number |
＊Wind speed | ＊Shape | *Speed and timing |
＊Solar flux | ＊Fractured faces | ＊Number of pass |
| dolores ＊Volume | ＊Lift thickness |
| Asphalt Binder | Other |
| *Chemical properties | ＊HMA production temperature |
| ＊Physical properties | *Haul distance |
| ＊Amount | ＊Haul time |
| | Foundation support |
| amalgamation | |
A Note on the Time Available for Compaction
加油英语怎么说
HMA temperature directly affects asphalt binder viscosity and thus compaction。 As HMA temperature decreas， 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 regardless of the applied compactive effort. The temperature at which this occurs, commonly referred to as cessation temperature, is often reported to be about 175°F for den-graded HMA （Scherocman and Martenson， 1984［9]； Hughes, 1989［8］)。 Below cessation temperature rollers can still be operated on the mat to improve smoothness and surface texture but further compaction will generally not occur。
ppc是什么意思Mat temperature is crucial to both the actual amount of air void reduction for a given compactive effort, and the overall time available for compaction。 If a mat’s initial temperature and cool—down rate are known, the temperature of the mat at any time after
laydown can be calculated. Bad on this calculation rolling equipment and patterns can be employed to:
