道路工程施工概况(英文).doc
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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 paction/consolidation have an overwhelming effect on pavement performance, which cannot be ignored or pensated for in mix or structural d
2、esign.pactionpaction 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 (Roberts et al.,
3、19961).Figure 1: A Steel Wheel and a Pneumatic Tire Roller Working Side-by-Side.paction 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, 19898). Inadeq
4、uate paction results in a pavement with decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture susceptibility (Hughes, 19847; Hughes, 19898).paction Measurement and Reportingpaction reduces the volume of air in HMA. Therefore, the character
5、istic of concern is the volume of air within the pacted 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 paring a test specimens density with the density it would theoretically ha
6、ve 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
7、 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 that there are 7 percent air void
8、s (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 pacted to the desired value under close scrutiny then used as the pacti
9、on 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 pacted HMA and sent to a laboratory to determine its density. Usually, core density results are available the next day a
10、t 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 (abou
11、t 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 tested. Usually nuclear gauges are calib
12、rated 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 paction measurement methods and equipment to be used on contracts under their jurisdiction.Figure 2: Core ExtractionFigure 3: Pa
13、vement CoreFigure 4: Thin Lift Nuclear Density GaugeFigure 5: Taking a Nuclear Density ReadingFactors Affecting pactionHMA paction 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 c
14、onstruction (see Table 1).Table 1: Factors Affecting pactionEnvironmental FactorsMix Property FactorsConstruction FactorsTemperatureAggregateRollers*Ground temperature*Gradation*Type*Air temperature*Size*Number*Wind speed*Shape*Speed and timing*Solar flux*Fractured faces*Number of passes*Volume*Lift
15、 thicknessAsphalt BinderOther*Chemical properties*HMA production temperature*Physical properties*Haul distance*Amount*Haul timeFoundation supportA Note on the Time Available for pactionHMA temperature directly affects asphalt binder viscosity and thus paction. As HMA temperature decreases, the const
16、ituent asphalt binder bees more viscous and resistant to deformation resulting in a smaller reduction in air voids for a given pactive effort. As the mix cools, the asphalt binder eventually bees stiff enough to effectively prevent any further reduction in air voids regardless of the applied pactive
17、 effort. The temperature at which this occurs, monly referred to as cessation temperature, is often reported to be about 175F for dense-graded HMA (Scherocman and Martenson, 19849; Hughes, 19898). Below cessation temperature rollers can still be operated on the mat to improve smoothness and surface
18、texture but further paction will generally not occur.Mat temperature is crucial to both the actual amount of air void reduction for a given pactive effort, and the overall time available for paction. If a mats initial temperature and cool-down rate are known, the temperature of the mat at any time a
19、fter laydown can be calculated. Based on this calculation rolling equipment and patterns can be employed to: Take maximum advantage of available roller pactive effort. Rollers can be used where the mat is most receptive to paction and avoided where the mat is susceptible to excessive shoving. Ensure
20、 the mat is pacted to the desired air void content before cessation temperature is reached. This can be done by calculating the time it takes the mat to cool from initial temperature to cessation temperature. All paction must be acplished within this “time available for paction”.MultiCool, developed
21、 by Professor Vaughn Voeller and Dr. David Timm, is a Windows based program that predicts HMA mat cooling. MultiCool can be used to predict the time available for paction and is available on the National Asphalt Pavement Associations A Guide for Hot Mix Asphalt Pavement CD-ROM or for download at: Un
22、iversity of California Pavement Research Center (http:/.ucprc.ucdavis.edu/SoftwarePage.aspx) National Asphalt Pavement Association (http:/.asphaltpavement.org/index.php?option=_content&task=view&id=178&Itemid=273)paction EquipmentThere are three basic pieces of equipment available for HMA paction: (
23、1) the paver screed, (2) the steel wheeled roller and (3) the pneumatic tire roller. Each piece of equipment pacts the HMA by two principal means:1. By applying its weight to the HMA surface and pressing the material underneath the ground contact area. Since this pression will be greater for longer
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