道路工程施工概况(英文)

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 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 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 pacted to the desired value under close scrutiny then used as the paction 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 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 gammasource (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 gaugesrequire 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 paction measurement methods and equipment to be used on contracts under their jurisdiction.Figure 2: Core ExtractionFigure 3: Pavement CoreFigure 4: Thin Lift Nuclear Density GaugeFigure 5: Taking a Nuclear DensityReading Factors 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 construction (see Table 1).Table 1: Factors Affecting pactionA Note on the Time Available for pactionHMA temperature directly affects asphalt binder viscosity and thus paction. As HMA temperature decreases, the constituent 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 effort. The temperature at which this occurs, monly referred to as cessation temperature, is often reported to be about 175°F for dense-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 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 mat’s initial temperature and cool-down rate are known, the temperature of the mat at any time after 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 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. Allpaction must be acplished within this “time available for paction”.MultiCool, developed by Professor Vaughn Voeller and Dr. David Timm, is a Windows based program that predicts HMA mat cooling. MultiCool can be used to predict thetime available for paction and is available on the National Asphalt Pavement Association’s A Guide for Hot Mix Asphalt Pavement CD-ROM or for download at:∙University of California Pavement Research Center(/SoftwarePage.aspx)∙National Asphalt Pavement Association(/index.php?option=_content&task=view&id=178&Itemid=273)paction EquipmentThere are three basic pieces of equipment available for HMA paction: (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 materialunderneath the ground contact area. Since this pression will be greaterfor longer periods of contact, lower equipment speeds will produce morepression. Obviously, higher equipment weight will also increase pression.2.By creating a shear stress between the pressed material underneath theground contact area and the adjacent unpressed material. When bined withequipment speed, this produces a shear rate. Lowering equipment speed candecrease the shear rate, which increases the shearing stress. Highershearing stresses are more capable of rearranging aggregate into moredense configurations.These two means are of pacting HMA are often referred to collectively as “pactive effort”.Steel Wheel RollersSteel wheel rollers (see Figures 6 and 7) are self-propelled paction devices that use steel drums to press the underlying HMA. They can have one, two or even three drums, although tandem (2 drum) rollers are most often used. The drums can be either static or vibratory and usually range from 35 to 85 inches in width and 20 to 60 inches in diameter. Roller weight is typically between 1 and 20 tons (see Figures 5 and 6).Some steel wheel rollers are equipped with vibratory drums. Drum vibration adds a dynamic load to the static roller weight to create a greater total pactive effort. Drum vibration also reduces friction and aggregate interlock during paction, whichallows aggregate particles to move into final positions that produce greater friction and interlock than could be achieved without vibration. As a generalrule-of-thumb, a bination of speed and frequency that results in 10 – 12 impacts per foot is good. At 3000 vibrations/minute this results in a speed of 2.8 –3.4 mph.Figure 6: Steel Wheel Rollers Figure 7: Steel Wheel Rollers Pneumatic Tire RollersPneumatic tire rollers are self-propelled paction devices that uses pneumatic tires to pact the underlying HMA. Pneumatic tire rollers employ a set of smooth tires (no tread) on each axle; typically four or five on one axle and five or six on the other. The tires on the front axle are aligned with the gaps between tires on the rear axle to give plete and uniform paction coverage over the width of the roller. pactive effort is controlled by varying tire pressure, which is typically set between 60 and 120 psi (TRB, 2000[10]). In addition to a static pressive force, pneumatic tire rollers also develop a kneading action between the tires that tends to realign aggregate within the HMA. Because asphalt binder tends to stick more to cold tires than hot tires, the tire area is sometimes insulated with rubber matting or plywood to maintain the tires near mat temperature while rolling (see Figures 8 and 9).Figure 8: Pneumatic Tire Roller Figure 9: Pneumatic Tires paction SequenceHMA paction is typically acplished by a sequence of paction equipment. This allows each piece of equipment to be used only in its most advantageous situation resulting in a higher quality mat (both in density and in smoothness) than could be produced with just a single method of paction. A typical paction sequence consists of some or all of the following (in order of use):∙Screed. The screed is the first device used to pact the mat and may be operated in the vibratory mode. Approximately 75 to 85 percent of TMD will be obtained when the mix passes out from under the screed (TRB, 2000[10]).∙Rollers. Generally a series of two or three rollers is used. Contractors can control roller paction by varying things such as the types of rollers used, the number of roller used, roller speed, the number of roller passes overa given area of the mat, the location at which each roller works, and thepattern that each roller uses to pact the mat. Approximately 92 to 95 percent TMD will be obtained when all rollers are finished pacting the mat. Typical roller position used in paction are:∙Breakdown Roller. The first roller behind the screed (see Figure 10).It generally effects the most density gain of any roller in thesequence. Breakdown rollers can be of any type but are most oftenvibratory steel wheel and sometimes pneumatic tire.∙Intermediate Roller. Used behind the breakdown roller if additional paction is needed (see Figure 10). Pneumatic tire rollers aresometimes used as intermediate rollers because they provide adifferent type of paction (kneading action) than a breakdown steelwheel vibratory roller, which can help further pact the mat or at thevery least, rearrange the aggregate within the mat to make itreceptive to further paction.∙Finish Roller. The last roller in the sequence (see Figure 11). It is used to provide a smooth mat surface. Although the finish rollerdoes apply pactive effort, by the time it es in contact with the mat,the mat may have cooled below cessation temperature. Static steelwheel rollers are almost always used as finishing rollers because theycan produce the smoothest surface of any roller type.Figure 10: Paving Operation Showing a Steel Wheel Breakdown Roller and a PneumaticTire Intermediate RollerFigure 11: Finish Roller∙Traffic. After the rollers have pacted the mat to the desired density and produced the desired smoothness, the new pavement is opened to traffic.Traffic loading will provide further paction in the wheel paths of a finishedmat. Traffic may pact the mat an additional 2 to 4 percent over the life of the pavement.Footnotes(↵ returns to text)1.Roberts, F.L., Kandhal, P.S., Brown, E.R., Lee, D.Y., and Kennedy, T.W. (1996). HotMix Asphalt Materials, Mixture Design, and Construction. National Asphalt PavingAssociation Education Foundation. Lanham, MD. ↵2.Scherocman, J.A. and Martenson, E.D. (1984). Placement of Asphalt ConcreteMixtures. Placement and paction of Asphalt Mixtures, F.T. Wagner, Ed. ASTM SpecialTechnical Publication 829. American Society for Testing and Materials. Philadelphia,PA. pp. 3-27. ↵3.Scherocman, J.A. (1984, March). Guidelines for pacting Asphalt ConcretePavement. Better Roads, Vol. 54, No. 3. pp. 12-17.↵4.Geller, M. (1984). “paction Equipment for Asphalt Mixtures.”Placement and pactionof Asphalt Mixtures, F.T. Wagner, Ed. ASTM Special Technical Publication829. American Society for Testing and Materials. Philadelphia, PA. pp. 28-47.↵5.Brown, E.R. (1984). Experiences of Corps of Engineers in paction of Hot AsphaltMixtures. Placement and paction of Asphalt Mixtures, F.T. Wagner, Ed. ASTM SpecialTechnical Publication 829. American Society for Testing and Materials. Philadelphia,PA. pp. 67-79.↵6.Bell, C.A.; Hicks, R.G. and Wilson, J.E. (1984). Effect of Percent paction on AsphaltMixture Life. Placement and paction of Asphalt Mixtures, F.T. Wagner, Ed. ASTMSpecial Technical Publication 829. American Society for Testing andMaterials. Philadelphia, PA. pp. 107-130.↵7.Hughes, C.S. (October 1984). “Importance of Asphalt paction.”Better Roads, Vol.54, No. 10. pp. 22-24.↵8.Hughes, C.S. (1989). National Cooperative Highway Research Program Synthesis ofHighway Practice 152: paction of Asphalt Pavement. Transportation Research Board,National Research Council. Washington, D.C.↵9.Scherocman, J.A. and Martenson, E.D. (1984). Placement of Asphalt ConcreteMixtures. Placement and paction of Asphalt Mixtures, F.T. Wagner, Ed. ASTM SpecialTechnical Publication 829. American Society for Testing and Materials. Philadelphia,PA. pp. 3-27.↵10.Transportation Research Board (TRB). (2000). Hot-Mix Asphalt Paving Handbook2000. Transportation Research Board, National Research Council. Washington, D.C.↵。